Study on Characteristics of Electromagnetic Coil Used in MEMS Safety and Arming Device

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Article Study on Characteristics of Electromagnetic Coil Used in MEMS Safety and Arming Device

Yi Sun 1,2,*, Wenzhong Lou 1,2, Hengzhen Feng 1,2 and Yuecen Zhao 1,2

1 National Key Laboratory of Electro-Mechanics Engineering and Control, School of Mechatronical Engineering, Beijing Institute of technology, Beijing 100081, China; [email protected] (W.L.); [email protected] (H.F.); [email protected] (Y.Z.) 2 Beijing Institute of Technology, Chongqing Innovation Center, Chongqing 401120, China * Correspondence: [email protected]; Tel.: +86-158-3378-5736

Received: 27 June 2020; Accepted: 27 July 2020; Published: 31 July 2020

Abstract: Traditional silicon-based micro-electro-mechanical system (MEMS) safety and arming devices, such as electro-thermal and electrostatically driven MEMS safety and arming devices, experience problems of high insecurity and require high voltage drive. For the current electromagnetic drive mode, the electromagnetic drive device is too large to be integrated. In order to address this problem, we present a new micro electromagnetically driven MEMS safety and arming device, in which the electromagnetic coil is small in size, with a large electromagnetic force. We ﬁrstly designed and calculated the geometric structure of the electromagnetic coil, and analyzed the model using COMSOL multiphysics ﬁeld simulation software. The resulting error between the theoretical calculation and the simulation of the mechanical and electrical properties of the electromagnetic coil was less than 2% under the same size. We then carried out a parametric simulation of the electromagnetic coil, and combined it with the actual processing capacity to obtain the optimized structure of the electromagnetic coil. Finally, the electromagnetic coil was processed by deep silicon etching and the MEMS casting process. The actual electromagnetic force of the electromagnetic coil was measured on a micro-mechanical test system, compared with the simulation, and the comparison results were analyzed.

Keywords: micro-electro-mechanical systems safety and arming device; electromagnetic coil; electromagnetic force; Maxwell energy balance formula

1. Introduction As one of the key components of ammunition, the miniaturization and intelligence of the fuse is of great significance for weapon systems. A smaller-sized fuse can provide additional space for micro-sensors, micro-actuators, and other electronic devices [1–4]. The traditional safety and arming device can fulfill the functions of safety and arming required by the fuse; however, the large size and numerous components of safety and arming devices create issues—such as difficulty in assembly, low precision, and poor anti-overload—which seriously affect the performance of the fuse [5,6]. The micro-electro-mechanical systems (MEMS) safety and arming device has been studied due to its small size, light-weight, and high anti-overload capabilities. The U.S. Army Armament Research, Development and Engineering Center has conducted research on MEMS fuse technology and produced smaller, safer, and less expensive safety and arming devices than ever before [7–11]. Three main methods are used to fabricate MEMS Safety and Arming devices. The first way is the ‘lithographie, galvanoformung, abformung’ (LIGA) method, which is based on metal substrates [12–15]. In this method, a MEMS metal spring and slide are needed, and the structures are set perpendicular to each other to form an interlocking mechanism.

Micromachines 2020, 11, 749; doi:10.3390/mi11080749 www.mdpi.com/journal/micromachines Micromachines 2020, 11, 749 2 of 16

Such devices can function well when driven by proper set back and rotation acceleration, and as most of the metals are elastic materials, they also have good explosion-proof characteristics [16–18]. However, the metal structures fabricated by LIGA are quite expensive. Although some researchers have used electroplating to replace LIGA, the large structure size and complex process method still limits the development of device miniaturization [19,20]. The second fabrication technique is the pyrotechnics method [21–24]. Based on micro-pyrotechnics, Robinson proposed a novel MEMS safety and arming device early in 2005, setting two MEMS springs perpendicular to each other to form an interlocking mechanism. This device can also function well when driven by proper set back and rotation acceleration [25–27]. However, using MEMS springs fabricated by LIGA are difficult to assemble. The V-shaped beam MEMS safety and arming device developed by Xi’an Jiaotong University, which functions under an electric heating environment with a driving voltage at 11 V [28], can achieve reliable function in electric heating, but this device requires larger energy module to operate—such as a Tantalum capacitor with voltage capacity of 16 V and a volume of 2.88 cm3—which has relatively large size and is not conducive to the miniaturization and integrated package of the system. Another disadvantage of this device is its high working voltage, which generates too much heat in the working process, wherein the excessive temperature can detonate the explosive by mistake. An electromagnetic driver of a safety and arming device designed by Nanjing University of Science and Technology [29] has a large displacement (3 mm) and sufficient electromagnetic force (3–50 mN). However, the size of the electromagnetic driver is 13 13 20 mm, which is too large for miniaturization and integration of × × systems. Another electromagnetically driven MEMS safety and arming device was also developed by Nanjing University of Science and Technology by using the fabrication method of UV-LIGA process based on SU-8 adhesive [30]. The maximum driving force of the lock pin of the two sets of safety and arming devices produced was 10 mN and 18 mN, and the maximum driving force of the MEMS slider was 13 mN, which could withstand impact acceleration of more than 20,000 g. However, the large size of the electromagnetic coil takes up too much space in this device. In this paper, a new micro electromagnetically driven MEMS fuse safety and arming (safety and arming) device is presented. As a key component of the MEMS safety and arming device, the impact release mechanism is designed parametrically and fabricated using silicon bulk micromachining technology. Two electromagnetic coils are embedded in the MEMS safety and arming device. The coils are manufactured by deep silicon etching and a die-casting process. The security mechanism conforms to the idea of integrated design, which not only meets the requirements of an intelligent and miniaturized MEMS fuse, but also reduces the space occupied by the fuse security system and improves the reliability of the system. The ultimate device size is successfully minimized to 15 9 0.5 mm. × × The electromagnetic coil is the most important driving component in the MEMS safety and arming device. Due to the small size and precise structure of the electromagnetic coil, when increasing the electromagnetic force of the electromagnetic coil as much as possible in the design, we also need to consider the factors of large current, high temperature, and processing technology of the electromagnetic coil. Therefore, we first carried out theoretical calculation on the electromagnetic coil model. Parametric simulation of the model was then conducted to optimize the structure of the model. Finally, it was verified by the experiment.

