MTS Material Test Systems - Manual (Page 23)

Table 3: Simple Control diagram
Description Item
Test command 1
Summing and conditioning 2
Station manager 3
Closed loop 4
Feedback source 5
When reduced to its basic form, a Series 793 test system typically includes these elements. (The command
can also be provided by a function generator or MultiPurpose TestWare).
The configuration of the devices provides a means of comparing a command (programmer output) signal
with a feedback (transducer output) signal to generate a signal that controls the servovalve. The servovalve
controls hydraulic flow to the actuator which moves the actuator piston rod. The actuator piston rod applies
the force required to load or displace the specimen being tested.
The control method is referred to as “closed-loop control” because the process of command, feedback,
comparison, and servovalve control is totally a function of the control circuitry and occurs without operator
interaction.
In closed-loop control, the programmer is used to generate the command signal. The controller provides the
necessary circuitry for the comparison of the command signal and the feedback signal. The controller also
contains the circuits that generate the servovalve control signal and condition the transducer output.
Control modes
Typical control modes that can be used to control the test include load, displacement, and strain.
The control mode uses the controller's summing junction to compare the command signal and the conditioned
feedback signal to produce a DC error signal. The polarity and magnitude of the DC error signal causes the
valve driver circuit to produce a signal that opens the servovalve spool in a direction and by an amount
necessary to allow fluid to flow into the actuator and cause the desired force, displacement, or strain. As the
command and feedback change, the summing junction continuously generates a DC error signal that drives
the servovalve to create the desired actuator response.
MTS Material Test System Operation 23
3.0 Introduction