Abstract:
A non-contacting capacitive position transducer comprises a stator substrate carrying two electrically conducting inverted wedge regions  14,16  whose width varies inversely in a sensing direction. A moveable pick-off  20  is capacitively coupled to both wedges. The wedges  14,16  are driven with respective distinguishable time varying periodic waveforms, e.g. a sine wave and a cosine wave, and the pick-off voltage at  20  processed to determine the position of the pick-off in the sensing direction. Various configurations of transducer are described, including those of linear, cylindrical and disc form. The pick-off voltage is preferably transferred from the pick-off  20  back to the stator by capacitively coupling the pick-off to a suitably screened pick-off track  36  on the stator.

Description:
This is a Continuation of: International Application No. PCT/GB99/02390 filed Jul. 22, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to capacitive position transducers and associated methods for position detection. The invention is particulary concerned with linear and rotary position transducers but is not limited to such transducers. 
     There is frequently a requirement in modern control systems for a positional transducer which provides a high resolution, absolute output, that is one in which the output indication is a unique expression of position. In applications where the accuracy requirements do not preclude their use, e.g. positional servo systems, potentiometers have traditionally been used, providing a readily available, cost effective solution where applicable. 
     However potentiometers do have several shortcomings. There is an inherent wear-out mechanism between the wiper and the resistive element which will ultimately result in failure of the device. They are prone to creating wiper noise, particularly under high rates of movement, and noise performance tends to deteriorate with life; this is a particular problem in high gain servo systems where the noise can interfere with correct system operation. Frequently the need is to monitor a linear motion and, whilst some linear travel potentiometers are available, these are almost invariably commercial parts unsuitable for anything other than consumer type applications. The application of a rotary potentiometer to such a requirement would therefore necessitate some kind of motional translation—a rack and pinion for example. 
     There is therefore a need for a potentiometer which obviates at least some of the above shortcomings. We have therefore designed a new form of transducer which does not require electrical physical contact between the mover and the stator and which provides an absolute output with high resolution, with an accuracy at least comparable with that of conventional servo-grade potentiometers. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, in one aspect, this invention provides a capacitive position transducer comprising: 
     a relatively fixed substrate including two spaced electrically conductive regions; 
     a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions and mounted for movement relative to said substrate in a sensing direction, the transverse extent of at least one of said electrically conductive regions varying with the position of said mover element in the sensing direction, 
     drive means for supplying respective different periodic time varying voltages to said electrically conductive regions, and 
     processing means for processing the electrical signal received by said pick-off means to determine the position of said mover element, 
     wherein said drive means applies respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference. 
     Preferably, the processing means determines at least one of the phase and magnitude of the electrical signal received by the pick-off means, to determine the relative position of the mover element. 
     In another aspect, this invention provides a capacitive position transducer comprising: 
     a relatively fixed substrate including two spaced electrically conductive regions; 
     a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions, and mounted for movement in a sensing direction, the electrically conductive regions being arranged such that at least one of the respective capacitances between said pick-off means and the electrically conductive regions varies with the position of said mover element in said sensing direction, and 
     drive means for applying respective voltages to each of said electrically conductive regions, and 
     position determining means for monitoring the position of said mover element relative to said substrate, 
     wherein said drive means supplies respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference. 
     In yet another aspect, this invention provides a method of position detection which comprises providing a relatively fixed substrate including two spaced electrically conducting regions and a pick-off means capacitively coupled to both of said regions, said pick-off being connected to a mover element for movement in a sensing direction, at least one of the respective capacitances between the pick-off and the electrically conductive regions varying with position in said sensing direction, the method further comprising monitoring at least one of said capacitances thereby to determine the position of said mover element, and 
     applying respective different sinusoidal voltages to said electrically conductive regions, said sinusoidal voltages having a relative phase difference. 
     Preferably, both of said electrically conducting regions vary in transverse extent in said sensing direction whereby the capacitance between each electrically conductive region and the pick-off means varies in said sensing direction. 
     Preferably, said electrically conductive regions vary linearly and inversely in transverse extent in said sensing direction. Thus, in one arrangement the electrically conductive regions may be arranged as two triangles in inverted relationship together defining a generally rectangular plan shape. 
     Alternatively, the electrically conductive regions may each vary in said sensing direction with a profile selected in accordance with the drive voltages to said electrically conducting regions, at least partially to compensate for non-linearities in the output signal. 
     The pick-off means may be connected directly to said processing means e.g. via an electrically conducting wire or the like. Alternatively, to avoid the need to provide a moveable electrically conducting element such as a wire or track, the pick-off means may be capacitively coupled to a track means which extends alongside the path of movement of said mover element, with suitable screening, so that the output signal may be taken from the track means. 
     The substrate may take any of a number of different forms. For example, the substrate may be generally flat or planar. Here the mover element may be mounted for linear movement in a straight line relative to said substrate, analogous to a conventional linear potentiometer. Alternatively, said mover element may be mounted for rotary movement about a rotary axis and the substrate may be of generally disc or annular form. In this instance the transducer is akin to a rotary potentiometer. 
     Alternatively, said substrate may comprise a cylindrical or part-cylindrical surface and said mover element may be disposed adjacent said cylindrical a part-cylindrical surface and mounted for movement about an axis generally coaxial with the principal axis of said cylindrical or part cylindrical surface. 
     The drive means may apply various forms of drive voltages to the electrically conductive regions such that the combined signal at the electrical pick-off means may be processed to determine the position of the mover element. Thus the drive means may apply respective different sinusoidal voltages. The sinusoidal voltages are preferably of substantially equal amplitude. Conveniently, said sinusoidal voltages have a relative phase difference of 90°, effectively to provide respective sin and cosine waveforms. However other phase differences may also be used. 
     The processing means preferably determines at least one of the relative phase and magnitude of the electrical signal received by said pick-off means, to determine the position of said mover element relative to a fixed datum position. 
     Where the electrically conductive regions vary linearly and inversely in axial extent in said sensing direction, the drive means may apply respective sinusoidal waveforms of the form V·Sin(ωt) and V·Cos(ωt) and the relative position of the mover element may be determined from the signal V o  at said pick-off, wherein 
     
