A wide variety of processes may be described as having one controlled parameter and two or more manipulated process inputs. The challenge from a control perspective is that there is no unique set of input values that are required to maintain the controlled parameter at setpoint. One of the most common ways of addressing multiple process inputs is known as split-range control. Using this approach, a splitter block is used to map the controller output to multiple manipulated process inputs. The splitter block may be used to define a fixed relationship between the controller output and each manipulated process input as illustrated below.
In this example, the controller output is shown on the x-axis. The input values to the two valves as calculated by the splitter are labeled in this figure as A and B. The splitter defines how each valve is sequenced as the controller output changes from 0 to 100%. From the controller’s perspective, it appears as though there is only one manipulated process input and the splitter is considered to be part of the process. Once the splitter has been configured, the PID block used with the splitter can be commissioned and operated in the same manner as a single-loop controller.
For split-range sequencing, the range of the controller output over which each valve opens and closes must be defined as part of the splitter configuration. By correctly defining the controller output regions over which each valve operates, then at any operating point, a change in the PID output should produce the same response in the controlled parameter. Several examples used in Chapter 13 of Control Loop Foundation – Batch and Continuous Processes to illustrate splitter configuration to provide a linear response.
The splitter block is designed to use the back calculation outputs provided by the analog output blocks to provide bumpless transfer when an analog output block mode is changed from Auto to Cascade. Also, the mode and limit status of the analog output block is reflected in the back calculation status and value provided to the PID block as illustrated below. Thus, bumpless transfer is provided when the splitter block mode transitions from Auto to Cascade. Also, the status information is used to avoid PID windup when the splitter output is limited.
Several split-range control application examples are detailed in chapter 13 of Control Loop Foundation – Batch and Continuous Processes. Also, the split-range control workshop included in that chapter provides several exercises that may be used to further explore split-range control. By accessing the book’s web site, you may complete this split-range control workshop using your web browser. The viewer below may be used to see the solution to this exercise.