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• To choose an appropriate sensor for a particular need, we have to consider
a number of different characteristics.
• These characteristics determine the performance, economy, ease of
application, and applicability of the sensor.
• In certain situations, different types of sensors may be available for the
• Therefore, the following may be considered before a sensor is chosen:
1. Cost: The cost of a sensor is an important consideration, especially when
many sensors are needed for one machine. However, the cost must be
balanced with other requirements of the design such as reliability
importance of the data they provide accuracy, life, and so on.
2. Size: Depending on the application of the sensor, the size may be of
primary importance. For example, the joint displacement sensors have to
be adapted into the design of the joints and move with the robot's body
elements. The available space around the joint may be limited. Therefore,
it is important to ensure that enough room exists for the joint sensors.
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Since robots are dynamic machine, the weight of a sensor is very important.
A heavy sensor adds to the inertia of the arm and reduces its overall payload.
4. Type of output (digital or analog):
The output of a sensor may be digital or analog and depending on the
application, this output may be used directly or have to be converted.
For example, the output of a potentiometer is analog, whereas that of an
encoder is digital.
If an encoder is used in conjunction with a microprocessor, the output may be
directly routed to the input port of the processor, while the output of a
potentiometer has to be converted to digital signal with an analog-to-digital
The appropriateness of the type of output must be balanced with other
5. Interfacing: Sensors must be interfaced with other devices such as
microprocessors and controllers. The interfacing between the sensor and the
device can become an important issue if they do not match or if other add-on
components and circuits become necessary (including resistors, transistor
switches, power source, and length of wires involved).
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6. Resolution: Resolution is the minimum step size within the range of measurement
of the sensor
7. Sensitivity: Sensitivity is the ratio of a change in output in response to a change in
input. Highly sensitive sensors will show larger fluctuations in output as a result of
fluctuations in input, including noise.
8. Linearity. Linearity represents the relationship between input variations and
output variations. This means that in a sensor with linear output, the same change
in input at any level within the range will produce a similar change in output.
Almost all devices in nature are somewhat nonlinear, with varying degrees of
9. Range: Range is the difference between the smallest and the largest outputs the
sensor can produce, or the difference between the smallest and largest inputs
with which it can operate properly.
10. Response rime: Response time is the time that a sensor's output requires to reach
a certain percentage of the total change. It is usually expressed in percentage of
total change, such as 95%. It is also defined as the time required to observe the
change in output as a result of a change in input. For example, the response time
of a simple mercury thermometer is long, whereas a digital thermometers
response time, which measures temperature based on radiated heat, is short.
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11. Frequency response:
Suppose you attach a very high-quality radio tuner to a small, cheap speaker.
Although the speaker will reproduce the sound, its quality will be very low,
whereas a high-quality speaker system with a woofer and tweeter will reproduce
the same signal with much better quality.
This is because the frequency response of the two-speaker system is very different
from the single, cheap speaker.
Reliability is the ratio of how many times a system operates properly, divided by
how many times it is used.
For continuous, satisfactory operation it is necessary to choose reliable sensors
that last a long time while considering the cost and other requirements.
Accuracy is defined as how close the output of the sensor is to the expected
If for a given input, the output is expected to be a certain value, accuracy is
related to how close the sensor's output is to this value.
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If the sensor's output is measured a number of times in response to the same
input, the output may be different each time.
Repeatability is a measure of how varied the different outputs are relative to each
In general, repeatability is more important than accuracy, since in most cases,
inaccuracies are systematic and can be corrected or compensated because they
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• Position sensors are used to measure displacements, both rotary and linear, as well
• The following arc common position sensors used in robotics:
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• A potentiometer converts position information into a variable voltage through a
• As the sweeper on the resistor moves due to a change in position, the proportion
of the resistance before or after the point of contact with the sweeper compared
with the total resistance varies
• Since in this capacity the potentiometer acts as a voltage divider, the output will be
proportional to the resistance as
• Potentiometers can be rotary or linear and thus can measure linear or rotary
• Potentiometers are generally used as internal feedback sensors in order to report
the position of joints and links. Potentiometers are used alone or together with
other sensors such as encoders.
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• An encoder is a simple device that can output a digital signal for each small portion
of a movement.
• To do this, the encoder wheel or strip is divided into small sections. as shown in Fig.
• Each section is either opaque or clear. (It can also be either reflective or
• A light source, such as an LED on one side provides a beam of light to the other side
of the encoder wheel or strip, where it is seen by another light-sensitive sensor,
such as a phototransistor.
• if the wheels angular position is such that the light can go through, the sensor on
the opposite side will be turned on and will have a high signal.
• If the angular position of the wheel is such that the light is stopped, the sensor will
be off and its output will be low.