       
           V   o   =K·V ·Sin(ω t θ); 
       
     
     
       
         θ=arctan(α/1−α); 
       
     
     
       
           K ={square root over ( )}(α 2 +(1−α) 2 ) 
       
     
     and α is the distance of the mover element from the highest capacitance part of the electrically conducting region driven by said V·Sin(ωt) waveform expressed as a proportion of the maximum range of sensing movement of said mover element. 
     It will be noted that α and θ vary non-linearly; this may be overcome by modifying the profiles of the electrically conductive regions to reduce or remove the non-linearity so that the phase difference (θ) varies linearly with the position (α) of the mover element. 
     Alternatively, the non linearity may be overcome by signal processing. Thus said processing means may be operable to multiply said output voltage V o  at said pick-off means with a D.C.-nulling periodic time varying voltage at the same frequency as said output voltage, and adjusting the relative phase of said D.C.-nulling periodic time varying voltage to null the D.C. term of said product, and thereby determine the position of said mover element relative to a fixed datum. 
     Preferably said D.C.-nulling periodic voltage is generated by multiplying the drive voltages applied to said electrically conducting regions. Thus said processing means may include generating means for producing said D.C.-nulling periodic voltage, said generating means including means for multiplying the time varying voltages (V·Sin(ωt); V·Cos(ωt)) supplied to said electrically conductive regions with respective inversely related voltages, (V ref −V con ) and (V con ), means for summing the product of said multiplying means and phase shifting the sum to obtain said D.C.-nulling periodic voltage. 
     Preferably said processing means includes integrator means for receiving the product of said pick-off output voltage V o  with the D.C.-nulling voltage V′ o , and adjusting said inversely related voltages, (V ref −V con ) and (V con ) to null said D.C. term. 
     In yet another aspect, there is provided a capacitive position transducer comprising: 
     a relatively fixed substrate including two spaced electrically conductive regions varying lineally and inversely in transverse extent; 
     a mover element comprising a common electrical pick-off means capacitively coupled to both of said electrically conductive regions and mounted for movement relative to said substrate in a sensing direction, the transverse extent of at least one of said electrically conductive regions varying with the position of said mover element in the sensing direction, 
     drive means for supplying respective different periodic time varying voltages to said electrical conductive regions, and 
     processing means operable to multiply an output voltage at said pick-off means with a DC nulling periodic voltage at the same frequency as said output voltage and to adjust the relative phases of said DC nulling periodic time varying voltage to null the DC term of said product, and thereby to determine the relative position of said mover element. 
     Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention may be performed in various ways and, by way of example only a specific embodiment and various modifications thereof now will be described in detail, reference being made to the accompanying drawings in which:- 
     FIG. 1 is a schematic plan view of a transducer in accordance with the invention; 
     FIG. 2 shows an equivalent electrical circuit of the embodiment of FIG.  1  and its output circuit, and 
