Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P15/0802—Details
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P15/12—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by alteration of electrical resistance
  • G01P15/123—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values by alteration of electrical resistance by piezo-resistive elements, e.g. semiconductor strain gauges
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
  • G01P15/00—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration
  • G01P15/02—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses
  • G01P15/08—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values
  • G01P2015/0805—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration
  • G01P2015/0822—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass
  • G01P2015/0825—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass for one single degree of freedom of movement of the mass
  • G01P2015/0828—Measuring acceleration; Measuring deceleration; Measuring shock, i.e. sudden change of acceleration by making use of inertia forces using solid seismic masses with conversion into electric or magnetic values being provided with a particular type of spring-mass-system for defining the displacement of a seismic mass due to an external acceleration for defining out-of-plane movement of the mass for one single degree of freedom of movement of the mass the mass being of the paddle type being suspended at one of its longitudinal ends

Definitions

• This invention relates generally to piezoresistive accelerometer devices for measuring
• acceleration forces and more particularly, to a new and improved non-servoed pendular piezoresistive accelerometer and method of fabrication, the accelerometer being of the type fabricated from a single piece of silicon substrate and having at least two 2 piezoresistive gages for providing very large electrical resistance changes from very small displacements between the pads associated with the gages.
• the accelerometer is a transducer comprised of a silicon substrate having a groove defined therein undercutting a gage which extends over connection pads at each end.
• the groove defines a hinge between a fixed end and a movable end of the substrate. 3 Q
• the movable end moves about the hinge relative to the fixed end creating a strain in the gage.
• Each silicon sensor must be individually cut out of the substrate before wire bonding.
• the fixed end of the sensor is thereafter bonded to a silicon support and this can be accomplished only at the rate of one sensor die at a time. This requirement negates the lower cost batch fabrication advantage of the silicon-based sensor.
• the seismic mass is defined by both chemical etching and diamond saw-cutting and the delicate gages are not protected from the attendant contamination and vibration damage during the cutting.
• the performance of the accelerometer device depends upon the leverage distance from the gage to the hinge. This leverage distance is generally limited by the thickness of the silicon substrate, thus limiting the optimal performance of the accelerometer.
• the present invention provides a new and improved non-servoed pendular piezoresistive accelerometer construction which substantially reduces the packaging costs of the accelerometer by simplifying wire connection procedures, and which significantly improves the fabrication process for etching the accelerometer from a single piece of silicon substrate in a single etching step which reduces damage to the seismic mass.
• the accelerometer construction of the present invention includes gages that are generally well matched in terms of resistance values and temperature coefficients, employs a base plate and a lid which encloses the sensor for protecting the moving parts, and provides a more flexible leverage distance between the gages and the hinged connection for optimizing accelerometer performance.
• the present invention is directed to a new and improved pendular piezoresistive accelerometer and method of fabrication for reducing the manufacturing costs. This is accomplished by redesigning the accelerometer such that all of the gages reside on just one side of the single silicon substrate which greatly simplifies the wire bonding procedure.
• a base structure is connected to a pendular seismic mass by a hinged connection for pivotal movement of the seismic mass about an axis along a sense direction perpendicular to the hinged connection and a piezoresistive force sensor connecting the seismic mass to the base.
• All of the components are sculptured from a single piece of silicon substrate.
• the sensor comprises at least two piezoresistive gages which are etched-free from the substrate to form "beams" between the seismic mass and the base.
• the gages are placed above and below the hinged connection on the same side of the substrate so that the accelerations along the sensitive axis creates compressive stresses on one gage and tensile stresses on the other.
• the gages are connected in a one-half Wheatstone Bridge circuit for an extremely precise reading of the change in voltage due to acceleration. Each gage requires only a very small strain energy since its volume can be very small.
• a single crystal silicon structure with the appropriate orientation is selected. Thereafter, an etch is selected which is both anisotropic and doping-selective. Additionally, mixtures of potassium hydroxide/water, hydrazine/water, and ethylene diamine/pyrocatechol/water may be selected for use depending upon the desired results.
• the etchants define a groove in the silicon substrate over which the gages extend. In the process, the gages and their terminals are also defined by a thermal diffusion or ion implantation through a silicon dioxide mask to a predetermined boron concentration. The boron doped gages are p-type while the substrate is n-type.
