Abstract:
The Resettable Latching MEMS Shock Sensor provides the capability of recording external shock extremes without consuming electrical power. The device incorporates a shock sensitive suspended proof mass, spring-loaded contacts, latches, and actuators for device reset. The device can be designed, hardwired, or programmed to trigger at various shock levels. The device can be fabricated in a simple micromachining process that allows its size to be miniaturized for embedded and portable applications. During operation, the device consumes no quiescent power. The device can be configured to close a circuit, switch an interrupt signal, or switch some other electrical trigger signal between devices at the time of a shock extreme being reached, or it can be configured to latch and be polled at some time after the shock limit has occurred.

Description:
REFERENCE TO RELATED APPLICATIONS 
   This application claims priority to Provisional Patent Application U.S. Ser. No. 60/619,496, entitled “Resettable Latching MEMS Shock Sensor” and filed on Oct. 15, 2004, which is fully incorporated herein by reference. 

   GOVERNMENT LICENSE RIGHTS 
   This invention was made with Government support under contract MDA972-03-C-0010, awarded by the Defense Advanced Research Projects Agency (“DARPA”). The Government has certain rights in the invention. 

   BACKGROUND 
   1. Field of the Invention 
   The present invention relates generally to a shock sensor and method for monitoring shock. More particularly, the present invention relates to a low-power and unpowered micro-electromechanical shock sensor using a micromechanical suspended proof mass structure. 
   2. Background of the Invention 
   Embedding miniature sensors in products, systems, storage and shipping containers, and other items allows the monitoring of those items to determine health, maintenance needs, lifetime, and other item characteristics. Information from miniature shock sensors can tell a user whether the item has been exposed to shock levels that can cause damage. In addition, miniature shock sensors can be used to “wake up,” from a low-power sleep mode, a more sophisticated sensing system to collect a more complete set of environmental data. 
   Current battery-powered embedded sensor systems that perform this type of monitoring often require a low power method of determining when a certain level of shock has been reached. Many other applications, such as in transportation and shipping monitoring, heating and air conditioning, and food storage, would benefit from the ability to monitor the shock environment with a completely unpowered sensor. In addition, these applications would benefit from the ability to poll that sensor to determine if a shock extreme was reached, and then reset the sensor for later use. In either case, an ultra-low power sensor, or even a sensor that consumes no quiescent power, would reduce the overall system power consumption enough to allow embedded sensors to operate for many years in portable battery powered applications, or in systems that scavenge small amounts of power from the environment. 
   Low power and unpowered shock sensors currently exist. However, they are large-scale devices such as the catches used in automotive seat belts. These devices operate in a similar fashion and provide a similar function as the present invention, but are not in a form factor suitable for integration with microdevices, and are not fabricated using techniques that are compatible with microelectronics or micro-electromechanical systems (“MEMS”) devices. 
   Micro-scale shock sensors, in the form of accelerometers, exist as well, but most of the previous work to develop low-power shock sensors has been focused on minimizing the power consumption of standard miniature devices, and using low-power analog electronics to determine when a specific shock level has been reached. Devices and systems would then create a low-impedance logic level signal for input to a sleeping microcontroller. The fundamental problem is that such a system must continuously power the sensor and analog trigger circuitry, creating a constant power draw on the batteries. Even using the latest in low-power devices and highest capacity batteries, systems that continuously power any sensor will only operate for 5-10 years. 
   As embedded miniature sensors get smaller, and as batteries are reduced in size and capacity, the use of lower power and unpowered devices will become more critical. Furthermore, maximizing the sensor functionality, without increasing power consumption, will enhance the capability of embedded sensing systems. 
   Other inventions have used suspended proof mass micro-machined devices to measure shock, and for switching, but, until the present invention, only one as had the advantages of the present invention in combining low- or no-power operation with a mechanical latching function. U.S. Pat. No. 6,737,979 discloses a MEMS shock sensor that achieves the goals of low- and no-power operation of a mechanical shock sensor with a mechanical latching function. In this prior art invention, as in the present invention, a moveable proof mass and a latching means are formed on the surface of a substrate. When the sensor is subject to a sufficient shock, the proof mass moves and latches with the latching means, and the latched condition is detected by external circuitry. 
   The present invention offers several improvements to the technology disclosed in U.S. Pat. No. 6,737,979 (“the &#39;979 invention”). First, in the &#39;979 invention, each separate device design can detect only one range of shock level because the distance between the proof mass and the latch is not variable. In the present invention, the latching distance is variable and a sensor can therefore be programmed to detect varying shock levels. Second, in the &#39;979 invention, the only electrical contact made between the proof mass and the latch to detect a shock level is through the latch itself. As is discussed in detail below, the present invention offers a contact that is separate from the latch so that a “triggering” condition (i.e., the proof mass contacting with the contact) can be made (and detected) prior to latching, if desired by the user. With this feature, the present invention can be programmed to detect a shock level smaller than that of the latching shock level. Third, although the &#39;979 invention offers an unlatching function so that the sensor can be re-used, the present invention improves upon this function with a mechanical linkage that applies no load to the latch during latching, thereby decreasing the necessary latching force and increasing the sensitivity of the sensor. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a low-power micro-machined shock sensor in which the sensitivity of the sensor can be adjusted. 
   It is another object of the present invention to provide a low-power micro-machined shock sensor which allows for detection of a shock level separate from and variable from the latching function (i.e., a triggering event separate from a latching event). 
   It is yet another object of the present invention to provide a micromachined shock sensor with an unlatching apparatus that does not apply a mechanical load on the latch during latching. 
   The present invention achieves these objectives with a micromachined proof mass connected to a substrate through micromachined flexures. The proof mass includes a contact area and a latching area. The contact area and latching area register respectively with spring-loaded contacts and a spring-loaded latch that are anchored to the substrate. Under a shock load of sufficient magnitude, the proof mass displaces to bring the contact area together with the spring-loaded contacts and to force the latch on the proof mass to engage with the spring-loaded latch. After latching, the contacts remain closed, allowing a voltage source to be connected to the input of a microcontroller, or allowing the completion of an external circuit. A thermal, capacitive, or other actuator can then be used to disengage the latch and return the proof mass to its original position. The sensor will use nearly zero power except when actually providing the trigger signal to the microcontroller or during any reset operation. The sensor can remain latched for interrogation at a later date, even if system power is lost, and the sensor can be reset to detect the next event. 
   For purposes of summarizing the invention, certain aspects, advantages, and novel features of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any one particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein. 
   These and other embodiments of the present invention will also become readily apparent to those skilled in the art from the following detailed description of the embodiments having reference to the attached figures, the invention not being limited to any particular embodiment(s) disclosed. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of the shock sensor and its components. 
       FIG. 2A  is a high-level flowchart for an embodiment of a process according to the present invention. 
       FIGS. 2B-2F  illustrate steps in the process of fabricating one embodiment of the present invention. 
       FIG. 3  is a top view of the illustrated embodiment of the sensor in its normal state and ready to sense shock extremes. 
       FIG. 4  is a diagram of the illustrated embodiment of the sensor in its latched and contacted state after a shock extreme has been reached. 
       FIG. 5  shows the definition of parameters used in the design of the sensor. 
       FIG. 6  is a diagram of electrical interconnection of the sensor. 
       FIG. 7  shows an embodiment of the invention with shock sensitive contacts that allow operation at lower shock levels. 
       FIG. 8  shows an embodiment of the invention with multiple contacts for detection of multiple shock levels. 
   

