Position sensor based on measuring capacitance

The armature of a position sensor has one or more electric current conductors adapted to be moved into and out of a gap between two capacitor electrodes. The capacitance between the capacitor electrodes is measured to determine the position of the armature. Electric current in the armature results from electric charges moving in the electric current conductors. The electric current conductors are adapted to restrict electric current therein to directions approximately perpendicular to the surfaces of the capacitor electrodes. Preventing current flow parallel to the surfaces of the capacitor electrodes in the electric current conductors makes the capacitance measurement insensitive to armature movement other than movement that moves the electric current conductors into or out of the gap. The position sensor is simpler and less expensive to make than known capacitance based position sensors.

FIELD OF THE INVENTION

This invention relates to linear or angular position sensors for applications in such as consumer appliances and automobiles.

BACKGROUND OF THE INVENTION

Variable capacitors are well known. Capacitors that vary their capacitance with rotation of an armature were used in radio tuners for many years. Combining a capacitor that varies its capacitance with rotation with a capacitance measuring device provides an angular position sensor.

CMOS integrated circuits for measuring capacitance are available from several sources. Micro Sensors of Costa Mesa, Calif. supplies the MS3110 integrated circuit which measures capacitance.

Of the known position sensing means, capacitance sensing is advantageous for being inherently insensitive to temperature, having an inherently linear output, not requiring permanent magnets, and allowing a wide variety of materials to be used in the sensor structure.

Prior to Applicant's invention, no low cost position sensors based on measuring capacitance were suitable for such as throttle position sensing in automobiles. Low cost capacitance position sensing has been limited to micromachined devices such as accelerometers and in diaphragm based pressure sensors in which the deflection of the diaphragm varies a capacitance. One reason sensors such as throttle position sensors and linear position sensors are not based on capacitance measurement is that known low cost capacitance based sensors are very sensitive to movements unrelated to the coordinate of the position being measured such as movements that place parts of the armature closer or farther from the capacitor electrodes.

Accordingly, there is an unmet need for an inexpensive linear or angular position sensor which is responsive by indicating positions along a direction or which indicates an angle of rotation but is unresponsive to other movements.

A reason prior art capacitance based position sensors are responsive to movements unrelated to the coordinates being measured is because such movements usually cause armature tilting, which moves a part of a movable electrical conductor of the armature nearer to one of the fixed capacitor electrodes while moving another part of the same movable electrical conductor nearer to a different fixed capacitor electrode, which greatly affects the capacitance.

Printed circuit boards are made in large numbers by chemically etching copper plated substrate to remove material from areas where conductors are not desired and drilling and plating holes to make connections between conductors on different layers. Turek et al. in U.S. Pat. No. 5,891,528 teaches that printed circuit boards with metallized holes can be formed by plasma spraying a conductive metal after preparing the substrate to keep the plasma sprayed metal from adhering to areas where metallization is not desired.

It is well known to plate metal on plastics by exposing plastic to vaporized metal. Plastics may also be metal plated by chemical vapor deposition as described by U.S. Pat. Nos. 5,191,099 to Gladfelter et al. and 6,399,772 to Shin et al.

A general object of this invention is to provide a low cost sensor offering superior performance and overcoming certain disadvantages of the prior art.

SUMMARY OF THE INVENTION

In accordance with the invention, an armature of a capacitance based position sensor restricts or prevents electric current flow in directions parallel to surfaces of capacitor electrodes while more freely permitting electric current flow in directions perpendicular to the surfaces of the capacitor electrodes. For controlling the electric current direction the armature may comprise a multiplicity of electric current conductors insulated from each other or, alternately, the armature may comprise a material having high impedance to electric current flow such as a high dielectric constant ceramic or an electrically conducting polymer. It is believed that providing a multiplicity of metallic electric current conductors in an insulating matrix is preferable to using a high impedance electric current conducting material because it is believed to be less expensive to make and to be more robust mechanically. Herein, “electric current” is defined to encompass conventional electric current in metallic and polymer electrical conductors and also to encompass displacement current in dielectric materials and “electric current conductor” is defined to encompass objects or materials that conduct conventional electric current and to also encompass objects or materials that have high dielectric constants that enable them to conduct displacement current.

Further, in accordance with certain embodiments of the invention, a capacitance based position sensor comprises two capacitor electrodes separated by a gap, an armature movable in an axial direction into and out of the gap, and a sensor responsive to the capacitance between the two capacitor electrodes. The armature is adapted to freely conduct electric current perpendicular to the capacitor electrodes and restrict or prevent electric current parallel to the capacitor electrodes, whereby the capacitance between the electrodes is proportional to the area of electricity conducting armature located between the capacitor electrodes.

Further, in accordance with the aforementioned certain embodiments of the invention, the aforementioned restricting or preventing electric current in directions parallel to the capacitor electrodes causes the response of the position sensor to armature movement in directions perpendicular to said axis (in cross-axis directions) to be small whereby the armature can fit loosely and move in cross axis directions without substantially affecting the measurement of the axial position of the armature.

