Peter Nachtwey
Member
This week a PID using the absolute or position form is used to tune a type 0 four pole system. Remember that a type 0 system is one where the PV or actual returns to the initial state when the output is turned off like a velocity or temperature system. A type 1 system is one that integrates the output like the velocity is integrated to become position in a position system. In this case when the output goes to 0 it just coasts to a stop and stays where it is.
Last week Vern showed how an absolute PID can be implemented, but integrator and output limiting were not used. This absolute PID has the proper limiting. This system should be a bit more challenging to tune than the other systems presented so far because there are more poles. ( lags, time constants ). The ITAE, integrated time absolute error, can be reduced to about 320 with some aggressive tuning. However, I would not really tune a system where the output is swinging all over. Even with just open loop or feed forward control the ITAE can be reduced to below 400. The ITAE can even be reduced below 200 with out overshooting with some tricks, but not with this PID. A system like this would very hard to tune with graphing and an ITAE evaluation.
Why is the ITAE being used instead of the ISE? See previous PID threads.
What should be noticed is that the absolute form of PID will always overshoot the target or set point. This is because the integrator is accumulated separately. Once the integrator winds up, it can’t be unwound without overshooting the set point where the error goes negative and unwinds the integrator. Don’t expect to achieve perfection or anything close to it. That is the point of the exercise. Some PIDs are better than others. Sometimes no PID is good enough. I will explain more as questions arise.
A challenge, tune a type 0 four pole system.
Last week Vern showed how an absolute PID can be implemented, but integrator and output limiting were not used. This absolute PID has the proper limiting. This system should be a bit more challenging to tune than the other systems presented so far because there are more poles. ( lags, time constants ). The ITAE, integrated time absolute error, can be reduced to about 320 with some aggressive tuning. However, I would not really tune a system where the output is swinging all over. Even with just open loop or feed forward control the ITAE can be reduced to below 400. The ITAE can even be reduced below 200 with out overshooting with some tricks, but not with this PID. A system like this would very hard to tune with graphing and an ITAE evaluation.
Why is the ITAE being used instead of the ISE? See previous PID threads.
What should be noticed is that the absolute form of PID will always overshoot the target or set point. This is because the integrator is accumulated separately. Once the integrator winds up, it can’t be unwound without overshooting the set point where the error goes negative and unwinds the integrator. Don’t expect to achieve perfection or anything close to it. That is the point of the exercise. Some PIDs are better than others. Sometimes no PID is good enough. I will explain more as questions arise.
A challenge, tune a type 0 four pole system.