Delta MMC120 - Manual (Page 25)

MMC120 Linear Motion Control Module Startup
Delta Computer Systems, Inc. 360/254-8688 25
Tuning
There is no substitute for experience when tuning an axis. This section offers some guidelines, tips, and
suggestions for tuning your system. While these steps will work for many systems, they may not be the best
for a particular system.
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. One way to adjust these
parameters is to set the DIFFERENTIAL GAIN and INTEGRAL GAIN to zero and the PROPORTIONAL
GAIN to a small value (between 1 and 5), then make long slow moves in both directions. Adjust the EXT
FEED FORWARD and RET FEED FORWARD until the axis tracks within 10% in both directions. In
hydraulic systems, the EXTEND and RETRACT FEED FORWARD terms will differ by the ratio of the
extend and retract piston areas.
Alternately, you can find the appropriate value for the FEED FORWARD terms by making moves with the
axis at a SPEED of 1,000. The amount of output drive required to maintain this SPEED is the correct value
for the FEED FORWARD parameter.
A third approach is to use the 'alt-F' command. This command, used after a move without oscillation or
overdrive on an axis, will automatically adjust the FEED FORWARD parameter for the direction of that move.
See Appendix A for more information.
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. You can adjust the
PROPORTIONAL GAIN by slowly increasing it and moving the axis. When you see a tendency to oscillate as
the axis moves or stops, reduce the gain by 10 to 30 percent.
Many hydraulic systems do not require INTEGRAL GAIN or DIFFERENTIAL GAIN. However, it is usually
desirable to have some INTEGRAL GAIN (5 to 50 counts) 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.
DIFFERENTIAL GAIN is used mainly on systems which 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. If you use DIFFERENTIAL GAIN,
you may be able to increase the PROPORTIONAL GAIN somewhat without causing the system to oscillate.
A disadvantage to DIFFERENTIAL GAIN is that it amplifies position measurement noise which can cause the
system to chatter or oscillate if the gain is too high or there is too much noise.
The ACCELERATION FEED FORWARD terms are particularly useful for axes which move large masses
with relatively small cylinders. This combination delays the start of movement, and the ACCELERATION
FEED FORWARD terms can help compensate for this delay. ACCELERATION FEED FORWARDS are
easiest to adjust with the PID gains set low and the VELOCITY FEED FORWARDS adjusted properly. After
commanding a move, plot the move using DCS120 and look for a following error during the acceleration.
Increase the ACCELERATION FEED FORWARD until the error disappears. For large masses the
ACCELERATION FEED FORWARD can be in the tens of thousands.
If the axis hunts around the set point, you can increase the DEAD BAND ELIMINATOR value slowly until
the hunting stops or the axis starts to oscillate. If it oscillates, reduce the DEAD BAND ELIMINATOR value.
If the axis gets no following errors, reduce the MAXIMUM ERROR until errors start to occur then adjust the
FEED FORWARD gains.
Increase the SPEED and ACCELERATION values gradually while making long moves. Use DCS120 to plot
the moves and look for following errors, overshoot, or oscillations. Eventually, when the SPEED and
ACCELERATIONS are too high, the moves will cause an error on the axis.
If an overdrive error occurs, there is not enough drive capacity to drive the axis at the requested SPEED.
Should this occur, reduce the SPEED. If a following error occurs, the appropriate FEED FORWARD must be