Abstract:
A device disclosed herein includes a feedback measuring circuit to measure a signal flowing through a movable MEMS mirror. Processing circuitry determines a time at which the signal indicates that a capacitance of the movable MEMS mirror is substantially at a maximum capacitance. The processing circuitry also determines, over a window of time extending from the time at which the signal indicates that the capacitance of the movable MEMS mirror is substantially at the maximum to a given time, a total change in capacitance of the movable MEMS mirror compared to the maximum capacitance. The processor further determines the capacitance at the given time as a function of the total change in capacitance, and determines an opening angle of the movable MEMS mirror as a function of the capacitance at the given time.

Description:
PRIORITY CLAIM 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 15/192,718 filed Jun. 24, 2016, the disclosure of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates to optical systems for scanning or deflecting light beams, and, in particular, to determining and controlling the opening angles of an oscillating mirror in a MEMS device. 
       BACKGROUND 
       [0003]    Certain devices such as wafer defect scanners, laser printers, document scanners, projectors and the like often employ a collimated laser beam that scans across a flat surface in a straight line path. These devices employ tilting mirrors to deflect the beam to perform the scanning. These tilting mirrors may be, or may include, Micro Electro Mechanical Systems (“MEMS”) devices. 
         [0004]    Common mirrors used in MEMS devices include a stator and a rotor, with the rotor or structures carried by the rotor being reflective. The stator and/or rotor are driven with a drive signal which results in the rotor rotating with respect to the stator, thereby changing the angle of reflectance of an incident light beam on the rotor. By oscillating the rotor between two orientations, an opening angle of the mirror is defined, and scanning of the light beam across the flat surface is accomplished. 
         [0005]    It is desirable to be able to precisely control movement of the rotor. To enable that precise control, collection of accurate data about the current position of the mirror is important. The collection of such accurate data, and consequently the precise control of the mirror, has proven troublesome due to changes in temperature, changes to the light beam itself, and other factors. This can result in commercially undesirable performance of the device. Thus, there is a commercial desire for the development of accurate ways to measure data about the position of the mirror, and precise ways to control the position of the mirror, are desirable. 
       SUMMARY 
       [0006]    This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
         [0007]    A device disclosed herein includes a feedback measuring circuit to measure a signal flowing through a movable MEMS mirror. Processing circuitry determines a time at which the signal indicates that a capacitance of the movable MEMS mirror is substantially at a maximum capacitance. The processing circuitry also determines, over a window of time extending from the time at which the signal indicates that the capacitance of the movable MEMS mirror is substantially at the maximum to a given time, a total change in capacitance of the movable MEMS mirror compared to the maximum capacitance. The processor further determines the capacitance at the given time as a function of the total change in capacitance, and determines an opening angle of the movable MEMS mirror as a function of the capacitance at the given time. 
         [0008]    A method aspect disclosed herein is a method of controlling an opening angle of a movable MEMS mirror. The method includes driving the movable MEMS mirror with a periodic signal such that the MEMS mirror oscillates, measuring a signal flowing through the movable MEMS mirror as it oscillates, and determining a time at which the signal indicates that a capacitance of the movable MEMS mirror is substantially at a maximum capacitance. 
         [0009]    The method also includes determining, over a window of time extending from the time at which the signal indicates that the capacitance of the movable MEMS mirror is substantially at the maximum to a given time, a total change in capacitance of the movable MEMS mirror compared to the maximum capacitance. The capacitance at the given time is determined as a function of the total change in capacitance. An opening angle of the movable MEMS mirror is determined as a function of the capacitance at the given time. The opening angle of the movable MEMS mirror is determined based upon the opening angle not being equal to a desired opening angle. 
         [0010]    Another method disclosed herein includes measuring a signal flowing through a movable MEMS mirror as it oscillates, and determining a time at which the signal indicates that a capacitance of the movable MEMS mirror is substantially at a maximum capacitance. This method also includes determining, over a window of time extending from the time at which the signal indicates that the capacitance of the movable MEMS mirror is substantially at the maximum to a given time, a total change in capacitance of the movable MEMS mirror compared to the maximum capacitance, and determining the capacitance at the given time as a function of the total change in capacitance. 
