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Control Systems Interview Questions

Q1. Where is servo mechanism used?


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?


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?


 No loading is present at output i.e. no power is drawn at the output of the system. The system is approximated by a linear lumped constant parameters .    

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


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.


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.

Q6. What is signal flow graph?


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?


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?


Effect of external disturbance is greatly reduced. Effect of parameter variation in controller and process parameters on the system performance is reduced.

Feed back in a control system improves the speed of its response.

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


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

Bandwidth increases (1 + K) times.

Feedback introduces the possibility of instability,negative feedback tends to stablize the system.

Q10. What is regenerative feed back?


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.

Q11.What do you mean by under damped and over damped systems?


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?


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

Q13. On what  factor steady state errors depends?


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?


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.Types of instrument cables?


IS cables & NIS cables

IS – Intrinsic safety & NIS – Non Intrinsic safety

Which cable to use, depends upon hazards condition.

Q17.Instrument JB’s ?


Instrument JB’s depend upon hazards area classification.

JB’s also can be IS or NIS

For IS signal IS JB’s used for NIS signal NIS JB’s used

For analog I/p & o/p signal we can use same JB.

But for Digital I/p & o/p we have to use separate JB’s. Because digital outputs are powered signal, by wrong connection there may be chances to damage the card.

For DCS I/p & o/p and PLC I/p & o/p we used separate JB’s.

Q18. List different types of control system?


There are several types of control systems, which can be widely described in two systems

linear control systems

non-linear control systems

Q19. What is time-invariant System?


Time invariant: A system is considered as time invariance if the behavior of the system is independent of the time at which input is applied. It is the property of the system that makes the system independent of time. For a system to be time-invariant, it must follow the same time shift (delay or advance) between the input signal and the output signal.





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