Coupling Folding Springs in Microelectromechanical Systems (MEMS) Devices

Abstract

The present invention relates to coupling folded springs in a microelectromechanical system (MEMS) device. A MEMS device is described, comprising a proof mass movably connected to a substrate via folded springs disposed on opposite sides of the proof mass, wherein a coupler couples two of the folded springs together. In some embodiments, the coupler is a rod and can be rigid. Thus, the coupler constrains the relative motion of the folded springs. In this manner, the motion of the proof mass can be constrained to a preferred type and frequency.

Description

[0001] This application is a divisional application of the Chinese invention patent application with application date of September 27, 2017, application number 201710885349.1, and invention name “Coupling Folding Springs in Micro-Electro-Mechanical Systems (MEMS) Devices”. Technical Field

[0002] The present disclosure relates to springs that couple a proof-mass to a substrate in a microelectromechanical system (MEMS) device. Background Art

[0003] Many microelectromechanical systems (MEMS) devices include a proof-mass movably coupled to a substrate. Such devices employ a variety of coupling structures, such as straight beam couplers, T-anchors, coil springs, or folded springs, to couple the proof-mass to the substrate. Summary of the Invention

[0004] A microelectromechanical system (MEMS) device is described, comprising a proof mass movably connected to a substrate via folded springs disposed on opposite sides of the proof mass, wherein a coupler couples two of the folded springs together. In some embodiments, the coupler is a rod and can be rigid. Thus, the coupler constrains the relative motion of the folded springs. In this manner, the motion of the proof mass can be constrained to a preferred type and frequency.

[0005] In certain embodiments, a microelectromechanical system (MEMS) device is provided, comprising: a substrate; a proof mass movably coupled to the substrate via first and second folding springs, wherein the proof mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and a rod coupling the first folding spring to the second folding spring.

[0006] In certain embodiments, a microelectromechanical system (MEMS) device is provided, comprising: a substrate; a proof mass movably coupled to the substrate via first and second folding springs, wherein the proof mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and a member for coupling the first folding spring to the second folding spring.

[0007] In certain embodiments, a system configured for harvesting energy is provided. The system includes an energy storage device; and an energy harvester coupled to the energy storage device and configured to deliver power to the energy storage device. The energy harvester includes a substrate; a mass movably coupled to the substrate via first and second folded springs, wherein the mass is disposed between the first and second folded springs along a compression direction of the first and / or second folded springs; and a rod coupling the first folded spring to the second folded spring. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Various aspects and embodiments of the present application will be described with reference to the following drawings. It should be understood that the drawings are not necessarily drawn to scale. Items appearing in multiple figures are represented by the same reference numerals in all figures in which they appear.

[0009] Figure 1A is a perspective view of a microelectromechanical system (MEMS) inertial device having a proof mass sprung to a substrate by folded springs, with a rod coupling two folded springs together, according to an embodiment of the present invention.

[0010] Figure 1B yes Figure 1A Top view of the MEMS inertial device.

[0011] Figure 1C 、 1D and 1E is Figures 1A-1B Three different operating modes of the MEMS inertial device.

[0012] Figure 2A is a perspective view of a MEMS inertial device having a proof mass sprung to a substrate by folded springs, wherein a plurality of rods couple together the folds of two of the folded springs, in accordance with an embodiment of the present invention.

[0013] Figure 2B yes Figure 2A Top view of the MEMS inertial device.

[0014] Figure 2C 、 2D and 2E description Figures 2A-2B Three different operating modes of the MEMS inertial device.

[0015] Figure 2F is with Figure 1A Shown is a top view of a MEMS device including alternative shapes for the folding spring.

[0016] Figure 3 is a system incorporating a MEMS inertial device of the type described herein.

[0017] Figure 4 According to a non-limiting embodiment of the present invention, a Figure 3 Shown is a sensor system for an automobile of the type shown. DETAILED DESCRIPTION

[0018] Aspects of the present invention provide a microelectromechanical system (MEMS) device having a movable mass coupled to a substrate via two folded springs coupled on opposite sides of the mass, with the masses being coupled together via a coupler. The two springs may be folded springs having a serpentine shape, extending along the direction of motion of the mass, and the coupler may be a rod extending along the direction of motion from one of the springs to the other. The rod may be rigid, thereby limiting the relative motion of the two springs. In this manner, undesirable vibration modes of the folded springs may be suppressed under normal operating conditions of the MEMS device.

