MTS Acumen - Manual (Page 171)

over-perform and become unstable (also referred to as “tuned hot”), the pointer will be positioned to the
right in the red zone.
Best Practices for Auto Tuning
Before auto tuning, read and understand the following:
• Tuning electrodynamic systems is fundamentally different than tuning servohydraulic systems. Therefore,
any best practices observed for tuning servohydraulic systems are likely not applicable when tuning
electrodynamic systems.
• To protect your specimen and fixturing, always enable specimen and fixturing limits and set the action
to C-Stop Interlock. When triggered, a C-Stop Interlock will instantly stop the actuator at its current
position to avoid damage to the specimen.
• To protect your specimen, enter Minimum and Maximum force values that are safe for your specimen.
— Force values for auto tuning can be much lower than test loads for linear specimens. Usually 10 to
20% is adequate.
— The force applied when the system auto tunes may be slightly greater than the value entered for
Maximum force.
— Ensure you enter Maximum and Minimum force values that represent your test type. For instance,
if you set polarity to Compression is Positive in the Install Fixturing node, then a
compression-compression test would have positive values.
• Always start auto tuning (Auto tune) and verification (Verify) from a near zero force value, and always
start them from the same values.
• Always use good fixturing and material testing practices.
• Be mindful that backlash in fixturing may cause inaccurate or unstable results.
• The software calculates stiffness, mass, and damping of the entire system, which includes the
characteristics of the entire load frame and actuator.
• The values returned for stiffness, mass, and damping will vary slightly every time you perform auto
tuning due to the nature of the random noise used for command.
• If you have a complex specimen, or one that you are concerned may be damaged during auto tuning,
you can scale down the tuning values that are automatically generated during auto tuning before you
perform the verify function.
• Be aware of specimens that may change stiffness during a test. The best practice for tuning this type
of specimen is to perform tuning in the stiff part of the test region. You can determine specimen
characteristics by performing a displacement ramp in the stable displacement control mode, which
works only on relatively compliant (“soft”) specimens. Very stiff specimens typically have a linear
response.
• Begin tuning with auto tuning at first, and then make manual tuning adjustments after auto tuning is
complete. Avoid manually adjusting the tuning prior to auto tuning (that is, tuning from scratch).
• If a particular specimen is not responding well to auto tuning, use the auto tuning values as a base from
which to perform manual adjustments instead of discarding the auto tuning values and starting manual
tuning from scratch.
• The system response to changes in tuning values is not linear; there is significant interaction between
all tuning parameters.
• The effect and sensitivity of P and I for the inner loop (displacement and force) and the outer loop
(velocity) will change based on the specimen.
MTS Acumen ™ | 171
Best Practices for Other System Configurations and Tests