Waters Corp Variable Aperture Slit Capstone Design Report

WATERS CORPORATION
VARIABLE APERTURE SLIT
MEIE 701-702
Capstone 2 Technical Design Report
December 4th, 2012
Department of Mechanical and Industrial Engineering
College of Engineering, Northeastern University
Boston, MA 02115
Waters Corp Variable Aperture Slit
Capstone 2 Design Report
Design Advisor: Prof. Mohammad Taslim
Design Team
Aaron Gill, Kevin McMorrow
Paul Ventola, Hanshen Zhang

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WATERS CORP VARIABLE APERTURE SLIT
Design Team
Aaron Gill, Kevin McMorrow
Paul Ventola, Hanshen Zhang
Design Advisor
Prof. Mohammad Taslim
Sponsor Contacts
Senthil Bala and Colin Fredette
Abstract
The goal of this project is to design a Variable Aperture Slit (VAS) system used for providing different
resolutions of light to be projected into a photodiode array (PDA) for examining the composition of a
fluid sample. The VAS will consist of different slit sizes that will be used to limit or refine the light from
an ultraviolet (UV) or deuterium light source. The VAS will attach to a holder aligned between a reflected
and refracting mirror to permit a certain amount of light with a specific resolution that is unique to each
slit size through from the light source to the PDA. Prior to passing through the slit, the UV light will pass
through a sample fluid, prepared via liquid chromatography separating the components of a substance,
which will absorb various wavelengths of light corresponding to the absorption pattern of the substance.
Based on the remaining wavelengths of the light which are passed through the slit and then refracted into
an optical detector, the composition of the sample fluid can be determined. In the detector, each slit width
will provide a different signal-to-noise ratio and resolution.
Based on the need for a VAS system, an accurate and repeatable design is required to ensure the quality
of the system. A simple design focused on movable holders for interchangeable slits provides
repeatability for the process and manufacturing. Current designs display the benefits of a movable holder
system. Physical models will be created based on the final design and tested to verify the system
maintains accurate and repeatable with a rotating motor. The device will be able to be calibrated based on
repeatable testing results for use in implementation for full scale manufacturing.

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Table of Contents
1 Acknowledgements .....................................................................................................................8
2 Project Statement ........................................................................................................................9
3 Background Technologies and Components ..................................................................................9
3.1 Liquid Chromatography .......................................................................................................9
3.2 Optical Bench....................................................................................................................11
3.2.1 Deuterium Lamp Light Source ....................................................................................12
3.2.2 Concave Mirror Set ....................................................................................................12
3.2.3 Aperture Slit...............................................................................................................13
3.2.4 Diffraction Grating .....................................................................................................14
3.2.5 Photodiode Array (PDA) Detector ...............................................................................15
4 Design and Sponsor Constraints .................................................................................................16
4.1 Patented VAS Systems.......................................................................................................19
5 Initial Designs ...........................................................................................................................22
5.1 Rotating Pin Wheel Design.................................................................................................22
5.2 Vertical Actuation Design ..................................................................................................23
5.3 Horizontally Sliding Slit Aperture.......................................................................................24
5.4 Rotational Holder System...................................................................................................24
5.5 Control System Requirements.............................................................................................26
6 Analysis and Testing .................................................................................................................26
6.1 Light Path Analysis............................................................................................................26
6.1.1 Benefits of different slit widths....................................................................................26
6.1.2 Diffraction error from tilt in the “Z” direction...............................................................27
6.2 Light Path Calibration and Verification Test Method............................................................29
6.3 Motion Control Analysis ....................................................................................................30
6.3.1 Motor Systems ...........................................................................................................30
6.3.2 Control Path Design....................................................................................................32
6.3.3 Control System Hardware ...........................................................................................32
7 Final Prototype..........................................................................................................................34
7.1 Optical Slit Aperture Holder ...............................................................................................34
7.2 Rotational Control System..................................................................................................35
7.3 Combined Prototype ..........................................................................................................37
7.4 System Operation ..............................................................................................................38

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7.5 Testing and Analysis ..........................................................................................................38
7.5.1 Optical Analysis and Testing.......................................................................................38
8 Project Management Overview...................................................................................................41
8.1 Design/Model/Construction Phases.....................................................................................41
8.1.1 Initial Design Phase....................................................................................................41
8.1.2 Modeling Phase..........................................................................................................42
8.1.3 Ordering Phase...........................................................................................................42
8.1.4 Initial Construction Phase............................................................................................42
8.1.5 Final Production Phase................................................................................................42
8.2 Report and Presentation Phases...........................................................................................43
9 Future Work..............................................................................................................................43
10 Summary ..............................................................................................................................43
11 Intellectual Property...............................................................................................................44
11.1 Description of Problem ......................................................................................................44
11.2 Proof of Concept................................................................................................................44
11.3 Progress to Date.................................................................................................................44
11.4 Individual Contributions.....................................................................................................44
11.5 Future Work......................................................................................................................44
.......................................................................................................................................... References
........................................................................................................................................................45
12....................................................................................................................................................45
13 Appendices ...........................................................................................................................47
13.1 Appendix A: System Part Drawings ....................................................................................47
13.2 Appendix B: Arduino Control Code ....................................................................................63
13.2.1 Code to define the Arduino in MATLAB:....................................................................63
13.2.2 Code to install arduino to work with MATLAB:...........................................................63
13.2.3 Code to run Control Program:......................................................................................64
13.3 Appendix C: Driver Shield Schematic .................................................................................65
13.4 Appendix D: Motor Wire Diagram......................................................................................66
13.5 Appendix E: Stepper Motor Specifications ..........................................................................67
13.6 Appendix F: Uno Schematic ...............................................................................................68
13.7 Appendix G: H Bridge Circuit Schematic ...........................................................................70
13.8 Appendix H: Microcontroller Schematic .............................................................................72

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13.9 Appendix I: Microcontroller Block Diagram........................................................................73

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List of Figures
Figure 1: Features of a Variable Aperture Slit [1]..................................................................................9
Figure 2: Liquid Chromatography Sample Pathway [2].......................................................................10
Figure 3: Full HPLC system including column and detector modules. [3].............................................11
Figure 4: How the detector displays sample peaks [3] .........................................................................11
Figure 5: Optical Bench Assembly, © Waters Corp. [3] ......................................................................11
Figure 6: Deuterium Light Spectrum..................................................................................................12
Figure 7: Diffraction pattern from a slit of width four wavelengths with an incident plane wave. ...........13
Figure 8: Diffraction Grating Spectrograph of Visible Light ................................................................14
Figure 9: Responsivity of silicon photodiode vs. wavelength of incident light. ......................................15
Figure 10: AutoCAD drawing of current aperture slit used in the PDA. All dimensions in millimeters. ..16
Figure 11: View of the PDA casting from the bottom side. Note the location of the current aperture
holder. .............................................................................................................................................17
Figure 12: View of the PDA casting from the top displaying modifiable section. ..................................17
Figure 13: View of the PDA casting from the top displaying the optical mirror area..............................17
Figure 14: Section view displaying the location of the aperture (displayed in red) within the casting. .....17
Figure 15: View of the PDA casting from the side, displaying bottom and top clearance values.............18
Figure 16: Approximate path of light travel with the PDA...................................................................18
Figure 17: Thermo Scientific Variable Slit Detector Layout [1] ...........................................................19
Figure 18: Fixed Position Aperture Slit Detector © Thermo Scientific Co. [1] ......................................20
Figure 19: Mechanism for US Patent 7170595 [10].............................................................................21
Figure 20: Slit mechanism for U.S. Patent 4612440 [11] .....................................................................21
Figure 21: Mechanism for U.S. patent 2852684 [12] ...........................................................................21
Figure 22: Mechanism for U.S. patent 5451780 [13] ...........................................................................21
Figure 23: Initial design emulating a rotating pin wheel.......................................................................22
Figure 24: Initial design displaying vertical actuation between two slits. ..............................................23
Figure 25: Initial design with two moving surface plates with an internal gap as an optical slit...............24
Figure 26: Initial design of hexagonal slit aperture array......................................................................25
Figure 27: Top-down view of hexagonal slit aperture array..................................................................25
Figure 28: Initial design of holder with the attached aperture slits on a rotational base...........................25
Figure 29: Experimental Set-up of a laser through an optical slit displaying a coordinate system [14].....27
Figure 30: Diffraction pattern in the plane x'o, y'o of a single slit inclined at an angle θ=45° to the
direction of propagation z’: lx = 0.02mm; D = 1m [14] .......................................................................28
Figure 31: Light Path Calibration Set-up ............................................................................................29
Figure 32: Slit image and intensity detecting tool................................................................................30
Figure 33: Linear Stage System.........................................................................................................31
Figure 34: Brushless Servo Motor......................................................................................................31
Figure 35: DC Stepper Motor with Offset Rotation Point ....................................................................32
Figure 36: Motion Control Diagram...................................................................................................32
Figure 37: Front view of Arduino Uno ...............................................................................................33
Figure 38: Adafruit motor shield stacked on Arduino Uno...................................................................34
Figure 39: Current VAS design, with the aperture slits secured to a rotational base. ..............................35
Figure 40: NEMA 11 Bipolar Stepper Motor ......................................................................................36
Figure 41: Assembled Control System and Prompts ............................................................................36