[0019] In some embodiments, the spring may include multiple folds. Multiple couplers may couple a spring to another spring, such as by connecting corresponding folds of one spring to the other spring. In some embodiments, the couplers may all be rigid rods.

[0020] In some embodiments, a MEMS device is an energy harvesting device capable of harvesting energy from the motion of a movable mass. In at least some embodiments, it is desirable that the displacement of the mass from its equilibrium position be large, for example greater than 200 microns. The greater the displacement, the more energy that can be harvested. The coupler described herein, which couples two folded springs together, can facilitate the use of such springs in MEMS devices where the mass is intended to undergo large displacements. The coupler can allow large displacements in a preferred vibration mode of the MEMS device while suppressing displacements associated with unwanted vibration modes.

[0021] Figure 1A 1 is a perspective view of a MEMS device having a proof-mass elastically connected to a substrate via folded springs, wherein two folded springs are connected together according to an embodiment of the present invention. The MEMS device 100 includes a proof-mass 102, a substrate 104 having a cavity 105, four folded springs 106a, 106b, 106c, and 106d having respective anchor points 108a, 108b, 108c, and 108d, and two couplers 110a and 110b. Figure 1B yes Figure 1A The top view of the structure is shown in FIG. 1 , in which the substrate 104 is omitted for ease of explanation.

[0022] The mass 102 can have any suitable size and shape and can be formed from any suitable material. As an example, the mass 102 can be rectangular (e.g., square) and can be formed from silicon. In some embodiments, the mass 102 is formed from the same material as the substrate 104. For example, the substrate 104 can be a silicon substrate and the mass 102 can be formed from the substrate 104 by suitable micromachining techniques (e.g., photolithography and etching). As a non-limiting example, the mass 102 can have a thickness T1 between 0.3 mm and 3 mm, or any value or range of values ​​within this range. As a non-limiting example, the mass 102 can have a length L1 between 3 mm and 20 mm, or any value or range of values ​​within this range.

[0023] As a non-limiting example, substrate 104 can be a silicon substrate or can be formed from other semiconductor materials. In some embodiments, substrate 104 forms part of a wafer so that multiple instances of MEMS device 100 can be formed simultaneously on the wafer using wafer-level fabrication techniques. In such embodiments, individual MEMS devices can be cut from the wafer. Cavity 105 can be formed in substrate 104 using suitable micromachining techniques so that proof mass 102 is suspended above cavity 105. It should be understood that alternative embodiments do not include a cavity in the substrate, as there are alternative ways to implement a movable proof mass relative to substrate 104.

[0024] Fold springs 106a-106d may be referred to herein as "accordion springs," "fold springs," "fold tethers," or other similar terms, and are substantially identical to one another in this non-limiting example, and each includes a single fold. In alternative embodiments, the fold springs may include more than one fold, an example of which is shown in FIG. Figure 2A and further described below. Figure 1A and 1B As can be seen, a pair of folding springs are included on opposite sides of the mass 102. A first pair, comprising folding springs 106a and 106b, is located on one side of the movable mass 102, and a second pair, comprising folding springs 106c and 106d, is located on the opposite side of the mass, such that the mass 102 is located between the pairs of folding springs.

[0025] The folding springs 106a-106d can expand (or compress) along the y-axis ( FIG. 2 ), allowing the proof mass 102 to move in that direction. The springs 106a-106d are coupled to the base plate 104 at respective fixed points 108a-108d, which are fixed. The anchor points can correspond to the ends of the respective folding springs 106a-106d. In one embodiment, the anchor points can represent posts extending downwardly to the surface of the base plate. Alternative configurations are possible. For example, in some embodiments, the ends of the folding springs represented by the anchor points 108a-108d can alternatively terminate on the sides (or sidewalls) of the base plate 104.

