MTS Series 247 Actuators - Manual (Page 17)

Functional Description
Series 247 Actuators Introduction 17
Functional Description
A linear actuator is used to push or to pull on a specimen. A linear actuator
consists of a cylinder that contains a piston and end caps which include the
hydraulic ports to control the piston.
The double-acting, single-ended actuator operates under precision servovalve
control in MTS closed-loop servohydraulic systems. The actuator is a
hydraulically powered piston that can extend or retract (double-acting). It has
unequal piston areas which causes it to develop higher compression forces than
in tension (single-ended).
The actuator has a hollow piston to allow an LVDT (linear variable differential
transformer) to be installed. An LVDT measures the displacement of the actuator
(extension or retraction) and sends the information to the system controller. It can
be replaced with other specialized measuring devices.
Actuator mounting
The actuator can be mounted to a wide variety of fixtures and assemblies
including force and displacement transducers, pedestal bases, clevis (also called
pivots), swivel rod ends, and swivel bases. Either end of the actuator can use
mounting fixtures. When equipped with the appropriate options and accessories,
the actuator can be configured for precision testing of materials, structures and
components. Component test systems usually have one end of the actuator
connected to a custom test frame base and the other end connected to the test
specimen.
Servovalve interface
The actuator requires a servovalve manifold to allow a servovalve to be mounted
to the actuator. The manifold connects to the actuator ports in the end caps and
route the extension and retraction ports to the required foot print of the
servovalve. Hydraulic power is also connected to the manifold and routed to the
servovalve.
Actuator operation
Actuator piston rod movement is accomplished by supplying high- pressure
hydraulic fluid to one side of the actuator piston and opening the other side to a
return line. High-pressure hydraulic fluid is ported into the cylinder through the
retraction port or the extension port. The differential pressure across the piston
forces the piston rod to move. The amount of hydraulic fluid and the speed and
direction of piston rod movement is controlled by a servovalve.
If the piston rod contacts some external reaction point, then a force is applied to
that point equal to the effective piston area times the actuating pressure. The
main criteria for selecting an actuator are the force and stroke (displacement)
required for the job.