Abstract:
A Micro Electronic Mechanical System (MEMS) scan controller generating clock frequency and a control method thereof are disclosed. The MEMS scan controller is for a MEMS mirror in a bi-direction laser scanning units (LSU). By detecting resonant frequency of the MEMS mirror, the scan controller sends frequency modulation signal and amplitude modulation signal of the MEMS mirror to a bridge circuit of the MEMS mirror for adjusting and stabilizing the MEMS mirror. A clock signal corresponding to the resonant frequency of the MEMS mirror at the moment is also sent so that scan data is sent within the effective scanning window in forward direction/reverse direction. Thus high-precision scanning is achieved.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a Micro-electronic-mechanical System (MEMS) scan controller generating clock frequency and a control method thereof, especially to a controller for micro-electric-mechanical mirrors (MEMS mirror) applied to bi-direction laser scanning units (LSU) and a control method that generate clock frequency signals so as to make a laser light source transmit laser beams within an effective scanning window according to the clock frequency signals. 
         [0002]    Most of LSU available now uses a polygonal mirror rotating at high speed to control reflection direction of laser beam. However, due to hydraulic driving, working rotational speed limits, high manufacturing cost, high noises and delayed initiation, such LSU is unable to meet requirements of high speed and high precision by using polygon mirror. In recent years, MEMS mirrors with torsion oscillators are getting known and are going to be applied to LSU of imaging systems, scanners or laser printers in future. The MEMS oscillatory mirror developed based on principle of torsion oscillators has higher scanning efficiency than conventional polygon mirror. 
         [0003]    In a laser scanning unit (LSU) a Micro Electronic Mechanical System (MEMS) oscillating mirror mainly consists of a control board with bridge circuit, a torsion oscillator and a mirror. A mirror driven by resonance magnetic field symmetrically oscillates along an axis. When a laser light is emitted to the mirror of the MEMS mirror, the MEMS oscillating mirror reflects the incident laser beam to the axis of the MEMS mirror at different angles for scanning along with different reflecting angles of the mirror surface that changes with time. Since the MEMS mirror scanning approach can neglect the wavelength effects, that the MEMS mirror has features of high resolution and large rotation angle so that has been applied broadly to commercial products, science and industries, such as devices disclosed in U.S. Pat. No. 5,408352, U.S. Pat. No. 5,867,297, U.S. Pat. No. 6,947,189, U.S. Pat. No. 7,190,499, TW Patent M253133 and JP 2006-201350. In order to improve scanning efficiency, a bi-directional LSU is developed yet associated control difficulties are raised. 
         [0004]    Due to resonant oscillation of the MEMS mirror, the rotation angles and stability of the MEMS have effects on precision of the LSU. In a controller for bi-directional LSU of the MEMS mirror, conventional technique focuses on stability control of the MEMS mirror such as adjustment of resonant frequency, working angle, or by means of a voltage controlled oscillator (VCO) to control the frequency. The frequency control of the VOC is based on changing permittivity of dielectric material by current or change of the capacitance by the voltage, as shown in US2006/00139113, US2005/0139678, US2007/0041068, US2004/0119002, U.S. Pat. No. 7,304,411, U.S. Pat. No. 5,121,138, and JP63-314965. Take a bi-directional LSU with 600 dots-per-inch (dpi) resolution per A4 size as an example, 5102 light spots are sent per each scanning in one directional. The 5102 light spots are sent completely during an imaging interval while the imaging interval should be invariant with the frequency or amplitude variations of the MEMS mirror that lead to deviation of the light spot and the image is not formed on the object. Thus the calculation frequency of the MEMS mirror for sending correct signal to the laser controller that emits laser light is a main point of control. Refer to US2006/0279364, a method for determining an operating point of an oscillation controller is disclosed. A table derived from a model built by the method is used to define the operating point. The operating point may be expressed in terms of clock counts by factoring in the clock rate of the oscillation controller. Refer to U.S. Pat. No. 6,891,572, an interpolation circuit writes a video signal into a frame memory in synchronism with the write system clock from the PLL circuit. Refer to U.S. Pat. No. 6,838,661, a torsion oscillator is stabilized in operation by a PD detector. Refer to U.S. Pat. No. 6,870,560 and U.S. Pat. No. 6,987,595, rotation of drum and laser scan frequency are controlled by a counter controller or dynamic adjustment of resonant frequency. As to bi-directional scanning, in order to let the scanning beam not deviated and an image is formed on the object within the scanning window, a more fast and effective method is required. 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore it is a primary object of the present invention to provide a MEMS scan controller applied to bi-direction laser scanning units (LSU). The MEMS scan controller is for detecting resonant frequency and amplitude of the MEMS mirror so as to generate signals to laser controllers and control bridge circuit of the MEMS mirror for adjusting resonant frequency and amplitude of the MEMS mirror and stabilizing the oscillation of the MEMS mirror. Thus the laser beam can perform scanning precisely and correctly within effective scanning window. 