[0026] The folding springs 106a-106d can have any suitable dimensions. Figure 1A The folded spring may have a thickness T2 between 10 microns and 2 mm, or any value or range of values ​​therein, as non-limiting examples. Figure 1B , they can have a length L2 in the y-direction between 200 microns and 3 mm, or any value or range of values ​​within this range, and the total meandering length will be longer. Length LS can determine the possible displacement of the proof mass and, therefore, can have an appropriate value to provide the desired amount of displacement of the proof mass, for example, between 150 microns and 2 mm. The length LD of the segment of the proof mass that provides the folding distance of the folded spring can be approximately half of length LS. The width W1 of the folded springs 106a-106d in the xy plane can vary along their length. For example, the width can be greater near proof mass 102 and smaller near the anchor point. Alternative configurations are possible. The value of width W1 can be selected to provide the desired resonant frequency of the MEMS device. In some embodiments, width W1 can be between 1 micron and 300 microns, or any value or range of values ​​within this range.

[0027] The dimensions of the folding spring can be selected to allow for large displacements of the proof mass in the y-direction. For example, the length of the folding spring and the positioning of the anchor point relative to the proof mass can be selected to allow the folding spring to expand and compress within a range of between 0.3 mm and 5 mm, between 1 mm and 3 mm, or any value or range within such ranges. Thus, in some embodiments, the range of motion of the proof mass 102 can be within such ranges or greater. Such large displacements can be beneficial in various applications, such as when the MEMS device 100 is an energy harvester that collects energy from the motion of the proof mass 102.

[0028] The MEMS device may include folding springs that are different from 106a-106d. That is, the shapes of the folding springs 106a-106d are non-limiting examples. Other examples are Figure 2F and further described below.

[0029] like Figures 1A-1D As shown, couplers 110a and 110b connect the folding springs on opposite sides of mass 102. In this example, coupler 110a couples folding springs 106a and 106c, while coupler 110b couples folding springs 106b and 106d. In this example, couplers 110a and 110b are substantially straight. They can have any suitable dimensions to provide the desired degree of rigidity. For example, they can have the same thickness T2 as the folding springs, but can have a width W2 that is greater than width W1. In some embodiments, width W2 is greater than 2 to 20 times W1, greater than 2 to 10 times W1, or any value or range of values ​​within this range. Width W2 can be selected to be substantially sufficient to resist bending of couplers 110a and 110b in the x or z directions and to resist tension or compression in the y direction under typical operating conditions of MEMS device 100. For example, width W2 can be between 50 and 500 microns, or any value or range of values ​​within this range. The width W3 of couplers 110a and 110b at the coupling point to the folded spring can be greater than width W2. In some embodiments, width W3 is between 1.5 and 5 times greater than W2, although other options are possible. The length L3 of couplers 110a and 110b is sufficient to extend around proof-mass 102. For example, length L3 can be between 1.25 and 3 times greater than the length L1 of proof-mass 102. At the coupling point between the coupler and the folded spring, length L4 can be sufficient to provide a strong mechanical coupling between the spring and the coupler. In some embodiments, L4 is between 300 microns and 1.5 millimeters, although alternatives are possible.

[0030] It should be understood that, in at least some embodiments, the inclusion of couplers 110a and 110b does not significantly increase the size of MEMS device 100. As described above, in some embodiments, the width of the couplers can be between 50 microns and 500 microns. The offset D1 of the couplers in the x-direction from mass 102 can be between 20 microns and 300 microns, or any value within this range. Thus, in at least some embodiments, the presence of couplers 110a and 110b can add less than 30% of the length L1 to the device in the x-direction. In some embodiments, by including couplers 110a and 110b, less than 20% or less than 10% of the length L1 is added in the x-direction. Thus, the benefits of the couplers can be achieved with a relatively small increase in device size.

[0031] As shown, in some embodiments, a coupler is provided that couples the outwardly facing surface of the folding spring. However, not all embodiments are limited in this regard, as the coupler can be configured to couple to an alternative configuration of the inwardly facing portion of the folding spring. Figure 2AAn example is shown in and described further below.

[0032] Couplers 110a and 110b can be formed from any suitable material, and in some embodiments are formed from the same material as the folding springs to which they couple. For example, they can be formed by the same patterning and etching process.

[0033] Figure 1C 、 1D and 1E shows Figures 1A-1B 1 and 2. The three different operating modes of the MEMS device 100. To simplify the illustration, the substrate 104 is omitted in these figures.

