Reference Manual

PMAC 2 Software Reference
PMAC I-Variable Specification 99
Note:
The units of this parameter are 1/16 of a count, so the value should
be 16 times the number of counts in the limit.
See Also
I-variables Ix11, Ix12, Ix68
Ix68 Motor x Friction Feedforward
Range
-32,768 .. 32,767
Units
DAC bits
Default
0
Remarks
Ix68 adds a bias term to the servo loop output of Motor x that is proportional to the sign of
the commanded velocity. That is, if the commanded velocity is positive, Ix68 is added to
the output. If the commanded velocity is negative, Ix68 is subtracted from the output. If
the commanded velocity is zero, no value is added to or subtracted from the output.
This parameter is intended primarily to help overcome errors due to mechanical friction. It
can be thought of as a “friction feedforward” term. Because it is a feedforward term that
does not utilize any feedback information, it has no direct effect on system stability. It can
be used to correct the error resulting from friction, especially on turnaround, without the
time constant and potential stability problems of integral gain.
If PMAC is commutating this motor, this correction is applied before the commutation
algorithm, and so will affect the magnitude of both analog outputs.
Note:
This direction-sensitive bias term is independent of the constant
bias introduced by Ix29 and/or Ix79.
Example
Starting with a motor at rest, if Ix68 = 1600, then as soon as a commanded move in the
positive direction is started, a value of +1600 (~0.5V) is added to the servo loop output.
As soon as the commanded velocity goes negative, a value of -1600 is added to the output.
When the commanded velocity becomes zero again, no bias is added to the servo output as
a result of this term.
See Also
Closing the Servo Loop
I-Variables Ix01, Ix29, Ix32, Ix35, Ix79
Ix69 Motor x Output Command Limit
Range
0 .. 32,767 (0 to 10V or equivalent)
Units
16-bit DAC bits
Default
20,480 (6.25V or equivalent)
Remarks
Ix69 defines the magnitude of the largest output that can be sent from PMAC’s PID
position/velocity servo loop. If the servo loop computes a larger value, PMAC clips it to
this value. When the PID output has saturated at the Ix69 limit, the integrated error value
will not increase, providing anti-windup protection.
For the Extended Servo Algorithm (ESA) that with the Option 6 firmware version, Ix69 is
used to multiply a normalized command (-1.0 <= Normalized Command < +1.0) before
outputting it or using it for commutation. As such, it acts as both a scale factor and an
output command limit for the ESA.
Ix69 is always in units of a 16-bit DAC, even if the actual output device is of a different
resolution, or the command value is used for PMAC’s own internal current loop
commands.