sensor

1. Sensors

2. SENSORS
 Human-beings collect information of the surroundings using their sensors, namely eyes,
ears, nose, skin etc., in order to perform various tasks.
 A sensor is used to take measurement of physical variable.
 A sensor requires calibration
 Sensors are used to build intelligent robots

3. What is a Sensor and Transducer?
The sensor is a device, that senses a physical quantity and converts it into an analogue quantity
which can be measured electrically such as voltage, capacitance, inductance and ohmic resistance.
The output needs to be operated, interfaced & regulated by the system designer.
There are different kinds of sensors are available, that are used in various applications.
The motion sensor is a one type of sensor, that is used in numerous systems like home security
lights, automatic door fixtures normally send out some kind of energy like ultrasonic waves,
microwaves or light beams and sense when the energy flow is interrupted by something entering
its lane.
The transducer is a device that is connected to sensor to convert the measured quantity into a
standard electrical signal such as 0-10V DC, -10 to +10V DC, 4 to 20mA, 0 to 20mA, 0-25mA
etc. The o/p of the transducer can be directly used by the system designer.

4. There are various types of sensors and transducers are available to choose from like analog,
digital, input and output. The type of i/p or o/p transducer being used, really depends upon the
kind of signal sensed or controlled. But, a sensor and transducer can be defined as they converts
one physical quantity to another.
A device which performs an i/p function is called sensor because they sense a physical change in
some characteristic that changes in response to some excitation.Transducer is also a device, that
converts the energy from one form to another.Examples for the transducer is microphone,
loudspeaker etc.

5. Common Sensors and Transducers

6. Classification of Sensors
 In the first classification of the sensors, they are divided in to Active and Passive.
 Active Sensors are those which require an external excitation signal or a power signal.
 Passive Sensors, on the other hand, do not require any external power signal and directly
generates output response.
 The other type of classification is based on the means of detection used in the sensor. Some of
the means of detection are Electric, Biological, Chemical, Radioactive etc.
 The next classification is based on conversion phenomenon i.e. the input and the output. Some
of the common conversion phenomena are Photoelectric, Thermoelectric, Electrochemical,
Electromagnetic, Thermooptic, etc.
 The next classification of the sensors are Analog and Digital Sensors.
 Analog Sensors produce an analog output i.e. a continuous output signal with respect to the
quantity being measured.
 Digital Sensors, in contrast to Analog Sensors, work with discrete or digital data. The data in
digital sensors, which is used for conversion and transmission, is digital in nature.

7. 7
SENSORS
Internal Sensors
(used to operate the drive
units)
Ex. Position sensors, velocity
sensors, Acceleration
sensors, Force/Moment
sensor
External Sensors
(used to collect information of
the environment)
Ex. Temperature sensors, Visual
sensor, Proximity sensor,
Acoustic sensor
 The next classification of the sensors are Internal Sensors and External Sensors

8. 8
SENSORS
Contact Sensors
hysical contact between sensor mounted
on robot and object)
Non-Contact Sensors
(No physical contact)
Proximity sensor
Visual sensor
Acoustic sensor
Range sensor
Touch sensor/ Tactile
sensor/ Binary sensor
(indicates presence or
absence of an object)
Ex. Micro-switch, Limit
switch
Force sensor/ Analog sensor
(not only the contact is made
but also the force is
measured)
Ex. Sensors using strain
gauges
The next classification of the sensors are Contact Sensors and Non-Contact Sensors

9. Sensors used in Robotics
The use of sensors in robots has taken them into the next level of creativity. Most importantly,
the sensors have increased the performance of robots to a large extent. It also allows the robots to
perform several functions like a human being. The robots are even made intelligent with the help
of Visual Sensors (generally called as machine vision or computer vision), which helps them to
respond according to the situation. The Machine Vision system is classified into six sub-divisions
such as Pre-processing, Sensing, Recognition, Description, Interpretation, and Segmentation.
Different types of sensors: There are plenty of sensors used in the robots, and some of the
important types are listed below:
 Proximity Sensor,
 Range Sensor, and
 · Tactile Sensor.

