Common questions and answers for PLC engineer interviews

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1. Summary of interview points

PLC engineer interviews are crucial for both job seekers and recruiting companies. It is not only a key link for companies to screen suitable talents, but also an important opportunity for job seekers to demonstrate their professional capabilities and experience. The interview content covers multiple aspects and comprehensively examines the comprehensive qualities of job seekers.

(I) Theoretical foundation issues

Basic electrical engineering knowledge is a necessary foundation for PLC engineers, including circuit principles, electrical safety and other aspects. The difference between servo motors and stepper motors is mainly reflected in control accuracy, response speed and applicable scenarios. Servo motors have higher control accuracy and faster response speed, and are suitable for occasions with high requirements for motion control; while stepper motors are relatively simple and low-cost, and are suitable for some occasions with low requirements for accuracy.
The basic principle of PLC is to control industrial equipment by receiving input signals, executing pre-written program logic, and then outputting control signals. Its role in automation cannot be ignored. It can improve production efficiency, reduce labor costs, and improve system reliability and stability.

(II) Practical Problems

In terms of project experience, job seekers need to give a detailed introduction to the projects they have participated in, including the scale, process, and equipment used. For the use of equipment, you need to be familiar with the characteristics and operation methods of different types of equipment. In terms of communication protocols, you need to understand common communication protocols such as Modbus, Profibus, etc., as well as their application scenarios and characteristics.

(III) Expansion Issues

PLC scanning cycle is generally around 10MS - 15Ms, which consists of steps such as reading input data, executing programming logic and updating output data. There are many types of sensors, such as temperature sensors and pressure sensors. Temperature sensors are divided into thermocouples and thermal resistors. Thermocouples are suitable for high-temperature measurements, while thermal resistors have better performance in medium and low-temperature measurements. Pressure sensors can be used to measure the pressure of liquids or gases.
The main advantages of PLC include high reliability, strong anti-interference ability, simple programming, and easy maintenance. Its modular design makes system expansion and upgrading more convenient, and can adapt to industrial automation needs of different scales and complexities.

II. Common Problem Analysis

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(I) Basic Concepts of PLC

  1. PLC (Programmable Logic Controller) is an electronic device specially used for industrial automation control. It can receive signals from various sensors and input devices, process these signals, and then control the machine or process through the output device. PLC is mainly composed of central processing unit (CPU), input module, output module, memory and other components. In automation, PLC plays a vital role. It can realize the precise control of industrial production process, improve production efficiency, reduce labor costs, and improve the reliability and stability of the system.
Different types of PLC mainly include integral type and modular type. Integral PLC integrates power supply, CPU, memory, I/O interface and other components in one chassis, with the characteristics of compact structure, small size and low price. Modular PLC makes each component into several separate modules, such as CPU module, I/O module, power module, etc. Various modules are installed on the rack or baseboard for easy expansion and maintenance.
Common PLC brands include Siemens, Mitsubishi, Omron, etc. PLCs of different brands vary in performance, function, price, etc. PLC programming languages ​​mainly include ladder diagram, statement list, function block diagram, sequential function diagram, and structured text. Ladder diagram is similar to electrical schematic diagram and is easy to understand and maintain; structured text is similar to traditional programming language and is suitable for complex logic and algorithms; function block diagram is suitable for modular design and reuse.
  1. Introduce the types, brands and programming languages ​​of PLC.
In addition to integrated and modular types, PLC types can also be classified according to their performance and functions. For example, some PLCs are specifically used in small control systems, with small size, low cost, simple functions, etc.; while some PLCs are suitable for large and complex control systems, with high performance, multi-functions, strong scalability, etc.
In terms of brands, in addition to the above-mentioned Siemens, Mitsubishi, and Omron, there are also Schneider, AB and other brands. Each brand has its own unique advantages and characteristics. For example, Siemens PLC is widely used in the field of industrial automation, with advantages such as high stability and powerful functions; Mitsubishi PLC performs well in small control systems, with simple programming and high cost performance.
PLC programming languages ​​are diverse, and each language has its applicable scenarios and advantages. Ladder diagram is the most commonly used programming language. It is intuitive and easy to understand, similar to electrical schematics, and suitable for beginners and on-site debuggers. Statement table is similar to assembly language and requires a certain programming foundation, but it has high execution efficiency and is suitable for occasions with high requirements for program execution speed. Function block diagram and sequential function diagram are suitable for modular design and complex sequential control logic, while structured text is suitable for complex algorithms and logical control.

