MTS Material Test Systems - Manual (Page 22)

Hydraulic power supply
The hydraulic power unit (HPU) provides pressurized hydraulic fluid to the servovalve. An HPU typically
includes a reservoir for the hydraulic fluid, a pump to pressurize the hydraulic fluid, a motor to run the pump,
a heat exchanger to cool the hydraulic fluid, and sensors to monitor the level, pressure, and temperature of
the hydraulic fluid.
Transducer(s)
The load cell transducer, mounted underneath the load frame crosshead, is used to measure the polarity and
magnitude of the force that is applied to the specimen.
The extensometer is a sensor attached to a specimen that measures a dimensional change (gage length or
strain) that occurs in the specimen while being tested. Extensometers use a Wheatstone bridge circuit to
detect the dimensional changes.
The LVDT (linear variable differential transformer) transducer is mounted inside the hydraulic actuator. The
LVDT is used to measure the axial position (or displacement) of the actuator piston rod. Encoders are used
in axial/torsional systems to measure rotation.
Specimen fixtures or grips
Either fixtures or grips are used to hold the specimen in alignment with the force train during the test. The
force train is comprised of the servohydraulic and mechanical components which receive the forces applied
by the hydraulic actuator.
Environmental products
Optional environmental products can be incorporated into the test system configuration. These include
temperature chambers, furnaces, and associated temperature controllers. When using products to control
specimen temperature, be sure to use other products (for example grips and extensometers) that can withstand
the test temperatures.
Test area enclosure
MTS recommends the use of test area enclosures. Protective enclosures such as cages or shields should
be used when you work with hazardous test specimens (for example, brittle or fragmenting materials or
materials that are internally pressurized). If a test area enclosure is not purchased form MTS< it is the
customer’s responsibility to ensure a safe working environment,
3.3.0 System Control Theory
The test system uses a control technique referred to as “closed-loop control.” The theory of this control
technique and the various control modes and interlock detection methods that can be used are described in
the following subsections.
Closed loop control
In a test system, the test controller (for example the MTS FlexTest 40) provides closed-loop control of the
system’s mechanical and hydraulic components.
22 MTS Material Test System Operation
3.0 Introduction