         [0011]    In an embodiment, a method is provided for determining an opening angle of a movable MEMS mirror. The method comprises: determining a total change in capacitance of the movable MEMS mirror from a time at which the capacitance of the movable MEMS mirror is at a maximum to a given time at which knowledge of the opening angle of the movable MEMS mirror is desired; and calculating the opening angle of the movable MEMS mirror at the given time based upon the total change in capacitance. 
         [0012]    In an embodiment, a picoprojector comprises: a housing; a laser generator carried within the housing; a movable MEMS mirror carried within the housing and upon which a laser generated by the laser generator impinges; mirror drive circuitry configured to generate a drive signal for the movable MEMS mirror; a feedback circuit configured to measure a mirror sense signal flowing through the movable MEMS mirror; and processing circuitry. The processing circuit is configured to, based upon the mirror sense signal: determine an opening angle of the movable MEMS mirror at a given time, as a function of a total change in capacitance of the movable MEMS mirror from a time at which the capacitance of the movable MEMS mirror is at a maximum to the given time, and as a function of the capacitance at the given time; and generate a control signal to cause the mirror drive circuitry to adjust the drive signal, as a function of the determined opening angle of the movable MEMS mirror. 
         [0013]    In an embodiment, a picoprojector comprises: a housing; a laser generator carried within the housing; a movable MEMS mirror carried within the housing and upon which a laser generated by the laser generator impinges; mirror drive circuitry for the movable MEMS mirror; an amplifier configured to amplify a mirror sense signal flowing through the movable MEMS mirror; an analog to digital converter configured to receive the amplified version of the mirror sense signal and to generate a digitized version of the mirror sense signal; and a microprocessor configured to: determine an opening angle of the movable MEMS mirror as a function of the digitized mirror sense signal at a given time at which knowledge of the opening angle of the movable MEMS mirror is desired, the digitized mirror sense signal at a time at which capacitance of the movable MEMS mirror is at the maximum capacitance, the given time, and the time at which the capacitance of the movable MEMS mirror is at the maximum capacitance; and generate a control signal for the mirror drive circuitry as a function of the opening angle of the movable MEMS mirror. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a top plan view of a movable MEMS mirror as may be used with the techniques described in this disclosure. 
           [0015]      FIG. 2  is a perspective view showing operation of a movable MEMS mirror scanning in accordance with the techniques described in this disclosure. 
           [0016]      FIG. 3A  is a schematic block diagram of a system for measuring and controlling the opening angle of a MEMS mirror that utilizes a hardware integrator in accordance with the techniques described in this disclosure. 
           [0017]      FIG. 3B  is a schematic block diagram of a system for measuring and controlling the opening angle of a MEMS mirror in accordance with the techniques described in this disclosure. 
           [0018]      FIG. 4  is a graph of capacitance vs opening angle of a movable MEMS mirror. 
           [0019]      FIG. 5  is a series of graphs that demonstrate typical capacitance changes as a movable MEMS mirror resonates. 
           [0020]      FIG. 6  is another series of graphs that demonstrate typical capacitance changes as a movable MEMS mirror resonates. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    One or more embodiments of the present disclosure will be described below. These described embodiments are only examples of the presently disclosed techniques. Additionally, in an effort to provide a concise description, all features of an actual implementation may not be described in the specification. 
         [0022]    When introducing elements of various embodiments of the present disclosure, the articles “a,” “an,” and “the” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Additionally, it should be understood that references to “one embodiment” or “an embodiment” of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Like reference numbers in the drawing figures refer to like elements throughout, and reference numbers separated by century, as well as reference numbers with prime notation, indicate similar elements in other applications or embodiments. 
