US 6,768,571 132
`(10) Patent N0.:
`(12) United States Patent
`Azarov et al.
`(45) Date of Patent:
`Jul. 27, 2004
`
`
`U3006768571B2
`
`(54) ORIENTATION STABILIZATION FOR MEMS
`DEVICES
`
`(75)
`
`Inventors: Alexei “Azamv, 0m uridgc, NJ (us);
`Randy c_ Giles, Whippany, NJ (US);
`Wendelin Weingartner, New York
`City, NY (US)
`
`3,5200] Greywall
`fi,20l,63l Bl
`3,1200] Bhalla elal.
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`5,275,325 Bl
`6,430,645 Bl “ 11/2002 Peale Cl 8L
`6,549,692 B] *
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`3593245
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`385MB
`385MB
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`250321
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`(73) Assignee: Lucent Technologies Inc., Murray Ilil],
`NJ {US}
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`* cited by examiner
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`(21) APPL N05 101'!261,167
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`8011.30 20,02
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`’
`Prior Publication Data
`US 2004HJIX1925 A1 A . l. 2004
`J
`Pr
`Int. Cl.7 ............................ G021r U01; GOZB 2008
`(51)
`(52) U.S. Cl.
`359(239; 3595224
`(58)
`Field of Search
`3591239, 212,
`359324, 223
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Acontro] circuit designed to control driving voltages applied
`to the actuating electrodes of a MEMS mirror. The control
`circuit
`is
`interfaced with a mirror position sensor and
`includes a variable gain amplilier whose output depends on
`a desired mirror equilibrium angle and a current mirror tilt
`angle determined by the sensor. The desired equilibrium
`.
`.
`.
`.
`angle can be changed by varying a retercnce Signal applied
`to the amplifier. The control circuit can stabilize the mirror
`31 “31511in “Hg“ 1i“ angle-‘5 and:
`515 a IESU“, CXIBUd ”16
`available angular range beyond lhc snap-down anglc- Since
`the number of MEMS mirrors that can be arrayed in an
`optical cross—connect is a function of the available angular
`range, the number of channels in a cross—connect may be
`substantially increased.
`
`5,648,618 A
`
`W199? Neukermans el al.
`
`73l862.08
`
`19 Claims, 9 Drawing Sheets
`
`120
`
`504
`
`
`
`1
`0001
`00°
`
`Capella 2006
`Capella 2006
`Fujitsu v. Capella
`Fujitsu V. Capella
`IPR2015-00726
`IPR2015-00726
`
`

`

`US. Patent
`
`Jul. 27,2004
`
`Sheet 1 of9
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`US 6,768,571 B2
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`US. Patent
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`Jul. 27,2004
`
`Sheet 2 of9
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`US 6,768,571 B2
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`FIG. 2
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`US. Patent
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`US 6,768,571 B2
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`US. Patent
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`Jul. 27,2004
`
`Sheet 4 of9
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`US 6,768,571 B2
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`14
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`US. Patent
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`Jul. 27, 2004
`
`Sheet 5 of 9
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`US 6,768,571 B2
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`US. Patent
`
`Jul. 27,2004
`
`Sheet 6 of9
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`US 6,768,571 B2
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`FIG.6
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`

`US. Patent
`
`Jul. 27,2004
`
`Sheet 7 of9
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`US 6,768,571 B2
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`US 6,768,571 B2
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`2
`
`FIG. 3 illustrates graphically the dependence, on the
`rotation angle, of absolute values of the electrostatic and
`mechanical torques generated in the device of FIG. 1;
`FIG. 4 shows graphically how the mirror in the device of
`FIG. 1 can be stabilized at a large rotation angle according
`to one embodiment of the invention;
`FIG. 5 shows a block diagram of a feedback circuit that
`can be used to bias the electrodes in the device of FIG. 1
`
`1f]
`
`‘15
`
`according to one embodiment of the invention;
`FIG. 6 shows a block diagram of a control circuit that can
`be used in the feedback circuit of FIG. 5 according to one
`embodiment of the invention;
`FIG. 7 shows a schematic diagram of a variable gain
`amplifier that can be used in the control circuit of FIG. 6
`according to one embodiment of the invention; and
`FIGS. 8A~B illustrate representative voltage profiles gen—
`erated by the control circuit of FIG. 5 using the variable gain
`amplifier of FIG. 7 according to one embodiment of the
`invention.
