This document presents the design and working of a capacitive rotary position encoder. The encoder detects the angular position of a rotating body from 0 to 360 degrees. It consists of two parallel copper plates divided into four segments each, with a perspex plate in between that acts as a dielectric. Different dielectric materials like teflon and aluminum foil are used on segments of the perspex plate. As the perspex plate rotates, the capacitance between the copper plates changes depending on the dielectric material in each position. A signal conditioning circuit converts these capacitance changes into voltage variations, allowing the angular position to be determined. The circuit uses a LM324 op-amp to generate a square wave excitation signal and a differentiator circuit to

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A Project On
A CAPACITIVEROTARY POSITION ENCODER
Venture by
NAME UNIVERSITY ROLL NO.
SwetaBanerjee 10601612039
Sanju Shaw 10601612029
ArobindoHore 10601612005
Tamal Chowdhury 10601612036
Dept. of Electrical Engineering
MurshidabadCollege of Engineering &Technology (MCET)
Under the supervisionof
Mr.T.S. Sarkar
AssistantProfessor
Dept. of Applied Electronics and Instrumentation Engineering
Murshidabad College of Engineering &Technology (MCET)
Murshidabad College Of Engineering & Technology(MCET) , Banjetia, P.O- Cossimbazar Raj
P.S- Berhampore, Dist- Murshidabad, West Bengal, pin- 742102[Approved By AICTE &
Affiliated to MoulanaAbulkalam Azad university of Technology (MAKAUT)]

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CONTENTS
Pg. No
I. ABSTRACT 3
II. INTRODUCTION 4-6
2.1Typesof rotary encoder
III. DESIGNING PROCEDURE 7-9
3.1Etching Process
3.2Dimension
IV.WORKING PRINCIPLE 10-16
4.1 SignalConditioning Circuit
4.2 Operation
V. RESULT ANALYSIS 17-19
VI. FUTURE SCOPE OF WORK 20
VII. CONCLUSION 21
VIII. REFERENCES 22

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I
ABSTRACT
A Capacitive Rotary Position Encoder which detects the position of the body (in 0◦ to 360◦) is
presented in this paper. The sensing part is made of parallel plates. For this purpose two
capacitive plates have been designed such a way that it has four major segments.From those
segments, capacitance values are to be calculated. Depending upon the types of dielectric (there
are three dielectrics) used; the value of the capacitor will differ for each position.Now whenever
the di-electric plate moves circularly, the value of the capacitance will change .Further this
process continue as the same manner .So from the changing of the value of the capacitance we
can easily detect position of the rotating body.

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II
INTRODUCTION
A Rotary Encoder, also called a shaft encoder, is an electro-mechanical device that converts
the angular position or motion of a shaft or axle to an analog or digital code. There are two main
types: absolute and incremental (relative).
The output of absolute encoders indicates the current position of the shaft, making them
angulartransducers. An "absolute" encoder maintains position information when power is
removed from the system[1].The position of the encoder is available immediately on applying
power. An absolute position measuring device is suitable for wide-angle range measurement
and providing the advantages of high precision, high resolution, and easy data processing. The
grid has an exclusive absolute displacement value within a Single pitch measurement
range[2].The system does not need to return to a calibration point to maintain position accuracy.
The output of incremental encoders provides information about the motion of the shaft, which is
typically further processed elsewhere into information such as speed, distance and position.
An incremental encoder works differently by providing an A and a B pulse output which
provides no usable count information in their own right. Rather than this,the counting is done in
the external electronics. The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder. A high-resolution wheel measures
the fractional rotation, and lower-resolution geared code wheels record the number of whole
revolutions of the shaft [3].
A rotary encoder disk has a conductive pattern which sums and couples the waveforms from
selected segments depending on rotary position [4].
2.1. Types of rotary encoder
There are 3 types of rotary encoder
a. Hall Effect or Magnetic Rotating Encoder
b. Optical Rotating Encoder
c. A Capacitive Rotary Encoder

