In CNC machine tools, many functions are controlled by a programmable logic controller (PLC, programmable controller for short). This unit mainly introduces the application of PLC in CNC machine tools. This allows readers who are ready to engage in this work to have a basic understanding of the control objects of the CNC machine tool PLC, the form of the CNC PLC, and the function and working principle of the commonly used input/output components of the CNC machine tool.
1. Classification of the control part of CNC machine tools
The control of a CNC machine tool consists of two parts: one part is the position control of the coordinate axis movement, and the other part is the sequence control of the CNC machine tool processing. When discussing the relationship between PLC, CNC and various mechanical parts of the machine tool, machine tool auxiliary equipment, and power lines, CNC machine tools are often divided into two parts: "NC side" and "MT side" (ie, machine tool side). "NC side" includes the hardware and software of the CNC system and the external equipment connected to the CNC system. "MT side" includes the mechanical parts of the machine tool and its hydraulic, pneumatic, cooling, lubricating, chip removal and other devices, the machine operation panel, the relay circuit, and the heavy current circuit of the locomotive. PLC is located between CNC and MT, and processes the input and output signals on the NC side and MT side. The final object of sequence control on the MT side is very different depending on the type, structure, and auxiliary equipment of the CNC machine tool. The more complex the CNC machine tool mechanism and the more auxiliary devices, the more final controlled objects. Generally speaking, the number of final controlled objects and the complexity of the sequence control program from low to high are CNC lathe, CNC milling machine, machining center, FMC, FMS.
(2) PLC In electrical control, PLC has m input/output (I/O points, as shown in Figure 1.1 b) above.
(3) The PLC is in the electrical control cabinet, and the input/output interface is on the side of the machine tool, as shown in Figure 1.1 c) above. This configuration method greatly reduces the number of cables connecting the CNC and the machine tool. The following figure 1.2 shows the schematic diagram of the input/output signal of the CNC machine tool PC.