• the heavily doped area is electrically isolated from the substrate by a p-n junction.
• a hinge, a seismic mass and a support rim are also defined in the substrate by protecting them from etching with a silicon dioxide mask.
• the new and improved pendular accelerometer and method of fabrication of the present invention permits reducing the manufacturing packaging costs by placing all of the gages on one side of the substrate simplifying the wire bonding procedure. Also, the present invention provides an improved fabrication process for etching the accelerometer from a single silicon substrate in a single etching step for reducing damage to the seismic mass.
• Fig. 1 is a perspective view of a piezoresistive transducer of the prior art in which force gages are arranged on opposite sides of the substrate;
• Fig. 2 is a perspective view of a pendular piezoresistive accelerometer in accordance with the present invention illustrating two force gages arranged on the same surface of the silicon substrate;
• Fig. 3 is a perspective view of the piezoresistive accelerometer of Fig. 2 showing the accelerometer positioned between a protective lid and base plate;
• Fig. 4 is another perspective view of the piezoresistive accelerometer of Fig. 2 showing an exploded view of the accelerometer positioned between the protective lid and base plate;
• Fig. 5 is a planar view of the piezoresistive accelerometer of Fig. 2 showing the physical orientation of the seismic mass, the hinge and the support rim with respect to a silicon substrate;
• Figs. 6a-6d are diagrammatic views in perspective of the piezoresistive accelerometer of Fig.
• Fig. 7 is a planar view of a first alternative embodiment of a pendular piezoresistive accelerometer of the present invention illustrating a pair of accelerometers placed back-to-back and in which the gages are electrically connected to form a full Wheatstone Bridge circuit;
• Figs. 8a-8b are wiring diagrams of the piezoresistive accelerometer of Figs. 2 and 7, respectively, illustrating a half Wheatstone Bridge circuit accelerometer in Fig. 8a and a full Wheatstone Bridge circuit accelerometer in Fig. 8b;
• Figs. 9a-9b are equivalent circuit diagrams of the piezoresistive accelerometer of Figs. 2 and 7, respectively, illustrating a half Wheatstone Bridge circuit accelerometer in Fig. 9a and a full wheatstone
• Figs. lOa-lOb are a wiring diagram and an equivalent circuit diagram, respectively, of a second alternative embodiment of a pendular piezoresistive accelerometer of the present invention showing the accelerometer positioned between a pair of protective lids.
• the invention is embodied in a pendular piezoresistive accelerometer 100 of the type having a single silicon substrate 102 upon which all components are formed and supported, and a sensor 104 comprised of at least two piezoresistive gages 106, 108 which are etched-free and formed on one side of the substrate 102 for forming "beams" between a seismic mass 110 and a base 112. Accelerations along a sensitive axis of the accelerometer 100 create compressive and tensile stresses on the two gages 106, 108 resulting in voltage differentials which are sensed by a Wheatstone Bridge circuit.
• a piezoresistive transducer 20 discloses a sensor 22 shown mounted on a base block 24 through the use of a clamp or the two components can be bonded together using an adhesive.
• Sensor 22 has a fixed end 26 bonded to the base block 24 while a movable end 28 is cantilevered from sensor 22.
• the movable end 28 reacts to forces in the direction of arrow 30 around a hinge 32 defined by a pair of upper and lower grooves 34, 36 respectively.
• Gages 38 are subjected to the strain during this movement in one direction and the electrical signal therein is picked up by a pair of contacts 40, 42 deposited on a pair of contact pads 44, 46 respectively at each end of the gages 38.
• This particular form of sensor includes identical gages deposited on the bottom surface of the sensor 22 for sensing movement of movable end 28 in the reverse direction of force 30.
• contact leads 48 extend from contacts 40 while contact leads 50 extend from additional contacts deposited on the lower surface of sensor 22 and in contact with gages mounted thereon.
• the assembly of the prior art shown in Fig. 1 may be employed as an accelerometer, for example, wherein the inertial force of the sensor movable end 28 is the force measured by the system.
• Certain disadvantages exist in the accelerometers of the prior art which includes expensive packaging costs.
• the gages 38 are located on opposite surfaces, e.g.; the top and bottom surface of both the fixed end 26 and the movable end 28. Such a design complicates the wire connections to the sensor.
