4. Concept of Control Systems
What is a Control System?
• A system which controls the output quantity is called a control system.
• To control means to regulate, to direct or to command.
• Control system is an arrangement of different physical elements connected in such a
manner so as to regulate, direct or command itself or some other system.
5. 1. Controlled Variable:
It is the quantity or condition that is measured & controlled.
2. Controller:
Controller means measuring the value of the controlled variable of the system & applying
the manipulated variable to the system to correct or to limit the deviation of the measured
value to the desired value.
3. Plant:
A plant is a piece of equipment, which is a set of machine parts functioning together. The
purpose of which is to perform a particular operation. Example: Furnace, Space craft etc.,
4. System:
A system is a combination of components that works together & performs certain objective.
6. 5. Disturbance:
A disturbance is a signal that tends to affect the value of the output of a system. If a
disturbance is created inside the system, it is called internal. While an external
disturbance is generated outside the system.
6. Feedback Control:
It is an operation that, in the presence of disturbance tends to reduce the difference
between the output of a system & some reference input.
7. Servo Mechanism:
A servo mechanism is a feedback controlled system in which the output is some
mechanical position, velocity or acceleration
7. Classification of Control Systems
• Linear and Non Linear Systems
• Time Variant and Time Invariant Systems
• Continuous and Discrete Systems
• Dynamic and Static Systems
• Open Loop and Closed Loop Systems
8. Linear and Non Linear Systems
• In a linear system, the principle of superposition can be applied.
• In non- linear system, this principle can’t be applied .
• Therefore a linear system is that which obeys superposition principle &
homogeneity.
Examples of Linear System:
Communication channels,
A network that is solely resistive and has a steady DC source
Filter circuits, and others.
Examples of Non-Linear System:
An example of a non-linear system is the triangulation of GPS signals.
A magnetization curve or the no load curve of a DC machine are well-known
examples of non-linear systems
9. Time Variant and Time Invariant Systems
• While operating a control system, if the parameters are unaffected by the time, then
the system is called Time Invariant Control System.
• Most physical systems have parameters changing with time.
• If this variation is measurable during the system operation then the system is called
Time Varying System.
• If there is no non-linearity in the time varying system, then the system may be called
as Linear Time varying System.
10. Continuous and Discrete Systems
Analog or Continuous System
• In these types of control systems, we have a continuous signal as the input to the
system. These signals are the continuous function of time.
Examples of continuous input signal
Sinusoidal type signal input source, square type of signal input source; the signal
may be in the form of continuous triangle etc.
11. Digital or Discrete System
In these types of control systems, we have a discrete signal (or signal may be in the form of pulse)
as the input to the system. These signals have a discrete interval of time.
Now there are various advantages of discrete or digital system over the analog system
Digital systems can handle nonlinear control systems more effectively than the analog type
of systems.
Power requirement in case of a discrete or digital system is less as compared to analog
systems.
Reliability of the digital system is more as compared to an analog system. They also have a
small and compact size.
Digital system works on the logical operations which increases their accuracy many times.
Losses in case of discrete systems are less as compared to analog systems in general.
12. Dynamic and Static Systems
Dynamic Systems
• A system whose response or output depends upon the past or future inputs in addition
to the present input is called the dynamic system.
• The dynamic systems are also known as memory systems.
• Any continuous-time dynamic system can be described by a differential equation or
any discrete-time dynamic system by a difference equation.
Examples
An electric circuit containing inductors and (or) capacitors is an example of dynamic
system. Also, a summer or accumulator, a delay circuit, etc. are some examples of
discrete-time dynamic systems.
13. Static Systems
A system whose response or output is due to present input alone is known as static
system. The static system is also called the memoryless system.
Examples
A purely resistive electrical circuit is an example of static system
14. Open Loop and Closed Loop Systems
Open Loop Systems
• A system is said to be an open loop system when the system’s output has no effect on
the control action.
• In open loop system, the output is neither measured nor fed back for comparison
with the input.
