Force responsive transducer

• US 4,295,376 A
• Filed: 12/01/1978
• Issued: 10/20/1981
• Est. Priority Date: 12/01/1978
• Status: Expired due to Term

• Current Assignee: ABB Instrumentation SpA (ABB Limited)
• Original Assignee: Besco Industries Incorporated
• Inventors: Bell, Robert L.
• Primary Examiner(s): Ruehl, Charles A.
• Application Number:
  US05/965,604
• Time in Patent Office: 1,054 Days
• Field of Search: 73/717, 73/718, 73/723, 73/724, 73/780, 361/280, 361/283, 361/290, 361/291
• US Class Current: 73/724
• CPC Class Codes: G01L 1/142 using capacitors
• CPC Class Codes: G01L 9/0075 using a ceramic diaphragm, e.g. alumina, fused quartz, glass

Abstract

A differential capacitive force responsive transducer provides a substantially linear relationship between transducer output and input variations. A pressure or load to be measured is applied to one surface of a deflectable diaphragm, the opposite surface of which includes at least a pair of electrodes at different positions, such that when the diaphragm flexes there is a differential deflection of the electrodes. A substantially rigid electrode-bearing plate is secured to the electrode-bearing face of the diaphragm by a spacer or spacers in an intermediate flexure region of the diaphragm, to define separate capacitive elements with the diaphragm electrodes. The plate thus moves with the diaphragm, maintaining a fixed spacing at the spacer elements, but with the spacings between the elements varying in opposite senses. With this construction, the transducer can be of low cost, small size, precision manufacture, and use readily deposited thick or thin film electrodes. Moreover the circuits may automatically be adjusted to provide selected nominal zero and range values, so that the transducer can be utilized interchangeably with other transducers in mass production systems. Furthermore, the differential capacitance values obtained from a single transducer can directly provide digital values to a processor, by oppositely varying two variable frequency oscillators, and using the difference frequency to generate pulse counts during successive time intervals.

Claims

1. A force responsive capacitive transducer comprising:

a flexible diaphragm having a face region deflectable in response to a deflecting force acting thereon and including first electrode means comprising electrically separate electrodes in at least two different regions on the deflectable face region thereof that are deflected differently in response to a deflecting force;

and movable plate means coupled to the deflectable face region of said diaphragm between the separate electrodes of said first electrode means and including second electrode means facing the first electrode means.

2. The invention as set forth in claim 1 above, wherein said diaphragm and movable plate means are of electrically insulative material and said first and second electrode means comprise deposited films, and including in addition spacer means coupling said movable plate means to said diaphragm between the separate electrodes on said diaphragm.

3. A force responsive capacitive transducer comprising:

a flexible diaphragm of electrically insulative material deflectable in response to a deflecting force acting thereon and including first electrode means comprising electrically separate deposited film electrodes in at least two different regions on one face thereof that are differently spaced from the center of deflection of the diaphragm;

movable plate means of electrically insulative material and including second deposited film electrode means facing the first electrode means;

spacer means coupling said movable plate means to said diaphragm between the separate electrodes on said diaphragm;

said movable plate means being substantially nondeformable in the region of said second electrode means during flexure of said diaphragm, and wherein said first and second electrode means define a pair of capacitive means changing in opposite senses in response to flexure of said diaphragm.

4. The invention as set forth in claim 3 above, wherein said movable plate means comprises means for absorbing torsional stresses induced in said spacer means during flexure of said diaphragm.

5. The invention as set forth in claim 4 above, including in addition means disposed on at least the diaphragm for limiting electrical interaction between the electrodes thereon.

6. The invention as set forth in claim 5 above, wherein said means for limiting electrical interaction comprises guard electrode means disposed on said diaphragm and plate between said first and second electrode means.

7. A variable capacitance device for providing readily processed signal variations in response to minute deflections of a diaphragm member comprising:

a diaphragm having a planar surface that is deflectable from the nominal plane within its interior regions, and including an integral peripheral member defining a concavity open to the deflection causing source, the peripheral member having a thickness substantially greater than that of the deflection region of the diaphragm to provide peripheral rigidity while permitting convex deflection of the central region of the diaphram, the planar surface further including first thin film electrode means thereon covering a portion of the interior regions;

a low mass plate member substantially coextensive with at least a portion of the planar surface of the diaphragm and substantially parallel thereto throughout the coextensive region when the diaphragm is not deflected, the plate member having second thin film electrode means thereon facing the diaphragm and being at a distance of the order of 1 mil from the diaphragm; and

spacer means comprising a number of elements disposed between and coupled to both the diaphragm and plate, the spacer means being disposed within the deflectable region of the diaphragm to provide a spacing such that the thickness of said thin film electrode means is less than 10% of the distance between the diaphragm and plate member and maintaining the plate in a position parallel to a selected plane during deflection of the diaphragm.

8. The invention as set forth in claim 7 above, wherein the diaphragm and plate member are insulative members, and further including enclosure means encompassing said plate member and coupled to said diaphragm for maintaining a selected pressure in the region between said diaphragm and plate member.

