Tuning a Torque Motor

The following procedure may be used to tune motors running in torque mode. Please read the Tuning Overview before following this procedure.

There is no substitute for experience when tuning an axis. This procedure offers some guidelines, tips, and suggestions for tuning your system. While these steps will work for some systems, they may not be the best for a particular system.

Tuning Procedure

Do Open Loop Move

This step is for verifying that the system wiring and setup is correct before doing any closed loop control. Issue an Open Loop (O) command with a small drive, such as 50-150. Increase the drive until the axis begins to move. A positive drive should yield increasing counts. Issue an Open Loop command again with a negative drive. This should yield negative counts.

Before continuing, verify that all the Gains and Feed Forwards are set to zero.

Check Dead Band

If your system has a large dead band, you will need to set the Dead Band Eliminator value. To find your dead band, give increasing amounts of drive to the system with the Open Loop command. The value of drive at which the system starts to move is your dead band. If this value is approximately 400 or greater, the Dead Band Eliminator should probably be used. If it is less, it is left to the discretion of the designer.

Adjust the Differential Gain

Torque motors generally do not have much damping. Damping must be provided for the system, or it will be difficult to control. Providing some Differential Gain will effectively dampen the system. Do the following:

Set all the gains to zero. Issue an Open Loop (O) command of zero.
Increase the Differential Gain. Issue a Set Parameters (P) command to put the axis in closed loop control.
Repeat b until sufficient damping is obtained. There are several methods this can be done, depending on the system:

For small motors, rotate the motor manually to get a feel for the resistance to movement (damping). Repeat step b until the damping is significant. If the motor chatters or oscillates, decrease the gain.
For systems that cannot be moved manually, repeat step b until the motor starts humming (or chattering or oscillating) and then back the Differential Gain off significantly, perhaps even 50%, to avoid oscillating later while making moves.
For finer adjustment on large systems, repeat step b, and then momentarily (e.g. 5 milliseconds) give a drive output to the motor. The event step table can be used to do this with the Open Loop command. This jolt to the system will provide a clear indication of whether the Differential Gain is too high.

Do not set the Differential Gain too high! Remember that the point here is only to provide some damping for continuing the tuning process. The Differential Gain will be fine-tuned later. Keep in mind that motors often require a very high Differential Gain.

Adjust the Proportional Gain

Adding Proportional Gain will now improve system performance. Adjust the Proportional Gain by slowly increasing it and making moves. If the system begins to oscillate, decrease the gain.

Adjust the Feed Forwards

In torque motor applications, feed forward parameters (Extend Feed Forward and Retract Feed Forward) often do not require high values. Adjust these parameters by making a long move without any oscillation or overdrive. Then issue the Set Feed Forward command. This command will automatically adjust the Feed Forward parameter for the direction of that move.

Adjust the Integral Gain

The Integral Gain is helpful for maintaining position. It will not significantly affect tracking during short, fast moves. Increase the Integral Gain until the system starts oscillating, then back it down some. Motors may require a high Integral Gain.

Readjust the Differential Gain

Differential Gain tends to dampen out oscillations and help the axis track during acceleration and deceleration. This will positively affect short, fast moves.

Important: If you increase the Differential Gain, you may be able to increase the Proportional Gain somewhat without causing the system to oscillate.

If the drive output during the constant velocity portion of the move is smooth, the Differential Gain is perhaps not set high enough. The drive output may look "fuzzy." This indicates that the drive is responding to the minute errors of the axis. Note that not all systems allow the differential gain to be set high enough for the drive to be "fuzzy".

A disadvantage of Differential Gain is that it amplifies position measurement noise. If there is too much noise or the gain is too high, this can cause the system to chatter or oscillate.

Adjust the Acceleration Feed Forwards

The Acceleration Feed Forward terms help minimize errors during acceleration and deceleration. Increase the Extend and Retract Acceleration Feed Forward terms until the errors disappear.

Increase System Speed

Gradually increase the Speed and Acceleration values while making long moves. Look for following errors, overshoot, or oscillations.

If an overdrive error occurs, there is not enough drive capacity to drive the axis at the requested Speed or Acceleration. Should this occur, reduce the Speed and/or Acceleration and Deceleration.
If a following error occurs during acceleration and deceleration and adjusting the Gains and Feed Forwards does not help, the Acceleration and Deceleration ramps are too steep for the response of the system.
Should the system seem a little sloppy, try adjusting the Proportional Gain.
If the Drive is not high, the gains can probably be increased for better control. If the Drive is too high, or an overdrive error occurs, the system is not capable of performing the requested move. The Speed, and/or Accelerations may need to be decreased.
If the system vibrates while in position, the Dead Band value may need to be increased.

The final tuning of the system should be made at the speed of intended operation.