The open design of five-axis linkage numerical control system

The open-architectured design of 5-axis CNC system

Wang Guangyan Zhao Jiandon Shuai Mei Wang Xiaochun

Abstract: In this paper,a 5-axis CNC system based on Industrial PC is introduced. Its architecture, hardware and software opening design method is discussed.From the developing procedure and actual working, we can make a conclusion that opening design is effective in Shorting the developing time and improving the quality of CNC system.
Key words: open-architectured system;ISA bus;CNC

1 Overview

Since MIT developed the first three-axis milling machine control system to the present forty years, the design method of the numerical control system has undergone tremendous changes. Especially in the past ten years, with the rapid development of computer technology, the numerical control system from the overall structure to the detailed design, from software design to hardware design, are very different from the early CNC system. For the early CNC system, due to efficiency considerations, many functions were implemented using hardware circuits. The specificity was very strong, and the maintainability and scalability were poor. On the other hand, the computing speed of general-purpose computers has increased exponentially over time, and the computing power of a microcomputer has now reached or surpassed that of early minicomputers. Moreover, general-purpose microcomputers are widely used, and there are perfect and open standards. Many peripheral hardware devices and rich software resources support. With the help of microcomputers, the development of numerical control systems can achieve a multiplier effect, thus becoming an international trend in the field of digital control.
The number of linkage axes of the five-axis linkage numerical control system is more, and at the same time two rotation movements are involved. The interpolation algorithm is complex, and its components, such as servo drive unit, position feedback unit, error compensation, electrical control, machine tool mechanical structure, etc. There are different characteristics in different applications. This should be fully considered in the overall design of the system. At present, most CNC systems cannot meet the needs of this diversity. For different applications, different types of CNC systems have to be selected, which will inevitably increase the development and maintenance costs. Studying the open CNC system and its functional components can easily re-combine the entire CNC system according to the user's needs to improve the system's portability, scalability, maintainability and compatibility.

2 Open Design of CNC System Hardware

2.1 General principles of hardware design <br> The hardware design of the traditional numerical control system is divided into two schools: a large-scale structure and a bus-type architecture using dedicated chips. The board structure is a closed system for the user, and the function expansion and system maintenance are limited. The bus structure has certain flexibility, but because this kind of bus is determined by the manufacturer himself, lacks the common industry standard, the products of different manufacturers are not interchangeable, so this kind of design method is not suitable for the modern manufacturing industry. Needs. On the other hand, with the development of computer technology, the speed of microcomputers is very different from that of more than ten years ago. In this situation of hardware and software design, the focus of attention has shifted from efficiency to interchangeability and maintainability. Affected by this, in the design process of the numerical control system, from the large-scale structure or dedicated bus to the standard bus, reconfigurable unit module has developed into an international trend.
The openness of hardware design is mainly reflected in the bus standard. The open numerical control system is composed of a variety of modules, and the modules are connected together by a standard bus. The choice of the bus line should meet three requirements: 1 There is a certain advanced nature in the technology, can meet the need of information exchange of various functional modules of the numerical control system. The 2 bus standard is completely open, and it is widely recognized and applied internationally. It is not a specific bus standard defined and used by a certain manufacturer. 3 has a high degree of reliability.
After selecting the appropriate bus standard, the design of each functional module can be performed. In the numerical control system, the main function 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 able to be reconfigured to avoid conflicts between different module I/O port addresses and interrupt types.
2.2 Hardware design of five-axis linkage numerical control system<br> In the development process of five-axis linkage numerical control system, we chose industrial control computer as the basis of design. The IPC itself meets a variety of industry standards and is an open computer system. It has good compatibility with commonly used microcomputers and is supported by a large number of hardware and software. At present, there are mainly two types of industrial chassis backplane slot bus: 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 already meet the needs of the numerical control system. At the same time, the high bus speed will impose higher requirements on the hardware of each functional module. Therefore, we use the ISA bus as the basis for all module designs.
Due to the complex five-axis interpolation algorithm and large number of floating-point operations, the real-time performance requirements are high. We chose the Pentium 166 CPU for interpolation. In addition, each coordinate axis in the system needs to have position control function. The real-time position control is very strong, and the number of control axes is relatively large. The sharing of a CPU with the task and the interpolation will lead to heavy burden on the host of the numerical control system and the real-time performance is not easy to guarantee. And the risk of failure is too concentrated. It is a good practice to use a separate CPU for each axis to control and use a hierarchical architecture to form the system. Based on the difference in the method of controlling the interaction between the CPU and the host, the location is divided into two structures (see Figure 1). The first structure inserts the position control board directly into the ISA slot of the IPC baseboard. In this case, the host computer communicates directly with the multiple position control boards. Due to the low speed of the CPU of the position control board, data communication occurs. The stage will waste host CPU resources. The more spindles are controlled, the lower the efficiency of the host CPU. In addition, the host must also take measures to ensure the accurate synchronization of multiple position boards in time. Therefore, we have chosen the second structure. The second structure uses a separate communication machine to complete the transfer of information between the host and the control board. On the one hand, the communication machine exchanges information with the host through the dual-port memory, and on the other hand, it exchanges information with the position control board through the self-built local bus. The dual-port memory has a capacity of 2kb and it also functions as a data buffer. This scheme greatly reduces the CPU time used by the host for information exchange.

