Abstract:
A hybrid impact sensor and method of operating the same. One sensor includes a support containing one or more mounts; a first sensor with first sensing properties and configured to generate a first output signal; a second sensor with second sensing properties and configured to generate a second output signal, wherein the second sensing properties are different from the first sensing properties; and a housing encasing the first sensor and the second sensor.

Description:
RELATED APPLICATIONS 
   The present patent application is a continuation-in-part of co-pending U.S. patent application Ser. No. 10/866,386, filed on Jun. 11, 2004, the entire contents of which are hereby incorporated by reference. 

   FIELD OF INTEREST 
   Embodiments of the invention relate to impact sensors including at least two types of sensors. 
   BACKGROUND OF THE INVENTION 
   Vehicles are often equipped with impact sensors so that air bags and other safety restraints can be triggered, for example, during an accident. Most sensors, however, can only sense impacts within a close proximity of the sensor. Safety sensor systems often include numerous accelerometers and/or door cavity pressure sensors separately or in combination. Numerous sensors are often employed since a sensor must be directly hit during an accident in order to detect impact. The sensors are often placed where impacts are common. Even though multiple sensors are used in detection systems, certain types of impacts are still difficult for the systems to identify. For example, impacts with narrow objects such as poles often pose a challenge for detection systems unless the pole directly hits a sensor. Thus, unless sensors completely cover all sides of a vehicle, the chance that an accident will be missed by the detection system still exists. 
   SUMMARY OF THE INVENTION 
   There is a need to provide sensors that can correctly detect an impact without having to be physically close to the point of contact. 
   In one embodiment, the invention provides a sensor that can measure the strain waves or stress waves traveling through a vehicle structure caused by deformation of the structure due to impact in a crash. The sensor is mounted onto a suitable vehicle structure, for example, the B-pillar of the vehicle or a reinforcing beam inside the door of the vehicle. The range of the sensor is adequate to allow only a single sensor to be placed along each side of a vehicle. The sensor includes a sensitive support that distorts when stress waves travel through it. A semiconductor element is mounted on the support such that it is distorted with the support. The semiconductor element, e.g., a silicon beam, may contain piezoresistors arranged in a Wheatstone-bridge configuration. The impedance of the piezoresistors changes as the physical characteristics of the attached support change. The sensor also contains a circuit capable of sensing the impedance of the piezoresistors. The change of the sensed impedance can be used to detect stress waves. By detecting the stress waves caused by impact and not the direct impact itself, the sensor can detect impacts that occur remotely from the location of the sensor. 
   In other embodiments, the stress wave sensor can be used to observe stress waves in other structures besides a vehicle structure. The sensor could be used to monitor stress applied to building or bridges or other compositions where unchecked stress strain can cause safety concerns. Any substance supporting the propagation of stress or force waves could be attached to the disclosed stress wave sensor. The material of the support contained within the sensor as well as the piezoresistant material used in the piezoresistors and semiconductor element can also be varied to create a specific sensor for specific types of stress waves. 
   In another embodiment, the invention provides a hybrid impact sensor. The sensor may include a support containing one or more mounts. The sensor may also include a first sensor with first sensing properties, which is configured to generate a first output signal and a second sensor with second sensing properties, which is configured to generate a second output signal. The second sensing properties are different from the first sensing properties. A housing encases the first sensor and the second sensor. 
   Another embodiment provides a method of sensing impact to a structure. The method may include providing a support with one or more mounts; providing a first sensor of a first sensing type; providing a second sensor of a second sensing type, wherein the second sensing type is different from the first sensing type; encasing the first sensor and the second sensor in a housing; generating a first output signal; and generating a second output signal. 
   Additional embodiments provide a hybrid impact sensor. The sensor may include a support containing one or more mounts and a semiconductor element mounted to the support between the mounts. The semiconductor element contains a plurality of piezoresistors. Each piezoresistor has an impedance and input and output terminals. The sensor may also include a circuit configured to be coupled to the input and output terminals of the plurality of piezoresistors. The circuit is capable of sensing the impedance of the plurality of piezoresistors. The sensor also includes a pressure sensor configured to generate a pressure signal. 
