Capacitor Transducer

Abstract

A capacitive transducer in which a pair of exciter electrodes connected to an AC source are disposed in common planes to present a gap. A pick-off electrode is disposed in another plane and extends on either side of the gap. A conductive screen is interposed between the exciter electrodes and the pick-off electrode and is connected to ground potential so as to block capacitive coupling between the exciter electrodes and the pick-off electrode except as between discrete areas of the exciter electrodes and an opposite area of the pick-off electrode exposed or coupled through an aperture in the screen.

Parent Case Text

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending application Ser. No. 7,187, filed Jan. 30, 1970, and now abandoned.

Description

BACKGROUND OF THE INVENTION

Capacitance devices are known in which the capacitive coupling between a pick-off electrode arrangement and a pair of exciter electrodes is varied such that a differential effect is attained. In such devices, however, the effects of stray capacitance between the exciter electrodes and the pick-off electrode may be significant, giving rise to inaccuracies, non-linearities or the like which may affect the usefulness of the device particularly in applications such as for measuring relative movements between machine parts. For example, it may be desirable to utilize a differential capacitance device to provide accurate and precise electrical signal outputs in response to minute feeding movements of a machine tool thereby to provide the basis for accurate and precise programming of tool movements so that sequentially produced parts each may be held to close tolerances.

BRIEF SUMMARY OF THE INVENTION

In a preferred embodiment of the invention, an electrically conductive screening plate is used electrically to isolate or decouple a pair of exciter electrodes, on the one hand, and a pick-off electrode on the other hand, except as between discrete areas of the exciter electrodes and an opposed area of the pick-off electrode. For this purpose, the screening plate is connected to ground potential and is interposed between the exciter electrodes and the pick-off electrode to mask the same from each other except through or across the dielectric region, in any operative position of the parts.

BRIFF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is a diagrammatic view of a simple form of differential capacitor embodying the invention coupled to a machine tool element;

FIG. 2 is a similar diagrammatic view of a multicycle differential capacitor having provision of calibration;

FIG. 3 is a diagrammatic view similar to FIG. 2 showing a different method of calibration, and an ambiguity resolver;

FIG. 4 is an exploded diagrammatic view of a differential capacitor similar to FIG. 2 except that it is arranged for rotary rather than linear movement;

FIG. 5 is a diagrammatic view of a modification of the capacitors of FIGS. 2 and 3;

FIG. 6 is an exploded diagrammatic view of a different kind of capacitor;

FIG. 7 shows the electrodes of the capacitor of FIG. 6 developed into a plane;

FIG. 8 is a diagrammatic view of parts cut away of a linear differential capacitor embodying the invention;

FIG. 9 is a diagrammatic showing of still a different form of capacitor for detection of linear movement and in which a stationary pick-off member is used;

FIG. 10 is a diagrammatic showing of still another form of capacitor for detection of angular movement or of longer linear movement; and

FIG. 11 is a longitudinal section taken through a preferred embodiment of the device.

DETAILED DESCRIPTION OF THE INVENTION

In general, differential capacitors have a pair of plates which are excited from the two ends of an AC supply, a movable member which may be described as an interpolator, and a pick-off which gives an electrical signal which changes with the position of the interpolator in relation to the excited plates. In many forms of differential capacitor the pick-off is also the interpolator.

In the differential capacitor shown in FIG. 1 exciter plates 11 and 12 are spaced apart by a gap 13 which is oblique to the direction of movement illustrated by arrow 14 of an interpolator 15, which also acts as a pick-off. The plates 11 and 12 are energized from the two ends respectively of a reference supply comprising a secondary winding 16 of a transformer 17 and it can be seen that the coupling between the interpolator 15 and each of the plates 11 and 12 depends upon the linear position of the interpolator in the direction of arrow 14 by virtue of the changing overlapping plate area. The voltage of this interpolator 15 is a measure of its position and a signal can be obtained from a terminal (not illustrated) connected to it. The interpolator 15 may be surrounded by, but spaced from a shield 20 for minimizing distortion at the ends of the range.

