Laser output power measurement and control design based on AD7896

0 Introduction
The modern digital laser color printing system converts the image information containing photoelectric signals stored in the computer into laser beam control signals, and controls the three semiconductor lasers R, G, and B to emit laser beams of different intensities, which are then converged into various colors to scan the photosensitive paper line by line to form an image. Although the laser emitted by the semiconductor laser has many advantages such as good monochromaticity, good directionality, small emission angle, high output power, stable operation, long life, and convenient use; however, as the light source of the photosensitive paper in the modern digital color printing system, each semiconductor laser will be affected by the changes in the surrounding temperature and its own life, and this influence will directly reflect on the color level, contour, clarity, and vividness of the printed color photos. In order to effectively prevent the color distortion caused by the change of the output light power of the semiconductor laser on the printed photos and improve the color purity of the printed photos, the output power of the three semiconductor lasers R, G, and B can be automatically measured and automatically controlled to ensure the quality of the color photos printed by the system, and at the same time, it can greatly reduce the maintenance time of the system equipment and improve the service capacity and economic benefits.

1 System Principle
1.1 Principle of Semiconductor Laser Structure
The electrical symbol and structure of semiconductor laser are shown in Figure 1. It consists of laser emitting part LD and laser receiving part PD. LD and PD are encapsulated in a tube shell, share a common terminal, and are connected to the metal shell of the tube. The laser driving voltage is connected to the 1st and 3rd pins of the laser through a resistor, which can control the working current of LD and the conversion current of PD after receiving light. The laser intensity emitted by LD is temporarily called the emission power.

1.2 Principle of power measurement and control of semiconductor lasers
Semiconductor lasers are thermal power devices. Under continuous operation, the junction temperature rises rapidly, and the threshold current changes accordingly. When the temperature rises, the laser emission power decreases accordingly. After the emitted laser converges, the color information deviates greatly from the color information contained in the storage file, so it is necessary to compensate the emitted light. The compensation method is to install a polarizer that can be adjusted by a stepper motor in front of the laser, adjust the angle of the polarizer to change the intensity of the emitted light (output power), and appropriately increase the emission power within the range of small laser temperature changes (adjusted by a semiconductor refrigeration device). The polarizer can be used to adjust the output and the light intensity information corresponding to the stored file, thereby ensuring the distortion of the output laser for the scanning exposure of the photo paper.
1.3 Block diagram of measurement control system
The main hardware circuits in the measurement control system are composed of a microprocessor AT89C52, a photodetector (one for each laser emission port and output port), an amplifier circuit, a 12-bit ADC conversion circuit, a stepper motor control module, etc. Its basic block diagram is shown in Figure 2.

The driving circuit provides the working current required by the semiconductor laser; the photodetector measures the changes in the laser emission power and the output power; the amplifier circuit amplifies the collected weak voltage change signal to suit the ADC conversion; the microprocessor AT89C52 receives the digital information converted by the ADC, and then compares it with the set value through software, sends out a PWM pulse control signal, drives the stepper motor to change the deflection angle of the polarizer at the laser output, and adjusts the output power of the semiconductor laser to achieve stability.
1.4 AD7896 characteristics and working mode
AD7896 is a low-power 12-bit high-speed serial A/D converter produced by the manufacturer. The product has three packaging forms: 8-pin Plastic DIP, Lead Cerdip and SOIC, and has an internal clock. Its peripheral wiring is extremely simple. The conversion time of AD7896 is 8μs, and it uses a standard SPI synchronous serial interface output and a single power supply (2.7~5.5 V) for power supply. Figure 3 is a timing diagram of AD7896 working in high-speed mode. In this mode, the start signal CONVST is generally at a high level. When a negative pulse is input to CONVST, its falling edge will start a conversion. This signal triggers BUSY to change from low level to high level and marks that the conversion is in progress. After a maximum of 8μs, the conversion is completed, BUSY automatically changes from high level to low level, and then 16 clock pulses are used to read out the conversion data for storage. The maximum clock pulse frequency is 10 MHz (+5 V power supply), and the shortest data read time is 1.6μs. After waiting for 400 ns, at the beginning of the next conversion, the data is serially shifted and output, and the shortest conversion time is 10μs.