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a.Hall Effect Or Magnetic Rotating Encoder:-
In Rotary magnetic encoder the shaft is coupled to a disc-shaped magnet having alternating
polarity on its face. The magnet rotates at the same speed as the motor. The motor positioned
close to the rotating magnet which is a Hall Effect device that can sense the alternating magnet
polarity. As the shaft rotates,the rotating magnet generates a sinusoidal waveform, one full
wave per revolution.
The main disadvantages in this encoder are-
This type of setup can be used only for a limited angular range because the output voltage (in
relation to the rotation angle) is ambiguous at angles >90° in both directions from the zero
crossing point. But in our encoder we can detect the position of the body in 0◦ to 360◦.Another
Disadvantage of using Hall Effect sensors includes potential interference from nearby wires or
other magnets. But in capacitive encoder, nearby wires and magnets didn’t effect on the encoder
output.
b. Optical Rotating Encoder:-
It has a shaft, mechanically coupled to an input driver which rotates a disc rigidly fixed to it.
Successions of opaque and clear segments are marked on the surface of the disc. Light from
infrared emitting diodes reaches the infrared receivers through the transparent slits of the
rotating disc and an analogue signal is created. Then electronically, the signal is amplified and
converted into digital form. This signal is then transmitted to the data processor.
The main disadvantages using an optical rotary encoder over a capacitive encoder are-
An optical encoder’s performance is influenced by dust&dirt and other contaminants gather on
the optical disk. This causes repeatability issues because the LED cannot pass light through the
Disk to the optical sensor. Once an optical disk is contaminated, the encoder must be replaced.
But in capacitive rotary encoder dust & dirt did not have effect the encoder output.
Just as contaminants have the possibility of influencing the incremental output of an optical
disk, temperature variations also impact the performance of an optical encoder.
The LEDs, and to some degree the optical disks used in optical encoders are susceptible to
thermal stress and have Limitations on both ends of the temperature range.
Optical encoders use an LED to generate the light signal that passes through the etched disk to
the optical sensor on the other side. The LED has a limited lifespan.
Vibration on the measuring environment may affect the output of the optical encoder but in
capacitive encoder it didn’t affect. A typical optical modular encoder meets a current
consumption range of 20~50 mA. But in case of capacitive encoder it is much less.

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3.A Capacitive Rotary Encoder:-
A sensor with a face having several tracks spaced apart from one another. One of these tracks
has a first electrode and also a second electrode, separated by an electrically nonconductive gap.
Also included is a detection device extending across the tracks to receive signals by capacitive
coupling. Capacitance probes are typically modeled as a parallel plate capacitor. If two
conductive surfaces are separated by a distance and a voltage is applied to one of the surfaces,
an electric field is created. This occurs due to the different charges stored on each of the
surfaces.
Here the shaft is mechanically coupled with the capacitor plate or the dielectric material. The
value of the capacitor will change when the body or the shaft rotates. A capacitive sensor just
gives the output in the form of capacitance at very small range(approximately in Pf range). So
an electronic signal conditioning circuit is needed to measure the variation of the capacitor in
the form of a voltage or current or in duty cycle. And the total arrangement is called capacitive
encoder or capacitive transducer. The advantages of capacitive transducer is smaller size,
reliability and high resolution.
Here In our project we change the position (circularly) of di electric materials which is coupled
with the body or shaft .when the dielectric materials rotate the capacitance value changes. As
we know the basic formula of the capacitor is
Capacitance = Area X Dielectric/Gap
If we assume the area and the distance between the plates remain constant for a specific probe,
then any change in di-electric is directly proportional to the change in capacitance. The change
in body position will reflect into change in capacitance. As we say earlier that the capacitance
value is in pF range so we add extra signal conditioning circuit which converts the capacitance
into voltage i.e. if the value of the capacitance will change then the voltage will also be change.
The details of sensors part is in ‘designing part’ and the details of signal conditioning circuit
part is in the “principle part’ of this project report.
Capacitive sensors have been used in a variety of sensing applications like measuring
acceleration, force, pressure, dielectric properties, liquid level, and displacement. The interface
electronics, which has been used in these applications, highly depends on the sensing element
design (e.g., single-ended versus differential).

