Pressure transducer
United States Patent Johnston l Apr. 29, 1975 l l PRESSURE TRANSDUCER Primary Ii.\&#39;umim&#39;rDonald O. Woodiel I75] Inventor: Samuel Andrew Johnston. Fontana. Amirm) AWN! m. fm&#34;Dlmd Wis Arbucklc [73} Assignee: Bunker Ramo Corporation. Oak 57] ABSTRACT Brook. lll. I Flexible conductive diaphragm is sealed to a flat di- [22] Filed: May 24, I973 electric substrate and spaced above a fixed plate on 1 I r N 3 3 44 the substrate. Diaphragm and plate constitute a capacitor. the capacitance of which varies as the spacing between diaphragm and plate changes. The variable cal52| Cl 73/398 73/407 pacitor. coupled to associated circuitry mounted on f&#34; 9H2 the substrate, causes corresponding changes in the Flew Search 73/398 0 398 output signal from the circuit. A cup. sealed to the 73/407 4311 317/246: Isl/3] R substrate. provides a chamber outside the diaphragm into which a pressure medium can be introduced References cued through a tubing connection. A second tubing connec- UNITED STATES PATENTS tion gives similar access to the space enclosed be- 17H7U3 WW5&#34; Rudcrrcr U 73 393 C tween the diaphragm and the substrate. The two con- 2.75l.53() 0/1956 Armstrong... 73/398 C nections permit the device to be used as a differential 2.999.385 9/196! Wolfe r 4 73/407 R gage. as a normal gage. as a barometer. or as an abso- 3.479.H79 1 H1969 Music 73/398 C l r gage 3.020.083 III/i969 Dimel&#39;f et al. 73/406 6 Claims. 3 Drawing Figures PR ESSI&#39;RI&#39;L TRANSDUCER BACKGROUND OF THE ll\&#39;\&#39;li.\l&#39;l ION Variations in pressure can he made to change the capacitance of an appropriate sensing device. Such a device can be provided by making one of the plates of a capacitor a movable or flexible member. such as a dia phragm. lfthe diaphragm is located closely adjacent to a second. fixed plate. and one side of the diaphragm is exposed to the pressure to be sensed. variations in that pressure will he reflected as changes in the capacitance between the diaphragm (movable plate] and the fixed plate. The variable capacitor thus constituted can be connected to an appropriate electronic circuit. to provide a circuit output which has parameters changing in accordance with the variance in pressure. For example. the changing capacitance may be used to change the operating frequency of an oscillator circuit. A transducer based on this general principle is the subject of Pat. No. 3.808380. filed Apr. lb. I973 assigned to the present assignee of this application.  
  The variable frequency signal can be passed through a discriminator to obtain an analog output. or through a gated counter to obtain a digital output. Systems of these types are disclosed in US. Pat. Nos. 3.295.360 and 3.595.084.  
  The present invention is a refinement in systems of the kind described. by which important advantages are gained in manufacturing economy. in versatility of application, and in obtaining a substantial change of capacitance fora given change of pressure. in a device of small size and weight.  
 BRIEF SUMMARY OF THE INVENTION A flat dielectric substrate is provided with a printed&#34; conductive land constituting a fixed plate on one surface of the substrate. An electrically conductive. flexible diaphragm mounted over the land has an upstanding peripheral flange which spaces the central portion of the diaphragm away from the fixed plate. The diaphragm is hermetically sealed to the substrate. with its flange surrounding and spaced from the fixed plate. The diaphragm in combination with the central fixed plate. comprises a variable capacitor. Circuitry for converting the varying capacitance to a correspondingly varying electrical signal is located on the substrate closely adjacent the diaphragm. A suitable housing or cap surrounds the diaphragm. is sealed to the substrate and provides a chamber by which the diaphragm may be subjected to a pressure medium as desired. A port is provided by which connection can be made to the chamber for introducing the pressure medium.  
 To enable the device to function as a differential pressure sensor. a second port is provided by which connection can he made to introduce a pressure medium into the space enclosed between the diaphragm and the substrate. This port is provided at or near the periphery of the fixed plate. where any necessary removal of the conductive land material comprising the fixed plate of the capacitor will have minimum effect upon the change in capacitance between the diaphragm and the fixed plate as a function of pressure changes.  
 DESFRIPTION OF THF DRAWINGS Fl(i. l is a plan view of a pressure sensor embodying the invention:  
  Fl(i. 2 is a side elevational view. partly in section taken on the line 22 of FIG. I; and  
  FIG. 3 is a view similar to that in FIG. I. but showing the reverse side of the pressure sensor.  