Further, in accordance with a first embodiment of the invention, two fixed coaxial tubes separated by a gap are capacitor electrodes. A tubular armature moves axially into and out of the gap and causes the capacitance between the two capacitor electrodes to vary linearly with the armature position. A sensor measures the capacitance. The combination provides an inexpensive linear position sensor.

Further, in accordance with a second embodiment of the invention, two fixed parallel plates separated by a gap are capacitor electrodes. An armature rotates about an axis into and out of the gap and causes the capacitance between the two capacitor electrodes to vary linearly with the angular position of the armature. A sensor measures the capacitance. The combination provides an inexpensive angular position sensor.

Further, in accordance with a third embodiment of the invention, two parallel plates separated by a gap are capacitor electrodes. An armature moves along an axis into and out of the gap and causes the capacitance between the two electrodes to vary linearly with the axial position of the armature. A sensor measures the capacitance. The combination provides an inexpensive linear position sensor.

Further, in accordance with certain embodiments of the invention, an armature comprises a multiplicity of electric current conductors movable into and out of a gap between two capacitor electrodes whereby the capacitance varies linearly with the fraction of the conductors in the gap between the capacitor electrodes and is insensitive to armature movements that do not change the fraction. The capacitance is measured to provide an inexpensive position sensor.

Further, in accordance with certain embodiments of the invention, an armature comprises a multiplicity of electric current conductors movable between two capacitor electrodes, and wherein each electric current conductor when it is between the two capacitor electrodes has two surfaces each extending parallel to and in close proximity to a respective one of the fixed capacitor electrodes. The capacitance between the capacitor electrodes is measured to provide a position sensor that is less sensitive to cross-axis armature movement than prior art capacitance based position sensors.

Further, in accordance with certain embodiments of the invention, the armature moves between two capacitor electrodes and the aforementioned certain embodiments of the invention comprise material adapted to minimize the effect of temperature on the capacitance between the capacitor electrodes.

Further, in accordance with certain embodiments of the invention, the capacitance sensor and the capacitor electrodes are incorporated into a unitary plastic molding.

Further, in accordance with certain embodiments of the armature of the position sensor of the invention, the armature comprises a printed circuit board on which two arrays of copper pads are formed on opposite sides of the printed circuit board by a chemical etching process suitable for making printed circuits. The pads are arranged as pairs, each pad of a pair located opposite its mate on the other side of the printed circuit board. Holes are made at the centers of the pads and are plated in the conventional way to electrically connect the two pads of the pair. The pairs of pads each connected by a plated hole constitute a multiplicity of electric current conductors each adapted to allow electric current to flow by way of the plating on the holes between pads of a pair in directions perpendicular to the surfaces of the printed circuit board. Electric current does not flow substantial distances parallel to the surface of the printed circuit board because adjacent pads do not contact each other. Therefore, the only current flow from a pad is to its mating pad, thereby providing substantially unidirectional electric currents in the armature of the invention.

Further, in accordance with a second embodiment of the armature of the position sensor of the invention, the armature comprises a matrix molded of plastic retaining an array of electric current conductors. The plastic matrix preferably stands proud of the conductors to provide low friction between the armature and the capacitor electrodes. It should be noted that the position measurement is not affected by a single electrical contact between an electric current conductor and a capacitor electrode. A conductor contacting both capacitor electrodes would cause failure.

Further, in accordance with the aforementioned second embodiment of the armature of the invention, the armature comprises a plastic molding shaped to fit between two capacitor electrodes, one side being in close proximity to one capacitor electrode and another side being in close proximity to the other capacitor electrode. The plastic molding also comprises an array of small holes extending between the two aforementioned sides. A metal such as aluminum is vapor deposited on both of the aforementioned sides and the surfaces of the holes, thereby forming the electrical equivalent of the plated feedthroughs formed by drilling and plating holes in printed circuit boards. To form pads that are electrically insulated from each other each hole is surrounded by a slight depression. The lands surrounding the depressions may be treated to prevent metal from being vapor deposited on the lands. Alternately, vapor deposited metal is selectively removed from the lands surrounding the depressions by a lapping operation or other suitable removal process. Either process provides a multiplicity of pairs of conducting pads, with the pads of each pair connected electrically by the plating on the hole therebetween and each pair of pads insulated from its neighboring pads by the metal free lands.

Further, in accordance with a third embodiment of the armature of the position sensor of the invention, the armature comprises high dielectric constant material or high resistance material that extends parallel to and in close proximity to the two fixed capacitor electrodes. In armatures of this design the electric current flow in the armature is primarily in directions perpendicular to the surfaces of the capacitor plates because the high impedances of the armature material provides greater opposition to current flow over the longer distances of flow parallel to the capacitor electrodes.

