Sensor is an electrical/mechanical device

1. Robotics sensors
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Sensors ????
• Collect information about the world
• Sensor - an electrical/mechanical/chemical device that
maps an environmental attribute to a quantitative
measurement.
• Each sensor is based on a transduction principle -
conversion of energy from one form to another.

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Conti..
Definition. 1. (Oxford dictionary)
• A device giving a signal for the detection or
measurement of a physical property to which it
responds.
Definition. 2.
• A sensor is a device that receives a signal or stimulus
and response with an electrical signal.

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Human sensing and organs
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• Vision: eyes (optics, light)
• Hearing: ears (acoustics, sound)
• Touch: skin (mechanics, heat)
• Odor: nose (vapor-phase chemistry)
• Taste: tongue (liquid-phase chemistry)

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Categorization of Sensor
Classification based on physical phenomena
• Mechanical: strain gage, displacement (LVDT), velocity
(laser vibrometer), accelerometer, tilt meter, viscometer,
pressure, etc.
• Thermal: thermal couple
• Optical: camera, infrared sensor

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Categorization of Sensor
Classification based on measuring mechanism
• Resistance sensing, capacitance sensing, inductance
sensing, piezoelectricity, etc.
• Materials capable of converting of one form of energy
to another are at the heart of many sensors.
• Invention of new materials, e.g., “smart” materials,
would permit the design of new types of sensors. .

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Classification of Sensors
1. Proprioception (Internal state) v.s. Exteroceptive (external state)
• measure values internally to the system (robot), e.g. battery level, wheel
position, joint angle, etc,
• observation of environments, objects
2. Active v.s. Passive
• emitting energy into the environment, e.g., radar, sonar
• passively receive energy to make observation, e.g., camera
3. Contact v.s. non-contact
4. Visual v.s. non-visual
• vision-based sensing, image processing, video camera

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Proprioceptive Sensors
1. Encoders, Potentiometers
• measure angle of turn via change in resistance or by counting optical pulses
2. Gyroscopes
• measure rate of change of angles
• fiber-optic (newer, better), magnetic (older)
3. Compass
• measure which way is north
4. GPS: measure location relative to globe
Touch Sensors
Whiskers, bumpers etc.
mechanical contact leads to
• closing/opening of a switch
• change in resistance of some element
• change in capacitance of some element
• change in spring tension

9. • Accuracy: error between the result of a measurement
and the true value being measured.
• Resolution: the smallest increment of measure that a
device can make.
• Sensitivity: the ratio between the change in the
output signal to a small change in input physical
signal. Slope of the input-output fit line.
• Repeatability/Precision: the ability of the sensor to
output the same value for the same input over a
number of trials
Specifications of Sensor
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10. Accuracy vs. Resolution
True value
measurement
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11. Accuracy vs. Precision
Precision without
accuracy
Accuracy without
precision
Precision and
accuracy
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13. Continue…..
• Measurements: ”The momentum and position of a particle can
not both be precisely determined at the same time.” Measuring
activity disturbs the physical process (loading effect).
• Measurement error: That is the difference between the
measured value and the true value.
error = measured value - true value
• Deterministic errors: They are repeated at every measurement,
e.g. reading offset or bias. Such errors can be corrected by
calibration.
• Random errors: They are caused by several parameters and
change in time in an unpredictable fashion. They can be
quantified by mean errors, standard deviation.
• Precision: Measurements with small deviation
• Accuracy: Measurements with small errors, i.e. small bias and
high precision 13

14. Sensor properties
• A sensor should represent a physical variable as fast and as
accurately as possible.
• A sensor is represented by its characteristic.
• Ideally, the sensor characteristic is a straight line
input
output
ideal
factual
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Force Sensors