• connection pads (as 44, 46 on the top surface) and the gages as 38 on the top surface exist on the bottom surface of the substrate, and contact lead 50 must be attached to permit electrical connection to the bottom surface. All of this must be completed prior to mounting the sensor substrate 22 to the base block 24. As should now be obvious, the manufacture of this assembly involves a cumbersome time consuming and expensive process. It must be recognized that during the final phase, chances for damage substantially is increased when the fixed end 26 of the completed sensor 22 is bonded to the base block 24.
• the movable end 28 is defined by both etching and saw-cutting and the delicate gages 38 are not protected from the attendant contamination and vibration damage during the cutting. Additionally, performance of the accelerometer transducer 20 depends upon the distance from the gage 38 to the hinge 32. This distance is generally limited by the thickness of the silicon transducer 20, thus limiting the optimal performance of the accelerometer transducer 20.
• the pair of gages 106, 108 mounted on the same side of the single silicon substrate 102 cooperate with the substrate to reduce the wire bonding and manufacturing packaging costs of the accelerometer 100 while the improved fabrication process for etching the accelerometer from the single silicon substrate 102 in a single etching step reduces damage to the seismic mass 110.
• the pair of gages 106, 108 are well matched with regard to resistance values and temperature coefficients.
• the completed accelerometer employs a base plate 114 and a lid 116 which enclose the sensor 104 for protecting the moving parts.
• the geometry of the seismic mass provides a more flexible leverage distance between the gages 106, 108 and a hinged connection 118 for optimizing accelerometer performance.
• the function of the piezoresistive accelerometer 100 is to detect acceleration in systems used to measure shock and vibration, such as, an automobile impact testing device. Towards such use, the accelerometer 100 is fabricated on a (110) orientation silicon substrate 102. An inverted C-shaped groove 120 on the silicon substrate 102 is defined therein undercutting the piezoresistive gages 106, 108. The gage 106 extends over the groove 120 to a plurality of gage pads 122, 124 and 126 on each end thereof while gage 108 extends over groove 120 to a plurality of gage pads 124, 128 and 130.
• a plurality of metal pads 132, 134 and 136 are connected to the gage pads 122, 124 and 128, respectively, to facilitate metal wire connection to the gages 106, 108.
• a pair of metal pads 138, 140, are connected to the gage pads 126, 130 for increasing the electrical conduction of the pads.
• the opened end of the inverted C-shaped groove 120 forms the hinged connection 118 which connects the base of fixed end 112 on a support rim 142 to the seismic mass 110.
• the thickness of the hinged connection 118 is determined by lithographic dimensions and tolerances and can be increased or decreased for varying the sensitivity of the accelerometer.
• Each of the gages 106 and 108 comprise a single link or beam which may be as small as 1 x 10 " xo cubic centimeters according to present practice, is placed on opposite sides of the hinged connection 118 and are etched-free from the substrate 102.
• Each of the gages 106, 108 requires only a very small strain energy since the gage volume is so small. It should be understood that these gages may be constructed larger or smaller depending on the requirements of the particular accelerometer.
• An acceleration is applied in the direction of an arrow 144 and thus a force is applied to the seismic mass 110 in the reverse direction.
• the force causes the seismic mass 110 to move around or rotate about hinged connection 118 relative to the fixed end 112. This action creates a compressive strain in piezoresistive gage 106 and a tensile strain in piezoresistive gage 108, or vica versa when the acceleration direction 144 is reversed.
• the gages 106, 108 are fabricated in the manner described in the present invention, they can be connected individually or in series and mounted in an electronic circuit and connected to a recording system.
• An example of such a mounting includes, but is not limited to, a Wheatstone Bridge which has the advantages of simplicity and superb accuracy.
• a circuit is disclosed in Figs. 9a and 9b in the present invention which shows one-half and full Wheatstone Bridge circuits.
• a single gage may be constructed using, for example, only 106 or 108 to measure either compression or tension as a quarter of a full Wheatstone Bridge circuits.
• the accelerometer 100 of the present invention retains all of the advantages of the prior art accelerometers while resolving the disadvantages associated therewith. Since the gages 106, 108 are positioned on the same surface, the wire connection procedure is simplified greatly reducing the packaging costs. This is the case since the connection wires are attached to one side of the substrate 102.
