Research status and development trend of MEMS S&A devices:A review

Mei Li ,Tengjing Hu

a School of Foreign Studies,Xi’an Medical University,China

b State Key Laboratory for Manufacturing System Engineering,Xi’an Jiaotong University,China

Keywords:MEMS S&A device Driven principle

ABSTRACT MEMS Safety-and-Arming(S&A)device is the new generation of S&A device which integrates the mechanism of actuation and barrier.The features of minimized structure and easy integration make it to be the indispensable support to the development of weapon miniaturization,integration and intelligence.As a key component in the new generation weapon system,the sound development of MEMS S&A devices will have a significant impact on the future national defense system.Herein,the research status and development trend of MEMS S&A devices are introduced in this paper.From literature review on various MEMS S&A devices,it can be seen that the researches have evolved from individual components to system integration,and many prototypes have the potential for live-fire testing.Different driven principles and structures of the MEMS S&A devices are compared and summarized.At present,the MEMS S&A device can realize the mutual integration of the driving mechanism and the blocking mechanism on the micron level.In the future,with the establishment of new design criteria,MEMS S&A devices will develop from prototypes to practical applications,which will further promote the integration and intelligent of weapon systems.

1.Introduction

Safety-and-Arming(S&A)devices are primarily used to control the energy transfer of energetic materials in ammunition,that is,to trigger normal detonation and prevent accidental detonation.The traditional S&A devices are mainly composed of spring-mass systems that complete the corresponding actions under the drive of specific environmental inertial forces.Since it is difficult to make breakthroughs in driving methods,control accuracy,and structural size,and it cannot meet the development requirements for the miniaturization,integration,and intelligence of new generation weapons,it is urgent to find new technical solutions to address the above problems.Owing to the high fabrication accuracy of MEMS technology at the micro-nano level,the concept of new generation S&A devices(MEMS S&A devices)come into being.The new generation S&A devices based on MEMS technology have the following features[1～5]:(a).Miniaturized structure.The size of device structure retains at the micro scale;(b).Intelligent control.By converting the passive driving method triggered by single environmental force into the active driving method controlled by electric driving signal,together combined with sensor technology,information recognition and fire control in a complex battlefield environment can be realized;(c).Integrated system.Guided by the integrated design,the integration of driving,detection and blocking systems can be achieved.As a key component in the new generation weapon system,the sound development of MEMS S&A devices will have a significant impact on the future national defense system.Because of this,it is essential to give a glimpse into the research status of MEMS S&A devices worldwide and shed some light on its future development trend.Hopefully,this paper can offer some inspiration to researchers and practitioners in this field.

2.The MEMS S&A devices based on different driven principles

With the characters of minimized size and versatile function,MEMS S&A devices have attracted great attention.Based on different driven principles,MEMS S&A devices can be divided into 5 categories:inertial,electro-magnetic,pyro-tech,electro-thermal,and multiphysics.

2.1.The inertial driven MEMS S&A devices

Charles H.Robinson[6,7]proposed an inertial driven MEMS S&A device in 2001,shown in Fig.1.The device mainly consists of a setback mechanism,a spin mechanism,and a time-delay mechanism.Among them,the setback mechanism and the pin mechanism form a vertical layout with each other,and the function of interlock can be realized.During the transportation or maintenance state,the spring force will keep the MEMS S&A device in safe place,thus avoiding the disarming caused by mis-operation;when the weapon is launched,the setback slider will move downwards under the inertia force and release the restriction of the pin slider.With the proper centrifugal force,the spin slider will move and complete the alignment of the explosive train.In order to enlarge the muzzle safety distance,a time delay mechanism formed by a sequence of zig-zag wall are also introduced in the device.All components of the MEMS S&A device are fabricated by LIGA process,and finally assembled together by a micro-assembly platform.Due to the need for micro-assembly of each component,the production efficiency will be greatly reduced.Moreover,with the assembly errors,the performance uniformity of each device can hardly be guaranteed.

Fig.1.The inertial driven MEMS S&A device(Charles H.Robinson).