   Repeat use of reference characters throughout the present specification and appended drawings is intended to represent the same or analogous features or elements of the invention 
   DETAILED DESCRIPTION 
   The illustrated embodiment of the invention is fabricated in a thick layer of silicon or other conductor material that has been released from a rigid substrate. Within this thick layer of material, a proof mass, a set of flexures, multiple contacts, multiple latch and pawl structures, multiple actuators, and multiple anchors and pads are fabricated. 
     FIG. 1  illustrates a schematic diagram of one embodiment of the invention. The invention utilizes a micromachined proof mass structure  1  attached to the substrate (not illustrated) via anchors  6  through proof mass flexures  5 . The proof mass structure  1  includes a contact area  2  and a latch  3 . Under a shock load, the inertia of the proof mass yields a force, F g , that displaces the mass sufficiently to force the latch  3  to engage with a similar latch on a thin pawl  4  attached to the substrate via anchor  6  through pawl flexure  8 . The force also causes the proof mass contact area  2  to connect with the contact  7  that is attached to the substrate via anchor  6  through contact flexure  9 . After latching, the contacts remain closed, and the shock sensor can then be interrogated by external circuitry (not illustrated). A thermal, capacitive, or other actuator (discussed below) can be used to develop a force, F a , and disengage the pawl  4  and return the proof mass  1  to its original position. 
     FIG. 2A  illustrates the high-level process flow for the process used to fabricate the suspended proof mass structure that is used in one embodiment of the invention. While the following discussion focuses on producing a silicon structure with the process discussed herein, other combinations of materials and other processes can be employed. 
   Employing the process of  FIGS. 2B and 2C , the starting material is a silicon-on-insulator (“SOI”) wafer  26  with a handle layer  20  and a 100-micron thick active silicon layer  22  separated by a 2 micron thick silicon dioxide layer  21 . With attention to  FIG. 2C , which illustrates step  12  in greater detail, the SOI wafer  26  is first patterned with photoresist  23  using standard lithography to define the footprint of a suspended proof mass  24  (illustrated in FIG.  2 D). As illustrated in  FIG. 2D , which shows step  13  of the fabrication process, a deep silicon reactive ion etch defines the structure of the suspended proof mass  24 . 
   As is illustrated in  FIG. 2E  (step  14 ), after the silicon pattern is transferred, the silicon dioxide layer  21  in between the silicon layers is removed with an isotropic oxide etch that allows portions of the layer  21 , specifically those underneath anchors and bond pads (not illustrated), to remain and hold the structure to the substrate. After the proof mass  24  and other components are released from the substrate, the entire device is coated at an angle with a metal layer system using a process that places metal  25  on the sidewalls of the structure, as illustrated in  FIG. 2F  (step  15 ). This metal is critical as it forms the contacts that the sensor uses. 
     FIG. 3  illustrates an embodiment of the invention that includes two “mirror-imaged” sets of latches/contacts on opposite sides of the proof mass  1  to monitor both positive and negative y-axis shock levels, and provides both a latch signal and a programmable trigger signal depending on the level of external shock. For example, the shock level may not be sufficient to cause the latches to engage (thus providing a “latch signal”), but may be sufficient for the contacts  2  and  7  to meet (this contact situation is discussed as a “trigger signal” for the purposes of this specification). The proof mass  1  responds to shock levels by displacing itself in the +y or −y direction. The latch  3  on the proof mass  1  is separated from the pawl  4  by a predetermined distance selected for the shock level at which the shock sensor is desired to latch. If that shock level is achieved, the proof mass  1  and latch  3  will move the distance required to engage the latch  3  with the pawl  4 . A very flexible beam  50  allows the pawl  4  to move easily in a direction perpendicular to the motion of the proof mass  1 , and to engage with the latch  3  which prevents the proof mass  1  from returning to its initial state. At this point, the shock sensor is in its latched state and a closed contact exists between the proof mass  1  and pawl  4 . This closed contact can function to signal a microcontroller or to allow interrogation by an external reader.  FIG. 4  illustrates the shock sensor in a latched state. 