     FIG. 3 is a diagram of an electrical circuit for processing the output received by the pick-off to provide an output signal which varies substantially linearly with the position of the mover element; 
     FIG. 4 is a plan view of a substrate for a further embodiment of non-contacting capacitive transducer of this invention; 
     FIG. 5 shows the substrate of FIG. 4 wrapped around to form a cylinder; 
     FIG. 6 is a cross-sectional view through the further embodiment showing the substrate of FIGS. 4 and 5, and a mover element; 
     FIG. 7 is a detailed view of the mover element; 
     FIG. 8 is a plan view of a substrate for another embodiment of capacitive transducer having a spiral arrangement of electrically conducting regions; 
     FIG. 9 is a side view of an arrangement including spaced upper and lower substrates of the form shown in FIG. 8, and 
     FIG. 10 is a plan view similar to that of FIG. 1 except with a modified arrangement for transferring the signal from the pick off to a stationary terminal. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring initially to FIG. 1, a first embodiment of position transducer  10  comprises a stator substrate  12  of flat planar form on which are deposited two triangular sections or tracks  14 ,  16  of copper, spaced by an insulating gap  18 . A mover element carrying a pick-off  20  extends across the width of the substrate and is spaced above the surface thereof by a small air gap such that the pick-off is capacitively coupled to both the triangular sections  14  and  16 . The two triangular sections of copper  14 ,  16  are driven with alternating voltage waveforms, one ( 14 ) with V·Sin(ωt) and the other ( 16 ) with V·Cos(ωt). 
     Referring to FIG. 2, the output from the pick-off  20  is fed to a high input impedance voltage buffer  22 . The values of C 1  and C 2  are proportional to the areas of the tracks  14 ,  16  under the pick-off  20 , so that with the pick-off at either end, one capacitor will be maximised whilst the other is reduced to zero. Thus at the Sin end, 
     
       
           C   1   =C   max   , C   2 =0, and  V   o   =V ·Sin(ω t ) 
       
     
     and at the Cos end, 
     
       
           C   1 =0,  C   2   =C   max , and  V   o   =V ·Cos(ω t ) 
       
     
     At some point in between the output voltage, V o , is given by: 
     
       
           V   o   =V ·Sin(ω t )/(1+ C   1   /C   2 )+ V ·Cos(ω t )/(1+ C   2   /C   1 ) 
       
     
     Now C 1 =C max ·(1−α) and C 2 =C max ·α, (where α is the ratio of the pick-off&#39;s distance from the sin end divided by the overall length of the tracks ( 14 ,  16 ), and substituting for C 1  and C 2  gives: 
       V   o   =V ·Sin(ω t )·(1−α)+ V ·Cos(ω t )·α  (Equation 1) 
     
       
           V   o   =K·V ·Sin(ω t +θ)  (Equation 2) 
       
     
     where 
     
       
         θ=arctan(α/(1−α))  (Equation 3) 
       
     
     
       
           K ={square root over ( )}(α 2 +(1−α) 2 )  (Equation 4) 
       
     
     It can be seen from Equations 2, 3, and 4 that the output voltage, V o , will be a Sin wave of varying phase and amplitude. At the mid point, (α=0.5), V o  will be 1/{square root over (2)} of the value at either end, and θ will be 45°. Between the mid point and either end however the relationship between θ and a becomes nonlinear. Thus, at a quarter distance from the Sin end, (α=0.25), θ=arctan(1/3)=18.435°, and not 22.5° as would be the case were the relationship linear. 
     This non-linearity may be removed by modifying the profile of the facing edges of tracks  14  and  16  but we describe below an alternative approach which multiplies the output voltage, V o  by a cosine term at the same frequency to obtain a D.C. term. Thus: 
     