   Yet another embodiment provides a method for sensing impact to a structure. The method may include providing a support with one or more mounts; attaching a semiconductor element containing a plurality of piezoresistors, each having impedance, to the support; connecting the support to the structure with the mounts of the support; providing a pressure sensor configured to generate a pressure signal; encasing the support and the pressure sensor in a housing; sensing the impedance of the plurality of piezoresistors; and sensing the pressure signal. 
   Additional embodiments further provide a hybrid impact sensor. The sensor may include a support containing one or more mounts; a semiconductor element mounted to the support between the mounts and containing a plurality of piezoresistors each piezoresistor having an impedance and input and output terminals; a circuit configured to be coupled to the input and output terminals of the plurality of piezoresistors and capable of sensing the impedance of the plurality of piezoresistors; and an acceleration sensor configured to generate an acceleration signal. 
   Another embodiment provides a method for sensing impact to a structure. The method may include providing a support with one or more mounts; attaching a semiconductor element to a support, the element containing a plurality of piezoresistors, each having an impedance; connecting the support to the structure with the mounts of the support; providing an acceleration sensor configured to generate an acceleration signal; encasing the support and the pressure sensor in a housing; sensing the impedance of the plurality of piezoresistors; and sensing the acceleration signal. 
   Yet another embodiment provides a hybrid impact sensor. The impact sensor may include a support containing one or more mounts and a magnetostrictive sensor. The magnetostrictive sensor is configured to provide a stress wave signal. A pressure sensor configured to provide a pressure signal is also included in the impact sensor. A housing encases the magnetostrictive sensor and the pressure sensor. Instead of a pressure sensor, an acceleration sensor may also be used. 
   Additional embodiments provide a method for sensing impact to a structure. The method may include providing a support with one or more mounts; providing a magnetostrictive sensor configured to generate a stress wave signal; providing a pressure sensor configured to generate a pressure signal; encasing the magnetostrictive sensor and the pressure sensor in a housing; sensing the stress wave signal; and sensing the pressure signal. If an acceleration sensor is used in place of the pressure sensor, the method includes sensing an acceleration signal. 
   Other features and advantages of the invention will become apparent to those skilled in the art upon review of the detailed description, claims, and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a top view of a first exemplary embodiment of the invention. 
       FIG. 2  is a front view of the exemplary embodiment illustrated in  FIG. 1 . 
       FIG. 3  is a top view of a second exemplary embodiment of the invention. 
       FIG. 4  is a front view of the second exemplary illustrated in  FIG. 3 . 
       FIG. 5  is a schematic illustration of a semiconductor element suitable for use in the embodiments illustrated in  FIGS. 1–3 . 
       FIG. 6  is a top-view illustration of the sensor of  FIG. 1  mounted to a vehicle structure. 
       FIG. 7  is a rear-view illustration of the sensor of  FIG. 1  mounted to a vehicle structure. 
       FIGS. 8 and 9  illustrate exemplary directions of bending of the support of the sensor of  FIG. 1  due to stress waves traveling through it. 
       FIG. 10  illustrates the support and semiconductor element of the sensor of  FIG. 1  bending due to stress waves traveling through it. 
       FIG. 11  is a top view of an exemplary embodiment of the invention. 
       FIG. 12  is a top view of another exemplary embodiment of the invention. 
       FIG. 13  is a front view of another exemplary embodiment of the invention. 
       FIG. 14  is a top view of the exemplary embodiment illustrated in  FIG. 13 . 
       FIG. 15  is a front view of yet another exemplary embodiment of the invention. 
       FIG. 16  is a top view of the exemplary embodiment illustrated in  FIG. 15 . 
