LOW COST RESISTANCE GAUGE ACCELEROMETER

• US 3,788,149 A
• Filed: 06/26/1972
• Issued: 01/29/1974
• Est. Priority Date: 06/26/1972
• Status: Expired due to Term

• Current Assignee: Leslie B. Wilner
• Original Assignee: Leslie B. Wilner
• Inventors: Wilner, Leslie B.
• Primary Examiner(s): Gill, James J.
• Application Number:
  US05/266,410
• Time in Patent Office: 582 Days
• Field of Search: 73/88.5R,398AR,517R,516R 338/43,46,277,2,6
• US Class Current: 73/514.12
• CPC Class Codes: G01P 15/0891 with indication of predetermined acceleration values G01P15/135 takes precedence
• CPC Class Codes: G01P 15/123 by piezo-resistive elements, e.g. semiconductor strain gauges

Abstract

A low cost accelerometer is provided for use in mounting on an automobile or other movable object to produce an output signal which will operate an emergency device if the acceleration of the object reaches or exceeds a predetermined unsafe value. The accelerometer employs a strain sensitive element in the form of a thin, and hence fragile, piezoresistive semiconductor element having its respective ends secured to a pair of hinged support members that rotate relative to each other thereby straining the element longitudinally in response to acceleration. An overload release engages the support structure to limit the hinge action to preclude straining the strain sensitive element beyond the breaking point when the accelerometer is subjected to acceleration forces in excess of the normal operating range for which the accelerometer is designed.

Claims

1. In an accelerometer:

an acceleration responsive device comprising a pair of mechanical members resiliently connected together, one of said mechanical members being a base member that is adapted for attachment to an object whose acceleration is to be detected, the other mechanical member being an inertia member which resists motion communicated to it by the base member through the resilient connection therebetween;

a strain gauge unit having a thin piezoresistive strain detecting element having its ends respectively bonded to each of a pair of enlarged support elements separated by a gap and having a hinge interconnecting said support elements, whereby the thickness of said gap is altered by relative pivotal movement of said support elements;

means for connecting said support elements to said mechanical members, said connecting means comprising a pair of fulcrum means and a spring for urging one of said support elements and one of said mechanical members into engagement through said fulcrum means, whereby said support elements flex about their interconnecting hinge to stress said piezoresistive element when said two mechanical members move relative to each other in response to acceleration of said base member, both said fulcrum means remaining engaged by pressure of said spring while the magnitude of acceleration lies in one range and one fulcrum means becoming disengaged and the other fulcrum means remaining engaged and acting as a pivot between said one mechanical member and said one support member when the acceleration lies in a range outside of said first mentioned range; and

a pair of electrical terminals connected to said piezoresistive element to detect changes in resistance thereof resulting from acceleration applied to said base member.

2. An aCcelerometer as defined in claim 1 wherein said piezoresistive element is composed of semiconductor material.

3. An accelerometer as defined in claim 1 wherein said intermediate portion of said piezoresistive element is a semiconductor element that has a reduced cross sectional area in the part thereof that extends across said gap.

4. An accelerometer as defined in claim 1 wherein said one mechanical member comprises a casing that includes said base member and encloses said other mechanical member, said strain gauge unit and said attaching means.

5. An accelerometer as defined in claim 1 comprising a leaf spring interconnecting said two mechanical members and acting as said resilient connection and defining a rotation axis therebetween, said rotation axis being substantially parallel to the axis about which said support elements flex.

6. An accelerometer as defined in claim 5 wherein said two axes are substantially coaxial.

7. An accelerometer as defined in claim 1 wherein said fulcrum means are provided by projecting means secured to said face member and one end of said spring is connected to said base member and the other end of said spring engages said one support element at a point located between said pair of fulcrum means.

8. An accelerometer as defined in claim 7 wherein said other support element is rigidly secured to said inertia member.

9. An accelerometer as defined in claim 4 in which said two mechanical members and said casing are composed of a plastic material.

10. An accelerometer as defined in claim 9 comprising overload stops arranged between said casing and said inertia member for limiting the movement of said inertia member relative to said case when the acceleration attains a predetermined value in excess of that which produces the lifting action referred to in claim 1.