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Figure 42: VAS assembly attached to rotary motor. ............................................................................37
Figure 43: Model of PDA casting displaying the new bore hole required for integration of the VAS
system..............................................................................................................................................38
Figure 44: Reference image for comparing test data to determine accuracy and repeatability .................39
Figure 45: Raw test analysis data.......................................................................................................40
Figure 46: Refined Data Table...........................................................................................................41
Figure 47: AutoCAD drawing for holder base.....................................................................................49
Figure 48: AutoCAD drawing for 50m aperture slit...........................................................................52
Figure 49: AutoCAD drawing for 100m aperture slit. ........................................................................55
Figure 50: AutoCAD drawing for 200m aperture slit. ........................................................................58
Figure 51: Drawing of Stepper Motor.................................................................................................59
Figure 53: AutoCAD drawing of new bore hole required for PDA casting (page 1 of 2). .......................61
Figure 54: AutoCAD drawing of new bore hole required for PDA casting (page 2 of 2). .......................62
Figure 56: Schematic of Driver Shield ...............................................................................................65
Figure 57: Motor Wire Diagram ........................................................................................................66
Figure 58: Stepper Motor Specifications.............................................................................................67
Figure 59: Arduino Uno Schematic....................................................................................................70
Figure 60: H Bridge Circuit Schematic...............................................................................................70
Figure 61: Microcontroller Schematic ................................................................................................72
Figure 62: Microcontroller Block Diagram.........................................................................................73
List of Equations
Equation 1: Beers Law......................................................................... Error! Bookmark not defined.
Equation 2: Foci Location Determination Equation................................ Error! Bookmark not defined.
Equation 3: Fraunhofer Diffraction Equation ......................................... Error! Bookmark not defined.
Equation 4: Diffraction Equation .......................................................... Error! Bookmark not defined.
Equation 5: Angle of Irradiance from Fraunhofer's Diffraction Equation .Error! Bookmark not defined.
Equation 6: Angular Spectrum from the Fourier Transform of a wave field transmitted on an aperture
........................................................................................................... Error! Bookmark not defined.
Equation 7: Electric field at the aperture of a single unit-amplitude monochromatic plane wave...... Error!
Bookmark not defined.
Equation 8: Integration of the Angular Spectrum from a Fourier Transform...........Error! Bookmark not
defined.
Equation 9: Direction condition for diffracted waves.............................. Error! Bookmark not defined.
Equation 10: Shift of a Slit Image ......................................................... Error! Bookmark not defined.
Equation 11: Angle of tilt of a slit ......................................................... Error! Bookmark not defined.
Equation 12: Intensity Drop/Gain ......................................................... Error! Bookmark not defined.

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1 Acknowledgements
Special thanks and acknowledgements to Colin Fredette for his assistance in providing materials and
information regarding the desired designs for the current Waters Corporation Optical Bench System, as
well as Professor Charles Dimarzio for use of the Northeastern University Optics lab and assistance with
explaining the fundamentals of optics.

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2 ProjectStatement
For optical detector systems,the amount of light permitted into a system affects the signal to noise ratio,
adjusting the resolution or accuracy of the composition of a test sample. Waters Corporation’s current
systems allow for only a single aperture,containing a precise slit that allows light through and into the
detector system by means of a reflecting mirror paired with a refraction mirror. To increase the
capabilities of current Waters’ products, a new system is required to allow various intensities of light to
enter the detector in order to examine a test sample. This system must be able to be integrated in a manner
that does not drastically alter the current configuration of Waters’ optical detectors and must not interfere
with the path of light outside of the intended use of the aperture.
Figure 1 displays a sample of the wavelengths and the change in a read-out for different slit widths. Note
that the wider slit contains less noise, but does not necessarily contain the most accurate peaks in the
system.
Figure 1: Features of a Variable Aperture Slit [1]
3 BackgroundTechnologies andComponents
3.1 Liquid Chromatography
Liquid Chromatography (LC) is the science of separating a sample into a liquid. Samples are
organized and prepared according to their natural properties before they enter the detector. Figure 2
displays a system pathway where solvents are pumped from containers to be mixed with a sample to
break the sample down into elements, which can be analyzed. This is called preparatory
chromatography. Rigorous study of analytical chemistry has allowed engineers to match historical
data to new LC test data from the output of a detector such as a photo-diode array. Incident light is

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then passed through a flow cell and the remaining wave lengths which gets diffracted into a detector
detail the composition of a sample in a graph called a chromatogram. In particular a substance is
defined by the absorbance of the incident light according to Beer’s Law:
A = log (Io/I) = bc (1)
where A is the absorbance, Io is the incident light intensity, I is the intensity transmitted,  is the molar
extinction coefficient, b is the path length of the cell in centimeters, and c is the molar sample
concentration. Changes to slit widths, mentioned in Section 2, will impact the incident light intensity
signified by Io in Beer’s Law.
High Pressure Liquid Chromatography (HPLC) uses high pressure pumps in order to enhance the
breakdown of the sample in the fluid and solvents. The sample mixture is then passed through filters in a
column stack releasing the waste byproducts into the filter. The Waters Corporation system uses an HPLC
column to break down samples for analysis. The specific pathway for Waters Corporation from solvent to
chromatogram analysis is displayed in Figure 3. Note that solvents are prepared and moved through a
pump system prior to the inclusion of the sample. Each individual sample is mixed with certain mix of
solvents to create the appropriate breakdown of the samples. In the final output for the sample the
chromatograph plots the sample data with intensity on the Y-axis and wavelength on the X-axis as shown
in Figure 4.
.
Figure 2: Liquid Chromatography Sample Pathway [2]

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Figure 3: Full HPLC system including column and detector modules. [3]
Figure 4: How the detector displays sample peaks [3]
3.2 Optical Bench
Figure 5 displays an optical bench consisting components used for optical analysis of a fluid sample. This
particular optics system uses light from a deuterium lamp light source,reflected off a concave mirror to
pass through a filter and fluid sample to a paired concave mirror. From the second concave mirror the
light is reflected through an aperture slit, represented as being 50µm wide in Figure 5. Light passed
through the slit will then refract off of a diffraction grating and into a detector system. [3] [4]
Figure 5: Optical Bench Assembly, © Waters Corp. [3]

