MTS Material Test Systems - Manual (Page 52)

• The input to the underpeak detector is the transducer feedback signal from a load cell with a full-scale
operating range of ±25 kN
• There is zero load after the specimen is installed
• Specimen failure is defined as a ±20% drop in load (in respect to a +20 kN initial force at the programmed
maximum and minimum displacements)
With these conditions, the following underpeak detector levels can be set.
Underpeak Max = +16 kN (+64% full-scale load)
Underpeak Min = –16 kN (–64% full-scale load)
and the underpeak detector interlock can be enabled. When enabled, the underpeak detector will stop the
test when the specimen fails, shutting down the system and minimizing the chance of equipment or specimen
damage.
The maximum underpeak level will typically be set at a level 5% to 10% less positive (more negative) than
the peak level of the selected input signal. The minimum underpeak level will typically be set at a level 5%
to 10% less negative (more positive) than the valley level of the selected signal.
Determining underpeak levels if test forces or displacements are not known
If the desired underpeak values are not known, the underpeak detector should be adjusted to prevent damage
to the test system and avoid undesired system shutdown. Then, they should be adjusted while the system
is operating.
6.4.0 Select Test Setup Methods
There are steps in the operating procedure that are optional or that provide optional setup/adjustment substeps.
These include zeroing transducer outputs and adjusting the servo loop controls. The following subsections
describe some of the factors to consider when determining how these setup adjustments should be completed.
6.4.1.0 Servo Loop Adjustments
The servo loop adjustments establish the response and stability of the servo loop. Only the controllers that
have been selected for active control of the test system need to be adjusted for servo loop response and
stability.
The required system response (DC error tolerance) and the specimen fragility should be considered when
adjusting the servo loop controls.
Sensitive specimens that cannot withstand potential actuator oscillation or unstable system operation, require
particularly careful consideration. Optional steps are provided in the operating procedure for sensitive
specimens. The following paragraphs explain how the servo loop controls affect the DC error.
Gain (P)
The gain adjustment determines how well the DC error will follow the program. The greater the gain, the more
the servovalve opens for a given DC error. As proportional gain is increased, the DC error decreases, indicating
closer tracking of the feedback to the command. The following figure shows the program and resulting
transducer feedback signal with a small gain.
52 MTS Material Test System Operation
6.0 Getting Started