11. An accelerometer including a casing adapted for mounting on a body whose acceleration is to be sensed, a leaf spring, an inertia member suspended within said casing by said leaf spring, an elongated supporting member formed of insulating material having a transverse groove formed in one surface midway of the length thereof to form a pair of interconnected arm portions, said supporting member having conductive coatings on each of said arm portions, a conductive coating on the opposite surface of said supporting member, and a conductive jumper electrically connecting the conductive coating on one of the arm portions with the conductive coating on the opposite surface thereof, the jumpered arm portion of said supporting member being affixed to said inertia member, a piezoresistive element having two end portions and an intermediate portion, said piezoresistive element mounted on said one surface of the supporting member to bridge said transverse groove by having its end portions bonded to the conductive coatings on the respective arm portions, a first conductive terminal element affixed to said casing and having a pair of projecting frets, and a second conductive terminal element affixed to said casing and including spring force means for holding the unjumpered arm portion of the supporting member against said frets, whereby when the acceleration force on said inertia member is within a predetermined normal range of operation said unjumpered arm portion is held by said spring force means against said frets to thereby transmit the strain produced by the relative movements of the arm portions across the piezoresistive element, and whereby when the acceleration force on said inertia member is in excess of the predetermined normal range of operation said unjumpered arm portion is tilted relative to said frets to thereby relieve the strain being transmitted across the piezoresistive element.

12. The invention in accordance with claim 11 wherein said intermediate portion of said piezoresisitve element is a semiconductor element and has a smaller cross sectional area than either of said end portions.

13. The invention In accordance with claim 12 wherein said leaf spring is a relatively large, thin rectangular sheet of resilient material having a centrally located opening, and wherein the frets on said first conductive terminal element are disposed in said centrally located opening.

14. The invention in accordance with claim 13 wherein said first conductive terminal element has an outer end portion forming one of two electrical terminals for the accelerometer and wherein said second conductive terminal element has an outer end portion forming the other electrical terminal for the accelerometer.

15. The invention in accordance with claim 11 wherein said casing and said inertia member are each molded of plastic.

16. The invention in accordance with claim 11 wherein at least the lower portion of said casing is filled with oil and said inertia member is formed with a depending member whose movements through said oil provides for damping said inertia member when the accelerometer is subject to vibration.

17. The invention in accordance with claim 14 wherein a utilization device is coupled to the two electrical terminals of said accelerometer.

18. The invention in accordance with claim 11 wherein said spring force means comprises a portion of said second conductive terminal forming an angularly bent relief spring having an inner end contacting and holding the unjumpered arm portion of the supporting member against said frets on said first conductive terminal element.

19. The invention in accordance with claim 11 wherein said spring force means comprises a helical spring which is mounted in said casing, and an electrically conductive ball on the end of said helical spring which contacts and holds said unjumpered arm portion against the frets on said first conductive terminal element.

20. The invention in accordance with claim 14 wherein said plastic inertia member is provided with bosses on the front and back surface thereof for arresting the movement of said inertia member within said plastic casing when said unjumpered arm portion is tilted about said one of said frets.

21. An accelerometer comprising a plastic casing adapted for mounting on a body whose acceleration is to be sensed, a leaf spring in the form of a wide thin metallic resilient sheet having an opening in the center thereof, a plastic inertia member affixed to one end portion of said leaf spring and suspended within said casing by affixing the other end portion of said leaf spring to said casing, an elongated supporting member having a transverse groove on one surface thereof midway of the length thereof to form opposite arm portions, a thin and fragile crystalline strain gauge element having two opposite end portions and an intermediate portion the electrical resistance of which varies in a predetermined manner in response to strains transmitted by the end portions thereof, said crystalline element being mounted on one surface of said supporting member so as to bridge said transverse groove by having the inner side of its two end portions respectively bonded to the two arm portions of said supporting member for transmitting strains produced by the relative rotation of said arm portions about said transverse groove to said end portions, said inertia member being affixed to the end of one of said two remote arm portions of said supporting member, a first terminal element affixed to said casing and provided with a portion having a flat projection surface, a relief spring the inner end of which exerts a force on the outer side of the other of the two remote arm portions for holding said other remote arm portion flat against said flat projection surface over a predetermined normal strain range applied to said crystalline element and permitting said other remote arm portion to be tilted relative to said flat projection surface in response to predetermined applied strains exceeding said normal strain range, and a second terminal element affixed to said casing, said terminal elements being electrically connected to the respective end portions of said crystalline element.

