robotics

1. Robot sensors and end effectors
Maj .Brhane G/m
Chapter Two
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2. Sensor
Definition:- a device for sensing a physical variable of a
physical system or an environment.
 Sensors are used in robots for both internal feedback control and
external interaction with the outside environment.
 The links and joints move sensors such as potentiometer,
encoders, and resolvers send signals to the controller that allow it
to determine where each joint is.
 There are many different types of sensors available such as
position, velocity, acceleration, pressure, force/torque, and range
finders.
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3. The need of sensor in robot
To accurately achieve a task in an intelligent environment, a robot
has to be able to react dynamically to changes its surrounding
Robots need sensors to perceive the environment
 Most robots use a set of different sensors
 Different sensors serve different purposes
 Information from sensors has to be integrated into the
control of the robot
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4. Classification of Sensors
 Sensors a physical element which produces a signal
relating to the quantity being measured.
Type of sensors: Analog, Digital, Active, Passive
Analog: Output is Continuous, requires ADC to interface.
Digital: the output is in the form of digital signal, can be
directly interfaced to digital system.
Active sensors: need separate power source to obtain the
output. radar, sonar, potentiometer,
Passive sensors: these are self generating, produces
electrical signal when subjected to sensed quantity.
Piezoelectric, thermocouple, radioactive….
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5. Continue
…
 Measure values internally to the system (robot), e.g.
battery level, wheel position, joint angle, etc,
 Observation of environments, objects
 Contact v.s. non-contact
Touch, proximity sensors
 Distance measurement sensors like IR sensor
 Visual v.s. non-visual
Vision-based sensing, image processing, video camera
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6. Characteristics of Sensor :-
 The main characteristics that determines the performance,
economy, ease of application, and applicability of the
sensor.
 Cost: the cost of sensor must be considered when a number
of sensors are needed for the robot. The cost of the sensor
must be balanced with the reliability, accuracy and life of
the sensor.
 Size: the size of the sensor has an effect on the movement of
the robot joint.
 Weight: the weight of the sensors is very important because
it will affect the over all inertia of the robot and payload.
 Type of output: it may be digital or analog. The nature of the
signal determines the requirement of accessories.
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7. Continued…
 Interfacing: the interfacing between the sensor and the
device can become an important issue if they do not
match or if other add-on circuits become necessary.
 Resolution: it is the minimum step size within the range
of measurement of the sensor. In a wire-wound
potentiometer, it will be equal to the resistance of one
turn of the wire.
 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
fluctuation in input, including noise.
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8. Continued…
 Linearity: the relationship between input variations and output variations.
 For linear sensor, the same change in input at any level within the range will produce the same
change in output.
 Range: the difference between the smallest and the largest outputs the sensor can produce.
 Response time: 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%.
 Frequency response: the frequency range of the sensor is important. The large the range of the
frequency response, the better the ability of the system to response to varying input.
 Reliability: the ratio of how many times a system operates properly divided by how many times it
is tried.
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9. Quantities to be measured?
 Linear displacement
 Angular displacement
 Velocity, acceleration
 Force, torque
 Angular rate etc…
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10. Sensors Used in Robot
 Resistive sensors
bend sensors, potentiometer, resistive photocells, ...
 Tactile sensors
contact switch,
 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, GPS, …
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12. IR Reflective Sensors
 Reflective Sensor:
 Emitter IR LED + detector photodiode/phototransistor
 Phototransistor: the more light reaching the phototransistor, the
more current passes through it
 A beam of light is reflected off a surface and into a detector
 Light usually in infrared spectrum, IR light is invisible
 Applications: Object detection, Line following, Wall tracking
 Drawbacks:
Susceptible to ambient lighting
Provide sheath to insulate the device from outside lighting
Susceptible to reflectivity of objects
Susceptible to the distance between sensor and the object
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13. IR Proximity Sensors
Proximity Sensors:
•Requires a modulated IR LED, a detector module with built-in
modulation decoder
•Current through the IR LED should be limited: adding a series
resistor in LED driver circuit
•Detection range: varies with different objects (shiny white card vs.
dull black object)
•Insensitive to ambient light
Applications:
•Rough distance measurement
•Obstacle avoidance
•Wall following, line following ,limiter
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14. Linear Variable Differential Transformer
It is a transformer whose core moves along with the distance
being measured and that output a variable analog voltage as a
result of this displacement.
 The electric energy into one coil creates a flux, which induces a
voltage in the second coil proportional to the ratio of the
number of turns in the windings.
 The output of an LVDT is very linear proportional to the input
position of the core.
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15. Potentiometer
 It converts position information into a variable voltage through
a resistor.
 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.
 Potentiometers are generally used as internal feedback sensors
in order to report the position of joints and links.
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16. Tachometer
 Is a generator that converts mechanical energy into
electrical energy.
 Its output is an analog voltage proportional to the
input angular speed.
 The back emf induced is proportional to the angular
velocity of the shaft.
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17. Strain Gauges
 Strain gauges can be used to measure forces. The output
of the strain gauge is a variable resistance, proportional to
the strain, which itself is a function of applied forces.
 Strain gauges are used to determine the forces at the end
effector and the wrist of a robot.
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18. Range Finder
 Time of Flight
 The measured pulses typically come form ultrasonic, RF and
optical energy sources.
 D = v * t
 D = round-trip distance
 v = speed of wave propagation
 t = elapsed time
 Sound = 0.3 meters/msec
 RF/light = 0.3 meters / ns (Very difficult to measure short
distances 1-100 meters)
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19. Ultrasonic Sensors
 Basic principle of operation:
 Emit a quick burst of ultrasound (50kHz), (human hearing:
20Hz to 20kHz)
 Measure the elapsed time until the receiver indicates that an
echo is detected.
 Determine how far away the nearest object is from the sensor
 D = v * t
 D = round-trip distance
 v = speed of propagation(340 m/s)
 t = elapsed time
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20. 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.
 Force sensor used as a touch sensors may not only send
touch information, but also repot 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 object size, shape and material type.
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21. Acceleration Sensor
 Accelerometer are very common sensors for measuring
accelerations. 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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22. Main elements of an accelerometer
 1.Mass 2. Suspension mechanism 3. Sensing element
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23. End Effectors
 The special tooling for a robot that enables it to perform a specific
task
 An end effector is the device that is at the end of a robotic arm.
 We can think of an end effector like a human hand. Even though a
human hand is very versatile, an end effector has one great
advantage that humans do not have and that is the
interchangablility of end effectors. If the end effector is not
suitable than it can be changed unlike the human hand.
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24. Types of an end effector?
 There are two main types of end effectors: tools and
Grippers.
 Tools – to perform a process, e.g., spot welding, spray painting
 Grippers – to grasp and manipulate objects (e.g., parts) during
work cycle
How Grippers work?
 Seven different methods to grip a part:
 grasp it
 hook it
 scoop it
 inflate around it
 attract it magnetically
 attract it by a vacuum
 stick to it
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26. Types of Grippers
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