   In addition, as is shown in  FIG. 3 , when the proof mass  1  is deflected by a shock, the proof mass contact  2  on the sidewall of the latch  3  may connect with contact  7 . The surface of the contact sidewalls ( 2  and  7 ) are designed to provide a reliable and low-resistance contact. The contact actuator  51  connected to the contact  7  allows the distance between the contacts  2  and  7  to be varied. The contact actuator  51  achieves this by deflecting (upon the application of a current through external circuitry, not illustrated) in a direction generally perpendicular to the direction of the movement of the proof mass  1  (in the + or −x direction). This ability of the sensor to vary the distance between the contacts  2  and  7  modifies the shock level required to make contact and thus provides user programmability. When the contacts  7  connect to the latch contacts  2 , a circuit can be closed that can provide a signal to a microcontroller or be interrogated by an external reader. The shock level for making a contact between the proof mass contact  2  and the contact  7  may or may not be the same as that for latching depending on the setting of the contact actuator  51 . In other embodiments of the invention, the latching shock can be adjusted as well by varying the distance between the latch  3  and the pawl  4  via adjustment of the reset actuator  53  in a manner similar to that of the contact actuator  51  discussed above. Therefore, in different embodiments and applications there could be instances in which the contacts  2  and  7  make contact before latching occurs (via latch  3  and pawl  4 ). Conversely, there could also be instances in which latching occurs (via latch  3  and pawl  4 ) before the contacts  2  and  7  make contact. 
   The shock sensor is designed to be reset after the sensor (in its latched state) is read or used to provide a signal to an external system. As shown in  FIG. 3 , the invention includes a mechanical linkage  52  on the pawl  4  that creates a unique mechanical connection to a reset actuator  53 . When the shock sensor is unlatched and ready to sense a shock event, the reset actuator  53  is not in contact with the mechanical linkage  52  or the pawl  4 . When a shock event occurs, the latch  3  on the proof mass  1  makes contact with the pawl  4  and forces the pawl  4  to move in a direction generally perpendicular to the motion of the proof mass  1 . If the shock is of sufficient magnitude, the latch  3  will push the pawl  4  until latching occurs. The mechanical linkage  52  is designed so that the reset actuator  53  does not apply a mechanical-load to the paws  4  during latching. Without such a mechanical linkage  52 , the reset actuator  53  would apply a stiff resistance to the latching motion, making the sensing of low shock levels difficult. Although  FIG. 3  illustrates one embodiment of such a mechanical linkage  52 , other mechanical designs within the scope of the present invention would also achieve the goal of permitting latching to occur without resistance from the reset actuator  53 . 
   While the sensor is in a latched state, as is shown in  FIG. 4 , the reset actuator  53  can be forced to deflect such that the reset actuator  53  engages with the mechanical linkage  52  and pulls the pawl  4  away from the latch  3 . The illustrated embodiment of the invention uses for the reset actuator  53  a thermal actuator that deflects when a specific amount of current is run through the device. Once the actuator  53  is engaged with the pawl  4 , the force from the reset actuator  53  will pull the pawl  4  away from the proof mass  1 . When sufficient force is applied, the latch  3  and pawl  4  disengage, thereby releasing the proof mass  1  and allowing it to return to its initial position. At this point, the sensor is ready to monitor another shock event. 
     FIG. 5  defines the primary parameters used to design one embodiment of the sensor to detect specific levels of shock. The mass of the proof mass defines the inertial force, and is given by the following expression:
   m=ρ*wm*lm*t,   
where m is the mass, ρ is the density of the material, wm is the proof mass width, lm is the proof mass length, and t is the thickness of the proof mass.
 
