Abstract:
Angular position detection sensors include a capacitive sensor embodiment and an inductive sensor embodiment. A non-rotating excitation element is electromagnetically coupled to a non-rotating receiver element. The electromagnetic coupling is varied by an electrically passive, rotating element disposed between the non-rotating excitation element and the non-rotating receiver element. Excitation signals applied to the non-rotating excitation element are electromagnetically coupled to the non-rotating receiver element, producing a single output signal directly indicative of the angular position of the rotating element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention is related to U.S. Pat. No. 5,872,408, Feb. 16, 1999 and Capacitive Rotary Coupling, U.S. Pat. No. 6,101,084, Aug. 8, 2000, both of which are fully incorporated herein by reference for all purposes. The present invention is also related to pending U.S. application Ser. No. 11/413,420, filed Apr. 28, 2006 which is fully incorporated herein by reference for all purposes. 
     BACKGROUND OF THE INVENTION 
     The present invention is directed generally to the detection of angular position, and in particular to the use of reactance sensors to indicate the angular position of the rotary elements of rotational devices. 
     Control of rotational electromechanical devices, including electrical motors, requires determining the position and speed of their axes and rotors. There are several ways to determine such parameters. First, the position of the rotor may be determined by an array of photo-transistors and a special shutter coupled to the rotor shaft, or by using Hall-effect sensors. Such systems are described in T. Kenjo, Electrical Motors and Their Controls, Oxford University Press (1994), pp 176 and following. Second, the speed informative signal may be obtained by using a small permanent magnet tachometer generator, attached to the shaft, or by using magnetic or optical sensors for generating pulses for each angular increment of the rotor. Such systems are described in W. Leonhard, Control of Electrical Drives, 2nd ed., Springer Verlag (2001), pp 420 and following. Third, a resolver may be used to determine the position of the rotor by a two-phase (sine/cosine) signal at a carrier frequency modulated sinusoidally by the rotation of the rotor. Such a system is described in J. R. Hendershot, Jr. and T. Miller, Design of Brushless Permanent—Magnet Motors, Magna Physics Publishing (1994), pp 1-19. All these methods require precise mechanical placement of sensors, or galvanic contact between moving parts. 
     Conventionally, the moveable element of a rotary device is the informative element that indicates rotational (angular) position; i.e., the information signal (an electrical signal) is generated on the moveable element. It is therefore necessary to have some means for transferring the information signal from this moveable element to external processing circuitry. This is usually accomplished by the use of rings and brushes, flexible connectors, and so on. The use of brushes can introduce noise into the information signal. Brushless solutions exist, but they suffer from low signal to noise ratios, and can be mechanically cumbersome. More significantly, brushes create problems with reliability and require constant maintenance. It is highly desirable to form and deliver signals to and from the rotating parts of mechanical or electromechanical devices without the use of mechanical or galvanic contact and a complex sensor supporting system. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides reactance (capacitive and inductive) sensors for measuring angular position of components of mechanical and electromechanical devices. 
     This invention refers to reactance (capacitive and inductive) sensors indicative of angular position of rotational devices, specifically axes of their moving parts, and more specifically of angular position of rotors of electrical motors of different types, and of other electromechanical motion devices. 
     In accordance with the present invention, a sensor includes a non-moving sensing element from which the informative signal indicative of the angular position is obtained. A passive moveable element is introduced between the non-moving sensing element and a source of electromagnetic energy. The passive element has special electromagnetic characteristics which affect the reactance coupling (i.e., electromagnetic coupling) of the electromagnetic excitation between the non-moving sensing element and the source of electromagnetic energy. 
     Following the principle of duality of electromagnetic fields, a sensor according to the present invention can be capacitive or inductive. The movable passive element can be characterized by having a dielectric constant for capacitive variants, or a magnetic constant for inductive variants. Both of these variants of reactance sensors have similar constructions, identical forms of excitation and output informative signals. 
     According to the present invention those elements of the angular position sensor that require an electrical connection do not rotate. This is a significant advantage because the present invention obviates the need for brushes or the like which would be used to provide electrical contact to rotating surfaces and the disadvantages of having to use such brushes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a planar embodiment of a capacitive angular position sensor according to the present invention. 
         FIG. 1A  illustrates a variation of the embodiment shown in  FIG. 1 . 
