Patent Publication Number: US-2004051033-A1

Title: Method of controlling deflection amplitude and offset of a resonant scanning mirror using photo detector timing

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates generally to micro-electro-mechanical system (MEMS) mirrors, and more particularly, to a method of controlling a resonant scanning mirror using only laser beam deflection timing.  
       [0003] 2. Description of the Prior Art  
       [0004] It would be desirable and advantageous in the MEMS mirror art to provide a technique for controlling the deflection amplitude and offset of a laser beam that is deflected off of a vibrating mirror galvanometer, given only beam deflection timing information.  
       SUMMARY OF THE INVENTION  
       [0005] The present invention is directed to a system and method for controlling the deflection amplitude and offset of a laser beam that is deflected off of a vibrating mirror galvanometer, given only beam deflection timing information.  
       [0006] According to one embodiment, a method of controlling a resonant scanning mirror comprises the steps of: measuring deflection timing associated with a laser beam deflected off the resonant scanning mirror in response to movement of the resonant scanning mirror; and controlling the deflection amplitude and offset of the laser beam in response to deflection timing measurements.  
       [0007] According to another embodiment, a method of controlling a resonant scanning mirror comprises the steps of: providing two photo detectors equally spaced apart from the center of the deflection range associated with the resonant scanning mirror; measuring a delta time associated with a deflected laser beam moving between the two photo detectors in response to movement of the resonant scanning mirror; and controlling the deflection amplitude and offset of the laser beam in response to the delta time measurements.  
       [0008] According to yet another embodiment, a system for controlling the deflection amplitude and offset of a laser beam that is deflected off of a vibrating mirror galvanometer comprises: a resonant scanning mirror; a pair of photo detectors spaced equally apart from the center of the deflection range associated with the resonant scanning mirror; timing detection logic configured to calculate a time sum and a time difference associated with a deflected laser beam moving between the pair of photo detectors; a digital processor configured to calculate a control effort in response to the time sum and time difference; a pair of digital to analog converters (DACs) configured to convert the control effort to a voltage; a sinewave generator configured to generate a sinewave in response to the control effort; and a voltage amplifier configured to generate a resonant scanning mirror motor coil voltage in response to the sinewave.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009] Other aspects, features and advantages of the present invention will be readily appreciated, as the invention becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing figures wherein:  
     [0010]FIG. 1 is a pictorial diagram illustrating two photo detectors near both ends of the deflection range associated with a resonant scanning mirror that is deflecting a laser beam;  
     [0011]FIG. 2 is a waveform diagram illustrating digital pulses generated by the two photo detectors depicted in FIG. 1 as the deflected laser beam sweeps from side to side;  
     [0012]FIG. 3 is a diagram illustrating the relationship between deflected laser beam amplitude and waveform timing for the digital pulses shown in FIG. 2;  
     [0013]FIG. 4 is a three-dimensional graph illustrating the functional relationship between a defined time t sum  and the laser beam deflection amplitude and offset for the system shown in FIG. 1;  
     [0014]FIG. 5 is a three-dimensional graph illustrating the functional relationship between a defined time t diff  and the laser beam deflection amplitude and offset for the system shown in FIG. 1;  
     [0015]FIG. 6 is a simplified schematic diagram illustrating a complete system for controlling the amplitude and offset of a deflected laser beam and that is suitable for use in association with the system shown in FIG. 1, to control the amplitude and offset of the deflected laser beam by measuring the time from the initial detection of the laser beam at the left sensor to the detection of the laser beam at the right sensor;  
     [0016]FIG. 7 shows a more detailed schematic of the timing detection logic circuit that is depicted in FIG. 6;  
     [0017]FIG. 8 shows a more detailed schematic of the state machine signal conditioner that is depicted in FIG. 7;  
     [0018]FIG. 9 is a system diagram illustrating the topology of a digital control loop for maintaining deflection amplitude and offset associated with the system depicted in FIGS.  6 - 8 ;  
     [0019]FIG. 10 is a pictorial diagram illustrating two mirrors and a single laser detector, each mirror located near one end of the deflection range associated with a resonant scanning mirror that is deflecting a laser beam;  
     [0020]FIG. 11 is a waveform diagram depicting a sinusoidal displacement of the laser beam deflected off the resonant scanning mirror that is seen by the laser detector as well a window function generated by the forcing function of the resonant scanning mirror shown in FIG. 10;  
     [0021]FIG. 12 depicts two output signals generated using the window function and detector output signal shown in FIG. 12;  
     [0022]FIG. 13 is similar to FIG. 3, and shows the relationship between deflected laser beam amplitude and the length of the positive-going pulses shown in FIG. 12;  
     [0023]FIG. 14 shows another more detailed schematic of the timing detection logic circuit that is depicted in FIG. 6 and that is suitable for use by the system shown in FIG. 10; and  
     [0024]FIG. 15 shows a more detailed schematic of the state machine signal conditioner that is depicted in FIG. 14.  
    
