The Enhanced Control Selector function block is ideal for providing automatic override control. DeltaV includes a faceplate and dynamo for the function block. This function block can take up to 16 control signals as input. You can set SEL_TYPE to Low, Middle, or High for various control configurations; you can select Middle only when there are exactly three inputs. OP_SELECT is a named set used by the default faceplate and dynamo.
In the following image, a Fuzzy Logic Control (FLC) function block is connected to an Enhanced Control Selector function block. Unlike the PID function block that uses a positional control equation, the Fuzzy Logic Control function block uses a velocity control equation and can cause output runaway on all function blocks connected to an Enhanced Control Selector function block if a downstream function block is limited. To prevent output runaway, put a Filter function block on the BKCAL wire between the Enhanced Control Selector function block and the Fuzzy Logic Control function block. For modules executing at one second, set the Filter function block TIMECONST parameter to 3 seconds.
Use the Enhanced Control Selector function block in a situation where flow is the primary control variable but pressure must be controlled in the event that it rises to a dangerous level. The following figure illustrates this example:
In this example, both the flow and pressure PID function blocks control a supply valve. The flow PID function block operates in a pressure regime below a safety limit. Since this is a low pressure for the pressure PID function block, it sends a high output signal to the supply valve, telling it to open more to increase the pressure.
At normal operating pressures, the pressure PID function block's high SP requests a maximum valve opening to increase pressure. The flow PID's signal is lower than that of the pressure PID function block. In this case, SEL_TYPE is set to Low so that the Enhanced Control Selector function block sends the flow PID function block's signal on to the supply valve and blocks the pressure PID function block's signal.
If the flow stream's pressure rises to a dangerous level, the pressure PID functino block sends a low signal to the supply valve, telling it to close. When this output signal falls below that from the flow PID function block, the Enhanced Control Selector function block begins to pass the pressure PID function block's signal and block the flow PID function block's signal.