         FIGS. 2A-2D  show views of components comprising the sensor of  FIG. 1 . 
         FIG. 3  illustrates examples of excitation signals, and an example of an output signal. 
         FIG. 4  illustrates operation of the sensor of  FIG. 1 . 
         FIG. 5  presents a schematic representation of the sensor of  FIG. 1 . 
         FIG. 6  is a circuit diagram that represents the sensor of  FIG. 1 . 
         FIG. 7  illustrates current waveforms produced in the circuit of  FIG. 6 . 
         FIG. 8  illustrates a cylindrical variant of the sensor of  FIG. 1 . 
         FIGS. 9A and 9B  are cross-sectional views of the embodiment of  FIG. 8 . 
         FIGS. 10A and 10B  illustrate variants of the sensors respectively illustrated in  FIGS. 1 and 8 . 
         FIG. 11  shows an illustrative embodiment of an inductive sensor of angular position according to the present invention. 
         FIG. 12  shows a perspective view of showing additional details of the sensor of  FIG. 11 . 
         FIG. 13  shows a circuit diagram that represents the sensor of  FIG. 11 . 
         FIG. 14  shows an alternative embodiment of the rotating element shown in  FIG. 11 . 
         FIG. 15  shows a variant of the sensor of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing problem was recognized and essentially solved in M. Rakov, Capacitive Sensor for Indicating Position, U.S. Pat. No. 5,872,408, Feb. 16, 1999, and in M. Rakov, Capacitive Rotary Coupling, U.S. Pat. No. 6,101,084, Aug. 8, 2000, both of which are fully incorporated herein by reference. The general approach was based on using the phase of output signal of sensor as a unified information parameter. Different configurations of reactance (capacitive and inductive) sensors were proposed. These sensors were characterized by uniformity of information signal, high reliability and low maintenance costs because of absence of contacts or brushes, and simple construction that allowed them to be practically implemented using modern methods of mass production. With these two patents as the foundation of present patent application, the present invention discloses a further improvement of such sensors. 
       FIG. 1  illustrates an embodiment of an angular position sensor  100  according to the present invention. In this particular embodiment, the sensor  100  is a capacitive sensor. The side-view illustration of  FIG. 1  is illustrative. The figure shows a sensor  100  having a planar construction, and in particular a disk-shaped construction. The sensor  100  comprises a rotating element  104  and a non-rotating excitation element  102  and a non-rotating receptor element  106 . In this particular embodiment, the rotating element  104  is positioned between the excitation element  102  and the receptor element  106 , and is spaced apart from the excitation element  102  and from the receptor element  106 . One of ordinary skill will readily appreciate that other shapes may be equally suitable. 
     The rotating element  104  is attached to a shaft  108 . Rotation of the shaft  108  about its axis of rotation causes the rotating element  104  to rotate. An opening is provided through a center portion of non-rotating excitation element  102 , through which the shaft  108  passes. This allows the shaft  108  to be mechanically connected to a device for which detection of its angular position is desired. For example, the shaft can be connected to the rotor of a brushless DC motor, whose operation requires knowledge of the angular position of its rotor. 
     Alternatively, the shaft  108  passes can through an opening provided in the receptor element  106 . Other embodiments for driving the rotating element  104  are possible, of course. For example,  FIG. 1A  shows an embodiment where the rotating element  104 ′ is a geared element that is driven by another gear  110 . A rotating shaft  108 ′ is attached to the gear  110  to drive the rotating element  104 ′. It should be apparent that this specific implementation detail is not relevant to the present invention. Any suitable configuration that allows for rotation of the rotating element  104  that does not require rotating the excitation element  102  or the receptor element  106  is appropriate. 
       FIG. 1  also shows a multiphase signal generator  122  for providing two or more input signals to the excitation element  104 . In this particular embodiment, the signal generator  122 , or other suitable source of signals, is configured to provide three input signals U A , U B , and U C  to the excitation element  102 . The figure further shows that the receptor element  106  provides a single output signal U OUT . 
