Sensors_2020.pptx

Military Technical College
Printing Engineering Department
Col. Dr. Ehab Said
Mechatronics I
Robotic Sensors
By Col. Dr. Ehab Said
Winter Term 2020 - 2021
Chapter 3

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Mechatronics I
Tuesday, August 22, 2023 Chapter 3 – Robotic Sensors
Sensor /
Transducer
Power
(for active sensors)
Mechanical System
(Plant, Process)
Response
Disturbance
Excitation
Actuator
Power
Drive
Excitation
Signal
Conditioning
Power
Controller
(Digital or
Analog)
Power
Control
Signal
Reference
Command
Signal
Conditioning
Power
Feedback
Signal
Measuring
System
Sensors: The BIG Picture
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Mechatronics I
Sensing : quantifying a system’s state
• Sensor : device that does sensing
– Receives and responds to a signal/stimulus/measurand
• A device transforms a stimulus/energy to a signal with information
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Mechatronics I
The World to Signal
A sensor is a device which measures a physical
quantity such as heat, light or motion and converts
it into a form suitable for measurement by an
observer or by an instrument. Because sensors are
a type of transducer, they change one form of
energy into another.
Signal Conditioning
Match sensor output to the microcontroller’s input
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Mechatronics I
For example, a mercury thermometer converts the measured
temperature into expansion and contraction of a liquid which
can be read on a calibrated glass tube. A thermocouple
converts temperature to an output voltage which can be read
by a voltmeter.
Examples
Temperature sensor
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Mechatronics I
Manometer
Bourdon tube:
A pressure sensing element consisting of a twisted tube of noncircular
cross section closed at one end. When a process stream is routed to the
open end of the tube, any increase in the pressure will cause the tube to
unwind.
Pressure sensor
Examples
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Pressure sensor
Examples
Bourdon tube:
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Mechatronics I
A good sensor obeys the following rules:
• The sensor should be sensitive to the measured property
• The sensor should be insensitive to any other property
• The sensor should not influence the measured property
Sensor Characteristics
Technology
Electric or magnetic, mechanical, electromechanical,
electro-optical, piezoelectric
Functional Performance Linearity, bias, accuracy, dynamic range, noise
Physical properties Weight, size, strength
Quality Factors Reliability, durability, maintainability
Cost Expense, availability, facilities for testing and maintenance
Considerations in sensor selection are:
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 Static characteristics:
– Define the performance criteria for the measurement of quantities
that remain constant or vary quite slowly. Static characteristics are
those that can be measured after all transient effects have been
stabilized to their final or steady state values.
– Static Performance Specifications:
These include: range, full scale, resolution, repeatability, accuracy,
sensitivity, hysteresis, linearity, and dead-band.
Sensor Characteristics
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Dynamic characteristics:
– Concern the relationship between the system input and output when the
measurand is varying rapidly.
– In the presence of a changing measurand, dynamic characteristics can be
employed to describe the sensing system’s transient properties. They can be
used for defining how accurately the output signal is employed for the
description of a time varying measurand.
– Dynamic Performance Specifications:
These include: response time, rise time, settling time, time constant,
stability, bandwidth, lower frequency limit and upper frequency limit.
Sensor
Characteristics
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Mechatronics I
1) Accuracy : The degree to which an indicated value matches the actual value of a measured
variable. To assess the accuracy, either the system is benchmarked against a standard
measurand or the output is compared with a measurement system with a superior
accuracy. considering a temperature sensing system, when the real temperature is 20.0C,
the system is more accurate, if it shows 20.1C rather than 21.0 C.
2) Precision (Repeatability):
It is the ability of the instrument to give the same output for the same input under the same
conditions. A temperature sensing system is precise, if when the ambient temperature is
21.0 C and it shows 22.0, 22.1, or 21.9 C in three different consecutive measurements. It is
not considered precise, if it shows 21.5, 21.0, and 20.5 C although the measured values are
closer to the actual temperature.
High Precision
Low Accuracy Low Precision
Low Accuracy
Static
Characteristics
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Mechatronics I
Static
Characteristics
4) Sensitivity: The sensitivity of the sensor is defined as the slope of the output
characteristic curve (DY/DX) or, more generally the minimum input of physical
parameter that will create a detectable output change. For example, in an electronic
temperature sensing system, if the output voltage increases by 1 V, when temperature
changes by 0.1  C, then the sensitivity will be 10 V/ C
0
2
4
6
8
10
12
0 1 2 3 4
Input, X
Output,
Y
Y1
Y2
Dead Band
3) Dead band: It describes how much change to the process is required before the
sensor actually responds to it or even detects it.