    
     [0025] While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.  
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0026] The particular embodiments of the invention discussed herein below with reference to FIGS.  1 - 9  are directed to a system and method for controlling the deflection amplitude and offset of a laser beam that is deflected off of a vibrating mirror galvanometer, given only beam deflection timing information.  
     [0027] Looking first at FIG. 1, a pictorial diagram illustrates two photo detectors  10 ,  12  near both ends of the deflection range associated with a resonant scanning mirror  14  that is deflecting a laser beam  16 . Each photo detector  10 ,  12  is a known, but equal distance from the center  18  of the deflection. As the beam sweeps from side  20  to side  22 , the photo detectors  10 ,  12  will generate pulses as shown in FIG. 2.  
     [0028]FIG. 2 is a waveform diagram illustrating digital pulses generated by the two photo detectors  10 ,  12  depicted in FIG. 1 as the deflected laser beam  16  sweeps from side  20  to side  22 . The deflection amplitude and offset are related to the sensor  10 ,  12  times by a relationship written as  
       det pos=ref=A  cos(ω t   n )+ b.   (1)  
     [0029] If the detectors  10 ,  12  are positioned near 70% of the desired full deflection amplitude, then each detector  10 ,  12  pulse appears in a different quadrant of the unit circle. For each respective quadrant, equation (1) becomes  
             (     ref   -   b     )     A     =     cos        (       -   ω                     t   0       )         ,     
              (     ref   -   b     )     A     =     cos        (       -   ω                     t   1       )         ,     
              (     ref   +   b     )     A     =     cos        (     π   -     ω                   t   2         )         ,   and               (     ref   +   b     )     A     =       cos        (       ω                   t   3       -   π     )       .                   
 
     [0030] When the amplitude of the deflection is low, the time from t 0  to t 1  and the time from t 2  to t 3  will become short. Likewise, when the deflection amplitude is large, these time deltas will increase; so measuring these times will generate a value that is a function of amplitude. FIG. 3 is a diagram illustrating the relationship between deflected laser beam amplitude and waveform timing for the digital pulses shown in FIG. 2. This function can be concisely written as  
         t   left     =         t   1     -     t   0       =         1   w          (         cos     -   1            (       ref   -   b     A     )       +       cos     -   1            (       ref   -   b     A     )         )       =       2   w            cos     -   1            (       ref   -   b     A     )                       t   right     =       2   w            cos     -   1            (       ref   +   b     A     )                       
 
     [0031] A value defined as t sum  can then be written as  
               t   sum     =         t   left     +     t   right       =       2   w          (         cos     -   1            (       ref   -   b     A     )       +       cos     -   1            (       ref   +   b     A     )         )                 (   2   )                       
 
