Parametric Study of the Centrifugal Insurance Mechanism in MEMS Safety and Arming Device

MEMS (Micro-electromechanical Systems) becomes important increasingly due to the smarter and smaller fuze used in OICW (Objective Individual Combat Weapon). MEMS Safety and Arming (S&A) device is employed in different platforms and regions for small caliber projectile. Therefore, it is necessary to make a parametric study of the MEMS S&A device in different apply environments and explore the main sensitive factors of the MEMS S&A device to provide reference for designs. In this paper, based on the MEMS S&A device designed by our term, theory and finite element models are established, and the centrifugal insurance mechanism of the MEMS S&A device is parametric studied under the different speeds, temperature and thickness of the model by nonlinear dynamic method. By comparing the experimental and predicted results, the established FEM model is verified, and the conclusion is that the temperature and the centrifugal force are the main sensitive factors in the centrifugal insurance mechanism. In summary, we can suggest that the application environment, which the MEMS S&A device is suitable for, is the temperature equal to or slightly greater than normal temperature and the rotating speed higher than35000r/min of small caliber projectile.


Introduction
The fuze S&A device is an important part of ammunition.The S&A device is intended as a key component in ensuring fuze safety in the service processing and launching process, and then to assure arming reliability under certain incentives after ammunition launching [1][2][3][4].With the development of the ammunition technology, the developing trends of the fuze technology should be miniaturization, smart, high reliability and low cost [5][6][7], so MEMS becomes important increasingly [8][9].Traditional S&A device can accomplish the safety and arming functions of fuze, but it is not available for small-caliber ammunition because of its large size, many parts and poor anti-overload capability [10][11][12][13].MEMS S&A device has small size, light weight and good anti-overload capability, it makes more space for conventional fuze to accommodate the multi-sensor detection circuit and the main power supply module, finally the precision and lethality of ammunition is improved [14][15][16][17].The miniaturization of S&A device takes priority over other tasks [18][19][20][21].Since 1990s, the superiority and approaches of MEMS fuze have been introduced [22][23][24][25].The most common MEMS S&A devices are shown in Figure 1.In recent years, the U.S. fuze annual conferences reported that the performance of on-chip-based S&A device is tested using the missile warhead at present [26][27][28][29][30], and the development trend of fuze technology is on-chip integration of machinery, electronics and pyrotechnics in the future.On this basis, our term developed a MEMS S&A device used in small-caliber ammunition innovatively, and the concrete structure as shown in Figure 2 [31][32][33].The size of the device isΦ14×15 mm (except detonating tube, electric detonator and electric thruster).In the launching state, MEMS S&A device withstand the setback overload and centrifugal force, and the centrifugal insurance mechanism (13) releases the explosion-proof slider.Because of the launch state lasts a long time during centrifugation environment, centrifugal insurance mechanism (13) can maintain the arming state.When ammunition flies a certain distance, electric thruster (4) works and the explosion-proof slider (12) in the long-distance safety insurance mechanism is released.The explosion-proof slider (12) overcomes the tension of MEMS spring (6) and moves quickly, and the latching mechanism (11) works, the fuze enters the state of waiting arming.The role of the centrifugal insurance mechanism is to ensure the safety in handling transportation and storage, and reliable firing with the launch overload.Because the MEMS S&A device is used in different platforms and different regions of small caliber projectile, it is necessary to study the MEMS S&A device and explore the main sensitive factors of the MEMS S&A device.Then, according to the main sensitive factors of the MEMS S&A device, the recommendations of the apply experiment and optimal design for the MEMS S&A device can be put forward.A few works have been carried out for detailed analysis of the MEMS S&A device.This paper aims to understand the main sensitive factors of the centrifugal insurance mechanism in MEMS S&A module.First, the simulation models for MEMS S&A module are established, which the influence of the temperature, the setback overload, the centrifugal force and the thickness of the model are studied based on the established models.Finally, the experimental results are presented, and the corresponding simulation models are verified quantitatively.The research can provide fundamental knowledge for optimizing model parameters.

