Common Control System Interview Questions (With Answers)

By Indeed Editorial Team

Published 26 May 2022

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

A control system refers to a set of devices that manage, command, direct and regulate the behaviour of other devices and systems to produce the desired result. Software professionals across industries use it to enhance the operations of domestic applications, the general military and every modern vehicle in the world. Knowing about the commonly asked control system interview questions can help you prepare effectively as a candidate in this field. In this article, we share some frequently asked control system interview questions and discuss their answers while providing tips to help you enhance your responses in the interview.

Common control system interview questions

Here is a list of additional control system interview questions to help you prepare better for the interview:

  • What is the time-invariant system?

  • What is an analogous system?

  • What are the essential components of the feedback control system?

  • What is the gain margin?

  • What is the resonant peak?

Control system interview questions and example answers

Some common control system interview questions include:

1. What is a control system?

Through this question, the interviewers aim to test your understanding of a vital aspect of the job and determine your skills and qualifications. When answering this question, consider presenting a clear and concise response defining the meaning and importance of a control system. You can share types and examples to add more depth to your answer.

Example: 'A control system refers to a set of mechanical or electronic devices that helps manage, control, regulate and command the behaviour of systems using a control loop. This system can range from a small to a large industrial device, depending on the specific purpose, project and industry. When I was working at ABC Technicals, it helped me efficiently perform controlling processes and functions better to manage machines and other relevant operations. There are two control systems, namely the open-loop and the closed-loop.

In an open-loop control system, the control action is independent of the desired output signal. You cannot compare the output and the input signal, so it has no feedback signal. An example of an open-loop system is an automatic washing machine where you can set the operating time manually. Contrarily, the control action in a closed-loop system depends on the desired output. Examples of such a system would be automatic electric irons, voltage stabilisers and air conditioners.'

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2. What are linear and non-linear systems?

These essential systems might form a significant part of your job. Interviewers may want to determine if you have the right expertise, exposure and skills to work on implementing such systems within your project and if you can conduct your daily tasks effectively. Consider sharing the essential features of each of these systems and describe how they are crucial to the overall functioning of a control system.

Example: 'Based on their methods of analysis and design, you can define systems as linear or non-linear. A linear system refers to a system with both homogeneity and superposition properties. In control systems, you can understand superposition through a hypothetical process where an input r1 produces an output c1, while another input r2 results in an output c2. If you apply these inputs together, the output would be the sum of the two outcomes. If your input–output relationship results in a straight line passing through the origin, you know that the system follows the superposition property.

In contrast, a non-linear system does not follow the superposition or the homogeneity property. Here, the output is not directly proportional to the system's input. The stability of a non-linear system depends on its initial status and input. The form of this system can change as you adjust the magnitude of the input at any specific time.'

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3. What is a transfer function, and what are the steps to get one?

This question aims to test your foundational understanding of essential control system concepts to help employers determine if you can successfully contribute to the role. The interviewers may use this question to assess your proficiency in understanding various types of control system functions and your knowledge and experience in deriving them. To answer this question, briefly define the function and share concise steps on how to achieve it.

Example: 'In a control system, the transfer function refers to the ratio between a Laplace transform of output to the Laplace transform of input, given that all the initial conditions are zero. The technician follows several steps to get a transfer function. The first step is to write a differential equation. Step two involves finding out the value of the Laplace transform of the equation by assuming zero as an initial condition.

The third step is to find the output ratio to the system's input, and the last step is to write the equation of gain or G (S). Poles and zeroes constitute a crucial aspect of transfer functions, where poles and zeroes refer to the frequencies of the transfer function when the value becomes zero.'

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4. What is a signal flow graph?

This question tests your conceptual understanding of this essential aspect of control systems. Employers may ask this question to assess if you are familiar with the application of this graph in a control system context and if you have the skills and expertise to create it efficiently. Respond with a concise definition and share a few significant characteristics of this concept to add more depth to your answer.

Example: 'A signal flow graph or SFG is a graphical representation of the relationship between several variables in a set of linear algebraic equations. The primary advantage of such a graph is that you can create it without using a reduction technique or process. With complicated systems, it became difficult and time-consuming to determine the transfer function of the control systems. S.J. Mason addressed this by developing this method to graphically relate the input and output system variables. In the SFG, you can call the transfer function transmittance.

One of the essential characteristics of this method is that it represents a network where you use nodes to define system variables connected by direct branches. Every node and branch of SFG comprises an arrow representing the signal's flow. This method only applies to linear systems. The node in this method refers to the system variable, which equals the sum of all the signals. Branch refers to a path from one node to another in the direction of the branch arrow.'

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5. What is servomechanism in control systems?

A servomechanism is an essential concept in control systems. Interviewers may ask this question to test your foundational understanding. They may also want to test your core theoretical and practical skills to assess if you fit the job description well. When responding, consider explaining the meaning and definition of this concept and highlight the areas where professionals use it to enhance the functioning of control systems.

Example: 'When you combine a servomotor with a rotary encoder or a potentiometer, the result is a servomechanism. In this type of setup, the potentiometer provides an analogue signal to show the position, and the encoder provides feedback on the position and speed of the system. When I was working as a systems engineer at Softforces, I used servomechanism in control systems to vary the mechanical position using the system's output.

During my time there, I also widely used servomechanism to control the governor value position, which power plants use to take the turbine's speed and process it using the transducer. Here, you can consider the final value as a mechanical movement of the value. Some of my colleagues would also use electronic controls that use power thyristors to control the governor's value position. You can also use this mechanism in robotic hand movements.'

6. What is a cable tray? What are its common types, and how do you decide on a cable tray size?

Employers may ask this question to determine if you have adequate knowledge and expertise in understanding and implementing essential control system features. They may also want to assess if you know the theoretical and practical aspects of cable trays to execute projects smoothly. When answering this question, consider explaining the meaning and types of trays and specific methodologies you use to choose a tray size.

Example: 'In control systems, a cable tray refers to the media or method for laying the field cables. You can make these trays using aluminium, steel or fibre-reinforced plastic, also known as FRP. There are six typical cable trays: ladder type cable tray, solid bottom cable tray, trough, channel, wire mesh and single rail cable trays. To choose an adequate size for these, you can consider the occupancy of a cable tray and the number of cables required. Some usual cable tray sizes are 80, 150, 300, 450, 600 and 900.'

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