   

   It is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary sensor  10 . The sensor  10  includes a housing  12  that encases the components of the sensor  10 . The housing  12  helps protect the sensor  10  from dust and debris and other environmental hazards that may interfere with the functioning of the sensor  10 . The sensor  10  also includes two mounts  14 ,  16  protruding out of the housing  12 . The mounts  14 ,  16  provide a mechanism to allow the sensor  10  to be mounted onto a component or structure requiring stress monitoring, such as the frame of a vehicle. The mounts  14 ,  16  are used as an interface to the component or structure so that any stress waves traveling through the component are transmitted to the sensor  10 . The mounts  14 ,  16  of the sensor  10  could be attached to a metal frame of a vehicle or a supporting beam of a building. Alternatively, the mounts  14 ,  16  could be studs capable of attaching to a component with screws or bolts. The sensor  10  also includes a connector  18  that may be used to transmit sensor measurements to other control units that may activate devices or mechanisms based upon the data collected by the sensor  10 . 
   Located inside the housing  12  is a support  20 . The support  20  contains the two mounts  14 ,  16  and, in the embodiment shown, is constructed with each mount on one end of the support, causing the support  20  to behave like a tuning fork. The two mounts  14 ,  16  act as tines of a tuning fork that are susceptible to stress waves, or vibrations. Stress waves or vibrations traveling through the beam or structure to which the sensor  10  is attached are transferred to the support  20  through the mounts  14  and  16 . 
   The stress waves or vibrations cause the support  20  to vibrate and distort. The support  20  is made from a flexible material or substance that is sensitive to stress waves. Aluminum, for example, may be used since it is light and flexible. The support  20  could also be constructed from steel or even high strength plastic. The thickness and composition of the support  20  determine the degree to which the support  20  distorts and, ultimately, the sensitivity of the sensor  10 . The support may also contain more or less mounts placed in various configurations, other than at ends of the support in order to facilitate the distorting of the support  20 . For example, a circular support could be provided with three, four, or more mounts that may be attached to more than one beam or structure. Each mount will transmit stress waves from the beam or structure, to which it is attached, to the circular support. 
   The support  20  also serves as a foundation for a semiconductor element  22 . The semiconductor element  22  is attached to the support  20  such that the support  20  transfers any distortions caused by stress waves traveling through the support  20  to the semiconductor element  22 . Just as the support  20  is flexible in order to distort due to the propagation of stress waves, the semiconductor element  22  has similar flexibility. The semiconductor element  22  is attached along a surface of the support  20 . In one embodiment, the semiconductor element  22  is attached flat to the surface of the support  20  so that the semiconductor element  22  will distort as the support  20  does. 
   The semiconductor element  22  includes piezoresistors  24 ,  26 ,  28 , and  30 . The piezoresistors  24 ,  26 ,  28 , and  30  are arranged in a Wheatstone-bridge configuration. The piezoresistors  24 ,  26 ,  28 , and  30  are constructed with a material whose resistivity is influenced by the mechanical stress applied to the material such as piezoresistant material. Examples of piezoresistant materials include, but are not limited to, silicon, polycrystalline silicon, silica glass, zinc oxide, and germanium. In one embodiment, the piezoresistors  24 ,  26 ,  28 , and  30  are divided into two categories. The piezoresistors  24  and  28  are used as sensing piezoresistors and are arranged horizontally along the major or longitudinal axis of the semiconductor element  22 . The piezoresistors  26  and  30  are used as reference piezoresistors, are smaller, and are arranged vertically or along the width of the semiconductor element  20 . The reference piezoresistors  26  and  30  have less impedance than the sensing piezoresistors  24  and  28 . The physical arrangement and characteristics of the two categories of piezoresistors make the sensing piezoresistors  24  and  28  more sensitive than the reference piezoresistors  26  and  30  to distortions of the semiconductor element  22  since they cover an area of the semiconductor element  22  that is more likely to distort in response to a stress wave passing through the support  20 . Likewise, the reference piezoresistors  26  and  30  are less sensitive to the distortions of the semiconductor element  22  since they cover less area of the semiconductor element  22  and are arranged closer to ends of the support  20  where the support  20  distorts less. When the support  20  and the attached semiconductor element  22  are distorted by stress waves, the impedance of the sensing piezoresistors  24  and  28  will change more than the impedance of the reference piezoresistors  26  and  30 . The difference between the changes of impedance of the two categories of piezoresistors can also be used to further estimate the characteristics of the impact or stress on the component that the sensor  10  is attached to. 
