Abstract:
A microelectromechanical system in which, operationally independent of the movable element and the component for moving it, a microelectromechanical sensor senses the position of the movable element. The microelectromechanical sensor adjoins the movable element, at least in part, and may be a strain gage, or a gage of a capacitive, piezoelectric, piezoresistive, or pressure type, among others. The resulting signal is fed back to control the component for moving the movable element. In an array of movable elements and sensors, the signal of each sensor is specific to one movable element.

Description:
TECHNICAL FIELD 
     This invention relates generally to the field of microelectromechanical devices and systems and in particular to communication, telemetry, and information processing using microelectromechanical devices and systems. 
     BACKGROUND OF THE INVENTION 
     Microelectromechanical devices have become increasingly used for applications for which no comparable non-mechanical electronic device is available. Even for switching applications for optical communication, telemetry, and information processing systems, for which non-mechanical electronic devices do exist, a need for augmented capabilities is frequently met by microelectromechanical devices. 
     Nevertheless, as increasing numbers of devices are fit into ever-smaller spaces, optical microelectromechanical devices (hereinafter, “MEMS”) and particularly their constituent actuatable MEMS elements present an increasing problem of sensing their respective actuated positions. One frequently used technique for handling this problem involves illuminating the constituent elements with infrared radiation, typically coherent radiation, detecting the pattern of reflected radiation with a high-resolution infrared video camera, and employing a computer to calculate the respective positions from the detected pattern. Unfortunately, as the number of MEMS elements in a given area increases, and as their speed of actuation increases, the computer software required for timely, accurate calculations becomes ever more complex and less reliable. 
     Such position detection techniques may be termed indirect techniques. It has become desirable to have reliable direct detection techniques. 
     While some MEMS sensors for directly detecting the position of a actuated element have been developed for some applications, their feasibility is not readily apparent for dense arrays of MEMS elements requiring a correspondingly dense array of electronic actuation circuits. Position sensing must not interfere with element actuation. 
     SUMMARY OF THE INVENTION 
     According to the invention, direct position detection of MEMS elements is achieved without interference with their electronic actuation. It has been recognized, according to the invention, that position detection circuitry can have elements in common with MEMS element actuation circuitry, so long as the detection circuitry and actuation circuitry are operationally independent of each other. 
     Thus, in a microelectromechanical device having a microelectromechanical movable element, a base with respect to which the element is movable, and means for moving the movable element with respect to the base, there is provided means independent of the moving means, but in proximity thereto, for sensing the actual position of the movable element with respect to the base. 
     Advantageously, according to one feature of the invention, this sensing arrangement is densely integratable with large numbers of MEMS elements and their actuation circuits. Preferably, each movable MEMS element is associated with at least one sensor specifically arranged for that movable MEMS element. Optical cross-connect switching circuits provide an exemplary application of this type. 
     According to a further feature of the invention, the sensing means can employ any of a plurality of sensing gages, for example, strain gages, capacitive gages, piezoelectric gages, and piezoresistive gages, among others. Advantageously, such gages can be employed whether the actuation movement is linear, rotary, or a combination of linear and rotary. 
     In a first specific embodiment of the invention, MEMS strain gages are mounted under the MEMS element. The amount of strain is directly proportion to the movement of the MEMS element. Without increasing the amount of area occupied by the MEMS element, a plurality of strain gages per MEMS element may be used. 
     In a second specific embodiment of the invention, MEMS capacitance gages are integrated with the MEMS element. Movement of the MEMS element changes the capacitance between a plate on the back of the MEMS element and a plate on the mounting base of the MEMS element. Advantageously, the two plates can be the same as those used for actuating the MEMS element. Independence of actuation and sensing resides in actuating movement by essentially DC or low frequency voltage, while the capacitance measurement is made at a higher frequency not affecting actuation. 
     It is a further advantage of the present invention that sensing arrangements for arrays of MEMS elements are relatively easy to design and have a shorter response time and greater reliability than the prior art reflected light arrangements. 
     Further features and advantages of the invention will become apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a pictorial illustration of a first embodiment of the invention; 
     FIG. 2 is a pictorial illustration of a second embodiment of the invention; 
     FIG. 3 is a block diagrammatic illustration of a control arrangement for the embodiment of FIG. 2; and 
     FIG. 4 is a pictorial illustration of an array implementation of the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Preferred implementations of the various aspects of the invention will now be described while referring to the figures, several of which may be simultaneously referred to during the course of the following description. 
     In FIG. 1 MEMS device  11  has a semiconductor wafer substrate forming a base  13  for a MEMS element array, of which MEMS steerable mirror  15  is a typical element. MEMS steerable mirror  15  is suspended on springs  19  which support it in a nominal position parallel to base  13 . Plate electrodes  17  form a pair of parallel plate capacitors with the conductive back of MEMS steerable mirror  15 . Electrodes  17  are arranged so that opposite polarity voltages on one side of mirror  15  apply a pulling deflection torque while the voltages on the other side are ‘off’. Each of a pair of strain gages  21  is mounted in proximity to electrodes  17  from base  13  to respective edges of MEMS steerable electrode  15 . As an alternative (not shown), Strain gages  21  may be mounted on springs  19 . In either case, as a strain gage is stretched or compressed a change in resistance occurs. This change in resistance has a direct, nearly linear, relationship to the deflection of MEMS steerable mirror  15 . It may be seen that, in this embodiment, the position detection circuitry, including strain gages  21 , is independent of the MEMS element actuation circuitry, including electrodes  17  together with the back of mirror  15 . 