• the attachment of the wires may be done after the sensor 104 is bonded to base 114 and lid 116.
• the attachment of many sensors 104 on one wafer to their base 114 and lid 116 wafers can be done in wafer scale. This reduces the number of steps involved in the manufacturing process and greatly increases the reliability and quality control of the final assembly.
• the leverage of the gages is no longer restricted by the thickness of the substrate but can freely be adjusted on the artwork thus facilitating the optimization of sensitivity and resonant frequency of the accelerometer 100. Additionally, since the gages
• the gages are generally well matched in terms of resistance values and temperature coefficients.
• the seismic mass 110, the hinged connection 118 and the gages 106, 108 are defined by one etching step.
• the seismic mass is surrounded by the protective support rim 142 as shown in Fig. 2 which is also formed in the same etching step.
• the moving parts such as the seismic mass 110, the hinged connection 118 and the gages 106, 108 are protected from the contaminated environment as is disclosed in Figs. 3 and 4.
• the lid 116 and the baseplate 114 can each be attached to the single silicon substrate 102 before the saw cutting occurs to form individual accelerometer components.
• the accelerometer 100 as disclosed in Fig. 3 is shown sandwiched between the base plate 114 and the lid 116.
• a single crystal silicon substrate 102 having (110) orientation is selected.
• An etch is then selected which is both anisotropic and doping-selective.
• Potassium hydroxide/water, hydrazine/water, and ethylene diamine/ pyrocatechol/water mixtures may be selected, depending upon the results desired. These mixtures attack the silicon rapidly in the [133] direction, moderately in the [110] direction, and very slowly in the [111] direction.
• the substrate orientation is (110) and the gages 106, 108 are aligned in the [111] direction so as to define a groove over which the gage extends. With such an orientation, the groove 120 is formed with walls which are nearly vertical.
• etchants are also dopant selective in that they attack very slowly silicon which is doped with boron atoms and having a concentration greater than 7 x 10 19 per cubic centimeter.
• the gages 106, 108 and their respective terminals are defined by a thermal diffusion or ion implantation through a silicon dioxide mask to a boron concentration of roughly 10 20 per cubic centimeters.
• the boron doped gages 106, 108 are also exposed to the etchant, but are not attacked thereby.
• the boron doped gages are p-type while the substrate is n-type.
• the heavily doped area is electrically isolated from the substrate 102 by a p-n junction.
• the hinged connection 118, the seismic mass 110 and the support rim 142 are also defined in the substrate by protecting them from the etchants with a silicon dioxide mask.
• the mask design must compensate for the fast attack by the etchant at the corner where two ⁇ 111 ⁇ planes meet.
• the edges of the inverted C-shaped groove 120 and thus the edges of the hinged connection 118 are perpendicular to the [111] crystalline direction.
• the seismic mass 110 and the hinged connection 118 are bounded by nearly vertical ⁇ 111 ⁇ planes which are virtually non-attacked by the anisotropic etchants.
• the gages 106, 108 are aligned in the [111] direction taking advantage of the largest piezoresistive gage factor in this direction.
• the hole 150 on the lid 116 can be seen as a dashed line in Fig. 5. The edges of the hole 150 are parallel to the [110] directions of the (100) [11Q] substrate used for the lid 116.
• the ⁇ 111 ⁇ plane can be seen as a 54.74 degree slope at the edge of the hole 150.
• two more substrate wafers may be bonded to the top and bottom of the sensor 104 to form the lid 116 and the base plate 114 to protect the sensor from the surrounding environment.
• the gage substrate 102 will still be (110) [111] silicon material, while the lid and the base plate wafers can be (100) [110] or (110) [111] silicon material.
• the hole 150 formed in the lid 116 permits accessing the terminals 132, 134 and 136 of the gages 106, 108.
• FIGs. 6a-6d A sequence of steps is shown in Figs. 6a-6d for processing a single-sided, etch-freed gage piezoresistive accelerometer 100 of the present invention.
• the substrate 102 has formed an oxidized layer 160 on the top surface and an oxidized layer 162 on the bottom surface.
• the oxidized layer 160 on the top surface is open for doping at position 164.
• a plurality of gage patterns 166, 168 are also defined in the opening 164.