   The semiconductor element  22  also contains input and output terminals  32 ,  34 ,  36 , and  38 . The input and output terminals  32 ,  34 ,  36 , and  38  are used to apply and measure voltage and/or current passing through the piezoresistors  24 ,  26 ,  28 , and  30 . The applied voltage and measured current can be used to calculate resistance by Ohm&#39;s law:
 
V=I R
 
where V represents the voltage applied to the circuit, I represents the current measured from the circuit, and R represents the resistance of the circuit.
 
   The support  20  may also be constructed from a semiconductor material and may directly contain the piezoresistors  24 ,  26 ,  28 ,  30  rather than a separate semiconductor element  22  attached to the support  20 . Any distortion of the semiconductor support created by stress waves traveling through the attached structure also causes the material of the embedded piezoresistors to distort. The semiconductor support may also contain input/output terminals used to apply and transmit voltage and/or charge flowing through the semiconductor support. 
   Applying voltage, measuring current, and calculating resistance can all be performed by a processor such as an application specific integrated circuit (“ASIC”)  40  attached to the semiconductor element  22 . The ASIC  40  is shown as being attached to a printed circuit board (“PCB”)  42  through the input and output terminals  44 ,  46 ,  48 , and  50 . Other connections and even other calculating mechanisms may be used. For example, a chip or microprocessor could also replace the ASIC  40 . The ASIC  40  could also be eliminated from the sensor  10  and the output and input terminals  32 ,  34 ,  36 , and  38  of the semiconductor element  22  could be directly coupled to the connector  18 . By directly coupling the semiconductor element  22  to the connector  18  the processing of the measurements taken by the sensor  10  (i.e., the calculating of resistance) can be carried out outside of the sensor  10  at a remote control unit. The connector  18  may provide amplification or filtering to improve the characteristics of any data sent from the sensor  10  or received by the sensor  10 , for example current or voltage values. However, the connector  18  does not process the data in order to deduce the meaning of the data such as to what degree the support  20  is stressed and distorted. The ASIC  40  may also act as a relay or amplifier for a sensed current measurement based on a constant application of voltage. The ASIC  40  could also process the sensed current of the piezoresistor arrangement and calculate a change in resistance, which could be used to further calculate a degree of stress applied to the support. 
     FIG. 2  illustrates the sensor  10  of  FIG. 1  from a front view. The connector  18 , shown with solid lines, is protruding toward the viewer. Two ends of the two mounts  14  and  16  are also protruding toward the viewer. The PCB  42  and attached ASIC  40  and the semiconductor element  22  are also displayed in phantom lines situated beneath the connector  18 . The input and output terminals  44  and  46  (input and output terminals  48  and  50  are hidden behind the ASIC  40 ) of the PCB  42  and the input and output terminals  32 ,  34 ,  36 , and  38  of the semiconductor element  22  are also shown in phantom lines along with the support  20  and the two mounts  14  and  16 . 
     FIG. 3  illustrates a second exemplary sensor  52  from a top view. The sensor  52  contains all the same components as the sensor  10 , but the semiconductor element  22  is not located on the top surface of the support  20 . As can be seen in  FIG. 3 , the semiconductor element  22  is attached along the front edge of the support  20 . The surface of the semiconductor element  22  containing the piezoresistors is positioned at a right angle to the ASIC  40  and PCB  42  rather than positioned parallel to the ASIC  40  and PCB  42  as in the sensor  10 . Similarly, the semiconductor element  22  may be placed on the back surface or edge of the support  20 . The location of the semiconductor element  22  can be varied to adjust the functionality of the sensor. The position of the semiconductor element  22  can also be varied to change the size and dimensions of the sensor. For example, placing the semiconductor element  22  on the front edge of the support  20  reduces the thickness of the sensor. The semiconductor element  22  may also be placed in a location where it can be easily replaced or tested, if needed. 