2. Design and Simulation

2.1. Working Principle A schematic of the silicon-based MEMS Safety and Arming device (15 9 0.5 mm) is shown × × in Figure1. The MEMS safety and arming device is used mainly for rotary ammunition. As shown in Figure1, the silicon-based MEMS safety and arming device is placed parallel to the rotary axis. Under the condition of a launch overload, the safety inertia pin, threshold mechanism, and the slider achieve arming status sequentially. First, the threshold mechanism at the connection between the safety inertia pin and the main slider breaks under the setback overload, and the safety inertia pin Micromachines 2020, 11, 749 3 of 16

Micromachines 2020, 11, x 3 of 15 moves to the bottom, where it is locked by the locking mechanism. At the same time, electromagnetic forcegenerated generated by the by switched the switched on coil on overcomes coil overcomes centrifugal centrifugal force forceto limit to limitthe movement the movement of the of main the mainslider. slider. When When the therelease release condition condition is satisfied, is satisfied, the the electromagnetic electromagnetic coil coil is is powered powered ooff,ff, andand thethe thresholdthreshold mechanism mechanism between between the the main main slider slider and and frame frame breaks breaks under under the centrifugal the centrifugal force. Theforce. slider The movesslider moves in the direction in the direction of the centrifugalof the centrifugal force and forc ise and finally is finally limited limited to a predetermined to a predetermined position position by a lockingby a locking mechanism, mechanism, whereupon whereupon the detonation the detonation transmission transmission hole is aligned hole is with aligned the electric with the detonator. electric Atdetonator. this point, At the this MEMS point, fuse the MEMS is armed. fuse is armed.

FigureFigure 1. 1.Working Working principle principle of of MEMS MEMS safety safety and and arming arming device. device.

2.2.2.2. DesignDesign ofof Electromagnetic Electromagnetic Actuator Actuator TheThe overall overall view view and and parameters parameters of of the the designed designed electromagnetic electromagnetic coil coil are ar showne shown in thein the Figure Figure2, Figure2, Figure3 and 3 Tableand Table1. The 1. working The working principle principle of the electromagnetic of the electromagnetic coil is that coil the is magnetic that the coremagnetic made core of softmade magnetic of soft materialmagnetic is material aligned with is aligned and magnetized with and magnetized by the magnetic by the field magnetic generated field by thegenerated switched by onthe coil. switched This strengthens on coil. This the strengthens generated magnetic the generate field andd magnetic exerts a field magnetic and forceexerts onto a magnetic the magnet force which onto isthe bonded magnet to thewhich slider is bonded of the safety to the and slider arming of the device safety with and aarming bonding device adhesive. with Ita bonding can be seen adhesive. in the FigureIt can2 bea, theseen electromagnetic in the Figure coil2a, the was electromagnetic formed by casting coil zinc-aluminum was formed by alloy casting into a zinc-aluminum silicon mold, which alloy wasinto made a silicon of deep mold, silicon which etching. was made Additionally, of deep silicon the silicon etching. mold Additionally, is divided into the two silicon substrates, mold whichis divided are bondedinto two together substrates, by bonding which adhesive.are bonded From together the cross-section by bonding of theadhesive. coil, itcan From be seenthe cross-section that there is a of layer the ofcoil, silicon it can between be seen the that wire there of the is coila layer and of the silicon magnetic between core. Thethe wire wasof the made coil outand of the a zinc-aluminum magnetic core. alloyThe wire whose was material made propertiesout of a zinc-a are closeluminum to copper alloy butwhose with material higher resistivityproperties and are betterclose to resistance copper but to current,with higher so that resistivity it produces and less better heat resistance in the process to current of working, so that than it copper. produces The less magnetic heat in core the inserted process in of theworking slot, as than shown copper. in Figure The2a, magnetic was a long core strip inserted type, and in the the slot, material as shown was IJ85 in quaternaryFigure 2a, was alloy a with long high strip permeability.type, and the The material material was parameters IJ85 quaternary of the alloy electromagnetic with high permeability. coil are shown The in Tablematerial2 and parameters the current of voltagethe electromagnetic of the electromagnetic coil are shown coil was in5 Table V. 2 and the current voltage of the electromagnetic coil was 5 V.

Table 1. MEMS drive performance comparison

Driving Type Driving Principle Displacement Force Security Electrostatic drive Electrostatic force short small good Electrothermal drive Thermal stress short large poor Electromagnetic drive Electromagnetic force long large good

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(a)

(b)

Figure 2. Integral structure diagram and cross section image of the electromagnetic coil. (a) Silicon Figure 2. Integral structure diagram and cross section image of the electromagnetic coil. (a) Silicon mold, coil and composite model of electromagnetic coil. (b) A schematic of the electromagnetic coil. mold, coil and composite model of electromagnetic coil. (b) A schematic of the electromagnetic coil Table 1. MEMS drive performance comparison. Table 2. Material parameters of the electromagnetic coil Driving Type Driving Principle Displacement Force Security Material Electrostatic drive Electrostatic forceSilicon short Zn-Al Alloy small IJ85 Alloy good PropertyElectrothermal drive Thermal stress short large poor ElectromagneticMagnetic drive permeability Electromagnetic force 1 long 1 large 250,000 good Electrical conductivity (S/m) 1 × 10−12 1.5 × 107 14.6 Relative permittivity 11.7 1 1 Saturation magnetic induction (T) — — 0.75

2.3. Theoretical Calculation The Maxwell energy balance formula is used to calculate the electromagnetic force because of the large working air gap (/ > 0.2 or /ℎ >0.2, see Figure 3). According to the principle of virtual displacement we can obtain

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air gap( )