         [0006]    As to the MEMS LSU, the laser light source is controlled by the laser controller. When the laser controller sends out the scanning data, the laser light source emits laser light toward the MEMS mirror that oscillates in resonant frequency f. Thus the laser light scans within effective scanning area being called as scanning window. After scanning, the laser light becomes scanning beams that pass the scanning lens to form images on the object. As to the scanning beams outside the scanning window, it is detected by a photoelectric detector (PD detector), the PD detector is excited to send a signal to the said controller and the MEMS scan controller. The MEMS mirror is controlled by the bridge circuit. When the MEMS mirror oscillates over a normal range, the MEMS oscillation of the MEMS mirror should be reduced by the bridge circuit. On the other hand, when the MEMS oscillation is getting weaker, the MEMS oscillation should be increased by the bridge circuit. When oscillation of the MEMS mirror becomes stable, a clock signal is sent to the laser controller of the printer or the multi-function printer for informing the timing and the frequency of scanning data. 
         [0007]    The clock signal is derived according to the frequency and amplitude of the MEMS mirror. Within the effective scanning window, a number of β or nβ beam spots are generated. Example to 600 dots-per-inch (dpi) resolution per A4 size, β is set as 5102 in one line. There are 5102 beam spots generated within the effective scanning window. 
         [0008]    The MEMS scan controller of the present invention comprises a control logic (logic unit), at least one D inverter, a phase locked loop and a counter comparator. The control logic receives triggering PD signals from the PD detector and calculates interval of each PD signal to generate frequency modulation signals and amplitude modulation signals. The phase locked loop generates clock signals. The frequency of the clock signal is f CLK (t), corresponding to the scanning frequency of the MEMS mirror at time t. Thus, when the laser controller receives the clock signal from the phase locked loop, the scan data is sent. 
         [0009]    The MEMS mirror oscillates in frequency f, a complete cycle period from the left side to the right side is T and the scanning angle is θ. The relationship between the scanning angle θ and time t is a sinusoid. Refer to  FIG. 2 , in order to prevent scanning deformation, within a cycle period T, two periods of time a˜b and a′˜b′ during which the curve is most close to linearity are selected. Refer to  FIG. 4 , T 2  and T 4  respectively are time of the forward scanning and time of the reverse scanning, in which the curve is the most close to linearity. The relationship among T 1 , T 2 , T 3 , T 4  is as following: 
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         [0010]    wherein T 1  is delay time, T 2  is time of the forward scanning, T 3  is delay time, T 4  is time of the reverse scanning, f is resonant frequency of the MEMS mirror, θ c  is scanning angle of the MEMS mirror, 2θ p  is the angle of the PD detector, and 2θ n  is an effective scanning window formed by effective scanning angle. 
         [0011]    It is another object of the present invention to provide a MEMS scan controller that simultaneously sends a data trigger signal while sending a clock signal with frequency of f CLK (t) for driving the laser controller to start sending scan data. Thus the transmission of scan data is more precisely. 
         [0012]    It is a further object of the present invention to provide a control method of the MEMS LSU that stabilizes oscillation of the MEMS mirror by control of resonant frequency and amplitude of the MEMS mirror. Moreover, the frequency f CLK (t) of the clock signal at t time is also calculated so as to transmit scan data with nβ beam spots precisely within the effective scanning window. 
         [0013]    The control methodology of resonant frequency f and amplitude A of the MEMS mirror by the MEMS scan controller according to the present invention includes the following steps:
       S1: set an initial value of duty D and set an initial value of period T;   S2: check the PD signal and whether there are two triggers within a half period; if yes, starts to adjust the frequency, while if not, starts to adjust the amplitude (step S5);   S3: check whether ratio of the time interval between two contiguous triggers of the PD signal to the whole period; If it&#39;s over 5%, starts to modify the amplitude. Wherein, the preset 5% deviation can be co-ordinated by the requirements of precision;   S4: while adjusting the amplitude, adjusts (increase or decrease) the value of the duty D so as to make the PD detector be triggered twice within a half-period;   S5: fine tune the frequency after adjusting the amplitude while the frequency is limited in the upper limit.       
 
         [0019]    As to the device with two PD detectors, the method includes the following steps:
       S1: set an initial value of duty D and set an initial value of period T;   S2: check the two PD signals and whether there are two triggers within a half period of each one PD detector; if yes, starts to adjust the frequency, while if not, starts to adjust the amplitude (step S5);   S3: check whether ratio of the time interval between two contiguous triggers of the PD signal to the whole period of each PD detector; If it&#39;s over 5%, starts to modify the amplitude. Wherein, the preset 5% deviation can be co-ordinated by the requirements of precision;   S4: while adjusting the amplitude, adjusts (increase or decrease) the value of the duty D so as to make the PD detector be triggered twice within a half-period;   S5: fine tune the frequency after adjusting the amplitude while the frequency is limited in the upper limit.       