17. Strain Gauges
• Foil strain gauge
– Least expensive
– Widely used
– Not suitable for long distance
– Electromagnetic Interference
– Sensitive to moisture & humidity
• Vibration wire strain gauge
– Determine strain from freq. of AC signal
– Bulky
• Fiber optic gauge
– Immune to EM and electrostatic noise
– Compact size
– High cost
– Fragile
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18. Strain gauge
• When external forces
are applied to a
stationary object, stress
and strain are the result.
• Stress is defined as
Strain is defined as the amount of deformation per unit length of
an object when a load is applied.
Strain (ε) = ΔL/L
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(68) What are Strain Gauge - Passive
Transducer-Bonded Strain Gauge-Transducers -
Electronics Engineering - YouTube

19. • Piezoelectric Strain Sensor
– Piezoelectric ceramic-based or Piezoelectric polymer-based
(e.g., PVDF)
– Very high resolution (able to measure nanostrain)
– Excellent performance in ultrasonic frequency range, very
high frequency bandwidth; therefore very popular in
ultrasonic applications, such as measuring signals due to
surface wave propagation
– When used for measuring plane strain, can not distinguish
the strain in X, Y direction
– Piezoelectric ceramic is a brittle material (can not measure
large deformation)
Strain Sensing
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25. Displacement Sensing
• LVDT (Linear Variable Differential
Transformer):
– Inductance-based sensor
– “Infinite” resolution
• limited by external electronics
– Limited frequency bandwidth (250 Hz typical
for DC-LVDT, 500 Hz for AC-LVDT)
– No contact between the moving core and coil
structure
• no friction, no wear, very long operating
lifetime
– Accuracy limited mostly by linearity
• 0.1%-1% typical
– Models with strokes from mm’s to 1 m
available
Photo courtesy of MSI
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(68) Construction & Working LVDT | Electrical
Engineering - YouTube

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29. LVDT
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(68) Construction & Working LVDT | Electrical
Engineering - YouTube

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Tactile Sensors

31. A tactile sensor is a device that measures information
arising from physical interaction with its environment.
What does it sense ?
Deformation of bodies (strain) or fields (electric or
magnetic).
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32. Types of human touch
• Cutaneous Sensations (internal sensors) - Cutaneous sense
receives sensory inputs from the receptors embedded in the
skin.
Senses : temperature, pressure, pain
• Kinesthetic Sensations (external sensors) - Kinesthetic sense
receives sensory inputs from the receptors located within
muscles, tendons and joints.
Senses : body position, movement, equilibrium
Tactile Sensor Cutaneous Sensory Receptors
Strengths Weakness
Linearity No Anticipation
Low Hysteresis
High Frequency of Signals
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33. Types of Signal in Human Touch Sensing
Basis of Classification :
• Type of Signal Frequency of Signal
FA = Fast Adapting, SA = slow adapting.
Type-1 tells us about the effective area that can be sensed by the receptor. Type 1 is small
area, type 2 is larger area. Hence FA1 means fast adapting, low area sensor.
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34. Tactile sensing: Methods of transduction
Usually an array of discrete sensing
elements.
Sensing elements can be many types:
• Resistive: strain gauge,
Piezoresistive
• Capacitive
• Piezoelectric
• Others like (magnetic, optical,
conductive rubber, ultrasonic
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35. Tactile sensing: Methods of transduction
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36. Resistive Sensing Elements
Mechanical deformation changes the
capacitance of parallel conducting plates
Strain gauge: a thin film having a metal pattern that changes resistance
when strained.
Piezoresistive element: Pressure on the element
causes the material to compress, changing it’s
resistance
Advantages: very simple construction, durable,
good dynamic range, easy readout
Disadvantages: non-linearity, hysteresis, low sensitivity
Capacitive Sensing Elements
Application area: Touchscreens.
Advantages: good dynamic range, linearity
Disadvantages: noise, measuring capacitance is hard!
(compared to measuring resistance) 36

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Proximity and range sensors

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41. Range Imaging Sensors
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Range imaging Sensor

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Velocity Sensors

43. Velocity Sensing
• Scanning Laser Vibrometry
– No physical contact with the test object; facilitate remote, mass-
loading-free vibration measurements on targets
– measuring velocity (translational or angular)
– automated scanning measurements with fast scanning speed
– However, very expensive (> $120K)
Photo courtesy of Bruel & Kjaer
Photo courtesy of Polytec
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