HEIDENHAIN ND 1300 CNC Setup - Manual (Page 46)

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3
CNC Setup
Increase integral factor and limit
Often in servo systems, as an axis approaches its final target position, the following error falls to such small
values that the proportional gain term alone is inadequate to drive the axis the final small distance to its
target. This is usually due to frictional forces opposing motion on the axis. The integral PID control term
can be added in these cases to increase PID loop gain for very small following errors. This additional drive
is achieved by summing a series of small following error samples at the end of the CNC move to create a
total motor drive capable of moving the axis to its final target position. The integral term consists of the
integral factor and the integral limit. The integral factor is a multiplier that amplifies each of the following
error samples, and the integral limit defines the maximum level of the sum of all amplified samples used
to drive the motor.
NOTE
Integral is used only in servo motor systems.
Integral factor
The size of the integral factor determines how quickly the motor will be driven by a series of small follow-
ing error samples. Large integral factors quickly add up to significant motor drive signals. This has the
advantage of moving the last small distance to the final target quickly, but has the disadvantage that too
much drive might cause the axis to move through the target position. If the axis moves through the target, it
must be driven back to the target again from the other direction. Very large integral factors can lead to axis
oscillations around the target
position. Integral factors are
typically 1% to 10% of Integral
limits. This ratio depends on
the static and dynamic coef-
ficients of friction on the axis
and can vary greatly. Begin by
entering a value of 1.
Integral limit
The integral limit prevents unstable PID loop behavior by limiting the overall integral control term to a
specified maximum value. Limiting the total drive of the integral term has the advantage of allowing rela-
tively large integral factors that produce rapid moves to the final target position while preventing excess
drive that could lead to oscillations around the target.
Tuning Flow Chart Block Descriptions