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3.2.1 Deuterium Lamp Light Source
A deuterium lamp, composed of a low-pressure gas-discharge, emits a continuous spectrum in the
ultraviolet (UV) region, traditionally used in spectroscopy. An electric arc is created from the tungsten
filament to an anode inside the lamp. The arc will excite the deuterium gas contained in the bulb to a
higher energy state. When molecules transit back to their initial states,the deuterium gas emits light. This
continuous cycle provides continuous UV radiation. A fused quartz envelope is used for a casing to
prevent blocking the UV lights.
The deuterium lamp has a spectra range from 112nm to 900nm with a continuous spectrum from 180nm
to 370nm as shown in Figure 6. Its continuous spectrum covers about 49% of UV light range. The
intensity between 250nm to 300nm does not actually decrease as shown in the plot. The decrease is due to
reduced efficiency at low wavelengths of the photo detector. [5]
Figure 6: Deuterium Light Spectrum
3.2.2 Concave Mirror Set
A concave parabolic mirror, in this case an elliptical reflector, has a reflecting surface that bulges inward
to reflect incident light emitted from the deuterium lamp inward to its focal point at where the fluid cell
locates. If the major and minor radiuses are known for an ellipse, the location of the foci can be found by
using the formula in Equation 2:
(2)
where F is the distance from each focus to the center of ellipse, j is the semi-major radius, and n is the
semi-minor radius. [6] Adjustments can be made to the values for j and n to focus the light to reflect
property. Note that the two concave mirrors should have identical foci locations as represented by
Equation 2 to focus the light through the path in the optical bench.
2 2
F j n 

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3.2.3 Aperture Slit
The aperture slit used in the Waters Corporation system is an etched piece of molybdenum, which is used
due to its lack of reflective properties. After passing through the slit, light travels in a circular wave as
shown in Figure 7. Each circular wave is uniform with the other waves in intensity at a given angle as the
waves travelfrom the slit itself so long as the slit width is smaller than the initial entering beam of light.
Using smaller slit sizes creates a more intense or refined beam of light passing through the slit. With a
smaller width of a slit there is a greater possibility for reflection between the edges of the slit, depending
on the thickness of the slit material, which can create additional interference or noise.
Figure 7: Diffraction pattern from a slit of width four wavelengths with an incident plane wave.
Based on the size of a wavelength being emitted from a fluid sample different diffraction patterns can
occur. A slit with a width less than or equal to one wavelength will produce a spectrum with very high
intensity at the center and almost zero intensities on the sides; a slit with a width wider than one
wavelength will produce a spectrum with less intensity at the center but some intensity on the sides. Due
to each sample emitting a variety of wavelengths of light, various resolutions and signal-to-noise ratios
can occur for each specific sample. To calculate the intensity of a specific wavelength for a given angle,
Fraunhofer diffraction equation (Equation 3) can be used:
(3)
where is the intensity at any given angle, I0 is the original intensity, d is the width of the slit, and is
the wavelength of the light. Fraunhofer diffraction allows for a user to determine the error or noise that
could be expected for a specific composition of a sample. Allowing for a sample to be examined by
multiple slit widths permits an accurate examination of various wavelengths in a single analysis of a
sample or analysis of a greater range of sample types. [7]
  2
0 sin ( sin )
d
I c 




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3.2.4 Diffraction Grating
A diffraction grating has a periodic structure which split and diffracts light into severalbeams in different
directions. The directions of the beams are related to the spacing of the grating and the wavelength of
incident lights. In this device, the grating is reflective.
The equation guiding the relationship between the grating spacing, the angles of the incident and
diffracted beams is known as the grating equation (Equation 4). An idealized reflective grating is made up
of a set of slits of spacing d. After the light with wavelength λ interacts with the grating, the diffracted
light is composed of the sum of interfering waves passing through each slit of the grating. Since the path
length to each slit in the grating varies, so will the phases of the waves at particular points from each slit.
The end phenomenon will be either constructive interference as peaks, or destructive interference as
valleys. When the path difference between two waves is equal to half of the wavelength, the waves will
canceleach other and create a point of minimum intensity; this is called out of phase. On the other hand,
when the path difference is one wavelength, the phase will add up and a maximum intensity will occur as
in phase. [8]
Figure 8 shows the end results when an incident visible light beam is reflected by diffraction grating. The
light is separated and the locations where maximum intensities occur are arranged into a nice order
according to their wavelengths. This principle works the same for UV radiation diffraction.
Figure 8: Diffraction Grating Spectrograph of Visible Light

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The reflecting angle θm where the maximum intensities occur satisfy this relationship
(4)
where θi is any arbitrary angle for an incident plane wave, d is the distance from the center of one slit to
the center of the adjacent slit, and m is an integer representing the propagation-mode of interest. Since the
angles of diffracted in phase light will be different for different wavelengths, the PDA detector will be
able to detect the intensity of a specific wavelength at a specific location. [8]
3.2.5 Photodiode Array (PDA) Detector
A photodiode is a photo detector which can convert light into either current or voltage. It is similar to a
semiconductor diode, but it focuses on detecting UV radiations and X-rays. When a photon with
sufficient energy strikes the diode, it transfers the energy to an electron and therefore creates an excited
free electron. In the junctions, holes with positive charge move toward the anode, and free electrons move
towards the cathode, resulting in the production of a photocurrent. Since the photocurrent is the sum of
both the dark current and light current, the dark current must be minimized to increase the sensitivity of
the detector. [9]
There are three critical performance parameters of a PDA detector,responsivity, dark current, and noise-
equivalent power (NEP). Responsivity is the ratio of generated photocurrent to incident light power with
unit (A/W). Depending on the material used to build the photodiodes, the responsivities are different for
different wavelengths. As an example, Waters’ 2998 PDA detector has a detective range from 190nm to
900nm which should be made from silicon with an electromagnetic spectrum wavelength from 190nm to
1100nm. The responsivity vs. wavelength graph is displayed in Figure. 9.
Figure 9: Responsivity of silicon photodiode vs. wavelength of incident light.

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The dark current is what is generated in the absence of light. It includes photocurrent generated by both
background radiation and saturation current of the semiconductor junction. It is an important source of
noise when a photodiode is operated in UV light detection. NEP is the minimum input light power in
order to generate photocurrent. It is basically the minimum detectable power. A signal light with power
lower than NEP will not be detected.
When hundreds or thousands of photodiodes are arranged into a one-dimensional array (PDA),they can
be used as an angle sensor or position sensor. In this case,the angles to be detected are reflecting angles
θm of diffracted lights. With the information about reflecting angles, computers can measure the
wavelengths of lights which interact with photodiodes by using Equation 3. Thus, the absorption
spectrum of the deuterium lamp can be determined. [4]
4 Designand SponsorConstraints
Waters Corporation has requested a design with a minimum of a 2 position automated aperture slit for a
model 2998 PDA detector. The current aperture slit, shown in Figure 10, is placed on a solid stand or
holder that is fixed into a casting. The aperture has a single slit with a width of 100±10µm which can vary
by what is requested by customers as far as the targeted inspection light spectrum. The aperture is
50±10µm thick and made of molybdenum with a black oxide finish.
Figure 10: AutoCAD drawing of current aperture slit used in the PDA. All dimensions in millimeters.
The variable aperture to be developed is required to have slit widths of 50±10µm and 200±10µm. If
possible, intermittent slit widths can be included as well. The height of the slit is not as critical, but should
be no less than approximately ¾”. The slit thickness, parallelism of the slit, and tolerances of the variable
aperture should reflect the current aperture.
The mechanisms to be developed for the variable aperture must fit into the existing PDA,therefore
presenting space constraints. Waters has asked that minimal alterations be made to the existing casting in
order to prevent interference with any of the current mechanisms of the PDA. However,there are a few
areas where materialon the casting can possibly be trimmed down to provide more space, as displayed in

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Figures 11 through 14. Looking from the top side, the casting can be modified in a cylindrical area around
the aperture up to some of the intersecting walls.
Figure 11: View of the PDA casting from the
bottom side. Note the location of the current
aperture holder.
Figure 12: View of the PDA casting from the top
displaying modifiable section.
Figure 13: View of the PDA casting from the top
displaying the optical mirror area.
Figure 14: Section view displaying the location of
the aperture (displayed in red) within the casting.
Figure 15 depicts available clearance space in the casting. From the top edge there is 2.118in of clearance,
and from the bottom edge there is 1.187in of clearance. Figures 13 and 11 respectively display the portion
of the casting the clearance measurements are taken from. The overall inside height of the casting is 7.020
in.