       FIGS. 2A to 2C  show additional details of the sensor  100  viewed along respective view lines  2 A- 2 A,  2 B- 2 B, and  2 C- 2 C shown in  FIG. 1 .  FIG. 2A  is a face-on view (viewed along view lines  2 A- 2 A), showing further detail of excitation element  102 . In accordance with the present invention, the excitation element  102  is configured to emit a plurality of electromagnetic fields during operation of the sensor  100 . The specifically disclosed embodiment of the excitation element  102  is a disk-shaped element comprising three electrically isolated emitting elements. It will be understood, however, that the excitation element  102  is not necessarily disk-shaped and can comprise more than three such emitting elements, or a few as two emitting elements. For purposes of explanation, the discussions which follow will assume a three-element configuration and three-phase excitation. 
     As shown in  FIG. 2A , the excitation element  102  comprises an underlying disk-shaped substrate  222  of nonconductive material. Disposed on the substrate  222  are three separate electrically conductive regions  224 ,  226 ,  228 , which divide the disk into equal areas and constitute the emitting elements in this particular embodiment of the invention. Each conductive region is substantially a 120° sector. The three electrically conductive regions  224 ,  226 ,  228  are electrically isolated from each other. Each of the input signals U A , U B , and U C  produced by the signal generator  122  is provided respectively to one of the electrically conductive regions  224 ,  226 , and  228 ; for example, by way of a wire connection. It will be appreciated that other implementations of the emitting elements are possible. 
       FIG. 2B  shows a face-on view (viewed along view lines  2 B- 2 B) of the receptor element  106 . In accordance with the present invention, the receptor element  106  is configured to output a single output signal. The particular implementation of the receptor element  106  shown in  FIG. 2B  illustrates a single disk-shaped element of electrically conductive material  262 . The electrically conductive material  262  can be disposed on a substrate (not shown) for mechanical stiffness. Although the shape of receptor element  106  is shown to match the shape of the excitation element  102 , it will be apparent that the present invention does not impose such a restriction of matching shapes. 
     Since the excitation element  102  and the receptor element  106  do not rotate, it is a simple matter to provide electrical connections to these elements to energize the emitter regions of the excitation element  102  and to measure or otherwise sense the single output signal U OUT  on the receptor element  106 . For example,  FIG. 2A  shows wires  202   a ,  202   b ,  202   c  soldered or otherwise electrically connected respectively to the conductive regions  224 ,  226 ,  228 . In this way, a source of signals such as signal generator  122  can provide individual signals respectively to the conductive regions  224 ,  226 ,  228  without the need for brushes as compared to conventional devices where the excitation element is a moving part. 
       FIG. 2B  similarly shows a wire  202   d  soldered or otherwise electrically connected to the electrically conductive material  262 . In this way, the output signal U OUT  that appears on receptor element  106  can be measured or otherwise detected by a suitable detection device (not shown) without the need for brushes as compared to conventional devices where the receptor element is a moving part. In a practical application of the present invention, the output signal can be fed into a controller (not shown) which would then perform control functions based on the measured output signal. 
       FIG. 2C  shows a face-on view (viewed along view lines  2 C- 2 C) of rotating element  104 . This disk-shaped element comprises a first portion  242  and at least a second portion  246 . The shaft  108  can be seen in cross-section. The first portion  242  is a 120° sector having proportions that match the proportions of each conductive region  224 ,  226 ,  228 . The first portion  242  is a dielectric material characterized by a dielectric constant ∈ 1 . Practically, the dielectric constant should be sufficiently greater than that of air which is defined by the constant of ∈ 0 . The second portion  246  is a material different from the material of the first portion  242 . The material of the second portion can be a non-dielectric material, or it can be a dielectric material having a dielectric constant ∈ 2  different from ∈ 1 . The rotating element  104  can in fact comprise three or more portions. 
       FIG. 2D  shows an alternate embodiment of the present invention wherein a rotating element  104 ′ comprises only a 120° sector  242 ′ having proportions that match the proportions of each conductive region  224 ,  226 ,  228 . The sector  242 ′ is connected to or is integral with the shaft  108 . When the shaft  108  is rotated, the sector  242 ′ rotates as well. 
     In operation, the input signals U A , U B , and U C  produced by the signal generator  122 , or other suitable source of signals, are applied respectively to the conductive regions  224 ,  226 ,  228  of the excitation element  102 . For example,  FIG. 2B  shows that input signal U A  is applied to conductive region  224 , input signal U B  is applied to conductive region  226 , and input signal U C  is applied to conductive region  228 . Since the conductive regions  224 ,  226 ,  228  are electrically isolated from each other, three separate electromagnetic fields will emanate from the excitation element  102 . 
