Safety enhancement for basic process control using 4-20 milliampere signal transmission

To enhance safety of basic process control using traditional 4-20 mA current loops with/without transmitting a return signal of actual actuator position back to a host, this article presents a useful technique to provide fault-state and fault-recovery actions in four patterns. The proposed technique is a design of control drawings configured in a distributed control system (DCS) host for conventional proportional-integral-derivative (PID) loops with/without feedforward path to improve actions of the control loops in response to sensor and actuator failures. The configuration design based on functions of failure status propagation and failure mode shedding for eight control drawings that contain software function blocks for running on the DCS host modeled CENTUM VP is described. Simulation results by employing the DCS virtual test function verify that all designed control drawings can successfully perform the desired fault-state and faultrecovery actions for process safety enhancement.