In the modern manufacturing industry, the degree of automatic control is getting higher and higher. Programmable logic controller is the most popular control mode in modern industrial manufacturing control. It is very convenient to use programmable logic controller to complete the sequence control law. This paper mainly studies the use of PLC timer and sequential control programming method, providing important reference for industrial production and manufacturing. PLC is called programmable logic controller. PLC is a new type of control equipment which takes microprocessor as the core and combines with traditional contactor. The structure of PLC is simple, the performance is excellent, the reliability is high, and the use is convenient. PLC is widely used in the control links of manufacturing, academic research and other industries. In the operation and control of electric machinery, the use of programmable controller is also a development trend. In many cases of sequential control, thermostatic element asynchronous motors are used to provide power. The principle is to convert electrical energy into mechanical energy. Solenoid valves and contactors are also used. The so-called sequential control means that according to the order set in advance in the production process, each executing unit operates automatically and sequentially. It is widely used in industrial manufacturing and daily life, such as automatic control of handling manipulator, automatic control of packaging production line, automatic control of traffic lights, etc. At the same time, sequence control has three factors: transfer conditions, transfer objectives and tasks. According to the characteristics of sequential control in order of arrival, sequential control can be classified into three types: logical sequential control, temporal sequential control and conditional sequential control.
Logical sequence control is to execute action instructions in logical sequence, which has no strict relationship with action time. The time sequence control is based on the action time, and the operation stop of each equipment is related to the time. Conditional sequential control is to execute the corresponding action instructions according to whether the action conditions are satisfied or not. In the traditional sequential control system consisting of relays, all the operations and logical relations are accomplished by relays.
Various relays, such as normal open and closed contacts, time relays, contactors, switches and other components, complete the logical functions required by the system. In the relay control system, because of the limitation of mechanical contacts and reliability of relays, the reliability of such control system is not very high and its service life is short. It is not particularly convenient to change the logic relationship, but can be used in some old or simple control systems. The first PLC in the world was manufactured by American Digital Equipment Company and successfully applied to the production line of American General Motors Company. After a period of development, programmable controllers have emerged and gradually been widely used, mainly composed of small and medium-sized integrated circuits. PLC replaces mechanical switches and electronic switches with memory, and replaces the state of switches with memory value, which greatly improves the reliability of switches and prolongs their service life. The storage value of memory can be used many times, so long as the control program of PLC is changed, the logical relationship can be changed. Many sequential controls in PLC programs are very important to timers.
The timer in the programmable controller acts as a time relay, which has a set value, current value and infinite contacts. The use of timers has the following characteristics. 1) Timer uses decimal coding, such as T0-T9, T10-T19; 2) Timer has constant open contact, constant closed contact and coil when writing ladder diagram. The timer accumulates clock pulses of 1ms, 10ms, 100ms etc. in PLC. When the set value is reached, the contact acts to output the signal. Mitsubishi series PLC timer has two kinds, universal timer and integrator timer. Control law: press the button, the motor will start automatically after 5 seconds. Control Law: There are three red, yellow and blue light bulbs. Press the button, the red light is on for 2 seconds, the yellow light is on for 2 seconds, the blue light is on for 2 seconds, and the blue light is on for 2 seconds. Control Law: Press the button, the motor starts, after 10 seconds, the motor starts automatically two, and after 10 seconds, the motor starts three. Press stop button, three motors stop at the same time.
With the progress of modern science and technology, the functions of PLC will be strengthened and the specifications will be more complete. Through high-quality man-machine interface, complete communication system and mature fieldbus communication ability can be better applied to various control sites. Sequential control is a very typical and common control law. It has many advantages to use PLC to realize sequential control. As an important part of automatic control system, PLC will occupy a more important position in production work and play more roles in serving people’s daily life and manufacturing.