Abstract:
Mechanical apparatus includes a base and a moving element, which is mounted to rotate about an axis relative to the base. A capacitive rotation sensor includes at least one first electrode disposed on the moving element in a location adjacent to the base and at least one second electrode disposed on the base in proximity to the at least one first electrode. A sensing circuit is coupled to sense a variable capacitance between the first and second electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application 61/929,140, filed Jan. 20, 2014, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to monitoring the motion of rotating mechanical devices, and particularly of scanning micromirrors. 
       BACKGROUND 
       [0003]    PCT International Publication WO 2014/016794, whose disclosure is incorporated herein by reference, describes scanning micromirrors, which are based on microelectromechanical systems (MEMS). Embodiments described in this publication provide scanning mirror assemblies that include a support structure; a base (also referred to as a gimbal), which is mounted to rotate about a first axis relative to the support structure; and a mirror, which is mounted to rotate about a second axis relative to the base. 
         [0004]    In one of the embodiments described in WO 2014/016794, capacitive sensing is used to monitor the rotation of the mirror, using plates of a capacitive sensor that are positioned in proximity to the mirror on opposite sides of the axis of rotation. In the disclosed embodiment, the plates are angled relative to the plane of the support structure, although in other implementations, the plates may be parallel to the plane of the support structure. Changes in the capacitance between the plates and the mirror are measured so as to monitor rotation of the mirror. 
       SUMMARY 
       [0005]    Embodiments of the present invention that are described hereinbelow provide improved techniques for capacitive sensing of miniature moving elements. 
         [0006]    There is therefore provided, in accordance with an embodiment of the present invention, mechanical apparatus, which includes a base and a moving element, which is mounted to rotate about an axis relative to the base. A capacitive rotation sensor includes at least one first electrode disposed on the moving element in a location adjacent to the base and at least one second electrode disposed on the base in proximity to the at least one first electrode. A sensing circuit is coupled to sense a variable capacitance between the first and second electrodes. 
         [0007]    In disclosed embodiments, the base defines a plane, and the moving element has a mechanical equilibrium position in the plane, such that the first and second electrodes are coplanar when the moving element is in the mechanical equilibrium position. The base and the moving element may be formed from a semiconductor substrate in a microelectromechanical systems (MEMS) process, wherein the electrodes and conductive traces connecting the electrodes to the sensing circuit are deposited on the semiconductor substrate as a part of the MEMS process. 
         [0008]    In one embodiment, the moving element includes a gimbal, and the base includes a frame on which the gimbal is mounted. Additionally or alternatively, the moving element may include a mirror, while the base includes a gimbal on which the mirror is mounted. 
         [0009]    Typically, the first and second electrodes have respective shapes that are elongated along a direction perpendicular and/or parallel to the axis about which the moving element rotates. 
         [0010]    In the disclosed embodiments, the sensing circuit is configured to output, responsively to the sensed capacitance, an indication of an angle of rotation of the moving element relative to the base. In some embodiments, the capacitance sensed by the sensing circuit varies nonlinearly with the angle of rotation of the moving element, and the sensing circuit is configured to apply both a magnitude of the capacitance and a slope of variation of the capacitance with rotation of the moving element in finding the angle of rotation as a function of the sensed capacitance. 
         [0011]    There is also provided, in accordance with an embodiment of the present invention, a method for sensing, which includes mounting a moving element to rotate about an axis relative to the base. At least one first electrode is disposed on the moving element in a location adjacent to the base, and at least one second electrode is disposed on the base in proximity to the at least one first electrode. A variable capacitance is sensed between the first and second electrodes as the moving element rotates about the axis. 