   The inertial force developed on the mass under acceleration is then given by:
 
 F=m*a, 
 
where F is the inertial force, m is the mass of the proof mass, and a is the applied acceleration.
 
   The stiffness of the suspension provides a force against the inertial force. The stiffness is given by: 
             k   =       2   *   kb     =       2   *   E   *   t   *     wb   3         l   ⁢           ⁢     b   3             ,         
where k is the entire suspension stiffness, kb is the stiffness of one beam in the suspension, E is the Young&#39;s modulus of the material the device is made in, wb is the width of a beam in the suspension, lb is the length of a beam in the suspension, and t is the thickness of the material.
 
   The distance the proof mass will move under the applied acceleration, neglecting the effects of the latch friction, is given by: 
           h   =     F   k           
A device will latch if the proof mass deflection is greater than the distance of the latch gap plus the distance across the tip of the pawl, and can be expressed by the following latching condition:
 
   
     
       
         
           a 
           &gt; 
           
             
               k 
               * 
               
                 ( 
                 
                   
                     1 
                     ⁢ 
                     g 
                   
                   + 
                   lp 
                 
                 ) 
               
             
             m 
           
         
       
     
   
   Table 1 below contains the shock levels required for latching the sensor given a set of design parameters and a material thickness of 100 μm and a latching gap of 7 μm. 
   
