MTS Material Test Systems - Manual (Page 60)

7.4.0 Adjust Displacement Servo Loop (optional)
Note:
If the test is to be run in displacement control, then displacement control loop should be tuned for
optimal performance.
1. Select displacement control.
2. Clear active interlocks.
3. Apply hydraulic pressure.
4. Adjust the displacement (P) and Rate (D) controls.
Note:
The following steps are provided as a guide to the servo loop adjustments. Adjustment of these
controls will vary depending on the test specimen and the components of the test system (for
example, fixtures, type of feedback, hydraulic components, etc). As operator experience is gained,
the steps can be modified to achieve the desired response.
a) Set the Span control to 000.
b) Set up the oscilloscope to monitor displacement feedback. Make the appropriate adjustments for
amplitude and time base on the oscilloscope.
c) Set up the selected programmer to output a low-frequency (1 Hz to 2 Hz) waveform. In most test
situations, a square wave can be used. It will provide the most broad-band, informative response.
When setting up the test system for a sensitive specimen, use a triangle or ramp waveform. A triangle
or ramp waveform should contain sufficient frequency and provide sufficient response to optimize
tuning without damaging the specimen.
d) Start the program.
e) Monitor the program command and slowly adjust the Span control for a small percentage (5% to 10%)
of the maximum amplitude of the test program.
f) Increase Gain (P) to get an overshoot without ringing.
g) While watching the rate of decay oscillation, add Rate (D) and then readjust Gain (P) and Rate (D)
iteratively.
h) Verify the system response at different amplitude levels. Readjust the servo loop controls as necessary.
i) Stop the program command.
60 MTS Material Test System Operation
7.0 Operation