         [0012]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a schematic frontal view of a scanning assembly comprising a gimbaled scanning mirror with a capacitive rotation sensor, in accordance with an embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic frontal view of a scanning assembly comprising a gimbaled scanning mirror with a capacitive rotation sensor, in accordance with another embodiment of the present invention; 
           [0015]      FIG. 3  is a plot showing calculated capacitance curves as a function of rotation angle of a gimbal, in accordance with an embodiment of the present invention; 
           [0016]      FIG. 4  is a block diagram that schematically illustrates a scanning system, in accordance with an embodiment of the present invention; and 
           [0017]      FIG. 5  is a flow chart that schematically illustrates a method for scanning, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0018]    Embodiments of the present invention that are described hereinbelow provide capacitive sensing of the rotation of a moving element relative to a base. In the disclosed embodiments, the moving element is a gimbal, which holds a scanning mirror, while the base is a frame on which the gimbal rotates. Alternatively or additionally, the mirror may be the moving element, while the gimbal is the base. Further alternatively, the principles of the disclosed embodiments may be applied to devices of other types that include moving elements, particularly planar devices, such as MEMS devices. 
         [0019]    The disclosed embodiments sense rotation of the moving element without electrodes outside the device plane, which in typical MEMS implementations is the plane of the wafer. Thus, in the case of a scanning mirror or gimbal, for example, sensing of rotation is accomplished using electrodes that have been formed only in the plane of the mirror structure. These embodiments use changes in the capacitance between a pair of electrodes that are positioned side by side in-plane on the mirror structures themselves. The capacitance in this case changes as the result of changes of the fringing electric fields with distance between the electrodes and thus provides a means for accurately monitoring rotation angle. 
         [0020]    This sensing approach not only enables accurate measurement, but is also inexpensive and simple to implement. Because the capacitance varies nonlinearly with angle, it can be used for absolute position measurement, based on the location of the peak in the capacitance curve (which typically corresponds to the in-plane, zero-torque angle of the rotating device), as well as the shape of the curve. This mode of measurement is thus resilient in the face of gain variations of amplifiers in the sensing circuit and other factors that could otherwise distort the scale of the capacitance measurement. 
         [0021]    In the embodiments that are shown in the figures and are described in detail hereinbelow, capacitive sensors of this sort are used in sensing relative motion between a rotating gimbal and a frame, which serves as the support structure for the gimbal and mirror. In alternative embodiments, not shown in the figures, capacitive sensors based on fringing electric fields may be used, additionally or alternatively, in measuring the rotation angle of the mirror relative to the gimbal. More generally, the principles of the present invention may be applied in monitoring rotating structures of other types, particularly in MEMS devices, in which sensors of this sort can be produced as part of the photolithographic manufacturing process that is used in fabricating the devices themselves. 
         [0022]      FIG. 1  is a schematic frontal view of a scanning assembly  20 , which comprises a gimbaled scanning mirror  26  with a capacitive rotation sensor, in accordance with an embodiment of the present invention. Mirror  26  rotates on a pair of torsion hinges  30  (oriented along the Y-axis in the figure) relative to a gimbal  24 , which in turn rotates on another pair of torsion hinges  28  (oriented along the X-axis) relative to a frame  22 . Rotation of mirror  26  and gimbal  24  may be driven, for example, by the sorts of magnetic drives that are described in the above-mentioned PCT publication, or by any other suitable sort of drive that is known in the art. 
         [0023]    Scanning assembly  20  may typically be produced from a semiconductor wafer by MEMS micro-fabrication processes, in which the borders of mirror  26 , gimbal  24  and hinges  28 ,  30  are defined by a photolithographic mask, and the wafer is then etched to release the moving mirror and gimbal from the surrounding parts of the wafer. As another step in this process, a reflective metal coating (not shown) is deposited on the surface of the mirror. In this same step, or in another metal deposition step, inner metal electrodes  34  and  38  are deposited along the edges of gimbal  24 , and outer metal electrodes  32  and  36  are deposited on an adjacent area of frame  22 , as shown in the figure. Conductive traces  40 ,  44 ,  46  are also deposited on the wafer surface, connecting electrodes  32 ,  34 ,  36 ,  38  to connection pads  48 . It may be desirable to deposit an insulating layer, such as an oxide layer, over the wafer before depositing the metal electrodes, in order to eliminate any possible ohmic coupling between electrodes  34  and  38  on gimbal  24  and electrodes  32  and  36  on frame  22 . 