Confined by the low resolution of the thick photoresist,the fabrication precision of LIGA is not satisfactory.In order to solve this technical problem,Robert Renz[8]used EFAB technology and minimized the metal device line width into 10μm.The structure of this MEMS S&A device is shown in Fig.2,which is composed of setback lock,setback slider,reduction gears and spin slider.During the proper detonation status,the setback lock firstly disarmed the restrict of the setback slider,then,through the torque amplification by the reduction gears,the spin slider will be actuated and complete the motion of the explosive train alignment.The device size after packaging is 10 mm×15 mm×10 mm,compared with the M550 traditional S&A device currently installed in the 40 mm caliber howitzer,the volume has been reduced by nearly 80%,and the mass is less than the original 20%.Compared with other similar products,the MEMS S&A device designed by Robert Renz has a more complex structure(metal based 3D micro structure),although it can be achieved by EFAB,this still places higher requirements on micro processing technology.

Fig.2.The inertial driven MEMS S&A device(Robert Renz).(a).The basic structure.(b).The safe mode.(c).The armed mode.

Dakui Wang[9]has proposed a MEMS S&A device for small caliber projectile platforms in 2017,and the structure is shown in Fig.3.Fabricated by LIGA process,the basic components of explosion-proof slider,spring and spin lock can be obtained separately.The micro spring and the explosion-proof slider is assembled together and fixed in the top and bottom plates.Under the launching state,the MEMS S&A device withstands the setback and centrifugal force,and the centrifugal insurance mechanism releases the explosion-proof slider.With a certain launching distance,the electric thruster works and deforms the latch,then the explosionproof slider will be released.When the fire hole in the barrier and the explosive train are aligned,the whole device will be in the armed mode.

Fig.3.The inertial driven MEMS S&A device(Dakui Wang).(a).The basic structure.(b).MEMS S&A device.

In order to reduce the fabrication difficulty,Jihun Jeong[10,11]has used the wet etching method to fabricate the metal structure and proposed an inertial delay MEMS S&A device in 2018.The device structure is shown in Fig.4.The proposed device is composed of a stainless steel frame,setback mechanism,spin mechanism,gear-chuck reduction mechanism,rack and barrier.During the launching condition,the setback force will drive the inertia mass to move downward and disarm the first restriction of the rack.Since the bottom of the inertia mass is designed as a round shape,this component will get jammed into the upper housing eventually,which will prevent the inertia lock from returning to the safe mode.Then,the spin lock will be moved away by the centrifugal force of the weapon system and disarmed the second restriction of the rack.Without the restriction of setback lock and spin lock,the rack can be moved under the control of the gearchuck reduction mechanism,and the arming delay function can be realized.In the final state of the arming delay,the rack drags the barrier and aligns the explosive train.

Fig.4.The inertial driven MEMS S&A device(Jihun Jeong).

Considering the limited inertial force at micro level,most of the current inertial driven MEMS S&A devices are based on metal substrate(Ni or Cu with higher density than Si).In addition,due to the sound explosion-proof performance of metal material[12],these metal devices can greatly improve the safety of weapon system.However,the fabrication processes on metal(LIGA or EFAB)are costly,and the fabrication precision is not satisfactory.

2.2.The electro-magnetic MEMS S&A devices

Walter H.Maurer[13,14]proposed an electro-magnetic MEMS S&A device in 2006,shown in Fig.5.This device is mainly composed of pre-compressed springs,barrier,slider,and lockpin magnetics.A pair of lockpin magnetics are set asymmetrically to the barrier and the function of interlock can be realized.In the launching state,the changing data of weapon velocity and acceleration will be obtained by sensors.When the parameters meet the arming requirements,the electro-magnetic coil will be actuated and pull the slider away.Without the restriction of the slider,the pre-compressed spring will push the barrier downwards and align the explosive train.The fabrication of the device is based on Ni material,through LIGA process,all of the components can be obtained.

Fig.5.The electro-magnetic MEMS S&A device(Walter H.Maurer).