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Mechatronics I
5) Hysteresis: The maximum difference between the calibrated sensor
outputs for continuously increasing and continuously
decreasing inputs.
6) Linearity: This is typically defined as the maximum error between the
mean values of the sensors outputs and the true values of the
quantity being measured.
Static
Characteristics
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Mechatronics I
7) Range: The range of the sensor is the maximum and minimum values of
applied parameter that can be measured
8) Resolution: is the smallest detectable incremental change of physical
parameter which can be sensed. Or it is defined as minimum
division of the instrument’s scale.
9) Drift: Is the variation of output, when the input is kept constant.
10) Zero Stability: Is the ability of the instrument to return to zero reading,
when there is no measurand
Static
Characteristics
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Mechatronics I
Dynamic Characteristics
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Mechatronics I
Dynamic Characteristics
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Mechatronics I
Sensor as a Transfer Function
Example:Signal (V) = 30 mV/°C * Temperature (°C) + 2.5 V
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Mechatronics I
Robot Sensors
• Why do Robots Need Sensors?
• What can be Sensed?
• What Sensors are Out There?
• What can They do?
• How Much do They Cost?
• How Easy are They to Use?
Col. Dr. Ehab Said

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Mechatronics I
Why Do Robots Need Sensors?
• Provides “awareness” of surroundings
– What’s ahead, around, “out there”?
• Allows interaction with environment
– Robot lawn mower can “see” cut grass
• Protection & Self-Preservation
– Safety, Damage Prevention, Stairwell sensor
• Gives the robot capability to goal-seek
– Find colorful objects, seek goals
• Makes robots “interesting”
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Mechatronics I
Sensors - What Can Be
Sensed?
• Light
– Presence, color, intensity, content (mod), direction
• Sound
– Presence, frequency, intensity, content (mod), direction
• Heat
– Temperature, wavelength, magnitude, direction
• Chemicals
– Presence, concentration, identity, etc.
• Object Proximity
– Presence/absence, distance, bearing, color, etc.
• Physical orientation/attitude/position
– Magnitude, pitch, roll, yaw, coordinates, etc.
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Mechatronics I
• Magnetic & Electric Fields
– Presence, magnitude, orientation, content (mod)
• Resistance (electrical, indirectly via V/I)
– Presence, magnitude, etc.
• Capacitance (via excitation/oscillation)
• Inductance (via excitation/oscillation)
• Other Things?
Sensors - What Can Be
Sensed?
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Mechatronics I
What Sensors Are Out There?
• Feelers (Whiskers, Bumpers) – Mechanical
• Photoelectric (Visible)
• Infrared (light)
• Ultrasonic (sound)
• Sonic
• Resistive/Capacitive/Inductive
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Mechatronics I
• Visual – Cameras & Arrays
• Color Sensors
• Magnetic
• Orientation (Pitch & Roll)
• GPS (location, altitude)
• Compass (orientation, bearing)
• Voltage – Electric Field Sensors
• Current – Magnetic Field Sensors
• Chemical – Smoke Detectors, Gas Sensors
What Sensors Are Out There?
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Mechatronics I
Proximity (Tactile) Sensors
(Used to tell us if we hit something)
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Mechatronics I
- Mechanical
- Optical
- Ultrasonic
- Inductive/Capacitive
Proximity: Determines if an object is next to or in between another a reference object.
• Usually digital (on/off) sensors detecting the
presence or absence of an object within some
particular distance of the sensor
• Widely used in general industrial automation
– Conveyor lines (counting, jam detection, etc)
– Machine tools (safety interlock, sequencing)
Proximity Sensor (Tactile)
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Mechatronics I
Feelers (Whiskers, Bumpers)
• Whiskers
– Piano wire suspended through conductive “hoop”
– Deflection causes contact with “hoop”
– Springy wire that touches studs when deflected
– Reaches beyond robot a few inches
– Simple, cheap, binary output
• Bumpers & Guards
– Impact/Collision sensor, senses pressure/contact
– Microswitches & wires or framework that moves
– Simple, cheap, binary output, easy to read
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Mechatronics I
Feelers - Whiskers
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Mechatronics I
Feelers - Bumpers & Guards
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Mechatronics I
•Contact sensor
•Mainly used for collision detection
•If the switch connects, electricity passes,
and we can detect a “hit”
Mechanical Proximity Sensor
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Mechatronics I
Optical Proximity Sensor
• These sensors are more commonly known as light beam sensors.
• Consist of a light source (LED) and light detector (Photoresistor).
• The light source generates light of a frequency that the light sensor is best able to
detect, and that is not likely to be generated by other nearby sources.
• Photoresistor: the resistance which depends on the intensity of the incident light
• Modulation of signal to minimize ambient lighting conditions.