2 Hardware Circuit Design
2.1 Amplifier Circuit Design
Since the current of the semiconductor laser to generate laser is very small, and generally only a few microamperes to tens of milliamperes during normal operation, the AD707 with relatively high performance and price is selected as the subtraction operational amplifier. Figure 4 shows the actual application amplifier circuit. In Figure 4, JGT is the output end of the red laser photodetector (connected to the third pin of the laser), which is input to the in-phase end of the operational amplifier AD707 through resistor R1, and the inverting end forms a feedback circuit through R40. The voltage divider circuit formed by R34 and R22 in the circuit is the inverting input end, which is used to provide voltage μA. This voltage can be adjusted by adjusting R34 to calibrate the ratio of the three primary colors. The voltage signal uJGT sampled by JGT is output from the sixth pin of AD707 after subtraction operation, and its size is:

The integrated operational amplifier is powered by dual power supplies of +8 V and -8 V, which can effectively reduce output voltage noise and improve system stability.

[page]

2.2 Design of ADC conversion circuit
The ADC conversion circuit is shown in Figure 5.

A CD4051 is used to select the six sampling signals of the three RGB lasers respectively, and the Y0 output is sent to the AD7896 for conversion. Among them, LDR, LDG, LDB are the detection voltage amplification values ​​of the laser emission end; LDRO, LDGO, LDBO are the detection voltage amplification values ​​of the laser output end, and the selection signal is controlled by P10, P11, P12 of the single-chip microcomputer AT89C52. The value of each laser detection voltage converted by ADC is compared with the standard value (used to calibrate the laser output light intensity) stored in AT24C08 (not shown in the figure). If the voltage measured at the laser output end is greater than the standard value, the stepper motor is controlled to rotate forward to reduce the laser output light; if the voltage measured at the laser output end is less than the standard value, the stepper motor is controlled to rotate reversely to increase the laser output light. The control of the stepper motor is controlled by the PWM pulse output from the P0 port of AT89C52.

3 Software Design
The design of the entire system software mainly includes the sampling signal selection control, the program control of the analog serial bus interface in ADC7896, the stepper motor PWM output control, and numerical analysis and logic processing, etc. The flow chart is shown in Figure 6.

The software design fully considers the convenience of production debugging and future maintenance. Due to the large performance differences between the three lasers, the correct three-primary color ratio value needs to be calibrated according to the parameters provided by the laser during debugging. After calibration, it is stored in the AT24C08 chip, and the value is taken out during operation to compare with the digitized value of the current laser emission light intensity. If there is a deviation, it is automatically corrected through the deflection mirror.
The following is the analog communication assembly program of AT89C52 and AD7896, port definition:

4 Conclusion
The design of semiconductor laser output power measurement and control based on AD7896 described here is a commissioned project of a digital equipment development company. After one year of development, it mainly solves the problem of the same type of domestic machines in the process of laser scanning paper imaging, which affects the color of the printed photos due to the change of laser output power. It also solves the problems of future machine maintenance and convenient adjustment. The service life of semiconductor lasers has been improved to a certain extent, and it also has good promotion value for the development of digital printing equipment in China.
The innovation of this article: 2 photoelectric detectors are used for laser power measurement, 1 for emission laser power measurement, and 1 for output power measurement; the adjustment of laser output power adopts stepper motor to drive polarizer rotation adjustment, which is very suitable for scanning applications with unfixed laser current; the standard voltage value of laser normal use is stored in ROM memory, and it is controlled and adjusted by microprocessor after monitoring changes, and the value can be changed in real time according to the current situation after the laser is used for a period of time, which greatly facilitates maintenance and repair.