# Control Systems Important interview questions Part 1

By Kajal Vats|Updated : May 18th, 2020

Q1. Where is servo mechanism used?

Ans.   Servo mechanism or servo is restricted to feedback control systems in which the controlled variable which is fed to input of servo is mechanical position or time derivatives of position e.g. velocity and acceleration and the output is error signal which is fed along with input to get the desired position, velocity or accelaration at the output.

Q2. What are lumped parameters and how are they modeled?

Ans.   The process of neglecting the spatial dependence of a parameter  by choosing a representative value is called lumping and the corresponding modeling is known as lumped parameter modeling. It is used were spatial variation of parameter is small and it can be ignored.

Q3. List 2 assumptions made in deriving the transfer functions of physical system?

Ans.   1. No loading is present at output i.e. no power is drawn at the output of the system.

1. The system is approximated by a linear lumped constant parameters .

Q4. What do you mean by order of the system?

Ans.   The highest power of the complex variable ‘s’ in the denominator of the open loop transfer function determines the order of the system.

Q5. Explain feed forward compensation.

Ans.   The presence of Disturbance at the input introduces error in the system performance. In systems where the disturbance input can be predicted then its effect can eliminated by a feed forward compensation.

Q6What is signal flow graph?

Ans.   A signal flow graph is a graphical representation of the relationships between the variables of a set of linear algebraic equations. nodes of the graph represents the system variables are connected by directed branches.

Q7. What is Mason’s gain formula?

Ans. Pk = gain of kth forward path.

∆ = determinant of the graph.

= 1 – (Sum of loop gains of all individual loops) + (Sum of gain products of all possible combinations of two non-touching loops) – (Sum of gain products of all possible combinations of three non-touching loops)                 + …………

Q8. What are the advantages of feedback?

Ans.  1.   Effect of external disturbance is greatly reduced.

1. Effect of parameter variation in controller and process parameters on the system performance is reduced.
2. Feed back in a control system improves the speed of its response.

Q9. How feedback effects gain, bandwidth and stability of the system?

Ans.   1.   Closed loop gain is less than open loop gain by (1+K) times in case of negative feedback.

1. Bandwidth increases (1 + K) times.
2. Feedback introduces the possibility of instability,negative feedback tends to stablize the system.

Q10. What is regenerative feed back?

Ans.   Regenerative feedback is also called positive feedback and is given by transfer function  =G(s)/(1-G(s)H(s))

The regenerative feedback is sometimes used for increasing the loop gain of feedback systems to make it work as an oscillator.

Q 11.What do you mean by under damped and over damped systems?

Ans.   In under damped systems, damping ratio is less than one. The time response of an under-damped second order system is a damped sinusoid. As damping ratio increases the response becomes less oscillatory and becomes over-damped for damping ratio greater than one. If we want highest possible speed of response yet non-oscillatory response, system should have damping ratio just less than one.

Q12. What is steady state error?

Ans.   Steady state error is difference between steady state output and desired output.

Q13. On what  factor steady state errors depends?

Ans.   Steady state errors depends on the input type of system .

Q14. What is ‘type’ of system?

Type denotes the number of open loop poles at origin i.e. s=0.

Q15. What is effect of adding a zero to a system?

Ans.   Zero on the real axis near the origin are avoided because the peak over shoot appreciably increases. However, in a sluggish system if a zero is added at  proper position can improve transient response.

Q16. What is PD controller? Why is it used?

Ans.  PD controller is proportional plus derivative controller. A system is said to posses derivative error compensation when the generation of its output depends in some way on the rate of change of actuating signal. The          PD controller can increase the damping factor while natural frequency remaining the same.

Q17. What is integral error compensation? Where is it used?

Ans.  In an integral error compensation, the output response depends in some manner upon the integral of the actuating signal. Integral compensation reduces the steady state error.

Q18. What is PID controller?

Ans.   It is seen that error integration in the forward path eliminates steady-state velocity error but it increases system’s order making it more susceptible to instability. To increase the damping ratio of the dominant poles           of a PI controlled system, we take advantage of combining it with derivative error scheme. Such controller is known as PID controller.

It improves both the steady state as well as transient response of the system.

Q19. System’s dynamic behavior is judged by standard test signals. Why?

Ans.   Since the input signals to control system are not known fully ahead of time,so it is  difficult to express the actual input signal’s mathematically by simple equations. The characteristics of actual signals which severely strain a control system are a sudden shock, a sudden change, a constant velocity and a constant acceleration system dynamic behavior for analysis and design is therefore judged and compared under application of standard test signals and impulse, a step a constant velocity and constant acceleration.

Q20. What is settling time?

Ans.   It is the time required for the response to reach and stay within a specified tolerance band (usually 2% or 5%) of its final value of a second order system. Q21. What do you mean by peak overshoot Mp?

Ans.   It indicates the normalized difference between the time response peak and the steady output and is defined as Peak percent overshoot. Q22. What is stability of a control system?

Ans.   A linear time invariant system is stable if

1. When the system is excited by a bounded input, the output is bounded.
2. In the absence of the input, the output tends towards zero irrespective of initial conditions.

Q23. What is Routh stability criterion for stability?

Ans.   For a system to be stable, it is necessary and sufficient that each term of first column of Routh array be positive i.e. a0 > 0. If this condition is not met, the system is unstable and the number of sign changes of     the terms of the first column of the Routh array corresponds to the number of roots of the characteristic equation in the right half of the s-plane that causes system to be unstable.

Q24. Can relative stability analysis done using Routh stability criterion?if yes how?

Ans.   Since settling time being inversely proportional to the real part of the dominant roots. The relative stability can be specified by requiring that all the roots of the characteristic equation be more negative than a certain value, i.e. all the roots must lie to the left of the lines
s = - s1(s1 > 0). The characteristic equation of the system under study is modified by shifting the origin of the s plane to s = - σ1 i.e. by substitution s = z – s1. If the new characteristic equation in z satisfies the Routh criterion then it implies that all the roots of the original characteristic equation are more negative than -σ1.

Q25. What is root locus?

Ans. Root locus provides a graphical method of plotting the locus of the roots in the s-plane as system parameter/ gain is varied over the complete range of its values. The roots corresponding to a particular value of the system parameter can than be located on the locus .

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