Basic electrical knowledge
Circuit principles:
To understand the input and output circuits of PLC, you must master the basic circuit principles, such as the characteristics of series and parallel circuits. For example, in the PLC input circuit, multiple sensors may be connected in series or in parallel. Understanding the principles of voltage division in series circuits and current division in parallel circuits helps to understand how input signals are processed. Knowing Ohm’s law (I = U/R) and being able to calculate current based on given voltage and resistance is important for analyzing the electrical parameters of PLC power supply circuits and input and output interfaces.
Mastering basic circuit analysis methods, such as Kirchhoff’s laws (including current law and voltage law), can be used to solve the current and node voltage of each branch in a complex circuit. In the circuit design and troubleshooting of PLC control cabinets, this knowledge can help analyze circuit fault points.
Relay control principle:
Be familiar with the working principle of relays, including the structure of relays (coils, contacts, etc.) and the action principle (power on, pull-in, power off, release). Because the ladder diagram programming language of PLC has evolved from the relay control circuit to a certain extent, its logical relationship is similar to relay control. For example, in the PLC program, the concepts of normally open contacts and normally closed contacts are the same as those in the relay circuit. Learning relay control can better understand the logic control part in the PLC program.
Understand the design methods of relay control systems, such as how to design start-stop control circuits, forward and reverse control circuits, etc. according to control requirements. These classic relay control circuits can provide ideas for the design of PLC control programs.
Electronic technology knowledge
Digital circuits:
Digital circuit knowledge is an indispensable part of learning PLC. It is necessary to understand the digital logic circuits inside the PLC, such as the functions and logical expressions of basic logic gates such as AND gates, OR gates, and NOT gates. In the PLC program, logical operations (such as logical AND, logical OR, and logical NOT) are the basis for realizing control logic, and these operations correspond to the logic gates in the digital circuit.
Master the principles of counters and timers. There are special timer and counter instructions in PLC to realize time control and counting functions. For example, these instructions are needed in product counting and processing time control on automated production lines. Understanding the principles of counters and timers in digital circuits helps to better use the corresponding functions in PLC.
Analog circuits:
For PLC applications that need to process analog input and output, analog circuit knowledge is critical. Understand the characteristics of analog signals (such as voltage signals and current signals), such as signal amplitude, frequency, phase and other concepts. Know how to use operational amplifiers for signal amplification, filtering and other operations, because in actual applications, the analog signals collected by the sensor may be very weak and need to be amplified and filtered before they can be effectively received and processed by the PLC.
Understand the principles of analog-to-digital (A/D) conversion and digital-to-analog (D/A) conversion. When processing analog quantities, the PLC needs to convert analog signals into digital signals (A/D conversion) for internal processing, and then may need to convert digital signals into analog signals (D/A conversion) for output. For example, in a temperature control system, the analog voltage signal output by the temperature sensor enters the PLC after A/D conversion. After the PLC is processed by operation, it outputs a control signal through D/A conversion to adjust the heating or cooling equipment.
Programming language knowledge
PLC programming language:
Master the commonly used programming languages for PLC, such as ladder diagram language (LAD), instruction list language (STL), function block diagram language (FBD) and structured text language (ST). Ladder diagram language is intuitive and similar to relay circuit diagram, suitable for beginners and logic control programming; instruction list language is closer to machine language and has high execution efficiency; function block diagram language is easy to understand the relationship between functional modules; structured text language is suitable for writing complex algorithms and large control system programs.
Learn the grammatical rules, instruction functions and programming skills of each programming language. For example, in ladder diagram language, you need to learn how to correctly connect contacts and coils, how to use timer and counter instructions; in structured text language, you need to master the definition of variables, data types, functions and procedures, etc.
Basics of computer programming language:
Understanding the basic concepts of computer programming language, such as variables, data types, operators, control structures (conditional judgment, loop structure), etc., is very helpful for learning PLC programming language. Because the structured text language of PLC has many similarities with computer programming language, with the foundation of computer programming language, you can understand and master the advanced functions of PLC programming language more quickly.
For example, when writing a complex PLC control program, you may need to use a loop structure to process batch data, or use a conditional judgment structure to perform different operations according to different situations. These are the same as the use of loops and conditional statements in computer programming languages.
Automatic control theory knowledge
Basic concepts of control systems:
Learn the basic components of control systems, including controllers, controlled objects, sensors, and actuators. In PLC automatic control, PLC acts as a controller, and the controlled object can be a motor, valve, and other equipment. The sensor is used to obtain the status information of the controlled object, and the actuator is used to control the controlled object according to the instructions of the PLC. Understanding the relationship and working principle between them can better design PLC control systems.
Master control methods, such as open-loop control and closed-loop control. Open-loop control means that the controller directly generates an output signal based on a given input signal without considering the actual output of the controlled object; closed-loop control means that the output of the controlled object is fed back to the controller through a sensor, and the controller makes adjustments based on the deviation between the feedback signal and the given signal. For example, in a temperature control system, open-loop control may only control the opening and closing of the heating equipment according to a preset schedule, while closed-loop control will dynamically adjust the power of the heating equipment based on the deviation between the actual measured temperature and the set temperature.
Control Algorithms:
Learn some basic control algorithms, such as proportional-integral-differential (PID) control. PID control is one of the most commonly used control algorithms in industrial control and is also widely used in PLCs. It adjusts the control signal through three links: proportional, integral, and differential, so that the output of the controlled object can quickly and accurately track the given signal. For example, in a pressure control system, the PLC uses the PID algorithm to control the valve opening based on the pressure signal fed back by the pressure sensor, so that the pressure is stabilized near the set value.
