introduction to instrument engineering

1. Sensing for Robotics and Control
ME 4135
R. R. Lindeke

2. General Ideas about Sensors
 Sensor are truly systems!
 Sensors systems consist of three separable
ideas:
 Informational sources: physically measurable data
sources (light beams, audio beams, electrical fields, etc)
 Detector areas: Devices that react to changes in the
informational sources
 Data Interpreters: devices (hard or soft based) that
convert informational changes into useful information

3. Sensor Topics
 Positional Control Sensing
– Kinesethic Sensing
 Resolvers
 Absolute Encoders
 Incremental Encoders
 Environmental Sensors
– Contact
 Point
 Field Sensors
– Proximity – typically single point
– Remote
 Single Point
 Field Sensors

4. Kinesethic Sensing
 These sensors provide feedback information
to the joint/link controllers (servo information)
 They use analog or digital informational
responses
 We will explore 3 generally used types:
– Resolvers
– Absolute Encoders
– Incremental Encoders

5. Resolvers
 Operating principle is
that a charged rotating
shaft will induce voltage
on stationary coils
 Secondary Voltages
are related to Primary
voltage as Sin and
Cos ratios of the
primary field voltage

6. Resolver Ideas:
Typically we use 2 stators
one (not shown) mounted
normal to an axis that is 90
away from the one thru
Winding A

7. Resolvers, cont.
 Position is determined for
computing stator ratio
 Winding A carries Sin
signal
 Winding B carries Cos
signal
 A/B = tan so
 Shaft position
=Atan2(B_Reading,
A_Reading)
 of interest

8. Resolver Issues
 These devices are susceptible to Electrical
Noise – must be highly shielded
 Usually use gearing to improve resolution
 Typically are expensive but very rugged for
use in harsh “shock motion” environments

9. Optical Encoder Positional Sensors
 Based on Photoelectric source/receiver pairs
 Looks for change of state as changing receiver
signal level (binary switching)
 Uses a carefully designed disk with clear and
opaque patches to control light falling on a fixed
sensor as disk rotates
 Can be made ‘absolute’ with several pairs of
emitters/receivers or Incremental with 2 ‘out of
phase’ photosensors

10. Optical Servo Measurement Systems
 Absolute Encoders
– Use Glass Disk marked for positional resolution
– Read digital words (0010111011) at receiver to
represent shaft position
– Commonly Available with up to 16 bits of
information (216
) to convert into positional
resolution

11. Operating Principle
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l

12. Absolute Encoders – optical disks

13. Absolute Encoder Variations – 8bit

14. Comparing Natural Binary to Gray
Code
 Natural Binary give actual position when read
– Actual position is known w/o analysis
 Gray code is designed so only one bit
changes “at a time”
– Where Bit change is subject to positional errors
as light “bleeds” around patch edges
 Gray codes are, therefore, less error prone,
but require an ‘intelligent converter’ to give
actual shaft position

15. Using Absolute Encoders Resolution:
360
2
: n is # of 'lines' on disk
Determine resolution if n = 5?
ABS n
here
φ
°
=
ABS for ‘5 liner’ = 360/25
=
360/32 = 11.25

16. To Improve Resolution:
 Add Gearing to shaft/encoder coupling
– New Resolution is:
 Increase # of Lines – this increases complexity and cost
of the encoder (can be a significant cost increase)
360
2
is gear ratio on encoder shaft
ABS n
φ
γ
γ
=
o
g

17. Absolute Encoder for 0.18 Resolution
( )
360
0.18
2
360log
0.18
log2
3.301 10.965 11
0.301
ABS n
n
n bits
φ = ° =
°=
= = →
o
o

18. Incremental Encoders
 This devices use
3 pairs of
Emitter/receivers
 Two are for
positional
resolution, the
third is a
‘calibrator’
marking rotational
start point
Sine wave is observed
due to leakage (light
bleeding) around opaque
patches!

19. Incremental Encoders
 The positional detector
uses what is called
“Quadrature”
techniques to look at
the changing state of
the 2-bits reporting
position for each
opaque/clear patch on
the optical disk

20. Incremental Encoders
 Notice the “square wave”
quadrature signals
– they are offset by “½ phase”
 Each patch resolves into 22
or 4 positions!
 Without hardware change,
resolution is a function of the
number of patches – or lines

21. Incremental Encoders
2
360 360
2 4
INC
patch patchC C
φ = =
o o
g
For 500 line Inc. encoders,
resolution = .18 (w/o gearing)
Consider a 500 ‘Line’ incremental encoder?

22. Comparing Absolute and Incremental
Encorders:
 Incremental are usually cheaper for same
level of resolution
 Absolute are able to provide positional
information at any time under power
– Incremental must be homed after power loss to
recalibrate count numbers
 Compared to resolvers, encoders are fragile
so must be shock protected during operation

23. Environmental Sensors
 These sensors provide ‘code decision making’ power
to the Manipulator
 These sensors can be simple
 Single point devices,
 Simple devices typically trigger yes/no decisions with switch
changes
 These sensors can be complex 2-D array (or even 3-
D field) devices
 Typically the receivers are complex arrays
 The data interpreters are sophisticated software and hardware
devices
 They can add “intelligence” for decision-making by the
manipulator

24. Contact Sensors – Force and
Deflection Sensing
 Force Sensors:
 Measure pressure
for gripping –
direct or indirect
 Measure
deflection during
contact – typical
of indirect contact
sensing

