Delta MMC120 - Manual (Page 116)

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with the axis using a SPEED of 1,000. The amount of output drive required to maintain
this speed should be used as the EXTEND FEED FORWARD parameter. If the axis lags
after the parameter has been set, the feed forward is too small or the system response is
too slow. If the axis leads, the feed forward is too large or the system response is too
slow. The 'F' command used after an extend move will automatically adjust the
EXTEND FEED FORWARD parameter. The EXTEND and RETRACT FEED
FORWARDS are also known as velocity feed forwards.
NOTE:Feed Forward Drive is limited to a maximum value of 10,000. Inserting that
value in the equation above and solving for the Feed Forward term gives the
follow relationship:
Feed Forward <= (10,000 * 1,000) / TARGET SPEED
That is, the larger the SPEED the smaller the maximum Feed Forward value.
For example, at 30 in/sec SPEED, the maximum Feed Forward is:
Max Feed Forward = (10,000 * 1,000)/30,000 = 10,000/30 = 333
If you set SPEED to 30,000 and enter a Feed Forward value larger than 333,
the value will be reduced to 333.
Think about this:
If the feed forward value is less than 300, an axis going at a speed of 65535 will not get a
full valve drive from the feed forward term alone. If the feed forward value is less than
200, the axis drive system is too big and the system may have trouble controlling
position. If this happens, then reduce the gain on the amplifier or use a smaller valve or
reduce pressure until the feed forward terms are over 300.
On hydraulic systems, the extend and retract feed forward terms will be different by the
ratio of the extend and retract surface areas of the position.
The Proportional Gain, Integral Gain and Differential Gain as well as the Integral Drive
Limit are adjusted internally based on the ratio of the Extend and Retract Feed Forwards.
The direction with the lower Feed Forward value will have effectively lower Gains and
Integral Limit. This is done to compensate for the different system dynamics in the two
directions.
For example, for a system with an Extend Feed forward of 100 and a Retract Feed
Forward of 200 the effective gains in the extend direction will be one half (100 / 200) as
big as the specified gains.
RETRACT FEED FORWARD
Default: 100