JUNE 2014 Solved Question Paper
1 a: Explain with examples open loop and closed loop control systems. List merits and demerits
of both. Jun. 2014, 10 Marks
Open & Closed Loop System - Advantages & Disadvantages [Control System].
In control systems engineering, a system is actually a group of objects or elements capable of
performing individual tasks. They are connected in a specific sequence to perform a specific
A system is of 2 types:
1. Open loop system which is also called as Manual control system.
2. Closed loop system which is also named as automatic control system.
In this post, we will be discussing various advantages and disadvantages of the 2 types of control
Open Loop System:
Advantages: 1. Simplicity and stability: they are simpler in their layout and hence areeconomical and stable too due to their simplicity.
2. Construction: Since these are having a simple layout so are easier to construct.
Disadvantages: 1. Accuracy and Reliability: since these systems do not have a feedback mechanism, so they are very inaccurate in terms of result output and hence they are
2. Due to the absence of a feedback mechanism, they are unable to remove the disturbances occurring from external sources.
Closed Loop System:
Advantages: 1. Accuracy: They are more accurate than open loop system due to their complex construction. They are equally accurate and are not disturbed in the presence of non-
2. Noise reduction ability: Since they are composed of a feedback mechanism, so they clear out the errors between input and output signals, and hence remain unaffected to
the external noise sources.
Disadvantages: 1. Construction: They are relatively more complex in construction and hence it adds up to the cost making it costlier than open loop system.
2. Since it consists of feedback loop, it may create oscillatory response of the system and it also reduces the overall gain of the system.
3. Stability: It is less stable than open loop system but this disadvantage can be striked off since we can make the sensitivity of the system very small so as to make the
system as stable as possible.
Open Loop Control System
A control system in which the control action is totally independent of output of the system then it
is called open loop control system. Manual control system is also an open loop control system.
Fig - 1 shows the block diagram of open loop control system in which process output is totally
independent of controller action.
Open-loop Motor Control
So for example, assume the DC motor controller as shown. The speed of rotation of the motor
will depend upon the voltage supplied to the amplifier (the controller) by the potentiometer. The
value of the input voltage could be proportional to the position of the potentiometer.
If the potentiometer is moved to the top of the resistance the maximum positive voltage will be
supplied to the amplifier representing full speed. Likewise, if the potentiometer wiper is moved
to the bottom of the resistance, zero voltage will be supplied representing a very slow speed or
Then the position of the potentiometers slider represents the input, i which is amplified by the
amplifier (controller) to drive the DC motor (process) at a set speed N representing the
output,o of the system. The motor will continue to rotate at a fixed speed determined by the
position of the potentiometer.
As the signal path from the input to the output is a direct path not forming part of any loop, the
overall gain of the system will the cascaded values of the individual gains from the
potentiometer, amplifier, motor and load. It is clearly desirable that the output speed of the motor
should be identical to the position of the potentiometer, giving the overall gain of the system as
However, the individual gains of the potentiometer, amplifier and motor may vary over time with
changes in supply voltage or temperature, or the motors load may increase representing external
disturbances to the open-loop motor control system.
But the user will eventually become aware of the change in the systems performance (change in
motor speed) and may correct it by increasing or decreasing the potentiometers input signal
accordingly to maintain the original or desired speed.
The advantages of this type of open-loop motor control is that it is potentially cheap and
simple to implement making it ideal for use in well-defined systems were the relationship
between input and output is direct and not influenced by any outside disturbances. Unfortunately
this type of open-loop system is inadequate as variations or disturbances in the system affect the
speed of the motor. Then another form of control is required.
Practical Examples of Open Loop Control System
1. Electric Hand Drier Hot air (output) comes out as long as you keep your hand under the
machine, irrespective of how much your hand is dried.
2. Automatic Washing Machine This machine runs according to the pre-set time
irrespective of washing is completed or not.
3. Bread Toaster - This machine runs as per adjusted time irrespective of toasting is
completed or not.
4. Automatic Tea/Coffee Maker These machines also function for pre adjusted time only.
5. Timer Based Clothes Drier This machine dries wet clothes for pre adjusted time, it
does not matter how much the clothes are dried.
