VALVE CONVERTER/POSITIONER WITH REMOTE FEEDBACK AND MEMORY

摘要

A valve positioning mechanism, configurable as either a positioner or a converter, acts in response to a control signal supplied over a two-wire transmission line by a remotely-located controller, and provides a feedback signal over the same two-wire line to the controller. Depending on the configuration used, this feedback signal verifies either that the valve has assumed the position directed by the controller or that the actuating pressure to the valve is as directed by the controller. The control signal triggers an electropneumatic switch which initiates the flow of air to operate a pneumatic mechanism driving the valve stem. A variable inductor, its inductance value being dependent upon the valve position/actuating pressure, forms part of an oscillator whose ouput frequency varies with the inductance value. The oscillator in turn provides a measurement signal to the controller over the two-wire transmission line, the frequency of the signal being proportional to the valve position/actuating pressure. The controller terminates adjustment of the valve when the measurement signal equals the controller setpoint. Upon loss of power, the valve can be maintained at a pre-existing position, or, in an optional arrangement, be returned to a fail-safe condition, with or without a predetermined delay.