Have you ever asked: “If it’s in ‘Auto’ and not ‘Manual’, then why do I have to give it a command?”
Engineering, operations, and maintenance personnel are often confused by the names of control modes on DeltaV faceplates.
To understand DeltaV’s naming convention, first consider the following “ANALOG (PID) LOOP” control application.
The “ANALOG (PID) LOOP” input (measurement) to DeltaV is the flow rate of a fluid through a pipe. DeltaV “commands” the control valve with an output signal to “position” the control valve in order to adjust the flow rate.
The 3 basic control modes for this loop, shown on the DeltaV faceplate are Manual (MAN), Automatic (AUTO) and Cascade (CAS).
Let’s assume nothing else, like interlocks, impacts this loop.
MAN Mode: The operator “commands” the output from DeltaV. In the PID loop example the operator commands the valve position (ex. 40 % open). DeltaV displays the status of the input (PV = Process Variable) of the loop (ex. 10 gal/min). The key part of MAN mode is DeltaV can no longer change the output to the valve based on the input (flow measurement) but will hold the valve position (ex. 40%) as commanded by the operator.
AUTO Mode: The operator “commands” DeltaV with a set point (or desired) value. DeltaV then “AUTO” matically determines what to do with the valve position based on what it gets from the input (measurement) relative to the set point. In the PID loop the operator “commands” a set point flow rate of 10 gallons per minute, for example, and DeltaV adjusts the output control valve position to read 10 gallons per minute input from the flow measurement input.
CAS Mode: DeltaV treats the loop just like AUTO mode with the exception that DeltaV logic determines the set point instead of the operator.
Next, let’s consider the “DISCRETE LOOP” where DeltaV turns on an output to start a pump motor and turns the same output off to stop the pump motor. Again assume no interlocks.
AUTO Mode: Again the operator issues a set point to start the pump. For the “DISCRETE LOOP”, DeltaV turns on the output to turn on the pump motor and waits a preconfigured amount of time (typically about 10 seconds) for an input indicating the pump motor is running. If it’s not on within 10 seconds DeltaV “AUTO”matically alarms and if configured, turns off the pump motor output. If it is on then DeltaV “AUTO”matically maintains the output signal to keep the pump motor running.
Note there is no MAN Mode for the “DISCRETE LOOP”. An AUTO set point command is just like a command in MAN mode. However since all commands are discrete (off or on), commands are limited to things like “Stop” and “Start”. During configuration, selections are made on how to use the loop input.
CAS Mode: Again, DeltaV treats the loop just like AUTO mode with the exception that DeltaV logic determines the set point instead of the operator.
Confusion over this naming convention originates in perception over who (DeltaV or the User) owns the loop. If the DeltaV user considers it theirs, then it is typically and understandably thought that anytime they issue a command or set point the loop is in MAN mode and anytime DeltaV determines what to do with the loop it’s in AUTO mode. The perspective DeltaV uses and therefore is preconfigured for, however, is that it owns the loop. It then considers MAN mode to be when the operator sets the output, AUTO mode to be when it “AUTO”matically determines the output based on the user’s set point, & then CAS to be when DeltaV “cascades” its own set point for control.
This situation presents a challenge to locations with highly experienced operations staff that use a naming convention different than what is preconfigured in DeltaV. The purpose of this article is to lend perspective for adopting the naming standard used by DeltaV. The alternative to this is designing what is then a custom naming convention that is costly, risky, and results in little added value.