6. Light Switch lamps glow whenever light switch is on irrespective of light is required or
7. Volume on Stereo System Volume is adjusted manually irrespective of output volume
Closed Loop Control System
Control system in which the output has an effect on the input quantity in such a manner that the
input quantity will adjust itself based on the output generated is called closed loop control
system. Open loop control system can be converted in to closed loop control system by
providing a feedback. This feedback automatically makes the suitable changes in the output due
to external disturbance. In this way closed loop control system is called automatic control
system. Figure below shows the block diagram of closed loop control system in which feedback
is taken from output and fed in to input.
Closed-loop Motor Control
Any external disturbances to the closed-loop motor control system such as the motors load
increasing would create a difference in the actual motor speed and the potentiometer input set
This difference would produce an error signal which the controller would automatically respond
too adjusting the motors speed. Then the controller works to minimize the error signal, with zero
error indicating actual speed which equals set point.
Electronically, we could implement such a simple closed-loop tachometer-feedback motor
control circuit using an operational amplifier (op-amp) for the controller as shown.
Closed-loop Motor Controller Circuit
Practical Examples of Closed Loop Control System
1. Automatic Electric Iron Heating elements are controlled by output temperature of the
2. Servo Voltage Stabilizer Voltage controller operates depending upon output voltage of
3. Water Level Controller Input water is controlled by water level of the reservoir.
4. Missile Launched & Auto Tracked by Radar The direction of missile is controlled by
comparing the target and position of the missile.
5. An Air Conditioner An air conditioner functions depending upon the temperature of the
6. Cooling System in Car It operates depending upon the temperature which it controls.
Comparison of Closed Loop And Open Loop Control System
Sr. No. Open loop control system Closed loop control system
1 The feedback element is absent. The feedback element is always present.
2 An error detector is not present. An error detector is always present.
3 It is stable one. It may become unstable.
4 Easy to construct. Complicated construction.
5 It is an economical. It is costly.
6 Having small bandwidth. Having large bandwidth.
7 It is inaccurate. It is accurate.
8 Less maintenance. More maintenance.
9 It is unreliable. It is reliable.
10 Examples: Hand drier, tea maker Examples: Servo voltage stabilizer,
1 b: Draw the electrical network based on torque-current analogy give all the perfor-mance
equations for Figure. 1 Jul. 2014, 10 Marks
T 1 k1
3 a: Draw the transient response characteristics of a control system to a unit step input
and define the following: i) Delay time; ii) Rise time; iii) Peak time;
iv)Maximum overshoot; v) Settling time 5 Marks
3 b: Derive the expressions for peak time tp for a second order system for step input. 5 Marks
Jun. 2014, 4 Marks
3 c: The response of a servo mechanism is c(t) = 1 + 0.2e 60t
to a unit step input. Obtain an expression for closed loop transfer function. Determine
the undamped natural frequency and damping ratio. 6Marks Jun. 2014, 4 Marks
3 d: The open loop transfer function of a unity feedback system is given by G(s) =K/S(ST+1)
s(Ts + 1)
where K and T are positive constant. By what factor should the amplifier, gain K be re-
duced so that the peak overshoot of unit step response of the system is reduced from 75%
to 25%. 4 Marks Jun. 2014, 6 Marks
4 a: Explain Routh-Hurwitz criterion in stability of a control system. What are the disad-
vantages of RH criterion on stability of control system? Jun. 2014, 4+4 Marks
4 b: The characteristics equation for certain feedback control system is given below. Deter-mine
the system is stable or not and find the value of K for a stable system
s3 + 3Ks
2 + (K + 2)s + 4 = 0. Jun. 2014, 6 Marks
4 c: The open-loop TF of a unity negative feedback system is given by
K(s + 3)
s(s2 + 2s + 3)(s + 5)(s + 6)
Find the value of K of which the closed loop system is stable. Jun. 2014, 6 Marks
5 a: For a unity feedback system, the open-loop transfer function is given by
. Jun. 2014, 15 Marks
s(s + 2)(s2 + 6s + 25)
(i) Sketch the root locus for K .
