In fields such as meteorological environment monitoring and product quality testing, it is often necessary to measure turbidity. Turbidity characterizes the degree of obstruction caused by suspended and colloidal substances in colorless transparent liquids to light transmission. In ideal conditions, turbidity can be obtained through theoretical calculations, but in practical engineering, factors such as the size, shape, surface structure, and surface properties of particles in liquids have a significant impact on turbidity values. So in order to obtain accurate turbidity parameters in engineering, actual measurements are required.

To solve this problem, based on the relationship between transmittance and absorbance, laser transmission method is used for measurement, and a high-speed and high-precision data acquisition card can be competent for data collection in this process, ensuring real-time online monitoring of the system. For this system, in addition to the stability of the laser driving circuit and the quality of the signal conditioning circuit, the reliability and working speed of the data acquisition platform will be important challenges for real-time online monitoring targets.

System design considerations

The system structure diagram is shown in Figure 1. After modulation, the laser passes through the splitter, and two beams of light are separated from the PPETH-PCU-C PCU2000ETH. One beam enters the receiving end 1 after passing through the tested turbidity, and the other beam directly enters the receiving end 0. After signal processing at the receiving end, two input signals are formed, which enter the data acquisition system and form a dual channel liquid turbidity measurement device.

Figure 1 Overall System Structure Diagram

Figure 1 Overall System Structure Diagram

Perform hardware design

(1) Light source driving circuit

The system uses a 635nm semiconductor laser as the light source. In order to reduce the interference of noise signals and facilitate the detection of useful signals, the laser source is modulated into a 1KHz square wave pulse. Choose a 1MHz active crystal oscillator with better stability and accuracy as the frequency source. Using a frequency divider circuit to perform 1000 frequency division on a 1MHz signal to output a 1KHz square wave signal with good frequency stability.

(2) Signal conditioning circuit

The system adopts an opposed receiving method and uses photodiodes as photodetectors. The PPETH-PCU-C PCU2000ETH photodiode converts the light signal into a current signal, performs I/V conversion and preamplification, and converts the current signal into a stronger voltage signal. In order to extract effective signals, a bandpass filter is used to filter the signal to obtain an effective signal of 1kHz. In order to further improve the strength of the collected signal, an in-phase proportional amplification circuit is used for amplification.

(3) Data acquisition card hardware

The data acquisition module of the system is designed using a high-performance data acquisition card PCI-9846H. The PCI-9846H data acquisition card is a high-performance data acquisition card, which is a 16 bit 4-channel type, as shown in Figure 2. The PCI-9846H data acquisition card can sample input signals with high frequency and dynamic range up to 20MHz, with a sampling rate of 40MS/s. Such a high sampling rate provides effective guarantee for real-time online measurement. The PCI-9846H comes with 512MB onboard memory as a cache, which is not limited by the transfer rate of the PCI bus and can record waveform information for a longer period of time, enabling a wider range of applications. The PCI-9846H series digitizer is equipped with four highly linearized 16 bit A/D converters, which can meet the ideal application requirements of devices with high dynamic range. PCI-9846H has four synchronous analog input channels, which can perform synchronous sampling at maximum sampling rates of 10MS/s, 20MS/s, and 40MS/s, effectively meeting the requirements of synchronous multi-channel data acquisition, ensuring the synchronization of data in each channel of the system, and providing effective guarantees for achieving high-precision measurement systems.