Aff (acceleration feedforward), Kdac – National Instruments NI-Motion User Manual

Page 34

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Chapter 3

Tuning Servo Systems

NI-Motion User Manual

3-6

ni.com

Velocity feedforward is an open-loop compensation technique and cannot
affect the stability of the system. However, if you use too large a value for
Vff, following error can reverse during the constant velocity portion, thus
degrading performance, rather than improving it.

Velocity feedforward is typically used when operating in PIVff mode
with either a velocity block amplifier or substantial amount of velocity
feedback (Kv). In these cases, the uncompensated following error is
directly proportional to the desired velocity. You can reduce this following
error by applying velocity feedforward. Increasing the integral gain (Ki)
also reduces the following error during constant velocity but only at the
expense of increased following error during acceleration and deceleration
and reduced system stability. For these reasons, increasing Ki is not a
recommended solution.

Tip

In PIVff mode, the Kd and Kv gains are set to zero.

Velocity feedforward is rarely used when operating in PID mode with
torque block amplifiers. In this case, because the following error is
proportional to the torque required, rather than the velocity, it is typically
much smaller and does not require velocity feedforward.

Aff (Acceleration Feedforward)

The acceleration feedforward gain (Aff) determines the contribution in the
16-bit DAC command output that is directly proportional to the
instantaneous trajectory acceleration. Aff is used to minimize following
error (position error) during acceleration and deceleration and can be
changed at any time to tune the PID loop.

Acceleration feedforward is an open-loop compensation technique and
cannot affect the stability of the system. However, if you use too large a
value of Aff, following error can reverse during acceleration and
deceleration, thus degrading performance, rather than improving it.

Kdac

Kdac is the Digital to Analog Converter (DAC) gain. Use the following
equation to calculate Kdac:

20 V represents the ±10 V range in the motion controller.

Kdac

20 V

2

16

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