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Figure 15: View of the PDA casting from the side, displaying bottom and top clearance values.
Figure 16 shows the approximate paths that light travels within the PDA. It is important that nothing
interferes with the path of the light.
Figure 16: Approximate path of light travel with the PDA.
Waters has recommended a simple adjustment mechanism for securing the aperture to the optical bench.
With respect to the optical bench, the position tolerance of the slit location will be on the order of a few
microns (~10µm). The bench has a reference datum that can be used to position the smallest slit width
during assembly. This will help provide a tight position tolerance (~10µm) between the slits when the
width is adjusted. The focus should be on the repeatability between the different positions. The variable
aperture slit must have a position repeatability of less than 25µm in the horizontal direction (sensitive
direction) and ±150µm along the vertical direction (less sensitive direction). The straightness of travel
should be in the order of < 10µm. The intensity a slit’s image cannot drop more than 27% of the original
intensity after switching to the other aperture with same slit width.

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The command to switch between slit sizes can be accomplished by either a software or hardware
mechanism for moving the system. Any developed software can be a standalone system and will not have
to be integrated into the Waters Corporation operating software. Any hardware used must fit into the
space constraints for the casting and optical bench body of the Waters Corporation system.
4.1 Patented VAS Systems
Based on similarities in analyzing light, systems used for mass spectrometry provide a good comparison
for analyzing HPLC systems. Figures 17 and 18 display light pathways for two Thermo Scientific
systems used for mass spectrometry. Similarities between mass spectrometry systems,such as the one in
Figure 17 and the Waters Corporation HPLC system allow for items designed for either system to be used
for the other style of system.
Figure 17: Thermo Scientific Variable Slit Detector Layout [1]

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Figure 18: Fixed Position Aperture Slit Detector © Thermo Scientific Co. [1]
A patent search revealed the designs for VAS systems usable in either mass spectrometry or HPLC.
Figure 19 displays a dual piston mechanism that would apply pressure to a plate in order to vary the
slit widths. This design proves to be a bulky design with several moving parts and possibilities for
error with uneven pressure from either piston. Figure 20 displays a slit system with motion on either
side of a plate, caused by a screw system in front of a hole. The screw based system again would
allow for additional error in the system because the screws if not turned evenly the system could be
off centered and one side would have a greater distance from the center point than the other. Figure
21 displays a mechanism being held up by tensioned wires to create the various slit widths. A wired
system could possibly have increased error over time, and once more if not tightened properly could
have a greater distance from the center point than the opposing side. In Figure 22 is an additional
screw based system, very similar to that in Figure 20, with the difference that only one side moves.
With only one side moving, it is possible that the system could be off-centered and cause additional
optical errors.

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Figure 19: Mechanism for US Patent 7170595 [10]
Figure 20: Slit mechanism for U.S. Patent 4612440
[11]
Figure 21: Mechanism for U.S. patent 2852684
[12]
Figure 22: Mechanism for U.S. patent 5451780
[13]

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5 Initial Designs
5.1 Rotating Pin Wheel Design
A rotary pin slit (shown in Figure 23) has a disc with multiple built-in slits. The incident light will pass
through the slit at the top of the disc. When a bandwidth needs to be adjusted, a motor will rotate the disc
until the next adjacent slit is vertically aligned at the top.
Figure 23: Initial design emulating a rotating pin wheel.
The intent behind this design is to overcome the limitation of having only a single slit in the vertical
direction. However, it is easier to control a precise vertical motion than a rotational motion. An imprecise
rotation can cause misalignment of the slit, which will affect the diffraction pattern of incident light. The
design and manufacturing of such a precise rotary wheel is considered to be a challenge that could be
overcome, but a rotary wheel system mathematically would not fit into the casting, needing an additional
0.25 inches in depth and height to be able to be removed from the optics wall.

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5.2 Vertical Actuation Design
The thought behind a vertical actuation slit (shown in Figure 24) is to maximize the use of the vertical
space within the casting. The original Waters design has a slit holder bolted directly onto the casting.
However,the vertical space under the slit holder is more than enough to contain a pressure pump or a
power screw system as a motion feature,as well as two vertically aligned slits.
Figure 24: Initial design displaying vertical actuation between two slits.
The benefit of this design is that the tolerance of position repeatability along the direction of motion (the
less sensitive direction) is 150µm, which is sufficiently larger than in the sensitive direction. If the length
of the slit increases,this tolerance will also increase. The major disadvantage of this design is being
limited to 2 to 3 slits to switch between,with limited opportunity for expanding the system. Additionally
custom made multi-slit apertures would have to be used for each customer, eliminating interchangeability.

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5.3 Horizontally Sliding Slit Aperture
Instead of a rectangular slice or disc, the design in Figure 25 is formed by placing two planes very close
to one another. Therefore, it is an open slit with the slices’ edges as its flanks. In order to vary the slit’s
width, the two planes will slide horizontally about a fixed center point. This is feasible because the
vertical length of the slit does not affect the diffraction pattern.
Figure 25: Initial design with two moving surface plates with an internal gap as an optical slit.
The benefit of this design is that it can incorporate a continuous variable aperture slit. Theoretically, the
slit can be adjusted to serve any bandwidth, which the PDA can detect,but it is extremely difficult to
adjust the planes 25µm. With this plate design, the system would have to be maintained perfectly parallel
as well to ensure accuracy,and system wear would have to be examined to determine the accuracy of the
system over its lifetime. In order to fit into the casting space available, this plate design would have to be
smaller than a 2” by 0.5” rectangular enclosure. Combining the size and accuracy constraints, and
examining preliminary cost estimates, this design proved to be too expensive to prototype as a system to
be integrated into all manufactured detectors.
5.4 Rotational Holder System
In the early stages of the design process, the team selected a semi-hexagonal array of apertures to contain
the various slit sizes for the current design by Waters (note: aperture dimensions are consistent with those
described in Figure 10). The proposed hexagonal array,displayed in Figures 26 and 27, allows for 3
different slit sizes to be used during the operation of the detector stack. The proposed widths of 50µm,
100µm, and 200µm allow for a great range of resolutions based on the light projected onto the detector
after passing through the slit and refracting mirror. Using the semi-hexagonal design, the 3 apertures can
be cycled or rotated through with a rotary motor, shifting the aperture 60° for each change to the next
available slit, which uses the center of a base holder as the datum for adjustment.

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Figure 26: Initial design of hexagonal slitaperture
array.
Figure 27: Top-down view of hexagonal slit
aperture array.
Figure 28 displays the initial model with a semi-hexagonal aperture holder (shown in blue) secured to a
rotational base (shown in red). Due to space constraints, the rotational base has a smaller diameter than
the circular base of the aperture holder currently utilized in the Waters PDA detector. Each aperture slit is
affixed to the holder by means of press fit pins (shown in gold), which are forced into press fit holes to
secure the apertures. The center of rotation is located in the geometric center of the rotational base and is
equidistant from each of the slits, allowing for an even rotation distance from the center of one slit to
another. With this semi-hexagonal design, the 3 apertures can be rotated through with a rotary motor,
shifting the apertures 60° to change to an adjacent slit option.
Figure 28: Initial design of holder with the attached aperture slits on a rotational base
This rotational base system had such thin walls (0.5mm) that the assembly could not be machined
accurately without the risk of failure in the walls due to the machining forces and stresses. This system
provided the greatest opportunities for interchangeability and motion, and further modifications were
necessary to conceptualize a more feasible design, described in detail in Section 7.