• An open loop control system utilizes an actuating device (or controller) to control the
process directly
15. Advantages
• Simple and ease of maintenance
• Less expensive
• Stability is not a problem
• Convenient when output is hard to measure
Disadvantages
• Disturbances and changes in calibration cause errors
• Output may be different from what is desired
16. Examples
• Washing Machine
• Electric Bulb
• Electric Hand Drier
• Time based Bread Toaster
• Automatic Water Faucet
• TV Remote Control
• Electric Clothes Drier
• Shades or Blinds on a window
• Stepper Motor or Servo Motor
• Inkjet Printers
• Door Lock System
• Traffic Control System
17. Closed Loop Systems
• A system that maintains a prescribed relationship between the output and the
reference input is called a closed-loop system or a feedback control system.
• The system uses a measurement of the output and feedback of the signal to compare
it with the desired output.
Feedback
Feedback control refers to an operation that, in the presence of disturbances, tends to
reduce the difference between the output of a system and some reference input and that
does so on the basis of this difference.
18. Advantages
• Can control for external factors
• More reliable and stable output
• Resilient to disturbances and changes
• More resource-efficient
Disadvantages
• More complex
• Requires tuning or integration
• Susceptible to oscillation or runaway conditions
• Sensor failure can cause unwanted system performance
19. • Automatic Electric Iron –Depending on the temperature of the iron heating
elements were controlled automatically.
• Servo Voltage Stabilizer – Stabilization in voltage is achieved by the feeding the
output voltage back to the system.
• Water Level Controller– Water level in the reservoirs decides the input water into
it.
• Air Conditioner –Air conditioner automatically adjusts its temperature depending
on its room temperature.
• In motor speed regulator using tachometer and/or current sensor , sensor senses the
speed and sends a feedback to the system to regulate its speed.
Examples
20.
21.
22. Effect of Feedback
When feedback is given the error between system input and output is reduced. However
improvement of error is not only advantage. The effects of feedback are
1) Gain is reduced by a factor
G
1±GH
2) Reduction of parameter variation by a factor 1 ± GH.
3) Improvement in sensitivity.
4) Stability may be affected.
5) Linearity of system improves
6) System Bandwidth increases
23. Open loop system Closed loop system
Any change in output has no effect on the
input. i.e., feedback does not exist
Changes in output, affects the input which is
possible by use of feedback.
Output is difficult to measure Output measurement is necessary
Feedback element is absent Feedback element is present
Error detector is absent Error detector is necessary
It is inaccurate and unreliable Highly accurate and reliable
Highly sensitive to the disturbance Less sensitive to the disturbances
Highly sensitive to the environmental
changes
Less sensitive to the environmental changes
Simple in construction and cheap Complicated to design and hence costly
System operation degenerates if the non-
linearities present
System operation degenerates if the non-
linearities present
24. Mathematical Modelling
• The set of mathematical equations, describing the dynamic characteristics of a
system is called Mathematical Modelling of the system.
• Most of the control systems are mechanical or electrical or both types of elements
and components.
• To analyze such systems , it is necessary to convert in mathematical models. This can
be done using
Transfer function approach- applicable to linear time variant systems
Steady state approach - non linear time varying systems and discreet systems
25. Transfer Function Modelling
The relationship between input and output of a system is given by the transfer function.
For a linear time−invariant system the response is separated into two parts : the forced
response and free response.
The forced response depends upon the initial values of input and the free response depends only on the initial
conditions on the output.
The transfer function P(s) of a continuous system is defined as
26. • The denominator is called the characteristic polynomial.
• The transform of the response may be rewritten as
Y(s) = P(s).U(s) + (terms due to all initial values)
• If all the initial conditions are assumed zero then
Y(s) = P(s) U(s)
• And the output as a function of time y(t) is simply
𝐿−1 𝑌 𝑠 = 𝐿−1[P 𝑠 . U 𝑠 ] = y(t)
27. Definition
The transfer function is defined as the ratio of Laplace transform of output to Laplace
transform of input under assumption that all initial conditions are zero.