Threshold mechanism Electromagnetic coil Magnet

Detonation transmission hole Spacer pin

Micro spring Threshold mechanism

Main slider

Frame 1mm Locking mechanism Safety inertial pin FigureFigure 3.3.Schematic Schematic ofof MEMSMEMS safetysafety and and arming arming device. device. Table 2. Material parameters of the electromagnetic coil. 3. Simulation and Discussion Material Silicon Zn-Al Alloy IJ85 Alloy The finiteProperty element simulation of the electromagnetically driven safety and arming device was carried out using MagneticCOMSOL permeability multiphysics field simulation 1 software 1 and the 250,000 simulation image is Electrical conductivity (S/m) 1 10 12 1.5 107 14.6 shown in Figure 4. The simulation mainly calculat× ed− the electromagnetic× force, resistance, current, and electric power Relativeof the electromagnetic permittivity coil at 5 V. 11.7 In the simulation 1 structure, the 1 wire width, wire Saturation magnetic induction (T) — — 0.75 thickness, wire clearance, thickness of the magnetic core, and coil turns of the electromagnetic coil were 40, 100, 25, 100, and 90 μm. In Figure 4, the coil with the magnetic core inserted is shown on the 2.3. Theoretical Calculation left, which means there is a long strip type magnetic core in the electromagnetic coil, while on the rightThe there Maxwell is no magnetic energy balance core, only formula air. isIt usedcan be to seen calculate from the the electromagnetic figure that the forceedge becauseof the magnet of the largeclose workingto the coil air has gap the (δ /highestd > 0.2 magneticor δ/h > flux0.2, δdensseeity. Figure Meanwhile,3). According by comparing to the principle the two of figures, virtual it displacementcan be found wethat can the obtain maximum magnetic flux density between the coil, which has magnetic core, and the magnet is more than 10 times that of 1the2 coildΛ δwithout a magnetic core. In addition, our Fx = Fmm (1) simulation calculation shows that the electromagneti−2 cd forceδ between the coil with magnetic core is where0.0174 theN, while negative the signforce indicates between thatthe coil the electromagneticwith no magnetic force core Fand is pointing the magnet in the is 6.62 direction × 10 ofN. the decreasing air gap, in other words, the electromagnetic force is suction. At the same time, air gap permeability also needs to be modiﬁed

µ0 Gδ = (d + Kδ)(h + Kδ) (2) δ Magnet Coil where Fmm represents the magnetomotive force and Fmm = NI, N represents the number of coil turns and I represents the current intensity, Λδ represents the air gap permeability, and K = 0.31/π. When the air gap is evenly distributed and leakage permeability does not change with the air gap, we can obtain dΛ S = µ0 (3) dδ − δ2 where S is the cross-sectional area of the magnetic ﬂux. Since the air gap between(a) the magnet and the electromagnetic coil is large,(b) the magnetic resistance of theFigure magnetic 4. Finite core element and non-working simulation of the air electromagne gap can betically ignored, driven under safety the and condition arming device. of constant (a) magnetomotiveFinite element force. simulation Thus, Formulaof electromagnetic (1) can be coil written with magnetic as core. (b) Finite element simulation of electromagnetic coil without magnetic core 1 2 dΛδ Fx = (IN) (4) 2 dδ

Micromachines 2020, 11, 749 6 of 16

Micromachines 2020, 11, x 6 of 15 If the pole edge eﬀect of air gap ﬂux is excluded, Formula (4) can be changed to

1 2 S 1 2 d h Fx = (IN) µ0 = (IN) µ0 × (5) 2 δ2 2 δ2 air gap( ) It can be seen that the electromagnetic force is inversely proportional to the square of the air gap value under the condition of constant magnetomotive force.Threshold mechanism Electromagnetic coil Electromagnetic coil resistance can be calculated as Magnet l Detonation transmission hole xq Spacer pin Rxq = ρ (6) Sxq Micro spring Threshold mechanism lxq = 2 (D + H ) N (7) × 1 1 × 1 Sxq = D L (8) 1 × 1 Main slider Then, the magnetomotive force can be obtained based on Formulas (6)–(8)

Frame 1mm U U Sxq = = × Locking mechanism Safety inertial pin NI N1 N1 (9) Rxq × ρ lxq × Electromagnetic forceFigure can 3 be. Schematic determined of MEMS by substituting safety and arming Formula device. (9) into (5).

3.3. Simulation Simulation and and Discussion Discussion

TheThe finite finite element element simulation simulation of of the the electromag electromagneticallynetically driven driven safety safety and and arming arming device device was was carriedcarried out usingusing COMSOLCOMSOL multiphysics multiphysics field field simulation simulation software software and theand simulation the simulation image image is shown is shownin Figure in4 Figure. The simulation 4. The simulation mainly calculated mainly calculat the electromagneticed the electromagnetic force, resistance, force, current,resistance, and current, electric andpower electric of the power electromagnetic of the electromagnetic coil at 5 V. Incoil the at simulation5 V. In the simulation structure, thestructure, wire width, the wire wire width, thickness, wire thickness,wire clearance, wire thicknessclearance, ofthickness the magnetic of the core, magnet andic coil core, turns and of coil the turns electromagnetic of the electromagnetic coil were 40, coil 100, were25, 100, 40, and100, 9025,µ 100,m. Inand Figure 90 μm.4, theIn Figure coil with 4, the the coil magnetic with the core magnetic inserted core is shown inserted on is the shown left, whichon the left,means which there means is a long there strip is typea long magnetic strip type core magnetic in the electromagnetic core in the electromagnetic coil, while on the coil, right while there on is the no rightmagnetic there core, is no only magnetic air. It cancore, be only seen air. from It thecan figurebe seen that from the the edge figure of the that magnet the edge close of to the the magnet coil has closethe highest to the magneticcoil has the flux highest density. magnetic Meanwhile, flux dens by comparingity. Meanwhile, the two by figures, comparing it can the be two found figures, that the it canmaximum be found magnetic that the flux maximum density betweenmagnetic theflux coil, density which between has magnetic the coil, core, which and has the magnetmagnetic is core, more andthan the 10 timesmagnet that is ofmore the coilthan without 10 times a magnetic that of th core.e coil In without addition, a ourmagnetic simulation core. calculationIn addition, shows our simulationthat the electromagnetic calculation shows force betweenthat the theelectromagneti coil with magneticc force corebetween is 0.0174 the coil N, while with themagnetic force between core is the coil with no magnetic core and the magnet is 6.62 10 5 N. 0.0174 N, while the force between the coil with no magnetic× − core and the magnet is 6.62 × 10 N.

