Research On The Open CNC System Of Five-Coordinate Virtual Axis Polishing Machine

Along with the development of science and technology, tremendous changes have happened to traditional manufacturing industry, which have gradually entered into the modern manufacturing models. However, CNC technology is the key in manufacturing systems. In some extent, the level of NC technology has become an important indicator of a country's manufacture. From the future developmental trend of numerical control system, traditional control system is a closed structure, because of the monopoly of technology and market. The hardware of this structure is exclusive, so the products, such as mainboard, the specialized servo circuit etc, have no interchangeability between different manufactures. Also the exclusive structure of CNC software has no portability and no scalability. As the strong-specific use, the control software relies on the hardware architecture and software design philosophy of specialized control system seriously. Therefore, in order to adapt the rapid changes of unborn technology, market and the organization structure of production etc, client-oriented modular characters and reconstruction of soft hardware are required, which should be quick, efficient and economic. NC system must change the direction of special purpose and closed types to universal and open architecture ones. So the open CNC system emerges as the times require.For CNC systems, the most important thing is to control the movement of all motors. It is CNC controller that controls all movements of each motor to realize NC machining in accordance with the instructions. The key indicators, such as precision of positioning control and performance of speed adjustment etc, are related to motion controller. Obviously, motion controller occupies a very important position in the open CNC systems. The architecture of multi-axis motion controller for the open numerical control machine tool is shown in Fig.1. As the host machine, PC issues all kinds of commands or the interpolation procedures, and finishes data communications by putting data on the single port RAM with PCI bus. After reading these data and progressing interpolation by TMS320F2812, theoretical position will be got. Simultaneous detection of servo motor position helps to consist of position feedback to adjust the location. The principle of location adjustment is: after reading the theoretical positions transmitted or interpolated by PC, and sampling feedbacks form position feedback module, the actual locations will be got through error compensation. After receiving the signals of motor encoder, position feedback module calculates the error between the theoretical position and the actual position. By outputting analog control voltage, which will be connected to the input interface of speed instructions, D/A conversion functional module controls the movement of servo motor. The design of driver procedures will be impacted gravely by the characteristics of hardware equipment, so meticulous analysis of hardware equipment and detailed understanding of the hardware features are required before the driver programs are designed. The using of WinDriver can greatly simplify the difficulties of device driver design, also can discover the hardware and generate driven codes automatically. Based on the kernel of WinDriver, the driver procedures are developed. According to the open-system characteristics, the procedures are compiled into API components format, which are provided to the applications in the way of non-MFC DLL.In general, master-slave mode of communication system has three ways: serial communications, parallel communications and shared memory approach. The shared memory approach is decided after analyzing the various forms of communications and features of motion controller. The shared memory method also has two ways: one way is that the host and DSP can not visit memory synchronously, which usually uses the DMA(direct memory access) function provided by DSP; the other is that they can visit memory at the same time, which is always achieved through dual-port memory. The former mode is used in this paper. The SARAM(CY7C1041) was expanded on controller, and was mapping of memories both on DSP and on the part of 9054. The mapping physical address of PC was: BAR2(Ox5000000～0x50fffff) and BAR3(0x5100000～0xfffff). In order to communicate efficiently and rapidly, also avoid the death cycles, because of visiting external SARAM at the same time between PC and motion controller, the means of interrupt response was used to visit the external SARAM. Through interrupt, both computers accessed control identifiers and then decided whether to visit expanded RAM at this time. When the data was transferred, the 9054 processor wrote the data into a designated area, and the control words into the interrupt control addresses. Then the interrupt signals of data transmission were generated by expanded SARAM. DSP responded to these signals, and read the state of control words. Thus the operation of response was finished, and the interrupt signals were removed at last. Similarly, this kind of interruption was used by the return data of DSP to finish the data processing with NC system of PC. Accessing the expanded RAM with the manner of interruption, can both transfer the data rapidly and efficiently, and maintain the results of real-time response. The expanded RAM was divided into four districts. Two control characters were used to complete the information transmission between PC and motion controller and the data format was structure. As is shown in Fig.2.A real sense of NC system based on PC should be such a system, which has the open architecture that enables all the functional components of the system easy to change. This requires consideration of the scalability and extensibility for the overall software, when the software is planning. Fig.3 shows that: the host PC provides a friendly alternating interface between man and computer, which includes all kinds of status setting, explaining of processing parameters, choices of motion control and detection of processing situations etc. After accepting the transmitted data, DSP achieves the specific motion control. This also could be subdivided into two smaller levels. The format of API interface can be developed following each functional level in order to build sound interface between layer and layer and facilitate later physical expansion of the functions.In the field of NC processing, parity is used as the method of data transfer check in current CNC machine tool. This method increases the volume of data redundancy, sends check code and data to receivers together. After identical verification of the date received, the checksum codes are got. Comparing this kind of code with another one, the transmission is considered to be correct if the two are the same. However, these methods have their own shortcomings, such as high misjudgments relatively. According to this situation, CRC (Cyclic Redundancy Check) is presented, which comes from branches of a linear code, and its main application is binary codes.CRC uses encoding method of polynomial expression. Processed data block can be seen as a binary n-order polynomial expression: a_n-1x^n-1+a_n-2x^n-2+…+a₁x+a₀Such as a binary data 10110101(8 bits) can be expressed as: 1x⁷+0x⁶+1x⁵+1x⁴+0x³+1x²+0x+1The processes of polynomial multiplication and division are the same with the general ones. The operations of polynomial addition and subtraction are D-2 to get remainder, no dislocation and carry, and agree with the logical XOR operations. The sender and receiver use the same generator polynomial g(x), whose coefficients of the first bit and the last bit must be1. The disposal of CRC method is: D-t(x) by g(x), the sender get remainder, which will be the checksum codes of CRC. Checking whether the calculated result of the correction is 0, the data frame errors will be judged during test.In summary, the driver procedures of servo motion controller based on the kernel of Windriver was developed. According the open system characteristics, the reading, writing and interrupted program of the controller were made into API software, which were provided to the applications in the way of non-MFC DLL. And based on the open-system features, CNC basic functional modules were classified, the system interfaces were developed. Against the rough interpolation, decoding and other functions, the reciprocal API components were developed to facilitate the users to develop and perfect. The error analysis of the rough interpolation was undergone in this paper. According to the experiment in hybrid virtual surface polishing machine tool, it can be seen that software system meets the design requirements.The method of CRC presented in this paper, can provide high and reliable fault location function, also can detect all the odd random errors by 100% or sudden mistakes where the length is less than k(k is the module of g(x)).