Distributed Control Systems’ Implementation of Mode

To ease operator transition from panel-based systems to monitor-based systems, the manner in which first generation Distributed Control Systems (DCS) displayed information often mimicked the way this information had earlier appeared on the control panel. Since the control interface provided by the control panel was familiar and had proven to be effective, a similar presentation was adopted in the screen design and display elements used by early distributed control systems. As detailed in Control Loop Foundation, the control faceplate used in distributed control systems can be traced to the manner in which information was displayed in panel based analog controllers. Details of a faceplate, such as showing the low-to-high engineering unit range of the measurement and displaying the value as a small vertical bar, replicated the presentation of the old analog controllers.

The implementation of mode in these early distributed control systems often based the way mode was implemented in old analog controllers. Thus, some DCS manufacturers implemented mode using two parameters; one to switch control from Auto to Man and the other parameter to establish the source of the setpoint or output ( local, cascade or remote). Where mode was implemented as a single switch on the analog controller then the DCS manufacturer often implemented mode as a single parameter.

In these early Distributed Control System the functionality associated with the mode implementation was basically no more than had been provided by an analog controller. Thus, on transition from manual to an automatic mode, it was the responsibility of the operator to make the needed adjustments to insure bumpless transfer. Also, it fell on the operator to detect and respond to an abnormal condition, such as loss of connection to a transmitter used in control. It was necessary for the operator to look at other parameters of the block to determine if the PID was operating in the requested mode. For example, a mode of Auto may have been requested by the operator but to determine if automatic control was possible it necessary to examine other parameters to determine if tracking was active, if the downstream block is in the wrong mode to allow the PID to function, etc. To address these issues, in the late 1980’s the ISA50 function block team lead by Dick Lasher, Exxon, proposed a more comprehensive implementation of mode. Today this definition of mode is fully implemented in all Foundation Fieldbus devices.

As manufacturers introduce their next generation Distributed Control System, some are adopting this more comprehensive implementation of mode. Over the next few blogs I will go into more detail on the mode implementation originally defined by ISA50 and how it impacts the design of a DCS.