     FIG. 4  illustrates the sensor  52  from a front view. Since the semiconductor element  22  is positioned along the front edge of the support  20  the piezoresistors  24 ,  26 ,  28 , and  30  contained within the semiconductor element  22  are seen when the sensor  52  is viewed from the front. When viewed from the front, the ASIC  40  and PCB  42  hinder the full view of the semiconductor element  22  since the semiconductor element  22  is positioned in a plane perpendicular to the plan containing the ASIC  40  and PCB  42 . The connector  18  is shown in phantom lines and is protruding toward the viewer. 
     FIG. 5  illustrates the semiconductor element  22  displayed in  FIGS. 1–4 . The semiconductor element  22  contains the four piezoresistors  24 ,  26 ,  28 , and  30  as well as the input and output terminals  32 ,  34 ,  36 , and  38 . As mentioned above, the sensing piezoresistors  24  and  28  are arranged length-wise in the middle of the semiconductor element  22 . Their position makes them more sensitive to distortions of the semiconductor element  22  than the reference piezoresistors  26  and  30  since they cover an area of the semiconductor element  22  that is more likely to distort in response to stress waves. The reference piezoresistors  26  and  30  are less sensitive to the distortions of the semiconductor element  22  since they cover less area of the semiconductor element  22  and are arranged closer to ends of the support  20  where the support  20  distorts less. The reference piezoresistors  26  and  30  may have higher impedance than the sensing piezoresistors  24  and  28 . Other constructions are also possible. All four resistors may have identical impedance or their impedance may be varied to better utilize and categorize a reading from the sensor. Each terminal  32 ,  34 ,  36 , and  38  of the semiconductor element  22  may have a designated data flow such as input only or output only or both may be bi-directional. The input and output terminals  32 ,  34 ,  36 , and  38  may be configured to be coupled to a variety of devices including a PCB, a microprocessor, or a connector. 
     FIG. 6  illustrates the sensor  10  shown in  FIGS. 1 and 2  mounted in a vehicle  60 . The sensor  10  and the components of the vehicle  60  are not drawn to scale. For the sake of clarity, the sensor  10  is illustrated without the housing  12 , the connected ASIC  40  and PCB  42 , and the connector  18 . The vehicle  60  contains a side sill  62  and a B-pillar  63  on each side. The side sills  62  are positioned parallel to a surface that the vehicle  60  travels on and supports the side doors and windows. The B-pillars  63  are attached to the side sills  62  and protrude upward toward the roof of the vehicle  60 . The B-pillars  63  may connect along the roof of the vehicle or may simply extend and connect to the roof. The sensor  10  is shown mounted on a B-pillar  63 . A single sensor  10  is shown mounted to the side of the vehicle  60  located next to a driver seat  64  for illustration purposes only. In practical use, each side of the vehicle  60  may include a sensor  10 . The sensor  10  may also be mounted to other structures of the vehicle  60  capable of transmitting stress waves such as the side sills  62 , roof, or other supporting frames. The mounts  14  and  16  are connected to the B-pillar  63  with screws  70 ,  72 . As indicated earlier, the screws  70 ,  72  could be replaced with bolts, rivets, or any other fastener. The mounts  14 ,  16  could also be soldered or welded to the B-pillar  63 . Other constructions are also possible depending on the composition and position of the mounts  14  and  16  and the structure to which the mounts  14 ,  16  are attached. 
   Once the sensor  10  has been attached to the B-pillar  63 , any stress waves traveling through the B-pillar  63  are transmitted to the sensor  10 . Stress waves travel from the B-pillar  63  and through the mounts  14  and  16  to the support  20 . The support  20  distorts according to the amplitude, frequency, or other characteristic of the stress waves, which also causes the semiconductor element  22  attached to the support  20  to distort. The distortion of the semiconductor element  22  causes the resistance of the piezoresistors  24 ,  26 ,  28 ,  30  to change. The change in the resistance of the piezoresistors  24 ,  26 ,  28 ,  30  can be processed by the ASIC or other processing device to monitor stress present in the B-pillar  63  of the vehicle  60 . Changes in the resistance of the piezoresistors can indicate a collision or accident that may require the activation of safety restraint devices such as seatbelts or airbags. 