It will be appreciated that the device need not be much longer than the desired range of movement of the interpolator 15, it being only necessary to provide provision for suitable mechanical bearings and appropriate shielding to define the electric field and prevent distortion by the mounting components, for exmple bearings. For example, if the range of movement is 5 inches, the device could be about 6 inches long. Also the angle of the gap 13 can be chosen in accordance with desired rate of change of the pick-off signal.

A simple means for calibration can be provided by a slope correcting plate 18 extending along the length of the capacitor and separated from the plate 11 by a gap 19. This plate 18 is energized from a separate secondary winding 21 of the transformer 17. The coupling between the interpolator 15 and the correcting plate 18 can be adjusted by screwing in or out of conducting mechanical pins 22 toward plate 18 carried by the interpolator 15 to adjust the value of a correcting signal. If a similar arrangement is used at the other side of the capacitor, that is to say a slope correcting plate 23 spaced from the plate 12 by a gap 24 calibration can be effected without moving the electrical center of the capacitor. Further, the plate 23 may be energized from a secondary winding (not shown) of the transformer 17, and conducting mechanical pins 25 may be provided to adjust the coupling between a plate 23 and the interpolator 15. Similarly, the coupling capacitance between the pick-off and each reference electrode can be adjusted to a standard value which can be of advantage when operating such transducers in a caliper mode.

Of course with adjustment, the mechanical pins 22 or the energized correcting plate 18, could be used by itself, rather than together as shown in FIG. 1.

The interpolator 15, as illustrated diagrammatically, is connected to a machine tool element 26, which may be a carriage for supporting a workpiece (not illustrated) or a grinding wheel not illustrated) or the like by mechanical means diagrammatically illustrated as dashed line 27.

Alternatively, the machine tool element 26 may be mechanically coupled to the exciter electrode assembly 11, 12 etc. while the interpolator 15 is held in a fixed position. Further, in this case, it may be advantageous to arrange for a multiplicity of pick-off elements 15 to be mounted on shield 20 and for a switch system to select the appropriate element 15 depending on the position of the moving exciter electrode assembly 11, 12, etc. as it moves past the pick-off elements 15.

The arrangement of FIG. 2 provides for a much greater linear movement, for example, 16 inches, and in this case the two exciter plates are of complementary saw-tooth shape as shown at 28 and 29 separated by a gap 30.

The interpolator 15A comprises a plate 31 genrally similar to the interpolator 15 of FIG. 1 in conjunction with a second similar plate 32, by a non-conductive mounting member diagrammatically illustrated at 33, to move with it and spaced so that the plate 31 will be moving over the central part of one inclined gap or ramp as shown at 34 while the other plate 32 is moving over the peak 35 joining two ramps. Then the second plate 32 will be moving over the central part of the next ramp 36 while the first plate 31 is moving over the peak 35. A simple electrically - or cam-operated switch 37 is arranged to switch a signal from either lead 38 or lead 39 connected respectively to the interpolator plates 31 and 32 to an output terminal 40 depending on which plate is over the central part of a ramp in the position of the interpolator concerned. The plates 28 and 29 are energized from an AC source by a pair of leads 41.

As with the emboidment of FIG. 1, calibration corrections can be made by use of correcting plates 42 and 42A along the edges of the capacitor, and plate 42 can be arranged to be connected to a selected tapping, for example 43, on a secondary winding 44 of a transformer 45 by operation of a selector switch 46. The switch 46 can be selected by an appropriate cam (not illustrated) or by an electrical logic circuit (not illustrated) and the tap that will be selected will depend on the correction required for the particular ramp 34 or 36 over which the interpolator plate 31 or 32 is moving when the cam or logic-circuit controlled switch is operated. There will be a similar arrangement for the correcting plate 42A which may include an additional secondary winding 47 and a switch 48. It will be appreciated, that the interpolator 15A may be mechanically coupled to a machine tool element in a similar manner to interpolator 15 as shown in FIG. 1.

