Flowserve Modutronic 20 II Modulating Position Controlle - Manual (Page 8)

Limitorque Actuation Systems Modutronic 20 II FCD LMENIM4002-00 – 08/06
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Operation
The Modutronic 20 II receives an input signal, normally 4-20 mA, from the external process
controller. This signal is compared (within the Mod 20 II circuitry) to the present position of the
actuator as indicated by the signal from the Feedback Potentiometer attached to the gear train of the
actuator. The comparison of these two signals generates an error signal that dictates the direction
and distance the actuator needs to be driven.
Upon generation of the error signal, the appropriate output triac is enabled and 115-volt control
power is directed to the output contactor (this may be an electromechanical contactor or a solid state
reverser depending upon the application).
As the actuator drives toward the set-point, it enters a region surrounding the set-point where the
control function within the Mod 20 II changes from a continuous power application to a pulsing mode
(proportional band). The Mod 20 II continuously compares the valve’s actual position to the desired
set-point through its circuitry; motor pulsing is continued until the actuator is accurately positioned at
the set-point. The point in the valve travel at which the pulsing mode is enabled and spacing/duration
of the pulses are all user adjustable on the Mod 20 II.