         [0024]    Each pair of metal electrodes—one electrode  34  or  38  on gimbal  24  and the other electrode  32  or  36  on frame—define a capacitor. The capacitance between the electrodes in each pair, due to the fringing fields of the electrodes, varies as a function of the gap between the electrodes and thus changes with the tilt angle of the gimbal. Frame  22  defines a plane, identified for convenience as the X-Y plane in  FIG. 1 . Gimbal  24  (as well as mirror  26 ) has a mechanical equilibrium position (zero torque angle) in the plane, such that electrodes  32 ,  34 ,  36  and  38  are coplanar when the gimbal is in the mechanical equilibrium position. Electrodes  23 ,  24 ,  26  and  38  have elongated shapes, with the long axes of the pairs of electrodes  32 / 34  and  36 / 38  oriented in the Y-direction, perpendicular to the axis of hinges  28  about which the rotation of gimbal  24  is to be measured. 
         [0025]    To measure the capacitance, and thus the angle of rotation (also referred to as the tile angle) of gimbal  24 , a sensing circuit  50  is connected to contact pads  48  and senses the variable impedance between electrodes  32  and  34  and between electrodes  36  and  38 . Sensing circuit  50  may sense the impedance, for example, by applying a modulated voltage between the electrodes, via conductive traces  40  and  46 , and sensing the resulting current (or vice versa). Sensing circuit  50  converts the sensed impedance to a corresponding value of rotation angle, typically based on a calibration function that is determined in advance. For these purposes, sensing circuit  50  may comprise, for example, a digital logic circuit with a frequency synthesizer and suitable digital/analog and analog/digital converters for analog coupling to the electrodes of assembly  20 , as well as a digital output, which outputs an indication of the angle of rotation. 
         [0026]    In typical applications, sensing circuit  50  outputs this indication of the rotation angle to a system controller (not shown in the figures), which may use the angular value, for example, in closed-loop control of the rotation of assembly  20 . Additionally or alternatively, the system controller may apply the angle measurements provided by sensing circuit  50  in calibrating and controlling the operation of a system based on scanning assembly  20 , such as a scanning LIDAR or projection system. Details of a system of this sort are shown in  FIG. 4 , while methods of control and calibration in such a system are shown in  FIG. 5  and are described hereinbelow with reference to these figures. 
         [0027]      FIG. 2  is a schematic frontal view of a scanning assembly  60 , in accordance with an alternative embodiment of the present invention. Assembly  60  is mechanically substantially identical to assembly  20  ( FIG. 1 ), but in the present embodiment, the metal pads that serve as electrodes  62 ,  64  and  66  of the capacitive rotation sensor are located in different areas of gimbal  24  and frame  22 . The long axes of electrode pairs  62 / 66  and  64 / 66  in assembly  60  are oriented along the X-direction, parallel to the axis of hinges  28 . In other respects, the operation of the capacitive rotation sensor in  FIG. 2  is similar to that in the preceding embodiment. The electrode configuration of  FIG. 1  is particularly effective for measuring rotation angle, while that of  FIG. 2  provides precise sensing of the in-plane, zero-torque position of the gimbal. In practice, the two embodiments may advantageously be combined, with electrodes deployed both perpendicular (as in  FIG. 1 ) and parallel (as in  FIG. 2 ) to the axis of hinges  28  about which gimbal  24  rotates. 
         [0028]    As noted earlier, the rotation of mirror  26  relative to gimbal  24  in assembly  60  can be monitored in similar fashion, by depositing electrodes on the mirror and on adjacent areas of the gimbal. Since the mirror has a reflective metal coating anyway, this metal coating may optionally also serve as an electrode of the capacitive sensor. 
         [0029]      FIG. 3  is a plot showing calculated capacitance curves  80 ,  82 ,  84 ,  86 ,  88  as a function of rotation angle of gimbal  24  relative to frame  22 , in accordance with an embodiment of the present invention. The calculation is based on a configuration that combines the electrodes of  FIGS. 1 and 2 , for different lengths L of the side electrodes ( 32 / 34  and  36 / 38 ). The lengths are smallest in curve  80  and increase in steps up to curve  88 , which represents the capacitance using the longest electrodes. As illustrated in  FIG. 3 , although  FIGS. 1 and 2  show particular electrode shapes and sizes, these features of the electrodes can readily be modified to give the desired capacitance range and behavior of the sensor. 