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III
DESIGNING
TheElements which are used:Coppersheet(plate),Perspex(di-electric
medium),Aluminumfoil(di-electric medium),Teflon(di-electric medium),Fecl3.
At the beginning, 2 coppersheets are cut into circular
shapes(outer diameter= 11cm ; ),Inner side of the copper
coatingis removed,along 7cm diameter circle using etching
process(**), and divided into 4 segments.Then these
twocopperplates are fixed5mm apart.A Perspex plate
of same sizeis placed in between the two copper
plates which is fabricated with a movableshaft.
The Perspex is divided into four segments.One half of a
segment is wrapped with Teflon and the other half is
wrappedwith aluminum foil.We made wire connections in eachfig: 1
Four segment of those two copper plates which is shown in fig1.
Fig: 2Aftercompleting the
etching processthe position of C1,C2 ,C3,C4 are shown in the
Figure above

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3.1. Etching process
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected
parts of a metal surface to create a design in intaglio (relief) in the metal. In modern
manufacturing, other chemicals may be used on other types of material. As a method of
printmaking, it is, along with engraving, the most important technique for old master prints, and
remains in wide use today.
In pure etching, a metal (usually copper, zinc or steel) plate is covered with a waxy ground
which is resistant to acid.The artist then scratches off the ground with a pointed etching needle
where he or she wants a line to appear in the finished piece, so exposing the bare metal.
The échoppe, a tool with a slanted oval section, is also used for "swelling" lines. The plate is
then dipped in a bath of acid, technically called the mordant (French for "biting") or etchant, or
has acid washed over it.
The acid "bites" into the metal (it dissolves part of the metal) where it is exposed, leaving
behind lines sunk into the plate. The remaining ground is then cleaned off the plate. The plate is
inked all over, and then the ink wiped off the surface, leaving only the ink in the etched lines.
The plate is then put through a high-pressure printing press together with a sheet of paper (often
moistened to soften it).The paper picks up the ink from the etched lines, making a print.
The process can be repeated many times; typically several hundred impressions (copies) could
be printed before the plate shows much sign of wear. The work on the plate can also be added to
by repeating the whole process; this creates an etching which exists in more than one state.

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3.2. Dimensions
Fig:3(a,b,c)
The Dimensionsof the two copperplatesare shown in these three figures

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IV
WORKING PRINCIPLE
In our project “A CAPACITIVE ROTARY POSITION ENCODER” detects the position of the
body by varying the di-electric medium. Here it has four segments on the both copper plate
which makes four capacitors (C1,C2, C3, C4) as shown in the fig3(a). At a time only two
capacitors give values as we vary the di-electric medium of those two capacitors. Depending on
this we can detect the position.
Let us assume the position of the di-electric (Teflon & aluminum) is in position ‘AB’, then
only C1 shows the capacitance and C2,C3,C4 gives the same capacitance value. Now when
there is deflection of angle dθ and the position is in A'B', the value of the capacitor C1 and C2
will change from its previous value but the value of C3 & C4 remain constant as
previous.Further these process will continue as the same manner. So from the changing value
of capacitance we can easily detect the angular position of the rotor.
Fig-4a: Di-electric medium in position AB Fig-4b: Di-electric medium in
position A'B'

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4.1. Signal conditioning circuit
Signal conditioning circuits convert capacitance variations into a voltage, frequency, or pulse
width modulation. Very simple circuits can be used, but simple circuits may be affected by
leakage or stray capacitance, and may not be suitable for applications with very small
capacitance sense electrodes as shown in the below fig:5
.
Fig: 5 Signal conditioning Circuit

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The above circuit consist of two part:-
a.Square wave generator
b. A differentiator circuit
a. Working of the square wave generator using LM324:-
Initially the voltage across the capacitor will be zero and the output of the op-amp will be high.
As a result the capacitor C1 (10nf) starts charging to positive voltage through ResistorR1
(4.7K). When the C1 is charged to a level so that the voltage at the inverting terminal of the op-
amp is above the voltage at the non-inverting terminal, theOutput of the op-amp swings to
negative. The capacitor quickly discharges through R1 and then starts charging to negative
voltage. When the C1 is charged to a negative voltage so that the voltage at the inverting input
more negative than that of the non-inverting pin, the output of the op-amp swings back to
positive voltage.
Now theCapacitor quickly discharges the negative voltage through R1 and starts charging to
positive voltage. This cycle is repeated endlessly and the result will a continuous square wave
swinging between +Vcc and -Vcc at the output.
The time period of the output of the LM324 square wave generator can be expressed using the
following equation:
The common practice is to make the R3 = R2.
Then the equation for the time period can be simplified as:
T=2.1976R1C1
The frequency can be determined by the equation:
F=1/T
HERE R1=4.7K R2=R3=100K AND C1=10nF so F=9.68174 kHz
But this is the theoretical value. In our project the frequency of the wave is nearly about 5.40k
(Time 185.606us)
The below mentioned figure is the trapezoidal wave of frequency 5.4 kHz of our project.