Magnet Coil

(a) (b)

FigureFigure 4. 4. FiniteFinite element element simulation simulation of ofthe the electromagne electromagneticallytically driven driven safety safety andand arming arming device. device. (a) Finite(a) Finite element element simulation simulation of electr of electromagneticomagnetic coil coil with with magnetic magnetic core. core. (b) (Finiteb) Finite element element simulation simulation of electromagneticof electromagnetic coil coil without without magnetic magnetic core core.

Micromachines 2020, 11, 749 7 of 16

The simulation data of resistance, electric current, electric power, and electromagnetic force were compared with the theoretical calculation. The comparison data are shown in Table3. It can be seen from the table that there was almost no difference between the theoretical calculation and simulation. Micromachines 2020, 11, x 7 of 15 Table 3. Experiment and simulation data comparison. The simulation data of resistance, electric current, electric power, and electromagnetic force were compared with the theoretical calculation. TheTheoretical comparison Calculation data are shown inSimulation Table 3. It can be seen from the table that there was almost no difference between the theoretical calculation and simulation. Resistance (Ω) 9.75 9.6 Current intensityTable 3. (A) Experiment and simulation 0.51 data comparison 0.52 Electric power (W) 2.56 2.60 Theoretical Calculation Simulation Electromagnetic force (N) 0.0178 0.0174 Resistance (Ω) 9.75 9.6 Current intensity (A) 0.51 0.52 Electric power (W) 2.56 2.60 Therefore, we used COMSOL multiphysics to build a parametric model of the electromagnetic Electromagnetic force (N) 0.0178 0.0174 coil, and analyzed the influence of coil turns on the electromagnetic force, current, resistance, and electrical powerTherefore, of the we electromagnetic used COMSOL multiphysics coil when to the bu wireild a parametric width of model the coil of the was electromagnetic 30, 40, 50, and 60 µm respectively.coil, Parametricand analyzed simulationthe influence wasof coil carried turns on out the on electromagnetic the model, force, the results current, are resistance, shown and in Figure5. As illustratedelectrical power in the of figure,the electromagnetic under the coil same when wire the widthwire width of the of the electromagnetic coil was 30, 40, 50, coil, and electromagnetic60 μm respectively. Parametric simulation was carried out on the model, the results are shown in Figure 5. force and resistance of the coil increased with an increase in the number of coil turns, while the coil As illustrated in the figure, under the same wire width of the electromagnetic coil, current andelectromagnetic electric power force displayed and resistance opposite of the coil trends. increased The with electromagnetic an increase in the number force, coilof coil current, turns, and coil power increasedwhile the with coil current the growth and electric of coil power width, displayed while opposite the coil trends. resistance The electromagnetic decreased. We force, can coil also see that the increasecurrent, of electromagnetic and coil power increased force with with the the growth number of coil of width, coil turns while was the coil not resistance obvious. decreased. However, due to We can also see that the increase of electromagnetic force with the number of coil turns was not the limitations of the overall size of the MEMS safety and arming device and the current resistance of obvious. However, due to the limitations of the overall size of the MEMS safety and arming device the electromagneticand the current coil, resistance it is expected of the electromagnetic to control thecoil,current it is expected below to control 1 A and the current the electric below power1 A below 4 W, whileand simultaneously the electric power increasing below 4 W, while the electromagneticsimultaneously increasing force the as electromagnetic much as possible. force as It much was observed that the coilas possible. worked It bestwas observed when the that wirethe coil width worked was best 40 whenµm the and wire the width number was 40 μ ofm coiland the turns number was 90. of coil turns was 90.

0.06 D1=30 μm 2.0 D1=30μm D1=40 μm D1=40μm 0.05 D1=50 μm D1=50μm D1=60 μm 1.6 D1=60μm 0.04 1.2 0.03 Force(N) 0.8 0.02 (A) Currentintensity

0.4 0.01

40 50 60 70 80 90 100 110 40 50 60 70 80 90 100 110 Coil turns Coil turns

(a) (b) D1=30 μm 10 D1=30 μm 20 D1=40 μm D1=40 μm D1=50 μm D1=50 μm D1=60 μm ） 8 D1=60 μm

16 W ）（ Ω （ 12 6

Resistance 8 4 Electric power Electric

4 2

30 45 60 75 90 105 120 30 45 60 75 90 105 Coil turns Coil turns (c) (d)

Figure 5. Variation of parameters of the electromagnetic coil with coil turns under diﬀerent wire

width. (a) Electromagnetic force of the electromagnetic coil with coil turns under different wire width. (b) Current intenstity of the electromagnetic coil with coil turns under different wire width. (c) Resistance of the electromagnetic coil with coil turns under different wire width. (d) Electric power of the electromagnetic coil with coil turns under different wire width. Micromachines 2020, 11, x 8 of 15

Figure 5. Variation of parameters of the electromagnetic coil with coil turns under different wire width. (a) Electromagnetic force of the electromagnetic coil with coil turns under different wire width. (b) Current intenstity of the electromagnetic coil with coil turns under different wire width. (c) MicromachinesResistance2020 of, 11 the, 749 electromagnetic coil with coil turns under different wire width. (d) Electric power8 of 16 of the electromagnetic coil with coil turns under different wire width In this paper, the overall length of the designed electromagnetic coil was no more than 6 mm. If the In this paper, the overall length of the designed electromagnetic coil was no more than 6 mm. If wire width of the electromagnetic coil increased, the number of coil turns decreased, given that the total the wire width of the electromagnetic coil increased, the number of coil turns decreased, given that length of the coil was fixed. However, the effect of the wire width and coil turns on the electromagnetic the total length of the coil was fixed. However, the effect of the wire width and coil turns on the force was the opposite. Therefore, it was necessary to analyze the influence of the coupling of wire electromagnetic force was the opposite. Therefore, it was necessary to analyze the influence of the width and number of coil turns on the electromagnetic force, resistance, current, and electric power of coupling of wire width and number of coil turns on the electromagnetic force, resistance, current, the electromagnetic coil. We set the wire clearance L2, wire width D1, and the number of coil turns N1 and electric power of the electromagnetic coil. We set the wire clearance L2, wire width D1, and the as 25, 40, and 90 µm, respectively. At this point, the total coil length L3 = (D1 + L2) N1 = 5.85 mm, number of coil turns N1 as 25, 40, and 90 μm, respectively. At this point, the total coil× length L3 = (D1 then the corresponding number of coil turns was calculated when the wire width of the coil was 30, 50, + L2) × N1 = 5.85 mm, then the corresponding number of coil turns was calculated when the wire width and 60 µm. Figure6 shows the relationship between the ratio of D1 and N1 of electromagnetic force, of the coil was 30, 50, and 60 μm. Figure 6 shows the relationship between the ratio of D1 and N1 of current, and electric power. electromagnetic force, current, and electric power.

