MTS Material Test Systems - Manual (Page 50)

Determining limits from known test forces or displacements
If the maximum and minimum test forces or displacements are known, the limit detector levels can be
determined and adjusted to the required levels before specimen installation.
The test configuration, test program, and fixturing are all factors to consider when determining the limit detector
levels. For example, consider the following test situation:
• The test will run in displacement control
• The actuator has a full-scale operating range of 6 inches (±3 inches)
• The actuator is at midstroke after the specimen is installed
• There is a 2-inch clearance between the fixtures and the specimen after the specimen is installed
• The program is a cyclic command of ±0.5 inch
With these conditions, the following displacement limit detector levels can be set:
Upper Limit = +0.75 inch (+25% of full-scale operating range)
Lower Limit = –0.75 inch (–25% of full-scale operating range)
and the limit detector interlock can be enabled. When enabled, the limit detectors will stop actuator rod
displacement when the specimen fails, shutting down the system and helping to protect the fixtures.
Determining limits if test forces or displacements are not known
As previously discussed, the upper and lower limits cannot always be determined before testing. For example,
the load applied to a specimen from a given displacement probably cannot be determined before testing.
If the maximum and minimum test force or displacement values are not known and cannot be determined
before testing, the limit detectors should be adjusted to avoid unwanted system shutdown. Then, they should
be properly adjusted while the test is running.
Determining limits for specimen installation
The upper and lower limit detector levels used for testing may prohibit specimen installation. For example,
displacement limits may prevent piston rod extension or retraction when installing the specimen. Therefore,
limit detector levels should also be determined for specimen installation.
6.3.5.0 Calculate Underpeak Detector Levels
The test controller contains an underpeak detector that monitors a selected signal. This signal is typically a
controller's transducer feedback signal. The underpeak detector can be enabled to shut down the test system
when the signal fails to reach a preset minimum or maximum level.
When the signal input to the underpeak detector does not reach the preset maximum detect level, the
underpeak detector will (if enabled during test setup) stop the test through the system interlock. Likewise, if
the signal does not reach the preset minimum detect level, the underpeak detector will (if enabled during test
setup) stop the test through the system interlock.
50 MTS Material Test System Operation
6.0 Getting Started