Design of high speed laser marking control system

2022-08-09
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Design of high-speed laser marking control system based on DSP

galvanometer scanning laser marking technology is to change the propagation direction of the laser by controlling the deflection angle of two high-speed galvanometers, and mark the surface of the workpiece through the focusing of f-theata lens on the surface of the workpiece. Compared with traditional marking technology, it has the characteristics of wide application (suitable for machined surfaces of different materials and shapes), no mechanical deformation of workpieces, no pollution, fast marking speed, good repeatability, high degree of automation, and has a wide range of applications in industry, national defense, scientific research and many other fields. High speed and high precision galvanometer marking has become the development direction of today's marking industry

the traditional galvo marking control system is connected to the single-chip control board through the serial port and parallel port ISA bus of PC. this way has simple interface, convenient connection and low development cost. However, due to the low transmission speed, it can no longer meet the real-time requirements of modern CNC system. This paper makes some new explorations in laser marking control technology: using the high-speed data transmission of PCI and the high-speed data processing ability of DSP, a method of "PC +pci bus +dsp control board card" is proposed for the galvanometer marking control system, so as to realize the accurate control of marking control, improve the control efficiency and ensure the real-time performance of the system. DSP control board is the core of the whole system, which directly determines the scanning speed and accuracy of the system. This paper will focus on the design of the control board

1 DSP chip

the main chip of DSP control board is TMS320C6205, a high-speed data processing chip of Texas Instruments C6000 Series. The chip is a high-performance fixed-point processor, which can speed up the upgrading of plastic granulator technology. The main frequency can reach 200MHz. Each cycle can execute 8 32-bit instructions, and the processing speed can reach 1600mi μ~ twelve μ The strength of PS will be high; Tms320c62xtm DSP core with high-performance VLIW structure has 8 independent functional units and 32 32-bit general registers; Provide 64K bytes of internal program ram and 64K bytes of internal data RAM; Provide 32-bit external memory seamless interface, including synchronous devices (such as SDRAM, sbsram, etc.), asynchronous devices (such as flash, SRAM, etc.) and addressable 52m bytes of external storage space; Provide flexible PLL and clock generator, and configure frequency doubling value; Provide PCI bus interface conforming to PCI 2.2 specification, and directly realize the bridge function between chip and PCI bus; Provide two 32-bit timers; Provide JTAG boundary scan interface for debugging. With this chip, high-speed data processing can be realized, and the real-time performance of the system can be guaranteed. Due to the PCI bridge function, the interface with PCI bus is provided, which is economical and reliable

2 hardware design

2.1 structure block diagram

Figure 1 shows the hardware structure block diagram of the system. DSP control board is connected with PC through PCI bus to realize high-speed communication. DSP processing module is the main control module, and TMS320C6205 chip with the main frequency of 200MHz is used as the main control chip. The DSP processing module makes full use of the fast computing power and high-precision timer of C6000 Series DSP, which can ensure that the galvanometer marking machine can mark at a constant speed and high speed. These are not possible for PC to customize special materials with different surface properties according to the needs of customers. The peripheral circuits of DSP include memory module, reset control, power control, clock system, JTAG port, digital to analog conversion module, CPLD logic control module and photoelectric isolation module. The storage module includes flash module and SDRAM module. Flash is used to store system startup code and software code, and SDRAM is used to provide additional storage space required for software operation. The DSP control board outputs two analog quantities to control the movement of the two galvanometers, outputs Q-switch control signals to control the switching light of the laser, and inputs/outputs 16 photoelectric isolation signals for function expansion

2.2 communication between PC and DSP

pci bus is a local bus that is not attached to a specific processor. In terms of structure, PCI is a primary bus inserted between CPU and the original system bus. Specifically, a bridge circuit realizes the management of this layer, and realizes the interface between the upper and lower levels to coordinate the transmission of data. The manager provides signal buffer to support 10 kinds of peripherals and maintain high performance at high clock frequency. PCI bus also supports bus master technology, allowing intelligent devices to obtain bus control when needed to speed up data transmission. Compared with ISA bus, PCI bus has the advantages of fast transmission speed and large transmission volume

TMS320C6205 is selected for this system. The chip comes with PCI bus bridging function in line with pci2.2 specification. The developer avoids the hardware and software implementation of PCI protocol, which brings convenience to the system design, shortens the development cycle, and saves the development cost. Developers only need to connect the bus signal on the PCI slot directly with the relevant PCI bus signal on the DSP chip. The DSP control board with "golden finger" can be directly inserted into the PCI card slot of PC to realize the communication between PC and DSP. PCI devices can access all internal RAM space, peripherals and external memory space

the PCI bus width used by the DSP control board is 32 (3.3V), the bus frequency is 33MHz, and the transmission rate is 33 × 32/4MB/s = 132MB/s 。 This transmission rate guarantees the high-speed operation of the whole system