 
         [0025]    The way to calculate frequency f CLK (t) of the clock signal is as following: after frequency and amplitude of the MEMS mirror becoming stable, the frequency of the MEMS mirror is f, and the sending time of the scan data within the effective scanning window is T 2  (or T 4 ). When the number of β or nβ spots are sent within the effective scanning window, the pulse frequency f CLK  of the clock signal at t time is: 
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         [0026]    The number of pulse generated in T 2  period is 
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         [0000]    and sent by the clock signal. 
         [0027]    The way of sending the scan data includes the following steps:
       S1: the MEMS scan controller checks the MEMS mirror in stable then calculates frequency f CLK  of the clock signal;   S2: after the MEMS mirror being stable, the MEMS scan controller sends a stable signal to laser controller;   S3: once the laser controller receives the stable signal, the scan data is sent in frequency of f CLK .   Thereby, after the oscillation of the MEMS mirror being stable, the MEMS scan controller sends a clock signal with frequency of f CLK  so as to transmit scan data within the effective scanning window (in the T 2  or T 4 ).       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a schematic drawing showing an embodiment of a bi-direction laser scanning unit (LSU) according to the present invention; 
           [0033]      FIG. 2  is a schematic drawing showing relationship between time and angle of the laser beam reflected by a MEMS mirror; 
           [0034]      FIG. 3  is a schematic drawing of an embodiment of a MEMS scan controller according to the present invention; 
           [0035]      FIG. 4  shows relationship among PD signal, scanning angle, scanning data and time; 
           [0036]      FIG. 5  shows a first modulation signal sent by a MEMS scan controller after the MEMS scan controller receiving signals from a laser controller and a PD detector; 
           [0037]      FIG. 6  shows relationship among PWM1 signal, PWM2 signal, and PWM3 signal; 
           [0038]      FIG. 7  shows resonant frequency of an embodiment according to the present invention; 
           [0039]      FIG. 8  shows PD signal of the present invention; 
           [0040]      FIG. 9  shows relationship among various signals of the present invention; 
           [0041]      FIG. 10  is a flow chart of a MEMS scan controller according to the present invention; 
           [0042]      FIG. 11  is a flow chart showing transmission procedures of scanning data according to the present invention; 
           [0043]      FIG. 12  is another embodiment of the MEMS scan controller according to the present invention; 
           [0044]      FIG. 13  is a further embodiment of the MEMS scan controller according to the present invention; 
           [0045]      FIG. 14  is a further embodiment of the MEMS scan controller according to the present invention; 
           [0046]      FIG. 15  is a further embodiment of the MEMS scan controller according to the present invention; 
           [0047]      FIG. 16  shows relationship between two PD signals of the embodiment in  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiment One 
       [0048]    Refer to  FIG. 1 , a MEMS LSU with one PD detectors is disclosed. A pre-scan laser  11  is controlled by a laser controller  23 . When the laser controller  23  sends out scanning data  318 , the pre-scan laser  11  emits laser light  111  toward a MEMS mirror  10  that oscillates in resonant frequency f. In this embodiment, the MEMS mirror  10  whose f=2500±5% HZ and maximum scanning angle±23° is used. The laser light  111  with scanning angle of ±23*2° (θ c =±23*2°) ranges from right-side scanning beam  115   a  to left-side scanning beam  115   b.  The scanning beam ranging from  113   a  to  113   b  is within angle of 2θ n . In this embodiment, the θ n =±19*2°, and is called effective scanning window. A PD detector  14   a  is disposed at θ p  while θ p =±21*2° for detecting scanning beam  114   a  and converting light into electric trigger signal. The scanning beams  113   a - 113   b  pass a post-scan lens  13  and form an image on an object  15  such as a photo conductor. In order to keep stability of the angle 2θ c , the MEMS mirror  10  is controlled by a bridge circuit  22  that sends driving signals  311  for driving the MEMS mirror  10  to oscillate. When the MEMS mirror  10  oscillates over the thresholds, the bridge circuit  22  sends out the driving signals  311 . In similar way, when the MEMS mirror  10  oscillation is under the thresholds, the bridge circuit  22  also sends out the driving signals  311 . The bridge circuit  22  is controlled by a first modulation signal  316   a  (Pulse Width Modulation signal(PWM1) a second modulation signal  316   b,  and a third modulation signal  316   c  from a MEMS scan controller  21 . Moreover, the laser controller  23  is a main controller of laser-printers or multi-function printers. The laser controller  23  sends the scanning data  318  for control of the pre-scan laser  11 , the enable (ENB) signals  313  that turns on the MEMS mirror  10 , and the adjust signals  314  that adjusts the MEMS mirror  10  so as to check whether the MEMS mirror  10  becomes stable, whether the scanning data  318  can be sent, and in what frequency the scanning data  318  is sent. 
