Abstract:
Disclosed is an apparatus and method for detecting whether rotatable MEMS elements are in the “on” or “off” position. Embodiments of the invention have application in devices switches that employ mirrors that move between an “on” or “off” position, wherein they reflect light from an input fiber into an output fiber in the “on” position, and allow the light to pass in the “off” position. Electrodes are positioned in the device such that the mirrors are close to, and therefor capacitively coupled to, a different electrode depending on whether they are in the “on” or “off” position. This invention is especially useful for switches that already employ electrodes for electrostatic clamping of mirrors in one or more positions, since those same electrodes can be used both to electrostatically clamp the mirrors and to sense their position. The method described in this invention comprises sensing of the capacitance between the mirrors and the one or more electrodes used to clamp the mirrors in its one or more position in order to detect which of the positions the mirrors are clamped in. Furthermore, the magnitude of the capacitances can be monitored to detect improper clamping.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on Provisional application No. 60/196,055, filed Apr. 10, 2000, which is herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to microelectromechanical systems (MEMS). More particularly, it relates to sensing a control state of MEMS devices. 
     BACKGROUND ART 
     Previous patents and publications have described fiber-optic switches that employ moveable micromirrors that move between two positions. An example is shown in FIG.  1 . Some of the prior art also employs electrostatic clamping of these mirrors at one or more of its two positions. For example, optical crossbar switches consisting of a series of moveable mirrors that are magnetically actuated are known in the art. The mirrors can be electrostatically clamped either in the horizontal position to the substrate or in the vertical position to the sidewalls of a separate chip. In the vertical position, the mirrors deflect light from an input fiber into an output fiber. 
     Previous work has described optical switches that use mirrors that are actuated between several discrete positions for switching light. These optical switches may rely on electrostatic comb drives to rotate the mirror. An electrostatic comb drive uses electrostatic forces between interdigitated fixed and movable comb members to rotate a device such as a mirror. It is possible to determine the relative angular position of the movable comb drive member with respect to the fixed comb drive member by measuring the capacitance between them. Unfortunately, comb drives have a limited range of angular movement and the capacitance may change only slightly over a desired range of discrete positions of the mirror. If each discrete position represents a different control state of an optical switch, it is difficult to correlate the capacitance measurement from the comb drive to the control state of the switch. Furthermore, not all optical switches use comb drive actuators. 
     Some of the prior art approaches require electrostatic clamping of a mirror structure to various electrodes in its different positions. For example, Behin et al. describe an optical crossbar switch consisting of a series of moveable mirrors that are magnetically actuated and can be electrostatically clamped either in the horizontal position to the substrate or in the vertical position to the sidewalls of a separate chip. Fujita et al. describe similar micromirrors that are electrostatically clamped against a shallow stop when deflected vertically. 
     FIG. 1 depicts an optical crossbar switch  100  that contains mirrors  102  that rotate between horizontal and vertical positions in order to switch optical signals  104  between one or more input fibers  106  and one or more output fibers  108 . The mirrors  102  are typically rotated by a combination of magnetic force and mechanical torsion in a hinge member (not shown) at the axis of rotation of the mirror. Optical switches like that shown in FIG. 1 are described in detail in U.S. Pat. No. 4,580,873, entitled “Optical Matrix Switch,” Issued Apr. 8, 1986 to Frank H. Levinson, which is incorporated herein by reference. Comb drives are generally not used to actuate this type of switch because it is difficult to create a comb drive that could directly move the mirror over the desired angular range without some additional mechanical linkage. One means for fault detection in optical switches involves monitoring of the optical signals received by the output fibers  108 . A splitter incorporated into the switch or fiber taps at the output fibers  108  can be used to monitor the output signals. This prior art method may also require monitoring of the input signal, since the criteria for failure is often a discrepancy between the input and the output signals. Unfortunately, monitoring the input and output optical signals incurs additional optical losses to the switch  100  since it requires tapping optical energy from the signals for monitoring. Furthermore, monitoring the input and output signals does not specifically indicate the cause of the failure, as the mirror position is not directly monitored. 
     There is a need, therefore, for an improved MEMS device with improved fault detection to directly detect faults in the control state of the mirror positioning mechanism. 
     OBJECTS AND ADVANTAGES 
     Accordingly, it is a primary object of the present invention to provide microelectromechanical system (MEMS) device having a fault detection system that directly measures mirror control state. 