2.3 CPLD logic control

the logic control of the whole high-speed system is realized by high-speed CPLD chip. The max7128e chip of Altera company is selected for implementation. The available programming logic gates are 2500, the number of macro units is 128, the number of logic array blocks is 8, the user can define 100 i/o pins, and the pin to pin delay is 5ns. MAX7000 series devices can be programmed through a programmer, or programmed. This design adopts programming (ISP). ISP allows rapid and convenient repetition of programming in the process of design and development, simplifies the manufacturing process, and allows devices to be assembled on the printed board before programming

in the system design, the address of CE1 space is used for LED signal lamp, flash, Da chip, 16 channel i/o photoelectric isolation interface, analog switch, Q switch, PWM output and software reset control. In order to prevent the mutual interference of these devices, the input address must be decoded. By judging the pa[2:6] and pa[16:21] input to CPLD, we can know the address area that DSP is accessing, and decode the address of CE1 space, so as to generate corresponding control signals to realize logic control and timing control

the high address of the register built on CPLD is the same, which is named DSP_ reg_ Addr is composed of pa16~21. If pa16~21 is set to "111000", it means the address 0x0178xxx

the low address is composed of pa2~6, addressing 10 registers, and the address correspondence is shown in Table 1

2.4 digital to analog conversion module

digital to analog conversion module converts the digital signal processed by DSP into analog signal to control the deflection of two galvanometer. As the requirements for marking accuracy are getting higher and higher, the traditional 8-bit analog-to-digital converter can no longer meet the needs of users. Therefore, the system selects the 16 bit high-precision digital to analog converter ad669 chip of ADI company, as shown in Figure 2. Ad669 is a 16 bit parallel input, two-level data cache structure. In the design, the direct grounding of/l1 signal is set to be effective, and the primary cache and secondary cache are controlled by controlling/cs and LDAC signals respectively. The voltage range of the control galvanometer signal is -10v ~ +10v to mark 100mm × For example, for a label with a size of 100mm, the accuracy can reach 100mm/216=0.0015mm, and the corresponding minimum output voltage is 0.00031v

through experiments, it is found that when powered on, the output of ad669 chip is an uncontrollable quantity, which will make the galvanometer deflect at the moment of power on. If the deflection amplitude is too large, long-term use will lead to the rupture of the galvanometer. In order to protect the galvanometer, an analog switch circuit can be designed to control the output of ad669 chip when it is powered on to make it 0V. The author puts the analog switch on the reference voltage input of ad669 chip, and controls the analog switch through CPLD to control the presence or absence of the reference voltage, so as to ensure that the galvanometer will not deflect when powered on

3 PCB design

the control board adopts a high-speed DSP processing chip with a main frequency of 200MHz. In the high-speed signal system, there are EMC problems, which will affect the performance of the system. In order to design a stable control with good anti-interference performance and obtain a certain positive effect, the following measures are taken

1. Reasonable arrangement of the board layer

the control board is a six layer board, and the board layer is designed as (from the top layer to the bottom) signal layer - layer - power layer - signal layer - layer - signal layer. Such a board layer structure arrangement makes each signal layer and power layer close to a layer, providing a shorter return path for the signal

2. Clock signal line processing

half of the PCI clock signal depends on the reflected wave to improve. Therefore, the CLK routing length of the clock signal is approximately 2500 mils, which is realized by using a serpentine line (this point is clearly specified in the routing requirements of pci2.2 specification). For DSP chip, the crystal oscillator circuit should be as close to the DSP chip as possible, and the clock signal should be as short as possible

3. Processing of SDRAM related signal lines

sdram working frequency is 100MHz. At high frequency, the transmission time of the signal is directly related to the length of the signal line, and this problem cannot be ignored. Therefore, the data line and address line of SDRAM should be of equal length to ensure the quality of signal transmission. In addition, crosstalk and ringing problems are also easy to occur at high frequencies. For the control signals, data and address bus signals of SDRAM and DSP interfaces, matching resistors are connected in series at the source end to improve the signal transmission quality and ensure that SDRAM can work normally at high frequencies

4. Isolation treatment of digital and analog circuits

there are digital circuits and analog circuits on the control board. In the layout, the isolation of digital and analog circuits must be considered, and the digital circuits and analog circuits should be arranged in blocks as far as possible to avoid the digital signal routing across the analog circuit area, so as to prevent the mutual interference between the two circuits. In addition, the digital circuit and the analog circuit are grounded at one point through a 0 ohm resistor

5. Use of capacitors

place a 1.01uf decoupling capacitor next to the power pin of each digital chip

4 summary

this system combines high-speed PCI bus with C6000 high-speed DSP processor, coupled with high-precision digital to analog conversion module, realizes a set of high-speed and high-precision control system, and successfully applies it to galvanometer laser marking system. The system makes full use of the high-speed processing ability of DSP and the internal high-precision timer to share the real-time task of PC, so as to realize the complementary advantages of PC and DSP control board, realize the real-time marking, and ensure the uniformity of marking quality. This paper also gives the attention points of DSP control board in PCB design stage. This board has been put into use in production practice and has good stability and anti-interference. (end)

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