10. Proximity Sensor
This type of sensor is capable of pointing out the availability of a component. Generally, the
proximity sensor will be placed in the robot moving part such as end effector. This sensor will be
turned ON at a specified distance, which will be measured by means of feet or millimeters. It is
also used to find the presence of a human being in the work volume so that the accidents can be
reduced.
Range Sensor:
Range Sensor is implemented in the end effector of a robot to calculate the distance between the
sensor and a work part. The values for the distance can be given by the workers on visual data. It
can evaluate the size of images and analysis of common objects. The range is measured using the
Sonar receivers & transmitters or two TV cameras.

11. Range Sensors:
The distance between the object and the robot hand is measured using the range
sensors Within it is range of operation.
The calculation of the distance is by visual processing. Range sensors find use in
robot navigation and avoidance of the obstacles in the path.
The - location and the general shape characteristics of the part in the work envelope ofthe
robot S done by special applications for the range sensors.
There are several approaches like, triangulation method, structured lighting approach and
time-of flight range finders etc. In these cases the source of illumination can be light-source,
laser beam or based on ultrasonic.

12. Triangulation Method:
 One of the simplest methods for measuring range is
through triangulation techniques.
 An object is illuminated by a narrow beam of light which
is swept over the surface.
 The sweeping motion is in the plane defined by the line
from the object to the detector and the line from the
detector to the source.
 If the detector is focused on a small portion of the
surface then, when the detector see the light spot, its
distance D to the illuminated portion of the surface can
be calculated from the geometry of Figure.

13.  The above approach yields a point measurement.
 If the source detector arrangement is moved in a fixed plane (up and down and side ways)
then it is possible to obtain a set of points whose distances from the detector are known.
 The distances are easily transformed to three-dimensional coordinates by keeping track of the
location and orientation of the detector as the objects are scanned.

14. Structured Lighting Approach:
 This approach consists of projecting a light pattern the distortion of the pattern to calculate the range. A pattern in
use today is a sheet of light generated narrow slit.
 As illustrated in. Figure, the intersection yields a light Stripe which is viewed through a television camera
displaced a distance B from the light source.

15.  The stripe pattern is easily analyzed by a computer to obtain range information. For example, an inflection
indicates a change of surface, and a break corresponds to a gap between surfaces.
 Specific range values are computed by first calibrating the system. One of the simplest arrangements is shown in
Figure, which represents a top view of Figure.
 In this, arrangement, the light source and camera are placed at the same height, and the sheet of light is
perpendicular to the line joining the origin of the light sheet and the center of the camera lens.
 We call the vertical plane containing this line the reference plane.
 Clearly, the reference plane is perpendicular to the sheet of light, and any vertical flat surface that intersects the
sheet will produce a vertical stripe of light in which every point will have the same perpendicular distance to the
reference plane. –
 The objective of the arrangement shown in Figure. is to position the camera so that every such vertical stripe also
appears vertical in the image plane.
 In this way, every point, the same column in the image will be known to have the same distance to the reference
plane.

16. Proximity Sensors: The output of the proximity sensors gives an indication of the presence of an
object with in the vicinity job operation.
In robotics these sensors are used to generate information of object grasping and obstacle
avoidance. This section deals with some of the important proximity sensors used in robotics.
Proximity sensor is a sensor, which senses the presence or absence of the object without having
physical contact between the objects.

17. Eddy current proximity sensors
Eddy current proximity sensors are used to detect non-magnetic but conductive materials. They comprise of a coil, an
oscillator, a detector and a triggering circuit. Figure shows the construction of eddy current proximity switch.
When an alternating current is passed thru this coil, an alternative magnetic field is generated. If a metal object comes
in the close proximity of the coil, then eddy currents are induced in the object due to the magnetic field. These eddy
currents create their own magnetic field which distorts the magnetic field responsible for their generation. As a result,
impedance of the coil changes and so the amplitude of alternating current. This can be used to trigger a switch at some
pre-determined level of change in current.
Eddy current sensors are relatively inexpensive, available in small in size, highly reliable and have high sensitivity for
small displacements.