Figure 1 Two Structures of Distributed Multi-CPU Control System

Communication machines play a role in the system. It receives various axis position commands from the Pentium 166 interpolation and distributes these instructions to the position control board through the parallel port. In addition, the communicator also provides management functions for the keys and indicators of the CNC panel. The design of the communication device is shown in Figure 2. The imitation ISA bus in the figure provides a channel for transmitting information between the communication device and the position control board. The "imitation ISA bus" at this time is designed based on the needs of the position control board and the common control card. It reconstructs part of the signals of the standard ISA bus, including various signal lines and interrupt signals required for I/O operation. Line, ready control, power cord, etc. The position control board designed in accordance with the "imitation ISA bus" is fully compatible with the standard ISA bus. This has two advantages: 1 In the system development phase, the design and debugging of the communication machine and the position control board can be done by the microcomputer alone, with no dependencies between the two; 2 In the system with fewer control axes The first structure in Figure 1 can be used to directly insert the position control board to the backplane of the IPC to facilitate the reorganization of the system.

Figure 2 Communication Design Principle

During the design process, we used a custom dedicated bus between the communication machine and the position control board. The dedicated bus has high efficiency, but the position control board designed according to the dedicated bus is not compatible with the industrial control computer, so the interchangeability is poor, and development, debugging and maintenance are all too much trouble. To this end, we redesigned this part, took the road of openness, and adopted the "imitation of the ISA bus" approaching the standard bus, receiving good results.

3 Open Design of CNC System Software

The software design of the open CNC system first needs to select the appropriate operating system and software development tools. Currently used operating systems such as DOS, Windows 3.1, Windows 95, and Windows NT are all applied to the CNC system. DOS is essentially a single-tasking operating system. Multitasking under DOS can only be achieved through interrupt techniques. The software function modules of the CNC system cannot generally be executed at the same time. To execute them at the same time, it is necessary to solve the scheduling problems between modules. However, the scale of DOS is very small, people have more knowledge of DOS, and the degree of development and application on DOS is relatively easy. Windows 3.1 is a non-preemptive multitasking operating system that can accomplish multiple tasks at the same time. The disadvantage is that a certain task, such as when task A gets CPU resources, whether other tasks can be successfully executed depends on whether A can finish processing this event in time, so real-time performance is not guaranteed. Windows 95 and Windows NT are preemptive 32-bit multitasking operating systems with excellent operating interfaces and are suitable for the needs of CNC systems in terms of functions. However, its use in industrial applications remains to be confirmed. As a result of comprehensive consideration, we chose the DOS operating system. Correspondingly, we chose Turbo C++ 3.0 as a software development tool.
Software design work is divided into three parts: host software, communications software and position control card software. Establishing a sound communication protocol is its primary issue. In order to ensure the reliable transmission of data, the communication machine components adopt the time-divisional processing method to complete the information transmission with the host and the position control card. The time synchronization relationship is shown in Figure 3. The division of the time slice and the synchronization relationship between the three parts are controlled by the communication machine. At the beginning of the first time slice, the communication machine sends a synchronization signal to the host computer and the position control card to inform the host to write new data into the dual-port RAM, and at the same time, the position control card starts the position control operation. In the second time slice, the communication machine takes the position command from the dual-port RAM and distributes it to each position control card. At the same time, the actual position data is collected from each control card and written into the dual-port RAM.