         [0049]    After receiving the ENB signal  313  and the adjust signal  314 , the MEMS scan controller  21  generates the first modulation signal  316   a  for modulating frequency, the second modulation signal  316   b  for modulating frequency, the third modulation signal  316   c  for modulating amplitude and a stable signal  315  that represents the MEMS mirror  10  has been stable. By the PD signal  312   a  from the PD detector  14   a,  the resonant frequency of the MEMS mirror  10  is calculated so as to provide the laser controller  23  a clock(CLK) signal  310  for driving the pre-scan laser  11  timely to send image signal. By calculation and phase of the MEMS scan controller  21 , the clock (CLK) signal  310  provided with correct frequency. Thus the scanning beams  113   a,    113   b  from scanning of the laser light  111  are within effective scanning window. That means the scanning beams  113   a,    113   b  generates nβ spots on the object  15 . 
         [0050]    The MEMS scan controller  21  comprises a control logic  211 , a D inverter I  212 , a D inverter II  213 , a phase locked loop (PLL)  214  and a counter comparator  215 . The control logic  211  receives trigger PD signals  312   a  from the PD detector  14   a  and calculates each PD signal  312   a  to generate frequency modulation signals (the first modulation signal and the second modulation signal  316   a,    316   b ) and amplitude modulation signals (the third modulation signal  316   c ). The PLL  214  generates the CLK signal  310 . When the laser controller  23  receives the CLK signal  310  from the PLL  214  of the MEMS scan controller  21 , the scanning data  318  is sent according to frequency of the CLK signal  310 . The details are as followings: 
         [0051]    Refer to  FIG. 2 , the MEMS mirror  10  oscillates around the Y-axis, along the X axis and oscillation angle is ±θ c  to the right and left. At any time t, the angle θ(t) between the optical axis  113   c  and scanning beam from reflection of the laser light  111  is a sinusoid. When the reflected scanning beam triggers the PD detector  14   a,  a first-time triggered PD signal  312   a  is generated. When the MEMS mirror  10  oscillates to the right edge with an angle θ c , the angle θ(t) is maximum. Then the MEMS mirror  10  oscillates back and the angle θ(t) is reduced. When the reflected scanning beam triggers the PD detector  14   a,  a second-time triggered PD signal  312   a  is generated. When the scanning beam arrives within the effective scanning window (from  113   b  to  113   a,  the point b′ to the point a′ in  FIG. 2 ), now the relationship between the angle θ(t) and the time t is most close to linear (but it&#39;s sinusoid intrinsically). This is the effective scanning window of the forward scanning. 
         [0052]    When the MEMS mirror  10  oscillates to the left edge with an angle-θ c , the angle θ(t) is maximum. Then the MEMS mirror  10  oscillates back and the angle θ(t) is decreased. When the scanning beam arrives within the effective scanning window (from  113   a  to  113   b,  the point a to the point b in  FIG. 2 ), this is the effective scanning window of the forward scanning. When the MEMS mirror  10  keeps oscillating to the right and the scanning beam triggers the PD detector  14   a,  a third-time triggered PD signal  312   a  is generated and a scan cycle±θ c  is completed. The MEMS mirror  10  oscillates back after arriving the maximum angle θ c  and the angle θ(t) is decreased. When the scanning light triggers the PD detector  14   a,  a fourth-time triggered PD signal  312   a  is generated. 
         [0053]    Refer to  FIG. 3 , the MEMS scan controller  21  in this embodiment is formed by a control logic  211 , two D inverters  212 / 213 , a phase locked loop (PLL)  214  and a counter comparator  215 . The MEMS scan controller  21  receives the PD signal  312   a  from the PD detector  14   a.  The MEMS mirror  10  oscillates at the frequency of f and time of a period from left to right is T(t), and this is called forward scanning and reverse scanning of a scan cycle, as shown in  FIG. 4 . In a scan cycle, when the θ(t) is reduced from position of the scanning beam  114   a,  it&#39;s delay time T 1 , now the relationship between the angle θ(t) and the time t is close to linear. The laser controller  23  sends the scanning data  318  and time of sending is T 2 . This is the effective scanning window of the forward scanning. After the delay time T 3 , the laser controller  23  sends the scanning data  318  and time of sending is T 4 . This is the effective scanning window of the reverse scanning. The T 1 , T 2 , T 3 , and T 4  are within the same scan period T(t). The relationship among T 1 , T 2 , T 3 , and T 4  is as following: when f=2500 HZ, T 1 =1.137×10 −5 , T 2 =T 4 =1.2377×10 −4 , T 3 =7.623×10 −5  (sec) obtained by performing calculation of equation (1) to equation (4). 