Further, in accordance with the invention, the capacitance sensor is manufactured in an assembly unitary with an electrical connector and the capacitor electrodes. This provides a particularly simple manufacture in which all electrical components are included in a single plastic molding.

A complete understanding of this invention may be obtained from the description that follows taken with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now toFIGS. 1,7, and13, three illustrative embodiments of the invention are shown. Each embodiment comprises: a plastic molding unitary with an electrical connector20; a capacitance sensor40connected to measure the capacitance between two capacitor electrodes (52-54,152-154, and252-254); and an electric current conducting armature (60,160,260) movable into and out of a gap between the electrodes whereby the capacitance between the electrodes is a measure of the fraction of the gap occupied by the electric current conducting portion of the armature. The armature is adapted to conduct electricity only in directions perpendicular to the electrodes by providing a multiplicity of electric current conductors insulated from each other. The electric current conductors (64,164,264) extend from a first surface in close proximity to one of the capacitor electrodes to a second surface in close proximity to the other of the capacitor electrodes.

Continuing with reference toFIGS. 1,7and13, linear position sensors10and210and angular position sensor110provide a position signal through connector pins26and28indicating the linear or angular position of the armature. Linear position sensor10comprises armature60, connector20and capacitance sensor assembly30. The output of linear position sensor10is an electric signal indicating the axial position of armature60with respect to capacitor electrodes52and54. Angular position sensor110comprises connector20, capacitance sensor assembly130, capacitor electrode assembly150, and armature160. Armature160is adapted to be rotated by a “D” shaft the angular position of which is to be measured. Linear position sensor210comprises armature260, connector20and capacitance sensor assembly30. The output of linear position sensor210is an electric signal indicating the axial position of armature260with respect to capacitor electrodes252and254. Capacitance sensor40of capacitive position sensor10,110or210indicates the fraction of the gap between the capacitor electrodes that is filled by the electric current conductors of the armature. That fraction is proportional to the position which is to be measured.

It will be appreciated as the description proceeds that the invention may be implemented in different embodiments.

Proceeding now to describe the first embodiment of the position sensor of the invention with reference to FIGS.1through6: Linear position sensor10comprise electrical connector20, capacitance sensor assembly30, capacitor electrode assembly50, and armature60that is axially movable relative to capacitor electrode assembly50.

Electrical connector20comprises connector pins26and28in a molded shroud24. Capacitance sensor assembly30is unitary with electrical connector20and capacitor electrode assembly50. Capacitance sensor assembly30includes extensions36and38of connector pins26and28respectively, reference capacitor42, extensions32and34of capacitor electrodes52and54respectively, and capacitance sensor40. Capacitor electrode assembly50comprises capacitor electrodes52and54. Armature60comprises a tube-shaped end62having openings for containing electric current conductors64, an extension66and a coupling68with opening68′ to which a movable part is connected for its movement to be sensed. The elements illustrated by dashed or hidden lines are hidden from view by the plastic molding.

Connector pins26and28are preferably made by the processes conventionally used for making connector pins from a material such as gliding metal or other pin material suitable for connection by wire bonding to capacitance sensor40.

Reference capacitor42, if required, is preferably a capacitor chosen for having a stable capacitance over the range of operating temperatures. Mica and certain ceramic capacitors are specified by their manufacturers to be stable over wide temperature ranges. Reference capacitor42has wire bonding pads44and46for connecting to capacitance sensor40by such as ultrasonic wire bonding.

Capacitor electrodes52and54comprise tubes made of a metal such as aluminum or copper alloy. If aluminum is selected it is preferably anodized to prevent galling and to minimize friction. Capacitor electrodes52and54are located on a common axis CL with electrode52inside electrode54. Capacitor electrode52may have a bevel at edge56over which armature60passes.

If present, the bevel at edge56is designed to smooth the capacitance deviations from linearity which otherwise happen because of the granularity of the conductivity of armature60from the multiplicity of conductors. Making the length of the bevel at edge56equal to approximately one half of the axial dimension of one of the conductors64provides smooth output with linear position changes. Capacitor electrodes52and54have extensions32and34respectively extending to the vicinity of capacitance sensor40to which they are connected. Extensions32and34may be unitary with capacitor electrodes52and54or they may be extensions welded to the capacitor electrodes. Capacitor electrodes52and54also have attachments52′ and54′ respectively which extend into the molding for attachment and structural integrity. The inside diameter of capacitor electrode54extends beyond capacitor electrode52to provide a bearing surface for bearing armature60in its movement.

