Sensors and Actuator

1
Sensors and Actuator
2
Sensor vs. Actuator

• Sensor A device that can sense its environment,
  and translate physical quantities such as light,
  sound, temperature, and motion into electrical
  signals.
• Actuator A device that converts electrical
  energy into a physical quantity.

3
Example Sensors and Actuators
4
Types of Sensors

• Contact vs. non-contact
• Contact
• Sensor must contact an object to sense it.
• Non-contact
• Sensor is achieved though interactions with
  electromagnetic fields.
• High speed
• Reliable
• Does not interfere with control process
• Digital vs. analog
• Digital
• Only has two state on, off
• Analog
• Sensor input and output can have infinite number
  of states.

5
Types of Sensors

• Functions
• Vision
• Temperature
• Pressure
• Strain
• Ranging and Motion
• Time

6
Sensor Characteristics

• Response function output vs quantity being
  sensed
• Sensitivity change in output/unit change in
  input
• Linearity error difference between sensor
  response and a best-fit line
• Accuracy error difference between measured
  quantity and the true value
• Stability repetition of results
• Noise the part of the signal that is independent
  of the quantity being sensed.
• Response time how quickly the output respond to
  abrupt changes in input.
• Temperature coefficient changes in sensors
  response per unit temperature.
  This characteristics applies to
  all sensors.
• Hysteresis (backlash) a sensor output depend on
  previous history.
• Common in magnetic and mechanical systems.

7
Position and Angle Sensors

• Applications
• Determine relative position and angle of machine
  tools and parts during machining
• Determine the position and angle of robot limbs
• Example sensors and actuators
• Potentiometer (sensor)
• Digital encoder (sensor)
• Step motor (actuator)

8
Potentiometer
Sliding metal shaft
d
D
-
VD Fixed voltage source
Voltmeter
Transformed position d into electric voltage Vd
9
Digital Encoder Gray Code
Decimal 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Binary 1111 1110 1101 1100 1011 1010 1001 1000 011
1 0110 0101 0100 0011 0010 0001 0000
Convert a position quantity into a digital number
10
The Stepper Motor
Phase 2
Phase 1
S N

• Controlled by multiple-phase currents
• The stepper motor rotates in discrete angular
  increments
• Commonly used mechanical actuator in digital
  control applications

11
The Step Motor Principle
Phase 1
0
2.5
0.5
1
1.5
2
3
3.5
4
Phase 2
2.5
0
0.5
1
1.5
2
3
3.5
4

N S
Step 1
N S
Step 0.5
Step 0
S N
S N
N S
S N
N S
S N
N S

N S
N S
12
Homework and Reading

• Handout5
• Reading P221-224, 226-230.
• Homework 11 Due Monday, 3/21
• 1. Create the gray code sequence for a 3-bit
  system
• 2. Handout 5, problem 4.1 a, b, c (this problem
  may require some tough work, please do not put it
  off until the last minute).
• 3. One more problem, see next page

13
Homework continued
3. The figure below showed the first three steps
of the phase currents and the rotor motion of a
step motor. If your systems requires the motor to
reverse its direction in step 1.5, 2, and 2.5,
how should we supply the phase currents? Draw
your phase current diagram for step 1.5, 2, and
2.5.