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5.5 Control System Requirements
The controller must fit within the cavity in between the center datum and the optical bench. It needs to
operate within the tolerances and have a high repeatability. It should also be rugged and outlast the
majority of the other components of the system before repair or replacement is required. The final design
will dictate the specifications of the control system focusing on both micro-controllers and actuators with
either rotational or linear capabilities.
6 Analysis and Testing
6.1 Light Path Analysis
Background research and analysis to determine the effects of errors and inaccurate motions in a slit
system were examined. Each discrepancy from the desired slit position has a different effect on the results
and these were tested to determine the accuracy of the system.
6.1.1 Benefits of different slit widths
According to the Fraunhofer diffraction equation, the first dark fringe or the minimum irradiance
occurring for an angle θ is given by:
(5)
where λ is the wavelength of incident light and is the width of the slit.
The system currently has a 50µm slit, and the variable slit width options range from 50µm to 200µm,
with 50µm or 100µm increments. Comparing the 100µm slit and the 400µm slit diffraction with 180nm
incident light; they output a 0.103o
and 0.0258o
angle respectively. This means the first dark fringe on
each side will be 0.206o
apart for the 100µm slit and 0.0516o
apart for the 400µm slit.
The intensity of the diffraction light depends on the angle θ, wavelength and slit width. In the case
described above, slit width and wavelength remained the same. With a larger diffraction angle, light with
the same wavelength and similar intensity will cover a larger area when hitting the diffraction grating. On
the other hand, a smaller diffraction angle will bring a concentrated light beam on to the diffraction
grating, covering less area and the intensity contrast will be sharper.
The diffraction grating will reflect the light with the same wavelength and the same reflection angle no
matter where it hits. However,the PDA only measures the intensity of the light, but not the wavelength of
it. Each photodiode will receive whatever wavelength of light is preloaded in the system. If the diffraction
beams from the slit spread far apart,it is more likely that light with a wavelength different from the
preload setup of a particular photodiode will be reflected on to it. These intake intensities from other
wavelengths are considered to be noise. [14]
sin
w

 
w

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With a small slit, the system can have a higher resolution, but lower signal-to-noise ratio. This is why it is
necessary to design a variable aperture slit. It is also the reason why there cannot be a V-shaped slit, since
the diffraction angle for each wavelength needs to remain constant.
6.1.2 Diffraction error from tilt in the “Z” direction
Figure 29 comes from a technical report and displays an experimental set-up used to examine the
phenomenon of light propagation through a tilted slit.
Figure 29: Experimental Set-up of a laser through an optical slitdisplaying a coordinate system [14]
In Figure 29, when the light wave E(x,y) at the points of an aperture t(x,y) in the plane of x and y are
known, the angular spectrum A(fx,fy) is the Fourier transform of wave field on the aperture. This is
represented by Equation 6:
(6)
where the approximation evaluates the light wave E(x,y) at the points of the plane x0 and y0 are parallel to
the plane x, y.
For a special case of a single, unit-amplitude wave illuminating the aperture normally, Equation 6 shows
the angular spectrum of the Fourier transform of the aperture. The problem can be extended to the case of
inclined incidence in the particular case of a single slit. When a single, unit-amplitude monochromatic
plane wave is incident along the z’ axis, the electric field at the aperture is given by Equation 7.
(7)
[ 2 ( ]
( , ) ( , ) ( , ) x yi f x f y
x yA f f E x y t x y e dxdy
 

 
2
[ ( cos ]
( , ) ( , )
i y
E x y e t x y






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According to Equation 6, the angular spectrum becomes the Fourier transform shown in Equation 7. After
integration, the result turns to be:
(8)
where . Referring to Figure 29, the angle is the angle between the y-axis and the
diffracted ray r.
Considering only the angular dependence of the diffracted rays,it shows the typical distribution of the
field diffracted by a single slit through normal incidence. The directions of the diffracted waves must
satisfy the following condition:
(9)
where the index d refers to the diffracted ray. If the space in regions A and B in Figure 29 have the same
refractive index, then so that Equation 9 gives (a) , (b) where (a) represents
transmission and (b) represents reflection. In both cases the direction of the propagation of the diffracted
rays lies on a conical surface with apex at zero and half-opening θ, because of the invariance of the angle
between the y-axis and the diffracted rays. This means the propagation will have an even distribution and
alignment. [14]
Figure 30 displays one of the diffraction patterns on a screen with a slit that is tilted at a 45º angle. If the
slit is installed at a tilted orientation, Figure 30 can be used as a reference to adjust it back to its normal
position.
Figure 30: Diffraction pattern in the plane x'o, y'o of a single slitinclined at an angle θ=45° to the direction of
propagation z’: lx = 0.02mm; D = 1m [14]
0
sin( )
( , ) ( )x x
x y x y
x
l f
A f f I f f
f



 
0 cos /f   d
cos / cos /d d   
d  d  d  

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6.2 Light Path Calibration and Verification Test Method
A simplified optical system in the Northeastern University Optics Lab has been created to perform
calibrations similar to those done in the full system of the Waters Corporation optical detector stack. The
simplified version consisted of a light source, a variable aperture slit, a concave lens, and a camera,which
detects light set-up in a straight line path. Having the light propagated in a straight path instead of a
reflected path eliminates possible errors from misaligned mirrors. The straight system also eliminates a
breakdown of the initial beam of light since visible light will be used instead of ultraviolet from a
deuterium for safety and to simplify the experiment. Figure 31 displays the instruments used for the
repeatability and calibration experiments.
Figure 31: Light Path Calibration Set-up
In the set-up the light source emits light through an adjustable focusing aperture onto the aperture slit. The
light then passes through the aperture slit and onto the magnification lens, creating a larger image for the
camera to read. Using the larger scale image the accuracy and position can be adjusted in a more precise
manner. Since the light passed through the slit is normal ambient light, the experiment is run in a dark
room to eliminate saturation and noise. The image captured by the software provides two benefits. The
first such benefit is the overall images can be compared to examine repeatability and tilt or error in the
system in a general manner. The image also can be converted into a bmp file and examined using
MATLAB to find a mathematical comparison of the accuracies of the system.
Initial testing performed on the original aperture holder and slit (provided as a sample from Waters)
revealed the image in Figure 32 with a display of the peaks in the light intensity.
Light Source and
Holder
Light Focusing Aperture
Aperture Slit
20x Magnification Lens
Light Detecting Camera

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Figure 32: Slit image and intensity detecting tool
The thin horizontal line seen in Figure 32 is the location at which the measurements for intensity were
taken from. This location was selected because it is close in proximity to the center point of the slit, and
because this location is in focus. From the intensity pattern of the slit, one can see the peaks in intensity
on the edges and then a drop to an average value of more uniform intensity in the center.
An intensity drop would be the result of the image going out of focus. Using a base with a micrometer
attached to either the aperture slit or the camera,the distance that a slit is off by can be determined and
then divided by the square of the magnification to determine the “actual” error in distance (or discrepancy
from the desired location) because the focused image is magnified. Using the error information, the
accuracy of the system can be confirmed and then converted to the number of steps taken by the stepper
motor. Adjustments can be made accordingly to account for any errors that may arise.
6.3 Motion Control Analysis
To properly rotate the system between various locations, a precise motor system is required for accuracy.
Using the design factors outlined in Sections 4 and 6.3.1, different motors were evaluated to determine
the optimal configuration as well as software and hardware systems required to accurately control them.
6.3.1 Motor Systems
Through research,three potential motor systems with rotational capabilities were compared and evaluated
based on feasibility of implementation with the VAS design selected by the team. The first option, a
linear actuator with a rotary stage (shown in Figure 33), would take the linear motion of the actuator and
translate the motion through the stage into a rotational motion on a scaled stage. The system is accurate
Point measurement
is taken from
Intensity Pattern