     In this particular embodiment, the input signals U A , U B , and U C  are harmonic signals which are 120° out of phase with respect to each other. This is illustrated in  FIG. 3 . Input signal U B  is seen to be shifted in phase by 120° relative to input signal U A . Input signal U C  is 120° phase shifted relative to input signal U B . Thus, excitation element  102  can be said to be driven by a polyphase excitation signal, and in this particular embodiment by a three-phase excitation signal, where each phase is defined as follows:
 
U A =U M  sin ωt
 
 U   B   =U   M  sin(ω t+ 120°)
 
 U   C   =U   M  sin(ω t+ 240°)  EQN 1
 
where U M  is a predetermined amplitude. However in general, excitation element  102  can comprise N emitter regions that are driven by an N-phase input signal.
 
     The embodiment of  FIG. 1  is a capacitive sensor, so the electromagnetic fields are electric in nature, and are also referred to as electric fields. The presence of the three electric fields emanating from the excitation element  102  causes charge variations on the conductive material  262  of the receptor element  106 . Since the electric fields are time-varying, the receptor element  106  will experience variations in charge. These variations in the charge can be measured to produce the single output signal U OUT  by measuring the voltage potential of the conductive material  262 . Since the input signals U A , U B , and U C  are time-varying signals, each of the three electric fields emanating from excitation element  102  is time-varying. The output signal U OUT  of the receptor element  106  is therefore a single time-varying voltage that is the result of the combined effect of the three electric fields on the conductive material  262 .  FIG. 3  shows an example of the output signal U OUT  of the receptor element  106 . 
     As with any capacitor, the dielectric material affects the rate of variation of charge. In accordance with the present invention, the rotating element  104  is the dielectric. Recall from  FIG. 2C , the rotating element  104  is not entirely of a single dielectric material. The first portion  242  of the rotating element  104  is dielectric material characterized by one dielectric constant ∈ 1 , while the remaining portion  246  is of another material and is characterized by a different dielectric constant ∈ 2 .  FIG. 2D  shows that the rotating element  104 ′ can simply comprise only the sector  242 ′ of a single dielectric material of dielectric constant ∈. 
     As the rotating element  104  (or  104 ′) rotates, the position of the dielectric material between the conductive regions  224 ,  226 ,  228  of the excitation element  102  and the electrically single conductive material  262  of the receptor element  106  will continually change. Consequently, different portions of the sensor  100  will have different capacitances as the rotating element  104  sweeps a circular path. The effect created by turning the rotating element  104  can be seen as phase shifts in the output signal U OUT  of the receptor element  106 . 
     This effect can be seen by the sequence illustrated in  FIG. 4 . To simplify the illustrations, the sequence shows turning of the rotating element  104 ′ as illustrated in  FIG. 2D . The rotating element comprises only a sector  242 ′ of dielectric material (shown here as the shaded element). At time t 0 , the sector  242 ′ is maximally aligned with conductive region  224  of the excitation element  102 , as energized by the configuration shown in  FIG. 2A . Consequently, the electric field produced by the conductive region  224  will be maximally coupled via the sector  242 ′ to the receptor element (not illustrated in this figure), while the electric fields from the other conductive regions  226 ,  228  will be coupled to the receptor element to a very much lesser extent. Assuming that the excitation element  102  is energized as shown in  FIG. 2A , the output signal U OUT  will be primarily determined by the response of the input signal U A  to the capacitance created by the three elements: conductive region  224 , dielectric material of the sector  242 ′, and receptor element (not shown). 
     At time t 1 , the sector  242 ′ is shown to have moved to a location that partially overlaps with conductive regions  224 ,  228 . Consequently, the electric fields produced by the conductive regions  224 ,  228  will be coupled via the sector  242 ′ to the receptor element (not shown), while the electric field from conductive region  226  will be coupled to the receptor element to a much smaller extent. The output signal U OUT  will be largely determined by the action of the input signals U A , U C  on their respective capacitances created by the conductive regions  224 ,  228 , the dielectric material of the sector  242 ′, and the receptor element. 
