Block 4 - lecture 2

Question 1

parts of a closed loop control approach?

Answer 1

• set point (sp)
• controller
• plant (actuator, process)
• disturbance (acting on process)
• sensor (for feedback)
• sensor noise (acting on sensor)

Question 2

input to the controller?

Answer 2

error (setpoint of variable - measured indication of variable)

Question 3

what’s the best way to express error?

Answer 3

as a percentage of the full range of measurement

Question 4

equation for percentage error?

Answer 4

ep = (r-b) / (bmax/bmin) *100%

setpoint-measurement / range of measurements *100%

Question 5

standard measurement ranges?

Answer 5

4 to 20 mA

0 to 10 V

Question 6

positive error?

Answer 6

measurement is below setpoint

Question 7

negative error?

Answer 7

measurement is above setpoint

Question 8

equation for percentage controller output?

Answer 8

p = u-min / umax-umin

output - min output / range of output

Question 9

how to calculate inputs for an output and vice versa

Answer 9

express max and min as
output = input*m + S₀
solve simultaneously
sub in value

Question 10

aspects that influence the choice of controller operating mode?

Answer 10

• process characteristics
• actuator characteristics
• sensor characteristics
• required accuracy, precision and control
• cost
• product rate

Question 11

two position controller mode?

Answer 11

a comparator with a deadband ie. the measurement needs to pass the setpoint by a certain amount, creating a neutral zone

Question 12

three position controller mode?

Answer 12

a middle value is given between two inputs

Question 13

an example of desirable histeresis?

Answer 13

neutral zone in 2 and 3 position controller modes

Question 14

what’s lag in controller output?

Answer 14

the time required for the process-control loop to make adjustments

Question 15

proportional controller mode?

Answer 15

the controller output changes smoothly in response to the error, each input value has a different output value

Question 16

bias?

Answer 16

the output has a constant offset from the input

Question 17

name of the range of controller inputs in proportional controller mode?

Answer 17

proportional band (PB)

Question 18

what happens when the inputs are outside of the proportional band?

Answer 18

saturation ie. max or min

Question 19

P controller?

Answer 19

proportional controller

Question 20

What happens is a P controller has a high Kp gain?

Answer 20

• the PB (proportional band) becomes very small so will act like an on/off controller
• can cause oscillations in transient responses