Samenvatting

The target of this project is to extend the range of Shinyei PPD42NJ sensor and outputs its results in units of mass concentration. The preliminary work of the project focused on understanding the working principle of the sensor and testing its current performance. Through experimentation and reading the conclusions of related studies, 500 μg/m3 is the limit of the concentration that the sensor can now measure.
After understanding the working principle of the sensor, it is concluded that to extend the range of the sensor it is necessary for the detection circuit to get more LPO (Low Pulse Occupancy) as a percentage of time and getting more LPO requires the photodiode to generate electrical signals more times or for longer durations. According to this analysis, this project carries out the expansion of the sensor range from three aspects. They are mechanical structure, optical design and response curve.
In terms of the mechanical structure, in addition to placing and protecting the sensor element, the housing of the sensor constitutes a gas chamber that needs to ensure the smooth movement of the gas. A SolidWorks model of the sensor housing was created and used for CFD simulation. The path of the gas flow inside the gas chamber and the velocities at various points in the chamber were obtained from the calculations of the CFD simulation. This housing can accommodate gas entry at the inlet of the gas chamber and exit without eddy currents during the movement.

In order to increase the number of times an electrical signal is generated in the photodiode, more dust particles need to be allowed to pass through the detection area, which can be done by increasing the velocity of the airflow within the air chamber. In the CFD simulation, a heat source was set up at the right side of the gas chamber inlet to heat up the airflow for the purpose of increasing the velocity of the airflow. This function is realized by a resistance wire in the actual sensor. The CFD simulation also gives the velocity of the airflow at various locations within the air chamber to verify the feasibility of increasing the temperature of the heat source to accelerate the airflow.

For the optical design part, the optical structure of the sensor is redesigned in this paper based on the Mie scattering principle. This design considers the effect of scattering angle on the scattered light by optical design. The optical path is optimized by the use of collimating lens to reduce the waste of scattered light and control the landing point of scattered light in the detection area of photodiode. The design of the optical part is carried out in the simulation software, where the effect of various optical parts is simulated by defining different touching surfaces.
The result of the optical design is shown in Figure 69. The light source, collimating lens, focusing lens and light trap are used in the design. the light from the source is optimized by the collimating lens, focusing lens and then passes through the light trap, where scattering occurs, and then the light is scattered by the special set of The light from the light source is optimized by collimating lens, focusing lens and light trap, and then scattered in this area, and finally focused on the detection area of the photodiode by a special set of lens for angle selection.

The optimization of the adjustment of the parameters and positional distances of the lens during the design simulation is done by the OPT optimization algorithm that comes with the simulation software, and the data from the calibration process is recorded.
For the response curve part, the response curve is a function used to correspond the LPO occupancy detected by the detection circuit to the mass concentration measurements, this paper explores the shortcomings of the currently used function and gives an equation from the quantity concentration measurements to the mass concentration measurements (M=43????????3n????????, r is radius of particles, n is numbers of particles, ???????? is average particle density, but due to the limitation of the project time and the experimental environment and equipment, the suitable response curve was not identified.