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Fig- 6Trapezoidal Wave Shape
Excitation frequency:-
The excitation frequency should be reasonably high so that electrode impedance is as low as
Possible. Typical electrode impedance is 1-100M ohms. Ideally, the excitation frequency will
be High enough to reject coupling to power waveforms and also high enough so that the overall
sensor Frequency response is adequate; about 50 kHz is usually acceptably high. The frequency
should also be low enough for easy circuit design. In our project the excitation frequency is near
about 5.4 kHz which will be enough for our project.
Excitation wave shape is usually square or trapezoidal, but a triangle waveform can be used to
allow a simpler amplifier with resistive feedback and a sine wave offers better accuracy at high
frequency. Square wave excitation produces an output bandwidth which can be higher than the
excitation frequency by 10x or more, other wave shapes usually result in an output bandwidth

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2x or 3x lower than the excitation frequency. Sensors excited with a continuous wave signal
usually use synchronous demodulators. This demodulator type offers high precision and good
rejection of out-of-band interference.
b.Differentiator Circuit:–
A differentiator circuit consists of an operational amplifier, resistors are used at feedback side
and capacitors are used at the input side. The circuit is based on the capacitor's current to
voltage relationship:
Where I is the current through the capacitor, C is the capacitance of the capacitor, and V is the
voltage across the capacitor. The current flowing through the capacitor is then proportional to
the derivative of the voltage across the capacitor. This current can then be connected to a
resistor, which has the current to voltage relationship:
Where R is the resistance of the resistor. Note that the op amp input has a very high input
impedance (it also forms a virtual ground) so the entire input current has to flow through R. If
Vout is the voltage across the resistor and Vin is the voltage across the capacitor, we can rearrange
these two equations to obtain the following equation:
From the above equation following conclusions can be made:
 Output is proportional to the time derivative of the input – Hence, the op amp acts as a
differentiator;
 Above equation is true for any frequency signal.
Thus, it can be shown that in an ideal situation the voltage across the resistor will be
proportional to the derivative of the voltage across the capacitor with a gain of RC.

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4.2. Operation
Input signals are applied to the capacitor C. Capacitive reactance is the important factor in the
analysis of the operation of a differentiator. Capacitive reactance is Xc = 1/2πfC. Capacitive
reactance is inversely proportional to the rate of change of input voltage applied to the
capacitor. At low frequency, the reactance of a capacitor is high and at high frequency reactance
is low. Therefore, at low frequencies and for slow changes in input voltage, the gain, Rf/Xc, is
low, while at higher frequencies and for fast changes the gain is high, producing larger output
voltages.
If a constant DC voltage is applied as input then the output voltage is zero. If the input voltage
changes from zero to negative, the voltage output voltage is positive. If the applied input
voltage changes from zero to positive, the output voltage is negative. If a square wave input is
applied to a differentiator, then a spike waveform is obtained at the output.
At high frequencies this simple differentiator circuit becomes unstable and starts to oscillate.
We see a variable capacitor in the differentiator circuit, this capacitor is our sensor
(ACAPACITIVE ROTARY POSITIONENCODER). The output of the capacitor is in pF range
(1-30pF).when we move the body circularly the capacitance of the capacitor is change
correspondingly, and the output voltage also be changed. Here the range of output voltage is 0-
8.76volt .The shape of the output waveform for capacitance of 10pF and for capacitance of
20pF is shown at the next page.