     SUMMARY 
     The objects and advantages are attained by an apparatus and method that allow for detection of whether a rotatable MEMS element is in a first or second position, , e.g., horizontal or vertical, and whether it is properly clamped in either of these two positions. This sensing capability is useful for fault detection. By sensing the mirror position, mirror failure can be immediately detected, and traffic through the switch can be appropriately re-routed. Embodiments of the invention provide apparatus and methods for detecting whether mirrors used in a certain type of optical switch are in the “on” or “off” position. Specifically, this invention applies to switches that employ mirrors that move between an “on” or “off” position, wherein they reflect light from an input fiber into an output fiber in the “on” position, and allow the light to pass in the “off” position. Electrodes are positioned in this system such that the mirrors are close to, and therefor capacitively coupled to, a different electrode depending on whether they are in the “on” or “off” position. This invention is especially useful for switches that already employ electrodes for electrostatic clamping of mirrors in one or more positions, since those same electrodes can be used both to electrostatically clamp the mirrors and to sense their position. The method described in this invention comprises sensing of the capacitance between the mirrors and the one or more electrodes used to clamp the mirrors in its one or more positions in order to detect which of the positions the mirrors are clamped in. Furthermore, the magnitude of the capacitances can be monitored to detect improper clamping. The apparatus may be incorporated into a MEMS mirror optical switch controlled by a computer processor. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 depicts a crossbar configuration for a MEMS optical switch according to the prior art; 
     FIG. 2A-2B depict simplified schematic diagrams of a MEMS device according to an embodiment of the present invention; and 
     FIG. 3 depicts a simplified block diagram of an MEMS apparatus according to an embodiment of the present invention; 
     FIG. 4A depicts simplified cross sectional schematics of the apparatus of FIG. 3 in three different positions; 
     FIGS. 4B-4C depict capacitance values corresponding to the three positions depicted in FIG. 4A; 
     FIG. 5 depicts a simplified timing diagram for operation of a MEMS device according to an embodiment of the present invention; and 
     FIG. 6 depicts a block diagram depicting an optical communications system according to an additional embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Although the following detailed description contains many specifics for the purposes of illustration, anyone of ordinary skill in the art will appreciate that many variations and alterations to the following details are within the scope of the invention. Accordingly, the following preferred embodiment of the invention is set forth without any loss of generality to, and without imposing limitations upon, the claimed invention. 
     FIGS. 2A-2B depict an apparatus  200  according to an embodiment of the invention. The apparatus generally comprises a rotatable element  202 , and first and second electrodes  204 ,  206 . The first electrode  204  is typically located adjacent to the element  202  when element  202  is in its vertical position. The second electrode  206  is typically located adjacent to the element  202  when element  202  is in its horizontal position. For the purpose of example, and without loss of generality, the rotatable element  202  may be a MEMS mirror that rotates about a substantially horizontal axis  201  relative to a static part  203 . The rotatable element may include a separate electrode for clamping or capacitance sensing. Alternatively, if the rotatable element  202  is electrically conductive, the element  202  itself may be regarded as an electrode. In the exemplary embodiment depicted in FIGS. 2A-2B the rotatable element  202  rotates between two positions that are substantially 90° apart. In particular, the rotatable element rotates between a vertical position, as shown in FIG. 2A, and a horizontal position, as shown in FIG.  2 B. The vertical position defines a first or “on” control state. The horizontal position defines a second or “off” control state. In the embodiments of the present invention the capacitance between the rotatable element  202  and the electrodes  204 , 206  depends on whether the rotatable element is in the first or second position. 
     The first electrode  204  can be placed so that it is disposed close to and substantially parallel with the rotatable element  202  in the vertical position. The capacitance between the rotatable element  202  and the first electrode  206  can be monitored to determine the control state of the rotatable element  202 . For example when the rotatable element  202  is flipped to the vertical position from the horizontal position, the capacitance between the element  202  and the first electrode  204  changes from a low value to a much higher value. At the same time, the capacitance between the rotatable element  202  and the second electrode  206  changes from a high value to a lower value. In a similar fashion, the capacitance between the second electrode  206  and the rotatable element  202  can be used to detect the control state of the when it is in the horizontal position. The magnitude of the “on”-state capacitance is known, and if the element  202  is somehow improperly positioned in the “on” state, the capacitance may not reach the known value, and a fault may be indicated. 
     In the device  200 , the electrodes  204 ,  206  may also serve as clamping electrodes as well as for capacitive control state sensing. Alternatively, the device  200  may include separate electrodes for sensing and clamping. In the case of an array of rotatable elements, e.g., MEMS mirrors, the electrodes for the “off”, or horizontal, state detection may be electrically shorted to each other. In such a case, the static part  203  may comprise a substrate to which the mirrors are mounted. Similarly, the “on”, or vertical, state electrodes may comprise a single component with features that define a vertical electrode for each mirror. 