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and responsive, but based on the size constraints in the casting, placing an actuator and a stage in the right
location for the appropriate motion would be limited in location. Additionally, attaching a rotational
holder to the stage would require sufficient material to bore and tap holes in to screw lock the holder to
the stage as well as attaching the system to the base. Based on these stipulations, the stage system would
necessitate significant alterations to the casting, which could prove to be ineffective in terms of cost.
Figure 33: Linear Stage System
A brushless servo motor system would provide the desired rotation for the system in a small vertical
cavity. The system would be able to be scaled to require simple bracket attachments to the casting of the
optical bench and a small servo motor would be able to fit into the available casting space. The servo
motor in Figure 34 displays a compact gearing system, which would minimize the size impact on the
system. The servo motor would constantly be moving to be controlled to the right angle during a control
system. Due to the constant motion, vibrations in the system could throw off readings for the optical
detectors,making the system unreliable and inaccurate.
Figure 34: Brushless Servo Motor
DC stepper motors rotate between set locations or steps and are able to end rotation and “hold” a position
at a certain step. Figure 35 displays a stepper motor that runs the system at an offset. The capabilities of
the motor system to be offset would permit the system to fit into the casting in numerous configurations.
A straight vertical stepper motor (similar to the servo motor shown in Figure 34) would be able to fit into

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the casting space. As a stepper rotates or steps,it moves between its set points to the next location
typically in a rotational motion of 1.8º, or 200 steps per 360º rotation. Using gearboxes, the rotation can
be adjusted to various desired rotational angles. The stepper motor system has the best capabilities for the
motor system.
Figure 35: DC Stepper Motor with Offset Rotation Point
6.3.2 Control Path Design
To control the motor motion, a hardware and software control system is necessary to maintain accuracy.
A simple position control system prompting a user input and relaying it into the hardware to move to a
set-point similar to the path in Figure 36 is a simple and straightforward control method.
Figure 36: Motion Control Diagram
6.3.3 Control System Hardware
Hardware to control a rotational system must be able to handle an input voltage and then translate the
voltage into motion in a system. Based on the possible motor systems, a basic open sourced hardware
system, such as an Arduino, is capable of controlling the VAS. Due to being open source, commonly
used, and advanced control requirements not being necessary an Arduino is a suitable hardware system

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for running the VAS. The Arduino Uno (shown in Figure 37) has USB capabilities to run off the voltage
of a computer system integration and is stackable with a motor driver shield to control a motor system.
Figure 37: Front view of Arduino Uno
To connect the motor system to the control system a standard open source motor shield from Adafruit,
shown in Figure 38, can operate the motor system for direction and rotation speed using the input leads
shown.
USB 2.0
Connectivity Via
Atmega8U2 (USB
to serial)
Sits under the Adafruit
driver product
(stackable)

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Figure 38: Adafruit motor shield stacked on Arduino Uno
7 Final Prototype
The final prototype of the system is broken down into the optical holder components and the rotational
control system. The two components are match machined to ensure fit and then combined together for the
final prototype.
7.1 Optical Slit Aperture Holder
The team has selected a semi-hexagonal array of apertures which incorporates the various slit sizes
desired by Waters. The proposed hexagonal array, displayed in Figure 39, gives the user the option of
selecting from 3 different slit sizes to be used during the operation of the detector stack. Standard size
widths of 50µm, 100µm, and 200µm will typically provide great range of resolutions, depending on the
specific application of the system. Compared to Waters’ original aperture slit design (as shown in Figure
10), the only difference with these new apertures is that 1mm has been removed from both the left and
right sides. This modification is necessary to ensure that space constraints within the casting were met.
With this semi-hexagonal design, a rotary motor is used to switch between the 3 different positions. When
switching between adjacent positions, the control system specifies for the motor to rotate 60°. This angle
of rotation is measured from a center datum located at the center of the holder base.
Using the rotation of the semi-hexagonal design, the prototype avoids interfering with the light paths
inside of the PDA. The two outer apertures will provide the greatest change of location, but will not
interfere with the light pattern due to the slits being perpendicular on their respective plane.
Stepper motor
inputs for the four
lead wires
L293 H-Bridge
Power Header

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Figure 39 displays the model of the variable aperture slit system placed on a rotational holder base. Each
slit is affixed to the holder base by means of press fit pins (shown in gold), which are forced into press fit
holes to secure the apertures. The center of rotation is located in the geometric center of the rotational
base and is equidistant from each of the slits, allowing for an even angle of rotation from the center of one
slit to another.
Figure 39: Current VAS design, with the aperture slits securedto a rotational base.
The aperture slits are made from molybdenum with a black oxide finish. The press fit pins are made of
aluminum. The holder is made of aluminum and is black anodized. It is necessary to anodize the
aluminum holder to ensure that the light beam passing through the VAS is not reflected and cause
undesired noise. Instead, the black anodized finish allows light to be absorbed, reducing noise. The
machining of all parts was performed by the machine shop at the Waters facility. Refer to Appendix A for
AutoCAD drawings of the parts in the VAS assembly, which were used for machining.
7.2 Rotational Control System
Using the Arduino control system (refer to Section 6.3.3) with a bipolar stepper motor for the prototype
control system, the holder is able to be rotated 60º in either direction from a centered position, measured
as 0. The bipolar motor system is able to make forward and backward movements by receiving a voltage
input for both direction and rotation speed using a four wire design, as shown by the motor in Figure 40.

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Figure 40: NEMA 11 Bipolar Stepper Motor
Using a National Electrical Manufacturers Association (NEMA) size 11 body style (11 representing the
size of the diagonal in millimeters) the motor will fit inside of the casting system. Using a standard body
style such as the NEMA 11 permits the use of a standard production size for mass production, as well as
for adding similar components to the system.
Integrating the motor system with the control hardware as shown in Figure 41, the system relies on a user
prompt to determine the translation command and then perform the movement action.
Figure 41: Assembled Control System and Prompts
The stepper motor performs steps in increments of 1.8º for moving. Based on the desired ±60º angle, the
entire system would have to be rotated around the base multiple times in order to reach the desired
location. To limit the motion, a gearbox was added to the system. Based on the standard sizes of
gearboxes available from various companies, the step angle, including backlash, was determined to select
a system that could create the rotation of 1.8º. A planetary gearbox small than 1.5” in height with a gear
ratio of 27:1 or 26.85 including the testing backlash was selected to create the 60º rotation in a manner of
895 geared steps. From this calculation the number of steps the control system will communicate to the
motor is determined.
User is prompted
Commands
translate to signals
Action is
completed
A B

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7.3 Combined Prototype
Figure 42 shows the VAS assembly attached to a rotary motor, which is used to switch between the 3
different positions. When switching between adjacent positions, the control system specifies for the motor
to rotate 60°. Using the rotation of the semi-hexagonal design, this VAS system avoids interfering with
the light paths inside of the PDA. The two outer apertures will provide the greatest change of location, but
will not interfere with the light pattern due to the slits being perpendicular on their respective planes.
Figure 42: VAS assembly attached to rotary motor.
In order to incorporate the semi-hexagonal VAS assembly and motor, it is necessary to remove some
material from the existing PDA casting. Figure 43 displays the new bore holes made in the casting. The
team verified that this modification to the casting abided by the design constraints specified by Waters.
Refer to Appendix A for AutoCAD drawings detailing the machining of these new bore holes.

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Figure 43: Model of PDA casting displaying the new bore hole required for integration of the VAS system.
To secure the motor to the casting and stabilize the VAS system, brackets were used to attach the motor to
the casting. This fixed positioning ensures accuracy and repeatability of the VAS system.
7.4 System Operation
To operate the rotational system with the control system, the code for the motor driver is loaded onto the
Arduino Uno. The code will then accept commands from the computer interface and send them to the
motor. For MATLAB to communicate with the Arduino and motor shield, the COM channel is updated
with a driver to operate the Arduino. Once the COM channel is updated, the Arduino definition file is run
in MATLAB to define the Hardware components into bit formats for MATLAB to understand. Once the
system is defined, the Arduino is installed to receive direct communications from MATLAB for
operation. Once the installation is complete, the “M File” designed to operate the rotational system can be
run to move the system between the designed set points based on the 895 step motion.
7.5 Testing and Analysis
To verify the accuracy and repeatability of the system a series of tests to compare the optics control
systems is performed.
7.5.1 Optical Analysis and Testing
Using a tolerance of 25µm in the “X” direction, (or 27% intensity drop) as the acceptable criteria the
system is tested using the calibration and analysis set-up discussed in Section 6.2. Using Figure 44 as a
reference image,each position is compared to test the accuracy and repeatability between each position.

