![]() Figure shows a tank with a valve that controls flow of liquid into the tank and some unspecified flow out of the tank. Consider for a moment the problem of liquid level in a tank.It can be viewed from the point of view of continuous control, discrete-state control and composite control. It can be shown that continuous variable regulation can be itself a part of a discrete-state system.To define the difference carefully, we will consider an example contrasting a continuous variable situation with a discrete-state variable situation for the same application.It is important to be able to distinguish between the nature of variables in a discrete-state system and those in continuous control systems. The characteristics are described by “Discrete State Variables”, “Process Specifications” & “Event Sequence Descriptions” In the sense of the previous statements, the discrete state process-control system is the master control system for the entire plant operation Other events may involve regulation of some continuous variable over time or the duration of an event.įor example, it may be necessary to maintain the temperature in some vat at a setpoint for a given length of time. Some of the events may involve the discrete setting of states in the plant-that is, valves open or closed, motors on or off, and so on. Some of these steps must occur in series and some can occur in parallel. Such a process will typically involve many operations or steps. The objective of an industrial process-control system is to manufacture some product from the input raw materials. ![]() In the following sections, we will consider how the event sequence is specified, how it is described, and how a controller can be developed to direct the sequence of events. The objective of the controller of Figure 1 is to direct the discrete-state system through a specified event sequence. For a simple heater, such a sequence might be With these definitions in mind, discrete-state process control is a particular sequence of events through which the process accomplishes some objective. This state is an event that will last until the temperature rises. The event lasts for as long as the input variables remain in the same state and the output variables are left in the assigned state.įor a simple oven, we can have the temperature low and the heater on. An event in the system is defined by a particular state of the system-that is, particular assignment of all output values and a particular set of the input variables. If there are three input variables and three output variables, then a state consists of specification of all six values.īecause each variable can take on two values, there is a total of 64 possible states. ![]() ![]() Each state is discrete in the sense that there is only a discrete number of possible states. Now we define a discrete state of the process at any moment to be the set of all input and output values. ![]() Let us suppose that all measurement input variables (S 1,S 2,S 3) and all control output variables (C 1,C 2,C 3) of the process can take on or be assigned only two values.įor example, valves are open/closed, motors are on/off, temperature is high/low, limit switches are closed/open, and so on. Definition and characteristics of discrete state process controlįigure 1 is a symbolic representation of a manufacturing process and the controller for the process. ![]()
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