1. The application form of PLC in CNC machine tools
PLC is used in CNC machine tools, and there are usually two forms: one is called built-in: the other is called free-standing. Built-in PLC is also known as integrated PLC, the CNC system that adopts this way, and the NC and PLC are considered at the beginning of the design.
The signal transmission between the NC and the PLC is carried out on the basis of an internal bus, so there is a high exchange speed and a wide information channel. They can share a CPU or be a separate CPU, this structure is considered from the software and hardware as a whole, there is no extra wire connection between PLC and NC, which increases the reliability of the system, and it is easy to realize many advanced functions between NC and PLC. The information in the PLC can also be displayed through the CNC display, which has great advantages for the use of the system. High-grade CNC systems generally use this form of PLC.
Stand-alone PLC, also known as external PLC, is independent of the NC device and has a PLC that completes the control function independently. In this application mode, the products of different professional PLC manufacturers can be selected according to the user's own characteristics, and the control scale can be adjusted more conveniently.
2. Information exchange between PLC and CNC system and CNC machine tools
In contrast to the PLC, the machine and the NC are external. The exchange of information between the PLC, the machine tool and the NC is very important for the function of the PLC. There are usually four parts to the exchange of information between the PLC and the outside:
(1) Machine tool side to PLC: The switching signal on the machine tool side is input to the PLC through the /O unit interface, except for a very few signals, the meaning of most of the signals and the configured input address can be defined by the PLC programmer or program user. CNC machine tool manufacturers can easily modify the PLC program and address assignment according to the function and configuration of the machine tool.
(2) PLC to machine tool: the control signal of PLC is sent to the machine tool side through the output interface of PLC, and the meaning and output address of all output signals are also defined by the PLC programmer or user.
(3), NC to PLC: CNC to PLC: CNC sent to PLC information can be directly sent to the PLC register by CNC, all CNC sent to PLC signal meaning and address (switch address or register address) are determined by CNC manufacturers, PLC programmers can only use can not be changed and added or deleted. For example, the M, S, and T functions of numerical control instructions are directly sent to the corresponding registers of the PLC after being decoded by CNC.
(4) The information sent from PLC to CNCLC to CNC is also completed by the switching signal or register, and the signal address and meaning of all PLC sent to CNC are determined by the CNC manufacturer, and the PLC programmer can only use it, and cannot be changed or added or deleted.
3. The workflow of PLC in CNC machine tools
The workflow of PLC in CNC machine tools is basically the same as the usual PLC workflow, which is divided into the following steps:
(1) Input sampling: Input sampling, that is, the PLC reads the signal status of all input ports in a sequential scanning manner, and reads this state into the input image register. Of course, these signal states do not change during the program run cycle, unless a new scan cycle arrives and the original port signal state has changed, and the signal state of the input image registers reads changes.
(2) Program execution: In the program execution stage, the system will scan the program in a specific order, and simultaneously read the relevant data of the input image register and the output image register, and after the relevant operations, the operation results will be stored in the output image register for output and the next operation.
(3) Out of the refresh stage: After the execution of the instruction is completed, the state of all output relays in the output image register (on/off) is transferred to the output latch in the output refresh stage, and the external load is driven by specific output.
4. The control function of PLC in CNC machine tools
(1) Control of the operation panel. The operation panel is divided into system operation panel and machine tool operation panel. The control signal of the system operation panel first enters the NC, and then the NC sends it to the PLC to control the operation of the CNC machine tool. The control signal of the machine tool operation panel directly enters the PLC to control the operation of the machine tool.
(2) The input signal of the external switch of the machine tool. The switching signal on the machine side is input to the input PLC for logic calculation. These switching signals include many detection element signals (e.g., travel switches, proximity switches, mode selector switches, etc.)
(3) Output signal control: The PLC output signal is output to the control object through the relay, contactor, solenoid valve, etc. in the peripheral control circuit.
(4) Functional realization. The system sends the T instruction to the PLC, which is decoded, retrieved in the data table, finds the tool number specified by the T code, and compares it with the spindle tool number. If it does not match, the tool change instruction is issued, the tool is changed, and after the tool change is completed, the system sends a completion signal.
(5) M function realization. The system sends M instructions to the PLC, and after decoding, the control signal is output, and the spindle is controlled to rotate forward and reverse, start and stop, etc. The M command is completed, and the system sends a completion signal.
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