28 Coil width(μm) Coil turns(μm) D1/N1 Resistance (Ω) 1.5 Coil width(μm) Coil turns(μm) D1/N1 Electricity (A) 25 117 0.213 19.5 25 117 0.213 0.26 30 106 0.283 14.9 30 106 0.283 0.36 24 40 90 0.44 9.61 40 90 0.44 0.56

50 78 0.64 6.49 (A) 1.2 50 78 0.64 0.75 ） 60 69 0.869 0.81 60 69 0.869 6.14 ty Ω 20 i （ 0.9 ntens 16 i

0.6 urrent Resisitance 12 C

8 0.3

0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 D1/N1 D1/N1 (a) (b) 0.060 8 Coil width(μm) Coil turns(μm) D1/N1 Electric power(W) Coil width(μm) Coil turns(μm) D1/N1 Force (N) 25 117 0.213 0.0068 25 117 0.213 1.28 30 106 0.283 0.0097 30 106 0.283 1.78 0.045 40 90 0.44 2.61 40 90 0.44 0.017 6 50 78 0.64 0.027 50 78 0.64 3.85 60 69 0.869 0.038 60 69 0.869 4.07

0.030 4 Force (N)Force Electric power (W)Electric 0.015 2

0.000 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0 D1/N1 D1/N1 (c) (d)

Figure 6. InfluenceInfluence of of wire wire width width and and coil coil turns turns on on the the parameters parameters of the of theelectromagnetic electromagnetic coil. coil. (a) (Influencea) Influence of wire of wire width width and and coil coilturns turns on Resistance on Resistance of the of electromagnetic the electromagnetic coil. coil.(b) Influence (b) Influence of wire of wirewidth width and coil and turns coil turns on Current on Current intensity intensity of the of electromagnetic the electromagnetic coil. coil.(c) Influence (c) Influence of wire of wire width width and andcoil coilturns turns on Electromagnetic on Electromagnetic forc forcee of ofthe the electromagnetic electromagnetic coil. coil. (d ()d Influence) Influence of of wire wire width width and and coil turns onon ElectricElectric powerpower ofof thethe electromagneticelectromagnetic coil.coil.

As illustrated in Figure6 6,, as as the the value value DD11/N1 increased,increased, that that is, with with the growth growth of the wire width D1, the number of coil turns decreased, and the electromagneticelectromagnetic force, current, and power increased obviously. This indicates that when the wire width and the number of coil turns change at the same time, thethe inﬂuenceinfluence ofof wirewire widthwidth on on the the coil coil parameters parameters plays plays a a major major role. role. It It can can be be seen seen from from Figure Figure6 that when the wire width of the coil was 40 µm and the number of coil turns was 90, it not only met the size requirement of the electromagnetic coil for the MEMS safety and arming device, but also meets the requirement that the current was less than 1 A and the electric power was less than 4 W. Micromachines 2020, 11, 749 9 of 16

Table4 shows the influence of wire clearance on the parameters of the electromagnetic coil under different wire width and the wire clearance increased from 20 µm to 35 µm. We can obtain from Table4 that the effect of wire clearance of the coil on the current and electric power was very small, and can be almost ignored.

Table 4. Inﬂuence of wire clearance on the parameters of the electromagnetic coil under diﬀerent wire width.

Resistance (Ω) Current Intensity (A) Electric Power (W) Force (mN) Min 12.72 0.39 1.95 9.8 Max 12.88 0.41 2.05 10.2 30 µm Average 12.81 0.4 2 10 Std. deviation 0.08 0.01 0.05 0.2 Min 7.78 0.66 3.3 27.2 Max 7.92 0.68 3.4 28.7 40 µm Average 7.85 0.67 3.35 27.9 Std. deviation 0.07 0.01 0.05 0.08 Min 7.78 0.66 3.3 27.2 Max 7.92 0.68 3.4 28.7 50 µm Average 7.85 0.67 3.35 27.9 Std. deviation 0.07 0.01 0.05 0.08 Min 6.14 0.81 4.05 37.7 Max 6.47 0.77 3.85 39.4 60 µm Average 6.3 0.79 3.95 38.6 Std. deviation 0.13 0.02 0.1 0.11

This is because the resistance of the coil is related to the resistivity, length, and the cross-sectional area of the wire, but not to the wire clearance. Figure7 shows the influence of wire thickness on coil resistance, current, electromagnetic force, and electrical power when D1 was 30, 40, 50, and 60 µm, respectively. As can be seen from Figure8, when the wire width and the number of coil turns remained unchanged, the resistance of the coil tended to decrease with the increase of the wire thickness, while the current, electromagnetic force, and electric power increased. At the same time, it can be seen that the larger the wire width, the lower the rate of change of resistance with the wire thickness, while the larger the rate of change of electromagnetic force, coil current, and electric power. This is because the resistance of the coil is lxq Rxq = ρ , and the cross-sectional area of the wire is Sxq = D1 L1. The larger the wire width D1, the Sxq × smaller the rate of change of resistance with wire thickness L1, and the change of resistance is directly related to the coil current, electromagnetic force, and electric power. Figure8 shows the influence of the air gap between the electromagnetic coil and the magnet on the electromagnetic force. It is shown in the figure that the electromagnetic force of the coil decreased with the increase of the air gap. In addition, we can also observe that the electromagnetic force changed significantly when the air gap was within 100 µm. With the increase of the air gap, the rate of change of electromagnetic force became increasingly smaller until it reached zero. To summarize, based on the theoretical calculation of the electromagnetic coil and the parametric simulation of relevant geometric dimensions, combined with the actual process capacity, the structure design and optimization of the electromagnetic coil was completed. Micromachines 2020, 11, x 10 of 16 Micromachines 2020, 11, 749 10 of 16

