Control Functions

Sequential, regulatory, and discrete control functions interface directly with elements and actuators to change the process. These control functions are defined as:

FIG. 3.13.9 Batch management model.

Discrete control maintains the process states at target values chosen from a set of stable states. Regulatory control maintains the measurements of a process as close as possible to their set-point values during all events, including set-point changes and disturbances.

Sequential control sequences the process through a series of states as a function of time.

The user implements these control functions using devices, loops, and equipment modules, which are defined as:

A device is an item of process equipment that is operated as a single entity and can have multiple states or values. The user initiates discrete states (using hardware and software) to control discrete devices such as solenoid valves, pumps, and agitators. A loop is a combination of elements and control functions which is arranged so that signals pass between the elements to measure and control a process variable. A PID control algorithm is a common control loop function. An example of an equipment module is the sequential control of dehydrogenator bed control valves which put one bed online while the other bed is being regenerated according to a time schedule.

Process interlocking and advanced control, in the form of feedforward, predictive, or model-based control, are ad ditional control functions which provide a higher level of automation for additional benefits.

Compared to continuous processing, additional control algorithms and control methodology are normally used in batch processing. Functions such as time-based PID (heat-soaked ramp), sequencers, and timers are required. Batch processing commonly uses techniques such as enabling and disabling control functions based on a phase state, enabling and disabling alarms on devices and loops, and employing antireset windup protection on PI or PID loops. Batch processes are device-oriented, while continuous processes are loop-oriented.

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