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FIG:-7Output of the differentiator circuit is shown for the capacitance of 20 pF (5.4 kHz)
FIG:-8Output of the differentiator circuit is shown for the capacitance of 20 pF (5.4 kHz)
V

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RESULT ANALYSIS
TABLE:-2 TABLE-3 TABLE-4
ANGLE VOLTAGE ANGLE VOLTAGE ANGLE VOLTAGE
90 -11.16 180 -11.16 270 -11.16
95 -9.93 185 -9.93 275 -9.93
100 -8.76 190 -8.76 280 -8.76
105 -7.56 195 -7.56 285 -7.56
110 -6.76 200 -6.76 290 -6.76
115 -5.43 205 -5.43 295 -5.43
120 -4.6 210 -4.6 300 -4.6
125 -3.25 215 -3.25 305 -3.25
130 -2.35 220 -2.35 310 -2.35
135 -1.41 225 -1.41 315 -1.41
140 -0.15 230 -0.15 320 -0.15
145 0.98 235 0.98 325 0.98
150 2.1 240 2.1 330 2.1
155 3.24 245 3.24 335 3.24
160 4.19 250 4.19 340 4.19
165 5.23 255 5.23 345 5.23
170 6.5 260 6.5 350 6.5
175 7.55 265 7.55 355 7.55
180 8.65 270 8.65 360 8.65
ANGLE VOLTAGE
0 -11.16
5 -9.93
10 -8.76
15 -7.56
20 -6.76
25 -5.43
30 -4.6
35 -3.25
40 -2.35
45 -1.41
50 -0.15
55 0.98
60 2.1
65 3.24
70 4.19
75 5.23
80 6.5
85 7.55
90 8.65

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FIG:-9 Angle VS Voltage curve
LINEARITY:-
From the graph we find the equation of linearity is y = 0.2182x - 11.025
this graph represent 0-135 slot.
Here the max non linearity at point (15°,-7.56)
so putting the value in the equation we find the value of y=0.2202*85-21.075=-2.538
so the percentage of non-linearity is
= [(standard- actual)/standard]*100
= [(-7.75+7.56)/(-7.56)]*100
=2.513%

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FIG:-10 Linearitychecking curve

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VI
FUTURE SCOPE OF WORK
Our experiment is totally based on the principle on differential capacitance theory, which will be
obtained by varying the di-electric medium. We have designed the project but yet it is not
completely tested. To achieve this we need to take more readings, and analyze those repeated
readings several times . So the reliability of our model will increase.From more analysis of
outputs we will try to find not only the angular position but the angular velocity, acceleration
and also the direction of the motion of any rotating body.Thereare several applications of our
developed model which yet to be founded.

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VII
CONCLUSION
A CAPACITIVEROTARYPOSITIONENCODER is a device which measure the angle or
angular position of a object. As the design of our project is hand made so there is little bit error
in the design which can be eliminated if this design is made by machine, then the linearity and
resolution is increase of our device, and we saw in signal conditioning circuit that the assume
output from the Multisim was not found properly e.g. the Schmitt triggers’ output is a pure
square wave but at high frequency we found a trapezoidal wave. So this is some problem we
face when our team made this project.

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VIII
REFERENCES
1. Eitel, Elisabeth. Basics of rotary encoders: Overview and new technologies | Machine Design
Magazine, 7 May 2014. Accessed: 30 June2014
2. Guangjin Li, Guilin (CN); Jian Shi, Guilin (CN); ANGLE-MEASURING DEVICEWITH
ANABSOLUTE-TYPEDISK CAPACITIVESENSOR”, US PatentNo. US 8,093,915B2, Dateof Patent:
Jan. 10, 2012
3. G. K. McMillan, D.M. Considine (ed.) Process Instrumentsand Controls Handbook Fifth
Edition, McGraw Hill 1999, ISBN978-0-07-012582-7, page5.26
4. James Edward Nelson. North Branch; Ronald Kenneth Selby. Burton; Raymond Lippmann,
Ann Arbor; Michael John Schnars. Clarkston. Allof Mich.” CAPACITIVEROTARYPOSITION
ENCODER” Patent Number: 5,736,865, Dateof Patent: Apr. 7, 1998
5. JeffryTola, Upland, CA (US); Kenneth A‘Brown’Banmng’ CA (Us)
“ENCODING TECHNIQUES FORA.CAPACITANCE-BASEDSENSOR”, US Patent No. US
7,119,718 B2 B1, Dateof Patent: Oct. 10, 2006

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THANK YOU
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