`
`1
`ORIENTATION STABILIZATION FOR MEMS
`DEVICES
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The invention relates to optical communication equip—
`ment and, more specifically,
`to micro—electromechanical
`devices for use in such equipment.
`2. Description ol‘ the Related Art
`Optical communication equipment ol'ten employs micro-
`electromechanical systems (MEMS). Atypical MEMS sys—
`tem may include an array of micro—machined mirrors, each
`mirror individually movable in response to an electrical
`signal. Such an array may be employed in an optical
`cross-connect, in which each mirror in the array receives a
`beam of light, for example, from an input optical fiber. The
`beam is reflected from the mirror and can be redirected to a
`
`dilTerent location, e.g., at which is located an output optical
`fiber, by rotating the mirror. More details on the principle of
`operation and methods of manufacture of MEMS devices
`including mirror arrays may be found, for example,
`in
`commonly assigned U.S. Pat. No. 6,201,631, the teachings
`of which are incorporated herein by reference.
`One problem with prior art MEMS devices is referred to
`as “snap-dOWI'I." More specifically, when [he voltage applied
`to an actuating electrode in such device approaches a critical
`value, the tilt angle of the mirror begins to increase rapidly
`and nonlinearly with the voltage. This behavior may cause
`a collision of the minor against the electrode andlor wafer,
`dam aging the mirror and rendering the MEMS device inop-
`erable.
`
`SUMMARY OF THE INVENTION
`
`DETAILED DESCRIPTION
`
`Reference herein to "one embodiment" or “an embodi—
`
`ment” means that a particular feature, structure, or charac—
`teristic described in connection with the embodiment can be
`included in at least one embodiment of the invention. The
`appearances of the phrase “in one embodiment" in various
`places in the specification are not necessarily all referring to
`the same embodiment, nor are separate or alternative
`embodiments mutually exclusive of other embodiments.
`FIG. 1A shows a crOSs-sectional view of a representative
`MEMS device 100 that may be used in an optical cross-
`connect. Device 100 has a movable mirror 102 formed in an
`
`overlayer 104 of a wafer 118 using, e.g., reactive etching.
`Wafer 118 has two additional layers: a substrate layer 106
`and a thin insulating layer 108.
`layer 108 electrically
`isolates overlayer 104 from substrate layer 106. ()verlayer
`104 and substrate layer 106 may be silicon, and insulating
`layer 108 may be silicon oxide. Mirror 102 is supported
`above a cavity 110 by a pair of springs 112, e.g., torsional
`members, connected to overlayer 104. Cavity 110 is defined
`in insulating layer 108 and substrate layer 106. A second
`wafer 114 includes electrodes llfia—b as well as electrical
`
`interconnections (not shown). Substrate layer 106 is
`attached to wafer 114 such that electrodes 116a—b are
`located beneath mirror 102 in cavity 110. Mirror [02 and
`electrodes 1169—!) form an actuator of device 100.
`
`35
`
`4D
`
`45
`
`The problems in the prior art are addressed, in accordance
`with the principles of the invention, by a control circuit that
`controls the voltages applied to the one or more actuating
`electrodes of a MEMS device. The control circuit receives a
`sensor signal, e.g., from a position sensor corresponding to
`a MEMS device,
`indicative of the current position of a
`movable part of the MEMS device relative to a stationary
`part ol‘ the MEMS device. Based on the sensor signal, the
`control circuit generates a control signal for the MEMS
`device actuator
`to achieve a desired orientation of the
`movable part with respect to the stationary part. The control
`circuit may include a variable gain amplifier whose output
`depends on the sensor signal and a reference signal corre-
`sponding to a desired equilibrium position for the movable
`part. The desired equilibrium angle can be changed by
`adjusting the reference signal applied to the amplifier.