15.0 4.0 D1=30 μm D1=30 μm D1=40 μm 3.5 D1=40 μm Micromachines12.5 2020, 11, x D1=50 μm D1=50 μm 10 of 15 D1=60 μm 3.0 D1=60 μm 10.0 (a) 2.5 (b) 0.30 18 D1=30 μm 2.0 D1=30 μm 7.5 D1=40 μm D1=40 μm Resistance(Ω) 0.25 D1=50 μm 151.5 D1=50 μm 5.0 Current intensity(A) D1=60 μm 1.0 D1=60 μm 0.20 12 2.5 0.5 0.15 100 150 200 250 9 100 150 200 250 Force (N) Force Wire thickness (μm) Wire thickness (μm)

(a) (W) Electric power (b) 0.10 6 0.30 18 D1=30 μm D1=30 μm 0.05 D1=40 μm D1=40 μm 3 0.25 D1=50 μm 15 D1=50 μm D1=60 μm D1=60 μm 0.00 0.20 12 100 150 200 250 100 150 200 250 Wire thickness (μm) Wire thickness (μm) 0.15 (c) 9 (d) Force (N) Force

Figure0.10 7. Influence of wire thickness on the parameters (W) Electric power 6 of the electromagnetic coil under different wire width. (a) Influence of wire thickness on the Resistance of the electromagnetic coil under 0.05 different wire width. (b) Influence of wire thickness on 3the Current intensity of the electromagnetic coil under0.00 different wire width. (c) Influence of wire thickness on the Electromagnetic force of the 100 150 200 250 100 150 200 250 electromagnetic coilWire under thickness different (μm) wire width. (d) Influence of wireW thicknessire thickness on (μ mthe) Electric power of the electromagnetic coil(c) under different wire width. (d) FigureFigure 7. Influence 7. Influence of wire of wire thickness thickness on on the the parameters parameters of of the the electromagnetic electromagnetic coil coilunder under didifferentfferent wire Figure 8 shows the influence of the air gap between the electromagnetic coil and the magnet on width.wire (a) width. Influence (a) ofInfluence wire thickness of wire onthickness the Resistance on the Resi of thestance electromagnetic of the electromagnetic coil under coil di ffundererent wire the electromagnetic force. It is shown in the figure that the electromagnetic force of the coil decreased width.different (b) Influence wire width. of wire (b) thickness Influence onof wire the Current thickness intensity on the Cu ofrrent the electromagnetic intensity of the electromagnetic coil under diff erent with wirethe increasecoil width. under (c )ofdifferent Influence the air wire ofgap. width. wire In thickness (caddition,) Influence on ofwe the wire Electromagneticcan thickness also observe on the force Electromagnetic that of thethe electromagneticelectromagnetic force of the coil force electromagnetic coil under different wire width. (d) Influence of wire thickness on the Electric power changedunder significantly different wire when width. the (d )air Influence gap was of wirewithin thickness 100 μm. on theWith Electric the increase power of of the the electromagnetic air gap, the rate of the electromagnetic coil under different wire width. of changecoil under of electromagnetic different wire width. force became increasingly smaller until it reached zero. Figure 8 shows0.04 the influence of the air gap between the electromagnetic coil and the magnet on the electromagnetic force. It is shown in the figure that the electromagnetic D1=30force of μmthe coil decreased with the increase of the air gap.0.035 In addition, we can also observe that D1=40the electromagnetic μm force changed significantly when the air gap was within 100 μm. With the increase of the air gap, the rate 0.030 D1=50 μm of change of electromagnetic force became increasingly smaller until it reached D1=60 zero. μm 0.03 0.025

0.020

Force (N) Force 0.015

0.010 0.02 0.005

Force (N) 20 40 60 80 100 the air gap (μm)

0.01

0.00 0 200 400 600 800 1000 1200 the air gap (μm) FigureFigure 8. 8. InfluenceInﬂuence of of the the air air gap gap on on parameters parameters of of the the el electromagneticectromagnetic coil coil under under different diﬀerent wire wire width. width.

To summarize, based on the theoretical calculation of the electromagnetic coil and the parametric simulation of relevant geometric dimensions, combined with the actual process capacity, the structure design and optimization of the electromagnetic coil was completed.

Micromachines 2020, 11, 749 11 of 16

4. Fabrication Process for the Electromagnetic Actuator Since the slot depth to width ratio of die-cast metal in the electromagnetic coil was 17:1, which is a structure with high aspect ratio, this paper adopted deep reactive ion etching technology and bonding Micromachines 2020, 11, x 11 of 15 technology to prepare the electromagnetic coil. A-A’ and B-B’ represent the main and side views. The eight steps of the manufacturing process of the electromagnetic coil structure used in this 4. Fabrication Process for the Electromagnetic Actuator paper are shown in Figure9. SinceA. The the Si slot (425 depthµm) substrateto width wasratio prepared;of die-cast metal in the electromagnetic coil was 17:1, which is a structureB. Silicon with oxide high of 500aspect nm ratio, thickness this waspape grownr adopted on the deep Si substrate; reactive ion etching technology and bondingC. Thetechnology magnetic to core prepare cavity the was electromagnetic etched with deep coil. silicon A-A’ and etching B-B’ technology represent the at an main etching and depthside views.of 50 µ m; TheD. Oneight the steps opposite of the side manufacturing of the silicon wafer,process the of preparationthe electromagnetic of slots and coil holes structure was completedused in this by papercomposite are shown mask in technology; Figure 9. A.E. The The Si surface (425 μ oxidationm) substrate process was wasprepared; used to make an insulation layer with a thickness of 500 nm; B.F. Silicon The structural oxide of 500 bonds nm thicknes in steps was E were grown formed on the into Si substrate; a complete spiral coil mold using bondingC. The adhesive; magnetic core cavity was etched with deep silicon etching technology at an etching depth of 50 μG.m; The ﬁlling of the metal zinc-aluminum alloy of the spiral coil was realized using MEMS-casting™ technologyD. On the to completeopposite side the preparation of the silicon of wafer, the metal the coils; preparation of slots and holes was completed by compositeFigure 10 maska is the technology; wafer diagram of the electromagnetic coil before cutting, and Figure 10 b,c are imagesE. The of thesurface electromagnetic oxidation process coil under was used the electron to make microscope. an insulation It canlayer be with seen a fromthickness the electron of 500 nm;microscope images that the wire width of the coil was 40 µm, and the wire clearance between the single-turnF. The structural of the coil bonds was 25 inµm. step In E addition, were formed due to into the cuttinga complete process, spiral it can coil be mold seen using from thebonding dotted adhesive;line in Figure 10b that there was a distance between the metal wire and the whole end of the coil, which mayG. cause The afilling gap between of the metal the electromagnetic zinc-aluminum force alloy measured of the spiral in the coil experiment was realized and using the theoretical MEMS- casting™calculation technology and simulation. to complete the preparation of the metal coils;