The arrangement of FIG. 3 shows how calibration correction can be affected in a different way by having a plurality of correcting plates 49, 49A, etc. one for each of the ramps 50, 51, etc. Each will be connected to an appropriate tap on a secondary winding (not illustrated) or to separate taps on secondary windings such as 44 and 47 as illustrated in FIG. 2. FIG. 3 also shows how ambiguity between the various ramps can be avoided by additional elongated wedge like plates 52 and 53 with its individual pick-off plate 54 ganged, by a non-conductive mounting member diagrammatically illustrated at 57, to move with the plates 58 and 59 and connected to a circuit (not illustrated) for indicating the number of complete ramps covered or cycles moved, along the length of the capacitor. Plates 58 and 59 are ganged together by the member 57 and comprise an interpolator 15B. The capacitor illustrated in FIG. 3 includes two exciter plates 55 and 56 separated by a saw-tooth gap 60. The two exciter plates are energized from an AC source via a pair of leads 61.

The interpolator 15B may be mechanically coupled to a machine tool element in a similar manner as interpolator 15 as shown in FIG. 1.

FIG. 4 shows how the capacitor can be arranged to detect angular or rotary movements. In this case the exciter plates 62 and 63 have complementary saw-tooth edges separated by a gap 64 and disposed around the periphery of a cylindrical shell 77. They are energized from a secondary winding 65 of a transformer 66, and the interpolator 15C includes plates 67 and 67A arranged on a rotor 68 to turn about the axis 69 of the device. It is believed that the analogy to the embodiment of FIG. 2 will be so clear that further description is not necessary and of course correcting plates 70 and 71 can be added in the same way as described. It is preferred that the angle between successive cycles of the saw-tooth will represent 60 angular degrees as indicated at 72 to produce 6 cycles in one revolution. While a pair of pick-off plates 67 and 67A, one for each of adjacent ramps are shown, it may be advantageous to provide two pairs of interpolator plates 180.degree. apart in order to provide corrections for slight eccentricity or other mechanical inaccuracy as illustrated by additional interpolator plates 73 and 74. The capacitor may include the calibration correction plates 70 and 71 separated from the exciter plates 62 and 63 by gaps 75 and 76 and arranged on the ends of the cylindrical shell 77. Of course, separate calibration correction electrodes may be provided for each ramp as illustrated in FIG. 3, and additional wedge-like plates and a pick-off plate can be provided for indicating the number of ramps moved as illustrated in FIG. 3.

It will be appreciated that the capacitor illustrated in FIG. 4 may be arranged so that interpolator 15C is mechanically coupled to a machine tool element or the like and thereby be suitable to provide signals indicative of the angular displacement or motion of the machine tool element or the like.

FIG. 5 shows an alternative arrangement with multi-ramp tracks but without having an interpolator comprising two pick-off plates spaced apart along the length of a single track.

It will be seen that there are three tracks having respective exciter plates 78 and 79, 80 and 81, and 82 and 83 separated respectively by saw-tooth gaps 84, 85 and 86 and energized from secondary windings 87, 88 and 89 of a transformer (partially illustrated). The first two tracks have gaps 84 and 85 which are of the same pitch, but are phased displaced by a quarter of a pitch. The interpolator 15D comprises pick-off electrodes 90, 91 and 92 which are ganged by means including shield 93 to move together over the respective tracks. Each electrode consists of a pick-off plate spaced within the shield 93. The tracks are separated by earthed strips 94, and earthed strips 95 are provided adjacent the endmost electrodes 78 and 83.

It can be seen that the pick-off electrode 90 will be moving over a ramp of its track as the pick-off electrode 91 is moving over a peak of its track and vice versa, so that by appropriate mechanical or electrical switching (not illustrated) or by appropriate alternate energization of the tracks a continuous signal can be derived from the two pick-off electrodes 90 and 91.

