Development of CNC System for CNC Flame Cutting Machine Based on PMAC

1 Introduction In recent years, the use of industrial PCs for machine tool control and the development of general-purpose CNC systems under the Windows operating system have become a new trend in the development of CNC technology. There are two ways to implement the CNC system under the Windows9x/NT operating system: one is a stand-alone mode consisting of one computer and some functional modules, and most of the numerical control functions are completed by software. The hardware cost of this mode is low. But you need to write the underlying driver (VxD) of the device to realize the real-time control environment under Windows. The software is written and maintained quite complicated, and the development cycle is very long. The other is the parallel dual-CPU host-computer communication mode. The non-real-time control part such as the machine interface, the lower-position machine is responsible for the real-time control part such as interpolation and servo update, and the mode is relatively more flexible and easier to implement. Among them, the numerical control system formed by inserting the PMAC motion control board into the industrial PC is proposed. The development cycle is short, the running speed is fast, and the control precision is high. This is an example of the second mode.

2 CNC system hardware design 2.1PMAC multi-axis motion controller Introduction DeltaTau's programmable multi-axis motion controller (PMAC) is one of the most powerful motion controllers in the world. It is powered by Motorola's DSP56001/56002. A digital signal processor that can manipulate 1 to 8 axes simultaneously. It can perform separate operations on programs stored in the internal, execute motion programs, PLC programs, update the servo loop, and communicate with the host computer in serial port and bus mode. Moreover, it can automatically prioritize the task for real-time multitasking, which makes it greatly reduce the burden on the host and programmer in processing time and task switching, and improves the running speed of the entire control system. control precision.

2.2 The hardware structure and working principle of the CNC system The CNC system is based on the industrial PC. The PMAC multi-axis motion controller is inserted into the ISA expansion slot of the industrial computer motherboard to form the control center of the machine tool. The CPU on the industrial computer and the CPU of the PMAC card form a master-slave dual-microprocessor structure. The two CPUs respectively implement the corresponding functions. Among them, the PMAC mainly completes the three-axis movement of the machine tool, the panel switch quantity and the digital acquisition control, and the industrial control machine Mainly realize the management function of the system. In order to realize the function of PMAC multi-axis motion control, it is necessary to expand the corresponding I/O board, servo drive unit, servo motor, encoder, etc. on the PMAC board to form a complete control system. The control system hardware consists of an industrial PC with a frequency of 233MHz, a PMAC-Lite1.5 motion controller, an I/O board, a servo unit, and an AC servo motor. The hardware block diagram of the CNC system is shown.

(1) The communication between the PMAC motion controller and the host computer adopts two methods, one is bus communication mode, and the other is data communication using dual port RAM (DPRAM). The host and PMAC motion controller communicate mainly through the PC bus. As for the status of the control card and motor, motor position, speed, following error and other data, the information is exchanged through the DPRAM. The bus communication mode refers to the host going to the specified address to find the PMAC motion controller, wherein the specified address is determined by the jumper of the PMAC. The built-in PLC function of the dual port (2) PMAC board is realized by the input and output of the intelligent I/O port. In the control system, the input signals input to the PLC mainly include: control panel and control buttons on the machine panel, selection switches, etc.; stroke switch of each axis, mechanical zero point switch, etc.; several bed electrical action, limit, alarm and other signals Although the signal in the cabinet, pneumatic switch contact and other signals; the working status of each servo module. These signals are optically isolated and sent to the intelligent I/O. The optical isolation effectively isolates the computer digital channel from the external analog channel, greatly reducing external factor interference and improving the reliability of the whole system. And stability. The signals from the PLC mainly include: indicator signals; control relays, contact energy signals, etc. These signals are sent to the corresponding PMAC-based CNC system via the I/O interface. The schematic RAM is mainly used for fast data and command communication with the PMAC. On the one hand, when used to write data to the PMAC, the dual port RAM can quickly download the location data information or program information in real time: on the other hand, when the dual port RAM is used to read data from the PMAC, The status information of the system can be obtained quickly and repeatedly. For example, data such as status, position, speed, and following error of the AC servo motor can be continuously updated and can be automatically written to the DPRAM by the PLC or by the PMAC. If the DPRAM is not used in the system, the data must use the PMAC online command ( For example, P, V, etc.) access data through the PC bus.

Since data access via DPRAM does not require a command to send commands and wait for a response through the communication port, much less time is required, so the response is much faster. In addition to the fast automatic access function, you can also use the M variable of the PMAC and the pointer variable of the host to specify the registers that have not been used in the DPRAM to realize the transfer of data between the host and the PMAC. When the PMAC uses the data acquisition function, the collected data is sent directly to the DPRAM. Instead of the conventional RAM, the selected appliance is connected to the transmitter, and finally controls the corresponding appliance.

3 software design of numerical control system 3.1 software structure of numerical control system The numerical control system adopts front and rear desktop structure, and correspondingly the whole software is divided into foreground program and background program. The foreground program is PMAC real-time control software, which mainly includes interpolation module, servo drive module, PLC monitoring module, machining program interpretation module, data acquisition and digital processing module, etc. It can also add some new control modules according to specific requirements. The foreground program function module is as shown. The background program is system management software, which mainly implements initialization, parameter input, machining program editing, system management and dual CPU communication. The daemon function module is as shown.

The background program function module CNC flame cutting machine processing technology features and the original track return function realization CNC flame cutting machine has the characteristics of general CNC machine tools, according to the CNC machining program, automatically complete from the ignition one preheating one oxygen one cutting one flame out one Returns the entire set of cutting processes at the origin. However, the CNC flame cutting machine is different from the general CNC metal cutting machine tool. It uses the oxygen-acetylene flame to heat the steel plate slit to the molten state, and uses high pressure oxygen to blow through the steel plate for cutting, unlike the metal cutting machine tool, which uses metal. The cutting tool is in rigid contact with the workpiece for cutting. Due to various factors, the phenomenon that the steel plate is not cut through sometimes occurs. At this time, the torch should be paused and returned to the uncut point accurately according to the original trajectory, and then re-cut according to the original trajectory. Therefore, the CNC flame cutting machine must have The ability to pause and return to the original track at any time.

During the cutting process, if there is any uncut condition, press the pause button to interrupt the cutting and automatically close the cutting oxygen; press the back button again, the torch will return to the original track accurately; the torch will be returned After the uncut point, press the pause button again and press the start button, the cutter will automatically open the cutting oxygen and re-cut. When the back button is pressed, the computer reads back the remaining number of pulses from the position counter on the PMAC card, compares it with the original number of pulses, sends the difference back to the position counter, and inverts the direction signal, thus making The torch returns as it was. At the end of the instruction, the system is directed back to the previous instruction to achieve a continuous return between the segments.

4 Conclusions Based on the analysis of the machining process of CNC flame cutting machine, the numerical control system of CNC flame cutting machine based on industrial PC and PMAC board was developed. The numerical control system not only contains all the functions of the ordinary numerical control system, but also has the functions of automatic ignition, preheating, cutting oxygen, machining end extinguishing and rapid retraction and advancement of the cutting torch according to the processing trajectory. Features. In addition, the system software also has a complete graphical programming, dynamic tracking display of processing trajectory, dynamic simulation of processing trajectory, fault diagnosis, processing program communication transmission and other functions. The system is very convenient to use and is suitable for industrial enterprises. It has been successfully used in the transformation of CNC flame cutting machine in Wuchang Shipyard.


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