[0034] Figure 1C The displacement of the MEMS device 100 associated with the fundamental vibration mode of the mass-spring system is shown. In this example, the mass moves along the y-axis, more specifically, Figure 1C The diagram shows a state where mass 102 is displaced from its equilibrium position in the negative y-direction. In this state, fold springs 106a and 106b are compressed in the y-direction, while fold springs 106c and 106d are expanded in the y-direction. It can be seen that couplers 110a and 110b do not expand or contract, so the distance between the coupled folds of the fold springs remains the same. That is, the distance between the coupling point of fold spring 106a and the coupling point of fold spring 106c remains unchanged from the equilibrium state of MEMS device 100. Similarly, the distance between the coupling point of fold spring 106b and the coupling point of fold spring 106d remains unchanged from the equilibrium state.

[0035] In some embodiments, Figure 1C The motion shown represents the desired motion of the MEMS device 100. For example, in response to experiencing vibration, acceleration, or some other condition of interest, the mass may be expected to experience a large displacement along the y-direction. In some embodiments, the displacement generates an electrical signal that is detected by suitable electrodes and captured and stored by suitable circuitry. Thus, the motion of the mass can be converted into harvested electrical energy. In alternative embodiments, the MEMS device 100 can be a sensor such as an accelerometer or gyroscope, and the mass 102 can be displaced in response to a condition of interest such as acceleration or rotation of the MEMS device 100. The displacement can be detected, thereby providing an indication of the experienced condition. In other embodiments, the MEMS device can be an actuator. In some embodiments, the system can include multiple such MEMS devices configured as different types of devices selected from harvesters, sensors, and actuators.

[0036] Figure 1D The vibration modes of the mass 102 moving along the z-axis are shown. In particular, Figure 1DThe mass 102 is shown displaced from its equilibrium position in the positive z-direction. Couplers 110a and 110b can prevent the folded springs 106a-106d from twisting freely in response to this motion of the mass and thus can suppress the vibration mode shown.

[0037] Figure 1E The vibration modes of the mass 102 are shown as they rotate about the y-axis. In particular, Figure 1E The mass 102 is shown rotated counterclockwise from its equilibrium position about the y-axis. Couplers 110a and 110b prevent the folding springs 106a-106d from twisting freely in response to this motion of the mass and thus suppress the vibration mode shown.

[0038] Figure 1D and 1E shows motion that may be undesirable in at least some embodiments, and thus the vibration modes shown may be considered spurious modes. Again, in some embodiments, it may be desirable for mass 102 to exhibit a Figure 1C In this case, the inhibition Figure 1D and 1E The motion shown may be beneficial. Couplers 110a and 110b may cause Figure 1D and 1E The resonant frequency of the vibration mode shown in the frequency domain is Figure 1C The resonant frequencies of the vibration modes shown are sufficiently shifted that they are unlikely to occur during typical operation of MEMS device 100. In other words, couplers 110a and 110b can suppress or completely prevent the occurrence of vibrations by providing a rigid connection between the folded springs. Figure 1D and 1E In some embodiments, the displacement type shown in FIG. Figure 1C The resonant frequencies associated with the vibration modes shown in Figure 1 are significantly higher than Figure 1C The resonant frequency of the vibration mode shown. For example, Figure 1D and 1E The vibration mode can have Figure 1C The resonant frequency of the vibration mode of the mass-spring system may be greater than the resonant frequency of the first vibration mode by a factor of 200 Hz to 3 kHz, or any value or range of values ​​therein. In some embodiments, the second and higher order vibration modes of the mass-spring system may have a resonant frequency that is 2 to 20 times greater than the resonant frequency of the first order vibration mode.

[0039] As previously described, the folded spring coupling the proof-mass to the substrate may include more than one fold. In such an embodiment, one or more couplers of the type described herein may connect together one or more folds of two folded springs. Figure 2A and 2BNon-limiting examples are shown.

[0040] The MEMS device 200 includes a front-side bonding Figure 1A and 1B 1. MEMS device 200 has many of the same components as those described above, so those components will not be described in detail here. However, a difference is that multiple folded springs 206a, 206b, 206c, and 206d are provided to couple proof mass 102 to substrate 104. In addition, MEMS device 200 includes six couplers 210a, 210b, 210c, 210d, 210e, and 210f.

