Resonant scanning mirror driver circuit

[0002] 1. Field of the Invention

[0003] This invention relates generally to micro-electro-mechanical system (MEMS) mirrors, and more particularly, to a MEMS resonant scanning mirror driver circuit.

[0004] 2. Description of the Prior Art

[0005] Movement of a resonant MEMS mirror is accomplished using a control loop that includes a sinusoidal driver circuit. This driver circuit must have low harmonic distortion characteristics in order to precisely and accurately control movement of the MEMS mirror. Such driver circuits generally require use of either a precision analog oscillator or a fast and therefore expensive digital-to-analog converter (DAC) devices and accompanied sine wave logic to control the amplitude of the drive signal to a MEMS resonant scanning mirror.

[0006] In view of the foregoing, it would be desirable and advantageous in the MEMS mirror art to provide a resonant scanning mirror driver circuit that allows use of an inexpensive means of generating the sine wave and an inexpensive DAC with a relatively slow sample rate commensurate with a microprocessor based control algorithm.

[0007] The present invention is directed to a MEMS resonant scanning mirror driver circuit. The driver circuit has separate amplitude and waveshape inputs which allows a relatively slow and therefore inexpensive DAC to be used to control the amplitude of the drive signal for the MEMS device.

[0008] According to one embodiment, a resonant scanning mirror driver circuit is provided that has an analog input to set the amplitude of a sinusoidal drive voltage and further has a PWM digital input that generates the sinusoidal waveform.

[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 figure wherein:

[0010] FIG. 1 is a timing diagram depicting a tri-level PWM pulse waveform;

[0011] FIG. 2 is a schematic diagram illustrating a resonant scanning mirror driver circuit according to one embodiment of the present invention;

[0012] FIG. 3 is a graph illustrating harmonic distortion with respect to pulse width and pulse width as a proportion of period for the resonant scanning mirror driver circuit shown in FIG. 2 when driven with a tri-level PWM pulse waveform such as depicted in FIG. 1;

[0013] FIGS. 4aand 4billustrate a digital pattern and a waveform diagram showing a single pulse width modulated digital signal wherein the single pulse width modulated digital signal shown in FIG. 4bis generated from the digital pattern shown in FIG. 4a;

[0014] FIG. 5 is a schematic diagram illustrating a resonant scanning mirror driver circuit according to another embodiment of the present invention and that can be driven with the single pulse width modulated digital signal shown in FIG. 4b; and

[0015] FIG. 6 is a schematic diagram illustrating a resonant scanning mirror driver circuit according to yet another embodiment of the present invention and that can be driven with the single pulse width modulated digital signal shown in FIG. 4b.

[0016] While the above-identified drawing figure sets 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.

[0017] As stated herein before, movement of a MEMS mirror is accomplished using a control loop that includes a driver circuit. This driver circuit must have low harmonic distortion characteristics in order to precisely and accurately control movement of the MEMS mirror. Such driver circuits generally require use of fast and therefore expensive digital-to-analog converter (DAC) devices to control the amplitude of the drive signal to a MEMS resonant scanning mirror. A resonant scanning mirror driver circuit that allows use of a relatively slow and therefore inexpensive DAC to control the amplitude of the drive signal to the MEMS resonant scanning mirror is now described below with reference to FIGS. 1-5.

[0018] FIG. 1 is a waveform diagram depicting a tri-level PWM pulse waveform 10 associated with the current passing through a MEMS resonant scanning mirror device. PWM pulse waveform 10 has a positive pulse 12, a nominal zero level 14, and a negative pulse 16.

[0019] FIG. 2 is a schematic diagram illustrating a resonant scanning mirror driver circuit 100 according to one embodiment of the present invention. Driver circuit 100 has an analog input 102 to set the amplitude of sinusoidal drive voltage to the MEMS resonant scanning mirror device 104. Driver circuit 100 further has PWM digital inputs 106, 108 that provide a tri-level PWM pulse waveform, such as waveform 10 depicted in FIG. 1, to generate the desired sinusoidal waveform signal to the MEMS device 104. Driver circuit 100 generates a voltage output, and therefore is sensitive to temperature induced changes in the MEMS device 104 coil resistance Rc. Those skilled in the resonant scanning mirror art will readily appreciate that such a voltage output will provide several advantages over a transconductance amplifier if the actual application includes a beam position feedback. Resonant scanning mirror driver circuit 100 was found by the present inventor to achieve less than −60 dB harmonic distortion in a laser beam deflection using its lowest possible PWM clock.