Materials
In this paper, the centrifugal insurance mechanism of the MEMS S&A device (Figure 2) is parametric studied, considering its overall characteristics and weak links in the multi physical field.The ANSYS/LS-DYNA explicit dynamic analysis software is used for simulating the model, wherein thereof the element type is MAT_PLASTIC_KINEMATIC.The material of the centrifugal insurance mechanism is beryllium bronze, and the mechanical parameters of the material are shown in Table 1.

Methods
A three-dimensional model and simplified finite element model of the MEMS S&A module are established respectively, as shown in Figure 3   For small caliber projectile, it has the highest centrifugal force and setback overload.Nonlinear dynamic mechanics simulation is built using ANSYS/LS-DYNA, under loading the centrifugal force and setback overload (shown in Figure 5) at the temperatures of 25℃, 50℃and -40℃, respectively.The simulation results are shown in Figure 6 and Table 2. (1)  According to the above simulation results, we can obtain that the initial motion time of centrifugal insurance mechanism under loading the centrifugal force is 1.57ms, and the initial motion time very close of centrifugal insurance mechanism under loading the centrifugal force and setback overload is 1.56ms;The arming time of centrifugal insurance mechanism under loading the centrifugal force is 2.02ms, which is more 0.04ms than that under loading the centrifugal force and setback overload, the difference is very small.In addition, it can be seen from the stress cloud that the stress of centrifugal insurance mechanism under loading the centrifugal force and setback overload is greater than that under loading the centrifugal force.So we can conclude that the setback overload can obviously increase the stress value of the material, but has almost no effect on the arming time of centrifugal insurance mechanism.The simulation results of different temperatures can be seen that centrifugal insurance mechanism cannot be effectively armed at low temperature (-40℃), the reason that the low temperature affects the material properties of the structure, instead, the centrifugal insurance mechanism can effectively be arming at a little higher temperature (50℃) than the normal temperature.
Therefore, in summary, the temperature is one of the main sensitive factors of the centrifugal insurance mechanism.
Furthermore, when loading the different centrifugal forces (the rotating speeds at the range of32000 r/min and 95000 r/min) to the MEMS S&A module, the simulation results are shown in Figure7, Figure8 and Table 3. (1) (2) (3)   As can be seen from table 3, when the rotating speed reaches 35000r/min, the centrifugal insurance mechanism can be effectively armed.At the same time, with the increase of rotating speed, the initial motion time of centrifugal insurance mechanism, the arming time and the arming process duration, all tend to be decreased.As can be seen from Figure 8, with the increase of rotating speed, the decreasing trend of the initial motion time of centrifugal insurance mechanism, arming time and arming process duration are decreased gradually.It is undoubtedly that the rotating speed is another main sensitive factor of the centrifugal insurance mechanism.
From the above analysis, it is shown that the critical load value of the centrifugal insurance mechanism which can be removed from the insurance is 35000 r/min.Loading the constant centrifugal force (35000 r/min), at the same time, reestablishing the different thickness of centrifugal insurance mechanism(between 0.24mm and 0.34mm)， and the results of final state are demonstrated in Figure 9 and Table 4. (1) (2) (3)  It can be obtained from the simulation results above, when the centrifugal insurance mechanism takes different thickness, it can effectively be arming at the same centrifugal force, so it can be argued that the thickness has no effect on the arming time of centrifugal insurance mechanism.
To verify the above statement, the centrifugal insurance mechanism is calculated theoretically.The deflection of the centrifugal insurance mechanism is analyzed by the beam deflection curve equation in material mechanics, as shown in figure 10. (1) When x=L, the deflection value at L is ( ) ( Then, centrifugal force F is Wherein, ρ is the material density of the centrifugal insurance mechanism， 0 s is surface area of the centrifugal insurance mechanism， h is the thickness of the centrifugal insurance mechanism ， A is the rotation radius of centroid in the centrifugal insurance mechanism and ω is the rotating speed.The section of beam as shown in Figure 11, so the moment of inertia for centrifugal insurance mechanism on the Z axis is Taking the cross section of the root for the centrifugal insurance mechanism as the research object, thus, we can get Wherein, b is the width of the beam section in the centrifugal insurance mechanism.According to the above formula, the formula does not contain the parameter of h , therefore the thickness has nothing to do with the motion of the centrifugal insurance mechanism, which is consistent with the simulation results.

