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Robotics: Unit-IV
Sensors
DEFINITIONS, CLASSIFICATIONS, GENERAL
REQUIREMENTS
Some general statements
Sensors/actuators are common
Usually integrated in a system (never alone)
A system of any complexity cannot be designed without them
Very difficult to classify
Difficult to get good data on them
Definitions and terms are confusing
Introduction
Industrial robot requires sensory feedback to:
1.
2.
3.
4.
5.
Locate randomly placed object;
Allow for variations in shape of objects;
Protect against dangerous and unexpected situations. Especially if the robot must work
close to humans:
Allow “intelligent” recovery form error conditions;
Perform quality control.
The main objective of incorporating sensors in robotic system is to enable
robots to work in nonstructural and random environments.
Sensors will make robots more intelligent. But the associated robotic software
must have the ability to receive data from the sensors and to process the
necessary real time information and commands needed for the decision
making.
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What is Sensing ?
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
Human sensing and organs
Vision: eyes (optics, light)
Hearing: ears (acoustics, sound)
Touch: skin (mechanics, heat)
Odour: nose (vapor-phase chemistry)
Taste: tongue (liquid-phase chemistry)
Counterpart?
Transduction to electronics
Thermistor: temperature-to-resistance
Electrochemical: chemistry-to-voltage
Photocurrent: light intensity-to-current
Pyroelectric: thermal radiation-to-voltage
Humidity: humidity-to-capacitance
Length (LVDT: Linear variable differential transformers) :
position-to-inductance
Microphone: sound pressure-to-<anything>
Sensors
Sensors that tell the robot position/change of joints:
odometers, speedometers, etc.
Force sensing. Enables compliant motion--robot just
maintains contact with object (video: compliant)
Sonar. Send out sound waves and measure how long
it takes for it to be reflected back. Good for obstacle
avoidance.
Vision systems
Extended ranges and modalities
Vision outside the RGB spectrum
◦ Infrared Camera, see at night
Active vision
◦ Radar and optical (laser) range measurement
Hearing outside the 20 Hz – 20 kHz range
◦ Ultrasonic range measurement
Chemical analysis beyond taste and smell
Radiation: a, b, g-rays, neutrons, etc
More confusion
Sensor can mean:
transducer
actuator
transducer can be part of a sensor
sensor can be part of a transducer
Many sensors can work as actuators (duality)
Many actuators can work as sensors
What is it then? - All of the above!
Our definitions:
Sensor
A device that responds to a physical stimulus.
Transducer
A device that converts energy of one form into energy of another
form.
Actuator
A device or mechanism capable of performing a physical action
Our definitions:
Stimulus
The quantity that is sensed.
Sometimes called the measurand.
Definitions - Sensors
Also called: transducer, probe, gauge, detector, pick-up etc.
Start with the dictionary:
A device that responds to a physical stimulus and transmits a resulting impulse.
(New Collegiate Dictionary)
A device, such as a photoelectric cell, that receives and responds to a signal or
stimulus. (American Heritage Dictionary, 3rd ed., 1996)
A device that responds to a physical stimulus (as heat, light, sound, pressure,
magnetism, or a particular motion) and transmits a resulting impulse (as for
measurement or operating a control) . (Webster, 3rd ed., 1999)
Definitions - Transducer
A device that is actuated by power from one system and supplies power
usually in another form to a second system. (New Collegiate Dictionary)
A substance or device, such as a piezoelectric crystal, that converts
input energy of one form into output energy of another. (from: Transducere – to transfer, to lead) (American Heritage Dictionary, 3rd ed.,
1996)
A device that is actuated by power from one system and supplies power
usually in another form to a second system (a loudspeaker is a
transducer that transforms electrical signals to sound energy) .
(Webster, 3rd ed., 1999)
Definitions - Actuator
A mechanism for moving or controlling something indirectly
instead of by hand. (New Collegiate Dictionary)
One that activates, especially a device responsible for
actuating a mechanical device such as one connected to a
computer by a sensor link (American Heritage Dictionary, 3rd
ed., 1996)
One that actuates; a mechanical device for moving or
controlling something. (Webster, 3rd ed., 1999)
Sensors - What Can Be Sensed?
Light
◦ Presence, color, intensity, direction
Sound
◦ Presence, frequency, intensity, content (mod), direction
Heat
◦ Temperature, wavelength, magnitude, direction
Chemicals
◦ Presence, concentration, identity, etc.
Object Proximity
◦ Presence/absence, distance, color, etc.
Physical orientation/attitude/position
◦ Magnitude, pitch, roll, yaw, coordinates, etc.
Sensors - What Can Be Sensed?
Magnetic & Electric Fields
◦ Presence, magnitude, orientation
Resistance (electrical)
◦ Presence, magnitude, etc.
Capacitance (via excitation/oscillation)
◦ Presence, magnitude, etc.
Inductance (via excitation/oscillation)
◦ Presence, magnitude, etc.
What Sensors Are Out There?
Feelers (Whiskers, Bumpers) – Mechanical
Photoelectric (Visible) – Active & Passive
Infrared (light) – Active & Passive
Ultrasonic (sound) – Active & Passive
Sonic – Active & Passive
Resistive/Capacitive/Inductive – Active & Passive
What Sensors Are Out There?