For example :
System g(t)
r(t) c(t)
r(t) = input
L r(t) = R(s)
c(t) = output
L c(t) = C(s)
g(t) = system function
Lg(t) = G(s)
∴ Transfer function G(s) :
G(s) =
𝐿𝑎𝑝𝑙𝑎𝑐𝑒 𝑇𝑎𝑛𝑠𝑓𝑜𝑟𝑚 𝑜𝑓 𝑜𝑢𝑡𝑝𝑢𝑡
𝐿𝑎𝑝𝑙𝑎𝑐𝑒 𝑇𝑎𝑛𝑠𝑓𝑜𝑟𝑚 𝑜𝑓 𝑖𝑛𝑡𝑝𝑢𝑡
G(s) =
𝐶(𝑠)
𝑅(𝑠)
28. Advantages of Transfer function
• If transfer function of a system is known, the response of the system to any input can
be determined very easily.
• A transfer function is a mathematical model and it gives the gain of the system.
• Since it involves the Laplace transform, the terms are simple algebraic expressions and
no differential terms are present.
• Poles and zeroes of a system can be determined from the knowledge of the transfer
function of the system.
Disadvantages of Transfer function
• Transfer function does not take into account the initial conditions.
• The transfer function can be defined for linear systems only.
• No inferences can be drawn about the physical structure of the system
29. Analysis of Mechanical Systems
In mechanical systems, motion can be of different types i.e. Translational, Rotational or
combination of both.
These systems are governed by Newton’s law of motion
Translational Systems
A system in which motion is taking place along straight line are Translational systems.
These systems are characterized by displacement, linear velocity and linear acceleration.
The following elements are dominantly involved in the analysis of translational motion
systems.
(i) Mass (ii) Spring (iii) Friction
30. Mass
Taking Laplace transform
F(s) = M 𝑠2
X(s)
A mass is denoted by M. If a force f is applied on it and it displays distance x, then
𝑓 = 𝑀
𝑑2𝑥
𝑑𝑡2
If a force f is applied on a mass M and it displays distance x1in the direction of f and
distance x2 in the opposite direction, then
𝑓 = 𝑀
𝑑2𝑥1
𝑑𝑡2 −
𝑑2𝑥2
𝑑𝑡2
Taking Laplace transform
F(s) = M 𝑠2[X1 s − X2 s ]
31. Linear Spring
A spring is denoted by K. If a force f is applied on it and it displays distance x, then
f = Kx
Taking Laplace transform
F(s) = K X(s)
If a force f is applied on a spring K and it displays distance x1in the direction of f and
distance x2 in the opposite direction, then
f = K(𝑥1 − 𝑥2)
Taking Laplace transform
F(s) = K X1(s) – X2(s)
32. Friction
A damper is denoted by B. If a force f is applied on it and it displays distance x, then
f = B
𝑑𝑥
𝑑𝑡
Taking Laplace transform
F(s) = B s X(s)
If a force f is applied on a damper B and it displays distance x1in the direction of f and
distance x2 in the opposite direction, then
f = B[
𝑑𝑥1
𝑑𝑡
-
𝑑𝑥2
𝑑𝑡
]
Taking Laplace transform
F(s) = B s [X1(s) – X2(s)]
35. The static equilibrium of a dynamic system subjected to an external driving
force obeys the following principle,
“For any body, the algebraic sum of externally applied forces resisting
motion in any given direction is zero”.
36. Rotational mechanical system
There are three basic elements in a Rotational mechanical system, i.e.
(a) inertia, (b) spring and (c) damper.
Inertia
A body with an inertia is denoted by J. If a torque T is applied on it and it displays
distance 𝜃, then
𝑇 = 𝐽
𝑑2𝜃
𝑑𝑡2
If a torque T is applied on a body with inertia J and it displays distance θ1 in the direction
of T and distance θ2 in the opposite direction, then
𝑇 = 𝐽
𝑑2𝜃1
𝑑𝑡2 -
𝑑2𝜃2
𝑑𝑡2
37. Spring
A spring is denoted by K. If a torque T is applied on it and it displays distance θ, then
T =Kθ.
