Multi-task real-time control of open CNC system based on Windows NT

With the rapid development of computer and communication technology, in the field of industrial control, the research and application of open CNC systems is an inevitable trend. The first item of the “Made in Manufacturing” in the current National Priority Development High-tech Industrialization Key Area Guide (Catalogue) issued by the National Planning and Development Committee and the Ministry of Science and Technology on July 14, 1999 is as follows: Applicable, advanced CNC machine tools And the open CNC system (total 112th item) can be seen that the state attaches great importance to the research and development of open CNC systems, because she is directly related to the competitiveness level of China's manufacturing field, and then determines whether the entire industrial base is solid. The open digital system facilitates system integration and end-user use of various control requirements with its excellent interoperability, portability, interchangeability and scalability.

In the research of open CNC system, it is feasible to use software to realize the core functions of the control system. The high real-time nature of the control system puts special requirements on the operating system. This paper will analyze the operating system platform of the full software open CNC system and give specific application implementation.

1 The real-time nature of the CNC system requires the current CNC system structure, whether it is a dedicated system or a PC-based computer, and is limited by computer performance and operating system, using a combination of hardware and software architecture.

The dedicated closed control system corresponding to the open control system currently occupies a considerable part of the market, including various numerical control devices of FANUC, AB and SIEMENS. The hardware structure and control software of these control systems are all designed to meet the requirements of real-time interpolation, position control, or dedicated hardware, or multiple CPUs. The PC-based CNC system benefits from a certain degree of openness in the PC. The system is built on a general-purpose operating system. The main system uses the Wimows+DOS operating system platform, because the current Windows products do not support real-time requirements. The scheduling management of high real-time tasks can only be used for human-machine interfaces and real-time weak tasks, while position control and interpolation operations are performed through hardware boards.

Since the numerical control hardware is generally a special design, the flexibility is poor, which restricts the opening pace of the numerical control system. On the contrary, the full software control system has the characteristics of convenience and flexibility, fully utilizing the popular operating system platform for support, standard application development environment, with good interoperability, portability, interchangeability and scalability to show its good. Open performance. Compared with the traditional PC-based structure, the full software CNC system should solve the software problem of interpolation operation and position control. The choice of operating system support for strong real-time functions becomes the key to full software CNC.

The functions of the CNC system can be divided into strong real-time, weak real-time and non-real-time according to real-time performance. The so-called strong real-time, that is, the system must end the processing of the control task within a given time frame, otherwise the consequences are unimaginable. Weak real-time is a response that can be delayed if the system cannot fully guarantee a response to the control requirements within the allowed time. Non-real time does not have strict requirements on system response time. For example, the interpolation operation performs a coarse interpolation for 2ms, and the position control takes a sampling for 1ms. If the scheduling management of the operating system cannot respond to them in time, the intermittent processing state occurs. Although decoding, tool compensation, and velocity calculations are closely related to interpolation, they are often weakly real-time tasks because they often wait for interpolation operations during automatic machining. The display update in the man-machine interface generally takes into account human visual factors, and is set to be more than 100ms, which is a non-real-time task.

2 Real-time analysis of the operating system In the open control system using the operating system, first consider the application range of the operating system, followed by the degree of support for real-time tasks.

The so-called openness is widely used and accepted. In the past real-time control systems, most of them used dedicated real-time operating systems such as VxWorks, RTOS and QNX, which did not meet the open trend and requirements. Currently widely used are Microsoft's Wm(i)ws and various versions of Unix (except Linux), but the various versions of these operating systems cannot fully support real-time control, which constitutes two contradictions.

Features such as are widely used in industrial control, but limited to weak real-time control systems. For Windows systems, it is a message-driven real-time multitasking operating system implemented by intercepting DOS's 08H interrupt. In the Widows message, the clock message WM-TIMER is considered to be a less important message. The message queue is always at the end, and when there are multiple WM-TIMER messages in the message queue, Wid (ws system itself will put All WM-TMER messages are merged into one WM-TIMER message. Simply relying on the clock message of the Windows system itself can not fully meet the requirements of the control system. In the other 5 or more systems, the system itself shields the hardware in the user application. Direct access to the hardware will generate an exception structure. This means that in the application under the Wimbws platform, direct access to the hardware is not feasible and the required clock precision cannot be obtained.

Modification, the introduction of WindowsCE based on real-time control. WindowsCE3.0 interrupt response speed of up to 50us, can fully meet the requirements of hard real-time, but its biggest drawback is that it only supports 1/3 of Win32API, can not meet GUI, network Required, therefore not fully competent for real-time control. In addition, a lot of Linux is currently discussed as a hope for real-time control of the operating system platform. The New Mexico University of Mining and Technology has developed Real-TimeLinux (RT-Linux) but it also has several shortcomings: security cannot be fully guaranteed; these shortcomings limit the application of Linux in real-time control systems, but with Linux research Its depth and scope of application continue to expand, and its operating system platform as a real-time control system is still very promising.

mature. The application of Windows NT's real-time extension system is indeed a good development direction. Many third-party software vendors have developed real-time extensions for Windows NT, such as Intime from RadSys and RTX from Ven-tuCom. Their solution is to use Win-d (wsNT to achieve weak real-time control tasks, take full advantage of NT's powerful graphical interface development capabilities, network and security, etc., while real-time tasks rely on real-time extensions.

3WindowsNT+ real-time extension platform application implementation We believe that the development of full software open control system should adopt the software structure of front and back end combined with multiple interrupts. The front and back control system structure is suitable for stand-alone environments, and multiple interrupts are suitable for distributed control environments.

However, in most front-end and back-end structures, there is often only one timed interrupt for real-time tasks, that is, interpolation operation, position control, and PLC are all completed in one timing cycle. This processing cannot effectively utilize system resources and improve system performance. The system is designed to operate in a single-machine environment. It adopts the structure of front and back, and sets three real-time timed interrupts for position control, interpolation operation and PLC. The priority is increased and the interruption period is decremented, which can achieve reasonable utilization of resources and improve system performance. purpose.

The main function of Windows NT real-time extension is to expand the real-time function and task priority of NT, and take over the scheduling management of NT by the real-time extended scheduling management, and add a series of real-time control functions, including: timer, interrupt, Management of physical memory, I/O operations, and lights.

The full software CNC system function is divided into two parts: Win32 process and real-time extension process. The Win32 process is responsible for real-time weak tasks such as man-machine interface, diagnosis, and interpolation preparation; the real-time extension process is responsible for positional control, interpolation operations, and PLCs with strong real-time tasks. Communication between them is achieved through a shared memory mechanism provided by real-time extensions, see .

The interrupt response speed of strong real-time tasks is the key to the all-software open CNC system. It is necessary to test the real-time performance of the system, which is the whole software CNC system structure 3) test. The running time is 1 hour. It can be seen that the latest cumulative time is 25.9. The interrupt response speed is related to the computer's main frequency, so the value of the low main frequency is large, and the value of the main frequency is small. Test results show that both can meet the requirements of strong real-time tasks.

4 Conclusion The full software control system based on popular operating system has good openness, free from the constraints of dedicated hardware architecture and special operating system, and brings convenience to end users in maintenance, use and system upgrade.


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