MTS Model 286-30 Pore Pressure Intensifier - Manual (Page 27)

Pore Pressure System Operation Considerations
286.30 Pressure Intensifier Operation 27
Use of Error Detectors and Limit Detectors While Pressurizing
Information under “Effects of Thermal Expansion of Pore Fluid” on page 26
describes two situations in which pore pressure can be, or can become, an
uncontrolled variable. These situations, and actually all “normal” pressurizing
procedures, warrant the use of whatever devices are available, on the electronic
controller equipment, that enable system conditions to be monitored and system
operation to be shut down in the event abnormal or undesirable operating
conditions occur.
In the first situation, the controller is controlling pressure but becomes unable to
compensate for additional pressure increase caused by further thermal expansion
if the piston of the pressure intensifier bottoms out and cannot retract any further.
In this case, two monitoring circuits, common to most controllers, would afford
some protection, if properly pre-adjusted.
Error detectors
Error detectors monitor the amplitude of the controller’s error signal (which is
proportional to the difference between command and feedback) and are therefore
always associated with the controlled variable. In the situation where the
controller becomes unable to compensate for further thermal expansion (because
the pressure intensifier is unable to retract any further), the error signal will begin
to increase in magnitude as soon as the controller can no longer compensate, if
further thermal expansion occurs. If the error detectors had been adjusted to be
sensitive to small error levels, the pressurization process would be terminated
very soon after the problem occurred.
Limit detectors
Limit detectors can be set up to monitor the level of variables, whether the
variables are independent (controlled) or dependent (uncontrolled). In the first
situation,
Thermal expansion when operating in pressure control mode , the
controller’s limit detector could also have been set up to terminate the
pressurization procedure in the event that pressure exceeded some pre-
established limit. With both error detectors and limit detectors in use monitoring
the controlled variable, the pressurization procedure would be terminated by
whichever detector reacted first.
In the second situation, Thermal expansion when operating in displacement
control mode , with volumetric displacement the controlled variable, the error
detector could be adjusted to react to some unusual level of error (associated with
the controlled variable, volumetric displacement) and the limit detector could be
set up to actuate if the dependent variable, pressure, exceeded some undesired or
unanticipated level.
Because an unanticipated high pressure level is nearly always of greater concern
than is high volumetric displacement, pressure should always be monitored by
either limit detectors or error detectors. Limit detectors provide some advantage
over error detectors in that precise limit levels are easily established. Error
detectors actuate when feedback (the actual level of the controlled variable)
deviates from command (the desired level of the controlled variable) by some
presettable amount and can detect such errors regardless of the level of the
controlled variable. Both detector types offer unique advantages and should be
used simultaneously and judiciously according to the test situation at hand.