     [0032] It can be seen that when there is a positive offset to the beam  16  deflection, the timing t left  will increase and the timing t right  will decrease. In view of the foregoing, a value that tracks deflection offset t diff  can then be defined as  
       t   diff   =t   left   −t   right   (3)  
     [0033] Solving equation (2) for amplitude A, and solving equation (3) for offset b then shows the relationship between the timing measurements and amplitude and offset as  
               A   =     ref       cos        (       ω   4          t   sum       )            cos        (       ω   4          t   diff       )             ,   and           (   4   )               b   =     ref                   tan        (       ω   4          t   sum       )              tan        (       ω   4          t   diff       )       .               (   5   )                       
 
     [0034] It can then be shown that around the desired operating point (when the offset b is zero and the reference value is 70.7% of the deflection amplitude A), equations (4) and (5) can be respectively approximated as  
         A   ≈     ref     cos        (       ω   4          t   sum       )           ,   and           b   ≈     ref                     tan        (       ω   4          t   diff       )       .                     
 
     [0035]FIG. 4 is a three-dimensional graph illustrating the functional relationship between the defined time t sum  and the laser beam deflection amplitude A and offset b for the system topology shown in FIG. 1. FIG. 5 is a three-dimensional graph illustrating the functional relationship between the defined time t diff  and the laser beam deflection amplitude A and offset b for the system shown in FIG. 1. In this case, deflection is measured in degrees of mirror  14  rotation; and time is in 1 MHz clock periods. The present inventors have found that in practice, a higher frequency clock can be used to increase resolution. Looking at FIGS. 4 and 5, it can be seen that at some amplitudes A or offsets b, the deflected laser beam  16  will not cross both detectors  10 ,  12 ; and so only two of the four detector pulses will be generated. In this case, the missing t left  or t right  values are defined as zero. The result then is that there are three regimes that a controller must consider. These can be described as 1) No detection: Operate open loop and step increase the amplitude control; 2) Left or Right detection only: Amplitude and offset gain is ˜half, so double controller gain; and 3) both detector times available: Use gain as described herein above.  
     [0036] With continued reference to FIGS. 4 and 5, it can be seen that the slope of the t sum  surface with respect to amplitude is around 40 clocks/degree; and the slope of the t diff  surface with respect to offset is around 60 clocks/degree (near the desired operating point). It can therefore be seen that a sufficient measure is available to use to control the deflection amplitude and offset of the laser beam.  
     [0037] In view of the above, a discussion regarding the MEMS mirror  14  coil driver is now set forth below. If the beam deflection equations are re-arranged to be in terms of the time the beam takes to cross the working area, that is from t 1  to t 2 , then amplitude can be expressed as  
             A   =       ref     sin        (     ω            t   2     -     t   1       2       )         .             (   6   )                       
 
     [0038] Therefore, if the deflection angle of the detectors  10 ,  12  and the sweep frequency of the mirror  14  are known, the amplitude from the time it takes the beam to swing from the left side detector  10  to the right side detector  12  can be calculated.  
     [0039] Since the sensitivity of the system to changing amplitude can be determined from equation (6), the change in amplitude that must be maintained can be determined as shown below if the change in period that can be tolerated is known and if the period is roughly ΔT.  
       T   =         2   ω            sin     -   1            (     y   A     )         =         2   ω            sin     -   1            (   a   )                     for                 y     =   aA                      T          A       =         -   2        y         ω   2              A   2          (     1   -       y   2       A   2         )         1   2                        T     =         -   2     ω          a       (     1   -     a   2       )       1   2                   A     A                     
 
     [0040] If the detectors  10 ,  12  are positioned such that they are at 70.7% of the full deflection, then the percent change in amplitude can be expressed as a function of an incremental change in timing as follows.  
                      A     A     =       π                 f           T       =     6283                      T           ,           (   7   )                       
 
     [0041] and for a 10 nsec change in timing, dT, equation (7) becomes  
              A     A     ≈       2     -   14       .                   
 