If a torque T is applied on a body with inertia J and it displays distance θ1 in the direction
of T and distance θ2 in the opposite direction, then
T = K θ1- θ2
Damper
A damper is denoted by D. If a torque T is applied on it and it displays distance θ, then
T = D
𝑑𝜃
𝑑𝑡
If a torque T is applied on a body with inertia J and it displays distance θ1 in the
direction of T and distance θ2 in the opposite direction , then
T = D
𝑑𝜃1
𝑑𝑡
-
𝑑𝜃2
𝑑𝑡
38. Electrical Analogous of Mechanical Translational systems
• Two systems are said to be analogous to each other if the following two conditions
are satisfied.
The two systems are physically different.
Differential equation modelling of those two systems are same.
• Electrical systems and mechanical systems are two physically different systems.
• There are two types of electrical analogies of translational mechanical systems.
Those are
Force Voltage analogy and Force Current analogy.
39. Force Voltage Analogy
In force voltage analogy, the mathematical equations of translational mechanical system
are compared with mesh equations of the electrical system.
Consider the following translational mechanical system shown in the following figure.
F=Fm+ Fb+ Fk
F = 𝑀
𝑑2𝑥
𝑑𝑡2 + B
𝑑𝑥
𝑑𝑡
+ 𝐾𝑥 (1)
40. • Consider the electrical system consists of a resistor, an inductor and a capacitor.
• All these electrical elements are connected in a series.
• The input voltage applied to this circuit is V volts and the current flowing through
the circuit is i amps.
V = 𝑅𝑖 + 𝐿
𝑑𝑖
𝑑𝑡
+
1
𝐶
𝑖 𝑑𝑡
Substitute, 𝑖 =
𝑑𝑞
𝑑𝑡
, then 𝑉 = 𝑅
𝑑𝑞
𝑑𝑡
+ 𝐿
𝑑2𝑞
𝑑𝑡2 +
1
𝐶
𝑞 (2)
41. Comparing equations ( 1) and (2)
Translational Mechanical System Electrical System
Force(F) Voltage(V)
Mass(M) Inductance(L)
Frictional Coefficient(B) Resistance(R)
Spring Constant(K) Reciprocal of Capacitance (1/C)
Displacement(x) Charge(q)
Velocity(v) Current(i)
42. Force Current Analogy
• In force current analogy, the mathematical equations of the translational mechanical
system are compared with the nodal equations of the electrical system.
• Consider the electrical system consists of current source, resistor, inductor and
capacitor.
• All these electrical elements are connected in parallel.
𝑖 =
𝑉
𝑅
+
1
𝐿
𝑉 𝑑𝑡 + 𝐶
𝑑𝑉
𝑑𝑡
(3)
Substitute, V =
𝑑∅
𝑑𝑡
, then
𝑖 = C
𝑑2∅
𝑑𝑡2 +
1
𝑅
𝑑∅
𝑑𝑡
+
1
𝐿
∅ (4)
43. Comparing equations ( 3) and (4)
Translational Mechanical System Electrical System
Force(F) Current(i)
Mass(M) Capacitance(C)
Frictional coefficient(B) Reciprocal of Resistance(1/R)
Spring constant(K) Reciprocal of Inductance(1/L)
Displacement(x) Magnetic Flux(Ø)
Velocity(v) Voltage(V)
44. BLOCK DIAGRAM REDUCTION
• In order to draw the block diagram of a practical system each element of practical
system is represented by a block.
• For a closed loop system, the function of comparing the different signals is indicated
by the summing point while a point from which signal is taken for the feedback
purpose is indicated by take off point in block diagrams.
• A block diagram has following five basic elements associated with it.
