Interview Questions & Answers - Control Systems Part 1

In this article, a set of interview questions and answers on basics of control systems is presented.

Read Interview Questions and Answers on Temperature Measurement

Question 1: What Is Meant By System?

Answer: A system is defined as any physical object comprises of various components or elements which are interconnected with each other in a sequence to perform a specific function.

Example: Automobile, Automatic washing machine

Question 2: What Is Meant By Control System?

Answer: Control system is a system that can control its output(s) to a particular value or perform a sequence of events or perform an event if the specified conditions are satisfied based on the input(s) given.

Question 3: List a few examples for a control system.

Answer: Control of a variable: In an automobile, the speed can be controlled by adjusting the accelerator to the desired level.

Control a sequence of events: In an automatic washing machine, a number of options are available like ‘normal’, ‘heavy’, ‘whites’ etc. The machine undergoes a sequence of washing cycle appropriate to the washing option chosen.

Control whether an event occurs or not: The automatic washing machine has a safety lock on the door so that the machine does not operate if the door is open. Here, the condition is that the door is to be closed for the machine to operate.

Question 4: What are the major types of control systems?

Answer: The two major types of control systems are open loop and closed loop systems.

Read here about the Comparison between open loop and closed loop control systems.

Question 5: What is an open loop system?

Answer: The control system in which the output is generated based on the reference input only is called as an open loop system. The actual value of the output is not measured and it does not modify the input to the process. Also, it has no influence on control action.

Example: Traffic control system where the traffic signals are operated based on the pre-programmed time slot given for each signal.

Question 6: What are the basic elements of an open loop system?

Answer: The basic elements of an open loop control system are, controller, final control element and process.

Question 7: What is a closed loop system?

Answer: The control system in which the actual value output is measured and is used to modify the input to the process and thereby maintain the output at a required value is called as a closed loop system.

Example: Traffic control system where the time slots of each traffic signal are decided based on the density of traffic.

Question 8: What are the basic elements of a closed loop system?

Answer: The basic elements of a closed loop system are, comparator, controller, a final control element, process, and measurement element.

Question 9: What is a feedback in control systems?

Answer: Feedback is a means by which the signal related to the output of the system is feedback to modify the input signal to a process.

When the feedback signal subtracts the input value, it is called negative feedback. It is the negative feedback that is required to control a system. 

Positive feedback occurs when the feedback signal adds to the input.

Question 10: Compare the effect of positive and negative feedback on control systems.
Answer: Positive feedback increases error signal and makes the system unstable. Sometimes, it is used in minor loops of control systems to amplify signals or parameters.
The negative feedback rejects disturbance signals and it is less sensitive to parameter variations. It reduces the system gain that results in better stability.

Question 11: Define the mathematical model of a system.

Answer: The mathematical model consists of a set of differential equations that describes the input-output relations of various physical components of a system. The output of the system can be obtained by solving the differential equations for various input conditions.

Question 12: Define Transfer Function.

Answer: Transfer Function of a control system is defined as the ratio of the Laplace transform of Output to the Laplace transform of the Input with zero initial conditions.

Also, try a short quiz on Control Systems.