Method and apparatus for detecting and latching the position of a MEMS moving member

An apparatus for detecting the position of an optical element includes an actuator coupled to the optical element. A sensor coupled to the optical element senses the movement of the optical element. The sensor includes a moveable electrode coupled to the optical element for outputting a position detection signal.

DETAILED DESCRIPTON OF THE PREFERRED EMBODIMENTS Systems integrators of optical MEMS devices having movable members wish to know the exact position of a movable member for control of the optical element; not merely that the movable member has been shifted between one of two particularly desired positions. The present invention measures the position of the movable mass utilizing electrodes and capacitors coupled to the movable mass, and determining the mass's position by measuring voltage differences across capacitors. Reference is specifically made to FIGS. 2A and 2B . Apparatus 100 includes a thermal actuator 101 having a heated beam 124 anchored between a first anchor 120 and a second anchor 122 such that when a voltage is applied across anchors 120 , 122 beam 124 heats and expands causing expansion of the beam in the direction of the left handed arrow of double headed arrow B. Conversely, when no voltage is applied, as beam 24 cools, it returns to an initial position moving in the direction of the right handed arrow of double headed arrow B. A movable mass 126 made out of silicon or the like is coupled to beam 124 by a link 127 so that movable mass 126 expresses movement in the directions of double headed arrow C with movement of heated beam 124 in the direction of double headed arrow B. An optical member, such as a high aspect ratio MEMS mirror, attenuator, shutter or the like is disposed on movable mass 126 and moves relative to an optical path (not shown) of an optical circuit upon actuation of actuator 101 . It should be understood that actuator 101 is an electro-thermal actuator by way of example, but may also be a piezo electric actuator, electrostatic actuator, or other conventional actuators as known in the art. Furthermore, it is also understood that optical element 128 is placed on a moving silicon mass by way of example in this embodiment, and may in fact be directly linked to link 127 or in fact, link 127 may be sized and dimensioned to act as the optical element itself. In this embodiment as will be seen below, it is preferred that a moving mass 126 be used for ease of coupling with a sensor 135 . Sensor 135 is operatively coupled to moving mass 126 , but could be just as easily coupled to link 127 , actuator 101 or, if properly sized and dimensioned, optical element 128 . Sensor 135 includes a first electrode 136 coupled to moving mass 126 . Electrode 136 is movable so as to move with a movement of moving mass 126 . The movement of electrode 136 defines a path of movement. A second electrode 132 is disposed on the movement path of electrode 136 at one end of the movement path. A third electrode 130 is disposed along the movement path at another end of the movement path so that as electrode 136 moves with moving mass 126 , it moves between second electrode 132 and third electrode 130 . A suspension member 134 , in electrical contact with first electrode 136 , is coupled to second electrode 132 across a first capacitor 138 and to third electrode 130 across a second capacitor 140 . As is known in the art, the voltage across the capacitor will be a function of the position of first electrode 136 relative to either of second or third electrodes 132 , 130 . Accordingly, because first electrode 136 moves with moving mass 126 , and because movement of the electrodes relative to each other causes changes in capacitance across capacitors 138 , 140 ; the change in capacitance across electrodes 138 , 140 is a function of the movement of moving mass 126 . Therefore, voltage differences across capacitors 138 , 140 indicate the position of movable mass 126 . It should be noted, that in a preferred embodiment electrodes 130 , 132 and 136 are comb electrodes with interlacing fingers allowing for close proximity of the electrodes to each other as moving mass 126 moves. However, it can be understood that the electrodes can be of other type, such as plate electrodes, as long as the electrodes maintain a spacing from each other no greater than that which allows a detection of a change in voltage which can be measured as a capacitance across capacitors 138 , 140 . Reference is now made to FIG. 2B in which one example of a sensing circuit for outputting a voltage signal corresponding to a movement of moving mass 126 is provided. Resistors 138 , 140 are coupled in series. Therefore, at the junction of capacitors 138 , 140 a net capacitance C X corresponds to the difference in capacitance across the two capacitors as a result of movement &Dgr;x of moving mass 126 . The C X is input to an amplifier 144 where it is input as a voltage signal. Amplifier 144 outputs an amplified voltage signal V o corresponding to the position of electrode 136 relative to electrodes 132 , 130 and in turn the position of moving mass 126 , and further in turn optical element 128 . More specifically, in accordance with the present invention, movement of moving mass 126 by distance &Dgr;x creates a differential change in capacitance