Armature60comprises tube-shaped end62, an extension66, and coupling68. Tube-shaped end62is made of a plastic with a matrix of openings for retaining a multiplicity of electric current conductors64. The preferred material for conductors64is the same material as capacitor electrodes52and54to minimize output change with temperature. The plastic may be any compound suitable for such as insert molded electric connectors. Molded nylon or polyphenylene sulfide with about 30% to 40% filler are believed to be good selections that provide good lubricity and temperature stability. The electric current conductors64may be made from short lengths of drawn metal wire. Alternately, tube shaped end62may be molded in a shape suitable for use with electric current conductors made by depositing vaporized aluminum or by other known processes such as the processes described hereinafter with reference toFIGS. 13 through 17for making electrically conducting pads and feedthroughs. For the case wherein conductors64are short lengths of metal wire or ceramic, they are made with rounded edges by which they are retained. The tube-shaped end62is preferably molded to have lips at the openings of the spaces that retain electric current conductors64. The lips are molded to fit the rounded edges of electric current conductors64for retaining the electric current conductors64.

If the electric current conductors64are made of a high dielectric constant ceramic or low conductivity material then two possible designs must be considered. The first design is exactly like the design with metal electric current conductors64except that the electric current conductors64are made of a different material. This design offers the advantages of totally preventing electric current parallel to the capacitor electrodes and possibly lower friction because of the lubricity of the thermoplastic polymer matrix. A preferred material for the first design is a ceramic selected to have the largest dielectric constant available at an acceptable cost. In the alternate design the tube-shaped end62and the electric current conductors64are a unitary ceramic or plastic molding. The material for the second design is selected to have sufficient mechanical strength and also to have an impedance no less than necessary to make the impedance across the gaps between the electric current conductors64and capacitor electrodes52and54much greater than the impedance across the electric current conductors64. Having no less than the necessary minimum impedance minimizes the electric current in directions parallel to the capacitor electrodes.

Electrical connector20, capacitance sensor assembly30, and capacitor electrode assembly50are preferably made by the following insert molding process: Connector pins26and28which are unitary with extensions36and38, capacitor electrode extensions32and34, reference capacitance42, and capacitance sensor40are placed in a fixture that may be used later as a part of the mold and are electrically connected together by such as ultrasonic wire bonding. A small amount of potting material is placed at and near the wire bonds to protect the capacitance sensor and bonded wires. Thermoplastic polymer is then injection molded around the aforementioned components to form a unitary assembly.

The materials and methods referred to hereinabove are only suggestions and others may be substituted by those skilled in the relevant arts.

The operation of the linear position sensing system10of the invention will now be described with reference toFIGS. 1 through 6. In operation of the system, the capacitance between capacitor electrodes52and54is small when the electric current conductors64of armature60are not between capacitor electrodes52and54. When electric current conductors64move into the gap between capacitor electrodes52and54, the gap between the capacitor electrodes52and54is effectively reduced where the electric current conductors64are in close proximity to the capacitor electrodes52and54. For example, the gap between capacitor electrodes52and54may be one or two millimeters which might result in a capacitance of approximately one picofarad between the two electrodes in the absence of armature60. The two gaps between electric current conductors64and capacitor electrodes52and54may total 0.05 millimeters to 0.1 millimeters so that the capacitance of the exemplary sensor may increase by a factor of five to twenty between an empty gap and when the electric current conductors64of the armature are in the gap between the capacitor electrodes.

The capacitance varies linearly with the position of armature60and is sensed by capacitance sensor40, which responds by providing a signal that indicates the axial position of armature60relative to the capacitor electrodes.

The armature also moves in directions perpendicular to the common axis of capacitor electrodes52and54. This cross axis movement will happen because the surfaces that rub together are not perfectly smooth and for other reasons. Cross-axis movement will cause each of the electric current conductors64to move within the gap between capacitor electrodes52and54. To the extent that the fringing field at the periphery of the electric current conductors64can be neglected, the capacitance will not be affected by the movement of electric current conductors64within the gap. By making the area of the fringing field small relative to the surface areas where electric current conductors64face the capacitor electrodes, the effects of cross-axis movement are made small. Therefore, the multiplicity of metallic electric current conductors64of armature60cause the capacitance between capacitor electrodes52and54to be highly independent of the cross-axis position of armature60while being linearly dependent on the axial position of armature60. Accordingly, the invention meets the unmet need for a capacitance based position sensor that is insensitive to movements that are distinct from movements in the axial direction.

The following example is presented to illustrate and explain the operation of a high impedance electricity conducting material to limit the electric current to the direction perpendicular to the capacitor electrodes in the case when tube-shaped end62conducts electric current and there are no discrete the electric current conductors64. Assume that the diameter of the tube-shaped end is one centimeter, it is one centimeter long, its thickness is one millimeter, and it is made of material having an impedance of one megohm-centimeter in operation. Assume the frequency is such that the capacitive reactance between capacitor electrodes52and54is one megohm when the gap is empty and that the capacitive reactance is 0.1 megohm when tube-shaped end62fills the gap between capacitor electrodes52and54. The surface area of tube-shaped end62is approximately 3 square centimeters. The material impedance times the thickness divided by the area is the reactance which calculates to be 0.03 megohms between its inside diameter and its outside diameter. This is small relative to the smallest reactance including the gap which is approximately 0.1 megohm. Therefore, the measured capacitance is approximately the same as if the reactance of tube-shaped end62were negligible (i.e. if end62comprised a number of metallic conductors64).