The third pick-off electrode 92 is merely for avoiding ambiguities and its track has the gap 86 which has a greater pitch than the gaps 84 and 85 associated with the first two tracks.

In an alternative arrangement the shield 93 may take the form of an apertured screen which does not have the pick-off electrodes 90,91 and 92 in its apertures, and is arranged to move along the tracks and is positioned between a set of fixed pick-off electrodes in spaced face-to-face relation with the tracks. The aperture in a screen, in this alternative arrangement, varies the coupling between each pick-off electrode and the exciter electrodes of respective tracks as it moves along the tracks. In this instance, outlet leads are connected to fixed pick-off electrodes rather than to moving electrodes.

In the capacitor of FIG. 6 the interpolator 15D includes pick-off electrode assembly 96 arranged to move axially within a cylindrical shell 97. The pick-off electrode comprises an external cylindrical conducting surface 98 on a cylindrical member 99. The exciter electrode arrangement consists of conducting surfaces 100 and 101 with complementary saw-tooth-like shapes separated by gaps 102 as can be seen best from the developed view in FIG 7. As the pick-off 96 moves over the range of measurement 103 (FIG. 7), the coupling with one of the surfaces 100, 101 will increase and the coupling with the other of the surfaces 100, 101 will decrease. The division of the exciter plates into two teeth each allows some compensation for slight eccentricity in assembling the pick-off electrode 96 with the cylindrical shell 97. Electrical connections to the conducting surfaces 100 and 101 are provided respectively by a wiring arrangement 104 and 105.

It will be appreciated that the interpolator 15D may be mechanically coupled to a machine tool element 106 or the like as diagrammatically illustrated by dashed line 107.

The differential capacitor of FIG. 8 has a pair of exciter capacitor plates 108 and 109 each insulated from and laced around the inner cylindrical surface of a fixed cylindrical housing 110 the two of them being connected across the ends of a secondary winding 111 constituting a reference AC supply and forming part of a transformer 112. There is a short annular gap 113 between the plates 108 and 109.

A pick-off signal representative of the linear position of an interpolator 15E shown as connected to a stylus 114 is derived from a fixed pick-off electrode 115 in the form of an elongated cylinder coaxial with the plates 108 and 109 and extending throughout the major part of the combined lengths of the plates being symmetrically disposed in relation to the plates. The pick-off electrode 115 is connected to a pick-off terminal or lead 115A.

The interpolator 15E whose movement is to be detected comprises a gap 116 between the components of an electrically conducting cylindrical shell screen consisting of two conducting shells 117 and 118 separated by the gap 116 but held together by a web 119 of uniform width, and electrically connected together. They are disposed coaxially with the rest of the capacitor surrounding the pick-off electrode 115 and within the plates 108 and 109. The gap 116 between the facing edges of the conducting shells 117 and 118 of the screen is substantially wider than the gap 113 but less than the length of the pick-off electrode 115. The screen consisting of shells 117, 118 and web 119 is earthed.

It will be appreciatpd that the position of a gap 116 in a screen between either exciter plate 108 or plate 109 and the pick-off electrode 115 will determine the coupling between the plates and the pick-off electrode 115 so that as linear movement takes place of the stylus 114 with the sreen, the coupling of the pick-off electrode 115 to one of the exciter plates 108 and 109 will increase while the coupling to the other will decrease and accordingly a differential output signal can be obtained from the terminal 115A representative of the linear position of the stylus 114.

The arrangement has the advantage that both the exciting reference supply and the pick-off signal are connected to fixed terminals while the moving electrical component only has to be grounded through some sort of flexible connection so that slip rings are not necessary for the signal output.