`In a MEMS device in which the movable part is a mirror
`that is rotatably coupled to the stationary part, the control
`circuit can stabilize the mirror at relatively large tilt angles
`and, as a result, extend the available angular range beyond
`the snapdown angle. Since the number of MEMS device .
`mirrors that can be arrayed in an optical cross—connect is a
`function of the available angular range, in accordance with
`lhe principles of the invention, the number of channels in a
`crosseonnect may be substantially increased.
`BRILLI" DESCRIPTION 01" TlIL-l DRAWINGS
`
`FIG. 1B illustrates how a beam of light 120 impinging on
`mirror 102 can be redirected from direction 120 (FIG. 1A)
`to direction 120 using mirror rotation. Mirror 102 rotates
`about the axis defined by springs 112 in response to voltages
`applied to electrodes llfia—b. For example, when electrode
`116!) is biased, mirror 102 rotates clockwise, as shown in
`FIG. 1B. Similarly, when electrode 116:: is biased, mirror
`102 rotates counterclockwise. Changing bias voltages
`changes the rotation angle {8)
`thus enabling a cross-
`connecting function of device 100.
`As seen in FIG. 1B, an increase of the rotation angle
`causes mirror 102 to approach the corresponding actuating
`electrode, e.g., electrode 116!) for the clockwise rotation
`illustrated in FIG. 18. As. a result, the electrostatic torque
`exerted onto mirror 102 by electrode 116!) at a constant
`voltage applied to that electrode increases nonlinearly with
`the angle while the mechanical torque exerted by springs
`112 increases substantially linearly. At relatively large rota-
`tion angles, this dill’erence in torque behavior may cause
`0011
`0011
`
`60
`
`lA—B illustrate cross-sectional views ol‘ a repre-
`FIGS.
`sentative MEMS device that may be used in an optical
`crosseortneet;
`
`FIG. 2 illustrates a simplified model describing the elec—
`trostatic and mechanical torques generated in the device of
`FIG. 1;
`
`65
`
`

`

`US 6,768,571 B2
`
`3
`mirror instability and/or snap-down as will be further
`described below.
`
`FIG. 2 illustrates a simplified model that may be used to
`qualitatively describe the behavior of the electrostatic and
`mechanical torques generated in device 100. In FIG. 2, a
`plate 202, a spring 212, an electrode 216, and coordinate x
`correspond to, e.g., mirror 102, springs 112, electrode 116.5,
`and angle 0, respectively, in device 100. The mechanical
`force, Jim, acting upon plate 202 is giVen by Equation (1) as
`follows:
`
`F...=~h'
`
`(l)
`
`where k is the spring constant of spring 212 and x=(l
`corresponds to the nonoeformed spring. The electrostatic
`force, lit, acting upon plate 202 is given by Equation (2) as
`follows:
`
`av:
`t-,= —ft _. I
`
`[21
`
`where A is a positive constant, V is the voltage applied to
`electrode 216, and h is the distance between plate 202 and
`electrode 216 when the plate is at x=0. In an equilibrium
`position, the total force (Iig,+lim) acting upon plate 202 is
`zero as expressed by the following equation:
`
`
`av:
`
`:31
`
`Thus, the coordinates of equilibrium positions can be found
`by solving Equation (3). Depending on the value of param—
`eter p, Equation (3) may have two, one, or no roots, where
`p is defined by the following equation:
`
`Ur
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`In particular, when p>0, Equation (3) has two roots (xI and
`x3) given by Equation (5) as follows:
`
`45
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`
`tm
`
`When V=Vm, the equilibrium position at x=bf2 is unstable
`and plate 202 will collapse against electrode 216 from any
`initial position. Furthermore, the case of negative p corre—
`sponds to a relatively large absolute value of V (i.e.,
`|V|>|V_,d|). In this case, plate 202 has no equilibrium posi-
`tions and will also collapse against electrode 216.