25. Contact Sensing
 Indirect contact sensors use Strain Gages (and
Hooke’s Law: Stress = E*Strain)
 The strain gage is a resistive device that exhibits a
change in resistance due to changes in shape
(length or width)
 The Strain Gage is mounted into a carefully built
(and calibrated) Wheatstone bridge
 small changes to the strain gages resistance, observed while
using a highly linear voltage source, are calibrated against
observed deflection
 This ‘bar’ deflection is strain and multiplying the strain times
the bar’s modulus of elasticity yields stress and hence applied
force!
 Stress = Force/Areabar

26. Contact Sensing
 Other contact sensor are “Direct
Reading”
 These devices use the piezoelectric
principle (effect) of the sensor material
 Piezoelectric effect states that in certain
material (quartz and some silicates)
applied forces (dynamically) will cause a
minute – but measurable – flow of
electrons along the surface of the crystal
based on di-polar disruption due to
shape change
 This flow is measure as a “Nano-current”
 The Current is linearized, amplified and
measured against a calibrated force

27. Contact Sensing
 A second general type would be the class of
“Micro-Switches”
 Like at the end of the Conveyor in the S100
cell
 Typically, applied forces directly move a
common contact between NC and NO contact
points

28. Examples of Micro-Switches:
 One Directional Reed
Switch:
 Omni-Directional Reed
Switch:
 Roller Contact Switch:
 Etc., etc., etc.!!!

29. Tactile Sensors – “feeler arrays”
 Potential Advantages of Tactile Sensors:
– They generate far fewer data bits (compared to
visual arrays) leading to simpler interpretation
analysis
– Collection is more readily controlled – we
completely control background and contrast
– The properties we measure are very close to
(exactly?!?) the properties we desire

30. Defining the “Ideal” Tactile Sensor
 They must be rugged and compliant to faults in the
manufacturing (operating) environment
 They should be “Smart” – That is able to process
most of the data into information for decision making
locally
– they send only results to the main controller
 Resolution should be on the order of about 100 mils
(about 10-4
inch)
 Sensors should respond to forces on the order of
about 5 -10 gmforce (0.1 N or 0.022 lbf)

31. Tactile Arrays:
 Machine Equivalent of
Human Skins
 Use arrays of micro-
sized switches or other
methods to detect
shapes and sizes due
to contact images of
“made” Switches

32. Tactile Arrays
 This device “measures”
shapes and sizes by
determining which of an
array of target points
have been charged
 Targets are “charged”
through contact with the
conductive Elastomer
skin and the PC ‘board’
targets

33. Tactile Arrays
 In this device, a series of thin
rods are pushed into an
object
 A “positive” image of the
object is produced by the
displaced rods
 In modern sensors,
displacement of each rod is
measured by the
detector/interpreter system –
this might be a vision system
located normal to the direction
of contact application or an
LVDT unit at each ‘rod’

34. Tactile Arrays
 The Anisotropic conductive
rubber sensor
 The ACR and gold contact
surface is separated when
unloaded
 As load is applied contact
patches grow indicating
shape and size of external
object and force being
applied

35. Proximity Sensors:
 Devices, including Photocells, Capacitance
sensors and Inductive sensors, that can be
used in areas that are near to but not
directly contacting an object to be sensed
 Like all sensors they use structured signal
sources, receive changes of state in their
energy (sensing) fields and interpret these
changes with signal changes to the “outside”

36. Photo Sensors
 The modern photosensor (in the
proximity range) emits
modulated light (at infrared or
near-infrared wavelengths). The
emitters are LED.
 The receivers (phototransistors)
are ‘tuned’ to be sensitive to the
wavelength of the source emitter
during the ‘on’ steps in the
modulated output stream
 The interpreters are (typically)
transistors that switch the power
(or ground) source on to the
output lead

37. Diffuse Mode Photosensor
 In proximity mode, the device is
looking for its own emitted beam
reflected back to its paired receiver
 The level of light falling on the
receiver to trigger positive response
can be ‘tuned’ to the task
 The sensors can be tuned to “Light-
Operate” or “Dark-Operate”
 Light operate means positive output
when reflective light is sensed
 Dark operate means positive output
when NO reflective light is sensed

38. Retro-Reflective Photosensors
 These devices rely on
“broken beams” to
detect
 They are “typically”
dark operate – that is
waiting for the object to
interrupt the light path
to the reflector

39. Thru-beam or Separated Systems
 The Emitter and
Receiver are
separate devices
 These again rely
on dark operate
mode (typically) –
that is a broken
beam indicates
objective present

40. Inductive Sensors
They typically oscillate
In ranges: 3 KHz – 1MHz

41. Inductive Sensors
Shielded types have slightly longer
range but smaller field of view

42. Uses:
 Inductive Sensors can (only) detect metals
as they draw power by induced surface
currents (eddy currents)
 The more magnetic the metal the greater the
sensor’s range

43. Principles of Capacitive Sensing

44. Uses And Capabilities
 Capacitive Sensors are able to detect any
material that raises the field dielectric in the
vicinity of the sensor
– In air this is nearly any other material!

45. Uses of Capacitive Sensors:
Typical Application of
Capacitive Sensor:
Detecting Liquid
(H2O) levels in bottles
When properly calibrated, the sensor can
detect any higher Dielectric Material thru
any lower Dielectric Material

46. Dielectric Values of
Various Materials: