PID control loops - closed loop feedback

[mightywombat]

PID control loops.... ahh. They seem to make sense sometimes but other times they don't... Here is the control loop that I have written up. Please take a look at it and critique it. The constants have not been tuned at all. I finally got to plug it into our second arm (with working pots!!) and somehow it was outputting numbers in the 400's....

Any help???

The two scaled values are pot values, one from our arm and one from the model. They are both scaled from 0 to 255 and are declared as unsigned longs... (don't ask, it has to do with how we scale them...)

sint = signed int

THIS IS WHERE IT IS CALLED:

//call the control loop to set the pwm values
Arm_Upper_Out = PID(scaled_highjoint_req, scaled_highjoint_act, lasthighjoint_act, sint_highjoint_errorsum);

//keep track of the error for the control loop
sint_highjoint_errorsum = sint_highjoint_errorsum + (scaled_highjoint_req - scaled_highjoint_act);

//keep track of the last position to be able to tell how fast it is moving for the control loop
lasthighjoint_act = scaled_highjoint_act;


THIS IS THE CONTROL LOOP:


unsigned char PID(signed int request, signed int actual, signed int lastactual, signed int sumerror)
{
#define PID_PROP 2
#define PID_DER 1
#define PID_INT 1

signed int pid, error;

request = request - 127;
actual = actual - 127;
lastactual = lastactual - 127;


pid = request;

error = request - actual; //find the difference of the two values

pid = pid + (error/PID_PROP); //add the proportional part of the error

pid = pid + ((actual - lastactual)/PID_DER); // add the derivative part of the error

pid = pid + (sumerror/PID_INT); //integral part of the pid

pid = pid + 127;

return(pid);
}