1.Introduction:
PLC (Programmable Logic Controller) engineers play a crucial role in the field of automation control, and their expertise and knowledge are essential for the stable operation of industrial systems. During the recruitment process for PLC engineers, interviews serve as a crucial step in evaluating candidates' skills and professional knowledge. This article will introduce some common PLC engineer interview questions and corresponding answers, aiming to help applicants prepare adequately and improve their chances of success in the interview.
2.Common Questions and Brief Answers:
2.1 General Questions:
Please introduce the basic principle and working mode of PLC.
PLC is a computer system specifically designed for industrial automation control. It achieves automation control by receiving input signals, executing program logic, and controlling output signals. The basic components of a PLC include input modules, a central processing unit, output modules, and communication modules.
What are bitwise logic and word logic in PLC programming?
Bitwise logic refers to operations on a single bit, with common bitwise logic operations including AND, OR, and NOT. Word logic, on the other hand, involves operating on multiple bits simultaneously, with common word logic operations such as addition, subtraction, and multiplication.
Please explain some commonly used instruction sets in PLC, such as bitwise logic instructions, counter instructions, and timer instructions.
Bitwise logic instructions are used for bit operations on inputs and outputs, such as turning outputs on or off, reading input statuses, etc. Counter instructions are used to implement counting functions, like incrementing, decrementing, and resetting counters. Timer instructions enable timing functionalities, such as delayed triggering, periodic triggering, and resetting timers.
Describe some common programming languages for PLCs.
Common PLC programming languages include Ladder Diagram (LD), Structured Text (ST), and Function Block Diagram (FBD). Ladder Diagram is the most widely used language, resembling electrical schematic diagrams and being easy to understand and maintain. Structured Text is similar to traditional programming languages, suitable for complex logic and algorithms. Function Block Diagram is suitable for modular design and reuse.
How do you handle faults and exceptions in PLC programming?
Handling faults and exceptions is an essential task for PLC engineers. This can be achieved by implementing appropriate error handling routines and exception detection mechanisms. For instance, using alarm indicators or messages to alert operators, logging error records for later analysis, or automatically switching to backup equipment.
What is PID control in PLC engineering, and what is its function?
PID (Proportional, Integral, Derivative) control is a commonly used closed-loop control method for precise regulation of systems. It calculates the control output based on the current error, the integral of the error, and the derivative of the error, aiming to achieve more stable and accurate system responses.
Please describe the troubleshooting steps for a PLC system.
Troubleshooting is a critical skill for PLC engineers. Common troubleshooting steps include checking the power supply and signal line connections, verifying the program logic, ensuring the input and output modules are functioning correctly, and using debugging tools and instruments for signal tracing and analysis.
2.2 Specific Questions Related to the Field:
Please introduce the PLC brands and models you have used in past projects and explain why you chose those PLCs.
Answers may vary depending on personal experience and project specifics. Candidates can share the PLC brands and models they have utilized in previous projects and the reasons for selecting those PLCs. For instance, it could be due to the brand's reliability and widespread application in a specific industry.
In industrial automation control, what are some common sensors and actuators you have used in PLC programming? Please explain their functions and applications.
Answers may vary depending on the industry and project. Candidates can mention some common sensors and actuators, such as temperature sensors, pressure sensors, motor drivers, etc., and explain their functions and applications in industrial automation systems.
Please describe a complex control logic you encountered in a past project and explain how you resolved the issue.
This question aims to evaluate the candidate's ability to solve problems and handle complex control logic. Candidates can share a specific situation they encountered in a previous project and describe the steps they took, including analyzing the problem, debugging the code, collaborating with the team, etc.
How do you ensure the stability and reliability of PLC programs in industrial automation control systems?
Stability and reliability are crucial elements in industrial automation control systems. Candidates can mention measures such as writing clear, modular, and maintainable program code, conducting rigorous testing and validation, utilizing error handling and fault detection mechanisms, etc., to ensure the stability and reliability of PLC programs.
Please describe an advanced feature or technology you have used in PLC programming and explain its application and advantages.
This question aims to assess the candidate's knowledge of advanced features and technologies in PLC programming. Candidates can share an advanced feature or technology they have utilized in past projects, such as data acquisition and analysis, remote monitoring and control, etc., and explain its application and benefits.
What is your understanding of safety control in the industrial automation field? Please describe your experience in applying safety control in projects.
Safety control is crucial in industrial automation systems. Candidates can mention their understanding of safety control, such as safety input/output modules, safety programming practices, safety monitoring, and emergency stop systems, and share their experience in applying safety control in projects.
Summary:
The PLC engineer interview is a crucial step in evaluating a candidate's skills and expertise. This article introduces some common PLC engineer interview questions and answers, covering both general topics and those specific to the field, such as commonly used instructions in PLCs, popular programming languages, handling faults and exceptions, PLC brand and model selection, sensor and actuator applications, and resolving complex control logic issues. By preparing and understanding these questions and answers, candidates will increase their chances of success in the interview.
Please note that interview questions and answers may vary depending on the company and job requirements. Therefore, before the interview, candidates should also research the target company's specific requirements and prepare relevant questions and answers accordingly.
Good luck in your PLC engineer interview!