Continuing, now, the example by considering current flow parallel to the surface of capacitor electrodes52and54. The circumference of the tube-shaped end is approximately three centimeters and the length is approximately one centimeter. These distances result in reactances of approximately three or more megohms. These reactances are for tangential paths from end to end or from the side at one diameter to a side at the opposite diameter. These reactances are much greater than the reactances seen by current flowing in the radial direction. It follows that the predominant current flow will be in the radial direction. The high impedance electric current conducting material operates to similar effect as if it were divided into pieces like conductors64and the pieces were insulated from each other as described hereinabove.

Proceeding now to describe the position sensor of the invention with reference to FIGS.7through12: Angular position sensor110comprises electrical connector20, capacitance sensor assembly130, capacitor electrode assembly150, and armature160rotatable between capacitor electrodes152and154.

Electrical connector20comprises connector pins26and28in a molded shroud24. Capacitance sensor assembly130is unitary with electrical connector20and capacitor electrode assembly150. Capacitance sensor assembly130includes extensions36and38of connector pins26and28respectively, reference capacitance42, extensions132and134of capacitor electrodes152and154respectively, and capacitance sensor40. Capacitor electrode assembly150comprises capacitor electrodes152and154and cover192. Armature160comprises a disk162retaining electric current conductors164, an extension and bearing166and a central opening168with flattened side168′ to which a rotatable part is connected for its angular position to be sensed. The elements illustrated by dashed or hidden lines are hidden from view by the plastic molding.

Shroud24, connector pins26and28, capacitance sensor40, and reference capacitor42may be the same as described hereinabove with reference toFIGS. 1 through 6.

Capacitor electrode assembly150comprises a plastic molding unitary with electrical connector20and capacitance sensor assembly130. Capacitor electrode assembly150also comprises capacitor electrodes152and154, openings182(only one is illustrated), channels184and186and cover192. Openings182accommodate a shaft for engaging armature160. Channels184provide entrance for disk162and surfaces for engaging extensions194and pawls196of cover192. Ledges (not illustrated) in channels184engage pawls196for latching cover192into place. Instead of or in addition to retention by pawls196, cover192may be attached by known means for attaching plastic covers which includes by adhesive, acoustic welding, friction welding, or other means known to be suitable by those skilled in the relevant arts. Channels186provide entrance for extensions and bearings166of disk162and for extensions198of cover192. Extensions198of cover192engage extensions and bearings166of disk162for keeping armature160in its predetermined location. Cover192covers the end of capacitor electrode assembly150and may include a seal (not illustrated) to keep dirt and liquid away from disk162.

Capacitor electrodes152and154comprise flat electrodes made of a metal such as aluminum or a copper alloy. If aluminum is selected it is preferably anodized to prevent galling and reduce friction. Capacitor electrodes152and154are positioned parallel to each other are spaced to form a gap. Capacitor electrodes152and154may have a slight bevel at the edges156and158over which the electric current conductors164of armature160pass. The bevel smooths the fluctuations in the capacitance signal which would otherwise happen when armature160moves because of the granularity of the conductivity of armature160. Alternately, capacitor electrodes152and154may have saw shaped edges156and158as illustrated inFIGS. 9 and 10. The saw shape at edges156and158smooths the fluctuations in the capacitance signal caused by the granularity of the conductivity of armature160.

Capacitor electrodes152and154have extensions132and134extending to the vicinity of capacitance sensor40to which they are connected. Extensions132and134may be unitary with capacitor electrodes152and154or they may be extensions welded to the capacitor electrodes. If extensions132and134are welded to the capacitor electrodes they may be any material known by those skilled in the relevant arts to be suitable for connection by wire bonding to capacitance sensor40and welding to the material selected for capacitor electrodes152and154.

Armature160comprises a disk162having a portion made conductive by electric current conductors164, two extension and bearing surfaces166, and a central opening168with a flat surface168′ for engagement to and rotation with a “D” shaped shaft for sensing the angular position of the “D” shaped shaft. Disk162is molded of electrically insulating plastic with an array of openings for containing a multiplicity of electric current conductors164. The plastic of which disk162is molded may be any plastic suitable for retaining electric current conductors164. A material having the same thermal expansion coefficient as the material of which capacitor electrode assembly150is molded is preferred. Disk162is preferably molded to have lips at the openings of the cavities shaped to fit the rounded edges at the ends of electric current conductors164to retain electric current conductors164in armature160.