It will be appreciated that rather than being connected to a stylus or in addition to the stylus, the interpolater 15E may be connected or coupled to a machine tool element or the like so that the output from terminal 115A may represent the position or movement of the machine tool element with respect to a workpiece or another part of a machine tool.

A preferred embodiment of the invention according to the principles shown in FIG. 8 is illustrated in FIG. 11 wherein a housing 160 of the transducer corresponds to the housing 110 of FIG. 8; exciter electrodes 161 and 162 correspond to the plates 108 and 109; the screening means comprises portions 163, 164 and an interconnecting web 165 corresponds to the shells 117, 118 and a web 119; the pick-off electrode 166 corresponds to the electrode 115; and the stylus 167 corresponds to the stylus 114.

The pick-off electrode 166 is carried by a support assembly which includes a tubular member 168 having an externally threaded reduced end portion 169 and an internally threaded counterbore 170 at its other end. A spindle or rod member 171 is provided with external threads and is engaged at one end in the counterbore 170 whereas its opposite end is in the form of an enlarged head 172 provided with a threaded recess receiving the screw fastener 173. The pick-off electrode 166 is electrically isolated from the spindle 171 by means of bevelled washers 174 and 175 of a nonconductive material. The washers 174 and 175 locate the pick-off electrode 166 radially when the member 168 and the spindle 171 are threaded together.

The housing 160 receives a cylindrical support plug 176 having a central opening through which the threaded end portion 169 projects and a nut 177 firmly attaches the entire support assembly within the housing 160. At its opposite end, the housing 160 is internally threaded and receives a threaded anchor ring 178 which supports the opposite end of the spindle 171 through the medium of a strap or guide post 179 which is held against the head end of the spindle 171 by means of the aforementioned fastener 173. Screw fasteners 180 secure the opposite ends of the guide post 179 to the ring 178.

The spindle 171 is provided with a longitudinal groove 181 of sufficient depth more or less and in alignment with a bore 182 of the tubular member 168 and thereby to provide a continuous passage through which a condutor 183 extends for connection to the pick-off electrode 166.

The exciter electrodes 161 and 162 present a gap 184 and are located within the housing 160 by means of respective bushings 185 and 186 of non-conductive material. The bushings 185 and 186 are of substantially equal lengths and the pick-off electrode 166 is essentially centered longitudinally with respect to them and may extend somewhat beyond the opposite ends thereof, as shown. The pick-off electrode 166 is of course substantially concentric with respect to the exciter electrodes 161 and 162.

The screening means includes the two electrically conductive portions 163 and 164 which define an aperture or a dielectric region therebetween through or across which discrete areas of the two exciter electrodes 161 and 162 provide a capacitive coupling with the opposed area of the pick-off electrode 166. The portions 163 and 164 extend well beyond the opposite ends of the pick-off electrode 166 so as always to mask the exciter electrodes 161 and 162 from the pick-off electrode 166 except for the capacitive coupling allowed as aforesaid. Thus, stray capacitive coupling between the electrodes 161 and 162 or connecting leads to the exciter electrodes 161 and 162 and to the pick-off electrode 166 is prevented such as might detrimentally affect the pick-off signal produced by the differential capacitance effect as the screening means is shifted longitudinally during measurements. Thus, it will be seen that the coupling path is a clearly defined area directly through a single dielectric region to permit the capacitor to operate with extreme precision, without the need to provide a direct electric connection to obtain an output signal which is a function of the position of the relatively moving member.

To this end, the dielectric region presented between the portions 163 and 164 is positioned always to bridge across the gap 184, and mechanical means are provided to limit the range of movement of the screening means with respect to the electrodes 161 and 162. The guide post 179 may function to limit the axial movement of the screening means by virtue of the locations and the finite lengths of slots 187 in the screen portion 164 through which the guide post 179 projects. The slots 187 may be of only slightly greater width than the guide post 179 so as to prevent relative rotation of the screening means and of the stylus 167.