as &Dgr;x increases. For example, as electrode 136 moves in the direction of the left handed arrow head of double headed arrow C, the capacitance of first capacitor 138 increases while the capacitance across capacitor 140 decreases. Therefore, if the capacitance value C 1 , C 2 of first capacitor 138 and second capacitor 140 are known then &Dgr;x can be determined. Reference is now made to FIG. 3 in which a circuit in which changes in capacitance can be converted to a voltage signal V out which allows the detection of the position of the movable mass 126 in response to the output voltage. The circuit of this embodiment, makes use of the following equation: V out &equals;V s ( C X −C ref )/( C fixed )   (1) It is possible to convert the voltage signal represented by the change of capacitance into a voltage out signal V out representing the position of mass 126 utilizing a circuit 200 , which includes a input 210 for receiving the capacitance differential voltage signal corresponding to C X . An input 212 receives a voltage input corresponding to a reference capacitance C ref . These inputs provide a first input to a gain amplifier 214 which is grounded at its second input and is coupled in parallel with a second reference capacitor 216 having a fixed capacitance C fixed . A reset switch 218 is coupled in parallel with fixed capacitor 216 . As a result, a voltage signal input relating to the change in capacitance between electrodes 132 , 136 and 130 can be compared with reference capacitance values to output a voltage signal V out which corresponds directly to movement of the mass 126 , as well as the position. As a result of this structure of apparatus 100 , the detection circuitry used to determine either the actuator position, or the optical element position can be simplified. The structure is particularly well suited for feedback control of an optical element which is particularly useful for attenuators and the like. By way of non-limiting example, one can measure the capacitance change resulting from movement of the MEMS device using a closed loop feedback circuit. Reference is now made to FIG. 4 in which a detection and control circuit 300 utilized to regulate the driving voltage which operates the actuator in order to equalize the two capacitances of the two capacitors, and thereby position the MEMS device precisely is provided. Like numerals are utilized to indicate like structure for ease of description. Circuit 300 includes the three electrodes 136 , 130 and 132 in which electrode 136 moves relative to fixed electrodes 130 , 132 , thus changing capacitance across capacitors 138 , 140 respectively coupled therebetween as described in detail above. The capacitance differential C X is input as a first input to a gain amplifier 320 . The output of gain amplifier 320 is also input to amplifier 320 as its second input to provide a buffer. The output of amplifier 320 is also input to a filter 322 which in turn provides one input to a gain amplifier 324 , the second input to gain amplifier 324 being coupled to ground. A diode 326 is coupled across the buffer 320 , filter 322 and gain amplifier 324 to form a feedback loop so that the output V out is continuously input at the C X input of amplifier 320 . In this way, V out is continually adjusted as a result of the relative capacitance of capacitors 138 , 140 , which is an effect the position of movable mass 126 . V out will keep changing until C X is equal to zero, so that the actuator control voltage will hit a steady state when C X equals zero. As a result of the structure of apparatus 100 and the complimentary circuits 200 and the associated circuits 200 and 300 by way of example, the invention provides a precise method for detecting changes in &Dgr;x of movable mass 126 . Furthermore, it becomes easy to calibrate the voltage V o representing the voltage corresponding to the capacitance differential C X . Therefore, it is very easy to calibrate V out as a function of &Dgr;x to obtain a V out signal for not only monitoring the position of movable mass 126 , but for controlling the drive voltage V out for precisely positioning the movable mass 126 and in turn optical element 128 . The position of an optical member can thus be determined by monitoring the capacitance between a moving electrode, coupled to a moving mass, and a second electrode and comparing that to the capacitance between the moving electrode and the third electrode and comparing the relative capacitances at the moving electrode to produce a voltage signal corresponding to the position of the electrode. Furthermore, utilizing a feedback loop, the derived voltage signal can be used to position the optical member by outputting the detection signal as the drive signals to the actuator. In such a way, the position of the optical member can be closely controlled. Once the position of the optical member can be determined and controlled with accuracy, it then becomes desirable to hold the optical member in a desired position. In known latched MEMS devices a movable member such as a mirror, shutter, attenuator or the like is often held in place utilizing an electrical charge across the device to maintain the heated beam or piezo electric device or electrostatic device in the activated position. Ideally there should be no voltage differential across the device. However, when