`FIG. 3 illustrates graphically the dependence, on angle 0,
`of absolute values of the electrostatic torque (curves 1
`through 5) and mechanical
`torque (line 6) generated in
`device 100. Curves 1—5 correspond to dilferent constant
`voltages, VJ—VS, respectively, applied to, e.g., electrode
`116b, where IV,|<|V3|<|V3|<|V4I<IV5L Intersection poian of
`each curve 1—5 with line 6 correspond to equilibrium angles
`of mirror 102. In agreement with the model of FIG. 2, for
`each voltage (curve), there may be two, one, or no equilib—
`rium angles (intersection points). For example,
`if \J’=\J'3
`(curve 3 in FIG. 3), there are two equilibrium angles (6,, and
`On) corresponding to points A and B in FIG. 3. If V=V,1
`(curve 4), there is one equilibrium angle (0C) corresponding
`to point C; and, if V=V5 (curve 5), there are no equilibrium
`angles. Angle SAWS degrees) is a stable equilibrium angle,
`whereas angles BB (#10 degrees) and Br. (”7.3 degrees) are
`unstable equilibrium angles. From angle 0=0,,, mirror 102
`will either move toward electrode 116!) or be drawn toward
`
`the stable equilibrium position at B=BA. Angle BC. and
`voltage V,. are the snap-down angle and voltage,
`respectively, for device 100.
`FIG. 4 shows graphically how mirror 102 of device 100
`can be stabilized at a relatively large angle, e.g., 0=03,
`according to one embodiment of the invention. In particular,
`to stabilize the mirror, device 100 is configured to vary the
`voltage applied, e.g., to electrode 116b, as a function of 6,
`Le, V=V(G). In one configuration, the voltage is varied with
`the angle such that the resulting electrostatic torque remains
`constant, e.g., as shown by dashed line 404- in FIG. 4. In
`another configuration, the voltage is varied linearly with the
`angle, e.g., according to the following equation:
`V(UJ=V-_r*(9—93)
`
`G)
`
`where r is the voltage ramp coellicient. Dotted line 406 in
`FIG. 4 illustrates the electrostatic torque corresponding to
`Equation (7). Both configurations result
`in substantially
`similar electrostatic torques between points I3 and I) in FIG.
`4. As indicated in FIG. 4, minor 102 can be stabilized at
`angle 6,, using either configuration because (i) the total
`torque is zero when 0=03 and (ii) the total torque rotates
`mirror 102 toward 03 when 0 deviates from 03 in either
`direction.
`
`When p=0, Equation (3) has a single doubly degenerate root
`xl_r.__=hi'2; and, when pd), Equation (.3) has no real roots.
`Physically, the case of positive p corresponds to small V.
`In this case, the first equilibrium position, corresponding to
`the minus sign in Equation (5), is stable, whereas the second ‘
`equilibrium position, corresponding to the plus sign,
`is
`unstable. In particular, in the vicinity of the first equilibrium
`position, the total force is directed toward that equilibrium
`position,
`thus stabilizing plate 202.
`In contrast,
`in the
`vicinity of the second equilibrium position, the total force is
`directed away from that position. As a
`result,
`a small
`perturbation of the plate coordinate at the second equilib—
`rium position will cause plate 202 to move either toward the
`first equilibrium position or toward electrode 216.
`The case of zero p corresponds to snap-down and the
`corresponding snap-down voltage (V_,,,.) is given as follows:
`
`60
`
`In one embodiment, device 100 is configured to vary the
`voltage applied to electrode 116b, for example, as follows.
`When 0=0, constant voltage V5 is applied to electrode 116.5
`to rotate mirror 102. As the angle begins to increase, the
`electrostatic torque changes according to curve 402 in FIG.
`4, which also corresponds to curve 5 in FIG. 3. When 6
`reaches or exceeds angle 0,, (corresponding to point D in
`FIG. 4 and given by GD=£IB—(V5—V3)i‘r),
`the voltage is
`changed according to Equation (7). Following this voltage
`profile, mirror 102 can bypass equilibrium angle 9,, (FIGS.