[0020] Resonant scanning mirror driver circuit 100 can be defined mathematically according to its transfer function written as $\frac{I_{\mathrm{coil}}}{\left(v_1-v_2\right)}=\frac{\left(\frac{1}{L_c}s+\frac{G{\omega }^2}{R_c}\right)}{s^2+\frac{R_c}{L_c}s+{\omega }^2}=\frac{\frac{1}{L_c}\left(s+\frac{G}{R_f{C}_f}\right)}{s^2+\frac{R_c}{L_c}s+{\omega }^2},$

[0021] where the DC voltage gain is $G=1+\frac{2R_f}{R_g}=5$

[0022] for the component values set forth in FIG. 2. Since the cutoff frequency is defined by ${\omega }^2=\frac{R_c}{R_f{L}_c{C}_f},$

[0023] the values R_c=60 Ohms, L_c=4.8 mH, and C_f=3300 pF shown in FIG. 1 yield a cutoff of 3.1 kHz. The Q of the poles for these component values is $\frac{\omega L_c{R}_c}{R_c^2+L_c^2{\omega }^2}\approx \frac{\omega L_c}{R_c}=1.5.$

[0024] The zero is then at ${\omega }_z=\frac{G}{R_f{C}_f}=24\mathrm{kHz}.$

[0025] The input network associated with driver circuit 100 is configured such that the voltage applied to the analog input Vr is also applied across the MEMS device 104 coil.

[0026] The mechanical transfer function according to one embodiment of the MEMS mirror 104 can be mathematically defined as $\frac{\mathrm{deflection}}{I_{\mathrm{coil}}}=\frac{{\omega }_{\mathrm{res}}^2}{s^2+\frac{{\omega }_{\mathrm{res}}}{Q}s+{\omega }_{\mathrm{res}}^2},$

[0027] for a normalized deflection gain, where the resonant frequency ω_resfor the MEMS mirror 104 was found to be 2,000 Hz with a Q=50. The harmonic distortion in deflection associated with the above MEMS mirror 104 can be mathematically defined as $10{\log }_{10}\left(\frac{\sum _{n=2}^9{\mathrm{harm}}_n}{{\mathrm{harm}}_1}\right).$

[0028] FIG. 3 is a graph illustrating harmonic distortion 200 with respect to pulse width, and pulse width as a proportion of period for the resonant scanning mirror driver circuit 100 shown in FIG. 2 when driven with a tri-level PWM pulse waveform such as depicted in FIG. 1. Specifically, the upper plot 202 shows the harmonic distortion with a high bandwidth on the amplifier, while the lower plot 204 shows the harmonic distortion with a bandwidth of 3.1 kHz. The best performance can be seen to occur when the positive and negative pulses are each about ⅓ of a period. The harmonics in the deflection can then be virtually eliminated by limiting the bandwidth of the amplifier.

[0029] FIGS. 4aand 4bdepict a waveform diagram showing a single pulse width modulated (PWM) digital signal 300. PWM signal 300 can also be used to generate a sine wave for the MEMS mirror 104 coil drive so long as PWM signal 300 has a sufficiently high clock rate so that the harmonics in the digital signal are attenuated by the transfer function of the associated mirror driver circuit. Specifically, FIG. 4adepicts a digital pattern that is repeated continuously to generate the PWM waveform with a 64× oversample clock seen in FIG. 4b, wherein the digital clock is set to 64× the resonant frequency of the MEMS device. The single pulse width modulated digital signal 300 was found by the present inventor to provide less harmonic distortion (<−100 dB) than that achievable using the tri-level scheme described herein before with reference to FIGS. 1-3.

[0030] FIG. 5 is a schematic diagram illustrating a resonant scanning mirror driver circuit 400 according to another embodiment of the present invention and that can be driven with the single pulse width modulated digital signal 300 shown in FIG. 4b. The bias input level most preferably is set to the middle of the operating range of the input signal.

[0031] FIG. 6 is a schematic diagram illustrating a resonant scanning mirror driver circuit 500 according to yet another embodiment of the present invention and that is also suitable to be driven with the single pulse width modulated digital signal shown in FIG. 4bthat is generated via the digital pattern shown in FIG. 4a. Driver circuit 500 can be shown to have a more symmetric output voltage. The output is defined according to the relationship $v_{\mathrm{out}}=\frac{s+\left(A+1\right)\omega }{s+\omega }{v}_{in},$

[0032] where ω is the pole of the driver, not including the mirror coil, and A is the amplifier's dc voltage gain. The topology of driver circuit 500 was found by the present inventor to have improved AC characteristics because both the positive (+) and negative (−) outputs have the same frequency transfer function.

[0033] In view of the above, it can be seen the present invention presents a significant advancement in the art of MEMS mirror driver circuits. Further, this invention has been described in considerable detail in order to provide those skilled in the resonant scanning mirror driver circuit 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.