The following procedure may be used to tune many position axes, hydraulic axes and motors in velocity 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 Proportional Gain
The Proportional Gain must be adjusted to gain some control over the system for continuing the tuning procedure. Adjust the Proportional Gain by slowly increasing it and making moves. When the system gets close to final position reasonably quickly, continue to the next step. If the system begins to oscillate, decrease the gain.
Adjust the Feed Forwards
In many hydraulic systems the feed forward parameters (Extend Feed Forward and Retract Feed Forward) are the most important parameters for position tracking during a move. These may be adjusted in 2 ways:
Make 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.
Set the Differential Gain and Integral Gain to zero and keep the Proportional Gain value from the previous step. Make long slow moves in both directions. Adjust the Extend Feed Forward and Retract Feed Forward until the axis tracks within 10% in both directions.
In non-regenerating hydraulic systems, the Extend Feed Forward will be less than the Retract Feed Forward. In regenerating systems, the opposite is true.
Readjust the Proportional Gain
Proportional Gain 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.
Slowly increase the gain. When you see a tendency to oscillate as the axis moves or stops, reduce the gain by 10 to 30 percent.
At this point, if you have gained sufficient control of the system, you may want to increase the speed, accel and decel of your moves and further adjust the proportional gain. A value of proportional gain that may seem good at low speeds and accels, may not work at higher speeds.
Adjust the Integral Gain
Many hydraulic systems do not require a large Integral Gain. However, it is usually desirable to have some Integral Gain (5 to 50 units) to help compensate for valve null drift or changes in system dynamics. Some systems may require larger Integral Gain, in particular if they are moving a large mass or are nonlinear. Too much Integral Gain will cause oscillations and overshoot. The Integral Gain is helpful for getting into position and for tracking during long, slow moves. It will not significantly affect tracking during short, fast moves.
Adjust the Acceleration Feed Forwards
The Acceleration Feed Forward terms are particularly useful for systems moving large masses with relatively small cylinders. Such systems often have a delay before the start of movement. The Acceleration Feed Forward terms can help compensate for this delay.
Look for following errors during acceleration and deceleration. Increase the Extend and Retract Acceleration Feed Forward terms until the errors disappear.
For large masses the Acceleration Feed Forward may be in the tens of thousands.
Adjust the Differential Gain
Differential Gain may greatly enhance performance on many hydraulic systems. It is used mainly on systems that have a tendency to oscillate. This happens when heavy loads are moved with relatively small cylinders. Differential Gain will tend to dampen out oscillations and help the axis track during acceleration and deceleration. This will positively affect short, fast moves.
Important: If you use 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.
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 Acceleration Feed Forward does not help, the Acceleration and Deceleration ramps are too steep for the response of the system.
If the actual position lags or leads the target position during the entire constant velocity section of the move, adjust the Feed Forwards.
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. However, if the oscillation is not caused by a deadband in the system then this will not help! A rule of thumb is to set the Dead Band Eliminator value to half of the peak-to-peak oscillation of the drive output while in position.
The final tuning of the system should be made at the speed of intended operation.
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