     FIG. 7  illustrates the sensor  10  mounted to a B-pillar  63  of a vehicle  60  from a rear view. The side sill  62  is shown supporting the B-pillar  63  that is positioned parallel and adjacent to the driver seat  64 . The sensor  10  is illustrated mounted to the B-pillar  63  with the screw  70 . Another screw may be used to mount the other end of the sensor  10  to the B-pillar  63  although it is not shown. 
     FIGS. 8–9  illustrates the support  20  of the sensor  10  distorted due to stress waves. The dashed lined illustrates the support  20  distorted from its original position, which is shown in solid lines. For purpose of illustration the support  20  is shown without the housing  12 , the semiconductor element  22 , the ASIC  40  and PCB  42 , and connector  18 . The stress waves cause the support  20  to distort into a U-shaped beam either upward toward the top of the sensor  10  or downward toward the bottom of the sensor  10 . 
   Referring to  FIG. 10 , as the support  20  distorts so does the attached semiconductor element  22 . The semiconductor element  22  contains the piezoresistors  24 ,  26 ,  28 , and  30  that also distort with the semiconductor element  22 . As shown in  FIG. 10 , the sensing piezoresistors  24 ,  28  are distorted more than the reference piezoresistors  26 ,  30  due to there position and size. Since the support  20  bends length-wise into a U-shape, the sensing piezoresistors  24 ,  28  are distorted while the reference piezoresistors  26 ,  30  are not. As sensing piezoresistors  24  and  28  distort, their associated impedance changes due to the physical change of the material of the sensing piezoresistors  24  and  28 . The ASIC  40  (not shown) can monitor the change of impedance of the sensing piezoresistors  24  and  28  so that the safety mechanisms may be activated when appropriate. 
   In the case of an accident at any point along a side of the vehicle  60 , the impact of the accident causes stress waves to propagate through the vehicle structure  50  and to the attached sensor. If the structure of the vehicle  60  is integral or unitary, a single sensor may be used to sense impact anywhere along the vehicle. It may be desirable, however, to place a sensor along each side of the vehicle  60  to reduce the travel distance and, therefore, also reduce the travel time of the stress waves. Such a configuration also increases the reaction time of the system. Using a sensor on each side of a vehicle also increases the sensitivity and accuracy of each sensor since the stress waves travel a shorter distance. This decreases the amount of time and the amount of material that the stress wave travels through. Certain characteristics of the waves may dissipate over time or as the waves travel through various media. 
   The support  20  returns to its original shape after the stress waves have passed through it. In severe accidents or collision the support  20  may be distorted to a point where it retains its distorted shape. In this case, the accident would likely cause damage to the vehicle that requires repair before the vehicle can be used again, and the sensor may also need to be repaired in this situation. 
   In some embodiments, the sensor is paired with another sensor with different sensing principals to provide a hybrid impact sensor. Combining two different sensors that sense different variables in a single hybrid sensor helps increase the range, accuracy, and efficiency in detecting vehicle impact. 
     FIG. 11  illustrates an exemplary hybrid sensor  75 . The hybrid sensor  75  includes the sensor  10  and a pressure sensor  80 . In some embodiments, the pressure sensor  80  is attached to the PCB  42  and includes a door cavity pressure sensor that is configured to measure door cavity pressure changes that may occur during impact. The pressure sensor  80  may include one or more micromachined silicon door cavity pressure sensors, which are manufactured, among others, by Analog Devices and Motorola. Door cavity pressure sensors are well known in the art and, therefore, not described in detail. The pressure sensor  80  may also include other components such as an ASIC, input/output terminals, and the like. 
   As shown in  FIG. 11 , the sensor  10  and the pressure sensor  80  are contained within the housing  12 . In order for the pressure sensor  80  to measure pressure changes, the housing  12  may include an opening  82 . The opening  82  may also be constructed to limit or prevent pressure changes due to changes in humidity or other air characteristics unrelated to changes due to impact. 
