Tuning a Position-Pressure or Position-Force System

The following procedure may be used to tune a system that uses pressure/force control or pressure/force limit. This procedure uses pressure/force control, but once an axis is tuned for pressure/force control, those gains can be used for pressure/force limit with few or no changes.

Please read the following topics before performing the tuning procedure:

Tuning Overview
Tuning a Position Axis

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.

Position/Pressure Tuning Procedure

Pre-Tuning Steps:

These steps come before actually tuning the gains.

1. Set the Pressure/Force Limits

Set the Positive Pressure/Force Limit and Negative Pressure/Force Limit. These specify the pressure/force range in which the axis will be allowed to control. If the Target Pressure/Force exceeds these values, an error bit will be set and will halt the axis if the AutoStops are set to do so.

2. Set the AutoStops

Set the Position Following Error and Pressure/Force Following Error AutoStop to Status Only to prevent following errors from halting the axis during the tuning. After the tuning is complete, set the AutoStop to a safe setting for machine operation.

Tuning Steps:

1. Tune the Position Gains

The position gains should be tuned before attempting to tune the axis for pressure/force. Obtaining control of the axis' position greatly simplifies the tuning of the pressure/force gains. If you have not yet tuned the position gains, follow the procedure outlined in the Tuning a Position Axis topic before continuing.

2. Enter Pressure/Force Control

Set the Pressure/Force Proportional Gain to a small value, such as 0.01.

Then enter pressure/force control with the Hold Current Pressure/Force (19) command. Issue the Ramp Pressure/Force (S-Curve) (41) or Ramp Pressure/Force (Linear) (42) or command to set the Command Pressure to the desired value.

You may wish to move the axis to where it encounters pressure/force before you issue the Hold Current Pressure/Force (19) command.

If the axis halts due to an Output Saturated error, reduce the Proportional Gain and repeat.

3. Adjust the Proportional Gain

Keep increasing the Pressure/Force Proportional Gain until the Actual Pressure/Force starts moving toward the Target Pressure/Force. It should not get all the way toward to Target Pressure/Force. Give just enough gain to make sure it is beginning to.

4. Adjust the Integral Gain

Add some Pressure/Force Integral gain, approximately twice as much as the Pressure/Force Proportional Gain. The Actual Pressure/Force should go towards the Target Pressure/Force.

Do not give a lot of Integral Gain, just enough to make sure it eventually gets to the Target Pressure/Force.

5. Ramp the Pressure/Force Up and Down

For the remainder of the tuning, repeat the following steps:

Ramp the pressure/force.
This can be done with the Ramp Pressure/Force (Linear) (42) or Ramp Pressure/Force (S-Curve) (41) command.
Change a gain.
View the plot.

Tip: You can use the Tuning Wizard to calculate a model and use the Gain Calculator to choose gains. After uploading a plot of pressure or force control, where the axis is controlling a changing pressure or force, click the Tuning Wizard.

The Tuning Tools make this very easy. In the Tuning Tools, set up the command buttons so that one button will ramp the pressure/force in one direction, and the other will ramp it in the other direction.

In the Tuning Tools, click one of the buttons labeled Click to set up.
Choose the Ramp Pressure/Force (Linear) (42) or Ramp Pressure/Force (S-Curve) (41) command and enter valid command parameters for your axis.

Repeat the previous step on the other command button. Enter the same command, except for a different Requested Pressure/Force.
Click OK. Now you will see that the command buttons are labeled.

To ramp the pressure/force, just click one of the buttons, and the plot will automatically be uploaded.

6. Adjust the Feed Forwards

Pressure/Force Rate Feed Forward

Ramp the pressure/force, and adjust the Pressure/Force Rate Feed Forward. Start with a small value. The Pressure/Force Rate Feed Forward should help the Actual Pressure/Force track the Target Pressure/Force during the ramp.

Pressure/Force Feed Forward

Most systems do not require the Pressure/Force Feed Forward. If your system exerts a pressure or force that is roughly linear to the amount of Control Output, you may need the Pressure/Force Feed Forward. To tune the Pressure/Force Feed Forward, move the Target Pressure/Force to various values, and adjust the Pressure/Force Feed Forward while the Target Pressure/Force is stationary. Do this for several value of pressure/force to ensure the best Pressure/Force Feed Forward.

7. Re-adjust the Proportional and Integral

Ramp the pressure/force, and re-adjust the proportional and integral gains. This should make it track better.

8. Adjust the Differential Gain

If you need better control, add some Differential Gain. The Differential Gain is typically a couple orders of magnitude less than the Proportional Gain.

9. Tune the Transition

Now that both position and pressure/force are tuned, you can tune the transition between position and pressure/force control or pressure/force limit.

Issue the commands for transitioning between position and pressure/force as you intend to do during normal machine operation. For some applications, this may involve creating a simple user program.

View the transition on a plot. If the pressure/force drops off at the transition, increase the Integrator Preload parameter in the Enter Pressure/Force command.

10. Fine-tune the System

The final tuning of the system should be made at the rate and pressure/force range of intended operation. Look for following errors, overshoot, or oscillations, and consider the following:

Should the system seem a little sloppy, try adjusting the Proportional Gain.
If the actual pressure lags or leads the target position during the entire constant velocity section of the move, adjust the Feed Forwards.
If an Output Saturated error occurs, there is not enough drive capacity to drive the axis at the requested rate of pressure/force change. Should this occur, increase the Ramp Time or decrease the speed of the system.
If the Control Output is not high, the gains can probably be increased for better control, depending on system stability.
Adding or changing the pressure/force feedback filter may help if noisy feedback is suspected.