User manual

PMAC User Manual
Getting Started with PMAC 17
Setting up the Servo Loop
Warning:
Make sure the motor is in open-loop mode before restoring the proportional gain.
Otherwise, it may lurch to an old commanded position.
This is enough to see if the motor is working. Make sure the motor can run free (preferably no plant
attached at this point) and that things can be stopped quickly so that no damage can be caused. Type
K<CR> to disable the output(s), then restore the proportional gain by setting I130 (try 2000 initially for
very fine resolution systems, 50,000 for very coarse systems, or somewhere in between for medium
resolution).
Closing the Loop
Now close the motor’s servo loop by typing J<CR> (the jog-stop command, which brings the motor into
zero-velocity position control). It should hold position at this point, resisting attempts to move it away, at
least gently. If it runs away, there is a mismatched polarity; re-run the above polarity tests. If there is loss
of control or the motor starts behaving wildly, type K<CR> to disable the motor.
Weak Loop
If it does not resist being turned, or does so very weakly, try increasing proportional gain (I130). Try
doubling it until there is some reasonable stiffness, but do not try yet to get the maximum possible stiffness.
The tests described below will help do that.
Oscillations
If the motor has a tendency to oscillate at low to moderate frequency, there is inadequate derivative gain.
Try doubling I131 and see if the oscillation goes away.
Buzzing
If the motor has a tendency to oscillate at high frequency (a "buzz"), there is too much proportional gain, or
maybe too much derivative gain. Try lowering I130 (or I131) until the buzz disappears.
Extensive Servo Loop Tuning
The PMAC Executive Program for PC-compatibles has a large section devoted to assisting the user in
optimizing the servo loop parameters for a motor. It allows performance of step moves and profiled moves
and the response is plotted to the screen with key statistics calculated, so that easy choices may be made
about changing gains. This process is documented in detail with examples in the manual for the PMAC
Executive Program.
In addition, there is an auto-tuning feature that lets the Executive program make the decisions about what
the gains should be. The program excites the system, evaluates the response, and calculates the gains
necessary to achieve the desired response. Remember that precise tuning cannot be done until the load has
been connected to the motor. Our goal at this point is simply to get the motor moving reasonably well
without a load.
Jogging Moves
With these two parameters (I130 and I131) at reasonable levels, good performance in moves should be
achieved. Try a jog move first. Before doing the move, set up the jog speed (I122, in counts/msec)
acceleration time (I120, in milliseconds), and S-curve time (I121, in msec) to desired values (to be safe, use
low speed and high acceleration times at first). Now type J<CR> — the motor should turn in the positive
direction.