A preferred material for electric current conductors64is drawn metal wire cut into short lengths, preferably by an orbital cutter which rounds the edges of the cut surfaces. The openings in disk162have molded lips to engage the rounded edges and retain the electric current conductors64. An alternate preferred material for electric current conductors164is an electrically conducting polymer having a thermal expansion coefficient approximately the thermal expansion coefficient of the plastic of which capacitor electrode assembly150is molded to minimize sensitivity to temperature. A second alternate preferred material for electric current conductors164is a high dielectric constant ceramic. Other alternates for electric current conductors64are plated areas made by the processes described hereinafter with reference toFIGS. 13 through 17for making electrically conducting pads and feedthroughs.

Two designs must be considered if the electric current conductors164are made of a high dielectric constant ceramic or a conductive polymer. The first design provides a multiplicity of ceramic or polymer electric current conductors164in the matrix of disk162which offers the advantage of preventing electric current flow in directions parallel to the capacitor electrodes and also offers the lubricity of the matrix standing proud of the ceramic electric displacement current conductors164and lubricating the movement of armature162. The preferred material in this first design for the electric current conductors of disk162is the material having the least resistance or reactance available at an acceptable cost.

In the second design for disk162for the case wherein electric current conductors164are made of high dielectric constant ceramic or a conductive polymer, the electric current conductors164are replaced by a single piece of ceramic or polymer. The material for the second design for disk162is selected to have sufficient mechanical strength and also have an impedance no less than is necessary to make the impedance across the gaps between the electric current conductors164and capacitor electrodes152and154much greater than the impedance across electric current conductors164. Having no less than the necessary impedance minimizes the electric current in directions parallel to the capacitor electrodes as described hereinabove.

Electrical connector20with capacitance sensor assembly130and capacitor electrode assembly150are preferably made by the following insert molding process: Connector pins26and28which are unitary with extensions36and38, capacitor electrodes152and154which are unitary with or welded to extensions132and134, reference capacitor42, and capacitance sensor40are placed in a fixture that may become part of the mold and are electrically connected together by such as ultrasonic wire bonding. A small amount of potting material is placed over the areas of wire bonds to protect the capacitance sensor and bonded wires. The assembly of the mold is completed and thermoplastic polymer is injection molded around the aforementioned components to form the completed assembly of electrical connector20, capacitance sensor assembly130and capacitor electrode assembly150.

The materials and methods referred to hereinabove are only suggestions and others may be substituted by those skilled in the relevant arts.

The operation of the angular position sensing system110of the invention will now be described with reference toFIGS. 7 through 12. In operation of the system, the capacitance between capacitor electrodes152and154is small when electric current conductors164of armature160are outside of the gap between capacitor electrodes152and154. When electric current conductors164move into the gap between capacitor electrodes152and154, the gap between the capacitor electrodes is effectively reduced where the electric current conductors164are in the gap between capacitor electrodes152and154. For example, the gap between capacitor electrodes152and154may be one or two millimeters which might result in a capacitance of approximately one picofarad between the two electrodes in the absence of conductors164of armature160. The gap between electric current conductors164and capacitor electrodes152and154may be a approximately 0.05 millimeters to 0.1 millimeters so that the capacitance contributed by the areas of capacitor electrodes152and154that are in close proximity to electric current conductors164are increased by a factor of five to twenty (in the case of this example) relative to the capacitance between the same areas of capacitor electrodes152and154without the electric current conductors164.

The capacitance varies linearly with the angle of rotation of armature160and is sensed by capacitance sensor40, which provides a signal that indicates the angular position of armature160relative to the capacitor electrodes.

Armature160may also move in directions perpendicular to the surfaces of capacitor electrodes152and154and in other ways that are not pure rotation about the axis of armature160. This movement will happen because the bearing surfaces166are not perfectly smooth and for other reasons. The perpendicular movement will cause each of the electric current conductors164to move within the gap between capacitor electrodes152and154in directions perpendicular to capacitor electrodes152and154. To the extent that the fringing field at the periphery of the electric current conductors164can be neglected, the capacitance will not be affected by the movement of electric current conductors164within the gap. The ratio of the area covered by the fringing field relative to the area opposed by conductors164is reduced if either: 1) the gap is made smaller, or 2) the area of electric current conductors164facing capacitor electrodes152and154is made larger. By making the area where the fringing field enters the capacitor electrodes small relative to the surface areas where electric current conductors164oppose the capacitor electrodes, the effects of cross-axis movement are made small. Therefore, the multiplicity of metallic electric current conductors of armature160cause the capacitance between capacitor electrodes152and154to be highly independent of the movements of armature160that are not pure rotation while being linearly dependent on the rotational position of armature160. Accordingly, the invention meets the unmet need for a capacitance based angular position sensor that is insensitive to movements that are distinct from pure rotation.

Proceeding now to describe the third embodiment of the position sensor of the invention with reference to FIGS.13through17: Linear position sensor210comprise electrical connector20, capacitance sensor assembly230, capacitor electrode assembly250, and an armature260that is axially movable relative to capacitor electrode assembly250. Alternately, armature360(illustrated inFIG. 17) may be substituted for armature260.