     At time t 2 , the sector  242 ′ is shown to have moved to a location that overlaps equal portions of conductive regions  226 ,  228 . Consequently, the electric fields produced by the conductive regions  226 ,  228  will be coupled via the sector  242 ′ to the receptor element, while the electric field from conductive region  224  will be minimally coupled to the receptor element. The output signal U OUT  will be largely determined by the action of the input signals U B , U C  on their respective capacitances created by the conductive regions  226 ,  228 , the dielectric material of the sector  242 ′, and the receptor element. 
     In accordance with the present invention, there is a one-to-one correspondence between the physical angular displacement of the rotating element  104  and the phase shift of the output signal U OUT  with respect to one of the input signals U A , U B , or U C  serving as a reference signal. This is explained in further detail in U.S. Pat. Nos. 5,872,408 and 6,101,084, and in pending U.S. application Ser. No. 11/413,420. For example, if the rotor is rotated by x° (geometrical), then the output signal U OUT  will be phase shifted by substantially x° (electrical) with respect to one of the input signals U A , U B , or U C . The present invention therefore, provides a direct indication of the angular position of the rotor. 
     This can be accomplished simply by monitoring changes in the phase difference between the output signal U OUT  and one of the excitation signals, e.g., U A , as the reference signal. At a given reference angular position of the rotating element  104 , there will be reference-position phase difference φ REF  (which could be zero) between the output signal U OUT  and the reference signal. As the rotating element  104  is turned, the change in phase difference between the reference signal U REF  and the output signal U OUT  will be substantially equal to the change in angular position of the rotating element from the reference position. 
     It is understood, of course, that in practice common signal processing will be required to obtain a usable signal. For example, A/D conversion may be needed to obtain a digital signal that a digital data processor can understand. There may be filtering of the measured output signal U OUT  to filter out noise and amplification to improve the signal-to-noise ratio. These signal processing steps are commonly performed on any measured signal in order to obtain a usable signal. It is noted that these signal obtaining steps are not performed for the purpose of determining angular position, but only for the purpose of obtaining a usable signal. In accordance with the present invention, the usable signal thus obtained requires no additional signal manipulations beyond being compared to a reference signal in order to ascertain a phase difference and hence angular position. 
       FIG. 5  shows a schematic illustration of the sensor  100 . The notation used in this figure is conventional notation used in describing multiphase systems. The figure illustrates a circuit diagram for driving the excitation element  102  and measuring the single output signal U OUT  of the receptor element  106 . Reference numeral  522  identifies a general representation of a multiphase signal source, in this case a three-phase source. Each phase serves as one of the input signals U A , U B , and U C . The output signal U OUT  is usually measured as a voltage across a load resistance R L . 
       FIG. 6  illustrates the circuit equivalent of the sensor  100 . The three emitter regions which comprise the excitation element  102  and the electrically single conductive material  262  of the receptor element  106  are equivalent to three variable capacitors CA, CB, and Cc connected in a star configuration to a common point  606 , where each capacitor is driven by a different excitation source. The signal generator  522  produces input signals U A , U B , and U C  as defined by equations EQN 1 given above. The capacitors CA, CB, and Cc are variable due to the changing location of the dielectric material of the first portion  242  of the rotating element  104  as it turns. 
     The amplitudes of the currents i A , i B , and i C , shown in  FIG. 7 , are produced using the simpler rotating element  104 ′ comprising only a single sector of dielectric material. In the case of the rotating element  104 , where there a first portion dielectric material  242  and a second portion  246  of another material, the current amplitudes will vary depending on the material of the second portion. In general, the current amplitudes will depend on the number of portions and materials used to construct the rotating element  104 . 
       FIG. 8  illustrates a second embodiment of a capacitive sensor  800  according to the present invention. The components of the sensor  800  are cylindrical elements rather than planar elements of the first embodiment; however, the operation is identical. Non-rotating excitation element  802  is configured to emit a plurality of electromagnetic fields, and corresponds to excitation element  102  of the embodiment shown in  FIG. 1 . Non-rotating receptor element  806  is configured to produce a single output signal in response to electromagnetic coupling of the electromagnetic fields emitted by the excitation element, and corresponds to the receptor element  106  shown in  FIG. 1 . As can be seen in the figure, the receptor element  806  is arranged concentrically with respect to the excitation element  802 . The electromagnetic coupling occurs through the rotating element  804 , which is concentrically disposed between the excitation element  802  and the receptor element  806 . Rotating element  804  corresponds to rotating element  104  in  FIG. 1 . Rotation of the rotating element  804  is provided by shaft  808  connected to the rotating element  804 . 