Experiment
Using nickel-based material and UV-LIGA processing technology to manufacture the MEMS spring, beryllium bronze material and low-speed wire-cutting electrical discharge processing technology are used to manufacture the other parts of MEMS S&A module that we have designed.The detailed structure of MEMS S&A module is shown in Figure 12.Through the experiment, we can get that the experiment results under different rotating speeds can be expressed as the form of the Table 5, and the experimental status of centrifugal insurance mechanism which can effectively arming is shown in Figure 14.According to the table 5, we can get the minimum rotating speed of arming for Centrifugal insurance mechanism about each experiment, and the average value of the minimum rotating speeds is 3600r/min.Comparing the experiment result with the simulation result (3500r/min), we can know that the experiment result is slightly bigger than the simulation result, the reason is that the friction and damping are ignored in the simulation.The experiment results and simulation results are basically consistent.Thus, it can be proved that the simulation result is reliable.

5.Conclusions
In this paper, based on the MEMS S&A device designed by our term, the main sensitive factors of the centrifugal insurance mechanism in the MEMS S&A device are explored by parametric studies.Theory model is established and analysis.Finite element models are used to study the influence of the temperature, the setback overload, the centrifugal force and the thickness of the model.At the same time, the centrifugal insurance mechanism under different speeds is parametric studied by experiment.Through the above simulation and experiment results, it can be obtained the following conclusions.
1) The setback overload has a little effect on the arming time of the centrifugal insurance mechanism, but it has a great effect on the stress of the elastic beam.Furthermore, temperature environment can affect the arming time of the centrifugal insurance mechanism, and the effect of low temperature on the arming time of the centrifugal insurance mechanism is bigger than that of high temperature through the Table 2.
2) As can be seen from table 3, when the rotating speed reaches 35000r/min, the centrifugal insurance mechanism can effectively be armed.At the same time, with the increase of rotating speed, the initial motion time of centrifugal insurance mechanism, the arming time and the arming process duration, all tends to be decreased.With the increase of rotating speed, the decreasing trend of the initial motion time of centrifugal insurance mechanism, arming time and arming process duration decreases gradually based on the results, as shown in Figure 8.
3) Comparing the experiment result with the simulation result, it can be obtained that the experiment result and the simulation result are basically consistent; it can be proved that the results are reliable and the centrifugal force is one of the main sensitive factors of the centrifugal insurance mechanism.
4) Under the critical centrifugal force, the centrifugal insurance mechanism can normally remove the insurance when the model has different thicknessby the Table 4. Compared with theoretical analysis, then, it is shown that the thickness has a little effect on the model.
In summary, the temperature and the centrifugal force are the main sensitive factors of the centrifugal insurance mechanism in the MEMS S&A device.

Figure 8 .
Figure 8. Fitting curve of the simulation data.

Figure 11 .
Figure 11.Beam section and corresponding coordinates.

Table 1 .
The mechanical parameters of the beryllium bronze

Table 2 .
Simulation results under different loads and temperatures.

Table 3 .
Simulation results under different rotating speeds.

Table 4 .
Simulation results under different thickness of the model.
Thickness (mm) Initial motion time of centrifugal insurance mechanism (ms) Arming time (ms)

Table 5 .
The experiment results under different rotating speeds.