To locate the screen portions 163 and 164 concentrically with respect to the pick-off electrode 166 and to allow for axial movement of the screen portions 163 and 164, bushings 188 and 189 are provided on the tubular member 168 and head 172 of the spindle 171, respectively. It will be understood that the screen portions 163 and 164 are connected to ground potential at the outer end of the bushing 188 or the bushing 189 by a coil spring (not shown) or by other electrical conducting means. The exciter electrodes 161 and 162 are connected in circuit as described in conjunction with FIG. 8. It will also be understood that the screening means may take the form of a tubular dielectric member having the electrically conductive portions deposited thereon to define an uncoated dielectric region therebetween through which the capacitive coupling takes place. Further, it will be appreciated that the pick-off electrode assembly 96 of FIG. 6 may be relatively fixed and a movable screen, as in FIGS. 8 and 11, utilized therewith.

FIG. 9 illustrates a differential capacitor similar, in some respects, to that shown in FIG. 1 and shows how the invention can be applied to a device for linear movement in which the exciter plates 120 and 121 and a pick-off electrode 129 are stationary. In this case, two exciter plates 120 and 121 are connected in series across a secondary winding 122 of a transformer 123, and have complementary edges separated by a gap 124 which is oblique to the direction of movement indicated by arrow 128.

Each of the exciter plates 120 and 121 is spaced from a pick-off electrode 129, which may be surrounded by, but spaced from a shield 125. The interpolator illustrated generally at 15F comprises a screen 126 having a gap or aperture 127 therein positioned beween the pick-off electrode 129 and the two exciter plates 120 and 121. The screen is electrically connected to a point of reference potential (ground) by a mechanically flexible connection means or other suitable arrangement.

Movement of the aperture 127 as indicated by the arrow 128 will increase the coupling between one of the plates 120 and 121 and the pick-off electrode 129 and at the same time decrease the coupling between the other plate and the electrode 129 so that the differential output signal can be derived from a terminal (not illustrated) connected to the electrode 129.

It will be understood that the transducer of FIG. 9, as is the case with those shown in FIGS. 8 and 11, is dimensioned such that the grounded screen 126 is interposed between or masks the exciter electrodes 120 and 121 from the pick-off electrode 129 except as between those discrete areas of the exciter electrodes which are coupled through the aperture 127 to the opposed area of the pick-off electrode 129. By inspection, it will be seen that a similar situation prevails for FIG. 10.

The shape of the output signal from the electrode 129 may be improved, so far as accuracy is concerned, by providing that exciter plates 120 and 121 be additionally energized respectively from secondary windings 122A and 122B of a transformer 123A, the secondary wingings 122A and 122B beng connected in series with the secondary winding 122. the transformers 123 and 123A are connected to the same source.

It will be appreciated that the capacitor as illustrated in FIG. 9 may be advantageously provided with a shield and/or slope correcting conductive screws in a similar fashion to the shield 20 and screws 22 shown in FIG. 1. In this case, the screws would be carried by the screen 126 and be adjustably movable into aperture 127 from an edge or edges thereof which are parallel to the direction of movement.

FIG. 10 illustrates a capacitor in which a three track arrangement is provided. An interpolator 15G comprises an electrically conductive cylinder 130 having three axially aligned apertures 131, 132 and 133 therein. The interpolator 15G is operatively arranged to rotate about an axis 134 as illustrated by the arrow 135. Positioned about and on the outside of the interpolator 15G are three tracks each of which includes two exciter plates of complementary saw-tooth shape and illustrated as exciter plates 136 and 137, 138 and 139, and 140 and 141 separated respectively by gaps 142, 143 and 144. The first two tracks have the gaps 142 and 143 which are of the same pitch, but are phase displaced by, for example, a quarter of a pitch. The pitch of the gap 144 is a greater pitch than the pitch of the gaps 142 and 143, and is for the purpose of avoiding ambiguities. The electrodes 136 and 137, 138 and 139, and 140 and 141 are respectively energized from secondary windings 145, 146, and 147 of a transformer 148.