[0041] In this example, with Figure 1A In contrast to the single-fold fold springs of FIG. 1 , each of fold springs 206a-206d has three folds. Couplers 210a-210f connect the corresponding folds of opposing fold springs. That is, couplers 210a-210c connect the corresponding folds of fold springs 206a and 206c, while couplers 210d-210f couple the individual folds of fold springs 206b and 206d. The couplers can have any suitable dimensions to provide a desired degree of rigidity. For example, the length, thickness, and width of couplers 210a-210f can be within the ranges listed above for couplers 110a and 110b, or any other suitable dimensions.

[0042] from Figure 2A and 2B As can be seen, couplers 210b, 210c, 210e, and 210f can have substantially the same shape as one another, but couplers 210b and 210e can have different lengths than couplers 210c and 210f to allow for coupled folding springs with different folds. Couplers 210a and 210d have different shapes than the other couplers because they couple facing surfaces of the folding springs. In other words, couplers 210a and 210d have wraparound segments that wrap around the folding springs. For example, coupler 210a includes wraparound segments 212a and 212b, and coupler 210d includes wraparound segments 212c and 212d. Thus, in some embodiments, folding of coupled folding springs is provided, with the folding springs facing different directions from one another. For example, coupler 210b is coupled to a folded portion of folded spring 206c consisting of segments 211c, 211d and 211e that faces a different direction than the folded direction of folded spring 206c coupled to coupler 210a consisting of segments 211a, 211b and 211c.

[0043] from Figure 2A and 2BIt should be understood that in some embodiments, a coupler is provided that couples each fold of the opposing fold springs. However, not all embodiments are limited in this respect. For example, Figure 2A and 2B Alternative configurations of the MEMS device shown may have two couplers or four couplers instead of six. Couplers 210b and 210e may be omitted in alternative embodiments.

[0044] Figure 2C 、 2D and 2E shows Figures 2A-2B Three different operating modes of the MEMS device. Figure 2C Shows the vibration modes of the mass-spring system for a mass moving in the y-direction. In particular, Figure 2C The state of mass 102 is shown displaced from its equilibrium position in the negative y direction. Fold springs 206a and 206b are compressed, and fold springs 206c and 206d are expanded. Couplers 210a-210f do not expand or compress, so the distance between the folds of the coupled springs does not change. In some embodiments, Figure 2C The vibration mode shown represents a fundamental vibration mode of the MEMS device 200 .

[0045] Figure 2D The operation mode is shown in which the mass 102 moves in the z direction. In particular, Figure 2D The state of mass 102 is shown displaced from its equilibrium position in the positive z-direction.Here, couplers 210a-210f resist the torsion of the folded springs associated with this vibration mode, thereby suppressing the mode of vibration.

[0046] Figure 2E The vibration modes of the mass 102 are shown as they rotate about the y-axis. In particular, Figure 2E The state in which the proof mass rotates counterclockwise about the y-axis is shown. Couplers 210a-210f also resist this mode of operation.

[0047] Like couplers 110a and 110b of MEMS device 100, couplers 210a-210f can affect the resonant frequencies of certain vibration modes. For example, Figure 2D and 2E The resonant frequencies of the vibration modes shown may be significantly higher than Figure 2C The resonant frequencies of the modes shown are reduced so that they are unlikely to occur during typical operation of the MEMS device 200. In this manner, the couplers 210a-210f can completely suppress or prevent such vibration modes. Figure 2D and 2E The resonant frequencies associated with the vibration modes can be compared with Figure 2CThe resonant frequency associated with the second and higher order vibration modes of the mass-spring system may be 200 Hz to 3 kHz higher. In some embodiments, the second and higher order vibration modes of the mass-spring system may have resonant frequencies that are 2 to 20 times greater than the resonant frequency of the first order vibration mode. Thus, during typical operation of the MEMS device 200, such undesirable vibration modes may be suppressed.

[0048] As mentioned earlier, Figures 1A-1D The shapes of the accordion springs in Figures 2A-2E are non-limiting examples. The accordion springs may have different shapes. Figure 2F Another non-limiting example is shown in Figure 2F FIG2 is a top view of a MEMS device 220. MEMS device 220 includes a mass 222, a first folded spring formed by segments 224a, 224b, 224c, and 224d (the first folded spring connects mass 222 to anchor point 226), and a second folded spring formed by segments 228a, 228b, 228c, and 228d (the second folded spring couples mass 222 to anchor point 230). Couplers 232a and 232b couple the folded springs together. Other folded spring shapes can be implemented in a MEMS device, including couplers that couple two or more folded springs together.