Electrical connector20comprises connector pins26and28in a molded plastic shroud24. Capacitance sensor assembly230is unitary with electrical connector20and capacitor electrode assembly250. Capacitance sensor assembly230includes extensions36and38of connector pins26and28respectively, reference capacitor42, extensions232and234of capacitor electrodes252and254respectively, and capacitance sensor40. Capacitor electrode assembly250comprises capacitor electrodes252and254insert molded into the unitary assembly comprising capacitance sensor assembly230, electrical connector20, and capacitor electrode assembly250. Armature260comprises a paddle like end262having electric current conductors264Armature260also comprises extension266having an opening268for connecting to an element that is to have its position sensed by linear position sensor210. Elements illustrated by dashed or hidden lines are hidden from the viewer by the plastic molding.

Shroud24, connector pins26and28, capacitance sensor40, and reference capacitor42, may be the same as described hereinabove with reference to connector20and capacitance assembly30.

Capacitor electrodes252and254comprise flat plates made of a metal such as copper or aluminum or other metal selected by those skilled in the relevant arts. If aluminum is selected, it is preferably anodized to minimize friction and galling. Capacitor electrodes252and254are located parallel to each other defining a gap in which armature260moves. Capacitor electrodes252and254may have bevels at edges256and258which make the response more linear by smoothing the variations from linearity which may be caused by the granularity of the conductivity of armature260. Making the axial length of the bevel at edges256and258extend approximately half of the axial dimensions of one of the electric current conductors264provides a smooth response. Capacitor electrodes252and254have extensions232and234respectively extending to the vicinity of capacitance sensor40to which they are connected. Extensions232and234may be unitary with capacitor electrodes252and254or they may be extensions welded to the capacitor electrodes. If extensions232and234are welded to the capacitor electrodes they may made of be any material known by those skilled in the arts to be suitable for wire bonding to capacitance sensor40and welding to the material selected for capacitor electrodes252and254.

In a first preferred design armature260is formed of double sided printed circuit board material comprising paddle like end262, an extension266, and opening268in extension266. A matrix of pads264is formed on paddle like end262by the processes used to make pads on printed circuit boards.

One method for forming pads264is by applying a photoresist on double sided printed circuit material. The photoresist is removed over the areas between pads264. The pads are then formed by etching away the copper plate where the photoresist is absent while the pads are protected by photoresist. The pads are sized and located to obtain approximately equal increments of area between the capacitor plates for equal armature movements. The pads264may have shapes other than the round shapes illustrated. Diamond shaped pads placed very close to each other provide proportional increments of area when the armature moves and make maximum use of the areas of the capacitor electrodes. However, in a linear position sensor made with round pads as illustrated inFIG. 13the granularity was not observable. Holes are drilled through each pair of pads and the holes are plated to make electrical connections between the pads. The material of which armature260is made preferably has the same thermal expansion coefficient as the material of which capacitor electrode assembly250is molded to minimize the effect of temperature on the output signal from linear position sensor210. The plastic used for armature260may be any printed circuit board material having a suitable thermal expansion coefficient and meeting other requirements for the application such as strength and consistent performance over the required range of operating temperatures. Glass filled epoxy or polyphenylene sulfide with about 30% to 40% glass filler are believed to be suitable.

A second method for manufacturing armature260is by injection molding a suitable polymer into the form of armature260with the holes between the pads formed in the molding process. An adhesion inhibitor is applied to the molding over areas where metallization is not desired, and pads264are formed and the holes are plated by plasma spraying as described by Turek et al. in U.S. Pat. No. 5,891,521.

Electrical connector20, capacitance sensor assembly230, and capacitor electrode assembly250are preferably made by the following insert molding process: Connector pins26and28which are unitary with extensions36and38, capacitor electrode extensions232and234, reference capacitance42, and capacitance sensor40are placed in a fixture that may become part of the mold and are electrically connected together by such as ultrasonic wire bonding. A small amount of uncured potting material is applied over the wire bonds to protect them. Additional potting material is applied in the vicinity of capacitance sensor40to protect and prevent relative movement between the assembled electrical and electronic components. The mold is completed and plastic is injection molded around the aforementioned components to form the unitary assembly comprising electrical connector20, capacitance sensor assembly230, and capacitor electrode assembly250.

Proceeding now with reference toFIG. 17, armature360is an alternate to armature260. Armature360is molded of thermoplastic polymer in the form of a paddle like end362and an extension366having opening368. Where each electric current conductor364will be a pair of slight depressions are molded with a hole connecting the centers of the depressions. The electric current conductors364are made by plating the depressions and the holes with a conducting metal. The electric current conductor364are sized and located to obtain approximately equal increments of area of electric current conductors364between the capacitor plates for equal armature movements. The electric current conductors364may have shapes other than round. Diamond shaped conductors364placed very close to each other provide proportional capacitance increments when the armature moves and make maximum use of the areas of the capacitor electrodes. The plastic material of which armature360is made preferably has the same thermal expansion coefficient as the material of which capacitor electrode assembly250is molded to minimize the effect of temperature on the output signal from linear position sensor210. The material used for armature360may be any moldable material on which conductors364can be plated, has a suitable thermal expansion coefficient, and meets other requirements of the application for such as strength and consistent performance over the required range of operating temperatures. Glass filled polyester with about 30% to 40% glass filler is believed to be suitable for many applications.

