Signals and Systems - Electrical Engineering

8 C H A P T E R 0: From the Ground Up!

for network services that it can offer to its users. Thus, SDR and CR are bound to change the way we

communicate and use network services.

0.2.3 Computer-Controlled Systems

The application of computer control ranges from controlling simple systems such as a heater (e.g.,

keeping a room temperature comfortable while reducing energy consumption) or cars (e.g., con-

trolling their speed), to that of controlling rather sophisticated machines such as airplanes (e.g.,

providing automatic flight control) or chemical processes in very large systems such as oil refineries.

A significant advantage of computer control is the flexibility computers provide—rather sophisticated

control schemes can be implemented in software and adapted for different control modes.

Typically, control systems are feedback systems where the dynamic response of a system is changed to

make it follow a desirable behavior. As indicated in Figure 0.4, the plant is a system, such as a heater,

car, or airplane, or a chemical process in need of some control action so that its output (it is also

possible for a system to have several outputs) follows a reference signal (or signals). For instance, one

could think of a cruise-control system in a car that attempts to keep the speed of the car at a certain

value by controlling the gas pedal mechanism. The control action will attempt to have the output of

the system follow the desired response, despite the presence of disturbances either in the plant (e.g.,

errors in the model used for the plant) or in the sensor (e.g., measurement error). By comparing the

reference signal with the output of the sensor, and using a control law implemented in the computer,

a control action is generated to change the state of the plant and attain the desired output.

To use a computer in a control application it is necessary to transform analog signals into digital

signals so that they can be inputted into the computer, while it is also necessary that the output of

the computer be converted into an analog signal to drive an actuator (e.g., an electrical motor) to

provide an action capable of changing the state of the plant. This can be done by means of ADCs

and DACs. The sensor should also be able to act as a transducer whenever the output of the plant is

Digital

computer

DAC Plant

Sensor

ADC

Clock

+

−

r(t)

w(t)

y(t)

v(t)

FIGURE 0.4

Computer-controlled system for an analog plant (e.g., cruise control for a car). The reference signal isr(t)(e.g.,

desired speed) and the output isy(t)(e.g., car speed). The analog signals are converted to digital signals by an

ADC, while the digital signal from the computer is converted into an analog signal (an actuator is probably

needed to control the car) by a DAC. The signalsw(t)andv(t)are disturbances or noise in the plant and the

sensor (e.g., electronic noise in the sensor and undesirable vibration in the car).