5. Control Flashcards
What two advantages are realised by relying on sensors for the accuracy of a system, rather than actuators?
- Sensors can be manufactured to a high degree of precision
- They do not have to cope with fluctuating loads, so they can do their job of measurement without being perturbed by external forces
Book 4 page 76
How does a reduction gearbox affect torque and angular velocity?
Reduces angular velocity while increasing the torque available
Book 4 page 77
What are the main disadvantages of using a potentiometer as a position sensor?
- Only a limited range of motion
- There is always some friction, which produces wear and affects control
Book 4 page 77
What is an absolute encoder?
One where the absolute position is always available and can be read directly from the stimulus
eg not affected by cycling power
Book 4 page 79
Name another type of encoder which isn’t the absolute encoder?
Incremental encoder
Book 4 page 79
Describe bang-bang control?
Simplest type of controller which turns the motor fully on or fully off, with no intermediate control of speed
Direction of rotation depends on whether the measured value is higher or lower than the desired value
When they are equal, the motor is switched off
Book 4 page 80
Give an example of where a bang-bang controller will be used?
Central heating thermostat which switches on the heating when the temperature is below a certain value
Book 4 page 80
Why might you introduce a deadband to a bang-bang controller?
The motor is driven at full speed and is likely to overshoot the desired value multiple times in both directions (hunting)
Book 4 page 80
What type of motor controller varies the motor power according to how far the measured value is from the desired value?
Proportional controller
Book 4 page 80
How does gain affect the performance of a proportional controller?
Too little gain and the power to the motor will be too low, even for large errors, meaning the motor may be very slow or stall due to friction
Too much gain and the controller will act more like a bang-bang controller
Book 4 page 81
How is accuracy in final positioning improved?
Integral control, which takes into account not only the size of the error but also the time over which the error has existed
Book 4 page 81
What does derivative control achieve?
Reduces the tendency of the motor to overshoot and oscillate
Does this by responding not to the size of the error signal, but to its rate of change
Book 4 page 82
What will happen to the voltage across a capacitor in an integrator circuit, if there is a constant current through it?
The capacitor will steadily charge, and the voltage across it will steadily increase
The op-amp is in an inverting configuration though, so if Vin is positive, Vout will steadily become more negative
Book 4 page 83 SAQ 6.3
How does the output voltage change with input voltage in a differentiator circuit?
- A constant input voltage (at any level) results in a zero output
- A falling input voltage results in a positive output
- A rising input voltage results in a negative output
- The size of the output depends on how fast the input changes, a faster change gives a larger output
Book 4 page 85
What type of controller combines proportional, integral and derivative control?
PID or three-term controller
Book 4 page 85