`3 and 4) and is stabilized at equilibrium angle Hg. Different
`parameters andtor different voltage profiles may be used to
`stabilize mirror 102 at different equilibrium angles. As a
`result, the available angular range of mirror 102 is extended
`beyond the snap-down angle (6337.3 degrees).
`0012
`0012
`
`65
`
`

`

`US 6,768,571 B2
`
`6
`Consequently, the output voltage of amplifier 604 (V614) is
`given as follows:
`
`UI
`
`v,,..=v,,_. it' v,,,‘:v,u.: and
`
`Van 4=V104 if Vamavrm
`
`(116.!)
`
`(111'!)
`
`5
`FIG. 5 shoWs a feedback circuit 500 that can be used to
`bias electrodes 116 in device 100 according to one embodi—
`ment of the invention. Circuit 500 includes a sensor 502 and
`
`a control circuit 504. Sensor 502 is configured to sense the
`current rotation angle of minor 102 and generate a signal
`512 corresponding to that angle. Signal 512 is preferably a
`monotonic function of the angle. Sensor 502 may be any
`suitable sensing device, such as, for example, a capacitive
`sensor, a piezo-resistive sensor, a piezo-voltage sensor, or a
`photo-sensor.
`In one embodiment, sensor 502 is imple-
`mented as a four-terminal piem-voltage torsion sensor dis-
`closed in U.S. Pat. No. 5,648,618, the teachings ofwhich are
`incorporated herein by reference. Signal 512 is applied to
`circuit 504, which also receives a signal 514 corresponding
`to a desired rotation angle. Based on signals 512 and 514,
`circuit 504 generates a signal 516, which is applied to one
`of electrodes ll6a—b and drives mirror 102 toward the
`
`desired rotation angle.
`FIG. 6 shUWs control circuit 504 of FIG. 5 according to
`one embodiment of the invention. The output of sensor 502
`(signal 512) is applied to an optional signal processing
`circuit 602. Alternatively, in a different embodiment, circuit
`602 may be part of sensor 502. Based on signal 512, circuit
`602 generates a signal 612 preferably of the following form:
`(8)
`Vet firm—""1B
`
`where V613 is the voltage of signal 612 and a0 and a1 are
`constants. In one configuration, an and al are both positive.
`Alternatively, circuit 602 may be configured with one or
`both constants negative. Signal 612 is applied to a variable
`gain amplifier 604, which also receives signal 514 and is
`designed to generate an amplified signal 614 based on those
`signals. Signal 614 varies as a function of current angle 0
`and the desired equilibrium angle, e.g., Hi:-
`In one
`configuration, signal 614 corresponds to curves 402 and 406
`of FIG. 4. Signal 614 is applied to a high voltage amplifier
`606, which amplifies it
`to generate signal 516.
`In one
`embodiment, amplifier 606 has a constant gain of about 20.
`In one embodiment, amplifier 604 is implemented as a signal
`processor, in which case circuit 602 may be removed from
`circuit 504.
`
`FIG. 7 shoWs variable gain amplifier 604 of FIG. 6
`according to one embodiment of the invention. Amplifier
`604 has (i) an operational amplifier (0A) 702 configured as
`an inverting adder and (ii) a voltage regulator 704. In the
`absence of regulator 704-, the output voltage sz of 0A 702
`is expressed as folloWs:
`
`R1
`R:
`V n : ——'V -\ — —V
`R]
`N:
`10..
`61;.
`514
`
`[9]
`
`where V514 is the voltage of signal 514 and R,- are the
`corresponding resistances in amplifier 604. Combining
`Equations (8) and (9), one llnds that the output of ()A 702
`is given by the following equation:
`
`N:
`R:
`N
`V702 = dfiflniv EVSI-1I‘I’ £5“?
