Design of silt density detector based on photoelectric detection

　　This detector uses the photoelectric method to detect the distribution and density of underwater silt. It has the characteristics of small size, low power consumption, strong safety, high accuracy, etc. It can distinguish silts of different densities and has little disturbance to the silt.

　　1 Basic principle and composition of silt density detector

　　The silt density detector uses the photoelectric method to detect silt density. The principle is that when the optical path length is constant, the amount of light absorbed by the medium is proportional to its density. Accordingly, a beam of light is sent by the light source. After being absorbed by silts of different densities, the light intensity received by the photoelectric sensor is different, and the size of the generated electrical signal is also different, so that the silt density can be converted into an electrical signal. According to the relationship between silt density and electrical signals obtained from the experiment, the silt density can be measured to distinguish different mud layers.

　　The silt density detector is mainly composed of a photoelectric sensor and a data conversion and transmission system. As shown in Figure 1, the photoelectric sensor detects the density of the silt, and the data conversion and transmission system converts the electrical signal collected by the photoelectric sensor and then transmits it to the computer via the RS485 bus.

　　

 

　　1.1 Photoelectric Sensor

　　Photoelectric sensors are mainly composed of light sources and silicon photocells. Silicon photocells are semiconductor devices that can directly convert light energy into electrical energy, and their absorption rate for effective incident light reaches more than 90%. The spectral characteristic curve of the photocell is shown in Figure 2, and its spectral response range is 400 to 1100 mm.

　　

 

　　As shown in Figure 2, silicon photocells are most sensitive to light with a wavelength of about 850nm, so an infrared LED with a wavelength of 850nm is selected as the light source. The photoelectric sensor is shown in Figure 3:

　　

 

　　The infrared LED emits infrared light, which is absorbed by silt of different densities. The light intensity received by the silicon photocell is different, thus generating electrical signals of different sizes. After being amplified by the transistor, it is transmitted to the microcontroller for signal processing.

2 Calibration of photoelectric sensors

 

　　2.1 Silt stratification

　　From the silt stratification theory, we know that the division criteria of different silt layers vary according to the mud quality in different regions. Generally speaking, they can be roughly divided into four layers according to the density variation range, as shown in Table 1.

　　

 

　　2.2 Calibration

　　Silt samples with four density ranges in Table 1 were prepared in the laboratory. The photoelectric sensor was placed in the pre-prepared mud sample so that the mud sample was evenly distributed between the infrared LED and the silicon photocell. The size of the electrical signal generated by mud samples with different densities was measured. Three tests were conducted and the experimental results are shown in Table 2.

　　

 

　　From Tables 1 and 2, we can derive the corresponding relationship between the electrical signal generated by the photoelectric sensor and the mud layer in which it is located, as shown in Table 3.

　　

 

　　According to the size of the electrical signal detected by the photoelectric sensor, the density of the mud layer where the photoelectric sensor is located can be obtained and then the layer can be divided.

　　3 Data conversion and transmission

　　The schematic diagram of the data conversion and transmission circuit is shown in Figure 4.

　　

3.1 Data Conversion

 

　　The core device of data conversion is the STC12C5410AD microcontroller. The program is directly burned into the microcontroller through the online programmable system, without the need for a dedicated programmer, which facilitates the modification of the program. The electrical signal generated by the photoelectric sensor is amplified and sent to the microcontroller, and then output as a digital quantity after AD conversion. The signal acquisition and conversion procedures are as follows:

　　

 

　　3.2 Data Transmission

　　The underwater measurement depth is considered to be 10m, and the RS485 level has good anti-interference performance and a long effective transmission distance. Therefore, it is chosen to transmit the digital signal serially to the computer via the RS485 bus. After further processing by the computer, the mud layer and the density of the mud layer are displayed, thereby guiding the staff to dredge.

　　4 Conclusion

　　The silt density detector is used to detect the underwater silt distribution and guide the silt removal personnel. Compared with other silt detection methods, this method has the advantages of fast speed, high accuracy, low cost, and little disturbance to the underwater silt. At the same time, the measured data can be saved for later analysis.