     FIG. 3 depicts a simplified cross-sectional schematic diagram of an apparatus  300  according to an embodiment of the present invention. The apparatus generally comprises a MEMS device  310 , and a device controller  320 . The device  310  typically includes a substrate  311  and a rotatable element  312 , such as a mirror. The substrate  311  includes a vertical stop  315  and a horizontal stop  317 . The rotatable element  312  rotates about an axis oriented substantially parallel to a plane of the substrate  311 . The rotatable element  312  may be attached to the substrate  311  by a torsional flexure  313 . The rotatable element  312  rotates, e.g. under magnetic actuation, between a vertical position proximate the vertical stop  315  and a horizontal position proximate the horizontal stop  317 . The substrate  311  further includes vertical and horizontal electrodes  314 ,  316  proximate the vertical and horizontal stops  315 ,  317 . The electrodes  314 ,  316  are typically electrically isolated from each other and from the rotatable element  312 . The controller  320  typically includes a processor  321 , a fault detector  322 , a state selector  323 , vertical and horizontal capacitance sensors  324 ,  326  and vertical and horizontal power voltage sources  327 ,  329 . The state selector  323  and fault detector  322  are coupled to the processor  321 . The capacitance sensors  324 ,  326 , are coupled to the electrodes  314 ,  316  respectively and to the fault detector  322 . Conditioning electronics  325 V,  325 H, such as amplifiers or analog to digital (A/D) converters, may optionally be coupled between the capacitance sensors  324 ,  326  and the fault detector  322 . In the embodiment shown in FIG. 3, the voltage sources  327 ,  329  are coupled to the electrodes  314 ,  316  respectively. The voltage sources  327 ,  329  supply clamping voltages to the electrodes  314 ,  316  to clamp the rotatable element to the vertical stop  315  or the horizontal stop  317 . Alternatively, the device  310  may include separate clamping electrodes coupled to the voltage sources  327 ,  329 . 
     For example, it is often the case that the capacitance sensors are coupled to conditioning electronics that interpret the signals from the capacitance sensors. Such conditioning electronics may include amplifiers, analog-to-digital converters, and the like. It is often desirable to ensure that the conditioning electronics receive signals from the sensors having an acceptable level of noise. The acceptable value of the noise level depends on the circuit and the required precision in the specific application. For very small capacitance signals, e.g., of order 10 −15  farads, this may affect the design of the apparatus. 
     For example, to reduce the signal to noise ratio, it may be important that the conditioning electronics be located in close proximity to the capacitive sensors. A short distance between the sensors and the electronics reduces the amount of wiring between them, thereby reducing noise. Close proximity between the sensor and the electronics may be ensured by placing the conditioning electronics in the same packaging as the sensor, e.g., on a die adjacent to a die containing a MEMS device with the sensors. The sensors and conditioning electronics may be connected by wire bonding across the die. Alternatively, the conditioning electronics may be integrated into the same die as the MEMS die itself. 
     Although only a single device  310  with a rotatable element  312  is shown in FIG. 3, those of skill in the art will recognize that the device  310  may include an array containing any number of such devices. Furthermore, the inventive concepts described herein may also be applied to micromirror architectures such as those described in H. Toshiyoshi and H. Fujita, “Electrostatic micro torsion mirrors for an optical switch matrix,”  J. Microelectromech. Syst ., vol. 5, no. 4, 231-7, Dec. 1996 and E. L. Goldstein, and R. W. Tkach, “Free-space micromachined optical switches with sub-millisecond switching time for large-scale optical crossconnects,” OFC&#39;98 and IEEE Photonics Technol. Lett., Apr. 1998, both of which are incorporated herein by reference. 