`
`[U
`
`E
`
`J
`
`Regulator 704 includes a zener diode (marked Z in FIG. 7),
`which is configured to operate in the breakdown regime. As
`a result, the output of 0A 702 is clipped when V7“: is more
`negative than V704, where VT04 is the base voltage of
`regulator 704- (voltage drop across the regulator). V704 is
`substantially independent of the output of ()A 702 and is
`related to the breakdown voltage of the zener diode.
`
`1f]
`
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`
`35
`
`4D
`
`In one embodiment, amplifier 604 may be implemented
`using the following parameters: R1=650 Q; R3=R3=4.7 k9;
`R4=20 £2; and V704=—8 V. In different ethdirnents, dilfer-
`ent parameters may be used.
`FIGS. BA—B illustrate representative voltage profiles gen-
`erated by control circuit 504 of FIG. 6 using amplifier 604
`of FIG. 7 and amplifier 606 of FIG. 6 whose gain is set to
`about 20. More specifically, three voltage profiles 302—806
`shown in FIG. 8A correspond to three different values of
`V5”. For example, profile 804 corresponds to V5M=8 V. In
`this case, signal 516 is about —160 V, if 0403 (FIG. SA) and
`increases linearly with the angle, if 0200. Voltage profile
`804 corresponds to the equilibrium angle of about 9 degrees.
`Similarly, voltage profiles 802 and 806 correspond to equi-
`librium angles of 8 degrees and 10 degrees, respectively, and
`each is obtained with the value of V514 that is lower and
`higher than 8 V, respectively.
`FIG. BB illustrates an alternative way of attaining dilfer-
`ent equilibrium angles using variable gain amplifier 604- of
`FIG. 7. More specifically, three voltage profiles 804, 808,
`and 810 shown in FIG. 8B correspond to a fixed value of
`V514 (=8 V) and dilferent settings of variable resistor R1 in
`amplifier 604 ofIiIG. 7.1I‘or example, profile 804- in FIG. BB
`is the same as profile 804 in FIG. 8A and corresponds to
`R1=650 9. Voltage profiles 808 and 810 are obtained with
`R1 set to the resistances that are higher and lower than 650
`$2, respectively. Similar to voltage profiles 802 and 806 of
`FIG. 8A, voltage profiles 808 and 810 of FIG. SB corre-
`spond to equilibrium angles of 8 degrees and 10 degrees,
`respectively.
`The invention may provide one or more of the following
`benefits. A control circuit of the invention may be used to
`provide differently shaped voltage profiles to the actuating
`electrodes. Such profiles may improve stability of operation
`with respect
`to noise and increase the switching speed
`between two different rnirnor positions. The switching speed
`increase is due to a relatively large voltage driving the mirror
`at low tilt angles, which results in faster mirror rotation. In
`addition, using the angle—dependent driving voltage may
`help to (lamp mirror oscillations near an equilibrium angle.
`Ilaving fewer oscillations at the equilibrium angle increases
`the switching speed due to a faster minor settling time.
`Furthermore,
`the invention extends the available angular
`range of MEMS mirrors beyond the snapcown angle. As a
`result, the number of MEMS devices that can be arrayed in
`an optical cross-connect may be substantially increased.
`While this invention has been described with reference to
`illustrative embodiments, this description is not intended to
`be construed in a limiting sense. Various position sensors
`may be interfaced with a control circuit of the invention. In
`certain embodiments, the control circuit andfor sensor may
`be integrated into a MEMS device analogous to device 100.
`Depending on the sensor, difi'erent voltage profiles may be
`generated and applied to the actuating electrodes. The volt—
`ages may he positive or negative. A desired angle of rotation
`may be specified by providing a reference electrical signal
`{e.g., signal 514) to a variable gain amplifier of the control
`circuit or adjusting a variable resistor in said amplifier, or
`both. Also, a MEMS device configured with a feedback
`circuit of the invention may be implemented in wafers
`dilferent from SO] wafers. In a MEMS device configured
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`US 6,768,571 B2
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`with a two-axis mirror, two feedback circuits may be used,
`one for each axis. Furthermore, a feedback circuit of the
`invention may be used with MEMS devices difi'erent from
`MEMS mirrors andfor applied to control translation as well
`as rotation. Various modifications of the described
`embodiments, as well as other embodiments of the
`invention, which are apparent to persons skilled in the art to
`which the invention pertains are deemed to lie within the
`principle and scepe of the invention as expressed in the
`following claims.
