Page 141 - MATLAB an introduction with applications

126 ——— MATLAB: An Introduction with Applications

Reference Input R(s): The reference input is an external signal applied to the control system generally at
the first summing input, so as to command a specified action of the process or plant. It typically represents
ideal or desired process or plant output response.
Primary Feedback Signal: The primary feedback signal is a function of the controlled output summed
algebraically with the reference input to establish the actuating or error signal. An open-loop system has
no primary feedback signal.
Actuating or Error Signal: The actuating or error signal is the reference input signal plus or minus the
primary feedback signal.
Positive Feedback: Position feedback implies that the summing point is an adder.
Negative Feedback: Negative feedback implies that the summing point is a subtractor.
Transducer: A transducer is a device that converts one energy form into another.
Disturbance or Noise Input: A disturbance or noise input is an undesired stimulus or input signal affecting
the value of the controlled output.
Time Response: The time response of a system subsystem, or element is the output as a function of time,
generally following the application of a prescribed input under specified operating conditions.

3.6 CONTROL SYSTEM CLASSES

Control systems are sometimes divided into two classes: (a) Servomechanisms and (b) Regulators.
(a) Servomechanisms: A servomechanism is a power-amplifying feedback control system in which the
controlled variable is a mechanical position or a time derivative of position such as velocity or
acceleration. An automatic aircraft landing system is an example of servomechanism. The aircraft follows
a ramp to the desired touchdown point. Another example is the control system of an industrial robot
in which the robot arm is forced to follow some desired path in space.
(b) Regulators: A regulator or regulating system is a feedback control system in which the reference input
or command is constant for long periods of time, generally for the entire time interval during which the
system is operational. Such an input is known as set point. An example of a regulator control system
is the human biological system that maintains the body temperature at approximately 98.6ºF in an
environment that usually has a different temperature.

3.6.1 Supplementary Terminology
(a) Linear System: A linear system is a system where input/output relationships may be represented by
a linear differential equation. The plant is linear if it can be accurately described using a set of linear
differential equations. This attribute indicates that system parameters do not vary as a function of
signal level.
Similarly, the plant is a lumped-parameter (rather than distributed parameter) system if it can be described
using ordinary (rather than partial) differential equations. This condition is generally accomplished if
the physical size of the system is very small in comparison to the wavelength of the highest frequency
of interest.
(b) Time-Variant System: A time-variant is a system if the parameters vary as a function of time. Thus, a
time-variant system is a system described by a differential equation with variable coefficients. A linear
time variant system is described by linear differential equations with variable coefficients. A rocket-
burning fuel system is an example of time variant system since the rocket mass varies during the flight
as the fuel is burned.

F:\Final Book\Sanjay\IIIrd Printout\Dt. 10-03-09