     [0042] At least 14 bits of resolution on the amplifier will therefore be necessary to control the mirror  14  deflection.  
     [0043]FIG. 6 is a simplified schematic diagram illustrating a complete system  100  for controlling the amplitude of a deflected laser beam  16  and that is suitable for use in association with the system shown in FIG. 1, to control the amplitude of the deflected laser beam  16  by measuring the time from the initial detection of the laser beam  20  at the left sensor  10  to the detection of the laser beam  22  at the right sensor  12 . System  100  comprises a left photo detector  10 ; a right photo detector  12 ; timing detection logic  102  that calculates the time sum and difference from the left and right detectors  10 ,  12 ; a digital processor  104  to calculate a control effort; an amplitude DAC  106  and an offset DAC  108  to convert the control effort values to voltages; a sinewave generator  110  having its amplitude modulated by the control effort; and a voltage amplifier  112  to drive the mirror motor coil  114 . According to one embodiment, the coil  114  is driven by an H-bridge voltage amplifier that employs a crystal controlled PWM signal to generate a sinusoidal drive waveform, wherein the amplitude of the drive signal is controlled via a 16-bit DAC, for example.  
     [0044]FIG. 7 shows a more detailed schematic of the timing detection logic circuit  102  that is depicted in FIG. 6. The timing detection logic circuit  102  is operational to measure the above described t left  and t right  time intervals (the time from left detector  10  to left detector  10  and from right detector  12  to right detector  12 ).  
     [0045]FIG. 8 shows a more detailed schematic of the state machine signal conditioner  116  that is depicted in FIG. 7. The state machine signal conditioner  116  design is based on Truth Table 1 shown below.  
                              Truth Table 1                                                 Current   Current   Next   Next                   Left   Right   Left   Right       Left   Detector   Pulse   Pulse   Pulse   Pulse       Detector   Detector   State   State   State   State       (LD)   (RD)   (LP)   (RP)   (LP)   (RP)   Comments               0   0   0   0   0   0   If signaling no pulses and none detected, continue signaling no pulses       0   0   0   1   0   1   If signaling right pulse and none detected, continue signaling right pulse       0   0   1   0   1   0   If signaling left pulse and none detected, continue signaling left pulse       0   0   1   1   0   0   If signaling both pulses, error, signal no pulse       0   1   0   0   0   1   If signaling no pulses and right detected, begin signaling right pulse       0   1   0   1   0   0   If signaling right pulse and right detected, stop signaling right pulse       0   1   1   0   0   1   If signaling left pulse and right detected, begin signaling right pulse       0   1   1   1   0   0   If signaling both pulses, error, signal no pulse       1   0   0   0   1   0   If signaling no pulses and left detected, begin signaling left pulse       1   0   0   1   1   0   If signaling right pulse and left detected, begin signaling left pulse       1   0   1   0   0   0   If signaling left pulse and left detected, stop signaling left pulse       1   0   1   1   0   0   If signaling both pulses, error, signal no pulse       1   1   0   0   0   0   If left and right detected simultaneously, error, signal no pulse       1   1   0   1   0   0   If left and right detected simultaneously, error, signal no pulse       1   1   1   0   0   0   If left and right detected simultaneously, error, signal no pulse       1   1   1   1   0   0   Of left and right detected simultaneously, error, signal no pulse                  
 
