MTS Series 793 Application - Manual (Page 69)

About Command Stop Behaviors and Automatic
MTS MultiPurpose TestWare® MPT Options Editor 69
About Command Stop Behaviors and Automatic Setpoint Adjustments
Note When you enable Setpoint, the Return to Zero and Taper to Zero
functions operate relative to the Setpoint, not to absolute zero.
If you enable Setpoint and Span and set Command Stop Behavior to Stay at
Level, you may find that Setpoint increases unexpectedly if you stop, unlock,
reset, and restart the test. This also applies if remove and then turn hydraulics on.
The reason for this is that, using the previously described conditions, Setpoint
automatically shifts by the amount of offset present when the procedure was
stopped.
Example
Suppose you run a procedure in which you run a ±10 mm cyclic command from a
mean level of 20 mm. You enable Span and Setpoint and select a Command
Stop Behavior of Stay at Level. You set Span to 100% and Setpoint to 0 mm.
When the actuator is at 20 mm, you stop the test. This causes the actuator to stop
and hold at 20 mm. Then you unlock the procedure and resume the test. In this
situation, the Setpoint value automatically increases to 20 mm, causing the
actuator to travel to mean level of 40 mm before resuming the ±10 mm cyclic
waveform.
Avoiding automatic
Setpoint adjustments
To avoid this behavior and run the test relative to the original Setpoint, select a
Command Stop Behavior of Ramp to Zero or Taper to Zero. This way, when
you unlock MPT, the commanded level is equal to the current Setpoint value.
Example Suppose you run a procedure similar to the one in the previous example, except
you select a Command Stop Behavior of Ramp to Zero. When the actuator is
at 20 mm, you stop the test. This causes the actuator to ramp to 0 mm at the
system ramp rate. Next, you unlock the procedure and resume the test. In this
situation, the Setpoint value remains 0 mm, and the actuator ramps at the same
rate back to 20 mm before executing the ±10 mm cyclic waveform.
Making manual
Setpoint adjustments
If the desired combination of Setpoint and Command Stop Behavior for your
situation results in a Setpoint shift, you have to manually adjust the Setpoint to
the desired position before you restart a test.