In order to discuss process control, in detail it is necessary to define several terms. A control loop is comprised of several variables. Among these are manipulated variables (MV), process variables (PV), and disturbance variables (DV). Manipulated variables refer to variables that are easily controlled, such as stream flowrates. Process variables are those that are desired to be set at a certain datum level, commonly a desired temperature. Disturbance variables create deviations in the process variable from the desired datum level or set point (SP). The controller receives process variable information and in turn attempts to maintain the process variable at the set point.

In particular there are two types of control systems, feedforward and feedback control. Feedback control operates by "feeding back" process variable data to the controller. In feedback process c

2) Babatunde, A. Ogunnaike; Ray, W. Harmon. Process Dynamics, Modeling, and Control, Oxford University Press, New York, NY, 1994.

Previously, only the design of a controller has been considered. Although this is an important element, in order for a controller to be effective, it must be fitted appropriately to the process at hand. This is commonly referred to as tuning. Only the qualitative aspects of tuning will be considered.

6) Perry, Robert H.; Green, Don W.; Maloney, James O.; Perry's Chemical Engineers' Handbook, 7th ed, McGraw-Hill, New York, NY, 1997.

The third type of control action is derivative control. The uniqueness of derivative action is that it anticipates the future response of the error signal. The derivative controller accomplishes this anticipation by taking into account the error's rate of change [4]. Consider the situation where a process variable, say pressure, increases ten-fold over a time period of only a few minutes. This is clearly a sharp rise in pressure and could indicate the onset of a predominately uncontrollable system. Were this a manual process the operator would recognize the potential danger and take action to prevent any further pressure increases. Should this process be operated under P or PI control a disaster would surely result. Proportional control and PI control provide no means of predicting or forecasting the future behavior of the process.

Some common words found in the essay are:
Despite PI, Figure II, DV Manipulated, Figure IV, PI PID, , Recall Kc=100/PB, York NY, process variable, derivative control, proportional control, derivative action, controller output, integral control, process control, reset windup, york ny, control action, process variable set, Engineers' Handbook, Cliffs NJ, final control element, process variable data, controller saturated controller, inc york ny,
Approximate Word count = 2097
Approximate Pages = 8 (250 words per page double spaced)