1 CNC system definition and structure The computer numerical control system (referred to as CNC system) is developed on the basis of hardware numerical control, which replaces the functions performed by the previous numerical control device with a computer. Therefore, it is a digital control system including a computer, and performs some or all of the numerical control functions according to a computer stored control program. According to the definition of the CNC standardization committee to which EIA belongs, the CNC uses a computer that stores programs to perform some or all functions of the numerical control device according to a control program stored in the read/write memory in the computer. The only device outside the computer is interface. At present, the computer used in the computer numerical control system is no longer a small computer, but a microcomputer. The system controlled by the microcomputer is called an MNC system, and is also collectively referred to as a CNC system.
It can be seen from the above definition that the CNC system differs from the conventional NC system in that the CNC system attaches a computer as part of the controller, and the hardware circuit performs other control operations.
2 CNC system software refers to the special software compiled for the realization of various functions of the CNC system, that is, the system program stored in the computer memory. It generally consists of an input data processing program, an interpolation operation program, a speed control program, a management program, and a diagnostic program.
3 The characteristics of the computer numerical control system Compared with the NC system, the main advantages of the CNC system are: flexibility: This is the outstanding advantage of the CNC system. For the CNC system, new functions can be supplemented and developed by changing the corresponding control program without having to manufacture new hardware.
The CNC system can evolve with the development of the plant and can adapt to the changing process requirements in the future. After the CNC equipment is installed, new technology can be added to the system, which extends the life of the system. Therefore, the CNC system has a lot of "flexibility" flexibility.
Versatility: In the CNC system, the hardware system adopts a modular structure and relies on software changes to meet various requirements of the controlled device. The use of standardized interface circuits brings many conveniences to machine tool manufacturers and CNC users. Therefore, with a CNC system, it is possible to meet the requirements of most CNC machine tools (including lathes, milling machines, machining centers, drill boring machines, etc.), and to meet certain other equipment applications. When the user asks for some special features, it is just a change to some software. Training and learning are also very convenient due to the same type of control system used in the factory.
Reliability: In the CNC system, the machining program is often sent into the computer memory once, avoiding the shutdown phenomenon caused by the failure of the tape input machine during the machining process. More than half of the failures of the ordinary numerical control device occur in the segment by segment. When photoelectric input).
4 CNC system hardware structure With the development of large-scale integrated circuit technology and surface mount technology, CNC system hardware modules and installation methods continue to improve. From the overall installation structure of the CNC system, there are two types of integral structure and split structure.
The so-called monolithic structure is to install a circuit board composed of a CRT and an MDI panel, an operation panel, and a function module board in the same chassis. The advantage of this approach is that it is compact and easy to install, but sometimes it can cause some signal connections to be too long. The split structure usually has a CRT and MDI panel, an operation panel, and the like as one component, and the circuit board composed of the functional modules is installed in a chassis, and the two are connected by wires or fibers. The CNC operation panel is mounted on the machine tool in the form of hanging type, bed type, control cabinet type, console type and so on. From the structural characteristics of the circuit board that constitutes the CNC system, there are two common structures, namely a large plate structure and a modular structure.
5 CNC system control software structure characteristics CNC system is a dedicated real-time multi-tasking computer system, in its control software combines many advanced technologies in today's computer software technology, the most prominent of which is multi-task parallel processing and multiple real-time Interrupted.
6 CNC system control software and its work process control software is dedicated software for the completion of specific CNC (or MNC) system functions, also known as system software (or system program). Because the functional settings and control schemes of CNC (or MNC) systems are different, various system softwares vary greatly in structure and scale.
The design of the system program has the most direct relationship with the realization of each function and its future expansion. It is the key and most important part of the development of the entire CNC (or MNC) system. As mentioned earlier, system software generally consists of input, decoding, data processing (pre-calculation), interpolation operations, speed control, output control, management procedures, and diagnostic programs. The following are introduced separately.
6.1 Input CNC system Generally, the part program is input through the tape reader, tape drive, disk and keyboard, and most of the input is interrupted. There are corresponding interrupt service programs in the system program, such as the tape reader interrupt service program and the keyboard interrupt service program. When the tape reader reads a character into the interface, it issues an interrupt to the host, which is sent to the memory by the interrupt service routine.
6.2 Decoding As can be seen from the previous discussion, the data segment has been sent to the part program memory via the input system. The next step is to translate the input part program data segment into a language that the system can recognize by the decoding program. The following steps are required for a data segment from input to transfer to the interpolation working register.
6.3 Pre-computation In order to reduce the burden of the interpolation work and improve the real-time processing capability of the system, the data is often pre-processed before the interpolation operation, for example, the calculation of the arc plane and the radius compensation of the tool (). When the digital integration method is employed, the normalization processing of the left shift and the calculation of the number of integration times can be performed in advance, so that the most direct and convenient form of data can be supplied to the interpolation operation.
6.4 Interpolation Calculation Interpolation calculation is one of the most important calculation tasks in the CNC system. In a conventional NC device, a hardware circuit (interpolator) is used to implement interpolation of various tracks. In order to calculate the required interpolation trajectories in the software system, these digital circuits must be simulated by a computer program.
The problem of using software to simulate a hardware circuit is that a system with three or more axes linked has three or more hardware circuits (such as a digital integrator per axis), and the computer uses several instructions to implement the interpolation work. of. However, it takes a certain amount of time for the computer to execute each instruction. However, the calculation speed of some small or micro computers is difficult to meet the requirements of the NC machine for the feed rate and frequency division.
The function of the 6.5 output output program is: (1) Servo control. As mentioned above. (2) When the feed pulse changes direction, the backlash compensation process is performed. If an axis changes from positive to negative, Q forward pulses are output before the reverse; otherwise, if negative from positive to positive, Q negative pulses are output before the reverse (Q It is a backlash value that can be preset by the program). (3) Perform screw pitch compensation. When the system has an absolute zero, the software displays the absolute coordinate value of the tool at any position. (4) Outputs of auxiliary functions such as M, S, and T.
6.6 Management and Diagnostics Software management software in the CNC (MNC) system involves only two items, namely CPU management and external device management. Since the machining of CNC machines is based on a single part, a part program can be divided into several blocks. The execution of each block is divided into data analysis, calculation, pass control, and control of other actions. In the actual system, usually a main program is used to string the whole process. After the main control program analyzes and judges the input data, it transfers to the corresponding subroutine processing. After the processing is completed, it returns to analyze and judge the data. Operation.
The ability to easily set up various diagnostic programs is also one of the features of the CNC and MNC systems. A more complete diagnostic program can prevent the occurrence or expansion of faults. After the fault occurs, the type and location of the fault can be quickly identified and the downtime can be reduced. The CNC system of some foreign companies can also perform communication (overseas) diagnosis, and the communication diagnostic center instructs the system or the operator to perform some test runs to find faults or fault locations.
7 Conclusion The above content discusses the structural characteristics and composition of the computer numerical control system. The computer numerical control system is a high-tech product that integrates high, precise and sharp technologies into one machine, electricity, light and liquid. It has many advantages such as high processing precision, stable and reliable processing quality, high production efficiency, strong adaptability and good flexibility. It has been widely welcomed in various industries and is receiving more and more attention in terms of use.
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