18. Applications of eddy current proximity sensors
•Automation requiring precise location
•Machine tool monitoring
•Final assembly of precision equipment such as disk drives
•Measuring the dynamics of a continuously moving target, such as a vibrating element,
•Drive shaft monitoring
•Vibration measurements

25. Hall effect sensor:
 Figure shows the principle of working of Hall effect sensor. Hall effect sensors work on the principle
that when a beam of charge particles passes through a magnetic field, forces act on the particles and the
current beam is deflected from its straight line path.
 Thus one side of the disc will become negatively charged and the other side will be of positive charge.
This charge separation generates a potential difference which is the measure of distance of magnetic
field from the disc carrying current.
 The typical application of Hall effect sensor is the measurement of fluid
level in a container. The container comprises of a float with a permanent
magnet attached at its top.
 An electric circuit with a current carrying disc is mounted in the casing.
When the fluid level increases, the magnet will come close to the disc and
a potential difference generates.
 This voltage triggers a switch to stop the fluid to come inside the container.
 These sensors are used for the measurement of displacement and the
detection of position of an object. Hall effect sensors need necessary signal
conditioning circuitry.
 They can be operated at 100 kHz. Their non-contact nature of operation,
good immunity to environment contaminants and ability to sustain in
severe conditions make them quite popular in industrial automation.

26. Ultrasonic Sensor
The principle of ultrasonic sensor is similar to sonar or radar in which interpretation of echoes from radio
or sound waves to evaluate the attributes of a target by generating the high-frequency-sound waves (around
40kHz). The transducer used for converting energy into ultrasound or sound waves with ranges above
human hearing range is called an ultrasonic transducer.
Application of Ultrasonic Sensor
The distance measurement at inaccessible areas is a typical application of ultrasonic sensors. The circuit
consists of an ultrasonic module, LCD display and microcontroller. The ultrasonic module is interfaced
with the microcontroller and this ultrasonic transducer consists of a transmitter and receiver.
The waves transmitted by transducer are received back again after the waves are reflected back from the
object. The velocity of sound is considered for calculating time taken for sending and receiving waves. The
distance is calculated by executing a program on microcontroller, and then it is displayed on the LCD
display.

32. Optical encoders
Optical encoders provide digital output as a result of linear / angular displacement. These are widely used in
the Servo motors to measure the rotation of shafts. Figure shows the construction of an optical encoder. It
comprises of a disc with three concentric tracks of equally spaced holes. Three light sensors are employed
to detect the light passing thru the holes. These sensors produce electric pulses which give the angular
displacement of the mechanical element e.g. shaft on which the Optical encoder is mounted.
The inner track has just one hole which is used locate the ‘home'
position of the disc. The holes on the middle track offset from the
holes of the outer track by one-half of the width of the hole. This
arrangement provides the direction of rotation to be determined. When
the disc rotates in clockwise direction, the pulses in the outer track
lead those in the inner; in counter clockwise direction they lag behind.
The resolution can be determined by the number of holes on disc.
With 100 holes in one revolution, the resolution would be,
360°/100=3.6°.

33. Tachometer:
A tachometer is a sensor device used to measure the rotation speed of an object such as the engine shaft in
a car, and is usually restricted to mechanical or electrical instruments. This device indicates the revolutions
per minute (RPM) performed by the object.
The device comprises of a dial, a needle to indicate the current reading, and markings to indicate safe and
dangerous levels. The word comes from the Greek ‘tachos’ meaning speed and ‘metron’ meaning measure
so tachometer and speedometer have become interchangeable and essentially both measure speed.
Historically, the first mechanical tachometers were designed based on measuring centrifugal force: an
inertial force directing away from an axis of rotation that acts on all objects as viewed from a rotating
frame of reference.
In 1817, it was adapted to be used for measuring the speed of machines and since 1840, it has been
predominantly used to measure the speed of vehicles; specifically locomotives.
Advanced tachometers have novel uses. For example, in the medical field, a haematachometer placed in an
artery or vein can estimate the rate of blood flow from the speed at which the turbine spins. The readings
can be used to diagnose circulatory problems like clogged arteries.