         [0023]    First, a movable MEMS mirror  100 , such as may be used in devices such as wafer defect scanners, laser printers, document scanners, projectors, and pico-projectors, will now be described with reference to  FIG. 1 . The movable MEMS mirror  100  includes a stator  102  having inwardly projecting fingers  103 . A rotor  104  is positioned within the stator  102  and has outwardly projecting fingers  105  that interleave with the inwardly projecting fingers  103  of the stator  102 . The rotor  104  spins about its axis, rotating its mirror surface with respect to the stator  102 . 
         [0024]    Either the stator  102  or the rotor  104  is supplied with a periodic signal, such as a square wave, while the other is supplied with a reference voltage. In the case where the periodic signal has an oscillating square voltage, for example, electrostatic forces cause the rotor  104  to rotate about its axis relative to the stator  102 . In the case where the periodic signal has an oscillating square current, for example, magnetic forces cause the rotor  104  to rotate about its axis relative to the stator  102 . Indeed, the movable MEMS mirror  100  may be driven according to any suitable way known to those of skill in the art. 
         [0025]    For use in scanning a light beam across a surface, the movable MEMS mirror  100  is driven so that it oscillates at its resonant frequency between two set or controllable rotation limits. Shown in  FIG. 2  is the movable MEMS mirror  100  scanning a light beam across a projection screen between two set rotation limits that define an “opening angle” θ of the movable MEMS mirror  100 . 
         [0026]    A system  200  for measuring and controlling the opening angle of the movable MEMS mirror  100  is now described with reference to  FIG. 3A . The system  200  includes a current sensing operational amplifier  202  having an inverting input coupled to the rotor of the movable MEMS mirror  100 , and a non-inverting input coupled to ground. A microcontroller  204  has an input coupled to receive the output of the operational amplifier  202 . It should be appreciated that although a microcontroller  204  is shown, any suitable circuitry, such as a programmable logic device or programmable logic controller, may be used instead. 
         [0027]    The microcontroller  204  includes a hardware integrator  212  that integrates the amplified signal received from the amplifier  202 . An analog to digital converter (ADC)  214  samples and digitizes the output of the integrator  212 , and delivers its output to a microprocessor  216 . It should be understood that in the case where a programmable logic device or programmable logic controller is used, the microprocessor  216  may represent electrical circuits such as logic gates, amplifiers, and various arrangements of transistors. The output of the microprocessor  216  is coupled to a driver  99 , which in turn is coupled to the movable MEMS mirror  100 . 
         [0028]    The operation of the system  200  will be described below, but first the relationship between a capacitance between the stator  102  and rotor  104  and the opening angle of the movable MEMS mirror  100  will be described. 
         [0029]    The capacitance between the stator  102  and rotor  104  varies as a function of the opening angle of the movable MEMS mirror  100 , as shown in  FIGS. 4-6 . Mathematically, the charge on a capacitor is equal to the capacitance across the capacitor multiplied by the voltage across the capacitor, which can be represented as: 
         [0000]        Q ( t )= C ( t )* V ( t ) 
         [0030]    The current through the capacitor is equal to the derivative of the charge with respect to time, which can be represented mathematically as: 
         [0000]        I ( t )= dQ ( t )/ dt=V ( t )* dC/dt+C ( t )* dV/dt, C ( t )= f (φ( t ))
 
         [0031]    Since V(t) is constant at the time of sampling, the C(t)*dV/dt term cancels out, and I(t) is dependent on the change in capacitance. 
         [0032]    The graph of  FIG. 4  shows how capacitance is a function of the opening angle, and typical values for dC/dt are shown in  FIGS. 5-6 . 
         [0033]    Operation of the system  200  will now be described referring back to  FIG. 3A . The amplifier  202  receives the output signal from the rotor of the MEMS mirror  100 , and amplifies it by a factor of K. After amplification, this signal can be represented as: 
         [0034]    Signal( t )= K·dC ( t )/ dt +Offset 
         [0035]    The offset can be ignored for the purposes of the techniques described herein. The purpose of the amplifier  202  and the amplification factor K is to amplify the signal such that changes due to capacitance changes of the MEMS mirror  100  are sampleable by the ADC  214 . 
