A new type of high-range micro-acceleration sensor for penetration testing

Introduction
High-range micromechanical piezoresistive accelerometers are widely used in impact tests and military fuzes. Reliability research is the key to the industrialization of micro-electromechanical systems (MEMS), especially for micro-accelerometers. Due to the particularity of the application environment, high-range micro-accelerometers must have extremely high overload resistance. At present, domestic research on the overload resistance of MEMS devices mainly stays in computer simulation and experimental testing. Considering that the test experiment cannot fully test the performance of the sensor in a special environment, the high-range accelerometer is tested and verified in the penetration environment, and the concrete target and steel target are penetrated by live ammunition respectively. The data and overload resistance of the sensor in the real impact environment are compared and analyzed, so as to obtain reliable data and provide a basis for in-depth research on the overload characteristics of projectile penetration.

2 Micro-accelerometer structure and packaging
Figure 1 is a cross-sectional view of a high-range accelerometer model, with a range of 150,000 g and a first-order natural frequency of 300 kHz. The beam width is consistent with the mass block, and the piezoresistors are symmetrically placed at the root of the four beams, suppressing the influence of lateral acceleration caused by the asymmetric structure. The overall structure is a "田"-shaped structure, made of single-crystal silicon. The active mass block in the center is suspended and connected to the frame by a cross beam. The frame is bonded to the glass substrate as an anchor area.

3 Live-fire penetration test experiment
The experiment used a 921A steel target with a target plate thickness of 26 mm and a muzzle velocity of 420 m/s. The volume of the reinforced concrete target was (2×2×1.5) m3 and the muzzle velocity was 895 m/s. Two sensors were installed in each shell, one was a 988 sensor with a sensitivity of 0.341 pc/g and the other was a test sensor. The system sampling rate was 200 kHz. A 130 mm caliber smoothbore gun was used to launch an elliptical head projectile to vertically penetrate the steel target and the reinforced concrete target, respectively, as shown in Figure 2.

Figure 3a is the overload curve of the steel target. The two dotted lines correspond to the effective overload curve (T=330μs) after the projectile hits the target. The minimum overload value is -106 391.2 g (t=065ms), and the maximum overload value is 256646.5 g (t=0.270ms). Figure 3b is the overload curve of the reinforced concrete target. The two dotted lines correspond to the effective overload curve (T=700μs) after the projectile hits the target. It can be seen that the sensor was damaged during the penetration of the concrete target, and the negative peak value of the recorded sensor data value was -130 000 g.

The filtered penetration steel target overload waveform is integrated once and twice. The time curves of the initial velocity and displacement (i.e., penetration depth) of the projectile are obtained, as shown in Figure 4a and b, respectively. In order to analyze the reliability of the measured data, the effective displacement of the projectile velocity and the measured displacement of the steel target velocity are obtained by comparing the penetration overload integral, and the acceleration values ​​measured by the two sensors are compared. The filtering frequency is 50 kHz. The analysis results are shown in Table 1.

As shown in Table 1, in the steel target penetration test, the velocity error obtained by the first integration of the measured overload waveform is 2.4%, the displacement error obtained by the second integration is 9.1%, and the acceleration value compared with the 988 sensor has an error of 11.3%, which has good overload resistance. The experimental results show that the sensor has a beam fracture phenomenon during the penetration of the concrete target test.

4 Conclusion
The study of the penetration characteristics of high-g micro-acceleration sensors is of great significance to the research on the penetration mechanism of projectiles, weapon warhead design, and bullet shielding technology. This new type of micro-acceleration sensor obtained valuable data in the live-fire penetration experiment: (1) The high-range sensor can measure acceleration values ​​above 100,000 g in the steel target penetration experiment with a small error; (2) During the penetration of the steel target, the pulse width is small and the energy is small, so the sensor has good overload resistance; when penetrating the concrete target, the initial velocity of the bullet is large and the energy is large. Due to packaging reasons, the sensor beam breaks. (3) The beam breakage is mainly due to the deformation of the packaging shell under high overload conditions. Therefore, the packaging needs to be further improved.