     In FIG. 2 MEMS device  31  has a semiconductor wafer substrate forming a base  33  for a MEMS element array, of which MEMS steerable mirror  35  is a typical element. MEMS steerable mirror  35  is suspended on springs  39 , which support it in a nominal position parallel to base  33 . Plate electrodes  37  form a pair of parallel plate capacitors with the conductive back of MEMS steerable mirror  35 . Electrodes  37  are arranged so that opposite polarity voltages apply a pulling deflection torque to MEMS steerable mirror  35  on just one side. 
     Position detection capacitors are formed with the back of MEMS steerable mirror  35  by auxiliary electrodes  41 . Auxiliary electrodes  41  may be separate from electrodes  37 , or an extension thereof To measure the capacitance of electrodes  41  with respect to the back of MEMS steerable mirror  35 , a relatively high frequency is applied (by a test signal generator not shown) between the back of MEMS steerable mirror  35  and electrodes  41 . Another relatively low frequency is applied between the back of MEMS steerable mirror  35  and electrodes  37 . That frequency is low enough to produce a deflection of MEMS steerable mirror  35 . 
     In operation, whether or not electrodes  41  are joined to adjacent electrodes  37  (as may be advantageous in an integrated circuit embodiment), the position detection operation remains independent of the actuation operation because the higher frequency used for position detection has substantially no effect on the actuation, which is the deflection of MEMS steerable mirror  35 . Further, this embodiment is adapted for application in an optical cross-connect switching system by an array of movable mirror like mirror  35  on a common substrate, the array being coupled to an array of optical fibers. In such an array of movable elements and sensors, the signal of each sensor is specific to one movable element. 
     FIG. 3 shows in block diagram form a preferred type of control circuitry for the embodiment of FIG.  2 . In signal generator  51 , signals from each combination of HEMS steerable mirror  35  and a sensing electrode  41  (there being four such electrodes for one mirror  35 ) are combined in known fashion or sent in a pair of signal paths to capacitance bridge  52 , which separately receives a signal representing a reference capacitance. This signal also represents a desired deflection of mirror  35 , as determined by external control circuitry. An error signal from the balance terminals of capacitance bridge  52  is sent to voltage controller  53 , which generates and applies a corrective signal to deflection electrode  37  for MEMS mirror  35 . Electrode  37  and MEMS mirror  35  form a MEMS control capacitor  54 . Each of signal generator  51 , capacitance bridge  52 , and voltage controller  53  can be implemented in any of several known manners, to the extent that they are mutually compatible. 
     In the operation of FIG. 3, the signal to set the reference capacitance of bridge  52  is calibrated for a particular steerable mirror  35  by observing the spots to which that mirror  35  reflects an optical beam. For an array (not shown) of steerable mirrors, separate calibrations for each mirror is stored in a memory (not shown) of the external control circuitry. 
     Feedback circuitry for the embodiment of FIG. 1 can also be implemented in any of several known manners, including using a bridge analogous to that of FIG.  3 . 
     In FIG. 4 MEMS device  61  has a semiconductor wafer substrate forming a base  63  for a MEMS element array, including MEMS steerable mirrors  65 ,  66 ,  67 , and  68 , as typical elements. These mirrors are suspended on springs  69 ,  70 ,  71 , and  72 , respectively, which support the mirrors in nominal positions parallel to base  63 . Plate electrodes  81 ,  82 ,  83 , and  84 , respectively form a pair of parallel plate capacitors with the conductive back of MEMS steerable mirror  65 ,  66 ,  67 , and  68 . Electrodes  81 ,  82 ,  83 , and  84  are arranged so that opposite polarity voltages apply a deflection torque to the respective MEMS steerable mirror. 
     Position detection capacitors are formed with the back of MEMS steerable mirrors  65 ,  66 ,  67 , and  68 , by auxiliary electrodes  91 ,  92 ,  93 , and  94 , which may be separate from electrodes  81 ,  82 ,  83 , and  84 , or may be extensions thereof To measure the capacitance of electrodes  91 ,  92 ,  93 , and  94  with respect to the back of MEMS steerable mirrors  65 ,  66 ,  67 , and  68 , a relatively high frequency is applied (by a test signal generator not shown) across each respective capacitor. Another relatively low frequency is applied between the back of each MEMS steerable mirror and the respective one of drive electrodes  81 ,  82 ,  83 , and  84 . That frequency is low enough to produce a deflection of the MEMS steerable mirror. 
     The operation of each element of the array of FIG. 4 is like that of the corresponding element of FIG.  2 . It should be apparent that the four-element array of FIG. 4 can be extended to a large number of elements. In such an array, each movable MEMS element has in direct association a plurality of sensors that serve only it. 
     It should be apparent that the foregoing embodiments can be modified, and other embodiments (e.g., embodiments having piezoelectric, piezoresistive, or pressure type sensing arrangements) can be implemented, without departing from the spirit and scope of the invention, as determined by the following claims and their equivalents.