[0049] MEMS devices of the type described herein may be used in various systems such as energy harvesting systems, sensor systems, tunable capacitor systems, and control systems. Figure 3 3 is a system that includes a MEMS inertial device of the type described herein. System 300 is a sensor system that includes a MEMS sensor 302, a power unit 304, a sensing circuit 306, and an input / output (I / O) interface 308. MEMS sensor 302 can be a MEMS accelerometer, pressure sensor, gyroscope, or other type of sensor to sense a characteristic of interest. Power unit 304 can include an energy harvesting device 310, such as MEMS device 100 or 200 described above and operating as an energy harvesting device, and an energy storage device 312. The energy storage device can be a battery, supercapacitor, or other suitable structure for storing energy harvested by energy harvesting device 310. The harvested energy can be used to power sensing circuit 306 that controls the operation of MEMS sensor 302 and / or power detection circuit 306 that processes signals output by MEMS sensor 302. I / O interface 308 can be a wired or wireless interface for communicating with external components such as a computer, server, or other system.

[0050] Systems incorporating MEMS devices of the type described herein can be implemented in a variety of settings. By way of example, such systems can be used in industrial settings. For example, industrial machinery may generate vibrations whose energy can be harvested by the MEMS devices described herein. The harvested energy can be used to power sensors that monitor the performance of the industrial machinery and / or processors that control the operation of the machinery.

[0051] As an alternative, Figure 3 The illustrated system may be implemented in a vehicle such as an automobile, a watercraft, or an aircraft. Figure 4 An example is shown in which a car 400 includes a sensor unit 401 and an onboard computer 402. The sensor unit 401 may be, for example Figure 3 The system shown includes a MEMS sensor, a power unit having an energy harvester of the type described herein, a sensor circuit, and an interface circuit. The sensor unit 401 may include a package or housing that is attached to a suitable portion of the car 400. As an example, the sensor unit may sense acceleration along the driving direction and / or perpendicular to the driving direction. Additionally or alternatively, the sensor unit 401 may be configured to sense vertical acceleration to monitor, for example, a pause state. The energy harvester of the sensor unit 401 may collect energy from the movement of the car. The sensor unit 401 may communicate with an onboard computer 402 and may provide a sensing signal to the onboard computer.

[0052] MEMS devices according to aspects of the present invention can, in at least some embodiments, be self-tested. For example, a MEMS energy harvester having coupled folded springs of the type described herein can be self-tested by applying a suitable drive signal, and the resulting motion of the proof mass can be evaluated. Self-testing of other types of MEMS devices having coupled folded springs, not just energy harvesters, can also be performed.

[0053] The term "about" can be used in some embodiments to mean within ±20% of a target value, in some embodiments to mean within ±10% of a target value, in some embodiments to mean within ±5% of a target value, and in some examples still within ±2% of a target value. The term "about" can include a target value.

Claims

1. A micro-electromechanical system (MEMS) device, comprising: substrate; a mass movably coupled to the substrate via first and second folding springs, wherein the mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and a rod coupling the folded middle portion of the first folded spring to the folded middle portion of the second folded spring, the rod being movable relative to the mass in the compression direction and movable relative to the base in the compression direction, Wherein the rod is coupled to a first side of the folded middle portion of the first folded spring, and a second side of the folded middle portion of the first folded spring is not coupled to the rod, the second side being opposite the first side.

2. The MEMS device of claim 1 , wherein each of the first and second folding springs comprises a plurality of folds, wherein the rod is a first rod and couples a first fold of the first folding spring to a first fold of the second folding spring, and wherein the MEMS device further comprises a second rod coupling a second fold of the first folding spring to a second fold of the second folding spring, the second rod being movable relative to the proof mass in the compression direction. 3 . The MEMS device of claim 2 , wherein a first fold of the first fold spring faces in an opposite direction of a second fold of the first fold spring. The MEMS device of claim 3 , wherein the first rod surrounds at least a portion of the first folded spring. 5 . The MEMS device of claim 1 , wherein the rod has a length from the first folding spring to the second folding spring and a width perpendicular to the length, and wherein the width of the rod is 2 to 10 times greater than the width of the first folding spring.

