Hi, Just wondering is anyone aware of a process plant control application where the control response to *setpoint* changes has to include derivative action. Presumably it would be the inner loop of a cascade that requires very snappy response.
Not process plants, no, but it's certainly common in servo applications where the whole point of the system is to follow the command -- and which is reflected by calling the input a command, and not a set point.
If nothing else, wouldn't you find it in servo valves (or whatever they're called)? Perhaps it's there, masquerading under the name "feed forward".
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Example plant: With disturbance z2
1,570587 v1'' + 1,262046 v1' + v1 = v2 Damping d = 0,503517Example 1: With derivative action Page 1
Example 2: Witout derivative action Page 2
If just using PID control there is no chance to find a better solution without derivative action!
JCH
Under damped systems should have a derivative gain to dampen the system response. If you want to place all the poles there needs to be one controller gain for each pole open loop pole. The integrator gain doesn't count because it comes with its own pole. In JCH's example the system has two poles and they are imaginary. Therefore the system is under damped so two gains are necessary, the P and D gains.
Not always. For example cascaded loop with an inner velocity loop and an outer position loop. The inner velocity loop may be just a simple PI controller because the error is in velocity units. The P gain in a velocity loop does about the same thing as the D gain in the position loop which uses error in position units.
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