Visual – Cameras & Arrays (Active & Passive)
Color Sensors (Active & Passive)
Magnetic (Active & Passive)
Orientation (Pitch & Roll)
GPS (location, altitude)
Compass (orientation)
Voltage – Electric Field Sensors
Current – Magnetic Field Sensors
Chemical – Smoke Detectors, Gas Sensors
Sensors Used in
Robot
Gas Sensor
Gyro
Accelerometer
Pendulum Resistive
Tilt Sensors
Metal Detector
Piezo Bend Sensor
Gieger-Muller
Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
Digital Infrared Ranging
CDS Cell
Resistive Light Sensor
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Pin
Diode
IR Sensor w/lens
Thyristor
IR Reflection
Sensor
Magnetic Sensor
Magnetic Reed Switch
IR Amplifier Sensor
Hall Effect
Magnetic Field
Sensors
Polaroid Sensor Board
IRDA Transceiver
Lite-On IR
Remote Receiver
Radio Shack
Remote Receiver
IR Modulator
Receiver
Solar Cell
Compass
Compass
Piezo Ultrasonic Transducers
Classification of Sensors and
Actuators
Based on physical laws
Based on any convenient distinguishing property
Possible to a certain extent - some devices defy
classification
1.
Active and Passive sensors
2.
Contact and non-contact sensors
3.
Absolute and relative sensors
4.
Other schemes
Sensors used in robot
navigation
Resistive sensors
◦ bend sensors, potentiometer, resistive photocells, ...
Tactile sensors
◦ contact switch, bumpers…
Infrared sensors
◦ Reflective, proximity, distance sensors…
Ultrasonic Distance Sensor
Inertial Sensors (measure the second derivatives of position)
◦ Accelerometer, Gyroscopes,
Orientation Sensors
◦ Compass, Inclinometer
Laser range sensors
Vision
Global Positioning System
Classification of Sensors
Internal state (proprioception) v.s. external state
(exteroceptive)
◦ feedback of robot internal parameters, e.g. battery level,
wheel position, joint angle, etc,
◦ observation of environments, objects
Active v.s. non-active
◦ emitting energy into the environment, e.g., radar, sonar
◦ passively receive energy to make observation, e.g., camera
Contact v.s. non-contact
Visual v.s. non-visual
◦ vision-based sensing, image processing, video camera
Robotic Sensor Classification
In general, robotic sensors can be divided into two classes:
i.
Internal state sensors - device being used to measure the
position, velocity and acceleration of the robot joint and/or
end-effector. These devices are potentiometer, tachometers,
differential transformers, optical interrupters, optical
encoders and accelerometer.
ii. External state sensors – device being used to monitor the
relationship between the robot kinematics and/or dynamics
with its task, surrounding, or the object being manipulated.
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1. Active and passive sensors
Active sensor: a sensor that requires external power to
operate. Examples: the carbon microphone, thermistors,
strain gauges, capacitive and inductive sensors, etc.
Other name: parametric sensors (output is a function of a
parameter - like resistance)
Passive sensor: generates its own electric signal and does not
require a power source. Examples: thermocouples,
magnetic microphones, piezoelectric sensors.
Other name: self-generating sensors
2. Contact and noncontact sensors
Contact sensor: a sensor that requires physical contact with
the stimulus. Examples: strain gauges, most temperature
sensors
Non-contact sensor: requires no physical contact. Examples:
most optical and magnetic sensors, infrared thermometers,
etc.
3. Absolute and relative sensors
Absolute sensor: a sensor that reacts to a stimulus on an
absolute scale: Thermistors, strain gauges, etc., (thermistor
will always read the absolute temperature)
Relative scale: The stimulus is sensed relative to a fixed or
variable reference. Thermocouple measures the
temperature difference, pressure is often measured relative
to atmospheric pressure.
4. Other schemes
Classification by broad area of detection
Electric sensors
Magnetic
Electromagnetic
Acoustic
Chemical
Optical
Heat, Temperature
Mechanical
Radiation
Biological
4. Other schemes
Classification by physical
law
Photoelectric
Photomagnetic
Magnetoelectric
Thermoelastic
Thermoelectric
Thermomagnetic
Photoconductive
Thermooptic
Magnitostrictive
Electrochermical
Electrostrictive
Magnetoresistive
Photoelastic
4. Other schemes (cont.)
Classification by
specifications
Accuracy
Resolution
Sensitivity
Linearity
Stability
Response time
Hardness (to environmental
conditions, etc.)
Hysteresis
Cost
Frequency response
Size, weight,
Input (stimulus) range
Construction materials
Operating temperature
4. Other schemes (cont.)
Classification by area of
application
Consumer products
Transportation
Military applications
Automotive
Infrastructure
Avionic
Energy
Marine
Heat
Space
Manufacturing
Scientific
Classification of actuators
All of the above
In addition:
Classification of actuators by type of motion
Linear
Rotary
One-axis
Two-axes
Three-axes
Classification of actuators
Low power actuators
High power actuators
Micropower actuators
Connection of sensors/actuators
The processor should be viewed as a general block
◦ Microprocessor
◦ Amplifier
◦ Driver
Matching: between sensor/processor and processor/actuator
Example - Temperature control
Sense the temperature of a CPU
Control the speed of the fan to keep the temperature
constant