User Manual Chapter 4
GFK-1742F Jan 2020
Configuration 126
5.
Commanded motor acceleration or motor deceleration that is greater
than system capability.
6.04 In Position Zone. In Position Zone (User Units). When the Position Error is less than
or equal to the active In Position Zone value, the In Zone %I bit will be ON. Default:
10.
6.05 Pos Loop Time Constant (0.1ms). Position Loop Time Constant (units = 0.1
milliseconds). The desired servo position loop time constant. This value configures
the amount of time required for the servo velocity output to reach 63% of its final
value when a step change occurs in the Velocity command. The lower the value, the
faster the system response. Values that are too low will cause system instability and
oscillation. Default: 1000 = 100 ms.
Note: For accurate commanded velocity profile tracking, Pos Loop Time Constant should be 1/4
to 1/2 of the MINIMUM system acceleration or deceleration time. For example, if the
fastest acceleration that must occur occupies 100msec of time the Pos Loop Time
Constant should be between 25 to 50msec. To maintain system stability, use the largest
value possible.
For users familiar with servo bandwidth expressed in rad/sec:
Bandwidth (rad/sec) = 1000 / Position Loop Time Constant (ms)
For users familiar with servo gain expressed in ipm/mil:
Gain (ipm/mil) = 60 / Position Loop Time Constant (ms)
Table 36: Gain / Bandwidth / Position Loop Time Constant
For applications that do not require feedback control or employ very crude
positioning systems, an Open Loop Mode exists. Setting a zero Position Loop Time
Constant, which indicates that the positioning loop is disabled, selects this mode.
Note that in Open Loop Mode, the only way to generate motion is to program a non-
zero Velocity Feedforward. The Position Error is no longer used to generate motion
because Position Error is based on position feedback and Open Loop Mode ignores
all feedback.
CAUTION
For Analog Axes, the Position Loop Time Constant will not be accurate unless the
Velocity at Max Cmd value is set correctly.
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