Based on the research of Walter,Zhiliang Wu[15]has made some improvements of the device in 2010.The structure is shown in Fig.6.This device is basically the same as the device proposed by Walter.The difference is that the barrier of the MEMS S&A device is driven by the electro-magnetic principle instead of releasing the pre-compressed spring.Therefore,the whole of device needs to introduce three electro-magnetic coils,which greatly increases the package size.

Fig.6.The electro-magnetic MEMS S&A device(Zhiliang Wu).

Compared with inertial driven principle,the electro-magnetic MEMS S&A devices are almost unaffected by the launch environment,which can improve the environmental adaptability of weapon system[16,17].However,the shortcomings of this kind of MEMS S&A devices are also obvious:confined by the driven principle(electro-magnetic)and substrate material(Ni),LIGA process seems to be the only feasible fabrication method,which will rise the production cost;Besides,electro-magnetic coil needs to be introduced in the system,which will increase the overall size of the device.

2.3.The pyro-tech MEMS S&A devices

Helene Pezous[18]proposed a pyro-tech MEMS S&A device in 2010.Shown in Fig.7,the architecture of the device is assembled by different wafers.The structure of mechanical arming function is designed in the bottom layer.The intermediary layer is Si-based safe initiator,on which are integrated micro switches to realize the electrical arming and disarming functions.The top layer is the electronic circuitry and power supply.In order to prevent misfunction caused by a shock,an inertial pin is introduced to block the barrier.The MEMS S&A device is stored in safe mode.The first order is for the mechanical arming:the inertial pin is removed by the acceleration;then the microcontroller sends an electrical order to the micro actuator.The gas generated by the pyrotechnical actuator moves the barrier in armed position.The device respects the STANAG 4187 norm(1 A/W during 5 min of not fire)and requires only 635 mW for ignition.

Fig.7.The pyto-tech MEMS S&A device(Helene Pezous).(a).The working principle.(b).The basic structure.

Peng Zhu[19]proposed a planar ignitor inserted with pyro-MEMS S&A device in 2018.The exploded view of the structural is shown in Fig.8,and the total size of this pyroMEMS S&A device is less than 2 cm3.The device is mainly composed of stainless steel output charge layer,silicon S&A layer and LTCC base.The output charge is filled in the output charge layer and set misaligned with the transfer charge that located in the S&A layer.S&A layer consists of C6H2(NO2)3OK/KClO4gas charge,B/KNO3transfer charge,silicon slider,double silicon solid beams,double silicon slots and double 65Mn steel elastic beams.LTCC base is composed of Ag circuit,Pd/Ag pad,RuO2resistor,Si/Pb3O4ignition charge and Si-S&A layer cavity.In safe mode,the silicon slider is fixed by the double solid beams and elastic beams.Thus,the motion caused by the axial and radial impact can be confined.After receiving the disarming command,large amounts of gas will break the solid beams.Then,the gas will push the silicon slider to the arming position,making the elastic beams fall into the slot and the slider locked in the armed position.This will lead to the main fire train in line.

Fig.8.The pyto-tech MEMS S&A device(Peng Zhu).

The pyro-tech MEMS S&A devices have some typical features:(a).Small volume and small mass;(b).No environmental force is required to disarm;(c).Large drive displacement achieved under small size conditions.However,due to the incompatibility between the fabrication process of the micro/nano energetic material and MEMS technology,the energetic material filling process is usually carried out after device fabrication[20,21].The energetic material will be filled in the chamber manually,thus the contact quality between the heater and energetic material can not be guaranteed.

2.4.The electro-thermal MEMS S&A devices

Robert A.Lake[22,23]proposed an electro-thermal MEMS S&A device in 2010,shown in Fig.9.The device is mainly composed of arrays of electro-thermal actuators,pawls,drive wheel and moveable plate.Among them,the arrays of electro-thermal actuators are arranged in two directions perpendicular to each other to complete the driving of the corresponding displacement.The engagement and disengagement between the pawl and drive wheel can be realized,thereby pushing the plate to move.Surface micromachining technology has been used in fabrication.Due to its complexity of the structure,different mechanisms need to cooperate with each other to complete the corresponding action.The response time of this MEMS S&A device can change with the frequency of the driven voltage.Under the 250 Hz operation frequency,it will take the device about 325 ms to generate the 785μm displacement.