• boron is diffused or implanted into the open aperture to a concentration of 10 ao boron/cubic centimeter.
• both sides of the substrate 102 are opened. This condition is shown as the C-shaped groove 120 on the top surface 160 and as another C-shaped groove 170 (not shown) on the bottom surface 162. Both of the C-shaped grooves, which are in mirror image to one another, are aligned precisely to each other. It is noted that alignment is critical to achieving low transverse sensitivity.
• the etching procedure is carried out, preferably with the ethylenediamine pyrocatechol- water mixture, as noted above. The etchant will not attack the oxide covered regions on the top surface 160, the bottom surface 162 z and the heavily doped boron regions 164, 166 and 168.
• the etchant attacks the openings 120 and 170 and eventually etches through the substrate 102 to completely form the inverted
• the groove 120 defines the hinged connection 118 and the seismic mass 110 and further undercuts the piezoresistive gages 106, 108.
• the formed gages 106, 108 extend over the C-shaped 5 groove 120 to the support rim 142 at one end and to the seismic mass 110 at the other end.
• the used oxide is stripped from the substrate 102 and a thin oxide coating is grown on both the top surface 160 and the bottom surface 162. 3
• the gages 106, 108 are eventually surrounded and protected by the thin oxide coating.
• contact windows 172, 174, 176, 178 and 180 are opened on the pads at each end of the gages 106, 108.
• a metal layer is 5 deposited on the top surface 160 of the substrate 102. The metalized deposit is then patterned to define the contacts or connecting links of metal pads 132, 134, 136, 138 and 140 formed at each end of the gages 106, 108.
• the finished piezoresistive accelerometer 100 is 0 then sandwich between bone plate 114 and lid 116 using solder glass frit as adhesive. Finally, the individual dice are cut from the substrate having been processed in accordance with the above procedure. Therefore, an additional advantage associated with the present invention is that the diamond saw-cutting procedure to define the size of the seismic mass employed in the prior art has been eliminated.
• the size of the seismic mass 110 is now defined completely by chemical etching which eliminates damage to the substrate wafers due to environmental contamination and vibration.
• a first alternative embodiment 200 of the pendular piezoresistive accelerometer of the present invention is illustrated in a planar view in Fig. 7.
• the planar view discloses a pair of accelerometers 202, 204 placed back-to-back in which a plurality of four gages 206, 208, 210 and 212 are electrically connected to form a full Wheatstone Bridge circuit. In essence, another pair of force gages have been added for providing higher sensitivity to the acceleration parameter being measured.
• the accelerometers 202, 204 are placed in juxtaposition on a common substrate 214.
• Each of the accelerometers 202, 204 includes one of the following: a seismic mass 216, 218; a hinged connection 220, 222; a common base 224; an inverted C-shaped groove 226, 228; a support rim 230, 232; and a plurality of gage pads and metal pads 234 for completing the electrical connections in the full Wheatstone Bridge circuit.
• the electrical wiring connections are distinguishable, the construction and fabrication of the gage pads and metal pads 234 are as described in the preferred embodiment 100. It is understood that one of the inverted C-shaped grooves 226, 228 may be rotated around the center of mass by approximately 90 degrees, along with the associated gages positioned around the resulting hinged connection.
• FIG. 8a Wiring diagrams disclosing the electrical connections for the Wheatstone one-half Bridge circuit shown in the accelerometer 100 of the preferred embodiment and the corresponding wiring diagram for the Wheatstone full Bridge circuit shown in the first alternative embodiment 200 are illustrated in Figs. 8a and 8b, respectively.
• the wiring connections for the half-bridge accelerometer 100 shown in Fig. 8a include the circuit elements as shown in Fig. 2. Additionally, two fixed resistors 238, 240 are connected in external circuitry with a first voltmeter V which is employed for detecting the change in resistance of the strain gages during the measurement of the acceleration parameter.
• the wiring connections for the full-bridge accelerometer 200 shown in Fig. 8b include the circuit elements as shown in Fig. 7. Furthermore, there is shown a second voltmeter V a incorporated into the circuit wiring between accelerometers 202 and 204. The second voltmeter V 3 is also employed for detecting the change in resistance in the strain gages of Fig. 7.
