© 2017 Baker Hughes, a GE company, LLC. All rights reserved.130 | BHGEPhysical and Operational DescriptionThe SVI II AP is housed in an industrial, tough, weatherproof, corrosion resistant aluminumhousing that has been designed for operation in hazardous areas as listed in Appendix B.Electrical connections are made through two 1/2” NPT conduit entries. Pneumatic connectionsare made through two or three ¼” NPT ports.Electronics ModuleThe Electronics module consists of an electronic circuit encapsulated in a housing. Theelectronics include a multiplexer, A/D, D/A, temperature sensor, Hall-Effect magnetic positionsensor, pressure sensors, a micro controller, and a power management/distribution circuit.The programs controlling the SVI II AP digital valve positioner are stored in a flash memorythat allows for the downloading of upgraded firmware.A separate non-volatile memory stores configuration information, and continuous diagnosticresults. Expansion capabilities include connectors for the addition of the optional local displaywith pushbuttons. Using the internal programmed positioner algorithm, the CPU computesthe required output based on information received from the measurement sensors. The basemodule has no user repairable components.Magnetic Position SensorA non-contact sensor uses a magnetic field to transfer the position through the wall of thehousing, without penetration, to sense the valve position. A Hall effect device, sealed withinthe electronics housing, senses the rotation of a magnetic assembly mounted on the end of arotary valve shaft or on a driven linkage mounted on a reciprocating valve.The output of the Hall sensor provides the position feedback signal to the position controlalgorithm. The magnetic assembly is environmentally sealed and is entirely external to theelectronics housing (See Figure 15 on page 45). The Hall effect sensor has a maximum travelrange of up to 140° rotation.Position RetransmitThe position sensor also provides, through the electronics module, readout of valve positionon the optional display and communication of valve position via HART® protocol.The position transmission option provides a 4 - 20 mA signal proportional to valve positiontransmitted on a separate pair of leads. A pair of contacts can signal high and low positionlimits.The 4-20 retransmit is galvanically isolated from the 4-20 input on the main board.Pressure SensorThe pressure sensor located in the Electronics Module measures the output of the singleacting relay. The pressure measurement is displayed on the local display or read by a HART®communication device.
