Implementing Mid Ranging in a DCS Environment

Mid ranging is an algorithm for controlling one control variable, such as flow or pressure, with two manipulated variables. Although mid ranging is a very well established technique there are a number of practical issues which must be addressed when implementing mid ranging in the DCS. These issues include handicapped operation, bump less transfer and proper handling of saturation conditions. In this article, an algorithm for mid ranging is presented which exploits the inbuilt functionality of the control blocks in a modern DCS. A specific feature of the algorithm is the ability to maintain control even when one of the manipulated variables is out of service. Finally the article also demonstrates how the algorithm is extended to handle three manipulated variables.