User`s manual
4-28 Motion Mate™ DSM314 for Series 90™-30 PLCs User's Manual
–
January 2001 GFK-1742A
4
3. Servo drives failure.
4. Mechanically forcing the motor/encoder shaft past the servo torque
capability.
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 4-11. Gain / Bandwidth / Position Loop Time Constant
Gain
(ipm/mil)
Bandwidth
(rad/sec)
Position Loop Time
Constant (ms)
0.5 8.5 120
0.75 12.5 80
1.0 16.6 60
1.5 25.1 40
2.0 33.4 30
2.5 41.8 24
3.0 50 20
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.