3D Printed Flexible Plastic Diaphragm for Optical Pressure Sensor Application

Abstract

Introduction Due to ongoing growth in automotive market, and also in order to respond the rising customers’ demands on driving the safe, comfort and green vehicle, the automotive sensor market expansion is inevitable. These components value about 20% of the total costs of the electronic systems in automobiles [1]. Among employed sensors in vehicles, pressure sensors have the significant role in providing the mentioned characteristics of the demanded automobiles. They are utilized at brake fluid, chassis, inlet manifold, etc. In 2000, a new optical pressure sensor was introduced by the by M. Fitzpatrick et al. [2] which used a silicon diaphragm to detect the applied pressure. Although the device was able to measure the in-cylinder pressure in a direct injection diesel engine, the provided results showed a slight drift in low pressure fields which reveals the opportunity to work on improvement of this type of sensor’s application [3].These days by developing the 3D printing industry, additive manufacturing is a promising technology to assist the low cost, fast and precise fabrication of macro/micro sensors either with complicated structures or specific materials. In this study prototyping of the mentioned optical pressure sensor’s diaphragm with using the SLA 3D printing technology is explained. Replacing the sensor’s silicon diaphragm with plastic material provides more mechanical flexibility and improvement in sensitivity of the sensor against low pressures. Optical pressure sensor mechanism This sensor includes two main parts (i) a sensor head (ii) an optical fiber and signal processing unit. The sensor head is a cavity with the depth of 50 to 100 μm which is capped with the diaphragm and connected to the optical fiber attachment from the other end, as shown in Figure 1. The external surface of the diaphragm exposes to the pressure field. The transmitted light from the source reflects back to the fiber end after hitting to the diaphragm’s internal surface and will be captured by the optical processing unit. The deflection of the diaphragm in presence of the external pressure reduces the internal gap between the fiber and membrane which results in the intensity alteration of the reflected light and finally determination of the pressure’s magnitude [3]. Method Five integrated plastic cavities and flexible diaphragms with thicknesses of 100, 200, 300, 500, 700 μm were 3D printed performing the Form 2 SLA 3D printer as shown in Figure 2. The process of printing took 130 minutes. The thickness of a layer at each step of SLA printing process was 50 μm. Table 1 shows the characteristics of the printing material. In order to mimic the function of the 3D printed diaphragms against the various external pressure fields, different magnitudes of deflection were applied at the center of the individual diaphragm gradually, until reaching the maximum of 100 μm. These tests were done by using FT-S100000 sensing probe which was mounted on FemtoTools FT-RS1002 Microrobatic Measurement System. The probe measured the induced forces corresponding to the applied deflections. Table 2 displays the size and performance characteristics of the probe. Figure 3 illustrates the setup used for the force measurement tests. Results and Conclusions According to the explained methodology multiple experiments have been performed to identify the linear stiffness, and the sensible average pressure of each type of sensor head diaphragms. Figure 4 displays the force-deflection diagrams of the diaphragms with respect to variation of their thicknesses. Since the imposed deflection was small enough compared to the surface of the plane, these diagrams show the bilinear relation between the generated forces and deflections. The average pressures proportional to induced force imposed by central deflection of diaphragms could be determined by using the classical plate theory [4]. Figure 5 illustrates these results. The diagram shows that at low pressures the thinner diaphragms are more sensitive to the variation of the pressure which is comparable to the work done in reference 3. This study determines the feasibility of using 3D printed flexible plastic diaphragm in optical pressure sensor for low to medium pressure measurement. In addition, performing the 3D printing technology for building the flexible plastic diaphragm results in providing the structural integrity of the built object, diaphragm-cavity, and reducing the fabrication cost and time.