Abstract: This paper proposes a dual-CPU open CNC system with a PMAC motion controller as the core of the control system and an industrial control computer as the system support unit. The system's functions are described, and hardware and software design block diagrams are provided. Practice has proven the feasibility of this CNC system. Keywords: PMAC; dual CPU; CNC system; gantry milling machine 1 Introduction Modular open CNC systems represent the current development direction of CNC technology. The most common approach to realizing multi-CPU open CNC systems is PC-based CNC systems. There are three ways to PC-based systems: 1) adding CNC modules to a PC; 2) adding PC modules to the CNC system; 3) managing CNC programs as floppy disk files. Inserting the CNC module into a PC is a major PC-based approach. The PMAC (Programmabb Moltiple Axes Controller) motion controller is such a CNC module. This paper proposes a dual-CPU open CNC system with a PMAC motion controller as the CNC module and an industrial control computer as the system support unit. This control system was applied to the development of a CNC digital measurement and machining system for a gantry milling machine, achieving good results. This paper introduces a parallel dual-CPU open CNC digital measurement and machining system based on PMAC, and provides hardware and software design block diagrams. The system is characterized by parallel operation of each microprocessor, with software operations distributed across different processor levels, achieving real-time multitasking in the Windows/NT environment and improving system execution speed. [b]2 Hardware Structure of the CNC System 2.1 Introduction to PMAC[/b] The programmable multi-axis controller PMAC is a product of Delta Tau Corporation in the United States. The PMAC motion controller is a high-performance servo motion controller that, with the help of Motorola's DSP56001/56002 digital signal processor, can simultaneously manipulate 1-8 axes. These eight axes can be linked together for fully coordinated motion; each axis can also be placed in its own coordinate system, resulting in eight completely independent motions; or other combinations thereof. The PMAC motion controller can perform individual operations and calculations through its internally stored program, and can automatically prioritize tasks for real-time multitasking. The PMAC's ability to execute multiple tasks simultaneously and correctly prioritize them significantly reduces the burden on the host computer and programmer in terms of processing time and task switching complexity, improving the overall control system's operating speed and control accuracy. In addition, the PMAC also has functions such as executing motion programs, executing PLC programs, servo loop updates, resource management, and communication with the host computer. 2.2 CNC System Hardware Design The gantry milling machine control system is based on an industrial PC (IPC). A PMAC multi-axis motion controller and a dual-port RAM (DPRAM) are inserted into the internal expansion slot on the IPC motherboard, forming the machine tool's control center. The IPC CPU and the PMAC CPU (DSP56001) form a master-slave dual-microprocessor structure. Each CPU performs its corresponding function; the PMAC mainly handles the motion control of the machine tool's three axes, the control of control panel switches, and the control of digital acquisition, while the IPC mainly performs system management functions. To realize the PMAC's multi-axis motion control function, corresponding I/O boards, servo drive units, servo motors, encoders, etc., need to be expanded on the PMAC board, ultimately forming a complete control system. The control system hardware consists of a 233MHz industrial control computer, a PMAC Lite 1.5 motion controller, I/O boards, dual-port RAM (DPRAM), servo units, and AC servo motors. The CNC system hardware block diagram is shown in Figure 1. 1) Communication between the PMAC motion controller and the host computer uses two methods: bus communication and DPRAM-based data communication. The host computer and the PMAC motion controller primarily communicate via the PC bus, while data such as the status of the control card and motor, motor position, speed, and following error are exchanged via DPRAM. Bus communication refers to the host computer searching for the PMAC motion controller at a specified address, determined by jumpers in the PMAC. The dual-port RAM is mainly used for fast data and command communication with the PMAC. On one hand, when writing data to the PMAC, the dual-port RAM can quickly and repeatedly download position data or program information in real-time; on the other hand, when reading data from the PMAC, the dual-port RAM can quickly and repeatedly obtain system status information. For example, data such as the status, position, speed, and following error of an AC servo motor can be continuously updated and automatically written to the DPRAM by the PLC or PMAC. If the system does not use DPRAM, this data must be accessed via the PC bus using PMAC online commands (such as ?, P, V, etc.). Since data access via DPRAM does not require sending commands and waiting