22. An accelerometer including a casing having a bottom compartment, said casing adapted to be mounted in an upright position on a body whose acceleration along a horizontal path is to be sensed, a leaf spring, an inertia member suspended within the bottom compartment of said casing by said leaf spring, an elongated supporting member having a transverse groove on one surface thereof midway of the ends thereof to form opposite arm portions, a small fragile piezoresistive element mounted across the transverse groove on said supporting member, said piezoresistive element having a neck portion the electrical resistance of which varies in a predetermined manner in response to the strains transmitted thereto by the relative rotation of said arm portions about said transverse groove, means for anchoring one arm portion of said supporting member to said inertia member, a first terminal element secured to said casing, said first terminal element provided with a flat projection portion having upper and lower edges, and a second terminal element secured to said casing, said second terminal element having a spring force portion for contacting the other arm portion of said supporting member and holding said other arm portion against the flat projection portion of said first terminal element, whereby when the moment of the acceleration force on said inertia member in one direction is less than the moment of the force produced by the spring force portion in the opposite direction with respect to either the upper or lower edge of said flat projection portion, said other arm portion is held by said spring force portion against said flat projection portion to thereby transmit the strain produced by the relative movements of the arm portions across the piezoresistive element, and whereby when the moment of the acceleration force on said inertia member in one direction is greater than the moment of the force produced by the spring force portion in the opposite direction with respect to either the upper or lower edge of the flat projection portion, said other arm portion is tilted about either the upper or lower edge of said flat projection portion to thereby relieve the transmission of any further strain across the piezoresistive element.

23. In an accelerometer an inertia member, a leaf spring having one end thereof affixed to a body whose acceleration is to be sensed and the other end thereof affixed to said inertia member, an elongated supporting member having a transverse groove on one surface thereof midway of the length thereof to form opposite arm portions, a strain gauge element comprised of a piezoresistive semiconductor material and having two end portions and an intermediate portion, said strain gauge element bridging said transverse groove and having its end portions bonded to respective arm portions on said supporting member, a terminal element provided with a projection, said terminal element being affixed to said body, said supporting member having one of its arm portions affixed to said inertia member and the other of its arm portions positioned against the projection of said terminal element, and a spring force means for providing a holding force for holding said other arm portion of said supporting member against said projection only when the acceleration to which the accelerometer is subjected lies within a normal range, whereby when the acceleration force on said inertia member is within said normal acceleration range said one arm portion having its end affixed to said inertia member is rotated about said groove relative to said other arm portion which is being held by the spring force means against said projection to thereby transmit a strain corresponding to the acceleration force across said strain gauge element, and whereby when the acceleraTion force on said inertia member is in excess of said normal acceleration range, said spring force means is overcome and said other arm portion is tilted relative to said projection to thereby enable both said arm portions to rotate together about said transverse groove to lift said other arm portion from part of said projection to prevent overload of said strain gauge element when such excess acceleration force occurs.

24. In an electromechanical transducer for producing an electrical signal in response to a mechanical force:

an elongated electrosensitive element responsive to bending moments about a midsection, said element being characterized by a normal operating range of bending moments and a higher bending moment at which damage occurs;

a mounting structure for the electrosensitive element at one side of the midsection, said mounting structure comprising a pair of spaced apart pivot means engaging part of said electrosensitive element on that side and spring means urging said part into engagement with said pivot means;

means for applying the force to the part of said electrosensitive element on the other side of said midsection to produce bending moments about said midsection, the spring force being such that for applied forces producing bending moments in the normal operating range the electrosensitive element is held in contact with said pivot means and such that when the applied force exceeds the normal range but is below that which produces a damaging bending moment the electrosensitive element is lifted free of one pivot means and rotates about the other pivot means; and

means connected to said electrosensitive element for detecting changes in the electrical properties thereof produced by such bending moments in the normal operating range.

25. An electromechanical transducer as defined in claim 24 comprising means including mechanical stops limiting gross overtravel of the parts of the transducer.

26. In an electromechanical transducer for producing an electrical signal in response to a mechanical force:

an elongated electrosensitive element responsive to bending moments about a midsection, said electrosensitive element comprising an elongated supporting member having a transverse groove on one side thereof midway between the ends thereof to form a Cardan hinge at said midsection and to form two support arms on opposite sides of said midsection, a piezoresistive semiconductor strain gauge element having two end portions secured to said support arms on opposite sides of said groove and having an intermediate portion, said electrosensitive element being characterized by a normal operating range of bending moments which cause the resistance of said strain gauge element to vary in a manner corresponding to variations in bending moment in said range and a higher bending moment at which damage to said strain gauge element occurs;

a mounting structure for the electrosensitive element at one side of the midsection, said mounting structure comprising two pivot means engaging the support arm on that side and spring means urging said support arm into engagement with said pivot means;

means for applying the force to the other support arm to produce bending moments about said midsection, the spring force chosen being such that for applied forces producing bending moments in the normal operating range the spring-urged support arm is held in contact with both said pivot means and such that when the applied force produces a bending moment outside the normal range but below that which produces a damaging bending moment, the electrosensitive element is lifted free of one pivot means and rotates about the other pivot means; and

means connected to said strain gauge element for detecting changes in the electrical resistance thereof produced by such bending moments in the normal operating range.

27. An electromechanical transducer as defined in claim 26 wherein said intermediate portion of said semiconDuctor strain gauge element has a reduced cross-section compared with said end portions.