(II) Instructions and functions

  1. Types and functions of timers and counters.
Timer is one of the commonly used instructions in PLC, which can realize the timing control function. According to the working mode of the timer, it can be divided into power-on delay timer, power-off delay timer and hold-on delay timer. The power-on delay timer starts timing when the input signal is connected, and when the timing time reaches the set value, the timer output signal is connected; the power-off delay timer starts timing when the input signal is disconnected, and when the timing time reaches the set value, the timer output signal is disconnected; the hold-on delay timer starts timing when the input signal is connected, and even if the input signal is disconnected, the timer will continue to count until the timing time reaches the set value, and the timer output signal is connected.
Counter is also one of the commonly used instructions in PLC, which can realize the counting control function. According to the counting mode of the counter, it can be divided into types such as add counter, subtract counter and reversible counter. The up counter counts up when the input signal rises, and when the count value reaches the set value, the counter output signal is turned on; the down counter counts down when the input signal rises, and when the count value decreases to 0, the counter output signal is turned on; the reversible counter can count up and down, and counts according to the direction of the input signal.
  1. Explain the concepts of ladder, rack, scan cycle, etc.
A ladder is a logical row in the ladder diagram, which consists of one or more logic elements and is used to implement specific control logic. The execution order of the ladder is from top to bottom and from left to right.
A rack is a physical structure in a modular PLC, which is used to install various modules, such as CPU modules, I/O modules, power modules, etc. Different types of modules can be installed on different racks to achieve system expansion and upgrade.
A scan cycle is the time required for a PLC to perform a complete program scan. The scan cycle includes three stages: input sampling, program execution, and output refresh. In the input sampling stage, the PLC reads the signal status in the input module; in the program execution stage, the PLC executes the logical operations and control instructions in the user program; in the output refresh stage, the PLC updates the signal status in the output module to the result of program execution. The length of the scan cycle depends on factors such as the performance of the PLC, the complexity of the program, and the number of I/O points.
    Introduce MCR, communication protocol and other instructions and concepts.
MCR (Master Control Relay) is an instruction used to control the execution flow of a program. It can realize segmented control of a program and improve the readability and maintainability of the program. When the input signal of the MCR instruction is connected, the program segment controlled by the MCR instruction is executed; when the input signal of the MCR instruction is disconnected, the program segment controlled by the MCR instruction is skipped.
Communication protocol is the rules and standards followed by PLC when communicating with other devices. Common communication protocols include Modbus, Profibus, Ethernet/IP, etc. Different communication protocols have different characteristics and application scenarios. For example, the Modbus protocol is simple and easy to use, suitable for small control systems; the Profibus protocol is fast and reliable, suitable for medium and large control systems; the Ethernet/IP protocol is based on Ethernet technology, open and universal, and suitable for industrial Ethernet environments.