Xiaodong Zhou[24]proposed an electro-thermal MEMS S&A device for laser interrupter in 2017.Shown in Fig.10,the device is mainly composed of V-shape electro-thermal actuator and optical fiber.The electro-thermal actuator acts as a switch to selectively couple optical power between input and output fibers using direct coupling technique.When the device is initiated,the electrothermal actuator will align the input fiber with the output fbier.With the conduction of laser energy,the energetic material can be ignited successfully.Switching time is 19 ms from the safety to the arming state with a maximum operational frequency of 34 Hz.The optical efficiency is found to be about 92.7% with a maximum power transfer of 2780 mW from a 3000 mW input,and the channel isolation is 57 dB.

Fig.9.The electro-thermal MEMS S&A device(Robert A.Lake).(a).The basic structure.(b).The working principle.

Fig.10.The laser interrupter for MEMS S&A device(Xiaodong Zhou).(a).The safe mode.(b).The armed mode.

The silicon based S&A devices have the features of miniaturization and integration.However,confined by the property of silicon material(fragility)[25,26],the devices can hardly stay intact when block the detonation.In order to improve the structure strength,a composite structure based on metal and silicon has been proposed[27],shown in Fig.11.A 300μm thick Ni plate has been electroplated on the movable silicon barrier.Under the control of 11 V driven voltages,the status of the S&A device can be changed smoothly(safe to armed,or armed to safe).In order to verify the effectiveness of the metal enhanced layer,a micro detonator has been integrated in the system.The test results show that the metal enhanced layer can stay intact after the detonation,which indicates the successfully design of the metal/silicon composite structure.

Most of the electro-thermal MEMS S&A devices are based on silicon substrate,which gives this kind of devices the features of low cost,mass production,and easy to fabricate[28].However,confined by the low thermal expansion coefficient of silicon,deform amplification mechanism(micro lever and micro spring)should be introduced in the whole system,which will increase the structure complexity[29].

2.5.The multiphysics-driven MEMS S&A devices

Taylor T.Young[30]proposed a kind of multiphysics-driven MEMS S&A device in 2016.The basic structure is shown in Fig.12.The entire device is fabricated on SOI wafer,and both of the device layer and the handle layer have movable structures.In the device layer,it is mainly composed of a command lock,a setback lock,a slider,an arming micro motor(electro-thermal actuator),and an initiator.The handle layer is integrated with energetic material and detonator.During the safe mode,the initiator and the energetic material are misaligned,and the system is blocked by the command lock,the setback lock and the micro motor.When the weapon is launched,the setback lock,micro motor and the command lock disarmed in sequence.Without restriction,the slider will be actuated by the weapon’s rotational acceleration,and then,the fire hole on the slider will be aligned with the initiator.At this time,the device is in the armed mode.This silicon-based MEMS S&A device now has installed on 40 mm caliber grenades,and performance tests have been taken on the prototype.

Electro-thermal principle and inertial driven principle are combined together in this kind of MEMS S&A device.With the introduction of multiphysics,it not only improves the safety of the weapon system,but also increases the diversity of arming signal and expands its application scope in complex battlefields.

3.The development trend of MEMS S&A device

The concept of MEMS S&A device was proposed in 2000.After nearly two decades’development,the device characteristics of integration,miniaturization and intelligence have been well proven.In order to have a clear understanding of the development trend of MEMS S&A devices,some typical literatures have been sorted in chronological order,shown in Table 1.