       FIGS. 9A and 9B  show cross-sectional views of the sensor  800  taken along view-lines  9 A- 9 A and  9 B- 9 B, respectively. As can be seen in the embodiment shown in  FIG. 9A , excitation element  802  is a cylindrical member comprising three emitter components  924 ,  926 ,  928  shaped to fit together as a cylinder. The emitter components  924 ,  926 ,  928 , which correspond to the emitter regions  224 ,  226 ,  228  shown in  FIG. 2A , are electrically isolated from each other. A non-conductive base  812  provides a supportive attachment for the emitter components  924 ,  926 ,  928 . The receptor element  806  is an electrically single conductive element that is attached to the base  812 . Wire attachments (not shown) are easily provided to the emitter components  924 ,  926 ,  928  of the excitation element  802  and to the receptor element  906  because these parts do not rotate; for example, passing wires through openings in the base  812  and connecting them to the emitter components  924 ,  926 ,  928  and the receptor element  906 . The sensor  800  is energized in the same way as described above for sensor  100 , and the output is detected in the same manner as in sensor  100 . 
       FIG. 10A  illustrates an example of a variation of the sensor  100  of  FIG. 1 , where the excitation element  1012 , the rotating element  1014 , and the receptor element  1016  each comprises a set of disks. Each of the disks of the rotating element  1014  is connected to a shaft  1018 . The excitation element  1012  and the receptor element  1014  can be attached to a suitable support structure (not shown) since these elements do not rotate. Assuming a three-phase embodiment, the disks of the excitation element  1012  each is driven (or energized) by input signals U A , U B , and U C  as discussed above for excitation element  102  of sensor  100  in  FIG. 1 . The disks of the receptor element  1016  are connected together to provide the single output signal U OUT . The disks of the rotating element  1014  each is constructed as described above for rotating element  104 . The sensor is assembled as shown in the figure by appropriately interleaving the disks. The resulting increase in surface area allows for increased charge accumulation on the receptor element  1016 , and hence produces a stronger output signal thus improving the signal to noise ratio of the sensor. 
       FIG. 10B  is a cross-sectional view that illustrates an example of a variation of the sensor  800  of  FIG. 8 , where the excitation element, the rotating element, and the receptor element each comprises a set of concentric cylinders. For example, cylinders  1082   a ,  1082   b  which constitute the excitation element are disposed on support base. Cylinder  1082   b  fits within and is concentrically aligned with cylinder  1082   a . Also disposed on the support base are cylinders  1086   a ,  1086   b  which constitute the receptor element. The receptor element cylinders  1086   a ,  1086   b  are arranged relative to the excitation element cylinders  1082   a ,  1082   b  such that concentric pairs of excitation and receptor cylinders are formed. For example, cylinders  1082   a  and  1086   a  form a pair and cylinders  1082   b  and  1086   b  form a pair. Cylinders  1084   a ,  1084   b  which constitute the rotating element are disposed on a rotating base, where cylinder  1084   b  fits within and is concentrically aligned with the outer cylinder  1084   a.    
     Again, assuming a three-phase embodiment, the cylinders  1082   a ,  1082   b  of the excitation element each is driven (or energized) by input signals U A , U B , and U C  as discussed above for excitation element  802  of sensor  800  in  FIG. 8 . The cylinders  1086   a ,  1086   b  of the receptor element are connected to provide the single output signal U OUT . The cylinders  1084   a ,  1084   b  of the rotating element each is sized and arranged on the rotating base to rotate between cylinder pairs  1082   a / 1082   a  and  1082   b / 1082   b , respectively. The resulting increase in surface area due to this interleaving of the elements allows for increased charge accumulation on the receptor element cylinders  1086   a ,  1086   b , and hence produces a stronger output signal thus improving the signal to noise ratio of the sensor. 