       
         2·Sin( A )·Cos( B )=Sin( A+B )+Sin( A−B ), 
       
     
     substituting 
     
       
           A =(ω t +θ) and  B =(ω t +φ), 
       
     
     the following expression results: 
     
       
         2·Sin(ω t +θ)·Cos(ω t θ)=Sin(2 ωt +θ+φ)+Sin(θ−φ)  (Equation 5) 
       
     
     Sin(2ωt+θ+φ) is an alternating term at twice the modulation frequency (2ωt) and can be removed with a low-pass filter. The remaining term Sin(θ−φ) is a D.C. term which becomes zero when θ=φ. 
     Therefore, if we multiply the Sin(ωt+θ) term from the output voltage of Equation (2) by Cos(ωt+φ) and adjust the result for zero D.C., then θ=φ. Knowing this, a value for φ can be obtained so that θ can be determined; α(the proportional distance of the slider along the track) is determined from Equation (3), to give a read out of the position of the slider. 
     Referring to FIG. 3, the terms Cos(ωt+φ) may be derived as follows: 
     Two multipliers  24 ,  26  are used to multiply the V·Sin(ωt) and V·Cos(ωt) waveforms by (V ref −V con ) and (V con ) respectively, where V ref  is a reference voltage and V con  is a control voltage derived from the output of an integrator 28 with the range 0→V ref . The outputs of the multipliers  24 ,  26 , are added in a summing amplifier  30  and the output is phase-shifted by a feedback capacitor  32 . 
     Were it not for the phase shift, the output of the summing amplifier  20  would be of the form: 
     
       
           V   o   ′=K ′·(( V   ref   −V   con )· V ·Sin(ω t ))+( V   con   ·V ·Cos(ω t )) 
       
     
     Where K′ is the gain through the summing amplifier. This reduces to 
     
       
           V   o   ′=K ″·( V ·Sin(ω t )·(1−β)+Cos(ω t )·β)  (Equation 6) 
       
     
     where 
     
       
         β= V   con   /V   ref  and  K″=K′·V   ref   
       
     
     The similarity to Equation (1) will be noted. This in turn reduces to 
     
       
           V   o   ′=K′″·V ·Sin(ω t +φ)  (Equation 7) 
       
     
     where 
     
       
         φ=arctan(β/(1−β))  (Equation 8) 
       
     
     
       
           K′″=K ″·{square root over ( )}(β 2 +(1−β) 2 ) 
       
     
     By introducing the 90° phase shift in the summing amplifier Equation (7) becomes: 
     
       
           V   o   =K′″·V ·Cos(ω t +φ)  (Equation 9) 
       