From Table 1,the current development trends of MEMS S&A devices can be summarized:(a).Enlarged output displacement.The output energy density of micro energetic material is much higher than that of traditional macro energetic material,therefore,MEMS S&A devices are required to provide larger output displacement to ensure the device overall safety performance.At present,output displacement at millimeter level is required;(b).Active driven method.MEMS S&A devices in early stage can only be actuated in specific inertial environment,and this passive driven method can hardly satisfy the current complex battlefield.The active driven methods such as electro-magnetic,electric-thermal or pyro-tech can better meet the development of intelligence for the next generation in weapon system.(c).Delicate structure.The fabrication method is gradually changed from LIGA process to silicon process,which makes it possible to design a more complex and highly integrated structure.In micro scale,silicon has similar mechanical properties to metal[31],although silicon may break into pieces when block the detonation,metal layer can be introduced to enhance the device structure strength,just as the solution proposed by Ref.[32]and Ref.[33].By summing up the current MEMS S&A devices,we can also find the shortcoming,which is the lack of the function of status detection.None of the MEMS S&A devices reported in the literature has the status detection function.As a key component in weapons system,the real-time detection of the device status will affect the safety performance of the entire system.Therefore,what kind of status detection principle is used and how to integrated with the MEMS S&A device will be a problem to be solved in the future development of MEMS S&A devices.

Table 1Performance comparison of MEMS S&A devices.

Fig.11.The electro-thermal MEMS S&A device with composite structure(Tengjiang Hu).

Fig.12.The multiphysics-driven MEMS S&A device(Taylor T.Young).(a).The basic structure.(b).The packaging.

In light of the variety of driven principles and structures of MEMS S&A devices,the devices can be designed to meet the needs for different battlefields.For the low-g rocket and missile that have limited available environment force,both electro-thermal and pyro-tech silicon based MEMS S&A devices can be applied.These kinds of devices can actively control the status of weapon through electrical signal.In coordination with certain sensors,these devices can further realize intelligent recognition of weapon’s flying height,speed and acceleration.For the high-g projectile,its overload acceleration during launching can reach such a high level as tens of thousands g.Therefore,the inertial driven metal based MEMS S&A device is more suitable for such hostile working conditions.This kind of device can realize status control mainly based on the inertial force of the structure itself.By adopting the passive packaging form,the device further improves the anti-overload ability of the system.

Although MEMS S&A devices are characterized by miniaturization,integration and intelligence,there still exists a certain gap from practical use.The bottlenecks faced by its development are:(a).The difficulties of fabrication.The fabrication process of MEMS S&A devices are mainly based on the MEMS techniques.Compared with the traditional fabrication method,MEMS techniques sets higher standards for equipment,sites,materials and designers.The research and development of MEMS S&A devices also necessitates a long-term and large amount of investments.Due to the high threshold of research and development on MEMS S&A devices,the qualified institutions in this field are limited,resulting in less research work.(b).The missing of design criteria.Compared with the criteria of traditional S&A devices(GJB373A in China,MIL-STD-1316D in USA,etc.),the standards for designing MEMS S&A devices are still in the conceptual stage.Due to the lack of design guidelines,most of the reported MEMS S&A devices are still in the stage of prototype.Therefore,in future research,corresponding criteria need to be developed to guide the design of MEMS S&A devices,e.g.,the input-output energy form,packaging form and the form of interference between explosion train,etc.

Table 2Performance comparison of different driven principles.

4.Conclusion and perspectives

By analyzing the research status and development trend of MEMS S&A devices in recent years,it can be seen that MEMS S&A devices are smaller in structure scale and have higher functional integration than traditional S&A devices.After nearly 20 years of development,the research on MEMS S&A devices has evolved from individual components to system integration,and many prototypes have the potential for live-fire testing.The driven principle and structure of the MEMS S&A device are various,and the performance comparison can be summarized in Table 2.Through reasonable combination,the devices can be designed to meet the needs for different battlefields.At present,the MEMS S&A device can realize the mutual integration of the driving mechanism and the blocking mechanism on the micron level.In the future,with the establishment of new design criteria,MEMS S&A devices will develop from prototypes to practical applications,which will further promote the integration and intelligent of weapon systems.

Declaration of competing interest

The authors declared that we have no conflicts of interest to this work.

We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgement

The work is supported by China Postdoctoral Science Foundation(2018M640977)and the Fundamental Research Funds for the Central Universities(xzy012019004).