(III) Programming and Debugging

  1. Common programming languages ​​and characteristics, such as ladder diagrams, etc.
In addition to the programming languages ​​mentioned above, such as ladder diagram, statement list, function block diagram, sequential function chart and structured text, there are some other programming languages, such as instruction list language, high-level language, etc.
The ladder diagram is intuitive and easy to understand, similar to the electrical schematic diagram, suitable for beginners and on-site commissioning personnel. Its programming method is simple, and only needs to connect the logic elements according to certain rules to realize the control logic. However, the execution efficiency of the ladder diagram is relatively low, and it is not suitable for occasions with high requirements for program execution speed.
The statement table is characterized by high execution efficiency and is suitable for occasions with high requirements for program execution speed. However, the programming method of the statement table is relatively complex, requiring a certain programming foundation, and is not suitable for beginners.
Function block diagram is characterized by modular design, easy maintenance and reuse. It encapsulates control logic in function blocks and implements control logic by connecting function blocks. The programming method of function block diagram is relatively simple and suitable for occasions with high requirements for modular design.
The characteristic of sequential function diagram is that it is suitable for complex sequential control logic. It divides the control logic into multiple steps in sequence, and each step has specific actions and transfer conditions. The programming method of sequential function diagram is intuitive and easy to understand, and is suitable for occasions with high requirements for sequential control logic.
The characteristic of structured text is that it is similar to traditional programming languages, has rich syntax and functions, and can implement complex algorithms and logical control. The programming method of structured text is relatively complex and requires a certain programming foundation, which is not suitable for beginners.
  1. How to debug PLC programs and troubleshooting steps.
There are two main methods for debugging PLC programs: simulation debugging and field debugging. Simulation debugging is to use simulation software or hardware to simulate actual input signals and output devices in a laboratory environment to check the correctness and stability of the program. Field debugging is to download the program to the PLC in the actual industrial site and check the correctness and stability of the program through actual input signals and output devices.
The troubleshooting steps mainly include the following aspects:
(1) Check whether the power supply and signal line connections are normal to ensure that the PLC can supply power and receive input signals normally.
(2) Check whether the program logic is correct. Check whether the program logic meets the control requirements by checking the program code, ladder diagram, etc.
(3) Check whether the input and output modules are working properly. Check whether the input and output modules are working properly by checking the indicator lights of the input and output modules and measuring the input and output signals.
（4）Use debugging tools and instruments to track and analyze signals, such as using oscilloscopes, logic analyzers and other tools to track input and output signals, analyze the execution process of the program to find out the cause of the fault.
（5）Perform corresponding processing according to the cause of the fault, such as modifying the program code, replacing the faulty module, etc.