maintaining the actuator position in the prior art, a voltage is continuously applied and voltage differentials occur internal to the MEMS device which can result in arcing and damage to the device. In the apparatus of FIG. 5, a mechanical latch is used to hold the movable member in place. Again, like numerals are utilized to indicate like structure. An apparatus 400 includes an actuator 101 similar in construction to that discussed above in which a heated beam 124 is anchored between anchors 120 , 122 and expands and contracts upon the application and removal of a voltage applied across anchors 120 , 122 . A movable mass 426 is coupled to beam 124 by a linkage 127 . Movable mass 426 has a main body 436 which is capable of motion in a path of motion in a direction shown by double headed arrow D. Extensions 428 extend from body 436 in a direction substantially orthogonal to the path of motion. Extensions 428 , 429 are disposed at one end of body 436 . Extensions 430 , 432 extend from body 436 in a direction substantially orthogonal to the path of motion at the other end of body 436 so that movable member 426 is substantially in an I configuration. Optical element 128 is disposed on movable mass 426 so that as movable mass 426 moves in the direction of arrow D optical element 128 moves into and out of an optical path. A mechanical latch is used to hold movable member 426 in place. By way of example, the mechanical latch is a movable stop 434 a , which by way of example may also be made of silicon for ease of manufacture. Stop 434 a is a shuttle member and moves in the direction of double headed arrow E to move into and out of the travel path of extension 430 by way of example. Stop 434 a is shaped so as to engage extension 430 when in the travel path of extension 430 . In an exemplary embodiment, silicon stop 434 a is moved into position by a thermal scratch drive as known from the art as discussed by Akiyama and Shono in their article, “Controlled Step-wise Motion in Polysilicon Microstructures,” J. Microelectromech. Syst., vol. 2, pp. 106-110, 1993 and by Akiyama et al. in their article “Scratch Drive Actuator with Mechanical Links for Self Assembly of Three Dimensional MEMS,” J. Microelectromech. Syst., vol. 6, pp. 10-17. As a result, through activation and deactivation of actuator 101 movable mass 426 will move in reciprocal motion in the direction of arrow D. At the same time, stop 434 a can move between a first position out of the path of movement of extension 430 to a second position within the path of movement of extension 430 . It is readily understood, that stop 434 a is shaped to engage extension 430 when stop 434 a is within the travel path of extension 430 and actuator 101 has been deactivated causing mass 426 to move in the direction of upper arrow double headed arrow D. Therefore, when energy is removed from actuator 101 the movable mass 426 is latched, held in place, by the engagement of stop 434 a and extension 430 . In a preferred embodiment, although not necessary, a second stop 434 b , also moved by a scratch drive mechanism, to move between a first position and a second position and back again in the direction of double headed arrow E, is provided to engage extension 432 when latching is desired. By providing two stops 434 a , 434 b less stress is placed upon extension 430 and stop 434 a and to provide more stability to the overall apparatus. It also should be readily understood from the above that to return movable mass 426 to an unlatched position the scratch drive moves stops 434 a , 434 b to withdraw stops 434 a , 434 b from the travel path of extensions 430 , 432 allowing movement of mass 426 in the direction of the upper arrow head of double headed arrow D. As a result, in order to latch the position of optical member 126 , it is not required to maintain a voltage across actuator 101 . Moveable stops 434 a , 434 b prevent the MEMS member from moving. Once the stops are in position, the electrical bias is no longer applied and the scratch drive may also be switched off. As a result, there is no bias applied from the moving mass contacting the stops. When the latch is actuated, the stops are held in compression. This arrangement is desirable because silicon, a prevalent material for MEMS, is much stronger in compression than tension. Additionally, all bias, both to the scratch drive and the thermal actuated beam 124 may be switched off when the stops are in place. As a result, optical member 128 stays in position in the absence of power. An additional feature of the embodiment is the use of stationery stops 436 a , 436 b permanently situated along the travel path of extensions 430 , 432 and 428 , 429 and between extensions 428 , 430 and 429 , 432 respectively. In the absence of the latching feature of stops 430 a , 434 b , stationery stops 436 a , 436 b will come in contact with extensions 428 , 430 and 429 , 432 respectively if beam 124 over extends itself (over flexes) in either direction of arrow D. As a result, stops 436 a , 436 b engage the extensions in either direction to prevent over shooting movement of optical member 128 . While there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the disclosed invention may be made by those skilled in the art without departing from the spirit and scope of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appending hereto.