D

#### 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.