6. The MEMS device of claim 1 , further comprising a third folding spring and a fourth folding spring, coupling the proof mass to the substrate and positioned so that the proof mass is between the third and fourth folding springs, wherein the rod coupling the first and second folding springs is a first rod, and wherein the MEMS device further comprises a second rod coupling the third and fourth folding springs, the second rod being movable relative to the proof mass in the compression direction.

7. The MEMS device of claim 1 , wherein the proof mass and the rod each have a width in a direction substantially perpendicular to the compression direction of the first and / or second folding springs, and wherein the width of the rod is 10% smaller than the width of the proof mass.

8. The MEMS device of claim 1, wherein the rod surrounds at least a portion of the first folded spring.

9. A micro-electromechanical system (MEMS) device, comprising: substrate; a mass movably coupled to the substrate via first and second folding springs, wherein the mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and means for coupling the folded middle portion of the first folded spring to the folded middle portion of the second folded spring, the means for coupling the first folded spring to the second folded spring being movable relative to the mass in the compression direction and movable relative to the base plate in the compression direction, wherein the member for coupling the first folding spring to the second folding spring is coupled to a first side of the folded middle portion of the first folding spring, and a second side of the folded middle portion of the first folding spring is not coupled to the member for coupling the first folding spring to the second folding spring, the second side being opposite to the first side.

10. The MEMS device of claim 9, wherein the means for coupling the first folded spring to the second folded spring comprises means for coupling a plurality of folds of the first folded spring to a plurality of folds of the second folded spring.

11. The MEMS device of claim 10, wherein the plurality of folds of the first fold spring include a first fold and a second fold facing in opposite directions from each other.

12. The MEMS device of claim 9, wherein the means for coupling the first folded spring to the second folded spring comprises means for creating a resonant frequency separation between a first mode of the proof-mass and a second mode of the proof-mass, wherein the second mode has a resonant frequency that is 2 to 20 times greater than the first mode.

13. The MEMS device of claim 9, further comprising a third folded spring and a fourth folded spring, coupling the proof mass to the substrate and positioned so that the proof mass is between the third and fourth folded springs, and a member for coupling the third folded spring to the fourth folded spring.

14. A system configured to harvest energy, comprising: Energy storage devices; and an energy harvester coupled to the energy storage device and configured to deliver power to the energy storage device, the energy harvester comprising: substrate; a mass movably coupled to the substrate via first and second folding springs, wherein the mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and a rod coupling the folded middle portion of the first folded spring to the folded middle portion of the second folded spring, the rod being movable relative to the mass in the compression direction and movable relative to the base in the compression direction, Wherein the rod is coupled to a first side of the folded middle portion of the first folded spring, and a second side of the folded middle portion of the first folded spring is not coupled to the rod, the second side being opposite the first side.

15. The system of claim 14, further comprising a sensor or actuator coupled to the energy harvester and configured to receive power from the energy harvester.

16. The system of claim 15, wherein the sensor or actuator is a microelectromechanical system (MEMS) sensor, comprising: substrate; a mass movably coupled to the substrate via first and second folding springs, wherein the mass is disposed between the first and second folding springs along a compression direction of the first and / or second folding springs; and The first folding spring is coupled to a rod of the second folding spring, wherein the rod is movable relative to the mass along the compression direction.

17. The system of claim 14, wherein each of the first and second folded springs comprises a plurality of folds, wherein the rod is a first rod and couples a first fold of the first folded spring to a first fold of the second folded spring, and wherein the energy harvester further comprises a second rod coupling a second fold of the first folded spring to a second fold of the second folded spring, the second rod being movable relative to the proof mass in the compression direction.

18. The system of claim 17, wherein a first fold of the first fold spring faces in an opposite direction of a second fold of the first fold spring.

19. The system of claim 14, wherein the energy harvester further comprises a third folded spring and a fourth folded spring, coupling the proof mass to the substrate and positioned such that the proof mass is between the third and fourth folded springs, wherein the rod coupling the first and second folded springs is a first rod, and wherein the energy harvester further comprises a second rod coupling the third and fourth folded springs, the second rod being movable relative to the proof mass in the compression direction.

20. The system of claim 14, wherein the rod surrounds at least a portion of the first folding spring.

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