Positioned within the interpolator 15G is a stationary cylinder 149 which has positioned on the outer surface thereof a plurality of electrodes 150, 151 and 152. The electrode 150 is associated with the aperture 131 of the interpolator 15G, the electrode 151 is associated with the aperture 132 and the electrode 152 is associated with the aperture 133 of the interpolator 15G. It can be seen that the aperture 131 will be moving over a ramp of its track as the aperture 133 is moving over a peak of its track and vice versa, so that by appropriate mechanical or electrical switching (not illustrated) or by appropriate alternate energization of the exciter electrodes 136 and 137, and 138 and 139 of the tracks a continuous signal may be derived from two of the pick-off electrodes 150 and 151. The third pick-off electrode 152 adjacent the moving aperture 133 will provide a separate signal for avoiding ambiguities for its track and is associated with the gap 144 which has a greater pitch than the gaps 142 and 143. It will be appreciated that the tracks may be separated from one another by earthed (grounded) electrodes and further that earthed (grounded) electrodes may be provided ajacent the endmost exciter electrodes 136 and 141 in a manner similar electrically to the capacitor illustrated in FIG. 5. An appropriate electrically operated or cam-operated switching means (not illustrated) can be arranged to select an output from one each of the electrodes 150 or 151, and/or 152 depending on the arcuate position of the apertures 131, 132 and 133 of the interpolator 15G. The electrically conductive cylinder 130 is earthed (grounded).

The interpolator 15G may be coupled to a machine tool element or the like so as to provide signals indicative of the angular displacement or motion of the machine tool element or the like.

It is to be understood that in many applications it is very desirable to provide the capacitors of the present invention with an electrostatic shielding for preventing the fields of the capacitor from being adversely affected by extraneous disturbances such as moving bodies or elements or electrical interference.

While the arrangements shown and described above represent illustrative and preferred forms of the invention, it will be readily apparent to one skilled in the art that variations may be made in such arrangements without departing from the scope and spirit of the invention as defined in the appended claims. For example, the capacitor illustrated as cylindrical may be alternatively constructed as flat capacitor and those illustrated as flat capacitors may be constructed as cylindrical capacitors.

Another possible modification is that additional secondary windings may be connected in series with secondary winding 111, shown in FIG. 8 for feeding a corrective voltage to either or both exciter electrodes 108 and 109 to improve the accuracy or modify the output from pick-off electrode 115; the arrangement is similar to the corrective arrangement illustrated in FIG 9.

Claims

I claim:

1. In a capacitive transducer, the combination comprising:

first and second exciter electrodes fixedly located in a common plane and having adjacent spaced-apart edges defining an electrical discontinuity therebetween,

a pick-off electrode fixedly located adjacent said first and second exciter electrodes in spaced relation from the common plan thereof and spanning the electrical discontinuity therebetween,

means for connecting said first and second exciter electrodes to sources of different potential to induce a signal on said pick-off electrode indicative of the portions of said first and second electrodes spanned by said pick-off electrode,

a shield located between said pick-off electrode and said first and second exciter elecrodes and connected to a ground potential to prevent a signal from being induced on said pick-off electrode,

said shield having a dielectric aperture therein bridging the electrical discontinuity for permitting portions of said first and second exciter electrodes adjacent the electrical discontinuity to induce a signal on said pick-off electrode,

said shield being movable relative to said first and second exciter electrodes about the electrical discontinuity to vary the portions of said first and second exciter elements inducing a signal on said pick-off electrode to produce a signal responsive to the position of said movable shield, and

said shield being dimensioned to prevent stray capacitive coupling between said pick-off electrode and said first and second exciter electrodes at all positions of movement of said shield.

2. A capacitive transducer as defined in claim 1 wherein said exciter electrodes each are of tubular form and are disposed in end-to-end spaced relation to define said electrical discontinuity and wherein said shield has electrically conductive portions of tubular configuration located in end-to-end spaced relation to define said dielectric region.