9. The invention as set forth in claim 8 above, wherein said spacer means comprises at least three elements bonded to both the diaphragm and plate member.

10. The invention as set forth in claim 9 above, wherein said plate means includes different integral torsional means coupled to each of said elements of said spacer means.

11. The invention as set forth in claim 10 above, wherein a vacuum is maintained within the enclosure means and wherein said elements of said spacer means are deposited material bonding the diaphragm and plate together.

12. A capacitive transducer comprising:

a flexible diaphragm having a fixed periphery and a deflectable surface area interior thereto and including separate electrode means disposed on at least two portions of the surface area that are differentially deflected in response to variations in force acting on said diaphragm;

a substantially rigid plate disposed adjacent the electrode means and including electrode means in facing relation thereto to define capacitors therewith; and

means coupled to the plate and the diaphragm for maintaining a fixed spacing between the plate and diaphragm between the separate electrode means, whereby the values of capacitors defined by opposed electrodes vary in opposite senses with diaphragm deflection.

13. The invention as set forth in claim 12 above, wherein said diaphragm has an essentially flat deflectable surface and a central axis and the separate electrode means are disposed at different radial positions relative to a central axis on the flat surface, and wherein said means for maintaining a fixed spacing comprises spacer elements disposed at radial positions intermediate the separate electrode means.

14. The invention as set forth in claim 13 above, wherein said diaphragm further has an integral peripheral member defining a pressure receiving concavity on the side opposite the electrode means.

15. The invention as set forth in claim 14 above, wherein said transducer further comprises enclosure means encompassing said plate and coupled to said diaphragm for maintaining a low pressure in the region of the electrode means.

16. The invention as set forth in claim 15 above, further including a body for supporting said diaphragm and wherein the peripheral member includes resilient seal means engaging said body.

17. The invention as set forth in claim 16 above, wherein said diaphragm and plate are of ceramic material and said electrode means are thin films.

18. The invention as set forth in claim 17 above, wherein said diaphragm has a rectangular configuration including an extension outside the peripheral member, wherein said plate is substantially coextensive at least with the electrode-bearing portion of the diaphragm and including in addition electronic circuit elements mounted on the extension of the diaphragm.

19. The invention as set forth in claim 14 above, wherein the electrode means on each of said diaphragm and plate comprise an outer ring and an inner disc, and means providing exterior electrical connection thereto.

20. The invention as set forth in claim 19 above, including in addition grounded guard electrode means on each of said diaphragm and plate disposed between said outer ring and inner disc.

21. A capacitive transducer comprising:

a substantially planar-faced member having first and second electrodes at spaced-apart regions thereon;

a deflectable diaphragm in facing relation to the planar face of the member and having electrode means facing the first and second electrodes respectively to define first and second capacitive means; and

spacer means disposed between the member and diaphragm and coupling the member to the diaphragm to maintain a fixed reference distance therebetween in the region between the first and second electrodes.

22. A capacitive transducer as set forth in claim 21 above, including in addition first and second variable frequency oscillator means coupled to said first and second capacitive means respectively and varying in frequency with the value thereof, and means responsive to said first and second variable frequency oscillator means for providing a varying frequency difference signal.

23. A capacitive transducer as set forth in claim 22 above, wherein said variable frequency oscillator means each includes a trim resistor in circuit therewith, said diaphragm having a circuit board extension and said trim resistors and oscillator means being mounted on the circuit board extension.

24. A capacitive transducer as set forth in claim 21 above, wherein said deflectable diaphragm has a circular deflection area concentric with a central axis with the first and second electrodes lying at different radial regions, and wherein said spacer means is disposed along a radius between the first and second electrodes, such that deflection of the diaphragm causes changes in opposite senses in the spacing between facing electrode pairs.

25. The invention as set forth in claim 24 above, including in addition body means for supporting said diaphragm, and wherein said diaphragm has a peripheral flange on the side opposite to the electrodes and coupled to said body means, and defining therewith an interior volume adjacent to the deflectable portion of the diaphragm for receiving an interior fluid pressure and communicating the interior pressure thereof to the deflectable diaphragm.

26. The invention as set forth in claim 25 above, wherein the diaphragm has a deflectable electrode bearing surface substantially less in thickness than the thickness of the peripheral flange, wherein said body means includes a base for receiving the peripheral flange of the diaphragm and means engaging the opposite side of the diaphragm for holding the diaphragm in position, and including in addition seal means coupling the peripheral flange of the diaphragm to the base of the body means.

27. The invention as set forth in claim 26 above, wherein said transducer further includes means hermetically enclosing the substantially planar-faced member to maintain a substantially constant low pressure thereabout.

28. A force responsive device for providing differential signal outputs, comprising:

a pair of insulating members having spaced apart, opposing surfaces, a first of the members being deflectable convexly toward the other in response to an applied force;

spacer means disposed between the insulating members and coupled thereto, the spacer means being positioned in the deflection region of the first member to maintain the second of the members in a plane symmetrically disposed to the first of the members and defining inner and outer deflection regions relative thereto;

first capacitive means disposed in the inner deflection region and varying in accordance with the spacing between the members thereat; and

second capacitive means disposed in the outer deflection region and varying in accordance with the spacing between the members thereat.

