Motion control question

This may seem very basic, but sometimes we need to always keep these things fresh in our minds.

If I am controlling position with a PID controller do I still need a motion profile. For example acceleration and velocity data to form a trapazoidal profile or would the unit step(point-to-point) profile be sufficent?

What I am asking is if the system is properly tuned with the unit step, would I still really need a motion profile to excute the move?

If the answer is YES, how much shorter would the sample time of the PID routine have to be then the acceleration of the movement?

Thanks in advance.

It depends

plc_user1973 said:

If I am controlling position with a PID controller do I still need a motion profile. For example acceleration and velocity data to form a trapazoidal profile or would the unit step(point-to-point) profile be sufficent?

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That depends on the needs and tuning. If you only care about point to point then you don't need a motion profile, BUT then you must detune your system or have the 'right stuff' in your PID so the motor doesn't start with a jerk when the PID outputs 10 volts because of the error.

What I am asking is if the system is properly tuned with the unit step, would I still really need a motion profile to excute the move?

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Like I said you must have the right kind of PID and the right kind of tuning. I do what you suggest all the time but it is not the main means of moving from point to point. We must also have one PID for the step jumps and one for following the motion profiles.

If the answer is YES, how much shorter would the sample time of the PID routine have to be then the acceleration of the movement?
Thanks in advance.

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The target or profile generator is separate from the PID but they run synchronously.

Here is an example of a point to point move without a profile generator. The lines are color coded.



You can see that the down side is that the actual position approaches the target position or set point exponentially at the end and this may take more time to get exactly on target. I can increase the gain of the tuning but then the acceleration at the beginning of the motion would be too extreme. When you not using a motion profile generator the system acts like it is one big filter which has its good points and its bad points. I have some tricks that allow me to limit the speed to a desired value. In this case the speed limit is 50000 which usually means 50 inches per second or 5 m/s depending on where your are and how the position units are interpreted.

BTW, I am doing all this from Ireland. The motion controller is actually in Vancouver, WA. Now why am I in Ireland when I can tune the actuators here from Vancouver, WA? Hmmm.

Generating proper motion profiles is very difficult and time consuming.

Peter Nachtwey said:

BTW, I am doing all this from Ireland. The motion controller is actually in Vancouver, WA. Now why am I in Ireland when I can tune the actuators here from Vancouver, WA? Hmmm.

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1. Because Ireland is a great place to visit.
2. They have great food and Guiness on tap.
3. The people are friendly and a lot of fun.