         [0054]    When the ENB signals  313  from the laser controller  23  is at high voltage, disabling the MEMS mirror  10 . When the high voltage turns into low voltage, enabling of the MEMS mirror  10  to start oscillating. Refer to  FIG. 5 , after enabling of the MEMS mirror  10 , the MEMS mirror  10  is not stable, the stable signal  315  from the laser controller  23  is at low voltage and so is the adjust signal  314  from the laser controller  23 . After a period of time, MEMS mirror  10  has been stable, the stable signal  315  as well as the adjust signal  314  becomes at high voltage and the first modulation signal  316   a  is sent. By the control of the bridge circuit  22 , the modulation signal  316   a  becomes into a driving signal  311  so as to make the MEMS mirror  10  oscillates to the left. After the MEMS mirror  10  oscillating forward and reverse, each scan period T(t) triggers two times of PD detector  14   a.  Thus a trigger period T(t) of the triggered PD signal  312   a  is obtained by calculation of the control logic  211 . While controlling T 1 , T 2 , T 3 , T 4 , the control logic  211  of the MEMS scan controller  21  receives the triggered PD signals  312   a  from the PD detector  14   a,  calculates each triggered PD signal  312   a  and generates the first modulation signal  316   a,  the second modulation signal  316   b  for modulating frequency of the MEMS mirror  10 , and the third modulation signal  316   c  for modulating amplitude of the MEMS mirror  10 . After the first, the second and the third modulation signals  316   a,    316   b,    316   c  being sent, the bridge circuit  22  receives the modulation signals for adjusting resonant frequency and amplitude of the MEMS mirror  10 . 
         [0055]    Refer to  FIG. 6 , pulse duty relation of the first, the second and the third modulation signals  316   a,    316   b,    316   c  are set as following: inside a resonant period T, pulse duration of the first and the second modulation signals  316   a,    316   b  respectively are TA 1  and TA 3  while TA 1  is set to be equal to TA 3  (for this embodiment, not restricted). Time interval of the first and the second modulation signals  316   a,    316   b  respectively are TA 2  and TA 4  while TA 2  is set to be equal to TA 4  (for this embodiment, not restricted). Where in one period, is TA 1 +TA 2 +TA 3 +TA 4 =T. That means within the resonant period T, the first and the second modulation signals  316   a,    316   b  respectively are sent once. Thus the first and the second modulation signals  316   a,    316   b  drive the MEMS mirror  10  to oscillate with resonant frequency of 1/T. There is no restriction on TA 1 /TA 4  ratio and the ratio varies according to control loops. In this embodiment, TA 1 /TA 4 =1/4. The third modulation signal  316   c  changes from high voltage to low voltage. The ratio between high-potential time TA 10  and high-potential time TA 9  is duty D of amplitude adjustment. If frequency of the third modulation signal  316   c  is set as 1K (in this embodiment, generally the frequency is not limited) thus, TA 11 =1/1000, D=TA 10 /TA 11 , TA 9 +TA 10 =TA 11 . By adjusting value of D, the waveform of the third modulation signal  316   c  can be modified. Thus the amplitude of the MEMS mirror  10  is changed through the bridge circuit  22 . After reflecting the laser light  111 , the MEMS mirror  10  oscillates from the left to the right and triggers the PD detector  14   a  twice. After the MEMS mirror  10  reflecting the laser light  111 , the MEMS mirror  10  oscillates from the left side to right side and triggers the PD detector  14   a  twice again. Refer to  FIG. 8  the time interval between two contiguous triggers of the PD detector  14   a  is TA 6  and it&#39;s ratio to T(t) is TA 6 /(T(t)/2). The period T(t) changes over time so that the ratio TA 6 /(T(t)/2) also changes over time. The PD detector  14   a  is fixed in a certain position and the angle between the scanning beam  114   a  that triggers the PD detector  14   a  and the optical axis is θ p  while maximum scanning angle of the MEMS mirror  10  is θ c . When the period is T, ′R=TA 6 /(T/2). According to the change of the ratio R, the change of the period T is obtained. The equation is as the following: 
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                   6 
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         [0056]    Because the MEMS mirror  10  oscillates by means of electromagnetic force or spring force, its resonant frequency f(t) and amplitude A(t) at any time t are not fixed values. The lower limit of the resonant frequency is f L , and the upper limit of the frequency is f H  so that f L ≦f(t)≦f H . In this embodiment, f L =2375, f H =2625. The MEMS mirror  10  oscillation is affected by environment or its structure so that the change of the resonant frequency f(t) will have effect on timing of sending scanning data while the change of the amplitude A(t) will affect the reflection angle θ(t) and further affect the effective scanning window formed by the scanning beam  113   a    113   b.  Therefore, the way of the MEMS scan controller  21  to control the resonant frequency f(t) as well as the amplitude A(t) of the MEMS mirror  10  includes the following steps, as shown in  FIG. 10 :
       S1: set a initial value of the duty D this embodiment, D=90%) set initial value of the period T (in this embodiment, T=1/f L =4.21×10 −4  sec), and make the pre-scan laser  11  emit laser light  111  under the control of the laser controller  23 ;   S2: check the PD signal  312   a  is and whether there are two triggers within a half period (the period is 4.21×10 −4 sec);   S3: set the first modulation signal  316   a,  the second modulation signal  316   b  and the third modulation signal  31   c  at low voltage for frequency adjustment;   S4: check whether the trigger time ratio TA 6 /(T(t)/2) of the PD signal  312   a  is within R±5%. Once TA 6 /(T(t)/2) is within the range, check whether it&#39;s continuous stable. If yes, the laser controller  23  sends the stable signal  315 . If TA 6 /(T(t)/2) is not within the range, starts to adjust the amplitude.   S5: before adjusting the amplitude, check the ratio TA 6 /(T(t)/2) is within the upper limit (+5%) and the lower limit (−5%)   S6: adjust the value of the duty D for changing the amplitude so as to make the PD detector  14   a  can be triggered twice within the half-period;   S7: fine tune the frequency after adjusting the amplitude and the frequency is no more than f H .       