     FIG. 12  illustrates another exemplary hybrid sensor  85 . The hybrid sensor  85  includes the sensor  10  and an acceleration sensor  90 . In some embodiments, the acceleration sensor  90  is attached to the PCB  42  and includes an accelerometer configured to measure acceleration or deceleration of a vehicle or other object. The acceleration sensor  90  may include one or more micromachined silicon acceleration sensors like those manufactured by Analog Devices and Motorola among others. Acceleration sensors are well known in the art and, therefore, not described in detail. The acceleration sensor  90  may also include other components such as an ASIC, input/output terminals, and the like. As shown in  FIG. 12 , the sensor  10  and the acceleration sensor  90  are also contained within the housing  12 . 
   The hybrid sensors  75  and  85  may provide early impact detection, and, as described for the sensors  10  and  52 , a single hybrid sensor  75  or  85  may be capable of detecting impact along an entire side of a vehicle. The dual sensing properties of the hybrid sensors  75  and  85  may also provide more accurate impact sensing, since one sensor can double-check the operation of the other sensor. For example, if the pressure sensor  80  or acceleration sensor  90  detects characteristics such as pressure changes or high rates of deceleration that may indicate impact while the sensor  10  does not detect corresponding stress waves indicating impact, safety equipment such as seat belt tensioning or air bags may not be activated. The multiple sensing properties may provide safing functionality to activate safety equipment when it is necessary and/or safe to do so. 
   It should be understood that the hybrid sensors  75  and  85  may include multiple sensors  10  and multiple pressure sensors  80  and/or acceleration sensors  90 . The hybrid sensors  75  and  85  may also include other types of sensors in place of or in addition to the pressure sensor  80  and/or acceleration sensor  90 . For example, the hybrid sensors  75  and  85  may include speed sensors, brakes sensors, steering wheel sensors, transmission sensors, and the like, to detect impact to a vehicle. The sensor  10  may also be replaced with the sensor  52 , as described above, as well as other configurations. The pressure sensor  80  and acceleration sensor  90  may also be placed at different locations on the sensor  10 . For example, the pressure sensor  80  or acceleration sensor  90  may be attached to a separate PCB (not shown). The opening  82  in the housing  12  for the pressure sensor  80  may also be located in various locations and have various configurations. 
   The sensor  10  may also be replaced with other stress wave sensors to create additional hybrid sensors.  FIGS. 13 and 14  illustrate a hybrid sensor  95  from a front and top view respectively. The hybrid sensor  95  includes the pressure sensor  80 , as described above, and a magnetostrictive sensor  100 . Magnetostrictive sensors useful in embodiments of the invention include sensors manufactured by Southwest Research Institute and are described in U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,767,766. Magnetostrictive sensors are configured to detect stress waves traveling through a structure by detecting variations in magnetization. Magnetostrictive sensors can actively or passively detect stress or strain to a structure. Active magnetostrictive sensors may introduce a stress wave with certain characteristics to a structure and may detect any modifications and/or reflections of the stress wave due to fractures or cracks in the structure. Modifications of the introduced stress waves may cause changes in magnetic flux detectable by a receiving coil of the sensor. Passive magnetostrictive sensors may monitor for stress waves introduced by structure cracking, breaking, impacts, or vibrations that also cause changes in magnetic flux detectable by a receiving coil of the sensor. As described for the sensors  10  and  52  above, magnetostrictive sensors  100  may be able to detect impacts fast and efficiently even if they are not located directly at the point of impact. The ability to sense impact over an extended area allows fewer magnetostrictive sensors  100  to be used to impact monitoring systems. When used in vehicles, a single magnetostrictive  100  sensor may be employed on each side of the vehicle to detect impacts along the entire side of the vehicle. When paired with an independent sensor, such as the pressure sensor  80 , the operation of the magnetostrictive sensor  100  can be verified or double-checked by the operation of the independent sensor. The independent sensor provides safing functionality for the magnetostrictive sensor so that safety equipment, which can often only be deployed once and may cause injury or damage if deployed unnecessarily, is only activated when required. 
