D
David Lawton
(Originally posted Tue 12/09/97)
I tend to agree with David B. on this one. I have found that in process
engineering, where speed of response tends to be less time-critical, a PI
controller normally provides perfrectly acceptable performance.
Just a few thoughts about the good old D term.
First off, the derivative term can produce a "violent" controller output in
response to a
step input e.g. change of setpoint, because d/dt (step) = infinity when it
"steps".
One way around this is clearly to "rate-limit" the step input.
Other stuff to consider if you're that way inclined..........
By doing an open-loop step response and measuring K (gain), T1 (time to reach
10%) and T2 (time to reach 90% of steady state), it's possible to use this
numbers to determine "rule-of-thumb" values for P, Ti and Td.
where P=(1.2*T2)/(K*T1)
Ti = 2*T1
Td = 0.5*T1
The other important thing that needs careful consideration is the SAMPLING
TIME of the PID controller. Again, rul of thumb is Td/2.
You could get sexier and start calculating Kalman controllers and such things,
although my PID controllers tend to be PLC-based, with "built-in" function
blocks to carry out the PID function. However, I have written Kalman control
loops using C before now (PC-based) - but this was generally Study based
rather than industry based.
Anyone else care to join this debate............
Regards
David Lawton.
I tend to agree with David B. on this one. I have found that in process
engineering, where speed of response tends to be less time-critical, a PI
controller normally provides perfrectly acceptable performance.
Just a few thoughts about the good old D term.
First off, the derivative term can produce a "violent" controller output in
response to a
step input e.g. change of setpoint, because d/dt (step) = infinity when it
"steps".
One way around this is clearly to "rate-limit" the step input.
Other stuff to consider if you're that way inclined..........
By doing an open-loop step response and measuring K (gain), T1 (time to reach
10%) and T2 (time to reach 90% of steady state), it's possible to use this
numbers to determine "rule-of-thumb" values for P, Ti and Td.
where P=(1.2*T2)/(K*T1)
Ti = 2*T1
Td = 0.5*T1
The other important thing that needs careful consideration is the SAMPLING
TIME of the PID controller. Again, rul of thumb is Td/2.
You could get sexier and start calculating Kalman controllers and such things,
although my PID controllers tend to be PLC-based, with "built-in" function
blocks to carry out the PID function. However, I have written Kalman control
loops using C before now (PC-based) - but this was generally Study based
rather than industry based.
Anyone else care to join this debate............
Regards
David Lawton.