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The detailed intensity for each image shown at the top of Figure 44 is also compared between the
reference image and the testing results.
Figure 44: Reference image for comparing test data to determine accuracy and repeatability
The horizontal X-axis coordinates of the edges of the slit image were recorded using the camera’s pixel
position. Figure 44 has an origin located at the bottom left pixel of the image. Comparing between pixel
positions, the shift of each position is numerically compared to determine the amount of shift in each
coordinate direction (X, Y, and Z). Using the intensity breakdown of each slit image, the intensity peaks
and average intensity were also compared to the reference image to determine the percent difference with
the original reference to determine the amount of intensity drop.
The team performed 3 sets of testing to the system. Each set contained two 60o
counterclockwise rotations
from the left slit to the center slit and to the right slit and two 60o
clockwise rotations back starting at the
leftmost slit. The data set included information from 5 groups of X, Y, and Z position data sets and 5
images. Figure 45 displays a table of the information collected as raw data from the tests.

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Figure 45: Raw test analysis data
Knowing that each pixel of the camera had a length of 7.4µm and that the magnification on the system is
at a ratio of 20:1, each pixel value in Figure 44 is representative of 0.37µm for the actualslit dimension.
The shifts of the slits’ X positions were calculated by using Equation 10:
(10)
where X’left, X’right were the X-coordinates of tested positions’ left and right edges, Xleft, Xright were the X
coordinates of the referenced slit’s left and right edges.
Using the values for the “Y” and “Z” direction, the angle of tilt in the Z direction of a slit is calculated
using Equation 11,
(11)
where Z’top,Z’bot were the Z coordinates of the tested positions’ top and bottom edges, Ztop,Zbot are the Z-
coordinates of the referenced slit’s top and bottom edges, and Ytop,Ybot were the Y-coordinates of the
referenced slit’s top and bottom edges.
Using the intensity graph for each image, a drop or gain in light intensity from the original image is
calculated using Equation 12:
(12)
where I is the referenced intensity and I’ is the measured intensity of each testing position. Using
Equations 10, 11, and 12, the data is refined as shown in Figure 46 to display comparisons between the
reference dimensions.
 the larger one of ' 0.37 ' 0.37shift left left right rightX X X X X           
1
( ' ) ( ' )
tan
top top bot bot
z
top bot
Z Z Z Z
Y Y
 
   
    
'I I
I
I

 

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Rotation between slits Average Intensity
Change
Z-direction shift (µm) X-direction shift (µm)
Nominal - Left -0.71% ± 8.25 ± 3.4
Nominal - Right -0.78% ± 6.45 ± 3.4
Left – Nominal -0.904% ± 5.0 ± 3.4
Left - Right -2.74% ± 8.5 ± 3.4
Right – Nominal -2.74% ± 10.8 ± 3.4
Right - left -3.84% ± 4.5 ± 3.4
Figure 46: Refined Data Table
From Figure 46, the rotational device error values were below the specification and tolerance values. The
greatest shift in sensitive direction is 3.4µm, in light path direction is less than 10.8µm, and the greatest
intensity drop is 3.84%. Repetition of the motion of the system displayed that the system never varied
greater than the tolerance limits from the desired location for each position and does not have an intensity
drop implying inaccuracies.
8 ProjectManagementOverview
The management of the project is divided into 5 design and construction phases for the development of
the prototype itself. In addition to these phases the project is broken into various sections for the
development of report and presentation deliverables.
8.1 Design/Model/Construction Phases
The Design/Model/Construction Phases are physical and digital deliverables related to the physical
prototype being developed. Each section of the phases are broken down below with additional details
related the work to be completed and current status.
8.1.1 Initial Design Phase
The initial design includes all preliminary design steps, including but not limited to design research,
patent research,competitor analysis, and other research items that provide background information for the
prototype. Each aspect of the research correlates to various aspects of the modeling phase of the project,
and will be completed prior to the beginning of the modeling phase. Many items included in the research
and documentation portion of the initial design phase provide information, which will be presented in the
initial design presentation phase later discussed.
Responsibilities for the initial design phase were broken up into 4 main categories, analysis of Waters and
key technologies that will be involved in the products associated with the project’s prototype completed
by Aaron Gill, an evaluation of the Waters “Stack System” performed by Kevin McMorrow, detailed
analysis the Waters optical detector, done by Hanshen Zhang, and a review of current design models and
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8.1.2 Modeling Phase
The modeling phase included all modeling, virtual design and development of the prototype and early
design iterations. During the modeling phase much of the mathematical analysis and design of the
prototype occurred. Each modeled component is ordered after the modeling phase is completed.
Evaluations of machinability for the holder system and gear ratios were determined as part of the
modeling phase. Final approval for the purchase and construction of these parts was verified with Waters
Corporation to ensure the items fit their intentions for the project.
Responsibilities for the modeling phase were separated between Aaron Gill and Paul Ventola performing
SolidWorks modeling and analysis to determine the appropriate motor systems to use, and Hanshen
Zhang evaluating how to calibrate the light path with integrated models.
8.1.3 Ordering Phase
The ordering phase includes a majority of the lead times associated with the purchasing, ordering, and
external vendor work necessary for the prototypes. This phase comprised a significant portion of the 6
month time period, as vendor timelines are outside of the control of the team and are planned to exceed
normal lead times.
The ordering phase relies heavily on the machinist at Waters Corporation responsible for the machining
of the modeled parts. Each part is tightly toleranced due to the nature of the optical systems and required
extra time in case items were made outside of tolerance and specifications and needed to be remade.
8.1.4 Initial Construction Phase
The initial construction phase is inclusive of all initial physical builds, a combination of initial
components and testing inside of the optical bench itself. This phase is also inclusive of several
debugging, modification, and fine-tuning phases to optimize the set-up, and determine the needs of any
changes or additional components that need to be purchased. The initial construction phase will require a
majority of the teams’ physical involvement and time. The project will not move into a final production
phase, including placing the components into the optical bench casting until the design has been fully
evaluated.
The initial construction phase involved the testing of the full motor control system performed and
verification of the aperture holder dimensions by Aaron Gill.
8.1.5 Final Production Phase
The final production phase is comprised of the final development of the prototype for the Waters VAS
system. The final production involves the placement into the final casting model for aperture system as
well as methods for integration into the various models and castings used by Waters. The final production
phase will additionally include some analysis of the design, debugging, and problem solving as necessary
for the proper integration into Waters designs.

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The final production phase involved attaching the system onto the mounting brackets and placing it into
the cavity and rotating the system through a series of step locations to ensure there is no interference with
the casting performed by Aaron Gill, Paul Ventola and Hanshen Zhang.
8.2 Report and Presentation Phases
The report and presentation phases are divided into sections based on the reports to be given by the team
over the development and production of the VAS prototype. The report and presentation will be
developed as needed for each individual report and presentation throughout the scope of the project.
9 Future Work
Based on the final designs, adding an additional vertical actuation into the semi-hexagon design permits
the addition of 3 additional slits as a second tier or level to the system. With a 6 aperture set-up a greater
variety of resolutions could be used for the optical detector. Based on customer needs and the availability
of time future efforts may include the introduction of the vertical actuation into the current design.
10 Summary
From the developed designs, a semi-hexagon formation of apertures placed on a rotating base or holder
will allow for a variety of slit sizes permitting light through to the optical detector. The design will not
interfere with the light patterns existing in the Waters optical detector. Using this design will also allow
for some interchangeability based on individual customer needs, placing a customer’s 3 preferred slit
sizes into the system they choose, without modifications to the system. Additionally this model permits
the possibility of upgrading the system to include a vertical motion, adding an additional row of possible
apertures possibilities.