         [0036]    As the rotor of the MEMS mirror  100  rotates, a maximum capacitance exists between the rotor and stator at the instant when the rotor is parallel to the plane of the stator. At this instant, the change in capacitance, and thus dC(t)/dt, is zero. Thus, the time at which the capacitance is at a maximum is known. 
         [0000]      Signal( t   maxCapacitance )= K·dC ( t   maxCapacitance )/ dt    
         [0037]    If the sum of the changes in capacitance from the time at which the capacitance is at a maximum until a given time T2 at which it is desired to know the opening angle of the MEMS mirror  100  is taken, the total change in capacitance from the maximum capacitance is known. This can be calculated as: 
         [0038]    Δ C ( t   2 )=1/ K∫   t     maxCapacitance     t     2   Signal( t ) dt    
         [0039]    This integration is performed by the integrator  212 . The ADC  214  samples the output of the integrator  212 , and then the microprocessor  216  calculates the capacitance at time T2 from ΔC(t 2 ) as: 
         [0000]        C ( t   2 )= C   max   +ΔC ( t   2 ) 
         [0040]    Having calculated the capacitance at time T2, the opening angle of the MEMS mirror  100  at time T2φ(t 2 ) can thus, as shown in  FIG. 4 , be determined as: 
         [0000]      φ( t   2 )= f ( C ( t   2 ))
 
         [0041]    Usually, K is stable and determined by the value of the resistor R in the feedback path of the amplifier  202 . However, in some instances, due to environmental conditions such as temperature, the amplification factor K may change. Therefore, optionally, normalization may be performed together with the integration. Thus, ΔC(t 2 ) may be calculated as: 
         [0000]      Δ C   normalized ( t   2 )=1/Signal( t   maxCapacitance )∫ t     maxCapacitance     t     2   Signal( t ) dt  
 
         [0042]    This helps to make the determination of the opening angle of the MEMS mirror  100  accurate and robust over a wide range of environmental conditions. 
         [0043]    It should be appreciated that the hardware integrator  112  need not be used, and that this function may be performed digitally by the microprocessor  116 . An embodiment where the microprocessor  116  may perform the integration is shown in  FIG. 3B . 
         [0044]    The system  200 ′ shown in  FIG. 3B  operates as the system  200  of  FIG. 3A , except the hardware integrator is not present, and the output of the amplifier  202  is fed directly to the input of the ADC  214 . As stated, here, the microprocessor  116  may perform the integration. 
         [0045]    In some instances, however, with this embodiment, a linear approximation may be used rather than an integration. As can be seen from  FIG. 6 , about the maximum capacitance, the signal remains substantially linear. Thus, where T2 is sufficiently close to the time at which the capacitance is at a maximum, the signal dC(t)/dt can be treated as linear, and ΔC(t 2 ) can be calculated as: 
         [0000]      Δ C ( t   2 )=1/ K∫   t     maxCapacitance     t     2   Signal( t ) dt= 1/ K ( t   2 −t maxCapacitance )·Signal( t   2 −Signal( t   maxCapacitance)/ 2)
 
         [0046]    This linear approximation technique may be advantageous when it is desired to save processing power, or to calculate the opening angle of the MEMS mirror  100  quickly without the use of a hardware integrator. 
         [0047]    The determined opening angle of the MEMS mirror  100  can be used for controlling the opening angle in real time. The microprocessor  116  can use this opening angle to determine a gain to be applied to the driving period signal by the driver  99  which will result in the opening angle changing to a desired value. Thus, the opening angle can be used to create a control loop capable of not only precisely keeping the opening angle at a desired value, but also for changing the opening angle to a different desired value. 
         [0048]    While the disclosure has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be envisioned that do not depart from the scope of the disclosure as disclosed herein. Accordingly, the scope of the disclosure shall be limited only by the attached claims.