• As the wheel rotates, it can continuously send signals. if the signals are counted,
the approximate total angular displacement of the wheel can be measured at any
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• Optical encoders come in both linear and rotary versions, and except for their type
of motion, they are exactly the same and work under the same principles.
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Displacement Sensor (Magneto Reflective)
• In this sensor, a pulse is sent through a conductor, which bounces
back as it reaches a magnet.
• The time of travel to the magnet and back is converted to the
distance if the speed of travel is known.
• By attaching the moving part to either the magnet or the conductor,
the displacement can he measured.
• A simple schematic of the sensor is shown in Figure
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• The following are the more common velocity sensors used in
• If an encoder is used for displacement measurement, there is no
need to use a velocity sensor.
• Since encoders send a known number of signals for any given angular
displacement, by counting the number of signals received in a given
length of time (dt),we can calculate velocity.
• A smaller length of time (dt) yields a more accurate calculated
velocity, once that is closer to the true instantaneous velocity.
• However, if the rate of rotation of the encoder is low, the velocity
measurement may become inaccurate.
• This velocity calculation is accomplished by programming the
controller to convert number of signals in a given length of time into
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• A tachometer is a generator that converts mechanical energy into
• Its output is an analog voltage proportional to the input angular
• It may be used along with potentiometers to estimate velocities.
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Differentiation of Position Signal
• If the position signal is clean, it is actually possible, and even simple, to
differentiate the position signal and to convert it to velocity signal.
• To do this, it is necessary that the signal be as continuous as possible in order to
prevent creation of large impulses in velocity signal.
• However, differentiation of a signal is always noisy and should be done very
• Figure shows a simple R-C circuit with an op-amp that can be used for
differentiation, where the velocity signal is
• Similarly, the velocity (or acceleration) signal can be integrated to yield position (or
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• Accelerometers are very common sensors for measuring
• However, in general, accelerometers are not used with industrial
robots, since no acceleration is generally measured in these robots.
• However, recently, acceleration measurements have been used for
high-precision control of linear actuators and for joint-feedback
control of robots.
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FORCE AND PRESSURE SENSORS
• Piezoelectric material compresses if exposed to a voltage and
produces a voltage if compressed.
• This is used in the phonograph to create a voltage from the variable
pressure caused by the grooves in the record.
• Similarly, a piece of piezoelectric can be used to measure pressures,
or forces, in robotics.
• The analog output voltage must be conditioned and amplified for
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2) Force-Sensing Resistor
• The Force Sensing Resistor(FSR) is a polymer thick-film device that
exhibits a decreasing resistance with increasing force applied normal
to its surface.
• For a changing force of 10 to 10,000 gram, its resistance changes
from about 500 KΩ to about 1 KΩ.
• Figure shows a typical force-sensing resistor.
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• Torque can be measured by a pair of strategically placed force
• Suppose that two force sensors are placed on a shaft, opposite of
each other, on opposite sides.
• If a torque is applied to the shaft, it generates two opposing forces on
the shafts body, causing opposite direction strains.
• The two force sensors can measure the forces, which can be
converted to a torque.
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• To measure torques about different axes, three pairs of mutually
perpendicular sensors must be used.
• However, since forces can also be measured with the same sensors, a
total of six force sensors can generally report forces and torques
about three axes, independent of each other, as depicted in Figure.
• Pure forces generate similar signals in a pair, while torques generate
pairs of signals with opposite signs.
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TOUCH AND TACTILE SENSORS
• Touch sensors are devices that send a signal when physical contact
has been made.
• The simplest form of a touch sensor is a micro switch which either
turns on or off as contact is made.
• The main switch can be set up for different sensitivities and ranges of
• For example, a strategically placed micro switch can send a signal to
the controller if a mobile robot reaches an obstacle during navigation.
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TOUCH AND TACTILE SENSORS
• More sophisticated touch sensors may send additional information.
For example, a force sensor used as a touch sensor may not only send
touch information, but also report how strong the touching force is.
• A tactile sensor is a collection of touch sensors that in addition to
determining contact can also provide additional information about
• This additional information may be about the shape, size, or type of
• In most cases, a number of touch sensors are arranged in an array or
matrix form, as shown in Figure.
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TOUCH AND TACTILE SENSORS
• In this design, an array of six touch sensors is arranged on each side
of a tactile sensor.
• Each touch sensor is made up of a plunger, an LED and a light sensor.
• As the tactile sensor closes and the plunger moves in or out, it blocks
the light from the LED projecting onto the light sensor.
• The output of the light sensor is then proportional to the
displacement of the plunger.
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TOUCH AND TACTILE SENSORS
• As the tactile sensor comes in contact with an object, depending on
the shape and size of the object, different touch sensors react
differently at a different sequence.