for responses through communication 13, the time required is much less, resulting in a much faster response speed. In this control system, data transfer between the host and PMAC utilizes the following functions provided by PMAC to DPRAM: ● DPRAM control panel function (from host to PMAC); ● DPRAM servo system status data feedback function (from PMAC to host); ● DPRAM background constant status data feedback function (from PMAC to host); ● DPRAM background variable status data feedback function (from PMAC to host); ● DPRAM ASCII communication buffer (bidirectional); ● DPRAM binary conversion program buffer (from host to PMAC); In addition to the above fast and automatic access functions, unused registers in DPRAM can be specified using PMAC's M variable and the host's pointer variable to achieve data transfer between the host and PMAC. When PMAC uses the data acquisition function, the acquired data is directly sent to DPRAM~I instead of conventional RAM. This control system utilizes DPRAM for automatic data access, improving the system's response speed and machining accuracy. The machine tool control system has a resolution of 1 ttm, and also facilitates fast communication between modules in the control system and the setting of address tables, making programming easier. 2) The built-in PLC function of the PMAC system is realized through the input and output of the intelligent I/O interface. In the control system, the input signals sent to the PLC mainly include: signals from the control panel and machine tool control buttons, selector switches, etc.; signals from the limit switches and mechanical zero-point switches of each axis; machine tool electrical action, limit, and alarm signals; contactor and pneumatic switch contact signals in the power cabinet; and working status signals of each servo module. These signals are sent to the intelligent I/O interface after opto-isolation. Opto-isolation effectively isolates the computer's digital signal channel from the external process analog signal channel, greatly reducing interference from external factors and improving the reliability and stability of the entire system. The PLC output signals mainly include: indicator light signals; action signals from control relays, contactors, solenoid valves, etc.; and drive enable and speed enable signals of the servo modules. These signals are sent to the corresponding relays via the I/O interface, ultimately controlling the corresponding electrical components. 3) The main function of the A/D interface board is to convert the rated analog voltage representing the bias amount sent by the contouring instrument into a digital quantity, which serves as a bias amount feedback signal for closed-loop contouring control or data acquisition. The D/A interface converts the speed command signals of each axis calculated by the CPU into analog signals and sends them to the AC servo modules of each axis to control the movement of the servo motors of each axis. The A/D converter of this system is built into the PMAC, and optocouplers are used for opto-isolation, effectively isolating the digital ground from the analog ground, thus improving the system's reliability and stability. 3 Software Design of the CNC System This CNC system adopts a front-end and back-end structure, and the entire software is divided into a front-end program and a back-end program accordingly. The design of the front-end program fully considers the openness of the software, so that functional modules can be added according to certain specific requirements. To achieve this function, a certain time slice must be reserved in the scheduling program. The PMAC application provides the function of calling these modules using interrupts. The front-end program mainly includes interpolation modules, servo drive modules, PLC monitoring modules, data acquisition and digital processing modules, etc., and some new control modules can also be added according to specific requirements. The functional modules of the front-end program are shown in Figure 2. [align=center] Figure 2 Front-end and Back-end Functional Module Diagram[/align] The back-end program mainly realizes human-machine dialogue, data processing and system management functions. Its functional modules are shown in Figure 3. [align=center] Figure 3 Background Functional Module Diagram[/align] 4 Conclusion This CNC system is based on a general industrial control computer and uses a powerful motion controller PMAC to undertake tasks such as interpolation calculation, position control, and speed control. Practice has proven that this open CNC system with a dual-CPU structure is completely feasible. This design scheme is of great significance for shortening the CNC development cycle and improving the machining accuracy, stability, and diversity of the CNC system. [References] [1] Delta Tau Data System Inc. PMAC USER'S MANUAL. 1998 [2] Delta Tau Data System Inc. PMAC/PMAC2 SOFTWARE REFERENCE. 1998 [3] Delta Tau Data System Inc. PMAC Option 2, Due Ported RAM User's Guide. 1998 Author Biography: Fu Xingzheng (1970-), male, from Dalian, Liaoning, engineer of Liaoning Radio Factory No. 2 (Group). Gantry Milling Machine CNC System Retrofit: PDF