1) Functional Blocks
2) Transfer functions of elements shown inside the functional blocks
3) Summing points
4) Take off points
5) Arrow
47. Rules for Block Diagram Reduction
Rule 1 : Associative Law
Now even though we change the position of the two summing points, output remains
same
Thus associative law holds good for summing points which are directly connected to each
other.
48. Rule 2:
Here G1 and G2 are in series and can be combined. But because of the take off point
G3 cannot be combined.
50. Time Response
In time domain analysis, time is the independent variable. When a system is given
an excitation, there is a response (output).
Definition: The response of a system to an applied excitation is
called “Time Response” and it is a function of c(t).
Time Response – Example
The response of motor’s speed when a command is given to increase the speed is shown
in figure,
As seen from figure, the motors speed gradually picks up from 1000 rpm and moves towards 1500 rpm. It
overshoots and again corrects itself and finally settles down at the last value
3
51. Generally speaking, the response of any system thus has two parts
Transient Response
Steady State Response
• That part of the time response that goes to zero as time becomes very large is called
as “Transient Response”
i.e.
• As the name suggests that transient response remains only for some time from
initial state to final state.
L c ( t ) 0
t
52. 4
From the transient response we can know;
When system begins to respond after an input is given.
How much time it takes to reach the output for the first time.
Whether the output shoots beyond the desired value & how much.
Whether the output oscillates about its final value.
When does it settle to the final value.
• That part of the response that remains after the transients have died out is called
“Steady State Response”.
From the steady state we can know;
How long it took before steady state was reached.
Whether there is any error between the desired and actual values.
Whether this error is constant, zero or infinite i.e. unable to track the input.
54. Standard Test Signal
• It is very interesting fact to know that most control systems do not know what
their inputs are going to be.
• Thus system design cannot be done from input point of view as we are unable to
know in advance the type input.
Need of Standard Test Signal
From example;
When a radar tracks an enemy plane the nature of the enemy plane’s variation
is random.
The terrain, curves on road etc. are random for a drives in an automobile system.
The loading on a shearing machine when and which load will be applied or
thrown of.
55. Thus from such types of inputs we can expect a system in general to get an input
which may be;
a) A sudden change
b) A momentary shock
c) A constant velocity
d) Aconstant acceleration
Hence these signals form standard test signals. The response to these
signals is analyzed. The above inputs are called as,
a) Step input - Signifies a sudden change
b) Impulse input – Signifies momentary shock
c) Ramp input – Signifies a constant velocity
d) Parabolic input – Signifies constant acceleration
Shadab. A. Siddique
56. Standard Test Signal
Step Input
Mathematical Representations
r(t) = R. u(t)
= 0
Graphical Representations
t>0
t<0
Shadab. A. Siddique
This signal signifies a sudden change in the reference input r(t) at time t=0
Laplace Representations L 𝑅 𝑢(𝑡) =
𝑅
𝑠
57. Unit Step Input
Mathematical Représentations
r(t) = 1. u(t)
= 0
t>0
t<0
Graphical Representations
This signal signifies a sudden change in the reference input r(t) at time t=0
Laplace Representations L 𝑢(𝑡) =
1
𝑠
58. Ramp Input
Mathematical Representations
r(t) = R.t
= 0
t>0
t<0
Graphical Representations
Signal have constant velocity i.e. constant change in it’s value w.r.t. time
Laplace Representations L 𝑅 𝑡 =
𝑅
𝑠2
Ramp signal is integral of step signal.
59. Unit Ramp Input
Mathematical Representations
r(t) = 1. t
= 0
t>0
t<0
Graphical Representations
Laplace Representations L 1. 𝑡 =
1
𝑠2
61. Impulse Input
r(t) = 𝛿(𝑡)=1
= 0
t>0
t<0
Graphical Representations
Mathematical Representations
The function has a unit value only for t=0.
In practical cases, a pulse whose time approaches zero is taken as an impulse function.
Laplace Representations L 𝛿(𝑡) = 1
