The openness of hardware design is mainly reflected in the bus standard. The open CNC system is composed of a variety of modules, and the modules are connected into one whole through a standard bus. The choice of bus should meet three requirements: 1 It has certain advancement in technology and can meet the needs of information interaction of various functional modules of CNC system. 2 The bus standard is completely open and widely recognized and applied internationally, rather than a special bus standard defined by a manufacturer. 3 has a high degree of reliability.
The design of each functional module can only be carried out after the appropriate bus standard has been selected. In the numerical control system, the main functional modules are: motion axis position control module, electrical control module, machine tool operation panel and numerical control panel interface module, communication module, display module and so on. Functional modules should be reconfigurable to avoid conflicts between different module I/O port addresses and interrupt types.
2.2 Hardware design of five-axis linkage CNC system
In the development process of the five-axis linkage CNC system, we chose the industrial computer as the basis of the design. The industrial computer itself meets various industrial standards and is an open computer system. It has good compatibility with commonly used microcomputers and has a large number of software and hardware support. At present, there are two main types of industrial baseboard slot bus types: ISA bus (industry standard bus) and PCI bus (peripheral device interface). The data transmission rate of the ISA bus is relatively low, but it can meet the needs of the numerical control system. At the same time, the high bus speed places higher demands on the hardware of each functional module. Therefore, we chose the ISA bus as the basis for all module design.
Due to the complexity of the five-axis linkage interpolation algorithm and the large number of floating-point operations, the real-time requirements are high. We choose the Pentium 166 CPU to perform the interpolation operation. In addition, each coordinate axis of the system also needs to have position control function, the position control is very real-time, and the number of control axes is relatively large. Sharing a CPU with the task and interpolation will cause the burden of the CNC system host to be too heavy, and the real-time performance is not easy to guarantee. Moreover, the risk of failure is too concentrated. It is better to use a separate CPU for each axis to control and use a hierarchical architecture to form the system. According to the position control CPU and the host interaction information method, it is divided into two structures (see Figure 1). The first structure inserts the position control board directly into the ISA slot of the bottom plate of the industrial computer. In this case, the information is directly transmitted between the host and the plurality of position control boards, and the data communication is low due to the low speed of the position control board CPU. The stage will waste host CPU resources. The more control axes, the lower the efficiency of the host CPU. In addition, the host needs to take measures to ensure accurate synchronization of multiple remote control panels in time. Therefore, we chose the second structure. The second structure uses a separate communication machine to complete the information transfer between the host and the position control board. On the one hand, the communication machine exchanges information with the host through the dual port memory, and on the other hand exchanges information with the position control board through the self-built local bus. The dual port memory capacity is 2kb, which also acts as a data buffer. This solution greatly reduces the CPU time that the host uses for information exchange.
Figure 1 Two structures of a distributed multi-CPU numerical control system
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