• They are used to detect near collision.
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Mechatronics I
When a metal object approaches into the inductive proximity sensor’s
field of detection, Eddy circuits build up in the metallic object,
magnetically push back, and finally reduce the Inductive sensor’s own
oscillation field. The sensor’s detection circuit monitors the oscillator’s
strength and triggers an output from the output circuitry when the
oscillator becomes reduced to a sufficient level.
Inductive Proximity Sensor
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Mechatronics I
• Metals affect sensor
• Current flows through inductor
• Magnetic field mostly ignores non-metals
• Inductance changes with metallic proximity
• Short range applications (~cm or mm)
Sensor performance can be affected by:
• Temperature
• Target material
• Target dimensions
Inductive Proximity Sensor
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Mechatronics I
Capacitive Proximity Sensor
Capacitive proximity sensors are similar to inductive proximity sensors.
The main difference between the two types is that capacitive proximity
sensors produce an electrostatic field instead of an electromagnetic field.
Capacitive proximity switches will sense metal as well as nonmetallic
materials such as paper, glass, liquids, and cloth.
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Mechatronics I
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The sensing surface of a capacitive sensor is formed by two metal electrodes of
an unwound capacitor. When an object approaches the sensing surface it enters
the electrostatic field of the electrodes and changes the capacitance in an
oscillator circuit. As a result, the oscillator begins oscillating. The trigger circuit
reads the oscillator’s amplitude and when it reaches a specific level the output
state of the sensor changes. As the target moves away from the sensor the
oscillator’s amplitude decreases, switching the sensor output back to its original
state. They typically have a short sensing range of about 1 inch, regardless of the
type of material being sensed.
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Mechatronics I
Time of flight Sensors
Used to tell us how far Objects are from
us
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Mechatronics I
Time of flight Sensors
-Send a signal and start a timer (t1 = 0 sec)
-Wait for echo signal and stop timer (t2 = 10 sec)
- Calculate difference (t2 – t1 = 10 sec)
-Use time difference to calculate distance (distance = speed * time)
(Speed of Light = 30 cm/nS)
( Speed of Sound = 30 cm/mS)
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Mechatronics I
IR - Sensor
-Works on Infra-Red light (invisible to humans)
-Measures the time it takes for light to go and
come back
-IR Sensors work by using a specific light sensor to
detect a selected light wavelength in the Infra-Red
(IR) spectrum.
-By using a LED which produces light at the same
wavelength as what the sensor is looking for, you
can look at the intensity of the received light.
-When an object is close to the sensor, the light
from the LED bounces off the object and into the
light sensor.
-Range = Approximately 4 – 26 inch.
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Mechatronics I
Laser Range - Sensor
A laser distance meter emits a pulse of laser at a target. The
pulse then reflects off the target and back to the sending
device (in this case, a laser distance meter). This "time of
flight" principle is based on the fact that laser light travels at a
fairly constant speed through the Earth’s atmosphere. Inside
the meter, a simple computer quickly calculates the distance
to target. This method of distance calculation is capable of
measuring the distance from the Earth to the moon within a
few centimeters. Laser distance meters may also be referred
to as “range finders” or “laser range finders.”
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Mechatronics I
Why Lasers?
Lasers are focused, intense beams of light, usually of a single frequency. They
are very useful for measuring distances because they travel at fairly constant
rates through the atmosphere and travel much longer distances before
divergence and without losing intensity. Compared with ordinary white light, a
laser pulse retains much of its original intensity when reflected off the target,
which is very important when calculating distance to an object.
Laser Range - Sensor
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Ultrasonic - Sensor
Ultrasonic sensors measure distance by using ultrasonic waves. The sensor
head emits an ultrasonic wave and receives the wave reflected back from the
target. Ultrasonic Sensors measure the distance to the target by measuring
the time between the emission and reception.
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Mechatronics I
Ultrasonic - Sensor
An example for ultrasonic sensor is a Car Parking sensor
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Ultrasonic - Sensor
 Ultrasonic sensor is a single transducer that send a pulse and receive the
echo.
 It sends an ultrasonic pulse out at 40kHz (Above the range of human
hearing) which travels through the air.
 The maximum range is around 20 meters (about 70 feet) and varies by
model.
 Relatively simple, not cheap, analog output.
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Mechatronics I
Position, Motion and
Displacement Measurement
(Linear, Angular)
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Resistive Displacement Sensors
Resistive displacement sensors are commonly termed potentiometers or “pots.” A pot is an
electromechanical device containing an electrically conductive wiper that slides against a fixed
resistive element according to the position or angle of an external shaft.