• the plurality of gage pads and metal pads 234 include a plurality of gage pads 242, 244 and 246 positioned on accelerometer 202 with corresponding metal pads mounted thereon and a plurality of gage pads 248, 250 and 252 positioned on accelerometer 204 with additional corresponding metal pads mounted thereon.
• the half-bridge accelerometer 100 when an acceleration is sensed, only the single seismic mass 110 rotates about the hinged connection 118 providing compressive and tensile stresses on the gages 106, 108 resulting on a voltage reading on voltmeter V x .
• both the seismic masses 216, 218 will rotate about the respective hinged connections 220, 222. Under these conditions, both strain gages 208 and 212 will experience compressive stress while strain gages 206 and 210 experience tensile stress or vica versa depending upon the acceleration force applied.
• Figs. 9a and 9b are equivalent circuit diagrams of the half-bridge accelerometer 100 and the full bridge accelerometer 200.
• Each of the equivalent circuits of Figs. 9a and 9b are connected to a battery at one end and to electrical ground at the other end.
• the equivalent circuit clearly shows the fixed resistances 238, 240 and the variable resistances ( ⁇ fR) of the stain gages 106, 108 shown in Fig. 2.
• the variation of the resistance of each of the strain gages is a function of the acceleration force applied.
• the voltmeter V x located at the center of the equivalent circuit in Fig. 9a detects the following value:
• the equivalent circuit shows the variable resistances ( R) of the strain gages 206, 208,
• the voltmeter V 2 located at the center of the equivalent circuit in Fig. 9b detects the following value:
• V V 2 ( ⁇ R/R) x battery (2)
• V 3 2 x V r (3)
• the full-bridge accelerometer circuit of Fig. 9b has twice the sensitivity to variations in the resistance ( ⁇ d * R) of the strain gages due to an acceleration force than does the half-bridge accelerometer circuit shown in Fig. 9a.
• the variations in strain gage resistance may be either increments or decrements.
• FIG. 10a and 10b A second alternative embodiment 300 of the pendular piezoresistive accelerometer of the present invention is illustrated in Figs. 10a and 10b.
• one pair of piezoresistive force gages are positioned on the top surface 160 while a second pair of force gages are positioned on the bottom surface 162.
• the force gages on the top and bottom surfaces are mirror images of one another.
• the base plate 114 is eliminated while a pair of through holes 150 on the bottom and top lids 116 provide access to the metal pads 132, 134 and 136 on the top surface 160 and the corresponding metal pads on the bottom surface.
• the wiring diagram for the second alternative embodiment 300 is shown in Fig. 10b.
• a plurality of strain gages 304, 306, 308 and 310 are represented by variable resistances, the value of which varies in accordance with the acceleration force applied.
• one end of the equivalent diagram is connected to the battery while the other end is connected to electrical ground.
• Connection wires 312, 314 and 316 are connected to the connection pads 132, 134 and 136, respectively, on the top surface 160 while connection wires 318, 320 and 322 are connected to the corresponding pads on the bottom surface 162 of the substrate.
• Terminals 312 and 322 in Fig. 10b are joined and connected to the positive terminal of the battery while terminals 316 and 318 are joined and connected to the negative terminal of the battery.
• a voltmeter V 3 is positioned at the center of the equivalent diagram in Fig. 10b between terminals 314 and 320 for detecting changes in the resistance of the full-bridge accelerometer 302 due to the force of acceleration.
• the pendular piezoresistive accelerometer 100 of the present invention substantially reduces the packaging costs of the accelerometer by simplifying wire connection procedures, thereby permitting the institution of economic batch manufacturing techniques. Further, the fabrication process for etching the accelerometer 100 from a single silicon substrate 102 in a single etching step has been significantly improved, thereby reducing environmental contamination and vibration damage to the seismic mass 110.
• the construction of the present invention includes stress gages 106, 108 that are generally well matched in terms of resistance values and temperature coefficients for minimizing drift caused by unbalance between components. Also, the utilization of a base plate 114 and a lid 116 which encloses the sensor 104 further protects the structure of the accelerometer 100. Finally, the flexibility in design size between the gages 106, 108 and the hinged connection 118 permits optimizing the performance of the accelerometer.

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Cited By (7)

Citations (4)

• 1992
  • 1992-01-21 WO PCT/US1992/000479 patent/WO1992015018A1/en active Application Filing

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