Reference Manual

PMAC 2 Software Reference
PMAC I-Variable Specification 91
Ix40 - Ix56 Motor x Extended Servo Algorithm I-Variables
(These variables are used only with the Option 6 Extended Servo Algorithm. Refer to the manual for the
Extended Servo Algorithm and the ACC-25 Servo Evaluation Program for details.)
Ix40 Motor x Net Desired Position Filter Gain {Option 6L Firmware Only}
Range
0.0 – 0.999999
Units
none
Default
0.0
Remarks
Ix40 permits the introduction of a first-order low-pass filter on the net desired position for
Motor x. This can be useful to smooth motion that comes from a “rough” source, such as
master following from a noisy sensor, or quantization error in very closely spaced
programmed points that are commonly found in lookahead applications.
If Ix40 is set to its default value of .0, this filter function is disabled. If Ix40 is set to any
value greater than 0.0, the filter is enabled.
Ix40 can be expressed in terms of the filter time constant by the following equation:
f
T
s
T
f
T
40Ix
+
=
where T
f
is the filter time constant, and T
s
is the servo update time.
The filter time constant can be expressed in terms of Ix40 by the following equation:
40Ix1
s
T*40Ix
Tf
−
=
Filter time constants can range from a fraction of a servo cycle (when Ix40 ~ 0) to infinite
(when Ix40 ~ 1). As with any low-pass filter, there is a fundamental trade-off between
smoothness and delay. Generally when the filter is used, filter time constants of a few
milliseconds are set. In an application where multiple motors are executing a path, the
same time constant should be sued for all of the motors.
Ix40 is available only with the special Lookahead option. If the Extended Servo
Algorithm option is selected along with the Lookahead option, this Ix40 filter is not
available. (In the ESA, Ix40 is used for another purpose.)
Example
To set a filter time constant of 2 msec on a system with the default servo update time of
442 µsec, Ix40 can be computed as:
819.0
2442.0
2
40Ix =
+
=