Basic structure of 1. programmable controller
Broadly speaking, PLC is a computer system, but it has stronger input and output interfaces connected with industrial processes than ordinary computers, is more suitable for control programming languages, and its anti-interference performance is more suitable for industrial environment. Its structure is very similar to that of a computer control system, and generally consists of a central processing unit (CPU), a memory, an input/output (I/O) interface and a power supply. Because the central processor of PLC is composed of microprocessor and single chip microcomputer, and the memory and 1/0 components are also various, according to the different structural forms, PLC can also be divided into integral structure and combined structure. The difference is that the integrated structure assembles CPU, RAM, ROM and I/O units into a main body to form the host, and its structural schematic diagram is shown in Figure 4. 1. The combined structure is to make all parts of PLC into corresponding circuit boards or modules, and the modules are connected with each other through the bus on the bottom plate. No matter what type of PLC it is, various external switching signals, analog signals and signals detected by sensors are input variables of PLC, which are input to internal registers through the external input terminals of PLC, and then sent to the output terminals after logical operation or other operation processing in PLC, and are output variables of PLC. The system program and user program of PLC are stored in the memory, the field input signal is transmitted to the CPU through the I/O unit, and the CPU performs logical or arithmetic operations according to the instructions in the user program memory.
The corresponding control instructions are sent to the site through the I/O unit to drive the corresponding actuators to complete the corresponding control tasks.
2.2. Basic working principle and main technical indicators. Programmable Logic Controller (PLC)
The working mode of PLC is quite different from the interrupt processing mode of microcomputer. Generally, the microcomputer is waiting for the command to work, and the PLC is scanning circularly. In PLC, user programs are stored in sequence, and the CPU executes the program from the first instruction until it returns to the first one after encountering the terminator, and so on. The scanning process of PLC is shown in Figure 4.2. This process is divided into three stages: data input sampling stage, program execution stage and output refresh stage. The time required for the whole process to be carried out once is called scanning period. In the data input sampling stage, PLC reads in the on/off states of all input terminals in a scanning mode and stores them in the input image storage area, and then switches to the user program execution stage. In the non-input sampling stage, no matter how the input state changes, the contents of the input image storage area remain unchanged, and PLC will not read the state of the input terminal into the input image storage area until it enters the input sampling stage of the next scanning cycle. In the program execution stage, according to the scanning principle of ladder diagram program from left to right and from top to bottom, the user program instructions are executed in turn. The program execution result is not directly output, but written into the output image storage area. Every bit in the output image storage area will change with the process of program execution. In the output data and processing stage, after the user program is executed, the on-off state of the relay in the output image storage area is centrally transmitted to the output state latch, and then isolated and power amplified by the output driving circuit to drive the external load.
In the above-mentioned scanning process of PLC, the scanning period is a very important index of PLC, and the scanning period of small PLC is generally ten milliseconds to tens of milliseconds. It takes some time from the change of an input signal at the input end of PLC to the response at the output end, which is called the response time or lag time of PLC. The main reasons that affect I/O lag are: the time constant of input filtering, the mechanical lag of output relay, the time of program execution, the additional influence of improper program design, etc. Millisecond scanning time is usually acceptable for general industrial equipment, and the response lag of PLC is also allowed, but for some equipment with fast I/O response, corresponding treatment measures should be taken. For example, high-speed CPU is selected to improve the scanning speed, and measures such as fast response module, high-speed counting module and different interrupt processing are adopted to reduce the lag time. For users, choosing PLC and making a reasonable program are the keys to shorten the response time.