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11 Intellectual Property
11.1 Description of Problem
The design of a Variable Aperture Slit (VAS) system used for providing different resolutions of light to
be projected into a photodiode array (PDA) for examining the composition of a fluid sample. The VAS
will consist of an aperture holder with attached aperture slits of various sizes that will be used to limit or
refine the light from an ultraviolet (UV) or deuterium light source. The VAS will be mounted onto a
rotational motor aligned between a reflected and refracting mirror to permit a certain amount of light with
a specific resolution that is unique to each slit size through from the light source to the PDA. Prior to
passing through the slit, the UV light will pass through a sample fluid, prepared via liquid
chromatography separating the components of a substance,which will absorb various wavelengths of
light corresponding to the absorption pattern of the substance. Based on the remaining wavelengths of the
light which are passed through the slit and then refracted into an optical detector, the composition of the
sample fluid can be determined. In the detector, each slit width will provide a different signal-to-noise
ratio and resolution.
11.2 Proof of Concept
A rotational system on a NEMA 11 stepper motor and gearbox configuration, rotating 895 steps between
60º set points maintains accuracy within 4µm. Repeatability examined images of optical slits, detailing
consistent intensities and focus of light supports the repeatability of the mechanism. Additional validation
of optical results from standard fluid samples supports the accuracy of the system.
11.3 Progress to Date
A functional rotational motor system connected to control software and an aperture holder containing
three slits has been built and tested. Initial testing on the accuracy of the system has been performed to
confirm overall repeatability.
11.4 Individual Contributions
The design and system layout has been created by Aaron Gill. 3D models of all components and parts
have been created by Aaron Gill and Paul Ventola. Accuracy and repeatability validation and verification
has been performed by Hanshen Zhang with the assistance of Aaron Gill and Paul Ventola. Control
software coding and interface creation has been completed by Kevin McMorrow.
11.5 Future Work
Integration of a vertical actuation system increases the number of slits capable of examining a single fluid
sample without disassembling the system. The system can also be integrated into other optical analysis
devices such as mass spectrometers.
.

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12 References
[1] ThermoScientific, "Thermo Scientific Dionex UltiMate 3000 Diode Array and Multiple-
Wavelength," Dionex, [Online]. Available: http://www.dionex.com/en-us/webdocs/69566-DS-DAD-
3000-12Apr2012-. [Accessed July 2012].
[2] Wikipedia, "Chromotography," Wikipedia, [Online]. Available:
http://en.wikipedia.org/wiki/Chromatography. [Accessed 15 September 2012].
[3] Waters Corporation, "HPLC - High Performance Liquid Chromatography Waters Inc. Primer,"
Waters Corporation, 2010. [Online]. Available:
http://www.waters.com/waters/nav.htm?cid=10048919. [Accessed 2012].
[4] Waters Corporation, "2998 Photodiode Array (PDA) Detector specifications," [Online]. Available:
http://www.waters.com/waters/nav.htm?cid=1001362. [Accessed 1 August 2012].
[5] Wikipedia, "Deuterium Arc Lamp," [Online]. Available:
http://en.wikipedia.org/wiki/Deuterium_arc_lamp. [Accessed 12 August 2012].
[6] Math Open References,"Optical Properties of Elliptical Mirrors," [Online]. Available:
http://www.mathopenref.com/ellipseoptics.html. [Accessed 12 August 2012].
[7] Wikipedia, "Diffraction," [Online]. Available: http://en.wikipedia.org/wiki/Diffraction. [Accessed 12
August 2012].
[8] Wikipedia, "Diffraction Grating," [Online]. Available:
http://en.wikipedia.org/wiki/Diffraction_grating. [Accessed 12 August 2012].
[9] Wikipedia, "Photodiode," [Online]. Available: http://en.wikipedia.org/wiki/Photodiode. [Accessed
12 August 2012].
[10] B. J. E. Smith and A. M. Woolfrey, "Optical Slit". United States Patent 7170595, 30 January 2007.
[11] C. Brunnee, P. Dobberstein and G. Kappus, "Device for adjusting slit widths in spectrometers".
United States of America Patent 4612440, 16 September 1986.
[12] J. H. Payne,"ADJUSTABLE SLIT MECHANISM". United States of America Patent 2852684, 16
September 1958.
[13] B. Laser,"Device for setting slit widths in the beam path of spectrometers". United States of
America Patent 5451780, 19 September 1995.
[14] C. V. Fel'De and P. V. Polyanskii, "Peculiarities of light diffraction by an inclined slit," Optics for
the quality of life; 19th Congress of the International Commission forOptics, no. 1,pp. 319-320,

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2003.
[15] S. Ganci, "Fourier Diffraction through a tilted slit," European Journal of Physics, vol. 2, no. 3, pp.
158-160, 1981.
[16] American Journal of Physics, "Journal of liquid Chromatography & Related Technologies 2010,"
American Journal of Physics, 2010.
[17] K.-L. Shu and P. R. Silverglate, "Spectrometer with planar reflective slit to minimize thermal
background". United States of America Patent 6310347, 30 October 2001.
[18] L. Dong, A. K. Agarwal, D. J. Beebe and H. Jiang, "Variable-Focus Liquid Microlenses and
Microlens Arrays Actuated by Thermoresponsive Hydrogels," Advanced Materials, pp. 1-5, 2007.

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13 Appendices
13.1 Appendix A: System Part Drawings

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Figure 47: AutoCAD drawing for holder base.

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Figure 48: AutoCAD drawing for 50m aperture slit.

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Figure 51: Drawing of Stepper Motor

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Figure 52: AutoCAD drawing of new bore hole required for PDA casting (page 1 of 2).

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drawing of new bore hole required for PDA casting (page 2 of 2).
13.2 Appendix B: Arduino Control Code
13.2.1 Code to define the Arduino in MATLAB:
This code will be provided as part of the transferred material to Waters Corporation. Defining the
Arduino is required for the first run of the MATLAB system to properly detail the Arduino’s hardware
capabilities for the software.
13.2.2 Code to install Arduino to work with MATLAB:
This code will be provided as part of the transferred material to Waters Corporation. The Arduino must be
installed inside of the MATLAB software to correctly align the paths from the COM ports for the voltage
transmission.

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13.2.3 Code to run Control Program:
The coded text for running the MATLAB control Program. The ‘COM’ port is adjusted based on what is
selected when installed into the computer system, but the remaining items remain constant for rotating the
system.
%a=arduino(‘COM6’)
a.stepperSpeed(2,20);
state=10;
state=input('Wide:1 Narrow:2 Nominal:3 Quit:4'); % user input
while(state~=4)
%Start While Loop{
if(state==1)%Code to move to the slit designated as “Wide” in position 1
a.stepperStep(2,'forward','single',255)
state=input('ENTER Wide:1 Narrow:2 Nominal:3 Quit:4.......?');
a.stepperStep(2,'backward','single',255)
end%close State #1
if(state==2)%Code to move to the slit designated as “Narrow” in position 2
state=input('Wide:1 Narrow:2 Nominal:3 Quit:4');
a.stepperStep(2,'forward','interleave',255)
end %close state #2
if(state==3)%Code to move to the slit designated as “Nominal” in position 3
state=input('Wide:1 Narrow:2 Nominal:3 Quit:4');
end%close state #3
end

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13.3 Appendix C: Driver Shield Schematic
Figure 54: Schematic of Driver Shield

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13.4 Appendix D: Motor Wire Diagram
Figure 55: Motor Wire Diagram

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13.5 Appendix E: Stepper Motor Specifications
Figure 56: Stepper Motor Specifications

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13.6 Appendix F: Uno Schematic

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Figure 57: Arduino Uno Schematic
13.7 Appendix G: H Bridge Circuit Schematic
Figure 58: H Bridge Circuit Schematic

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13.8 Appendix H: Microcontroller Schematic
Figure 59: Microcontroller Schematic

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13.9 Appendix I: Microcontroller Block Diagram
Figure 60: Microcontroller Block Diagram

Waters Corp Variable Aperture Slit Capstone Design Report

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