`Although the steps in the following method claims, if any,
`are recited in a particular sequence with corresponding
`labeling, unless the claim recitatiOns otherwise imply a
`particular sequence for implementing some or all of those
`steps, those steps are not necessarily intended to be limited
`to being implemented in that particular sequence.
`What is claimed is:
`1. Apparatus, comprising:
`a MEMS device; and
`a controller adapted to:
`receive, from a position sensor corresponding to the
`MEMS device, a sensor signal indicative of a current
`position of a movable part of the MEMS device with
`respect to a stationary part of the MEMS device; and
`generate a control signal as a function of the sensor signal,
`wherein an actuator of the MEMS device is adapted to
`achieve a desired position of the movable part with
`respect
`to the stationary part based on the control
`signal, wherein the control signal has a voltage profile,
`wherein a first portion of the voltage profile has voltage
`having an absolute value greater than a snapazlown
`voltage [or the MEMS device.
`2. The invention of claim 1, wherein a second portion of
`the voltage profile has voltage varying substantially linearly
`with a current
`rotation angle between the movable and
`stationary parts.
`3. The invention of claim 1, wherein a second portion of
`the voltage profile has voltage con'esponding to a substan-
`tially constant electrostatic torque applied to the movable
`part with respect to the stationary part.
`4. The invention of claim 1, wherein the MEMS device
`forms part of an optical switch.
`5. Apparatus, comprising
`a MEMS device; and
`a controller adapted to:
`receive, from a position sensor corresponding to the
`MEMS device, a sensor signal indicative of a current
`position of a movable part of the MEMS device with
`respect to a stationary part of the MEMS device; and
`generate a control signal as a function of the sensor signal,
`wherein an actuator of the MEMS device is adapted to
`achieve a desired position of the movable part with
`respect
`to the stationary part based on the control
`signal, wherein:
`the movable part is rotatably coupled to the stationary
`part;
`the sensor signal is indicative of a current rotation angle
`between the movable and stationary parts;
`the control signal is a function of the current rotation
`angle;
`the desired position corresponds to an equilibrium
`rotation angle between the movable and stationary
`parts; and
`the control circuit is adapted to generate the control
`signal based on the sensor signal and a reference
`signal corresponding to the equilibrium rotation
`angle.
`
`8
`6. The invention of claim 5, wherein an adj ustmenl of the
`reference signal corresponds to a change in the cqu ilibriunn
`rotation angle.
`7. The invention of claim 5, wherein the equilibrium
`rotation angle is greater than or substantially equal
`to a
`snap—down angle for the MEMS device.
`8. The invention of claim 5, wherein the MEMS device
`forms part of an optical switch.
`9. The invention of claim 5, wherein the control circuit
`comprises a first amplifier adapted to:
`receive the reference signal and a fourth signal based on
`the sensor signal; and
`generate a fifth signal, wherein the control signal is based
`on the fifth signal.
`10. The invention of claim 9, wherein the fourth signal is
`a substantially linear function of the current rotation angle.
`11. The invention of claim 9, wherein the control circuit
`further comprises a second amplifier adapted to amplify the
`fifth signal to generate the control signal.
`12. The invention of claim 9, wherein the first amplifier
`comprises:
`an operational amplifier adapted to operate as an inverting
`adder; and
`a voltage regulator connected to an output of the opera—
`tional amplifier, wherein:
`is adapted to receive the
`the operational amplifier
`reference and fourth signals; and
`the fifth signal
`is a regulated output signal of the
`operational amplifier.