     The relationship between the position of the rotatable element  312  and the capacitance values measured by the sensors  314 ,  316  is illustrated in FIGS. 4A-4C. When the rotatable element is in the vertical state, as shown on the left in FIG. 4A, a large capacitance is detected between the rotatable element  312  and the vertical electrode  314 , and a small capacitance is detected between the rotatable element  312  and the horizontal electrode  316  as shown on the left of FIGS. 4B and 4C. This combination of capacitances indicates that the rotatable element  312  is in an “up” digital control state. When the rotatable element  312  is switching and is in between the vertical position and the horizontal position (or vice versa), as shown in the middle in FIG. 4A, a small capacitance is detected between the rotatable element  312  and both the horizontal electrode  316  and the vertical electrode  314  as shown in the middle of FIGS. 4B and 4C. When the rotatable element  312  is in the horizontal position, as shown on the right in FIG. 4A, a large capacitance is detected between the rotatable element  312  and the horizontal electrode  316 , and a small capacitance is detected between the rotatable element  312  and the vertical electrode  314  as shown on the right of FIGS. 4B and 4C. This combination of capacitances indicates that the rotatable element  312  is in a “down” digital control state. The capacitance can be measured across the same electrical connections that are used to supply the electrostatic clamping voltages from the voltage sources  327 ,  327  and the electrodes  314 ,  316 . 
     The processor  321  determines the appropriate control state for the rotatable element  312  and supplies a control signal to the state selector  323  and the fault detector  322 . The state selector  323  determines which voltage source  327 ,  329  applies a clamping voltage based on a control signal from the processor. Those of skill in the art will recognize that the state selector  323  may be implemented in either hardware, software or a combination of both. Although two voltage sources  327 ,  329 , are depicted in FIG. 3, the control state selector may alternatively be connected to a single voltage source, which is selectively coupled to the electrodes  314 ,  316  by a switch. The fault detector  322  compares the control signal from the processor to a measured control state determined by measurements from the capacitance sensors  324 ,  326 . In either the horizontal or vertical position, the sensors  324 ,  326  can detect exact magnitude of the capacitance to indicate improper clamping of the rotatable element  312 . For example, if a particle (e.g. a piece of dust) lands on one of the clamping surfaces and causes the mirror to clamp at an improper angle to the vertical sidewall, the capacitance detected between the rotatable element  312  and the vertical electrode  314  will be different than that normally detected in the vertical control state. In such a situation the fault detector  322  would signal a fault to the processor  321 . Those of skill in the art will recognize that the fault detector  322  may be implemented in either hardware, software or a combination of both. 
     The apparatus  300  may operate according to a method according to an embodiment of the present invention. The method  500  is set forth in the flow diagram of FIG.  5 . In the method  500  begins at step  502  with the provision of an apparatus with a rotatable element and such as the apparatus  300 . Electrodes, such as the vertical and horizontal electrodes  314 ,  316  are provided at step  504 . At step  506  a capacitance between the rotatable element and one or more of the electrodes is measured, e.g. with sensors such as the sensors  324 ,  326 . In the apparatus  300 , the capacitance sensors  324 ,  326  measure the capacitance between the rotatable element  312  and the electrodes  314 ,  316  to monitor the control state of the rotatable element  312 . Various methods exist for detecting the capacitance between the rotatable element  312  and the electrodes  314 ,  316 . For example, in step  506 , a small AC signal may be superimposed on top of a DC signal that is supplied by one or more of the voltage sources  327 ,  329  for electrostatic clamping. The sensors  314 ,  316  can monitor a current arising from this small AC signal to indicate the capacitance. An alternative method employs time-division multiplexing of actuation and sense signals. In this scheme, the DC actuation signal is periodically turned off and replaced by a small AC or DC sense signal. The sensors  314 ,  316  measure the sense signal to monitor the capacitance. Preferably, the time-multiplexing is done at a rate much faster than the natural frequency of the device 
     The capacitance signals from the sensors  314 ,  316  can be used to properly time the electrostatic clamping signals used for clamping the rotatable element  312  in its two positions. For example, when the rotatable element  312  is actuated up to a position near the vertical stop  315 , the processor signals the state selector to apply a voltage to the vertical electrode  314  to pull the rotatable element  312  in to the vertical stop  315  and clamp it there electrostatically. After the rotatable element  312  is pulled in, the voltage can be reduced to a lower value, since a lower voltage is needed to hold the rotatable element  312  next to the electrode  314  than that needed to pull it in. Monitoring of the capacitance signal can allow proper timing of these signals. That is, the clamp voltage would be lowered only when the capacitance value from the vertical capacitance sensor  324  indicates that the rotatable element  312  has reached the vertical position. 
     In the descriptions above, it is assumed that the electrodes used for clamping are also used for sensing. It is also possible to divide the electrode structures into several isolated regions, in which case one set of electrodes can be used for electrostatic clamping or actuation, and another set for capacitive sensing. 