• This information is then used by the controller to determine the size
and the shape of the object.
• Figure shows three simple setups: one touching a cube, one touching
a cylinder, and one touching an arbitrary object.
• As can be seen, each object creates a different unique signature that
can be used to detect it.
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• A proximity sensor is used to determine that an object is
close to another object before contact is made.
• This noncontact sensing can be useful in many situations,
from measuring the speed of a rotor, to navigating a robot.
• There are many different types of proximity sensors, such
as magnetic, eddy current and Hall-effect, optical,
ultrasonic, inductive, and capacitive.
• The following is a short discussion of some of these
– Magnetic Proximity Sensors
– Optical Proximity Sensors
– Ultrasonic Proximity Sensors
– Inductive Proximity Sensors
– Capacitive Proximity Sensors
– Eddy Current Proximity Sensors
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1) Magnetic Proximity Sensors
• These sensors are activated when they are close to a magnet.
• They can be used for measuring rotor speeds (and number of rotations), as well as
for turning on or off a circuit.
• Magnetic sensors may also be used to count number of rotations of wheels and
motors, and thus they can be used as position sensors as well.
• Imagine a mobile robot, where the total displacement of the robot is calculated by
counting the number of times a particular wheel rotates, multiplied by the
circumference of the wheel.
• A magnetic proximity sensor can be used to track wheel rotations by mounting a
magnet on the wheel (or its shaft) and having the sensor stationary on the chassis.
• Similarly, the sensor can be used for other applications, including for safety.
• For example, many devices have a magnetic proximity sensor that send a signal
when the door of the machine is open, and thus the controller stops the rotating or
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2) Optical Proximity Sensors
• Optical proximity sensors consist of a light source called an emitter (either internal
or external to the sensor) and a receiver, which senses the presence or the absence
• The receiver is usually a phototransistor, and the emitter is usually an LED.
• The combination of the two creates a light sensor and is used in many applications,
including optical encoders.
• As a proximity sensor, the sensor is set up such that the light, emitted by the
emitter, is not received by the receiver, unless an object is close-by.
• Figure is a schematic drawing of an optical proximity sensor.
• Unless a reflective object is with in the range of the switch, the light is not seen by
the receiver, and therefore, there will be no signal.
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3) Ultrasonic Proximity Sensors
• In these sensors, an ultrasonic emitter emits frequent bursts of high-
frequency sound waves (usually in the 200-kHz range).
• There are two modes of operations for ultrasonic sensors, namely,
opposed mode and echo (diffused) mode.
• In opposed mode, a receiver is placed in front of the emitter, whereas
in echo mode, the receiver is either next to, or integrated into, the
emitter and receives the reflected sound wave.
• If the receiver is within range or if the sound is reflected by a surface
close to the sensor, it will be sensed and a signal is produced.
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3) Ultrasonic Proximity Sensors
• Otherwise, the receiver will not sense the wave, and there is no
• All ultrasonic sensors have a blind zone near the surface of the
emitter in which the distance or the presence of an object cannot be
• Ultrasonic sensors cannot be used with surfaces such as rubber and
foam that do not reflect the sound waves in echo mode.
• Figure is a schematic drawing of this type of sensor.
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4) Inductive Proximity Sensors
• Inductive sensors are used to detect metal surfaces.
• The sensor is a coil with a ferrite core, an oscillator-detector, and a
• In the presence of a metal object in the close vicinity of the sensor,
the amplitude of the oscillation diminishes.
• The detector detects the change and turns the solid state switch off.
• When the part leaves the range of the sensor, it turns on again.
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5) Capacitive Proximity Sensors
• The capacitive sensor reacts to the presence of any object that has a
dielectric constant more than 1.2.
• In that case, the material acts as a capacitor, raising the total
capacitance of the sensor's probe.
• This will trigger an internal oscillator to turn on the output unit,
which will send out an output signal.
• Thus, the sensor can detect the presence of an object within a range.
• Capacitive sensors can detect nonmetal materials such as wood,
liquids, and chemicals.
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6) Eddy Current Proximity Sensors
• As we know that, when a conductor is placed within a changing
magnetic field, an electromotive force (emf) is induced in it, which
causes a current to flow in the material.
• This current is called eddy current.
• An eddy current sensor typically has two coils, where one coil
generates a changing magnetic flux as reference.
• In the close proximity of conducting materials, an eddy current is
induced in the material, which in turn, creates a magnetic flux
opposite of the first coils flux, effectively reducing the total flux.
• The change in the total flux is proportional to the proximity of the
conducting material and is measured by the second coil.
• Eddy current sensors are used to detect the presence of conductive
material, as well as nondestructive testing of voids and cracks,
thickness of materials, etc.
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