     
     which is the required term for Equation (5). 
     A third multiplier  33  takes these two terms and multiplies them to generate the terms to the right of the equality in Equation (5) and the output of this multiplier consists of an A.C. term at 2·ω plus a D.C. term. The alternating component is removed by a low-pass filter  34  and the integrator 28 adjusts V con  until the D.C. term becomes zero. 
     Because the forms of Equations (1) and (6) are the same, the non-linearity that exists in the relationship between α and θ is exactly cancelled by the same relationship between β and φ and β is therefore linearly related to α. 
     Furthermore, since it is β and not V con  which is important, V con  will be scaled by V ref , thereby giving an output which is potentiometric in nature. Throughout the circuit actual signal values are unimportant, so long as signal levels are sufficient to maintain adequate signal to noise, and it is purely the relative phases of the signal which matters. 
     Furthermore it can be shown that, as opposed to excitation waveforms of Sin(ωt) and Cos(ωt), (which is actually the same as Sin(ωt+90°)), Sin(ωt) and Sin(ωt+ε) could be used, where e is some phase angle other than 90°, with no change in performance other than, in the limit, a degradation of signal to noise. (ε tending to zero for example). 
     This means that the circuit will be tolerant of variations in the phase/frequency of the drive waveforms and variations of pick-off voltage due to discrepancies in the size of the air gap. Likewise, so long as the relative amplitudes of the two drive waveforms remain constant, the actual levels are not important. 
     In the embodiment described above with reference to FIGS. 1,  2 , and  3 , the voltage from the pick-off  20  is passed by a trailing wire to the voltage buffer amplifier  22 . In particular applications this may not be desirable and so in an alternative arrangement shown in FIG. 10, a third track or rail  36  is provided alongside the substrate  12  and the pick-off  20  is extended at  38  so that it is capacitively coupled to the track  36 , thus allowing the signal to be picked off from a static position. To prevent the track  36  “seeing” the signal on the adjacent track section  14 , suitable screening is provided including the provision of an electrical barrier  40  between the track  36  and the track  14 . 
     As a further modification, instead of being flat planar, the copper tracks,  14 ,  16  can be made circular, either in the form of a cylinder or a disc, of proportions suitable for embodiment in a traditional potentiometer housing. It should be noted that the tracks need not be separated by a straight diagonal gap but instead the gap may be of sinusoidal form, so that the output voltage from the buffer amplifier  22  is linearly related to the position of the pick-off. In this instance, the position is determined by observing the value of V con  relative to V ref  in a similar way in which the position indicated in a conventional contacting potentiometer is measured by the wiper voltage relative to the voltage across the whole of the track. 
     It should also be appreciated that several similar transducers may be stacked together one above the other with a common actuator, e.g. a common axle or rotor shaft to which an appropriate number of pick-off rotors are attached. 
     In each of these embodiments, the excitation voltage for the potentiometer would be used to power the electronics, incorporated inside a housing in the form of a lowpower analogue ASIC (application specific integrated circuit), and the output would be a D.C. voltage which behaved exactly like a wiper voltage, ranging from zero to reference voltage, but without the attendant potentiometer problems outlined at the start. 
     Alternatively, a 5-terminal approach could be adopted whereby two additional terminals provide power to the electronics whilst the other three terminals simulate a potentiometer, two of the three being used for the potentiometer reference and the third being the wiper output voltage. 
     Whilst in the above embodiment the drive waveforms have been sine waves, tests have shown that triangular waveforms give results comparable to those using sine waves. Also waveforms which are not pure sine waves, i.e. those containing some harmonic content or distortion do not within quite wide limits, appear to make any significant difference to the results obtained, provided they contain incremental phase information, from which a position dependent phase difference may be obtained. 
     Referring now to FIGS. 4 to  7 , a firther embodiment of non-contacting capacitative transducer will now be described. In this embodiment, a pattern of copper tracks making up the wedges  14 , 16  and the pick-off strip  40  are provided on a thin, flexible, insulating substrate  42 , for example by etching, plating, deposition, or other suitable accurate photographic method or the like. The substrate  42  is then folded round to form a cylinder as shown in FIG. 5. A centre shaft  44  carrying a first wedge pick-off  46  and a second cylindrical pick-off  48  electrically connected thereto, is mounted coaxially within the cylinder for rotation about the cylindrical or sensing axis. As seen more clearly in FIGS. 6 and 7, the wedge pick-off  46  is of axial length equivalent to the maximum width of each wedge  14 , 16 . The cylindrical pick-off  48  is capacitively coupled to the pick-off strip  40 . The substrate  42  is housed within a metal cylindrical housing  50  which includes a disc-shaped screen  52  which screens the upper and lower sections of the substrate. The electric connections to the wedges  14 ,  16  and the pick-off strip  40  are made through the housing walls, with suitable layers of screening to prevent coupling between the wedges and the pick-off strip  40 . 
     Referring now to FIGS. 8 and 9, in this embodiment  10  the wedges  14  and  16  are formed on a first substrate disc  54  with the gap  18  therebetween describing a spiral, and a radial insulating gap  55 . 
     The first substrate disc  54  is mounted on one side of a support disc  56  which includes a metallic screen element  58  in its mid region. The lower surface of the support disc  56  carries a second substrate  60  which is a plane pick-off disc. A pick-off shaft  52  is coaxially mounted with respect to the support disc  56  and carries a radially extending phase pick-off  64  and a coupling disc  66  electrically connected to the pick- off  64  through the shaft  62 . 
     Electrical connections are made to the wedges  16 ,  18  and the pick-off substrate  60  via the support disc  56 . As previously, the rotary position of the shaft  62  determines the proportions of the widths of the wedges  16  and  18  capacitively coupled to the phase pick-off  64 . The wedges  16  and  18  are driven by respective sinusoidal voltages or other suitable periodic time-varying waveforms, and the resultant voltage picked up by the pick-off  64  processed to determine the angular position of the shaft.