34. Types of Tachometers
The types of tachometers commonly found are:
•Analog tachometers - Comprised of a needle and dial-type of interface. They do not have provision for
storage of readings and cannot compute details such as average and deviation. Here, speed is converted to
voltage via use of an external frequency to voltage converter. This voltage is then displayed by an analog
voltmeter.
•Digital tachometers - Comprised of a LCD or LED readout and a memory for storage. These can
perform statistical operations, and are suitable for precision measurement and monitoring of any kind of
time-based quantities. Digital tachometers are more common these days and they provide numerical
readings instead of using dials and needles.
Contact and non-contact tachometers – The contact type is in contact with the rotating shaft and uses an
optical encoder ot magnetic sensor. The non-contact type is ideal for applications that are mobile, and uses
a laser or optical disk. Both of these types are data acquisition methods.
•Time and frequency measuring tachometers – Both these are based on measurement methods. The time
measurement device calculates speed by measuring the time interval between the incoming pulses;
whereas, the frequency measurement device calculates speed by measuring the frequency of the incoming
pulses. Time measuring tachometers are ideal for low speed measurements and frequency measuring
tachometers are ideal for high speed measurements

35. Working principle:
The working principle of an electronic tachometer is quite simple. The ignition system triggers a voltage
pulse at the output of the tachometer electromechanical part whenever the spark plug fires. The
electromechanical part responds to the average voltage of the series of pulses and it shows that the average
voltage of the pulse train is proportional to engine speed. The signal from the perception head is
transmitted by standard twin screened cable to the indicator.
The tachometers are temperature compensated to be able to handle operations over an ambient temperature
range of – 20 to +70°C (-4 to +158°F).
The tachometer in a vehicle enables the driver to select suitable throttle and gear settings for the driving
conditions as prolonged use at high speeds can cause insufficient lubrication which will affect the engine.
It enables the driver to prevent exceeding speed capability of sub-parts such as spring retracted valves of
the engine, and overheating, thereby causing unnecessary wear or permanent damage and even failure of
engines.
Applications
•Automobiles, airplanes, trucks, tractors, trains and light rail vehicles
•Laser instruments
•Medical applications
•Analog audio recording
•Numerous types of machinery and prime movers
•To estimate traffic speed and volume.

36. Tactile Sensors:
A sensing device that specifies the contact between an object, and sensor is considered as the
Tactile Sensor. This sensor can be sorted into two key types namely:
· Touch Sensor, and
· Force Sensor.
The touch sensor has got the ability to sense and detect the touching of a sensor and object.
Some of the commonly used simple devices as touch sensors are micro – switches, limit
switches, etc. If the end effector gets some contact with any solid part, then this sensor will
be handy one to stop the movement of the robot. In addition, it can be used as an inspection
device, which has a probe to measure the size of a component.
The force sensor is included for calculating the forces of several functions like the machine
loading & unloading, material handling, and so on that are performed by a robot. This
sensor will also be a better one in the assembly process for checking the problems. There are
several techniques used in this sensor like Joint Sensing, Robot –Wrist Force Sensing, and
Tactile Array Sensing.

38. 38
Touch Sensor
 Used to indicate whether contact has been made between two objects without regard to the
magnitude of the contacting force.
 Included within this category are simple devices such as
limit switches
Micro-switches
 The simpler devices are frequently used in the design of interlock systems in robotics.
Presence or absence of parts in a fixture or at the pick up point along a conveyor
 Another use for a touch-sensing would be as part of an inspection probe which is manipulated
by the robot to measure dimensions on a work part.
 A robot with 6-DOF would be capable of accessing surfaces on the part that would be difficult
for a three axis coordinate measuring machine the inspection system normally considered for
such an inspection task.

