Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2000-05-25
2001-06-05
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S198100, C359S212100, C359S213100, C359S214100, C359S223100, C359S224200, C359S226200, C359S900000, C250S234000
Reexamination Certificate
active
06243186
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to optical scanner devices, and more particularly to a mechanical resonant scanner having a mirror which moves to deflect light along a scanning pattern.
Mechanical resonant scanners are used in retinal display devices to scan an image onto the retina of an eye. In an exemplary configuration one scanner is used to provide horizontal deflection of a light beam, while another scanner is used to provide vertical deflection of the light beam. Together the two scanners deflect the light beam along a raster pattern. By modulating the light beam and implementing multiple colors, a color image is scanned in raster format onto the retina.
Scanning rate and physical deflection distance characterize the movement of the scanner's mirror. In the context of a retinal display the scanning rate and deflection distances are defined to meet the limits of the human eye. For the eye to continually perceive an ongoing image the light beam rescans the image, or a changing image, in periodic fashion. Analogous to refreshing a pixel on a display screen, the eye's retinal receptors must receive light from the scanning light beam periodically. The minimum refresh rate is a function of the light adaptive ability of the eye, the image luminance, and the length of time the retinal receptors perceive luminance after light impinges. To achieve television quality imaging the refresh rate is to be at least 50 to 60 times per second (i.e., ≧50 Hz to 60 Hz). Further, to perceive continuous movement within an image the refresh rate is to be at least 30 Hz. With regard to the deflection distance, the mirror is deflected to define a raster pattern within the eye. System magnification and distance between the scanner and an eyepiece determine the desired deflection distance.
To define a raster pattern in which millions of bits of information (e.g., light pixels) are communicated onto a small area (i.e., eye retina), the position of the mirror needs to be known to a high degree of accuracy. In a mechanical resonant scanner, the resonant frequency defines the scanning rate. The resonant frequency is determined by a natural frequency of the scanning structure. Conventionally, a mechanical turn-screw is used to tune the resonant frequency to be equal to an image data drive signal (e.g., HSYNC or VSYNC). The resonant frequency, however, changes with environmental changes (e.g., temperature, barometric pressure). This change in resonance changes the phase relationship between the phase of the image data drive signal and the position phase of the mirror position. Accordingly, there is a need to monitor the position of the mirror.
SUMMARY OF THE INVENTION
According to the invention, two piezoelectric sensors are mounted on a spring-plate of a mechanical resonance scanner. The spring-plate supports a mirror or has a polished surface embodying a mirror used for deflecting a beam of light.
According to one aspect of the invention, the two piezoelectric sensors are mounted on respective sides of a center line on the back of the spring plate. Such center line is in parallel with the mirror's axis of rotation. As the mirror rotates back and forth the two sensors are accelerated and decelerated generating sensor output voltages at a 180° phase difference. A sensor output voltage crosses a zero level when the acceleration is unchanging. Both sensors cross the zero level at the same time but with opposite voltage polarity swings.
According to another aspect of the invention, a differential amplifier or other device detects the zero crossing. Such zero crossings correspond to the mirror being at a known position. Specifically, the mirror undergoes zero acceleration at its maximum velocity. Maximum velocity occurs when the mirror is at a level orientation relative to its mirror support structure. Detection of the zero crossover corresponds to the mirror being at this known position.
According to another aspect of this invention, acceleration of the scanner as a whole is differentiated from the accelerations of the mirror within the scanner. The piezoelectric sensors respond to acceleration to define a voltage output signal. In one application the scanner is part of a virtual retinal display worn by a user. Such user is able to move with the scanner. Such motion or other external vibrations or shocks induce voltage onto the piezoelectric sensors. By processing the two piezoelectric sensor output signals at a differential amplifier the common modes of the respective sensors are canceled out. Such common mode rejection eliminates the non-rotational accelerations associated with the external vibrations and shocks, and prevents masking the mirror's zero-crossings.
According to another aspect of the invention, the phase of an image data drive signal used for feeding image data onto the light beam being reflected by the scanner is locked to the position phase of the mirror oscillation action.
According to one advantage of the invention, detection of when the mirror is at the known position is useful for identifying phase difference between the phase of the image data drive signal and the position phase of the mirror. Mirror position phase changes are caused, for example, by changes in temperature. The resulting phase difference is corrected to keep the drive signal and mirror oscillation in phase. By doing so, a uniform raster scanning pattern is defined by one or more scanners. These and other aspects and advantages of the invention will be better understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5172002 (1992-12-01), Marshall
patent: 5526165 (1996-06-01), Toda et al.
patent: 5982528 (1999-11-01), Melville
Koda Steven P.
Phan James
University of Washington
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