`13. The invention of claim 12, wherein the voltage
`regulator is adapted to clip the output signal of the opera—
`tional amplifier if the output signal is more negative than a
`base voltage of the voltage regulator.
`14. Apparatus, com prising:
`means for receiving, from a position sensor corresponding
`to a MEMS device, a sensor signal indicative of a
`current rotation anile beIWeen a movable part of the
`MEMS device and a stationary part of the MEMS
`device; and
`means for generating a control signal based on the sensor
`signal and a reference signal, wherein:
`the movable part is rotatably coupled to the stationary
`part;
`the control signal is a function of the current rotation
`angle;
`the reference signal corresponds to an equilibrium
`rotation angle between the movable and stationary
`parts;
`an actuator of the MEMS device achieves the equilib-
`rium rotation angle based on the control signal; and
`the equilibrium rotation angle is greater than or sub—
`stantially equal to a snap-down angle for the MEMS
`device.
`
`15. Apparatus, comprising:
`means for receiving, from a position sensor corresponding
`to a MEMS device, a sensor signal indicative of a
`curI'Cnt rotation angle between a movable part of the
`MEMS device and a stationary part of the MEMS
`device; and
`means for generating a control signal based on the sensor
`signal and a reference signal, wherein:
`the movable part is rotatably coupled to the stationary
`part;
`the control signal is a function of the current rotation
`angle;
`the reference signal corresponds to an equilibrium
`rotation angle between the movable and stationary
`parts;
`
`_.—,
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`US 6,768,571 B2
`
`9
`an actuator of the MEMS device achieves the equilib-
`rium rotation angle based on the control signal; and
`means for adjusting the reference signal to change the
`equilibrium rotation angle.
`16. A method comprising:
`receiving,
`from a position sensor corresponding to a
`MEMS device, a sensor signal indicative of a current
`rotation angle between a movable part of the MEMS
`device and a stationary part of the MEMS device; and
`generating a control signal based on the sensor signal and
`a reference signal, wherein:
`the movable part is rotatably coupled to the stationary
`part;
`the control signal is a function of the current rotation
`angle;
`the reference signal corresponds to an equilibrium
`rotation angle beIWeen the movable and stationary
`parts; and
`an actuator of the MEMS device achieves the equilib—
`rium rotation angle based on the control signal; and
`adjusting the reference signal to change the equilibrium
`rotation angle.
`17. The invention of claim 16, wherein the MEMS device
`forms part of an optical switch.
`
`UI
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`18. A method comprising:
`receiving, from a position sensor corresponding to a
`MEMS device, a sensor signal indicative ol‘ a current
`rotation angle between a movable part of the MEMS
`device and a stationary part of the MEMS device; and
`generating a control signal based on the sensor signal and
`a reference signal, wherein:
`the movable part is rotatably coupled to the stationary
`part;
`the control signal is a function of the current rotation
`angle;
`the reference signal corresponds to an equilibrium
`rotation angle between the movable and stationary
`parts;
`an actuator of the MEMS device achieves the equilib-
`rium rotation angle based on the control signal; and
`the equilibrium rotation angle is greater than or sub—
`stantially equal to a snap—down angle for the MEMS
`device.
`19. The invention of claim 18, wherein the MEMS device
`forms part ol‘ an optical switch.
`ncncsaaasa
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`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`
`CERTIFICATE OF CORRECTION
`
`PATENT N0.
`DATED
`INVENTOR(S}
`
`: 6,768,574 B2
`: July 27, 2004
`: Azarov el a].
`
`Page 1 ol‘
`
`1
`
`It is certified that error appears in the above—identified patent and that said Letters Patent is
`hereby corrected as shown below:
`
`Column 8,
`
`Line 36, please replace “current rotation anile” with —— current rotation angle ——.
`
`Signed and Sealed this
`
`Seventh Day of December, 2004
`
`my
`
`
`
`JON W. DUDAS
`Dimemr ofn'rr Urrr'i‘rrd Starr's Parrot: and Trademark Oflfrre
`
`0016
`0016
`
`