     FIG. 6 depicts a block diagram depicting an optical communications system  600  according to an additional embodiment of the invention. In the system  600 , a method having features in common with step  506  of method  500  of FIG. 5 is implemented as a computer program code  605  running on a processor of a computer controlled apparatus having features in common with the apparatus  300  described above with respect to FIG.  3 . In the exemplary embodiment shown, the program code  605  controls the operation of one or more MEMS mirrors M in a crossbar optical switch S. The switch S may have features in common with the type of switch  100  shown in FIG.  1 . One or more input fibers IF and output fibers OF are coupled to the switch S. Each mirror M is rotatably coupled to a substrate and actuated by electrostatic or magnetic actuators A. The mirrors M are clamped in the vertical or horizontal position by voltages applied to clamping electrodes CE. 
     The system  600  includes a controller  601 . The controller  601  includes a programmable central processing unit (CPU)  602  that is operable with a memory  604  (e.g., RAM, DRAM, ROM, and the like) an optional mass storage device,  606  (e.g., CD-ROM hard disk and/or removable storage), and well-known support circuits  610  such as power supplies  612 , clocks  614 , cache  616 , input/output (I/O) circuits  618  and the like. All of the above elements may be coupled to a control system bus  608 . 
     The memory  604  contains instructions that the processor unit  602  executes to facilitate the performance of the apparatus  600 . The instructions in the memory  604  are in the form of the program code  605 . The program code may conform to any one of a number of different programming languages. For example, the program code can be written in C+, C++, BASIC, Pascal, JAVA or a number of other languages. The mass storage device  606  stores data and instructions and retrieves data and program code instructions from a processor readable storage medium, such as a magnetic disk or magnetic tape. For example, the mass storage device  606  can be a hard disk drive, floppy disk drive, tape drive, or optical disk drive. The mass storage device  606  stores and retrieves the instructions in response to directions that it receives from the processor unit  602 . The processor unit  602  operates the apparatus  600  using data and program code instructions that are stored and retrieved by the memory  604  and/or the mass storage device  606 . The data and program code instructions may be first retrieved by the mass storage device  606  from a medium and then transferred to the memory  604  for use by the processor unit  602 . 
     The apparatus  600  may optionally include a user interface  620 , such as a keyboard, mouse, or light pen, coupled to the processor unit  602  to provide for the receipt of inputs from an operator (not shown). The apparatus  600  may also optionally include a display unit  622  to provide information to the operator in the form of graphical displays and/or alphanumeric characters under control of the processor unit  602 . 
     The control system bus  608  provides for the transfer of data and control signals between all of the devices that are coupled to the control system bus  608 . Although the control system bus  608  is displayed as a single bus that directly connects the devices in the processor unit  602 , the control system bus  608  can also be a collection of busses. For example, the display unit  622 , user interface  620  and mass storage device  606  can be coupled to an input-output peripheral bus  608 , while the processor unit  602  and memory  604  are coupled to a local processor bus. The local processor bus and input-output peripheral bus are coupled together to form the control system bus  608 . 
     The system controller  601  is coupled to the elements of the apparatus  600 , for turning off a source of optical power in accordance with embodiments of the present invention via the system bus  608  and the I/O circuits  618 . These elements include the following: one or more clamping voltage sources CV and capacitance sensors CS coupled to clamping electrodes CE in the switch S, and one or more actuator drivers AD coupled to the actuators A. For the sake of clarity, connection is shown to only one of the clamping electrodes CE and one of the actuators A. In practice, all the clamping electrodes CE and actuators A could be coupled to the I/O circuits  618 . The system controller  601  provides signals to the above elements to switch optical signals between the input fibers IF and the output fibers OF. 
     The steps of the method of the method described above with respect to FIG. 5 could be implemented by a suitable computer program running on the CPU  602  of the controller  601 . The CPU  602  forms a general purpose computer that becomes a specific purpose computer when executing programs such as the program  505  of the embodiment of the method of the present invention depicted in the flow diagram of FIG.  2 . Although the invention is described herein as being implemented in software and executed upon a general purpose computer, those skilled in the art will realize that the invention could be implemented using hardware such as an application specific integrated circuit (ASIC), microcontroller or other hardware circuitry. As such, it should be understood that the invention can be implemented, in whole or in part, in software, hardware or both. 
     Those skilled in the art would be readily able to devise a computer program  605  to implement step  506  described above with respect to FIG.  5 . The program  605  is suitable for monitoring and controlling the switch S in accordance with embodiments of the present invention. Although the program  605  is described herein with respect to a MEMS optical switch, those skilled in the art will recognize that programs embodying the method of the present invention can be applied to any MEMS device. 
     It will be clear to one skilled in the art that the above embodiments may be altered in many ways without departing from the scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their legal equivalents.