Biomedical control systems -FREQUENCY RESPONSE ANALYSIS (Short Questions & Answers) - ITS DEALS WITH PEAK RESPONSE, BODE PLOT, POLAR PLOT, NYQUIST CRITERION, PHASE MARGIN, GAIN MARGIN, FREQUENCY DOMAIN SPECIFICATION, CUT OFF FREQUENCY, BANDWIDTH, CORRELATION, DAMPING RATIO, ETC.,

1. BIOMEDICAL ENGINEERING – BCS – UNIT IV - SHORT QUESTIONS AND ANSWERS
MATHANKUMAR.S, COORDINATOR/BME, V.M.K.V.E.C
BIOMEDICAL CONTROL SYSTEMS
UNIT IV-FREQUENCY RESPONSE ANALYSIS
PART-A
1. Define resonant peak.
It is the maximum value of magnitude of the closed loop frequency response. Larger
the value of the resonant peak, more is the value of the peak overshoot of system for
step input. It is a measure of relative stability of the system.
2. List the advantages of Bode plots.
 Transfer function of system can be obtained from bode plot.
 Data for constructing complicated polar and Nyquist plots can be easily obtained
from Bode plot.
 It indicates how system should be compensated to get desired response.
 Relative stability of system can be studied by calculating gain margin and phase
margin from Bode plot.
3. What is meant by frequency response of system?
The magnitude and phase function of sinusoidal transfer function of a system are
real function of frequency ω and so they are called frequency response.
4. How do you determine the stability of a system by using Nyquist criterion?
For a closed loop system to be stable, G(s) H(s) plot must encircle -1+j0 points as
many number of times of poles that are right hand side of s-plane and encircle if
any must be in clockwise direction.
5. Define gain margin.
In root locus gain K is increased, the system stability reduces and for a certain value
of K, it becomes marginally stable. (Except first and second order systems). So gain
margin is defined as the margin in gain allowable by which gain can be increased till
system on the verge of instability.
6. What is cut-off rate?
The slope of the resultant magnitude curve near the cut-off frequency is called
cut-off rate.

2. BIOMEDICAL ENGINEERING – BCS – UNIT IV - SHORT QUESTIONS AND ANSWERS
MATHANKUMAR.S, COORDINATOR/BME, V.M.K.V.E.C
7. How the gain margin and phase margin be improved?
The easiest way to improve gain margin and phase margin is to reduce the gain.
However this increases steady state error and makes the system sluggish. Better
methods are available. These methods are adding compensating networks are
compensators.
8. What are the two methods of specifying the performance of control system?
i. By a set of specifications in time domain or in frequency domain such as peak
overshoot, setting time, gain margin, phase margin, steady state error etc.
ii. By optimality of certain function
9. List the frequency domain specifications.
The frequency domain specification indicates the performances of the system in
frequency domain are as follows
(i) Resonant peak
(ii) Cut-off rate
(iii) Resonant frequency
(iv) Gain margin
(v) Phase margin
10. Define Bandwidth.
The Bandwidth is the range of frequencies for which the system gain is
more than -3db.
11. What is minimum phase system?
The minimum phase systems are systems with minimum phase transfer functions.
In minimum phase transfer functions, all poles and zeros will lie on the left half of
s-plane.
12. Compare Bode plot and Nyquist plot analysis.
Bode plot is suitable only for minimum transfer functions. But Nyquist plot analysis
is suitable for both non-minimum and minimum transfer.
13. Write short note on the correlation between the time and frequency response.
Correlation exists between time and frequency response of first or second order
systems. The frequency domain specification can be expressed in terms of the time
domain parameters ξ and ωn. For a peak overshoot in time domain there is a
corresponding resonant peak in frequency domain.

3. BIOMEDICAL ENGINEERING – BCS – UNIT IV - SHORT QUESTIONS AND ANSWERS
MATHANKUMAR.S, COORDINATOR/BME, V.M.K.V.E.C
For higher order systems, there is no explicit correlation between time and frequency
response. But if there is a pair of dominant complex conjugate poles, then the
system can be approximated to second order system and the correlation between
time and frequency response can be estimated.
14. What happens to the damping ratio and rise time if the band width is
increased?
A large band width corresponds to a small rise time or fast response so
band width in inversely proportional to the speed of response, so as
bandwidth is increased damping ratio and rise time both reduces.
15. List the frequency domain methods to find the stability of the system.
The commonly used frequency domain methods to sketch the frequency response of
the systems are
 Bode plot
 Polar plot
 Nyquist plot
 Nichol’s chart
16. Define phase margin.
Phase margin is similar to the gain, it is possible to introduce phase lag in
the system. i.e. negative angles without affecting magnitude plot of
G(jω)H(jω). The amount of additional phase lag, which can be introduced in
the system till the system reaches on the verge of instability is called phase
margin.
17. What are the advantages of frequency response analysis?
 Without the knowledge of the transfer function, the frequency response of stable
open loop system can be obtained experimentally.
 For difficult cases, such as conditionally stable systems, nyquist plot is probably
the only method to analyze stability.
 Frequency response can be precisely applied to the systems those do not have
rational transfer function. i.e. 1-TS etc.
 It can be extended to certain non-linear systems.
 The apparatus required for obtaining frequency response is simple and
inexpensive and easy to use.

4. BIOMEDICAL ENGINEERING – BCS – UNIT IV - SHORT QUESTIONS AND ANSWERS
MATHANKUMAR.S, COORDINATOR/BME, V.M.K.V.E.C
18. Give the factors of G(jw) used in the construction of bode plots.
The factors of G(jw) used in the construction of bode plot are:
 Constant gain K
 Poles at the origin
 Poles on the real axis
 Zero on the real axis
 Complex conjugate poles, zeros.
19. What is polar plot?
The polar plot of a sinusoidal transfer function G(jω) is a plot of the magnitude of
G(jω) versus the phase angle/arguments of G(jω) on polar or rectangular
coordinates as ω is varied from zero to infinity.
20. Write the drawback of the Nyquist stability criterion.
The drawback of the Nyquist stability criterion is that unlike the root locus method,
the Nyquist criterion does not give the exact location of the characteristic
equation roots.