Control Objectives

The objectives of any design problem are a statement of what the final system is to achieve, and this applies to control system design as well. Control systems usually have to satisfy many different objectives across a wide spectrum of issues:

• Product quality : targets on quality and maximum acceptable variation may be set
• Inventory control: flow rates through the system and allowable accumulations may be specified
• Safety : limits may be placed on certain key variables; trips and failsafes may need to be designed
• Equipment protection: limits may be placed on some variables to prevent equipment damage, e.g. flowrates through pumps may have a minimum allowable value.
• Environmental protection: quantities and qualities of effluents, and occasional omissions, need to be kept within acceptable limits.
• Efficiency and optimisation: the process needs to operate in a way that maximises its contribution to profitability.

As well as covering a wide range of issues, control objectives also exist at a variety of different levels. For example, a control objective for a complete plant could be to produce 900-1,110 tonnes/year of product. This objective could be decomposed to give control objectives to individual process units, e.g. the exit temperature of a heat exchanger should be held between 95 and 105 ^oC.

Intimately related to objectives in design are constraints. Without constraints design problems wouldn't exist (for example supposed the objective is to land a human on Mars; if there are no constraints of cost, time or the number of people that get killed, the problem isn't really that difficult!). Constraints usually lead to additional control objectives (for example, limits on discharges are imposed by Government; the thickness of metal on a vessel imposes a maximum limit on operating pressure).

Finally, for a control design specification to be useful the most important disturbances have to be identified and quantified. Often this is done by including the disturbance in the control objective; e.g. the outlet concentration should be held within 1% of its nominal value when subjected to inlet concentration changes of 5%, or flowrate changes of 10% of their nominal values.