 
         [0064]    In this embodiment, the PD detector  14   a  is disposed at the angle θ p =21°. When f=2500 HZ, it is got from the Eq.(5): R=0.26745. When the laser controller  23  adjusts resonant frequency f(t) as well as the amplitude A(t) of the MEMS mirror  10  and checks the trigger time ratio TA 6 /(T(t)/2) of the PD signal  312   a,  R ranges from lower limit 0.25408 to upper limit 0.28082 (R=0.25408˜0.28082). 
         [0065]    After adjusting the frequency T(t) and the amplitude A(t) of the MEMS mirror  10 , and the laser controller  23  sending the stable signal  315 , scan data  318  is going to be transmitted. The MEMS scan controller  21  further comprises: at least one D inverter I  212 , at least one inverter II  213 . The D inverter I  212  and the D inverter II  213  receive the frequency modulation signals, the first modulation signal  316   a  and the second modulation signal  316   b,  from the control logic  211  and generates a resonant frequency signal  321  as well as feedback signal. 
         [0066]    Or after receiving trigger signals  322  from the counter comparator  215 , the D inverter  212 / 213  generates internal oscillation Q signal as well as feedback signal. The low voltage time T 12  and high voltage time T 13  of the resonant frequency signal  321  are shown in  FIG. 9 . The phase locked loop  214  receives the resonant frequency signals  321  and/or internal oscillator signals  323 , feedback signals from the D inverter and then generates the CLK signal  310 . The CLK signal  310  depends on the T 12 /T 13  ratio of the resonant frequency signal  321 . nβ pulse is generated within a half period. The counter comparator  215  receives the CLK signal  310  from the phase locked loop  214  while the CLK signal  310  is a pulse with f(t) frequency. After accumulating pulses of the CLK signal  310  to a certain number, the counter comparator  215  generates a trigger signal  322  and deletes the accumulated CLK signal  310 , reset for next feedback. 
         [0067]    When frequency and amplitude of the MEMS mirror  10  have become stable, the transmitting time of the scan data  318  within the effective scan window is T 2  (or T 4 ) with frequency of f(t) at time t. nβ=1*5102 spots should be sent within the effective scan window, as shown in  FIG. 9 . At this moment (time t), pulse frequency of the CLK signal  310  is f CLK (t). At t time and the frequency of the MEMS mirror  10  is 2500 HZ, f CLK =41.22 MHZ is got from Eq.(4). The counter comparator  215  generates 8244 pulses within T 2 . 
         [0068]    After frequency T(t) and amplitude A(t) of the MEMS mirror  10  being stable, the laser controller  23  starts to sending scan data and the way of sending scan data includes the following steps, as shown in  FIG. 11 :
       S1: if the ENB signal  313  from the laser controller  23  is at low voltage, the MEMS scan controller  21  will not output the CLK signal  310  and the data trigger signal  317   a.  Once the laser controller  23  outputs the ENB signal  313  or the adjust signal  314 , the MEMS scan controller  21  sends the first, the second and the third modulation signals  316   a,    316   b,    316   c  for adjusting and checking whether the MEMS mirror  10  becomes stable; now start-up procedure of the MEMS mirror  10  is completed;   S2: the MEMS scan controller  21  sends a stable signal  315  after the MEMS mirror  10  being stable;   S3: the MEMS scan controller  21  sends a clock signal  310  whose frequency f CLK (t) is calculated by Eq.(5);   S4: the laser controller  23  sends the scan data  318  with the frequency f CLK (t) of the clock signal  310 .       
 
         [0073]    The frequency f CLK (t) of the clock signal  310  is determined by the MEMS scan controller  21  according to the resonant frequency f(t) of the MEMS mirror  10  at any time t. Thus the laser controller  23  sends the scan data  318  according to this frequency (f CLK (t)) and the number of β or nβ beam spots are sent in a T 2  or T 4  period. The present invention provides a MEMS scan controller  21  that sends the clock signal  310  with frequency of f CLK (t), after oscillation of the MEMS mirror  10  becoming stable so as to send the scan data  318  within the effective scan window in the T 2  or T 4 . 
