1, working principle of PLC

After PLC starts running, its working process is generally divided into three stages, namely, input sampling, user program execution and output refresh. The completion of the above three stages is called a scanning cycle.

2, PLC classification

1) integrated PLC

Integrated PLC is used to install power supply, central processing unit, input/output, etc. In a box, it is usually called the basic unit, as shown in the figure below. Integral PLC can also be equipped with special functional units, such as analog unit and position control unit. And expand its functions. This PLC is characterized by compact structure, small volume and low price. Small PLC generally adopts this overall structure.

2) Modular PLC

Modular PLC is to make each component of PLC into several independent modules, such as CPU module, I/O module, power module (partially included in CPU module) and so on. , the module is installed on the socket of the frame or substrate, as shown in the figure below. This modular PLC is characterized by flexible configuration, which can configure systems of different sizes according to needs, and is convenient for assembly, expansion and maintenance. Large and medium-sized PLC generally adopts modular structure.

3. The function and application of 3.PLC

PLC is designed and developed on the basis of integrating many advantages of relay contactor control and computer control, and is widely used in metallurgy, energy, chemical industry, transportation, electric power and other fields.

1) switch logic control

PLC replaces the traditional relay circuit to realize logic control and sequence control. It can be used for single machine control, multi-machine group control and automatic production line. Such as injection molding machine, printing machine, stapler, modular machine tool, grinder, packaging production line, electroplating production line, etc.

2) Analog control

In the process of industrial production, there are some continuously changing quantities (namely analog quantities), such as temperature, pressure, flow, liquid level and speed. A/D module can convert analog quantities such as temperature, pressure, flow and speed into digital quantities, which are then processed by the microprocessor in PLC and then controlled. Or it can be converted into analog by D/A module, and then the controlled object is controlled, so that PLC can control the analog. Analog control is widely used in metallurgy, chemical industry, heat treatment, boiler control and other occasions.

3) data acquisition

PLC has the functions of mathematical operation (including matrix operation, function operation and logic operation), data transmission, data conversion, sorting, table lookup and bit operation, and can complete data acquisition, analysis and processing. Data processing is generally used in large-scale control systems, such as unmanned flexible manufacturing systems;

4) Timing and counting control

PLC has powerful timing and counting functions, and can provide users with dozens or even hundreds or thousands of timers and counters. The timing time and counting value can be set arbitrarily by the user when writing the user program, and can also be set by the operator through the programmer in the industrial field to realize the control of timing and counting. If users need to count high-frequency signals, they can choose high-speed counting module.

PLC programming principle

1, security, using PLC control will have some automatic operation parts, so it is necessary to be foolproof, not to do it after doing it, and not to let the controlled object get out of control. The manual part of the program should also be chained or time-limited. Many mechanical equipment has its own safety mechanism in the organization. You'd better forget this and ensure the absolute safety of people and machines in the program.

2. It is fully functional and meets the functional requirements of customers as far as possible on the premise of ensuring safety. It's time to show your professionalism. Don't wait for the equipment to arrive, so that the sales staff can't carry out their work. Communicate with customers as soon as possible for the parts that cannot be realized due to hardware or other reasons.

3. The logic is meticulous, and our program not only needs to successfully complete every action under the condition that all parts are normal. When some signals are lost, they should still be installed, and there are safety tips.