Design of milk composition detector using laser diffusion technology

1 Introduction

Milk is becoming more and more common in people's daily life and diet. Real-time, fast and accurate detection of milk ingredients is of great significance to improving milk quality and realizing the automated management of dairy production process. There are many methods to detect milk ingredients. Chemical analysis method is still the most accurate test method, but it is difficult to adapt to the needs of short-term testing. Physical instrument testing methods mainly use ultrasonic principle and spectral analysis. At present, foreign technology is relatively mature, but the instrument is expensive and it is impossible to popularize it in China, especially in small and medium-sized enterprises and dairy farms. The milk ingredient tester introduced in this article uses laser transmission ratio for detection, which is more accurate and has lower cost.

2. Detection principle

Laser scattering ratio detection is to use the ratio of the scattered light intensity Is at 90° in the incident plane of the laser to the transmitted light intensity It at 0° to characterize the optical parameters of the test milk protein content. However, due to the presence of two scattering macromolecules in milk, it is difficult to accurately measure the protein and fat content separately. Through chemical research, a fast protein dissolving solution (dilute ethylenediaminetetraacetic acid solution) was found to dissolve protein into small molecules, so that only fat in the dilute milk solution is an insoluble macromolecule in milk. The fat is measured, and then the dilute milk solution in which fat and protein coexist is measured. The protein content can be calculated by establishing a theoretical correlation model. During the measurement process, the photocurrent signal is relatively weak, so it is very important to design a light intensity detection circuit with good performance. The most critical part of the circuit design of this instrument is the design of the light intensity sampling circuit. The rationality of the design of the light intensity sampling circuit directly affects the accuracy and precision of the milk component detection. The block diagram of the milk component detector is shown in Figure 1.

The semiconductor laser emits non-polarized light with a wavelength of 635 nm, which is collimated and incident on the sample box in parallel. The photodiode converts the light signal into an electrical signal in the direction of the transmitted light and scattered light. The light intensity detection circuit consists of an amplifier circuit and an A/D conversion circuit. The amplifier circuit amplifies the weak electrical signal converted by the photodiode into an analog voltage suitable for A/D conversion. The A/D conversion circuit converts the analog voltage into a digital quantity. The single-chip microcomputer is the controller of the instrument, which is used to process the buttons, read the sample values, calculate and display the measurement results.

3. Factors affecting the accuracy of photoelectric conversion output signals

From the structure diagram of this instrument, we can see that the factors that affect the accuracy of the photoelectric conversion output signal are as follows:

(1) The light emitted by semiconductor lasers is not consistent and is distributed in a certain wavelength range. Semiconductor lasers are sensitive to temperature. Changes in ambient temperature and the thermal effect of injected current will cause changes in the threshold current and output optical power of the laser. This instrument design uses a distributed feedback (DFB) semiconductor laser, which has good wavelength stability, a temperature drift of about 0.08 nm/℃, and low frequency and intensity noise.

(2) The performance parameters of the photodiode directly affect the stability and accuracy of the output signal. The S5226 silicon photodiode of Hamamatsu Photonics Co., Ltd. of Japan is used in the design of this instrument. The performance parameters of this device are as follows: effective receiving area 5.8×5.8 mm2; terminal capacitance 430 pF; shunt resistance 1 GΩ; dark current 100 pA; spectral response range 190～1 000 nm; peak sensitivity wavelength 740 nm; peak sensitivity about 0.36 A/W; sensitivity at 635 nm is about 0.32 A/W; From the above parameters, it can be seen that when the light emitted by the laser is consistent, the error is very small.

4 Design of photoelectric conversion circuit

The photoelectric conversion circuit amplifies the micro-electric signal output by the sensor photodiode. The photoelectric conversion circuit is shown in FIG2 .