Figure 3 Relationship between the various processes of CNC system

The host software is mainly composed of NC program editing module, manual operation, electrical control module, communication module, automatic processing, machine parameter adjustment, system positioning, screw torque compensation and other functional modules. The following uses the electric control module as an example to illustrate the open design method of the software module.
Electrical control is an integral part of all machine tools. In the numerical control machine tool, there are three methods to realize it: Externally mounted PLC, embedded PLC and virtual PLC. Various existing PLCs on the market generally have communication functions and can be integrated with the CNC system through a communication interface. This type of electrical control is called an external PLC; in addition, an intelligent I/O interface card can also be designed to pass through. The bus is directly integrated with the CNC system. The CPU on the card completes the switching logic operation and control. This mode is implemented as embedded. Can also use the host computer of the numerical control system to carry on the logical operation cyclically, cooperate with the ordinary switch quantity I/O card to realize the control of the electrical switch, this kind of method is called the virtual PLC.
If conventional program design methods are used, different software interfaces must be designed for the above three types of electrical control methods. The main body of the digital control system software will directly relate to the implementation mode and details of the electrical control. Once the control mode changes, it will be forbidden. There are no major changes to the software. Software written in this way is less versatile and difficult to adapt to unexpected changes. In order to increase the independence between software and hardware, we use object-oriented technology to open the design of the system.
Obviously, regardless of the type of control, the goals are the same. After careful analysis, we have found the common ground among the three, and we have obtained an abstract class CPlc, which provides all the external features of the electrical control of CNC machine tools and provides a complete message processing function for the main software of the CNC system. The rest of the CNC system simply sends a message to the PLC object to make the electrical switch act accordingly. This section does not deal with the details of electrical operation.
Based on the abstract class CPlc, the derived classes CExernalPlc, CEmbedPlc, and CVirtualPlc are defined for three ways. In these classes, the message interpretation and hardware operation are completed. The electrical control software obtained according to this design idea has the structure shown in FIG. As can be seen from the figure, this design method adds an abstract class hierarchy between the main software of the numerical control system and the electrical control hardware, weakening their interdependence and becoming relatively independent two parts. The numerical control system software obtained by this method has device-independent features. When a new hardware device appears, simply derive a new object class from the original abstract class, interpret the message according to a common standard, and manipulate the hardware to make a corresponding action, without making any changes to the rest of the software. The efficiency of software design has been greatly improved. In fact, after a clear specification of the message's data structure and its meaning, anyone else can refer to this specification to design new electrical control hardware and corresponding drivers. System. This is also the main purpose of open software design.

Figure 4 Abstract design of electrical control software

The above describes the idea of ​​open design of CNC system software with PLC as an example. According to this idea, we completed the software design of the CNC system. Although repeated analysis and discussion were conducted in the early stages of development, the hardware and software development was relatively smooth after the system structure was determined. From the perspective of development process and online debugging, open design can effectively shorten the software development cycle and improve the quality of CNC system software.

Thin Cylinder

Thin Cylinder,Thin Walled Cylinder,Pneumatic Thin Type Cylinder,Thin Hydraulic Ejector Cylinder

Dongguan Jinzhuang Hydraulic Technology Co., Ltd. , https://www.jinzhuanghydraulic.com