Lecture 10 - Actuators Flashcards
What is the use for a motor and encoder?
Refer to slides
What are some of the gears that exists?
Spur gear
Rack and Pinion
Helical gear
Spiral bevel
Hypoid gear
Worm gear
Planetary gear
What is motor speed and direction determined by?
- 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)
How does an encoder and sensors work with gears?
- 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
What is a H-bridge?
Allows a motor to be driven in both directions -> Refer to slides
How is a H-Bridge implemented with a microcontroller?
- 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
H-Bridge with relays
Refer to slides for image
Power Amplifier
Refer to slides for images and exmple using code (NEED TO KNOW HOW TO CODE)
How can we convert a digital output (a number) into an analog voltage (e.g. range 0..5V) ?
- Hardware:
Voltage divider (“resistor grave”) - Software:
Switching power on/off in intervals
This is called “Pulse-Width Modulation” (PWM)
Refer to slides for examples images/diagrams
How does Pulse-Width Modulation work?
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)
PWM Output with RoBIOS
Refer to slides
What is a servo?
- 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
What are Stepper Motors?
- 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
What are the characteristics of Stepper Motors?
- 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
How can you drive a stepper motor?
- 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)