 DETAILED DESCRIPTION The pressure sensor shown in FIG. I is designated generally by the reference numeral 10. It is constructed on a flat substrate 12 which may be a dielectric material of the kind used for conventional printed circuits; any material having the necessary electrical and mechanical properties throughout the range of environmental conditions to which the device will be exposed. can be used. It must not be adversely affected by the pressure medium with which the device is to he used. and must be impermeable to it. For many purposes. a glass-filled melamine is a suitable material. The substrate carries terminal contact conductor areas 14. which are shown as inserted in a conventional printed circuit connector I6. through which connections are made for power supply and signal output.  
  The substrate 12 carries a land of conductive material constituting a fixed plate 22. formed on the substrate by conventional printed circuit methods. Surrounding the fixed plate 22 and separated from the pe riphery thereof by a space 24, is a diaphragm 26 of an electrically conductive material. preferably a thin. flexible metal, sealed to the substrate. The diaphragm has an axial peripheral flange 28 by which its central portions are spaced away from the fixed plate sufficiently so that it will not touch the fixed plate under the maximum excursions of the diaphragm in response to the pressure changes which are to be sensed. A cap [8 preferably made of a dielectric material is hermetically sealed to the substrate. enclosing the diaphragm 26 and forming around it a chamber 20.  
  In the form of the device shown in the drawings. a short section of tubing 30 is affixed to the substrate. in communication with a hole aperture 32 running through the substrate into the annular area between the periphery of plate 22 and the flange 28 of diaphragm 26. The tube 30 provides a port by which a suitable hose or other tubing connection can be made. to introduce a pressure medium into the space enclosed between the diaphragm 26 and the substrate I2.  
  A similar tube 34. communicating with another opening running through the substrate 12, gives a port into the chamber 20 outside the diaphragm 26 and provides means for introducing a desired pressure medium into that space. It will be understood that it is not essential to have this port extend through the substrate. it can equally well be made through a wall 37 of the cap 18.  
  The diaphragm is thus subject to the difference of the pressures communicated through the tube connections 30 and 34. and due to its flexible nature. will change its spacing with respect to fixed plate 22. in response to the pressure differentials which it experiences.  
  The necessary components 35 for an electronic circuit utilizing the variable capacitance arising from this change in spacing. are mounted on the substrate l2 closely adjacent to the cap 18. as shown particularly in FIG. I. Appropriate circuits for this purpose are well known. and do not constitute a part of the present in- \eltllull A suitable circuit has been shown in US. Pat. N .rsosasu. referred to above. Printed circuit or other conductors can be prmidcd on the reverse side of substrate II to interconnect the circuit elements. One such conductor. 3h. shown in Fl(i. 3. leads from a wire 38. soldered to the diaphragm at 40. and makes connection to appropriate elements of the oscillating circuit at point 42. (&#39;onductor 44 leads from a connection to the oscillating circuit at 46 to a connection through the substrate at 48 leading to the lixed plate 22.  
  &#39;lhc structure described provides in very small si/e a de\icc which is compact. light in weight. economical to manufacture. and which has considerable \crsatility. It can be used in the manner which has been described. to sense the difference between two pressures communicated to the tube connections 30 and 34. thus functioning as a differential gage. It may also be used as a normal pressure gage. by leaving one connection open to atmosphere. and applying a pressure to be sensed to the other connection. As still another alternative. one ofthe connections may be pumped down and sealed off as a partial vacuum. in which case. if the other connection is exposed to atmosphere. the device functions as a barometer. or if a pressure medium is connected to it. it functions as an absolute pressure gage.  
  ln description of the tubing connection 30. it was noted that the opening through the substrate is into the area at or near the periphery of the fixed plate 22. This arrangement is preferred. because to the extent that removal of some ofthe plate 22 is required to give access to the contained space. it is desirable to have that reduction in plate area made in a region where it causes a minimum loss in the capacitance change occurring with a given change in pressure. Maximum movement of the diaphragm with respect to the fixed plate takes place at the center of the diaphragm. so it is not desir able to remove conductive material from the fixed plate for a port in that region; the location at the periphery. where little change takes place in spacing between facing parts of the diaphragm and fixed plate. is preferable.  
  Making the cap I8 of a dielectric material has the virtue of avoiding an undesirable stray capacitance from the diaphragm to the housing. such as would be present if a metal cover were used. On the other hand. if shielding from external fields is necessary. a metal cap may be used: in such a case it would preferably be made larger. so that its walls would be spaced further from the diaphragm than the cap shown in Fl(i. 2. in order to reduce the stray capacitance between cap and diaphragm.  