FigureFigure 9. 9. ProcessProcess flow ﬂow for for the the fabricatio fabricationn of of the the electromagnetic electromagnetic coil. coil.

Figure 10a is the wafer diagram of the electromagnetic coil before cutting, and Figure 10 b,c are images of the electromagnetic coil under the electron microscope. It can be seen from the electron microscope images that the wire width of the coil was 40 μm, and the wire clearance between the single-turn of the coil was 25 μm. In addition, due to the cutting process, it can be seen from the dotted line in Figure 10b that there was a distance between the metal wire and the whole end of the coil, which may cause a gap between the electromagnetic force measured in the experiment and the theoretical calculation and simulation.

Micromachines 2020, 11, x 12 of 15 Micromachines 2020, 11, 749 12 of 16 Micromachines 2020, 11, x 12 of 15

Figure 10. Structure of the electromagnetic coil (a) and scanning electron microscope images (b,c). FigureFigure 10. 10. StructureStructure of of the the electromagnetic electromagnetic coil coil ( (aa)) and and scanning scanning electron electron microscope microscope images images ( (bb,,cc).). 5. Electromagnetic Force Test 5. Electromagnetic Force Test 5. ElectromagneticTo test the mechanical Force Test properties of the electromagnetic coil and understand the difference betweenTo test the theexperimental mechanical results properties of the electromagne of the electromagnetictic coil and coil the andtheoretical understand simulation, the di weﬀerence used To test the mechanical properties of the electromagnetic coil and understand the difference betweenthe micro-mechanical the experimental test results system of the to electromagnetic analyze the relationship coil and the theoreticalbetween simulation,the magnitude we used of the between the experimental results of the electromagnetic coil and the theoretical simulation, we used micro-mechanicalelectromagnetic force test systemand the to displacement. analyze the relationship The test platform between theconsisted magnitude of a ofpower the electromagnetic supply, force the micro-mechanical test system to analyze the relationship between the magnitude of the forcesensor, and clamp, the displacement. control platform, The test and platform computer consisted system. of The a power schematic supply, diagram force sensor, of the clamp,test system control is electromagnetic force and the displacement. The test platform consisted of a power supply, force platform,provided andin Figure computer 11. system. The schematic diagram of the test system is provided in Figure 11. sensor, clamp, control platform, and computer system. The schematic diagram of the test system is provided in Figure 11. Power Supply

Power Supply Electrode Electromagnetic coil

Electrode Electromagnetic coil Sensor Control Platform

Sensor Control Platform Clamp Circuit Magnet(IJ85) Clamp

Clamp Circuit Magnet(IJ85) Clamp ComputingComputing SystemSystem

ComputingComputing SystemSystem Figure 11. Schematic diagram of the electromagnetic coil testing system.

Figure 12 showsFigure the 11. experimentalexperimental Schematic diagram test image. of the electromagnetic As shown in Figurecoil testing 1212a,a, system. the electromagnetic coil was fixedfixed on on the the circuit circuit board board by by silver silver paste paste welding, welding, the electrodethe electrode was connected was connected to the powerto the supplypower Figure 12 shows the experimental test image. As shown in Figure 12a, the electromagnetic coil throughsupply through a wire, a the wire, circuit the boardcircuit wasboard fixed was by fixed a clamp by a clamp at one at end, one and end, the and magnet the magnet IJ85 was IJ85 fixed was was fixed on the circuit board by silver paste welding, the electrode was connected to the power onfixed the on clamp the clamp at the at other the other end. end. The controlThe control platform platform could could be adjusted be adjusted by the by computerthe computer so that so that the supply through a wire, the circuit board was fixed by a clamp at one end, and the magnet IJ85 was cross-sectionthe cross-section of the of the magnet magnet was was aligned aligned with with the th cross-sectione cross-section of of the the electromagnetic electromagnetic coil,coil, andand the fixed on the clamp at the other end. The control platform could be adjusted by the computer so that distance betweenbetween thethe magnet magnet and and the the electromagnetic electromagnetic coil coil could could be be adjusted adjusted to theto the micron micron level. level. At theAt the cross-section of the magnet was aligned with the cross-section of the electromagnetic coil, and the samethe same time, time, the force the sensor force wassensor connected was connected to the computer to the system, computer and thesystem, image and of electromagnetic the image of distance between the magnet and the electromagnetic coil could be adjusted to the micron level. At electromagnetic force and displacement could be obtained according to the distance between the the same time, the force sensor was connected to the computer system, and the image of electromagnetic coil and the magnet. electromagnetic force and displacement could be obtained according to the distance between the electromagnetic coil and the magnet.

