Peter Nachtwey
Member
Two spread sheets with different forms of PID
The first PID uses only the error for all three terms of the PID.
Control engineers would say that all the gains are in the forward path. This is the same PID and system used last week.
The second PID is what many call a I-PD. I-PD uses the error only for the I term and it uses only the PV for the P and D terms. In this case only the I gain is in the forward path. The P and D terms are in the feedback path because they only depend on the PV or feedback. Hence the name I-PD.
The point of the exercise is to compare the two PIDs and to realize that NO MATTER HOW MUCH YOU TURN UP THE GAINS YOU CAN NOT REDUCE THE ERROR TO ZERO. The PID is much better at following the sine wave and the I-PD is much better at responding to the step changes. This means that you should have the right PID for the right application. If the application uses a target generator with smooth changes to the SP, the PID with all the gains dependent on the error should be used. However, for most temperature systems where the SP is changed in steps, the I-PD form should be used.
This spread sheet is not a tuning exercise. The tuning exercises are over for now. You may have noticed that you can get the ITAE or ISE much lower than what is prudent by increasing the gains. In some cases the gains were so hot that they could go unstable if anything in the system changes. The PID has one major flaw because it doesn't provide any output unless there is, or has been, an error. It doesn't make any difference how much to crank up the gains.
There is a way to reduce error to almost 0. This technique is called feed forward. I will release a feed forward spread sheet in a day or two.
H
The first PID uses only the error for all three terms of the PID.
Control engineers would say that all the gains are in the forward path. This is the same PID and system used last week.
The second PID is what many call a I-PD. I-PD uses the error only for the I term and it uses only the PV for the P and D terms. In this case only the I gain is in the forward path. The P and D terms are in the feedback path because they only depend on the PV or feedback. Hence the name I-PD.
The point of the exercise is to compare the two PIDs and to realize that NO MATTER HOW MUCH YOU TURN UP THE GAINS YOU CAN NOT REDUCE THE ERROR TO ZERO. The PID is much better at following the sine wave and the I-PD is much better at responding to the step changes. This means that you should have the right PID for the right application. If the application uses a target generator with smooth changes to the SP, the PID with all the gains dependent on the error should be used. However, for most temperature systems where the SP is changed in steps, the I-PD form should be used.
This spread sheet is not a tuning exercise. The tuning exercises are over for now. You may have noticed that you can get the ITAE or ISE much lower than what is prudent by increasing the gains. In some cases the gains were so hot that they could go unstable if anything in the system changes. The PID has one major flaw because it doesn't provide any output unless there is, or has been, an error. It doesn't make any difference how much to crank up the gains.
There is a way to reduce error to almost 0. This technique is called feed forward. I will release a feed forward spread sheet in a day or two.
H