（四）Project experience related

  1. PLC brand and model used in past projects and the reasons for selection.
In past projects, PLCs of different brands and models may have been used, such as Siemens S7-300, Mitsubishi FX3U, Omron CP1H, etc. The reasons for choosing these PLCs may include the following aspects:
(1) Project requirements: Select the appropriate PLC brand and model based on the control requirements, scale and complexity of the project. For example, for a large and complex control system, you may need to choose a PLC with powerful performance and rich functions, such as Siemens S7-400, Mitsubishi Q series, etc.; for a small control system, you may need to choose a PLC with low cost and simple functions, such as Mitsubishi FX1S, Omron CP1E, etc.
(2) Reliability and stability: Select PLC brands and models with high reliability and stability to ensure the normal operation of the system. For example, PLCs of brands such as Siemens, Mitsubishi, and Omron are widely used in the field of industrial automation and have high reliability and stability.
(3) Convenience of programming and debugging: Select PLC brands and models that are easy to program and debug to improve development efficiency. For example, ladder diagram is the most commonly used programming language. For engineers who are familiar with electrical schematics, it will be more convenient to select PLC brands and models that support ladder diagram programming, such as Siemens, Mitsubishi, and Omron.
(4) After-sales service and technical support: Choose a PLC brand and model with good after-sales service and technical support to ensure that you can get timely help in solving problems during use. For example, PLCs of brands such as Siemens, Mitsubishi, and Omron have extensive sales and service networks in China and can provide timely after-sales service and technical support.
  1. Common sensors and actuators and their functional applications.
In industrial automation control systems, commonly used sensors include temperature sensors, pressure sensors, flow sensors, liquid level sensors, etc. Temperature sensors are used to measure the temperature of an object. Common types include thermocouples and thermal resistors. Thermocouples are suitable for high-temperature measurement, with the characteristics of fast response speed and wide measurement range; thermal resistors are suitable for medium and low temperature measurement, with the characteristics of high accuracy and good stability. Pressure sensors are used to measure the pressure of liquids or gases. Common types include strain gauge pressure sensors and piezoresistive pressure sensors. Flow sensors are used to measure the flow of liquids or gases. Common types include electromagnetic flowmeters, turbine flowmeters, vortex flowmeters, etc. Liquid level sensors are used to measure the liquid level of liquids. Common types include float type liquid level sensors and pressure type liquid level sensors.
Common actuators include motors, valves, cylinders, etc. Motors are used to drive mechanical equipment. Common types include AC motors, DC motors, stepper motors, etc. Valves are used to control the flow of liquids or gases. Common types include ball valves, butterfly valves, regulating valves, etc. Cylinders are used to achieve linear motion. Common types include single-acting cylinders and double-acting cylinders.
  1. Sharing the experience of solving complex control logic.
In past projects, you may have encountered some complex control logic, such as multi-axis motion control, complex sequential control, etc. The experience of solving these complex control logic can be shared as follows:
(1) Analyze the problem: First, conduct an in-depth analysis of the complex control logic, understand the control requirements and process flow, and find out the key to the problem.
(2) Design plan: According to the analysis results, design a reasonable control plan and select the appropriate PLC product.
（3）Programming implementation: According to the design plan, use appropriate programming language and programming tools to write the control program. During the programming process, pay attention to the readability, maintainability and scalability of the program.
（4）Debugging and testing: Download the written program to the PLC for debugging and testing. During the debugging and testing process, pay attention to observe the changes in input and output signals, and discover and solve problems in a timely manner.
（5）Optimization and improvement: During the debugging and testing process, some problems and deficiencies may be found, and the program needs to be optimized and improved. The direction of optimization and improvement can include improving the execution efficiency of the program, enhancing the stability and reliability of the system, etc.
  1. Measures to ensure the stability and reliability of PLC programs.
In order to ensure the stability and reliability of PLC programs, the following measures can be taken:
(1) Write clear, modular and maintainable program codes to avoid complex logic and lengthy programs.
(2) Carry out strict testing and verification, including simulation debugging and on-site debugging, to ensure the correctness and stability of the program.
（3）Use error handling and fault detection mechanisms, such as alarm indicators, alarm information prompts, error log records, etc., to promptly discover and handle faults.
（4）Periodically maintain and service the PLC system, including checking the connection status of hardware devices, cleaning equipment, and updating software.
（5）Back up the PLC program so that the system can be restored in time when a fault occurs.
  1. Application and advantages of advanced functions or technologies.
In PLC programming, some advanced functions or technologies can be applied, such as data acquisition and analysis, remote monitoring and control, fault diagnosis and prediction, etc. The application of these advanced functions or technologies can bring the following advantages:
(1) Improve production efficiency: Through data acquisition and analysis, the status and parameters of the production process can be understood in real time, problems can be discovered and adjusted in time, thereby improving production efficiency.
(2) Reduce maintenance costs: Through remote monitoring and control, remote maintenance and management of equipment can be achieved, reducing the workload of on-site maintenance personnel and reducing maintenance costs.
（3）Improve system reliability and stability: Through fault diagnosis and prediction, equipment failure hazards can be discovered in time, maintenance and care can be carried out in advance, equipment failures can be avoided, and system reliability and stability can be improved.
（4）Achieve intelligent control: By applying advanced functions or technologies, intelligent control of the production process can be achieved, and the automation level and intelligence level of the production process can be improved.
  1. Understanding of safety control and project application experience.
In the field of industrial automation, safety control is crucial. Safety control includes safety input/output modules, safety programming specifications, safety monitoring and emergency shutdown systems.
Safety input/output modules have special designs and functions to ensure that power is cut off or equipment is stopped in time in dangerous situations to protect the safety of personnel and equipment. For example, in some dangerous industrial environments, such as chemical and metallurgical industries, safety input/output modules can detect danger signals and take quick measures to avoid accidents.
Safety programming specifications are some rules and standards that must be followed during PLC programming to ensure the safety and reliability of the program. For example, when writing a safety control program, strict programming specifications must be followed to avoid logical errors and loopholes and ensure the correctness and stability of the program.
The safety monitoring system can monitor the operating status and safety parameters of the equipment in real time, discover potential safety hazards in a timely manner, and take appropriate measures to deal with them. For example, in some large industrial equipment, the safety monitoring system can monitor the temperature, pressure, speed and other parameters of the equipment. Once an abnormal situation is found, an alarm will be immediately issued and emergency shutdown measures will be taken.
The emergency shutdown system is a system that can quickly cut off the power supply or stop the operation of the equipment in an emergency. For example, in some dangerous industrial environments, such as chemical and metallurgical industries, the emergency shutdown system can quickly cut off the power supply or stop the operation of the equipment in the event of fire, explosion and other dangerous situations to protect the safety of personnel and equipment.
In project applications, it is necessary to select appropriate safety control equipment and technology according to the specific industrial environment and safety requirements, and strictly follow the safety programming specifications and safety operation procedures to ensure the safety and reliability of the system. At the same time, safety control equipment should be regularly maintained and serviced to ensure its normal operation.