The silicon photodiode is in reverse bias, so that the silicon photodiode works in the third phase limit of its volt-ampere characteristic. The light intensity is linearly related to the photocurrent. Compared with the zero bias circuit, it has lower noise and better linearity. Since the output current of the silicon photodiode is small, in order to reduce the influence of the bias current of the op amp on the measurement, an op amp with low bias current must be selected; in addition, parameters such as temperature drift, offset current, and offset voltage must also be considered. After comprehensive consideration, Maxim's ICL7650 op amp is selected. This chip is a chopper-stabilized high-precision op amp made using dynamic zeroing technology and CMOS technology. The input bias current is 1.5 pA at 25°C, the input offset voltage is 1μV, the offset voltage temperature coefficient is 0.01μV/°C, and the input resistance can reach 10×12 Ω. In addition, its common mode rejection ratio reaches 130 dB. When ICL7650 is used, two 0.1 μF zero adjustment capacitors need to be connected. In order to stabilize the DC component of the operational amplifier output signal, the clamp terminal (CLAMP) needs to be connected to the input and output terminals of the operational amplifier. In this way, the chip will establish a current channel between the clamp terminal and the output terminal before the output reaches saturation, thereby preventing the charge from continuing to accumulate on the zero adjustment and storage capacitors, reducing the charge and discharge recovery time of the capacitor, and stabilizing the output voltage. Since it is a chopper-stabilized zero device, the internal crystal oscillator of the chip generates an internal beat frequency of 200 Hz. In order to reduce the noise of the output signal, a 0.1 μF capacitor C4 can be connected to the output terminal to remove the high-frequency signal. In order to prevent self-oscillation, a 0.1 μF compensation capacitor C1 is connected between the input and output. For the gain resistor, a high-precision adjustable resistor can be used. The output signal amplitude is proportional to R1. A larger value of R1 can increase the signal-to-noise ratio. However, the value of R1 is limited by the output voltage amplitude. The reference voltage of a typical high-resolution A/D converter is 3.3 V, and its analog input range is 0 to 3.3 V. In order to match the A/D conversion circuit, the Uo output of the photoelectric conversion circuit should not exceed 3.3 V.

5. Design of A/D conversion circuit

The A/D conversion circuit uses a high-precision ∑-type A/D converter, which has a high sampling frequency. The chip is mainly composed of digital circuits, and the analog part has fewer circuits, which is easy to achieve high precision and low cost. It is widely used in instrumentation, industrial data acquisition and other occasions. In this system, TI's ADS1100 is used. ADS1100 is a precise continuous self-calibration analog/digital A/D converter with differential input and up to 16-bit resolution, packaged in a small SOT23-6. The conversion is performed proportionally with the power supply as the reference voltage. ADS1100 uses a compatible I2C serial interface and operates on a single power supply of 2.7 to 5.5 V. ADS1100 can sample 8, 16, 32 or 128 times per second for conversion. The on-chip programmable gain amplifier PGA provides up to 8 times gain, allowing smaller signals to be measured with high resolution. In the single-cycle conversion mode, the ADS1100 automatically powers off after a conversion, greatly reducing current consumption during idle time. In addition, the light intensity sampling circuit can be placed in a metal shielding box and placed near the output position of the photoelectric sensor, and connected to the single-chip circuit through a two-wire serial output interface. This design separates the analog and digital circuits, minimizes the measurement error caused by external interference, and improves the stability and accuracy of the measurement system. The A/D conversion circuit and the single-chip interface are shown in Figure 3.

It is relatively easy to use PCB wiring technology for ADS1100, and 16-bit performance is not difficult to achieve. Any data converter is actually only equivalent to its benchmark. For ADS1100, its benchmark is the power supply, so the power supply must be clean enough to achieve the desired performance. If a power supply filter capacitor is used, the capacitor should be placed close to the VDD pin, there is no path between the capacitor and the VDD pin, and the path leading to the pin should be as wide as possible. The output digital quantity can be directly connected to the MSP430 series microcontroller interface. ADS1100 has few external components, which simplifies the system design and improves the reliability of the circuit.

6. Conclusion

Through the analysis of the laser transmission ratio method for detecting milk ingredients and the analysis of various factors affecting the precision and accuracy of the light intensity sampling circuit, the design of this sampling circuit can meet the requirements of analog signal accuracy. In addition, this measurement scheme uses the chopper-stabilized high-precision op amp conversion device ICL7650 to improve the detection accuracy; the use of the high-precision ∑-type A/D converter ADS1100 simplifies the circuit and improves reliability.