EC2255- Control Systems
UNIT I-CONTROL SYSTEM MODELING
Basic Elements of Control System – Open loop and Closed loop systems - Differential
equation - Transfer function, Modeling of Electric systems, Translational and rotational
mechanical systems - Block diagram reduction Techniques - Signal flow graph
1.1. What is meant by a system?
It is an arrangement of physical components related in such a manner as to form
an entire unit.
1.2. What is a command input?
It is the excitation applied to a control system from an external source. It is also a
motivating input signal to the system, which is independent of the output of the system.
1.3. Define output.
It is the actual response obtained from a control system, which must be
maintained at a prescribed value.
1.4. What is an actuating signal?
It is the difference between the reference input and feedback signal. It is also
called as actuating signal.
1.5. Define plant.
It is the process/body/ machine, of which a particular quantity or condition is to
be controlled.
1.6. List the two types of control systems?
The two types of control system are, open loop systems and closed loop system.
1.7. What is an open loop system?
The control system in which the output has no effect upon the input quantity, is
known as open loop control system.
1.8. What is meant by a controller?
It is the component required to generate the appropriate control signal applied to
the plant.
1.9. Define manually controlled systems?
Systems that involve continuous manual control by a human operator are called
manually controlled system.
1.10. For what purpose feedback element is used?
It is the component required to generate the appropriate control signal applied to
the plant.
Prepared by A.Devasena, Associate Professor, Dept/ECE,
Dhanalakshmi College of Engg. Chennai
,EC2255- Control Systems
1.11. What is a closed loop system?
A system in which output has some effect upon the input quantity, in such a
manner as to maintain the desired output value.
1.12. What is feedback?
The feedback is a control action, in which the output is sampled and a
proportional signal is given to input for automatic correction of any changes in desired
output.
1.13. Give the types of feedback?
Negative feedback
Positive feedback.
1.14. What type of feedback is employed in control system?
Negative type of feedback is employed in control system.
1.15. Define - Servomechanism
It is the power amplifying feedback control system in which the controlled
variable (output) is mechanical position, or a time derivative of position such as velocity
or acceleration. Occasionally it refers to a mechanical system in which the steady state
error is zero for a constant input signal.
1.16. Define - Regulator.
It is the system, in which the output is a steady state value for a variable input
signal.
1.17. When will feedback exist in a system?
Feedback is said to exist in a system, when a closed sequence, of cause and effect
relations exist between systems available.
1.18. Define transfer function.
Transfer function of a given system is defined as the ratio of the laplace transform
of output variable to Laplace transform of input variables at zero input conditions.
1.19.Define order of the system.
The highest power of the complex variables in the denominator of the transfer
function determines the order of the system.
1.20. Define linear system.
A system is said to be linear if it obeys the principle of superposition and
homogeneity. The principle of superposition states that the response of the system to a
weighted sum of the responses of the system to each individual input signals.
1.21. Give the important characteristics of open loop control system.
Their ability to perform accurately is determined by their calibration, which
implies to establish the input-output relation to obtain a desired system accuracy.
Prepared by A.Devasena, Associate Professor, Dept/ECE,
Dhanalakshmi College of Engg. Chennai
,EC2255- Control Systems
1.22. When will the system is said to be linear?
The system is said to be linear, if it satisfies the following two properties:
Adaptive property that is for any x and y belonging to the
domain of the function f, we have
F(x+y) = f(x) +f(y)
Homogeneous property that is for any x belonging to the
domain of the function f and for any scalar constant α
We have
F(αx) = αf(x)
These two properties together constitute a principle of superposition.
1.23. Why most of the systems are non-linear in nature?
Most of the systems are non-linear in nature because of different non-linearities
such as saturation, friction, dead zone etc. present in the system.
1.24.Give the classification of control system.
Broadly control systems are classified as
Natural control systems, manmade control systems, combinational control
systems, time varying and time-invariant systems, linear and non-linear systems,
continuous time and discrete control systems, deterministic and stochastic control
systems, single input single output (SISO) and multiple input multiple output systems,
open loop and closed loop systems.
1.25. Give the advantages of open loop system.
The advantages of open loop system are
1. Such systems are simple in construction.
2. Very much convenient when input is difficult to measure.
3. Such systems are easy for maintenance point of view.
4. Generally these are not troubled with problems of stability.
5. Such systems are simple to design and hence economical.
1.26. Give the disadvantages of open loop system.
The disadvantages of open loop system are
1. Such systems are inaccurate and unreliable because accuracy of such
systems are totally dependent on the accurate precalibration of the
controller.
2. Such systems give inaccurate results if there are variation in the
external environment i.e. systems cannot sense environmental changes.
3. Similarly they cannot sense internal disturbances in the system, after
the controller stage.
1.27. Give the advantages of closed loop system.
The advantages of closed loop system are:
1. accuracy of such system is always very high because controller
modifies and manipulates the actuating signal such that error in the system will be zero.
Prepared by A.Devasena, Associate Professor, Dept/ECE,
Dhanalakshmi College of Engg. Chennai
, EC2255- Control Systems
2. Such systems senses environmental changes, as well as internal
disturbances and accordingly modifies the error.
4. In such system, there is reduced effect of non-linearity and distortions.
1.28. Compare open loop system and closed loop system.
OPEN LOOP SYSTEM CLOSED LOOP SYSTEM
Output measurement is not required Output measurement is
for operation of the system. necessary.
Highly affected by non-linearity Reduced effect of non-linearity
Highly sensitive to the disturbances Less sensitive to disturbances
and environmental changes and environmental changes.
Feedback element and error detector Feedback element and error
are absent detector are absent
Generally stable in nature Stability is the major
consideration while designing.
1.29. Give the difficulties associated with feed forward system.
In some systems, the disturbance may not be measurable.
The feed forward compensation is an open loop technique and if actuator
transfer function is not known accurately, then such compensation cannot be achieved.
1.30. Mention the basic elements of closed loop system {{Madras univ.apr.98}
The basic elements of closed loop system are Command input, reference
transducer, error detector, controller, process to be controlled, and feedback element.
1.31. Define Laplace transform.
The Laplace transform is defined as
Let f(t) be a real function of a variable t defined for t>0, then
f(s) = L{F(t) } = integral of( f(t) e-st) dt.
1.32. For what purpose Laplace transform is used in control system?
The output of any control system, for any input can be obtained by solving integro
differential equations. Mathematically it is very difficult to solve such equations in time
domain. The laplace transform of such integral differential equations converts them into
simple algebraic equations.
1.33. What is meant by modeling?
The process of obtaining the desired mathematical description of the system is
known as modeling.
1.34. The basic models of dynamic physical systems are differential equations.
1.35. List the steps involved in obtsaining the transfer function.
To write the differential equations governing the system.
Prepared by A.Devasena, Associate Professor, Dept/ECE,
Dhanalakshmi College of Engg. Chennai
