Programmable logic controllers (PLCs) have become an indispensable component of CNC equipment due to their valuable characteristics (such as variable control programs, good flexibility; convenient programming using procedural languages; comprehensive functions; easy expansion and flexible configuration; simple system structure, convenient installation and debugging; and high reliability). There are two types of PLCs used in CNC machine tools: built-in PLCs and stand-alone PLCs.
The characteristics of a built-in PLC are:
(1) Its performance indicators are determined by the performance specifications of the CNC device to which it belongs. Its hardware and software are designed as the basic functions of the CNC device, and it has the advantages of compact structure and strong adaptability.
(2) It has two types: one that shares a microprocessor with the CNC and the other that has a dedicated microprocessor. The former uses the spare capacity of the CNC microprocessor to complete the functions of the PLC, and has fewer I/O points; the latter has an independent microprocessor and is mostly used in situations where the sequential program is complex and the speed of action is required to be fast.
(3) It is installed in the same chassis as other CNC circuits and shares a power supply and ground wire.
(4) Its hardware circuit can be made on the same printed circuit board as other CNC circuits, or it can be made into an additional printed circuit board separately.
(5) It does not have a separate I/O interface circuit configured for external use, but uses the I/O interface circuit of the CNC device itself.
(6) The built-in PLC expands the window communication function of the CNC to be directly processed. It can use ladder diagram editing and transmission of advanced control functions, and the cost is low, which improves the performance/price ratio of the CNC.
The characteristics of a stand-alone PLC are:
(1) The control functions of the CNC machine tool can be flexibly selected or developed independently according to the requirements of the CNC machine tool.
(2) It has its own I/O interface circuit. The connection between PLC and CNC device, and between PLC and machine tool side are all through the I/O interface circuit. The PLC itself adopts a modular structure and is installed in a plug-in type cage. The number of I/O points can be flexibly configured by adding or removing I/O modules or plug-in boards.
(3) It can expand the control function of CNC and can form two or more additional axis controls.
(4) In terms of performance/price ratio, it is not as good as the built-in PLC.