       FIG. 11  shows an example of an inductive sensor of angular position  1100  embodied in accordance with the present invention. A three-phase sensor will be described; however, it will be apparent that, in the general case, the present invention can be embodied in an N-phase sensor. The sensor  1100  comprises three emitter elements  1102   a ,  1102   b ,  1102   c  which collectively constitute the non-rotating excitation element of the sensor. In this embodiment of the present invention, the emitter elements  1102   a ,  1102   b ,  1102   c  are electromagnets. Each emitter element  1102   a ,  1102   b ,  1102   c  comprises a metallic core about which a coil is wound. The harmonic multi-phase input signals U A , U B , and U C  disclosed above are respectively provided to the coils of emitter elements  1102   a ,  1102   b ,  1102   c . The resulting electromagnetic fields are magnetic in nature, and can also be referred to simply as magnetic fields. 
     The rotating element  1104  is shown in  FIG. 11  as a 120° wedge-shaped element (sector) and is configured for rotation about an axis. The view lines A-A represents a view of through the sensor  1100  in a direction perpendicular to the drawing in the figure. The view along view lines A-A is shown in the inset in  FIG. 11 . The rotating element  1104  is connected to a shaft  1108 , which in turn can be connected to a rotating device for which knowledge of its angular position is desired. The rotating element  1104  is connected to the shaft  1108 . The rotating element  1104  and the shaft  1108  each is of a material that is characterized by a magnetic permeability μ that may or may not be equal to each other, thus providing a path of magnetic flux between the rotating element and the shaft. 
     As can be seen in the inset in  FIG. 11 , the non-rotating receptor element  1106  comprises a stationary bobbin  1116  about which a coil of wire  1126  is wound. The bobbin  1116  is disposed around and spaced apart from shaft  1108 , allowing the shaft to rotate while the bobbin remains stationary. An induced current flow through the coil of wire  1126  can be measured and used to provide the single output signal U OUT . The perspective view of this portion of the sensor  1100 , shown  FIG. 12 , illustrates some of the construction details in more detail. 
     Operation of the inductive sensor  1100  is similar to operation of capacitive sensor  100  discussed above. When the coils of the emitter elements  1102   a ,  1102   b ,  1102   c  of sensor  1100  are respectively energized by the time-varying multiphase input signals U A , U B , and U C , each emitter element will emit an electromagnetic field. In the case of inductive sensor  1100 , the electromagnetic fields are magnetic in nature. As the rotating element  1104  rotates in proximity to each of the emitter elements  1102   a ,  1102   b ,  1102   c , the magnetic fields emanating from the emitter elements will be coupled by magnetic induction to the rotating element. This creates a magnetic field in the rotating element  1104  and in the shaft  1108 . Since the input signals U A , U B , and U C  are time-varying signals, the magnetic field created in the rotating element  1104  and the shaft  1108  likewise is time-varying. The varying magnetic field emanating from the shaft  1108  induces a current in the coil of the receptor element  1106 . The resulting current flow in the coil is measured as the single output signal U OUT . 
     Since the rotating element  1104  is wedge-shaped, there will be times when there is no magnetic coupling, or very reduced magnetic coupling, of the magnetic fields emanating from one or more of the emitter elements  1102   a ,  1102   b ,  1102   c . For example, when the rotating element  1104  is aligned adjacent to emitter element  1102   a , then the magnetic coupling between the element  1102   a  and the rotating element will be maximal, while the magnetic coupling between the rotating element and the elements  1102   b  and  1102   c  will be minimal. The magnetic field created in the rotating element  1104 , and hence the output signal U OUT , will result primarily of the contribution of the magnetic coupling of the magnetic field produced by element  1102   a . As the rotating element  1104  continues to rotate and is aligned partially adjacent to elements  1102   a  and  1102   b , the magnetic field created in the rotating element and hence the output signal U OUT , will result from contributions from the magnetic fields of elements  1102   a  and  1102   b . The graphs in  FIG. 3  characterize this behavior. 
       FIG. 13  illustrates the circuit equivalent of the inductive sensor  1100 . The three emitter elements  1102   a ,  1102   b ,  1102   c  which comprise the excitation element of the sensor and receptor element  1106  coupled via the rotating element  1104  are represented as three variable inductors L A , L B , and L C  connected in a star configuration to a common point  1306 . The signal generator  1322  produces the three-phase input signals U A , U B , and U C  as defined by equations EQN 1 given above. The inductors L A , L B , and L C  are variable due to the changing location of the rotating element  1104  as it rotates. The amplitudes of the currents i A , i B , and i C  are substantially similar to those shown in  FIG. 7  for capacitive sensor  100 . 
