Design of Laser Marking Controller Based on DSP

With the continuous expansion of the application scope of laser marking machines, the speed and accuracy requirements for laser marking are getting higher and higher. TI (Texas Instruments)'s TMS320F2812 DSP is a high-speed processor designed specifically for industrial control applications. It is of practical significance to use it to develop laser marking controllers. The laser marking controller designed in this article has broad application prospects. 1 Working principle of the controller The upper computer of the galvanometer laser marking controller is a computer with marking software installed. The text and graphics are processed into a large amount of marking data through image processing, and the effect diagram is displayed on the marking software interface. The marking data is transmitted to the extended memory RAM by the USB bus, and then taken out by the DSP in sequence and sent to the D/A conversion chip. After conversion, the D/A chip outputs an analog voltage of -5 to 5 V to drive the scanning galvanometer and control the power of the laser power supply. A GPIO (general input/output) pin controls the switch of the laser energy. The x and y axis galvanometers control the laser focus to move orderly on the two-dimensional plane to complete various forms of text and graphic marking. 2 Hardware circuit design of galvanometer laser marking controller The principle block diagram of the system is shown in Figure 1. The galvanometer laser marking controller mainly includes USB communication circuit, expansion memory circuit, D/A conversion circuit and CPLD circuit.

2.1 USB communication interface
The USB communication module uses Cypress's CY7C68013. There are two ways to connect USB to DSP: FIFO and GPIF. In this system, FIFO is used, and the read and write signals are provided by CPLD and DSP. The SLOE of the USB chip is connected to the external interrupt of the DSP. Other control signals are decoded by the DSP through the CPLD and connected to the USB chip. The data line and two address lines of the DSP are directly connected to the USB.
2.2 High-speed D/A conversion circuit The digital
-to-analog conversion part is the key part of the controller. Three D/A channels are required in this system to control the x-axis and y-axis of the galvanometer and the laser power respectively. The main technical parameters of AD7836 for this system meet the requirements:
(1) Single-chip 4-channel 14-bit D/A conversion; (2) Voltage output, the maximum voltage output range is ±10 V; (3) The typical value of the output voltage settling time is 16 μs.
The D/A voltage resolution is: 5 V/213 = 0.61 mV. Compared with bipolar and unipolar voltage output, the circuit omits the operational amplifier that changes the voltage polarity, making the circuit simpler.
Ad7836 supports interfaces with microprocessors and DSPs above 16 bits, including 14-bit data lines, 3-bit address lines A0, A1, A2, and control signals CS, CLR, WR, and SEL. When CS is at a low level, AD7836 is selected; only when CLR is at a high level can the internal data register value of the D/A converter control the analog voltage output value; WR is valid at a low level and can be combined with CS to write data into the input buffer. When SEL is at a high level, the user-set register E value is output to VOUT, so it can be grounded. When the system is working, due to the use of the external interface XINTF, the operation of the D/A chip is the same as reading and writing data from SRAM. The starting address of the external interface 2 area that controls the D/A chip is 0X08 0000. The corresponding value can be changed at any time in the program to control the voltage value of the D/A conversion, and then control the position of the marking point. If you want to mark an image, you can output the position and laser energy of each marking point in a line-by-line scanning manner.

The pin connection between DSP and AD7836 is shown in Figure 2.

Since the driving voltage range of the galvanometer used is also ±5 V, the reference voltage VREF(+) and VREF(-) of each channel in this system are connected to ±2.5 V respectively. The precise ±2.5 V reference voltage uses Microehip's dedicated 2.5 V voltage reference source MCPl525 and OP operational amplifier MCP606 in the hardware circuit design. As shown in Figure 3, after the +2.5 V voltage reference is generated by MCPl525, in order to reduce noise, RC low-pass filtering and MCP606 are used. Here, MCP606 is used as a voltage follower. As shown in Figure 4, the -2.5 V voltage is generated by +2.5 V through a voltage divider composed of two equal-value resistors and MCP606.

The pin voltages of AD7836 and DSP are different, so they cannot be directly connected. The SN74ALVCl64245 chip is selected in the system to isolate the data buses on both sides. Its output enable control pins OE1 and OE2 are both connected to low level, and the direction of the data line DIR1 and DIR2 are both connected to high level to ensure that the chip output changes instantly with the input.
2.3 Extended data memory
The memory is used to temporarily store the marking data transmitted from the host computer. The external extended memory uses IS61LV51216, which is a 512 kB, 16-bit SRAM memory with a total of 19-bit address bus and 16-bit data bus. In this system, the SARAM access time is 10 ns, CMOS process, 3.3 V power supply, input and output are TTL compatible, suitable as an external extended memory for DSP. The memory is expanded through the external interface XINTF of DSP, and the /XZCS6AND7 pin of F2812 is used as the chip select, so it corresponds to the external interface 6 area of ​​the memory map, and the address range is 0X10 0000~0X17FFFF.
2.4 CPLD system
In this system, the CPLD uses the EPM3256A chip of the MAX3000A series of Altera, and the firmware of the CPLD is designed with the software MAX+plusII. The CPLD is mainly used to control the data transmission of the USB chip, and it is expanded to the interrupt button and LCD display module. In addition, the DSP system reserves multiple GPIO interfaces, which can control the multi-dimensional motion control platform composed of stepper motors. By reserving GPIO ports and using CPLD chips, the scalability of the system is enhanced.

3 Controller system software development
The control system software consists of two parts. The host computer uses the visual object-oriented language VB 6. O design, mainly responsible for the generation and packaging of marking data, as well as the user interface to achieve human-computer interaction; the lower computer DSP is programmed in C/C++, using the efficient C compiler and integrated development environment provided by TI: Code Studio (CCS) to develop. The main flow chart of the control system is shown in Figure 5.

After the marking machine is turned on, the lower computer program initializes each chip and memory according to the parameters on the user interface. The parameters include laser energy, number of scans, text filling method, etc. Then wait for the upper computer to transmit the marking data through the USB interface, and the marking data is first saved in the data memory. At this time, no matter whether the marking start command is issued from the user interface or the keyboard, the DSP takes out the marking data from the data memory and sends it to the D/A chip. The D/A chip outputs the corresponding voltage to drive the galvanometer until the marking is completed. The same marking graphic only needs to transmit data once, and the start processing button can be selected from the software or keyboard.
When marking graphics, the graphic file is imported and processed, and then converted into marking position data; when marking text, the Windows API function GetGlyphOutline is called in VisualBasic to obtain the outline of the text, which can be scaled, rotated, etc. without affecting the text display quality, and its display effect is displayed on the user interface through the DrawGlyph function. Figure 6 shows the display effect of Chinese characters in the software user interface.

4 Conclusion
This paper introduces a laser marking controller based on TMS320F2812 DSP. It uses a 4-channel output D/A converter to control the galvanometer and laser power supply. The high-performance D/A conversion module greatly improves the speed and accuracy of laser marking. The USB interface makes the marking controller plug-and-play and improves the stability of data transmission. The upper computer of the system is programmed in Visual Basic, and the lower computer is programmed in C language, which makes the program easy to transplant. The developed marking system has achieved ideal results in speed and accuracy. This paper describes the software and hardware system, which has a general reference value for the design of galvanometer laser marking controller.