29. The invention as set forth in claim 28 above, further including means cooperating with the second of the members to absorb torsional forces acting on the spacer means as the first of the members deflect.

30. The invention as set forth in claim 29 above, further including grounded electrode means disposed between said first and second capacitive means.

31. A pressure transducer comprising:

a diaphragm responsive to a differential pressure applied to the faces thereof;

a plate substantially overlying said diaphragm;

spacer means disposed between said diaphragm and plate in an interior deflecting portion of the diaphragm, for providing a fixed spacing thereat such that the distance between the diaphragm and said plate changes differentially on opposite sides of said spacer means;

first capacitive means mounted between said diaphragm and plate and responsive to relative position changes thereof in a region on one side of said spacer means; and

second capacitive means mounted between said diaphragm and plate and responsive to relative position changes thereof in the region on the other side of said spacer means.

32. A system for providing, to a digital processor, digitally valued signals representative of force variations comprising:

force transducer means comprising a deflectable diaphragm having at least two differentially deflected electrode means thereon and an electrode bearing reference plate affixed to the diaphragm between the two electrode means and movable therewith, the transducer means providing oppositely varying capacitive values in response to force variations;

first and second variable frequency oscillators, each coupled to the transducer means and responsive to the capacitive value of each for providing first and second variable frequency signals representative of force variations; and

means responsive to the first and second signals for generating a third variable frequency signal representative of the difference between the first and second signals, whereby the processor can count the number of cycles in successive time samples of the third signal to determine successive force values.

33. A system for providing, to a digital processor, digitally valued signals representative of force variations comprising:

force transducer means including different capacitance means providing oppositely varying capacitive values in response to force variations, said transducer means comprising a deflectable diaphragm having at least two differentially deflected electrode means thereon;

first and second variable frequency oscillators, each coupled to the transducer means and responsive to the capacitive value of each for providing first and second variable frequency signals representative of force variations;

means responsive to the first and second signals for generating a third variable frequency signal representative of the difference between the first and second signals, whereby the processor can count the number of cycles in successive time samples of the third signal to determine successive force values; and

said force transducer means further including externally trimmable resistors in circuit with the different capacitance means coupled to the different oscillators and additionally varying the time constant to determine the frequency of the different oscillators, whereby the zero and range of the transducer means can be adjusted to selected nominal values by trimming the resistors, to enable different transducer means to be used interchangeably.

34. In a transducer having a deflectable diaphragm bearing at least one electrode, the combination comprising:

an electrode-bearing member disposed opposite the diaphragm and forming variable capacitance means therewith;

substantially rigid spacer means disposed in the deflection region of the diaphragm and coupling the diaphragm to the electrode-bearing member; and

means coupled to said electrode-bearing member for absorbing bending and torsional forces exerted on said spacer means during deflection of said diaphragm.

35. The invention as set forth in claim 34 above, wherein said deflectable diaphragm has a circular periphery and at least two spaced-apart electrodes at different radii from the central axis thereof, wherein said electrode-bearing member has electrodes facing the electrodes of said diaphragm to define a pair of capacitance means at different radii positions, and wherein said spacer means is disposed at a radius intermediate the pair of capacitance means.

36. The invention as set forth in claim 34 above, wherein said means for absorbing bending and torsional forces comprises means defining thin web portions in parts of said electrode-bearing member in engagement with said spacer means.

37. The invention as set forth in claim 34 above, wherein said means for absorbing bending and torsional forces comprises elongated means formed integrally with said electrode-bearing member.

38. The invention as set forth in claim 37 above, wherein said spacer means comprises at least two spaced apart arc segment lengths of deposited fused material bonding the diaphragm and plate together.

39. The invention as set forth in claim 38 above, wherein said elongated means comprises torsional columns in said electrode-bearing member at least coextensive with and coupled to each of said different arc segment lengths and defined at least partially by adjacent arc shaped slots in said electrode-bearing member.

40. The invention as set forth in claim 39 above, wherein said torsional columns have a thickness less than that of said electrode-bearing member.

41. The invention as set forth in claim 38 above, wherein said elongated means comprises elongated cantilever elements in said electrode-bearing member at least coextensive with and coupled to each of said different arc segment lengths and defined at least partially by substantially U-shaped orifices in said electrode-bearing member.

42. In a pressure transducer having a flexible diaphragm that responds in a deflection region to a differential pressure and incorporates electrode means for sensing deflection on one side thereof, the improvement comprising:

a ceramic diaphragm having a flat surface on the side facing the electrode means and an integral peripheral member defining a concavity open to the pressure medium on the opposite side from the electrode means, with the peripheral member having a thickness substantially greater than that of the deflection region of the diaphragm to provide peripheral rigidity while permitting convex deflection of the central region of the diaphragm;

a body member facing the peripheral member on the side opposite the electrode means;

a resilient element seated between the peripheral member and the body member to seal the peripheral member to the body member; and

second electrode means on the flat surface of the diaphragm.