0195136047.pdf

782 BASIC CONTROL SYSTEMS

Activating

signal u

Controlled

variable c

Reference

input r

Controller Controlledprocess

Figure 16.2.1Elements of an open-loop control system.

The elements of an open-loop control system may be divided into two parts: the controller

and the controlled process, as shown by the block diagram of Figure 16.2.1. An input signal

or commandris applied to the controller, whose output acts as the actuating signalu. The

actuating signal then controls the controlled process such that the controlled variablecwill

perform according to some prescribed standards. The controller may be an amplifier, mechanical

linkage, or other basic control means in simple cases, whereas in more sophisticated electronics

control, it can be an electronic computer such as a microprocessor.

Feedback control systems can be classified in a number of ways, depending upon the purpose

of the classification.

1. According to the method of analysis and design.
   
   
   • Linear versus nonlinear control systems.Linear feedback control systems are idealized
     models that are conceived by the analyst for the sake of simplicity of analysis and
     design. For the design and analysis of linear systems there exist a wealth of analytical
     and graphical techniques. On the other hand, nonlinear systems are very difficult to treat
     mathematically, and there are no general methods that can be used for a broad class of
     nonlinear systems.
   
   
   • Time-invariant versus time-varying systems.When the parameters of a control system are
     stationary with respect to time during the operation of the system, the system is known
     as a time-invariant system. Even though a time-varying system without nonlinearity is
     still a linear system, the analysis and design of such a class of systems are generally
     much more complex than that of linear time-invariant systems.
   
   
   
   


2. According to the types of signal found in the system.
   
   
   • Continuous-data and discrete-data systems.A continuous-data system is one in which
     the signals at various parts of the system are all functions of the continuous-time variable
     t. When one refers to anac control system, it usually means that the signals in the system
     aremodulatedby some kind of modulation scheme. On the other hand, adc control system
     implies that the signals areunmodulated, but they are still ac signals according to the
     conventional definition. Typical components of a dc control system are potentiometers,
     dc amplifiers, dc motors, and dc tachometers; typical components of an ac control system
     are synchros, ac amplifiers, ac motors, gyroscopes, and accelerometers.
   
   
   • Sampled-data and digital control systems.These differ from the continuous-data systems
     in that the signals at one or more points of the system are in the form of either a pulse
     train or a digital code. Sampled-data systems usually refer to a more general class of
     systems whose signals are in the form of pulse data, whereas a digital control system
     refers to the use of a digital computer or controller in the system. The termdiscrete-data
     control systemis used to describe both types of systems.