New version of incPID available.
Hi,
First, bugfix....
I've discovered a bug/problem with the original incPID routine. When there's a persistant error present at the filter input the integrator for the I-term is supposed to accumulate and over time produce an output that will drive the error away. However, a small error in conjunction with low I-gain may actually never accumulate enough to produce an output signal like it's supposed to. This was due to truncation of the numbers during calculations. This version implements a fix for this issue.
Be careful if you move from the previous version to this as the I-term may behave quite differently depending on how your particular application is set up. I've tested it here and it seems to work but please let me know if you find any problems.
Second, new feature...Feed-forward
While a PID filter by itself works on the difference between the "setpoint" and the "target" feed-forward works directly on the command signal. Let's say you're controling the speed of a motor, the friction in the bearings, brushes and in the load the motor is driving causes it to need a certain "baseline" amount of power for any given speed in order to compensate for its mechanical and electrical losses. Feedforward can provide this "baseline" which helps the actual filter and can increase the overall system response.
There are three new "external" variables that you need to use with feed-forward:
pid_Vel_Kff This is the velocity feedforward gain.
pid_Acc_Kff This is the acceleration feedforward gain
pid_Velocity_cmd This is the actual command signal.
As motor control is my personal use for the incPID routine I've used the terms velocity and acceleration but it's applicable to whatever you are controling. Lets take an example where we want to control the temperature in an oven.
By imperical testing you have determined that in order to maintain your desired temperature (lets say 150 degrees) under normal/optimal/stable conditions you need an "output" of 800 to your "powerstage". If you set pid_Vel_Kff to $0600 and pid_Vel_cmd to 150, this will result in a "baseline" output of 800. Then you calculate the difference between the setpoint (150) and the current temperature and send it to filter in pid_Error. The output from the actual PID-filter then gets added to or subtracted from that 800 number in order to compensate for any disturbances that occurs.
The acceleration feedforward works in the same way as the derivative term of the PID filter but instead of acting on the derivative (ie. the difference) of pid_error it acts on the derivative of pid_Vel_cmd.
More info is available in the file itself.
I've tested this quite a bit but as always I'm sure there are things I've overlooked. If you have problems, please post in the thread and we'll try to work it out. If you don't have problems and it just works that's always nice to hear as well ;-)
Sincerely,
/Henrik.
(Change file extension from .txt to .pbp and place it in your project folder or in the PBP installation folder).
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