       Embodiment Two 
       [0074]    This embodiment is applied to a MEMS LSU with a PD detector. The control method of the MEMS scan controller  21  according to this embodiment is the same with that of the above embodiment. In order to send the scan data  318  more precisely, when the MEMS scan controller  21  sends the clock signal  310  with frequency of f CLK (t), the data trigger signal  317   a  is also sent simultaneously thereby for driving the laser controller  23  to start sending the scan data  318 . Refer to  FIG. 12 , once the control logic  211  of the MEMS scan controller  21  receives the ENB signal  313 , it sends the clock signal  310  as well as the data trigger signal  317   a.  The method of this embodiment to send the scan data includes following steps:
       S1: if the ENB signal  313  from the laser controller  23  is at low voltage, the MEMS scan controller  21  will not send the CLK signal  310  as well as the data trigger signal  317   a.  Once the laser controller  23  sends the ENB signal  313  or the adjust signal  314 , the MEMS scan controller  21  sends the first, the second and the third modulation signals  316   a,    316   b,    316   c  for adjusting and checking whether the MEMS mirror  10  becomes stable; now start-up (setting) procedure of the MEMS mirror  10  is completed;   S2: the MEMS scan controller  21  sends a stable signal  315  after the MEMS mirror  10  being stable;   S3: the MEMS scan controller  21  sends a clock signal  310  and a data trigger signal  317   a;  the frequency f CLK (t) of the clock signal  310  is calculated from Eq.(5);   S4: while receiving the data trigger signal  317   a,  the laser controller  23  sends the scan data  318  with the frequency f CLK (t) of the clock signal  310 .       
 
       The Embodiment Three 
       [0079]    This embodiment is applied to a MEMS LSU with a PD detector. The control method of the MEMS scan controller  21  according to this embodiment is the same with that of the first embodiment. The MEMS scan controller  21  of this embodiment further comprises a RF delay circuit  216  that delays the input resonant frequency signal  321  and not sending the data trigger signal  317   b  until generation of pulse of the first modulation signal  316   a.  The data trigger signal  317   b  drivers the laser controller  23  starting to send the scan data  318 . As shown in  FIG. 13 , once the control logic  211  of the MEMS scan controller  21  receives the stable signal  315 , it sends the clock signal  310  as well as the data trigger signal  317   a.  The method of this embodiment to send the scan data includes following steps:
       S1: if the ENB signal  313  from the laser controller  23  is at low voltage, the MEMS scan controller  21  will not send the CLK signal  310  as well as the data trigger signal  317   b.  Once the laser controller  23  sends the ENB signal  313  or the adjust signal  314 , the MEMS scan controller  21  sends the first, the second and the third modulation signals  316   a,    316   b,    316   c  for adjusting and checking whether the MEMS mirror  10  becomes stable; now the star-up procedure of the MEMS mirror  10  is completed;   S2: the MEMS scan controller  21  sends a stable signal  315  after the MEMS mirror  10  being stable;   S3: the MEMS scan controller  21  sends a clock signal  310  and a data trigger signal  317   b  while the frequency f CLK (t) of the clock signal  310  is calculated from Eq.(5);   S4: while receiving the data trigger signal  317   b,  the laser controller  23  sends the scan data  318  with the frequency f CLK (t) of the clock signal  310 .       
 
       Embodiment Four      
       [0084]    This embodiment is applied to a MEMS LSU with a PD detector. The control method of the MEMS scan controller  21  according to this embodiment is the same with that of the first embodiment. The MEMS scan controller  21  of this embodiment further comprises a data trigger delay circuit  217  that sends the input resonant frequency signal  321  when pulse of the first modulation signal  316   a  generates. In order to send the scan data  318  more precisely, when the MEMS scan controller  21  sends the clock signal  310  with frequency of f CLK (t), the data trigger signal  317   c  is sent simultaneously by the data trigger delay circuit  217  so as to drive the laser controller  23  starting transmission of the scan data  318 . Refer to  FIG. 14 , once the control logic  211  of the MEMS scan controller  21  receives the stable signal  315 , it sends the clock signal  310  as well as the data trigger signal  317   c.  The method of this embodiment to send the scan data comprises following steps:
       S1: if the ENB signal  313  from the laser controller  23  is at low voltage, the MEMS scan controller  21  will not send the CLK signal  310  as well as the data trigger signal  317   c.  Once the laser controller  23  sends the ENB signal  313  or the adjust signal  314 , the MEMS scan controller  21  sends the first, the second and the third modulation signals  316   a,    316   b,    316   c  for adjusting and checking whether the MEMS mirror  10  becomes stable; the start-up procedure of the MEMS mirror  10  is completed at this moment;   S2: the MEMS scan controller  21  sends a stable signal  315  after the MEMS mirror  10  being stable;   S3: the MEMS scan controller  21  sends a clock signal  310  and a data trigger signal  317   c  while the frequency f CLK (t) of the clock signal  310  is got from Eq.(5);   S4: while receiving the data trigger signal  317   c,  the laser controller  23  sends the scan data  318  with the frequency f CLK (t) of the clock signal  310 .       