         [0030]    As shown by the curves in  FIG. 3 , the variation of capacitance is not linear in angle. Consequently, both the magnitude of the capacitance and the local slope of the curve can be used in measuring the rotation angle, and the accuracy of measurement can thus be enhanced. Because the two sets of electrodes—those on gimbal  26  and those on frame  24 —are formed on the same wafer, any temperature variations will have the substantially same effect on both sets of electrodes and thus will have no more than minimal impact on the measurement accuracy. 
         [0031]    Moreover, the nonlinearity of the variation of capacitance with angle can be used for absolute position measurement, based on the location of the central peak (corresponding to the in-plane, zero-torque angle) and the shape of the curve. This mode of measurement is thus resilient in the face of gain variations of the amplifiers and other factors that could otherwise distort the scale of the capacitance measurement. Compensating for such factors in a linear sensing configuration can require a difficult calibration procedure. 
         [0032]    Furthermore, although  FIGS. 1 and 2  show certain particular arrangements of the capacitive sensing electrodes on frame  22  and gimbal  24 , any other suitable arrangement of one or more pairs of electrodes may be used for this purpose, so long as the sizes of and spacing between the electrodes are such as to give a substantial capacitive response that varies with rotation of the gimbal or other structure. 
         [0033]      FIG. 4  is a block diagram that schematically illustrates a scanning system  100 , in accordance with an embodiment of the present invention. System  100  comprises an optical head  102 , which incorporates scanning assembly  20  and sensing circuit  50 , as described above. An optical transmitter/receiver  104  transmits pulses of light toward mirror  26  in scanning assembly  20  and receives light returned from the mirror. Alternatively, optical head  102  may comprise only the transmitter or only the receiver. Driver circuits  106  control the scanning frequency, phase and amplitude of scanning assembly  20 , as well as controlling operation of transmitter/receiver  104 , such as the amplitude and repetition rate of the transmitted pulses. 
         [0034]    A controller  108  comprises control circuits  110 , which receive signals from sensing circuit  50  and provide control outputs accordingly to drivers  106  under the command of a system processor  112 , which comprises one or more processing units. The control outputs may, for example, cause drivers  106  to adjust the frequency, phase and/or amplitude of scanning assembly  20  as necessary. Processor  112  may also use the readings of scanning angle provided by sensing circuit in processing the signals output by the receiver in optical head  102 . Controller  108  typically comprises ancillary circuits, such as a power supply  114  and other components that are known in the art. Although the functional elements of controller  108  are shown in FIG.  4 , for the sake of conceptual clarity, as separate blocks, some or all of these elements may be combined in a single integrated circuit. 
         [0035]      FIG. 5  is a flow chart that schematically illustrates a method for scanning using system  100 , in accordance with an embodiment of the present invention. Controller  108  receives a sequence of input signals or data from sensing circuit  50 , indicating the rotation angle of scanning assembly  20  as a function of time, at a sense input step  120 . Based on these signals or data, the controller computes the actual rotation angle as a function of time, at an angle computation step  122 . The computed angles may be used for (at least) two purposes:
       Based on the angle readings, controller  108  computes the frequency of rotation of scanning assembly  20 , as well as the phase and amplitude of rotation, at a frequency computation step  124 . The controller checks these values against corresponding benchmarks, such as preset frequency and amplitude targets, at a parameter checking step  126 . If the computed values deviate from the benchmarks, controller  108  sends an appropriate command to driver circuits  106 , so as to cause the driver to adjust the scanning parameters. The control loop (regardless of the result of step  126 ) then returns to step  120  for the next iteration.   Controller  108  may also use the angle readings in calibrating the signals received from transmitter/receiver  104  in optical head  102 , at a signal calibration step  130 . For example, the angle readings may be used in order to ascertain accurately the angle at which each signal from the receiver is received.       
 
         [0038]    It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.