     As in the case of the capacitive sensor  100 , the inductive sensor embodiment of the present invention is suitable for use in detecting angular position. Like the capacitive sensor  100 , the inductive counterpart can provide a direct indication of the angular position of the rotating element  1104 . As in the case of the capacitive sensor, this can be accomplished simply by monitoring changes in the phase difference between the output signal U OUT  and one of the excitation signals, e.g., U A , as the reference signal. At a given reference angular position of the rotating element  1104 , there will be reference-position phase difference φ REF  (which could be zero) between the output signal U OUT  and the reference signal. As the rotating element  1104  is turned, the change in phase difference between the reference signal U REF  and the output signal U OUT  will be substantially equal to the change in angular position from the reference position. 
       FIG. 14  show another embodiment of the rotating element  1104 . In  FIG. 14 , rotating element  1104 ′ comprises a first portion  1424  of a material having a first magnetic permeability μ 1 , and at least a second  1426  of another material having a second magnetic permeability μ 2  different from μ 1 . The rotating element  1104 ′ may comprise additional portions as well. The principle of operation is the same. As the rotating element  1104 ′ turns, the amount electromagnetic coupling of the electromagnetic fields produced by the emitter elements  1102   a ,  1102   b ,  1102   c  to the rotating element will vary. The resulting induced current in the receptor element  1106  will likewise vary and can be measured. This configuration would be suitable where a rotationally balanced structure is required. 
       FIG. 15  illustrates another embodiment of an inductive sensor  1500  according to the present invention. The inductive sensor  1100  had a “bent” inductive path, where the magnetic fields of the emitter elements  1102   a ,  1102   b ,  1102   c  were coupled to the rotating element  1104  in one plane and the magnetic field created in the rotating element and shaft is coupled to the receptor element  1108  in a different plane. In the case of inductive sensor  1500 , the inductive path is “straight”, all of the magnetic coupling occurs substantially in the same plane. 
     The emitter elements  1502   a ,  1502   b ,  1502   c  constitute the excitation element, and are similar to the embodiment disclosed above. The rotating element  1504  is substantially similar to the foregoing disclosed embodiments and is connected to a shaft  1508 . The rotating element  1504  and the shaft  1508  each is of a material that is characterized by a magnetic permeability μ that may or may not be equal to the other material, thus providing a path of magnetic flux between the rotating element and the shaft The shaft  1508  passes through a receptor element  1506  which is collinearly arranged with respect to the emitter elements  1502   a ,  1502   b ,  1502   c . Of course, it can be appreciated that other variations of the inductive sensor can be constructed. 
     Operation of sensor  1500  is similar to that of sensor  1100 . The magnetic fields from the emitter elements  1502   a ,  1502   b ,  1502   c  couple to the rotating element  1504 . A corresponding magnetic field is created in the shaft  1508 . The magnetic field created in the shaft  1508  induces a flow of current in the coil comprising the receptor  1506  which is then measured and serves as the output signal U OUT . 
     Both the capacitive and inductive sensors disclosed above are governed by the principles of electromagnetic theory. However, differences in technical properties and construction will dictate their suitability in different areas of use. For example, the capacitive sensor is simple in construction, and lends itself to implementation using mass production techniques including microelectronic manufacturing and nanotechnology fabrication. 
     Inductive angular position sensors, on the other hand, require the manufacture of coil windings. It is well known fact, that systems with windings are not easily adapted for microelectronic manufacturing techniques. Nonetheless, inductive position sensors still find use in certain applications, including for example electromechanical devices such as brushless DC motors where angular position information is necessary for proper operation. Inductive sensors also produce a stronger output signal, and thus generally offer better signal to noise performance as compared capacitive sensors. 
     In some applications (e.g., motors), the inductive sensor according to the present invention can simplify the design because the function of torque production and the function of obtaining an information signal indicative of angular position of the rotor use the same system of stator poles and windings. This advantageous aspect of the present invention arises from the constructional similarities between a motor and a sensor according to the present invention. Both utilize a multi-phase excitation system and have corresponding poles and windings. In this case, the sensor may not have separate poles with windings, and the general construction of a combined motor and sensor can be effectively simplified. This make the inductive sensor of the present invention an attractive design element for use in control of electromechanical devices.