A preferred process for making electric current conductors364is to apply vaporized aluminum heated by a tungsten coil in a high vacuum onto the surfaces of the slight depressions and the holes formed when paddle like end362is molded. This process is preferably done in a conventional flash vaporizing machine. Aluminum deposited on the lands between the electric current conductors364is removed by lapping or any other known process for removing the aluminum not deposited in the depressions and the holes. Alternately, the aluminum may be applied by chemical vapor deposition or by plasma spraying. Alternately to removing unwanted aluminum from the lands, an adhesion inhibitor may be applied to the lands before the vaporized metal is applied. Other known methods for applying electrically conducting material to form electric current conductors364on armature360may be substituted by those skilled in the relevant arts.

The materials and methods referred to hereinabove are only suggestions and other materials and methods may be substituted by those skilled in the relevant arts.

The operation of the linear position sensing system210of the invention will now be described with reference toFIGS. 13 through 17. In operation of the system, the capacitance between capacitor electrodes252and254is small when the electric current conductors264or364of armature260or360respectively are not between capacitor electrodes252and254. When electric current conductors264or364move into the gap between capacitor electrodes252and254, the gap between the capacitor electrodes252and254is effectively reduced at the locations of the electric current conductors264or364between the capacitor electrodes. For example, the gap between capacitor electrodes252and254may be one or two millimeters which might result in a capacitance of approximately one picofarad between the two electrodes in the absence of armature260or360. The gap between electric current conductors264or364and capacitor electrodes252and254may be approximately 0.05 millimeters to 0.1 millimeters so that the capacitance may be increased by a factor of five to twenty (in the example) when the electric current conductors264or364of armatures260or360respectively are moved into in the gap between the capacitor electrodes252and254.

The capacitance varies linearly with the position of armature260or360and is sensed by capacitance sensor40, which provides a signal that indicates the axial position of armature260or360relative to the capacitor electrodes. Capacitance sensor40, as do all capacitance sensors, operates by applying an electric signal across capacitor electrodes252and254and then sensing the resulting electric current. When an electrical signal is applied between capacitor electrodes252and254electric charges move from one of the pads of an electric current conductor264or364that is located in the gap between capacitor electrodes252and254; through the plating on the hole between the pads of the electric current conductor264or364that is located in the gap; and to the other pad of the electric current conductor264or364.

Particular electric charges may move in directions that are not perpendicular to the surfaces of capacitor electrodes252and254, particularly as they move in an electric current conductor264or364between the outer periphery of a pad and the center of a pad in their passage to and from the plating on the holes between the pads. However, the movements in an electric current conductor264or364of charges parallel to the surfaces of capacitor electrodes252and254cancel as electric charges from opposite diameters of a pad simultaneously move between the outer diameters of the pads and the plating on the holes at the center of the pad. Accordingly, in the aggregate, the movement of the electric charges in an electric current conductor264or364is along a path collinear with the perpendicular to capacitor electrodes252and254that passes through the center of the hole between the pads of the electric current conductor264or364.

The armature260or360may also move in directions and ways that are not aligned with the direction of center line CL indicated in FIG.13. These movements may include movement perpendicular to center line CL and may include rotation. Movements in directions or ways other than in the direction of the axis CL along which the position of armature260or360is to be measured are called “cross-axis movements”. Cross-axis movement is limited by capacitor electrodes252and254and by the molded plastic at the sides the gap in which armature260or360moves. Cross-axis movement causes each of the electric current conductors264or364to move within the gap between capacitor electrodes252and254but not into or out of the gap. To the extent that the fringing field at the periphery of the electric current conductors264or364can be neglected, the capacitance will not be affected by the movement of electric current conductors264or364within the gap. Therefore, the areas of the fringing field are made small relative to the surface areas where electric current conductors264or364oppose the capacitor electrodes. The area of the fringing fields is made small by making the gap between conductors264or364and capacitor electrodes252and254small and by minimizing the space between electric current conductors264or364. Therefore, the multiplicity of metallic electric current conductors264or364of armature260or360respectively cause the capacitance between capacitor electrodes252and254to be highly independent of the cross-axis position of armature260or360while being linearly dependent on the axial position of armature260or360. Accordingly, the invention meets the unmet need for an inexpensive capacitance based position sensor that is insensitive to movement not in the axial direction in which position measurement is desired.

Although the description of this invention has been given with reference to particular embodiments, it is not to be construed in a limiting sense. Many variations and modifications will now occur to those skilled in the art. For a definition of the invention reference is made to the appended claims.