Modelling and Experimental Investigation of a Current to Pressure Converter

A Current-to-Pressure (I/P) converter is a device which converts a current input, typically between 4mA and 20mA, to a proportional pressure output. Such devices are often used in valve positioners and regulators to provide accurate and flexible control. This paper describes a physics based mathematical model of an I/P converter implemented using Matlab which is capable of simulating response under dynamic conditions. Detailed physical conditions such as thermal conduction and convection (and their effects on components), compressible orifice flow and inertial movement of components have been incorporated into the model using an explicit time-stepping lumped-parameter scheme. Parameters were obtained for the model using a series of novel experimental procedures. Experimental tests were carried out on a set of I/P converters over a range of operating temperatures and input current sequences. It is shown that the correlation between the simulated results (based on only measured physical characteristics of components) and the experimental test results is quantitatively accurate. It is further shown that the simulation allows the effects of significant design changes to be predicted, and that comparison between experimental and simulated results reveals areas where complex flow behaviour modifies pressure output significantly.