(ii) At what value of K the system becomes unstable.
(iii) At this point of instability, determine the frequency of oscillation of the system.
5 b: Consider the system with G(s)H(s) =
using angle condition find
s(s + 2)(s + 4)
whether s = 0.75 and s = 1 + j4 are on the root locus or not. Jun. 2014, 5 Marks
s = 0.75 and s = 1 + j4 are not on the root locus.
6 a: Explain the procedure for investigating the stability using Nyquist criterion. Jun. 2014, 8 Marks, 6 Marks
Nyquist Stability Criterion The Nyquist plot allows us also to predict the stability and performance of a closed-loop system
by observing its open-loop behavior. The Nyquist criterion can be used for design purposes
regardless of open-loop stability (remember that the Bode design methods assume that the
system is stable in open loop). Therefore, we use this criterion to determine closed-loop
stability when the Bode plots display confusing information.
The Nyquist diagram is basically a plot of G(j* w) where G(s) is the open-loop transfer function
and w is a vector of frequencies which encloses the entire right-half plane. In drawing the
Nyquist diagram, both positive and negative frequencies (from zero to infinity) are taken into
account. We will represent positive frequencies in red and negative frequencies in green. The
frequency vector used in plotting the Nyquist diagram usually looks like this (if you can imagine
the plot stretching out to infinity):
However, if we have open-loop
poles or zeros on the jw axis, G(s)
will not be defined at those points,
and we must loop around them
when we are plotting the contour.
Such a contour would look as
Please note that the contour loops
around the pole on the jw axis. As
we mentioned before, the Matlab
nyquist command does not take
poles or zeros on the jw axis into
account and therefore produces an
The Cauchy criterion
The Cauchy criterion (from complex analysis) states that when taking a closed contour in the
complex plane, and mapping it through a complex function G(s), the number of times that the
plot of G(s) encircles the origin is equal to the number of zeros of G(s) enclosed by the frequency
contour minus the number of poles of G(s) enclosed by the frequency contour. Encirclements of
the origin are counted as positive if they are in the same direction as the original closed contour
or negative if they are in the opposite direction.
When studying feedback controls, we are not as interested in G(s) as in the closed-loop transfer
1 + G(s)
If 1+ G(s) encircles the origin, then G(s) will enclose the point -1.
Since we are interested in the closed-loop stability, we want to know if there are any closed-loop
poles (zeros of 1 + G(s)) in the right-half plane.
Therefore, the behavior of the Nyquist diagram around the -1 point in the real axis is very
important; however, the axis on the standard nyquist diagram might make it hard to see what's
happening around this point
To view a simple Nyquist plot using Matlab, we will define the following transfer function and
view the Nyquist plot:
s - 0.5
Closed Loop Stability
Consider the negative feedback system
Remember from the Cauchy criterion that the number N of times that the plot of G(s)H(s)
encircles -1 is equal to the number Z of zeros of 1 + G(s)H(s) enclosed by the frequency contour
minus the number P of poles of 1 + G(s)H(s) enclosed by the frequency contour (N = Z - P).
Keeping careful track of open- and closed-loop transfer functions, as well as numerators and
denominators, you should convince yourself that:
the zeros of 1 + G(s)H(s) are the poles of the closed-loop transfer function
the poles of 1 + G(s)H(s) are the poles of the open-loop transfer function. The Nyquist criterion then states that:
P = the number of open-loop (unstable) poles of G(s)H(s)
N = the number of times the Nyquist diagram encircles -1
clockwise encirclements of -1 count as positive encirclements
counter-clockwise (or anti-clockwise) encirclements of -1 count as negative encirclements
Z = the number of right half-plane (positive, real) poles of the closed-loop system The important equation which relates these three quantities is:
Z = P + N
Note: This is only one convention for the Nyquist criterion. Another convention states
that a positive N counts the counter-clockwise or anti-clockwise encirclements of -1. The
P and Z variables remain the same. In this case the equation b...