Lecture 12 - Control

Question 1

Why do we use control?

Answer 1

Why use control at all?
* Correct or wrong?
Supplying a certain voltage / pulsewidth will make the motor spin at a certain speed.
* Not always!
This will only work in idle condition, not under load (e.g. driving up a slope)

Question 2

What is motor control and how is it done?

Answer 2

• Using a feedback control will make sure the desired speed is maintained under load.
• If motor goes too slow → increase speed
• If motor goes too fast → reduce speed
• How is this done?
  (1) Read current speed
  (2) Compare with desired speed
  (3) Change motor output accordingly

Question 3

What are some things to consider when working with motor control?

Answer 3

• Time delay of controller (for measurement and calculation)
• Time delay of motor (physical reaction time of actuator in real world)
• Inaccuracy of encoder data
• Inaccuracy of actuator movement

Question 4

What is an On-Off controller

Answer 4

Simplest case: Only on/off of control value, “Bang-Bang Controller
Refer to slides (NEED TO KNOW HOW TO CODE)

Question 5

What is a timer?

Answer 5

• We need a timer function that calls our control routine regularly at exactly the right time intervals
• Background timer = interrupt from clock
• Most microcontrollers contain a number of timers that can be set by software.
  Refer to slides (NEED TO KNOW CODE)

Question 6

What are some parameters to consider for background timer frequency

Answer 6

Parameters to consider
* Encoder ticks per meter
* Lowest controlled speed

Refer to slides

Question 7

What is a watchdog timer?

Answer 7

A watchdog is a specialized timer/counter
* It is initialized to a certain value and keeps counting down
* If the watchdog counter reaches zero, an interrupt is raised
➢ Correctly running program will reset the watchdog timer in regular intervals to its initial value, so no interrupt will occur
➢ Effective tool for fault detection, e.g. if program “hangs”
➢ Can be used to detect hardware errors and software errors

Refer to slides

Question 8

On-Off Controller with Hysteresis

Answer 8

Refer to slides

Question 9

What is a proportional controller?

Answer 9

P-controller (proportional controller)
R(t) = Kp * (vdes(t) - vact(t))
Linear change of control value to reduce error → better controller
Where:
* Kp is a constant value, “controller gain”
* Kp must be selected to achieve fast goal speed without overshoot
* Note: No equilibrium when desired velocity is reached: if vdes(t) = vact(t) then R(t) = 0
Refer to slides

Question 10

What is an intergral controller?

Answer 10

Adding I-Controller (Integral Controller) Part
* Problem: P-Controller may reach equilibrium without reaching the target velocity → steady state error
* Solution: Integral part is used to eliminate steady state error (REFER TO SLIDES FOR EQUATION)

Question 11

What is dertivative controller?

Answer 11

Adding D-Controller (Derivative Controller) Part
* Problems: P-Controller responds slow to change in input P-Controller with high gain tends to oscillate
* Solution: Add a derivative term for response/damping (REFER TO SLIDES FOR EQUATION)

Question 12

What is a PID controller?

Answer 12

REFER TO SLIDES (FOR EQUATIONS)
PID-Controller is combination of P, I, and D controller
* Simple, universal controller
* Trade-offs: Response time vs. stability
* Noise limits max. proportional gain
* Note physical limits of actuator e.g. max acceleration, velocity

Question 13

What are the tuning method for PID Paramter Tuning?

Answer 13

Manual Tuning
Ziegler-Nichols method
Cohen-Coon method

Question 14

What is Manual tuning?

Answer 14

1. Select typical operating setting for desired speed, turn off integral and derivative part, then increase KP to max. or until oscillation occurs.
2. If system oscillates, divide KP by 2.
3. Increase KD and observe behavior when changing desired speed by about 5%. Choose a value of KD which gives a fast damped response.
4. Slowly increase KI until oscillation starts. Then divide KI by 2.5

Question 15

What is velocity control?

Answer 15

Velocity Control is what we have achieved so far We can make a motor drive at a certain speed and make it maintain that speed
Refer to slides

Question 16

What is Position control?

Answer 16

Position Control also incorporates a start/stop phase and makes the motor stop at a certain location E.g. exactly after one full revolution
Refer to slides

Question 17

What is a practical way to achieve position control?

Answer 17

• Use velocity control in start phase and continuous phase
• Monitor current position and calculate begin of end phase
• Continuously update speed while decelerating
• Important: Once stopped (v=0), cannot easily adjust pos. anymore