     
       
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Table of design parameters versus shock trigger levels 
             
           
        
         
             
               Shock 
               Flexure 
               Flexure 
               Proof Mass 
               Proof Mass 
             
             
               Level 
               Width, wb 
               Length, lb 
               Width, wm 
               Length, lm 
             
             
                 
             
             
                20 g 
               3.5 μm 
               800 μm 
               1500 μm  
               1100 μm  
             
             
               250 g 
                   5 μm 
               800 μm 
               515 μm 
               515 μm 
             
             
               500 g 
               6.5 μm 
               800 μm 
               545 μm 
               545 μm 
             
             
               1000 g  
               8.5 μm 
               800 μm 
               581 μm 
               581 μm 
             
             
                 
             
           
        
       
     
   
   In one embodiment of the invention, the shock sensor is used to wake up a microcontroller in an embedded sensing application. In other embodiments, the device is used in standalone applications where the sensor is connected to an RFID tag or other transmitter for remote determination of the shock environment experienced by shipping containers and products. Similar devices for other environmental variables such as temperature, humidity, and chemical concentrations can be developed using the principles disclosed herein. 
     FIG. 6  illustrates a wiring schematic for an embodiment of the invention that is used for waking up an embedded microcontroller from a sleep mode when a certain shock level is experienced. In this embodiment, a voltage difference is applied across actuators  53  and  51 . In operation a single bias signal is applied to the proof mass  1  of the device. The bias signal could be a voltage or current depending upon the type of readout circuit used. Connections to the external contacts and pawls would be outputs to which the bias signal is connected. These outputs could be connected to microcontroller interrupt lines, to a wireless transceiver, to a large circuit network that performs some function, or a number of other connection and circuits. 
   Although several embodiments and forms of this invention have been illustrated, it is apparent that those skilled in the art can make other various modifications and embodiments of the invention without departing from the scope and spirit of the present invention. For example, other configurations of the sensor are possible that utilize varying surface features on the contacts, multiple movable contacts, and different actuator types. 
   One particular embodiment of the invention, shown in  FIG. 7 , uses the sidewall  40  of a second proof mass  41  as a moving contact to connect with the contact  42  on the sensor&#39;s main latching proof mass  1 . When a shock load is applied, the moving contact  41  will move out of the way of the main proof mass  1  during the latching operation, thereby reducing the amount of force required to meet the latching condition. After the latching occurs and the shock load is removed from the device, the moving contact  41  will return to its original position and make a connection with the contact  42  on the device&#39;s main latching proof mass  1 . This configuration is useful when designing low shock trigger devices where the inertial force may be insufficient to overcome the retarding force created by the stationary electrical contacts. 
   Another embodiment, shown in  FIG. 8 , includes multiple contacts  70  and multiple latches  71  to allow one sensor device to trigger at and latch at multiple shock levels that the proof mass  1  is subjected to. 
   Another embodiment of the device (not illustrated) uses a capacitive actuator for reset functions instead of a thermal actuator. A capacitive actuator consumes less power but would be suitable only for lower force and lower shock level applications. The configuration would require additional capacitive actuators on the proof mass to move it out of contact with the pawl, thereby eliminating the friction that holds the pawl in contact with the latch. Only then could another capacitive actuator move the pawl out of position, after which the actuator on the proof mass is released, followed by the release of the pawl, at which point the sensor is unlatched and ready for another sensing operation. 
   Furthermore, other fabrication processes for the device are possible. Any fabrication process that realizes a single thick micromechanical structural layer with 1) conducting sidewalls that can make electrical contact, and 2) large amounts of suspended inertial mass. Examples include bulk micromachining and wafer-bonding fabrication approaches in silicon, silicon dioxide, nickel, titanium and other conductors, as well as LIGA-type fabrication processes using electroplated metals. 
   This invention may be provided in other specific forms and embodiments without departing from the essential characteristics as described herein. The embodiment described is to be considered in all aspects as illustrative only and not restrictive in any manner. The following claims rather than the foregoing description indicate the scope of the invention. 
   As described above and shown in the associated drawings, the present invention comprises a micro-electromechanical shock sensor. While particular embodiments of the invention have been described, it will be understood, however, that the invention is not limited thereto, since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is, therefore, contemplated by the appended claims to cover any such modifications that incorporate those features or those improvements that embody the spirit and scope of the present invention.