 I claim;  
  I. A \ariable capacitance type pressure transducer comprising;  
 a substrate of dielectric material.  
 a land of electrically conductive material affived thereto.  
 a flexible electrically conductive diaphragm covering said land. spaced therefrom. and sealed to said sub strate. said land and said diaphragm forming two plates of a capacitor.  
 a cap covering said diaphragm. spaced therefrom and sealed to said substrate.  
 a port providing communication for a pressure medium to the otherwise closed volume surrounding said diaphragm and defined by said cap and said substrate. from the space external thereto.  
 a port providing communication for a pressure medium to the otherwise closed volume defined by said diaphragm and said substrate. from the space external thereto. and electrical circuit means mounted on said substrate. connected to said diaphragm and to said land. and adapted to produce a signal having characteristics varying as a function of the capacitance between said diaphragm and said land.  
 2. A variable capacitance type pressure transducer comprising;  
 a substrate of electrically non-conductive material impermeable to gas. said substrate having first and second faces spaced apart by the thickness of said substrate.  
 a land of electrically conductive material affixed to said first face.  
 a thin flexible electrically conductive diaphragm impermeable to gas. hermetically sealed to said first face in spaced apart relation to and covering said land. said land and said diaphragm forming two plates of an electrical capacitor.  
 a cap comprised of a gas impermeable material hermetically scaled to said first face in spaced apart relation to and covering said diaphragm.  
 an aperture running through the thickness of said substrate at a position on said first surface which is embraced by said diaphragm.  
 gas porting means for the otherwise closed volume surrounding said diaphragm which is defined by said cap and said substrate. and  
 electrical circuit means mounted on said substrate. connected to said land and said diaphragm. and adapted to produce a signal having characteristics varying as a function of the capacitance between said land and said diaphragm.  
 3. A variable capacitance type pressure transducer comprising:  
 a substrate of electrically non-conductive material impermeable to gas. said substrate having first and second faces spaced apart by the thickness of said substrate.  
 a land of electrically conductive material affixed to said first face.  
 thin flexible electrically conductive diaphragm impervious to gas. hermetically sealed to said first face in spaced apart relation to and covering said land. said land and said diaphragm forming two plates of an electrical capacitor.  
 cap comprised of a gas impermeable material hermetically sealed to said first face in spaced apart relation to and covering said diaphragm.  
 a first aperture running through the thickness of said substrate at a position on said first face which is embraced by said diaphragm.  
 a second aperture running through the thickness of said substrate at a position on said first face between the inner extremity of said cap and the outer extremity of said diaphragm.  
 and tubular fittings mounted on said second face. each connecting respectively with one of said apertures,  
 4. A variable capacitance type pressure transducer comprising:  
 3ssun09 a substrate of electrical! non-conducthe material riphcr} of said land.  
 6. A variable capacitance type pressure transducer comprising:  
 a substrate ol&#39;electrically nonconducthc material ima land of electrically conductive material al&#39;fised to q permeable to gas. said substrate liming first and said first face. second faces spaced apart b the thickness of said a thin flexible electrically conductive diaphragm imsubstrate.  
 perrious to gas. hermetically sealed to said first a land ofelectrically conductive material affi\ed to face in spaced apart relation to and covering said said first face. land. said land and said diaphragm forming tno to a thin llexible clectricallt conductive diaphragm implates of an electrical capacitor. penious to gas. hermeticall) sealed to said first a cap comprised of a gas impermeable material her face in spaced apart relation to and covering said mctically sealed to said first face in spaced apart land. said land and said diaphram forming two relation to and covering said diaphragm. plates of an electrical capacitor.  
 a first aperture running through the thickness of said s a cap comprised of a gas impermeable material hersubstrate at a position on said first face which is mctically sealed to said first face in spaced apart embraced by said diaphragm. relation to and covering said diaphragm.  
 a second aperture running through the thickness of a first aperture running through the thickness of said said substrate at a position on said first face besubstrate at a position on said first face \shich is t\\cen the inner extremity of said cap and the outer 3o embraced by said diaphragm. estremit of said diaphragm. a second aperture running through the thickness of electrical components mounted on said substrate said substrate at a position on said first face bc forming a circuit rcsponsixe to capacitance tween the inner extremity of said cap and the outer changes to develop an output signal the characterextremity of said diaphragm. istics of which change as a function of capacitance 35 electrical components mounted on said substrate change. and  
 electrical conductors running from said land and said diaphragm to said electrical components.  
 5. Apparatus according to claim 3 wherein said first aperture runs through the thickness of said substrate at a position on said first face which is embraced by said diaphragm and is immediately adjacent the outer pcfrom said diaphragm to said electrical components.  
 (ill