39. 39
Connected to robot’s wrist
Micro – switches
Figure: Micro – switches placed on two
fingers of a robotic hand
Finger

40. Force Sensors:
 Force and torque sensors are used primarily for measuring the reaction forces developed at the
interface between mechanical assemblies.
 A joint sensor measures the Cartesian components of force and torque acting on a robot joint
and adds them vectorially.
 For a joint driven by a dc motor, sensing is done by simply by measuring the armature current.
 In wrist sensor the sensors are mounted between the tip of a robot arm and the end effector.
 Wrist sensors are small, sensitive, light in weight and relatively compact in design-on the order
of 10 cm in total diameter and 3 cm in thickness, with a dynamic range up 200 lb.
 In order to reduce hysteresis and increase the accuracy in measurement, the hardware is
generally constructed from one solid piece of metal, typical aluminum.

41.  In Figure uses eight pairs of semiconductor strain gauges mounted on four deflection bars one
gauge on each side of a deflection bar.
 The gauges on the opposite open ends of the deflection bars are wired differentially to a
potentiometer circuit whose output voltage is proportional to the force component normal to
the plain of the strain gauge.

42.  The differential connection of the strain gauges provides automatic compensation for variations
in temperature.
 The eight pairs of strain gauges are oriented normal to the x, y and z axes of the force
coordinate frame, the three components of force F and three components of moment M can be
determined by properly adding and subtracting the output voltages, respectively.
 Most wrist force sensors function as transducers for transforming forces and moments exerted
at the hand into measurable deflections or displacements at the wrist.
 It is important that the wrist motions generated by the force sensor do not affect the positioning
accuracy of the manipulator.

43.  High stiffness: The natural frequency of mechanical device is related to its stiffness; thus, high
stiffness ensures that disturbing forces will be quickly damped out to permit accurate readings
during short time intervals. Furthermore, it reduces the magnitude of the deflections of an
applied force/moment, which may add to the positioning error of the hand.
 Compact Design: This ensures that the device will not restrict the movement of the
manipulator in a crowded workspace. It also minimizes the collision between the sensor and
other objects present in the workspace. Within the compact force sensor, it is important to place
the sensor as close to the tool as possible to reduce positioning error as a result of the hand
rotating through small angles. In addition, it is desirable to measure as large a hand
force/moment as possible; thus minimizing the distance between the hand and the sensor
reduces the lever arm for forces applied at the hand.

44.  Linearity: Good linearity between the response of force sensing elements and the applied
force/moments permits resolving the forces and moments by simple matrix operations.
 Low hysteresis and internal friction: Internal friction reduces the sensitivity of the force
sensing elements because forces have to overcome this friction before a measurable deflection
can be produced. It also produces hysteresis effects that do not restore the position measuring
devices back to their original readings.

45. Proximity and Range Sensors:
 Proximity sensors are devices that indicates when one object close to another object. How close
the object must be in order to activate the sensor is dependent on the particular device.
 The distances can be any where between several millimeters and several feet.
 Some of these sensors can also be used to measure the distance between the object and the
sensor, and these devices are called range sensors.
 Proximity and range sensors would typically be located on the wrist or end effector since these
are the moving parts of the robot.
 One practical use of a proximity sensor in robotics would be to detect the presence or absence
of a work part or other object.


46. But generally, all types of sensors can be classed as two kinds, either Passive Sensors or Active
Sensors. Generally, active sensors require an external power supply to operate, called
an excitation signal which is used by the sensor to produce the output signal. Active sensors are
self-generating devices because their own properties change in response to an external effect
producing for example, an output voltage of 1 to 10v DC or an output current such as 4 to 20mA
DC. Active sensors can also produce signal amplification.
A good example of an active sensor is an LVDT sensor or a strain gauge. Strain gauges are
pressure-sensitive resistive bridge networks that are external biased (excitation signal) in such a
way as to produce an output voltage in proportion to the amount of force and/or strain being
applied to the sensor.
Unlike an active sensor, a passive sensor does not need any additional power source or excitation
voltage. Instead a passive sensor generates an output signal in response to some external stimulus.
For example, a thermocouple which generates its own voltage output when exposed to heat. Then
passive sensors are direct sensors which change their physical properties, such as resistance,
capacitance or inductance etc.