3. A capacitive transducer as defined in claim 2 including a tubular housing within which said exciter electrodes are fixed, said pick-off electrode being in the form of an elongated sleeve concentrically disposed with respect to said exciter electrodes, and said shield being of a length to completely mask said exciter electrodes from said pick-off electrode except through said dielectric region.

4. A capacitive transducer as defined in claim 3 including a support assembly located axially within said housing and concentrically locating said pick-off electrode therewithin, and means for slidably supporting said shield on said support assembly.

5. A capacitive transducer as defined in claim 4 including a guide member disposed diametrically within said housing, said shield having slots formed therein for slidably receiving said guide member to prevent rotation of said shield means relative to said housing.

6. A capacitive transducer as defined in claim 1 wherein said pick-off electrode, said exciter electrodes and said shield are in the form of flat plates.

7. A capacitive transducer as defined in claim 1 wherein said exciter electrodes are each of annular form and having zig-zag edge portions disposed in spaced relation to define said electrical discontinuity, said pick-off electrode being concentrically disposed with respect to said exciter electrodes, and said shield being rotatable between said exciter and pick-off electrodes.

8. A capacitive transducer as defined in claim 1 wherein the electrical discontinuity defined by the adjacent edges of said first and second exciter electrodes is a straight gap of uniform width.

9. A capacitive transducer as defined in claim 1 wherein the electrical discontinuity defined by the adjacent edges of said first and second exciter electrodes is a saw-tooth gap of uniform width.

10. A capacitive transducer as claimed in claim 1 including a compensatory electrode means positioned near one of said exciter electrodes and separated therefrom by a gap, said compensatory electrode means being coupled to said pick-off electrode.

11. A capacitive trasducer as claimed in claim 10 including a second compensatory electrode positioned near the other of said exciter electrodes and separated therefrom by a gap, said second compensatory electrode being coupled to said pick-off electrode.

12. A capacitive transducer as claimed in claim 11 including means for varying the coupling between at least one of said compensatory electrodes and said pick-off electrode.

13. A capacitive transducer as defined in claim 1 including a second pair of exciter electrodes defining a second electrical discontinuity therebetween, a second pick-off electrode, and said shield having a second dielectric aperture therein bridging said second electrical discontinuity.

14. A capacitive transducer as defined in claim 13 wherein the electrical discontinuity defined by the adjacent edges of said first and second exciter electrodes is a saw-tooth gap of uniform width.

15. A capacitive transducer as claimed in claim 14 including a compensatory elecrode means positioned near one of said exciter electrodes and separated therefrom by a gap, said compensatory electrode means being coupled to said pickoff electrode.

16. A differential capacitor as claimed in claim 15 including a second compensatory electrode positioned near the other of said exciter electrodes and separated therefrom by a gap, said second compensatory electrode being coupled to said pick-off electrode.

17. A capacitive transducer as defined in claim 1 wherein the electrical discontinuity defined by the adjacent edges of said first and second exciter electrodes is a zig-zag gap along the path of relative movement between said shield and said exciter electrodes, and further including a second pick-off electrode and a second pair of exciter electrodes associated therewith, said second pair of exciter electrodes having adjacent edges defining a second electrical discontinuity therebetween extending at an acute angle of said path of relative movement, and said shield defining a second dielectric aperture therein bridging the second electrical discontinuity between said second pair of exciter electrodes.

18. A differential capacitor as claimed in claim 17 including a compensatory electrode means positioned near one of said exciter electrodes and separated therefrom by a gap, said compensatory electrode means being coupled to said pick-off electrode.

19. A differential capacitor as claimed in claim 18 including a second compensatory electrode positioned near the other of said exciter electrodes and separated therefrom by a gap, said second compensatory electrode being coupled to said pick-off electrode.