Lecture 10 - Actuators

Question 1

What is the use for a motor and encoder?

Answer 1

Refer to slides

Question 2

What are some of the gears that exists?

Answer 2

Spur gear
Rack and Pinion
Helical gear
Spiral bevel
Hypoid gear
Worm gear
Planetary gear

Question 3

What is motor speed and direction determined by?

Answer 3

• Motor speed determined by: supplied voltage
• Motor direction determined by: polarity of supplied voltage
• Cannot generate analog power signal (1A ..10A) directly from microcontroller
  → Use external amplifier (pulse-width modulation)

Question 4

How does an encoder and sensors work with gears?

Answer 4

• Encoder disk is turned once for each rotor revolution
• Encoder disk can be optical or magnetic
• Single sensor can determine speed
• Dual sensors can determine speed and direction
• Using gears on motor shaft increases encoder accuracy

Question 5

What is a H-bridge?

Answer 5

Allows a motor to be driven in both directions -> Refer to slides

Question 6

How is a H-Bridge implemented with a microcontroller?

Answer 6

• 2 Digital output pins from microcontroller, one for switches 1+2, one for switches 3+4 feed into relays or power amplifier
• Alternative: use 1 digital output pin plus inverter, then relays or power amplifier

Question 7

H-Bridge with relays

Answer 7

Refer to slides for image

Question 8

Power Amplifier

Answer 8

Refer to slides for images and exmple using code (NEED TO KNOW HOW TO CODE)

Question 9

How can we convert a digital output (a number) into an analog voltage (e.g. range 0..5V) ?

Answer 9

1. Hardware:
    Voltage divider (“resistor grave”)
2. Software:
    Switching power on/off in intervals
    This is called “Pulse-Width Modulation” (PWM)

Refer to slides for examples images/diagrams

Question 10

How does Pulse-Width Modulation work?

Answer 10

How does this work?
* We do not change the supplied voltage
* Power is switched on/off at a certain pulse ratio
matching the desired output power
* Signal has very high frequency (e.g. 1kHz – 20kHz)
* Motors are relatively slow to respond
*  The only thing that counts is the supplied energy
 Integral (Summation)
* Pulse-Width Ratio = ton / tperiod

Refer to slides for diagrams and code (NEED TO KNOW HOW TO CODE)

Question 11

PWM Output with RoBIOS

Answer 11

Refer to slides

Question 12

What is a servo?

Answer 12

• A servo is a unit combining motor and simple feedback electronics for position control
• A servo is set by supplying a PWM signal of a certain ratio
• Ratio determines servo position, not speed!
• Servos are usually used in model airplanes, etc.
• Servos usually have three cables: power, ground and
  PWM-signal
• Servos require a PWM signal with 50Hz frequency (20ms)
• The pulse should be between 0.5 ms and 2.0 ms long this sets the servo to its extreme left or right position
  Remember:
• Servo speed cannot be set -> servo tries to get to new position as fast as possible
• Servos do not provide feedback to the outside

Question 13

What are Stepper Motors?

Answer 13

• Stepper motors are another kind of motors that do not require feedback
• A stepper motor can be incrementally driven, one step at a time, forward or backward

Question 14

What are the characteristics of Stepper Motors?

Answer 14

• Stepper motor characteristics are:
  – Number of steps per revolution (e.g. 200 steps per revolution = 1.8°per step)
  – Max. number of steps per second (“stepping rate” = max speed)
  NOTE:Step sequence can be very fast, the the resulting motion appears to be very smooth

Question 15

How can you drive a stepper motor?

Answer 15

• Driving a stepper motor requires a 4 step switching sequence for full-step mode
• Stepper motors can also be driven in 8 step switching sequence for half-step mode (higher resolution)

Question 16

What are the advantages and disadvantages of stepper motors?

Answer 16

Advantages
* No feedback hardware required
Disadvantages
* No feedback ! but often feedback is still required, e.g. for precision reasons, since a stepper motor can “lose” a step signal
* Requires 2 H-Bridges plus amplifiers instead of 1