     [0046]FIG. 9 is a system diagram illustrating the topology of a 5 th  order digital control loop for maintaining deflection amplitude associated with the system depicted in FIGS.  6 - 8 . The blocks below the dashed line represent functions implemented in code.  
     [0047]FIG. 10 is a pictorial diagram illustrating a system  200  that comprises a far mirror  202 , a near mirror  204 , and a single laser detector  206 , wherein each mirror is located near one end of the deflection range associated with a resonant scanning mirror  208  that is deflecting a laser beam  210  generated by a laser generator. The resonant scanning mirror  208  generates a sinusoidal displacement of the laser beam  210  that is greater than the printer optics  212  range. When the deflected laser beam (enumerated as  230  and  240  in FIG. 10) crosses the far or near mirrors  202 ,  204  (which are fixed position mirrors), a beam  214 ,  216  is reflected to the single laser detector  206 .  
     [0048]FIG. 11 is a waveform diagram depicting a sinusoidal displacement of the laser beam deflected off the resonant scanning mirror  208  that is seen by the laser detector  206  as well a window function  242  generated by the forcing function of the resonant scanning mirror  208  shown in FIG. 10.  
     [0049]FIG. 12 depicts two output signals  244 ,  246  generated using the window function  242  and detector  206  output signal  250  shown in FIG. 12. If the amplitude of the sinusoid  252  shown in FIG. 11 is represented by the length of the positive-going pulses  244 ,  246  from the “At or Beyond” signals, then a longer pulse signifies a larger amplitude.  
     [0050]FIG. 13 depicts a diagram that is similar to the diagram shown in FIG. 3, and shows the relationship between deflected laser beam amplitude and the length of the positive-going pulses  244 ,  246  shown in FIG. 12. The total amplitude of the sinusoid  252  is then represented as the length of the pulse from the far mirror  202  added to the length of the pulse of the near mirror  204 . The result is subtracted from some expected total length to generate an amplitude error that can then be fed back to a controller in order to manage the amplitude of the sinusoid  252  by modifying the amplitude of the forcing function on the resonant scanning mirror  208 .  
     [0051] If the sinusoid  252  is not centered between the far and near mirrors  202 ,  204 , the pulse  244 ,  246  widths from the far and near mirrors  202 ,  204 , will be dissimilar in length. Subtracting one pulse length from another yields an effective offset of the sinusoid from the center  220  shown in FIG. 10. This offset can similarly be fed back to a controller to manage the offset of the sinusoid  252  by modifying the offset of the forcing function on the resonant scanning mirror  208 .  
     [0052]FIG. 14 shows a detailed schematic for another embodiment of the timing detection logic circuit  102  that is depicted in FIG. 6. The control system  100  is also suitable for use by the detector system  200  shown in FIG. 10 when the timing detection logic circuit  102  employs the structure shown in FIG. 14. Detector system  200  can be seen to be responsive to a single detector signal  250  as well as the window signal  242 .  
     [0053]FIG. 15 shows a more detailed schematic of the state machine signal conditioner  300  that is depicted in FIG. 14. The state machine signal conditioner  300  design is based on Truth Table 2 shown below.  
                              Trith Table 2                                                 Current   Current   Next   Next                   Left   Right   Left   Right               Pulse   Pulse   Pulse   Pulse       Detector   Window   State   State   State   State       (D)   (W)   (LP)   (RP)   (LP)   (RP)   Comments               0   0   0   0   0   0   If signaling no pulses and none detected, continue signaling no pulse       0   0   0   1   0   1   If signaling right pulse and none detected, continue signaling right pulse       0   0   1   0   1   0   If signaling left pulse and none detected, continue signaling left pulse       0   0   1   1   0   0   If signaling both pulses, error, signal no pulse       0   1   0   0   0   0   If signaling no pulses and none detected, continue signaling no pulses       0   1   0   1   0   1   If signaling right pulse and none detected, continue signaling right pulse       0   1   1   0   1   0   If signaling left pulse and none detected, continue signaling left pulse       0   1   1   1   0   0   If signaling both pulses, error, signal no pulse           0   0   0   1   0   If signaling no pulses and left detected, begin signaling left pulse           0   0   1   1   0   If signaling right pulse and left detected, begin signaling left pulse           0   1   0   0   0   If signaling left pulse and left detected, stop signaling left pulse           0   1   1   0   0   If signaling both pulses, error, signal no pulse           1   0   0   0   1   If signaling no pulses and right detected, begin signaling right pulse           1   0   1   0   0   If signaling right pulse and right detected, stop signaling right pulse       1   1   1   0   0   1   If signaling left pulse and right detected, begin signaling right pulse       1   1   1   1   0   0   If signaling both pulses, error, signal no pulse                  
 
     [0054] In view of the above, it can be seen the present invention presents a significant advancement in the art of MEMS mirror controllers. Further, this invention has been described in considerable detail in order to provide those skilled in the resonant scanning mirror controller art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow.