1.2 Software structure of CNC PDA
There are many embedded operating systems currently used on handheld computers, such as Windows CE, Pocket OS, Palm OS, Hopen, and embedded Linux. Windows CE.NET as the current Pocket PC is an operating system based on the Win32 Application Programming Interface (API). We use Windows CE.NET and Visual Studio.Net as the operating system platform and application development platform of the CNC PDA. The entire software system is designed according to functional modules. The system implementation is divided into three levels, namely the human-machine interface and the system management layer, the data organization and the management layer, and the communication layer for communication with the superior server and the lower-level embedded machine tool controller. The communication layer includes the establishment of a communication link, the implementation of a data link communication protocol, and the specific implementation of the presentation layer. Database management completes the creation, sorting, retrieval and maintenance of data to achieve true mobile transmission. The control management layer of the system is the core of the whole software system. For the CNC PDA, the difference between its function and the degree of intelligence is reflected in this layer. The increase and decrease of system functions are also carried out at this level, including the program. Compile interpretation, adjustment, automatic, diagnostic, parameter setting, simulation and other functional sub-modules related to user operations.
Each functional module exchanges information with the underlying layer through a wireless communication program. The structure of its software function module is shown in Figure 3. The communication layer uses three types of communication methods: serial port, USB and wireless communication port. The serial communication mode adopts RS232 serial bus technology, full-duplex, the transmission baud rate is 57600bps; the USB communication mode adopts USB2.0 high-speed universal serial bus, the transmission rate can be up to 480Mbps; the wireless communication mode follows the IEEE802.11 communication protocol. Supports a maximum transfer rate of 54Mbps. The application software of the whole system has functions of control, data processing, human-computer interaction, and network communication.
2. New digital communication wireless communication technology
2.1 802.11 communication mechanism
At present, the wireless networking methods of commonly used handheld computers include GPRS, IEEE802.11 (Wi-Fi), Bluetooth technology, and home network HOMERF. These methods have their own advantages and disadvantages. IEEE 802.11 is the latest version of the wireless local area network (WLAN) standard in 1999. Compared with the HOME RF standard of the Bluetooth protocol and the home network, the IEEE802.11 protocol has the advantages of long transmission distance and fast transmission speed, and is the mainstream of wireless communication technology development. Like other IEEE 802.x standards, the 802.11 standard focuses on the physical layer (PHY) and medium access control (MAC) sublayers. The MAC layer of IEEE 802.11 provides access control to wireless shared media. In addition, it provides the network connectivity, authentication, security, and power management features necessary to support wireless transmission and site mobility. In recent years, the 802.11 standard has expanded to include 802.11b, 802.11a, and 802.11g. These extended standards use different physical layer standards to increase the communication rate to 11 Mbit/s and 22 Mbit/s to 54 Mbit/s. But they have not changed significantly for the MAC layer protocol.
2.2 The specific implementation of wireless communication in the new numerical control
In the new CNC architecture, the CNC PDA communicates with the shop floor server and machine tool controller via the IEEE802.11x communication protocol. According to the different configurations of the machine control system, the implementation of wireless communication is also different. There are mainly the following:
(1) A wireless communication interface circuit connected to the wireless module of the digital PDA and the single-chip numerical control system;
(2) The wireless module of the CNC PDA is connected to the RS232/802.11 converter of the traditional CNC system;
(3) Communication with the 802.11x wireless network card of the embedded machine controller through the wireless module of the CNC PDA.
In the single-chip numerical control system, the wireless communication between the digital PDA and the single-chip microcomputer is realized through a wireless communication interface circuit based on single-chip microcomputer. The single-chip wireless communication interface circuit is composed of a single-chip transmitter chip (MICRF102) and a single-chip receiver chip (MICRF007) introduced by Micrel. Working in the international ISM frequency band 300 ~ 400MHZ, with ASK modulation and demodulation capabilities, strong anti-interference ability, very suitable for industrial control. At the same time, it has the advantages of good frequency stability, high sensitivity, and low power consumption. It can facilitate the serial data wireless communication between the CNC PDA and the single chip microcomputer. The working principle of its interface circuit is shown in Figure 4.
For traditional CNC machines, the communication interface provided by these devices is usually a common RS-232 serial interface. How to make these devices communicate with CNC PDA? An economical and fast solution is to connect an RS-232/802.11 converter to the serial interface of the traditional CNC system, and connect the serial RS-232 signal of the CNC device with wireless communication. The signals are converted to each other to conform to the IEEE 802.11x communication protocol. It enables the numerical control equipment to be networked with the wireless local area network, effectively expands the networking communication capability of the numerical control device, and finally enables the fixed numerical control device to realize wireless communication with the freely-moving and controlling CNC PDA.
In short, a wireless serial port based on the IEEE 802.11 protocol is an important trend in digital manufacturing under the network environment. Its wireless communication structure is shown in Figure 5. The realization of wireless communication between the two can actually be seen as replacing the wired serial connection with a wireless module, and also regards the wireless channel composed of two wireless modules as a two-port device. For the two devices, the correspondence between the wired connection and the wireless connection terminal is the same in serial communication. The communication programming for replacing the wired connection with the wireless channel is also basically the same. But you need to pay attention to the following points:
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