Representative of linear-motion (a) and rotary (b) potentiometers.
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Mechatronics I
Linear Displacement Contact-type Sensor
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The Fig. illustrates what happens when the LVDT's core is in different axial positions. The LVDT's
primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus
developed is coupled by the core to the adjacent secondary windings, S1 and S2. If the core is located
midway between S1 and S2, equal flux is coupled to each secondary so the voltages, E1 and E2,
induced in windings S1 and S2 respectively, are equal. At this reference midway core position,
known as the null point, the differential voltage output, (E1 - E2), is essentially zero. As shown in
Fig., if the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2, so the
induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2).
Conversely, if the core is moved closer to S2, more flux is coupled to S2 and less to S1, so E2 is
increased as E1 is decreased, resulting in the differential voltage (E2 - E1).
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Mechatronics I
Linear Variable Differential Transformer (LVDT)
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Slotted Disk and Opto-Interrupter
One relatively inexpensive method to measure
the angular displacement or angular position
of a shaft is to use a simple slotted disk and
opto-interrupter. This device is constructed of
a circular disk (usually metal) mounted on the
machine shaft. A small radial slot is cut in the
disk so that light from an emitter will pass
through the slot to a photo-transistor when the
disk is in a particular angular position.
As the disk is rotated, the photo-transistor
outputs one pulse per revolution.
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Mechatronics I
Incremental Encoder
Although the slotted disk encoder works well for complete revolution
indexing, it may be necessary to measure and rotate a shaft to a precise
angular position smaller than 360 degrees. When this is required, an
incremental optical encoder may be used.
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Mechatronics I
Incremental Encoder
This solution with only a sensing head is not able to identify the direction of disc
rotation. To solve this problem another optical sensing head is used, with its output
signal offset from the first by 90°; in other words, the optical sensing head signals
are in quadrature. This layout produces two square waves in quadrature,
corresponding each to one sensing head (channels A and B), as shown in Figure.
How can we detect the direction of disc rotation?
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Mechatronics I
Incremental Encoder
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Mechatronics I
An incremental encoder can be used to extract Three pieces of information about a
rotating shaft.
First, by counting the number of pulses received and multiplying the count by the
encoder’s resolution, we can determine how far the shaft has been rotated in
degrees.
Second, by viewing the phase relationship between the phase A and phase B
outputs, we can determine which direction the shaft is being rotated.
Third, by counting the number of pulses received from either output during a fixed
time period, we can calculate the angular velocity in either radians per second or
RPMs.
Incremental Encoder
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Mechatronics I
Incremental Encoder
Incremental encoders are specified by the number
of pulses per revolution that is produced by either
the phase A or phase B output. By dividing the
number of pulses per revolution into 360 degrees,
we get the number of degrees per pulse (called the
resolution). This is the smallest change in shaft
angle that can be detected by the encoder. For
example, a 3600 pulse incremental encoder has a
resolution of
360 degrees / 3600 = 0.1 degree.
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Mechatronics I
Absolute Encoder
Unlike the incremental encoder, the absolute
encoder provides digital values as an output signal.
The output is in the form of a binary word which is
proportional to the angle of the shaft. Absolute
encoders have a unique code for each shaft position.
Since each position is distinctive, the verification of
true position is available as soon as power is
switched on. The absolute encoder does not need to
be homed because when it is energized, it simply
outputs the shaft angle as a digital value.
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Mechatronics I
Absolute Encoder
Example Problem:
A 12 bit binary absolute encoder is outputting the number
101100010111.
a) What is the resolution of the encoder.
b) what is the range of angles indicated by its output.
Solution:
a) A 12 bit encoder will output 4095 binary numbers for one revolution.
Therefore the resolution is 360 degrees / 4095 = 0.087891 degree/Pulse.
b) Converting the binary number to decimal, we have 101100010111 =
2839. The indicated angle is between 2839 x 0.087891 = 249.52 degrees
and
249.52 + 0.087891 = 249.60 degrees.
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Mechatronics I
Tuesday, August 22, 2023
Thank You
Chapter 3 – Robotic Sensors
Col. Dr. Ehab Said

Sensor Deadband
A Sensor deadband is device specific. The sensor itself will not send a value change
unless the variation exceeds the deadband limit. For example, a data acquisition board
receives a voltage signal, but a change is only registered if the change in voltage exceeds
the resolution.
Sensor deadbands are usually dictated by hardware limitations, factory configuration or
can sometimes be configured with the sensor driver provided by the manufacturer of the
device.

Hysteresis is the deviation of the sensor’s
output at any given point when approached
from two different directions
• Caused by electrical or mechanical systems
– Magnetization
– Thermal properties
– Loose linkages

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