ABSTRACT
A schematic diagram of a control system is shown in Figure 12.2. The physical dynamic system (e.g., a mechanical system) whose response (e.g., speed) needs to be controlled is called the
plant
or
process
. The device that generates the signal (or, command) according to some scheme (or, control law) and controls the response of the plant, is called the
controller
. The plant and the controller are the two essential components of a
control
system
. Usually the plant has to be monitored and its response needs to be measured using
sensors
and
transducers
, for feeding back into the controller. Then, the controller compares the sensed signal with a desired response as specified externally, and uses the error to generate a proper control signal. This is a
feedback
control
system
. Let us examine the generalized control system represented by the block diagram in Figure 12.3. We have identified several discrete blocks, depending on various functions that take place in a typical control system. In a practical mechatronic control system, this type of clear demarcation of components might be difficult; one piece of hardware might perform several functions, or more than one distinct unit of equipment might be associated with one function. Nevertheless, Figure 12.3 is useful in understanding the architecture of a general control system. This is an analog
control system because the associated signals depend on the continuous time variable; no signal sampling or data encoding is involved in the system. As noted before,
plant
is the system or “process” that we are interested in controlling. By
control,
we mean making the system respond in a desired manner. To be able to accomplish this, we must have access to the
drive system
or
actuator
of the plant. We apply certain
command signals,
or inputs, to the
controller
and expect the plant to behave in a desirable manner. An important factor that we must consider in any practical control system is noise,
including external disturbances. Noise may represent actual contamination of signals or the presence of other unknowns, uncertainties, and errors, such as parameter variations and modeling errors. Furthermore, weak signals will have to be amplified, and the form of a signal might have to be modified at various points of interaction. Identification of the hardware components (perhaps commercially available off-the-shelf items) corresponding to each functional block in Figure 12.3 is one of the first steps of instrumentation of a control system. For example, in process control applications off-the shelf analog proportionalintegral-derivative (PID) controllers are commonly used. These controllers for process control applications have knobs or dials for control parameter settings-that is, proportional band or gain, reset rate (in repeats of the proportional action per unit time), and rate time constant. The control bandwidth (frequency range of operation) of these control devices is specified (see Chapter 5). Various control modes-such as on/off, proportional, integral, and derivative, or their combinations are provided by the same control box.