III. Interview preparation suggestions

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(I) Knowledge reserve

  1. Review the basic knowledge of PLC, including concepts, types, brands, programming languages, etc. For example, understand the characteristics of PLCs of different brands, such as Siemens PLCs are known for their high stability and powerful functions, and Mitsubishi PLCs perform well in small control systems, are easy to program, and are cost-effective. At the same time, be familiar with the characteristics and applicable scenarios of various programming languages, such as ladder diagrams are intuitive and easy to understand for beginners, and structured text is suitable for complex algorithms.
  1. In-depth mastery of PLC instructions and functions, such as the different types and functions of timers and counters. The power-on delay timer starts timing when the input signal is connected, the power-off delay timer starts timing when the input signal is disconnected, and the hold-type power-on delay timer continues timing even if the input signal is disconnected. The up counter counts up on the rising edge of the input signal, the down counter counts down on the rising edge, and the reversible counter can count according to the direction of the input signal.
  1. Understand MCR, communication protocol and other instructions and concepts. MCR can control the program execution process and improve program readability and maintainability. Common communication protocols include Modbus, Profibus, Ethernet/IP, etc. Different protocols have different characteristics and application scenarios. Modbus is simple and easy to use and suitable for small systems. Profibus is fast and reliable and suitable for medium and large systems. Ethernet/IP is based on Ethernet technology and is open and universal.

(II) Project Experience Review

  1. Review the PLC brands and models used in past projects and the reasons for their selection. For example, select the appropriate PLC based on project requirements, reliability and stability, programming and debugging convenience, after-sales service and technical support. For large and complex control systems, you may choose the powerful Siemens S7-400, while for small systems, you may choose the low-cost, simple-function Mitsubishi FX1S.
  1. Organize common sensors and actuators and their functional applications. Temperature sensors include thermocouples and thermal resistors. Thermocouples are suitable for high-temperature measurements and have fast response speeds; thermal resistors are suitable for medium and low-temperature measurements, with high accuracy and good stability. Pressure sensors include strain gauges, piezoresistive types, etc. Flow sensors include electromagnetic flowmeters, turbine flowmeters, etc. Level sensors include floats, pressure types, etc. Actuators include motors, valves, cylinders, etc. Motors are used to drive equipment, valves control flow, and cylinders achieve linear motion.
  1. Summarize the experience of solving complex control logic. Analyze the problem, understand the control requirements and process flow; design the solution and select the appropriate equipment; implement the program, pay attention to the readability, maintainability and scalability of the program; debug and test, observe the signal changes and solve the problem in time; optimize and improve, improve the efficiency of program execution and the stability and reliability of the system.

(III) Improve interview skills

  1. Self-introduction should highlight the highlights and use a storytelling method to connect past experiences. For example, first state your disadvantages, such as low education, then describe the biggest project you have done to create a contrast and conflict, then divide your work experience into sections, use small stories to describe your growth at each stage, and show the main line and highlights of your growth.
  1. Preset question points on your resume to show your strengths. Prepare your words for the preset technical directions or successful projects, so that the interviewer's questions are within the scope of your preparation, and use routines to better show yourself.
  1. Speak concisely during the interview and answer every question. Be concise in your expression, avoid being sloppy, and give people a reliable impression. When answering questions, you can mention the company information you obtained before the interview to prove that you have done your homework on the position and are familiar with the company instead of sending resumes everywhere. You can also summarize the highlights through writing, think and organize your ideas while writing, and improve the fluency of expression; or lower the standards and find companies to practice interviews to improve your coping ability.
  1. Show learning attitude and work ability. You can show your willingness to learn and the ability to learn on your own in the interview, but you cannot express that you are here to learn. Emphasize that you can complete your work better while learning and create value for the company.

(IV) Mentality Adjustment

  1. Correctly understand that interviews are two-way choices. After entering the interview company, observe the overall atmosphere, including the office environment, the mental outlook of the employees, the tone of the interviewer, etc. Not only being interviewed, but also interviewing the company, find a suitable platform for development for yourself, and achieve a win-win situation for the company and the individual.
  1. Relieve tension. You can adjust your mind by understanding the interview requirements and types, giving yourself positive suggestions to increase your confidence, practicing more interviews to improve your coping ability, not taking one interview too seriously, taking deep breaths before the interview to ease your tension, and taking a relaxed attitude towards sleep to ensure a good mental state. You must relax.