Micromachines 2020, 11, 749 13 of 16 force and displacement could be obtained according to the distance between the electromagnetic coil and the magnet. Micromachines 2020, 11, x 13 of 15 Micromachines 2020, 11, x 13 of 15

(a) (b) (a) (b) Figure 12. Photograph showing the components of the micro force testing system. (a) The control FigureFigure 12. Photograph12. Photograph showing showing the the componentscomponents of of the the micro micro force force testing testing system. system. (a) The (a) control The control platform of the micro force testing system. (b) The computing system of the micor force testing platformplatform of the of microthe micro force force testing testing system. system. (b) The(b) The computing computing system system of theof the micor micor force force testing testing system. system. system. The testing process of the electromagnetic coil was as follows: The testing process of the electromagnetic coil was as follows: The testing process of the electromagnetic coil was as follows: (1) The system was powered on, and the supply voltage of the electromagnetic coil was 5 V; (1) The system was powered on, and the supply voltage of the electromagnetic coil was 5 V; (1) The system was powered on, and the supply voltage of the electromagnetic coil was 5 V; (2) (2)The The computer computer system system drove drove thethe control platform platform to to align align the the cross cross section section of the of magnet the magnet with with the (2) The computer system drove the control platform to align the cross section of the magnet with centertheof center the crossof the sectioncross section of the of electromagnetic the electromagnetic coil. coil. The The initial initial distance distance was zero; zero; the center of the cross section of the electromagnetic coil. The initial distance was zero; (3) (3)The The control control platform platform was was set set by by the the computer computer system system to drive to the drive magnet the magnetto move in to the move Y in the Y (3) The control platform was set by the computer system to drive the magnet to move in the Y direction, and the displacement was 2 mm, when the electromagnetic force was almost zero; direction,direction, and and the the displacement displacement was was 22 mm, when when the the electromagnetic electromagnetic force force was was almost almost zero; zero; (4) The computer provided the image of the relationship between the electromagnetic force and (4) (4)The The computer computer providedprovided the the image image of ofthe the relationship relationship between between the electromagnetic the electromagnetic force and force and displacement with an accuracy of 1 mN. displacementdisplacement with with an an accuracy accuracy of of 11 mN.mN. The extracted experimental data are plotted and compared with the simulation, the graph line The extracted experimental data are plotted and compared with the simulation, the graph line Thewas as extracted follows: experimental data are plotted and compared with the simulation, the graph line was as follows: was as follows:As can be seen from Figure 13, the electromagnetic force measured in the experiment was As can be seen from Figure 13, the electromagnetic force measured in the experiment was Assmaller can bethan seen the fromtheoretical Figure simulation 13, the electromagnetic value. One reason force for measuredthis is that inthere the was experiment a small distance was smaller smaller than the theoretical simulation value. One reason for this is that there was a small distance thanbetween the theoretical the metal simulation wire in the value. coil and One the reasontail of th fore coil this as is a thatwhole. there It was was also a smallcaused distance by magnetic between between the metal wire in the coil and the tail of the coil as a whole. It was also caused by magnetic flux leakage in the air gap during the experiment. However, as can be seen from Table 5, the the metalflux leakage wire in in the the coil air and gap the during tail of the the experiment coil as a whole.. However, It was as also can caused be seen by from magnetic Table flux 5, the leakage difference between the current and the resistance was not significant, and was under the 5 V driving in thedifference air gap duringbetween the the experiment. current and the However, resistance as was can not be seensignificant, from Table and was5, the under di ff theerence 5 V driving between the voltage. currentvoltage. and the resistance was not significant, and was under the 5 V driving voltage. 0.020 0.020 Simulation Simulation Experiment Experiment 0.015 0.015

0.010 0.010 Force(N) Force(N)

0.005 0.005

0.000 0.000 0 200 400 600 800 1000 1200 1400 0 200 400 600 800 1000 1200 1400 the air gap(μm) the air gap(μm) Figure 13. Comparison of experiment and simulation of electromagnetic force. FigureFigure 13. 13.Comparison Comparison of of experiment experiment and and simu simulationlation of ofelectromagnetic electromagnetic force. force.

Micromachines 2020, 11, 749 14 of 16

Table 5. Experimental simulation data comparison.

Resistance (Ω) Current Intensity (A) Electric Power (W) Simulation 9.7 0.49 2.5 Experiment 9.83 0.50 2.46

6. Conclusions In this paper, we presented an electromagnetically driven MEMS safety and arming device that was small in size, low in power consumption, and in line with the idea of integrated design. As an important driving component of the MEMS safety and arming device, the electromagnetic coil was small in size and precise in structure. To this end, we firstly designed and calculated the geometric structure of the electromagnetic coil and analyzed the model using COMSOL multiphysics field simulation software. The resulting error between the theoretical calculation and the simulation of the mechanical and electrical properties of the electromagnetic coil was less than 2% under the same size. A parametric simulation of the electromagnetic coil was then carried out and combined with the actual processing capacity to obtain the optimized structure of the electromagnetic coil. Finally, the electromagnetic coil was processed by deep silicon etching and the MEMS casting process. The actual electromagnetic force of the electromagnetic coil was measured using a micro-mechanical test system and compared with the simulation. The results showed slight differences, mainly due to the coil processing and air gap. Compared with the electro-thermal driven safety and arming device designed by Xi’an Jiaotong University, it needs a minimum 11 V drive voltage, but the output displacement is only 28.4 µm, which is too small for safety and arming devices. Meanwhile, the 11 V drive voltage needs to be matched with a large energy module, which is not conducive to the miniaturization and integration of the whole system. The electromagnetically driven MEMS safety and arming device of Nanjing University of Science and Technology has a driving voltage of 5 V and a large enough electromagnetic force (30–50 mN), but the size of the electromagnetic coil is too large (6 8 6 mm) to × × achieve miniaturization and integration. The electromagnetic drive S&A device designed in this paper has enough driving electromagnetic force (17–40 mN), small size of driving coil (2.5 6 0.8 mm), low × × drive voltage (5 V), small volume of power supply module, and can realize system miniaturization. How to improve the mechanical and electrical properties of MEMS electromagnetic coils and test the overall structure of the electromagnetic driven MEMS safety and arming device will be further studied in the future.

Author Contributions: W.L. conceived the problem and designed the solution; Y.S. and H.F. designed the structure and simulation; Y.S. and H.F. designed the experiments; Y.S. and Y.Z. analyzed the data; Y.S. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: The research was funded by the State Key Laboratory of Mechatronics Engineering and Control and sponsored by National Projects (grant number Z092014B001, B3320132011). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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