Specifications

8
MX Protection, Control and Monitoring Features FCD LMENTB2300-01 – 07/08
2. Protection Features
LimiGard
™
Circuit Protection
MX actuators include LimiGard
™
(US Patent 5,719,559 dated February 17, 1998) circuit protection.
LimiGard
™
consists of dedicated circuitry that continually monitors the motor contactor, control
relays, internal logic circuits, and external command signals. When the recommended wiring connec-
tions are made, it virtually eliminates unexpected erroneous actuation caused by internal electronic
failures and erratic external command signals. Additionally, in the event of malfunction, LimiGard
™
supervises the actuator response, detects the source of the failure, and signals an alarm.
The voltage across the control solid-state output drivers is monitored by LimiGard
™
. If the voltage
level remains valid and no motor movement is sensed or motor contractor feedback is detected, then
a motor contactor failure is diagnosed. Motor control solid-state output drivers and series-redundant
solid-state output drivers are controlled by signals that must have the proper form to permit turn-on.
If either the direction driver or the safety driver is damaged, or if the drive signals do not have the
proper form, then the motor contactor will not be able to turn on. Any open circuit, short circuit, or
component failure will alter the waveform, disabling the output and setting a monitor relay alarm.
This unique protection feature prevents inadvertent, unexpected or dangerous valve movement that
could otherwise occur when other circuit components fail. Additionally, LimiGard
™
uses dual series-
redundant solid-state drivers for motor contactor control. These consist of an open or close driver
and an operation enable driver. Both drivers must be on for the motor to run, but only one needs to
be off to stop the valve. The use of two drivers prevents an inability to stop the actuator due to a relay
failure. All LimiGard
™
alarms are signalled by de-energizing the monitor relay and communicated via
the diagnostic screens. Valve operation is inhibited until the failure is corrected; referred to as “Fail No
Action.”
LimiGard
™
requires that external control signals (Open-Stop-Close-ESD-Inhibit) have a minimum
pulse width of 250–350 ms to be considered valid. This minimum pulse width ensures that brief,
noisy signals will not cause the valve to move. The signal must have a level of 17–22 V to turn on the
input, and the input will not turn off until the level drops below 5–10 VDC. This “signal hysteresis”
(> 8 VDC) prevents weak and erratic signals from stopping or initiating valve operation. Additionally,
LimiGard™ incorporates optical couplers to protect the input circuits from high voltage transients on
the remote control signals.
The LimiGard
™
feature may also be extended to external wiring (customer supplied). One method of
securing the external wiring is shown in Figure 12.5. Proper function depends upon setting Remote
Input mode to “LimiGard,” which configures redundant inputs for the OPEN, CLOSE, and STOP
inputs. Both OPEN or CLOSE signals must be present to allow motor motion, while loss of either
STOP signal will stop the actuator and prevent further operation in REMOTE mode. The customer is
responsible for external wiring system safety. LimiGard has been enhanced with the addition of the
18-bit encoder with BIST and redundant sensing circuits.
Autophase Protection and Correction
The phase rotation of the incoming three-phase supply is continuously monitored. In the event
that field wiring is reversed, MX controls automatically correct to ensure the valve operates in the
commanded direction. In addition, the detector circuit monitors the presence of all three phases. If
a phase is lost, valve starting will be prevented and the user alerted via an LCD error message and
Monitor Relay Alarm.
Jammed Valve Protection
If the actuator cannot overcome the required valve starting torque, a jammed valve condition occurs.
Jammed valve protection senses the lack of valve movement and initiates a brief reverse/forward