Sensors and actuators are important components for robots. Sensors can be analog or digital and include sensors for position, orientation, distance, light, and more. The right sensor must match the application needs. Actuators allow robots to move and interact with their environment. Common actuators include DC motors, stepper motors, and servos, which can be controlled through techniques like pulse-width modulation. Together, sensors and actuators enable robots to perceive and interact with the world.

1. Sensors and Actuators

2. Sensors
• What is important is to find the right sensor for a particular application.
• The right measurement technique
• The right size and weight
• The right operating temperature range
• Power consumption
• The right price range
• Data transfer from the sensor to the CPU
• CPU initiated  polling
• Sensor initiated  interrupt

3. Sensor output

4. Sensor classification

5. Analog versus Digital Sensors
• More sensors produce analog signals rather than digital signals.
• Need A/D converter
• Ex:
• Microphone
• Analog infrared distance sensor
• Analog compass
• Barometer sensor
• Digital sensors
• More accurate and more complex
• Analog sensor packed with A/D converter
• Output  parallel interface, serial interface or synchronous serial

6. Sensors (Shaft encoders)
• Fundamental feedback sensor for motor control
• Most popular types  magnetic encoders , optical encoders
• Magnetic encoders
• Hall-effect sensor and a rotating disk on the motor shaft with a number of
magnets (16 magnets for 16 pulses or ticks)
• Optical encoders
• A sector disk with black and white segments together with a LED and photo-
diode.
• 16 white and 16 black segments (for 16 pulses)

7. Sensors (Shaft encoders)
• Incremental encoders  they can only count the number of
segments.
• They are not capable of locating absolute position of the motor shaft.
• Solution: Gray code disk with set of sensors.
• 3 sensors (23 = 8 sectors)

8. Sensors (Shaft encoders)
• To detect that the motor shaft is moving clockwise or counter-
clockwise, two sensors are used.
If encoder 1 receives the signal
first, then the motion is
clockwise; if encoder 2 receives
the signal first, then the motion is
counter-clockwise

9. A/D Convertor
• Convert an analog signal into a digital signal.
• Characteristics of an A/D convertor

10. A/D Convertor
Two types of interfaces:
Parallel interface or a synchronous serial interface
(the latter has the advantage  it does not impose any limitations on
the number of bits per measurement)

11. Position Sensitive Device
• In the past, most robots have been equipped with sonar sensors.
• A typical configuration to cover the whole circumference of a round
robot required 24 sensors, mapping about 15° each.
• Measurements are repeated about 20 times per second

12. Position Sensitive Device
• Sonar sensors have been replaced by either infrared sensors or laser
sensors.
• Laser sensor
• Perfect local 2D map from the viewpoint of the robot, or even a complete 3D
distance map.
• Too large and heavy
• Too expensive
• IR PSD (Position Sensitive Detector)
• Cannot measure less than 6cm
• IR proximity sensor
• Cover obstacles closer than 6cm
Infrared sensor
(PSD)

13. Compass
• How to track the robot’s position and orientation.
• Standard method:
• “dead reckoning” using shaft encoders
• Error will grow lager and lager over time (ex: wheel slippage)
• Global positioning system (GPS)
• The simplest modules are analog compasses that can only distinguish
eight directions, which are represented by different voltage levels.
• Digital compasses are considerably more complex, but also provide a
much higher directional resolution.

14. Gyroscope, Accelerometer, Inclinometer
• Accelerometer
Measuring the acceleration along one axis.
• Gyroscope
Measuring the rotational change of orientation about one axis.
• Inclinometer
Measuring the absolute orientation angle about one axis

15. Digital Camera
• Digital cameras are the most complex sensors used in robotics.
• For mobile robot applications, we are interested in a high frame rate,
because our robot is moving and we want updated sensor data as fast
as possible.
• Since there is always a trade-off between high frame rate and high
resolution, we are not so much concerned with camera resolution.

16. Actuators

17. DC Motors
• DC electric motors are arguably the most commonly used method for
locomotion in mobile robots.
• Standard DC motors revolve freely.
• Motor–encoder combination

18. H-Bridge
• For most applications we want to be able to do two things with a motor:
1. Run it in forward and backward directions.
2. Modify its speed.
• An H-bridge is what is needed to enable a motor to run forward/backward.
(“pulse width modulation” to change the motor speed. )

19. H-Bridge
• There are two principal ways of stopping the motor:
• set both x and y to logic 0 (or both to logic 1) or
• set speed to 0

20. Pulse Width Modulation
• Pulse width modulation or PWM

21. Stepper Motors
• Stepper motors differ from standard DC motors in such a way that
they have two independent coils (or four) which can be
independently controlled.
• A typical number of steps per revolution is 200, resulting in a step size
of 1.8°.
• Some stepper motors allow half steps, resulting in an even finer step
size.

22. Servos
• A servo has three wires: VCC, ground, and the PW input control
signal.
• Unlike PWM for DC motors, the input pulse signal for servos is not
transformed into a velocity.
• It is an analog control input to specify the desired position of the
servo’s rotating disk head.
• Internally, a servo combines a DC motor with a simple feedback
circuit, often using a potentiometer sensing the servo head’s current
position.

23. Servos
• The width of each pulse specifies the desired position of the servo’s
disk.

24. Summary