   The magnetostrictive sensor  100  and pressure sensor  80  may be encased within a housing  108 . The housing  108  may also include an opening  109  for the pressure sensor  80 . In some embodiments, the housing  108  is constructed from an insulating material such as plastic. The housing  108  may also be constructed from aluminum. 
   The positions of the magnetostrictive sensor  100  and the pressure sensor  80  and/or acceleration sensor  90  may be fixed by a potting material (not shown) that fills the interior of the housing  108 . The magnetostrictive sensor  100  and pressure sensor  80  may also be attached to a support  102 . The support  102  may be constructed of a ferromagnetic material that has a magnetostrictive property that causes physical and/or dimensional changes associated with variations in magnetism. Stress waves traveling through the support  102  may cause changes in magnetic flux detectable by the magnetostrictive sensor  100 . 
   The housing  108  may includes two mounts  104  and  106 . The mounts  104  and  106  protrude may provide a mechanism for the hybrid sensor  95  to be attached onto a component or structure requiring stress monitoring such as the frame of a vehicle. The mounts  104  and  106  may also extend and be connected to the support  102  when present. 
   The mounts  104  and  106  are used as an interface to the component or structure so that any stress waves traveling through the component are transmitted to the hybrid sensor  95 . The mounts  104  and  106  of the hybrid sensor  95  could be attached to a metal frame of a vehicle or a supporting beam of a building. In some embodiments, the hybrid sensor  95  may be mounted on an inner door panel with the pressure sensor  80  or opening  109  facing the door cavity. The hybrid sensor  95  may also be mounted on a reinforcing beam typically provided in door panels to increase door stiffness against intruding or impacting objects. Alternatively, the mounts  104  and  106  could be studs capable of attaching to a component with screws, bolts, or rivets. 
   The hybrid sensor  95  may also include a connector  110  that may be used to transmit sensor measurements or detections to a control unit that may activate devices or mechanisms based upon the data collected by the hybrid sensor  95 . 
     FIG. 16  shows another exemplary hybrid sensor  115  from a top view. The hybrid sensor  115  includes the magnetostrictive sensor  100  and the acceleration sensor  90 . As described for the hybrid sensor  95 , positions of the magnetostrictive sensor  100  and acceleration sensor  90  may be fixed by a potting material (not shown) or by attaching the magnetostrictive sensor  100  and the acceleration sensor  90  to the support  102 . The magnetostrictive sensor  100  and the acceleration sensor  90  are also encased within the housing  108 , which includes the mounts  104  and  106 . The hybrid sensor  115  may also include the connector  110 . 
   It should be understood that the hybrid sensors  95  and  115  may include multiple magnetostrictive sensors  100  and multiple pressure sensors  80  and/or acceleration sensors  90 . The magnetostrictive sensor  100 , pressure sensor  80 , and acceleration sensor  90  may also be located in various configurations. Although the magnetostrictive sensors  100  are illustrated next to the pressure sensor  80  and acceleration sensor  90  in  FIGS. 13 and 15  respectively, other configurations are possible. In some embodiments, the pressure sensor  80  or acceleration sensor  90  is positioned on top of the magnetostrictive sensor  100 . The sensors may be placed on top of each other to decrease the size of the hybrid sensor  95  or  115 . The sensors may also be located on separate supports or components. The opening  109  in the housing  108  for the pressure sensor  80  may also be located in various locations and have various configurations. 
   It should also be understood that the hybrid sensors  95  and  115  may also include other types of sensors in addition to or in place of the magnetostrictive sensor  100 , the pressure sensor  80 , and/or the acceleration sensor  90 . In some embodiments, an acoustic wave sensor is used in place of the magnetostrictive sensor  100 . Acoustic wave sensors detect the properties of acoustic waves traveling through a component. If the component is modified (i.e., bent or dented during a collision) acoustic waves travel differently through the component. The detected changes of acoustic waves traveling through a component can be used to sense structure modifications and determine if impact has occurred. 
   Various features and advantages of the invention are set forth in the following claims.