The control mode of plc belongs to the storage program control, and its control function is realized by the program stored in the memory. To make necessary modifications to the control function, it is only necessary to change the control program, which realizes the softwareization of the control. The advantage of the programmable controller is the word "can". In terms of software, its control program can be edited and modified. From the hardware, its external device configuration is variable. The focus of building a PLC control system lies in the preparation of the control program, but the selection of external equipment will also have an impact on the programming. Therefore, in the design of the program should be combined with the actual needs, hard and software comprehensive consideration. In this paper, the hard and soft aspects are chosen. The ladder diagram is selected as the programming language. Taking the Panasonic Electrician FPO-C32 PLC as an example, several problems and solutions that are easy to appear in the process of PLC are analyzed.
First, PLC programming components
The various functions of the PLC are mainly realized by running a control program. When programming, you need to use the programming components (ie, devices) provided by the PLC. There are two types of programming components commonly used in FPO type PLCs: bit elements and word elements. The bit component is actually a binary bit unit provided by the PLC memory area. It is also called a soft relay. It is mainly used as a programming component for basic sequential instructions, such as input relay Xn, output relay Yn, internal general-purpose relay Rn, timing (counting The device, etc., participates in the control mainly by affecting the logical operation result by the change of the on-off state of the corresponding contact.
The word component is a word unit (16bit) in the PLC memory area. It is mainly used as a programming component for function instructions and advanced instructions. It is usually used to store data, such as data register DTn, timing (counter) setting value SVn, After the value EVn and so on. Word components have no contacts and are usually controlled by the overall content.
It is worth noting that the input (X) area, the output (Y) area, and the internal general-purpose (R) area in the memory, each bit in the area can be used as a bit element, and each 16-bit can constitute a word element, such as WRIO is a word element composed of 16 bit elements R100 to R10F. Once the content of the word element changes, the state of these 16 bits also changes.
Second, the selection of external input devices and the use of PLC input relays
1. Acquisition of external input signals
The external equipment of PLC mainly refers to the input and output equipment in the control system. The input equipment is the main electric appliance that sends various control signals to the system. When writing the control program, it must be noted that the external input equipment is normally open or normally closed. The contacts are programmed on the basis of this. Otherwise, control errors are easy to occur.
There is an input relay area dedicated to the input state storage in the PLC internal memory. The status of each input device (switch, button, travel switch or sensor signal) is stored in the area via the input interface circuit, and each input relay can store one input. device status. The "relay" used in the PLC is not a physical relay, but a "soft relay" that provides a myriad of normally open, normally closed contacts for programming. Each "soft relay" only corresponds to one bit in the PLC memory unit. The bit status is "1", indicating that the "soft relay coil" is energized, and all the contacts of the relay in the program are activated. The input relay acts as a device for the PLC to receive an external master signal, and is connected to an external input device by wiring. Its "coil" state can only be driven by an external input signal.
2. Parking button uses normally closed type
Since the PLC determines the on-off state of the contact in the running program, it only depends on the state of the input relay coil in its memory, and does not directly identify the external device. Therefore, when programming, the selection of the external device is closely related to the type of contact in the program. This is a problem that is easy to occur when programming PLCs against the electrical control schematic. The most typical example is the basic control - the parking control in the "start and stop control".
3. Parking button uses normally open type
If you want to create a ladder program that meets our usual reading habits, you need to use the normally open contact of button SB0 to connect to X0 when using an external parking device.
Third, the sequential control multi-step and output programming method
Sequential control is a common type of control task in the production site. The stepping instructions are dedicated to sequential control in the PLC instruction library. When programming step instructions, the program is divided into individual blocks according to the process flow. When executed, the CPU strictly follows the ladder programming sequence. Only after executing the previous program can the next program be activated and before the next program is executed. , reset the previous block. And the syntax requires that the output used by each block does not allow duplicates. This brings certain difficulties when solving the situation that there are multiple steps and outputs in the sequential control task. This problem can be easily solved with the aid of an internal general-purpose relay.
Fourth, PLC "serial" operation mode and the preparation of control procedures
One of the important differences between PLC and relay contactor control is the way it works. The relay contactor control system works in a "parallel" manner, that is, it operates in a simultaneous manner. As long as a current path is formed, several appliances may operate simultaneously. The PLC works in the "serial" mode. When the PLC executes the program cyclically, it performs logical operations from top to bottom according to the writing order of the statement, and the result of the previous logical operation affects the logical operation result of the subsequent statement. Therefore, when programming ladder diagrams, the position of each statement also has a critical impact on the control function.
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