The following procedure may be used to tune a system that transitions from position control to pressure control.
Please read the following topics before performing the tuning 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.
Position/Pressure Tuning Procedure
Tune the Position Gains
The position gains should be tuned before attempting to tune the axis for pressure. Obtaining control of the axis' position greatly simplifies the tuning of the pressure gains. If you have not yet tuned the position gains, follow the procedure outlined in Tuning a Position Axis before continuing.
Set Null Drive.
When the axis is holding position (stopped) in closed loop, issue a Set Null Drive to Integral Drive (n) command. For best accuracy, this should be done after the axis has been in position for a while, such as 1 second.
The null drive is the drive required to hold the axis in position. In some systems, such as hydraulics with servo valves, this value may change with time. Therefore, this step should be performed periodically.
For the remainder of the tuning procedure, use an Event Step sequence as described in the Position/Pressure Setup topic and illustrated in the Position/Pressure Example.
Adjust the Proportional Gain
The Proportional Gain should be adjusted first to gain some control over the pressure before continuing the tuning procedure.
Note: If negative drive causes an increase in pressure, use negative values throughout the tuning procedure.
Set the Proportional Gain to a small value, such as 2.
Use an Event Step table that makes a position move with the Monitor Pressure bit set so the axis will transition to pressure control when the Pressure Set A threshold is crossed. Use a fairly long ramp time initially, such as 1000 msec.
Once the axis is in pressure control, ramp the pressure between two pressures (this is best done in Event Steps). Gradually increase the proportional gain to minimize the following error. When you see a tendency to oscillate as the axis moves or stops, decrease the gain 10 to 30 percent.
Add Filter if Necessary
If the pressure feedback signal is excessively noisy, it may cause instability and will likely have been noticed already in the previous step. Adding a filter may help. This is done using the Filter TC parameter. See Filter TC for detailed information.
Adjust the Integral Gain
The Integral Gain helps get to the command pressure quickly.
Use the same Event Step sequence as previously to ramp between two pressures. You may want to increase the delay between the steps to more clearly see the effect of this tuning step.
Adjust the Integral Gain so that the pressure gets to commanded pressure quickly.
Too much Integral Gain will cause oscillations and overshoot. If this happens, decrease the Integral Gain.
Adjust the Differential Gain
Differential Gain greatly enhances performance on many hydraulic systems. Differential Gain will tend to dampen out oscillations and help the axis track during acceleration and deceleration. This will positively affect short, fast moves.
Continue ramping between two pressures.
If the drive output is always smooth, the Differential Gain can probably be increased. The drive output should look "fuzzy." This indicates that the drive is correctly responding to the minute errors of the axis.
A disadvantage of Differential Gain is that it amplifies measurement noise. If there is too much noise or the gain is too high, this can cause the system to chatter or oscillate. In this case, decrease the Differential Gain.
Readjust the Proportional Gain
Once the Differential Gain has been adjusted, the Proportional Gain may be readjusted. It affects the responsiveness of the system. Low gains make the system sluggish and unresponsive. Gains that are too high make the axis oscillate or vibrate.
Continue ramping between two pressures.
Slowly increase the gain. When you see a tendency to oscillate as the pressure changes or stops, reduce the gain by 10 to 30 percent.
Tune the Transition.
Now that both the position and pressure gains have been tuned, the transition may be tuned.
Use the Event Step sequence to transition from position to pressure.
If necessary, adjust the previously tuned pressure parameters for a smoother transition.
If there is oscillation and overshoot or other undesirable behavior at the transition that cannot be avoided with tuning the gains, the Ramp Time may need to be changed, or the speed going into the transition may be need to be slower.
Selecting Calculate Ramp Time in the Mode word allows the RMC to automatically calculates the Ramp Time. This is useful if it is difficult to determine a Ramp Time.
The following three steps also address parameters affecting the transition.
Adjust the Feed Forwards
The Extend and Retract Feed Forwards provide extra drive when extending or retracting.
Adjust the Feed Forwards to help the actual pressure track the target pressure when it is changing.
If the actual pressure leads the target pressure, decrease the Feed Forwards.
The Feed Forwards may need to change for different rates of pressure change. See Feed Forwards for more information.
Integrator Preload
Upon transition from position control to pressure control, the drive normally goes immediately toward zero. In some cases, this is undesirable. The integrator preload can be set to some value (positive or negative) to provide some drive immediately upon the transition. If your system overshoots or undershoots when entering pressure control, you may want to adjust this value. See Integrator Preload for more information.
Using the Integrator Preload is useful for systems with predictable position-to-pressure transitions. It always provides the same amount of drive.
Drive Transfer Percent
The Drive Transfer Percent acts similarly to the Integrator Preload. The difference is that the Integrator Preload places a constant value into the integral drive term, while the Drive Transfer Percent places a certain percentage (positive or negative) of the current drive into the integral drive term. If your system overshoots or undershoots when entering pressure control, you may want to adjust this value. See Drive Transfer Percent for more information.
Using the Drive Transfer Percent is useful for systems with unpredictable position-to-pressure transitions, such as entering with different speeds. Because it is a percentage, the transferred drive will vary with the drive required immediately prior.
Fine-tune the System
The final tuning of the system should be made at the speed of intended operation.
Look for following errors, overshoot, or oscillations.
Should the system seem a little sloppy, try adjusting the Proportional Gain.
If the actual position lags or leads the target position during the entire constant velocity section of the move, adjust the Feed Forwards.
If an overdrive error occurs, there is not enough drive capacity to drive the axis at the requested rate of pressure change. Should this occur, increase the Ramp Time or decrease the speed of the system.
If the Drive is not high, the gains can probably be increased for better control, depending on system stability.
Adding or changing the Filter TC value may help if noisy feedback is suspected.
Note that tuning pressure is often very difficult because of the large change in pressure for a small change in position.
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