 
       Embodiment Five 
       [0089]    This embodiment is applied to a MEMS LSU with two PD detectors. As shown in  FIG. 1 , the other PD detector  14   b  is disposed at the position of θp=−21°. In this embodiment, the MEMS mirror  10  has the frequency of 2500±5% HZ(f=2500±5% HZ) and maximum scanning angle of ±23°. The MEMS scan controller  21  receives the ENB signal  313  from the laser controller  23 , the adjust signal  314 , the first, the second and the third modulation signals  316   a,    316   b,    316   c  from the laser controller  23 . The resonant frequency of the MEMS mirror  10  is detected by the PD signals  312   a,    312   b  from the PD detectors  14   a,    14   b  so as to generate the clock signal  319  that is provided to the laser controller  23  for driving the pre-scan laser  11  timely. Thus the scan beam  113   a - 113   b  after scanning of the laser light  111  is within the effective scanning window even the scanning beam  113   a - 113   b  generates a number of nβ=5102 (when n=1) beam spots on the object  15 . 
         [0090]    The MEMS scan controller  21  comprises a control logic  211 , D inverter I  212 , D converter II  213 , a phase locked loop (PLL)  214  and a counter comparator  215 . The control logic  211  receives triggering PD signals  312   a  from the PD detector  14   a  (and PD signals  312   b  from the PD detector  14   b,  not showing in  FIG. 1 ) and calculates each PD signal  312   a,    312   b  from the PD detectors  14   a,    14   b  to generate frequency modulation signals (the first modulation signal and the second modulation signal  316   a,    316   b ) and amplitude modulation signals (the third modulation signal  316   c ) for the MEMS mirror  10 . The PLL  214  generates the CLK signal  310 . When the laser controller  23  receives the CLK signal  310  from the PLL  214  of the MEMS scan controller  21 , the scanning data  318  is sent according to this CLK signal  310 . 
         [0091]    When the MEMS mirror  10  oscillates forward and backward, in each scan period T(t), the scan beam  114   a  triggers the PD detector  14   a  twice (the scan beam  114   b  triggers the PD detector  14   b  twice). Thus the trigger period T(t) of the PD signal  312   a,    312   b  is obtained by the control logic  211 . While controlling T 1 , T 2 , T 3 , T 4 , the control logic  211  of the MEMS scan controller  21  receives the PD signal  312   a  from the PD detector  14   a  and the PD signal  312   b  from the PD detector  14   b,  calculates each trigger PD signal  312   a,    312   b  and generates the first modulation signal  316   a,  the second modulation signal  316   b  and the third modulation signal  316   c  of the MEMS mirror  10 . After the first, the second and the third modulation signals  316   a,    316   b,    316   c  being sent, the bridge circuit  22  receives the modulation signals for adjusting resonant frequency and amplitude of the MEMS mirror  10 . 
         [0092]    After the MEMS mirror  10  reflecting the laser light  111 , the MEMS mirror  10  oscillates from the left side to the right side and triggers the PD detector  14   a  as well as the PD detector  14   b  twice. Refer to  FIG. 16 , the time interval between the second triggering of the two contiguous triggers of the PD detector  14   a  and the first triggering of the PD detector  14   b  is TA 6  and it&#39;s ratio to T(t) is TA 6 /(T(t)/2). The period T(t) changes over time so that the ratio TA 6 /(T(t)/2) also changes over time. The PD detector  14   a  ( 14   b ) is fixed in a certain position and the angle between the scanning beam  114   a  that triggers the PD detector  14   a  and the optical axis is θ p  while maximum scanning angle of the MEMS mirror  10  is θ c . When the cycle is T(t), R=TA 6 /(T(t)/2). Or by calculation of change of the ratio R, the change of the period T is also obtained. The calculation is as following: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
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         [0000]    the same as (6) 
         [0093]    The method of the MEMS scan controller  21  to control resonant frequency f(t) and amplitude A(t) of the MEMS mirror  10  is the same with the first embodiment, as shown in  FIG. 10 . In this embodiment, the PD detector  14   a  and the PD detector  14   b  are disposed at the position the angle θ p =21°. When f=2500 HZ, it is calculated from the Eq.(7): TA 6 =1.4651×10 −4  sec R=0.73255. When the laser controller  23  controls resonant frequency f(t) as well as the amplitude A(t) of the MEMS mirror  10 , the trigger time ratio TA 6 /(T(t)/2) of the PD signal  312   a  ( 312   b ) is checked and R ranges from 0.17398 to 0.19230 (R=0.17398˜0.19230). 
         [0094]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.