Low vibration resonant scanning unit for miniature optical display apparatus

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LOW VIBRATION RESONANT SCANNING UNIT
FOR MINIATURE OPTICAL DISPLAY
APPARATUS

FIELD OF THE INVENTION

This invention relates to display devices which generate a raster scan image by using a line of light-emitting devices displayed by means of an oscillating mirror. More particularly, the invention relates to means for reducing vibration in such devices caused by the rapid oscillation of the mirror.

BACKGROUND OF THE INVENTION

There are many different types of display devices which can visually display information usch as figures, numbers and video information. These devices include the ubiquitous cathode ray tube in which a raster is created by repetitively sweeping an electron beam in a rectangular pattern. The image is created by selectively modulating the beam to generate light and dark spots on the raster.

Another display device is an electromechanical scanning system in which a line of light-emitting devices is modulated with the information to be displayed. The 25 illuminated line is converted into a raster by means of an oscillating mirror thereby generating a virtual raster image. These latter devices have the advantage that a full "page" display can be created from a much smaller number of light-emitting devices than is necessary to 30 generate a normal full page real image.

In operation, an enlarged, virtual image of the illuminated devices is reflected from a mirror as the niirror is being physically pivoted about a fixed axis by means of an electromagnetic motor. Although it is possible to directly drive the mirror to produce oscillations, in order to reduce the power necessary to drive the mirror, it is possible to use a resonant electromechanical oscillator to move the mirror. In such an oscillator, the niirror is mounted on a spring attached to a frame so that the mass of the mirror and the spring create a mechanical resonator. An electromagnetic motor oscillates the mirror mass at the resonant frequency of the spring/mirror system. In this manner, only a small amount of power is needed to produce a relatively large oscillation. Such a conventional resonant oscillator is shown in U.S. Pat. No. 4,632,501 in which the rnirror is attached to the base by a thin sheet of spring material.

A problem with the conventional mirror/spring oscillator system is that the rapid angular oscillation of the mirror requires a large spring force to accelerate and decelerate the rnirror. The spring force is also applied to the base of the device and constitutes a "reaction force". When the base is rigidly secured to a relatively massive object, this force is not a serious concern. However, when it is impossible or undesirable to attach the display device to a massive object, as is the case for hand-held, eyeglass-mounted or "heads-up" displays, the force causes vibrations which are, at best, annoying and, in some cases, may cause the resulting image to be blurred or even unintelligible. In addition, the vibration can disrupt the function of an accompanying instrument, such as a microscope, that is sensitive to vibration. Further, even if the vibration is acceptable, the power required to oscillate the mirror increases when the vibration is transmitted to an external structure. This extra power means a larger motor is required to insure that the motor can drive the display with suffi

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cient amplitude, in turn, resulting in increased battery drain for portable displays.

This problem is even more serious if the system design requires that the mirror pivot about a point near the end of the rnirror as opposed to near the center of the rnirror. Such a design is desirable in a hand-held display application because it permits use of smaller lenses and results in a more compact display.

Accordingly, it is an object of the present invention to provide a resonant scanning unit for an optical display device in which the net reaction force transferred to the mounting base is reduced.

It is a further object of the present invention to provide a resonant scanning unit which reduces vibration in a resonant-scanning optical display device.

It is yet another object of the present invention to provide a resonant scanning unit which allows the electromagnetic motor which drives the mirror to operate in an efficient manner.

It is yet another object of the present invention to provide a resonant scanner construction which uses a counter-balanced mass construction to reduce the net reaction force transmitted to the mounting base.

It is still a further object of the present invention to provide a resonant scanning unit in which the pivot point about which the mirror oscillates is located at a distance from the mirror center.

SUMMARY OF THE INVENTION

The foregoing objects are achieved and the foregoing problems are solved in one illustrative embodiment of the invention in which a resonant scanning unit comprises a mirror and a counterbalance mass which move in opposing directions. As both masses are attached to the same mounting base, it is possible to configure the arrangement so that reaction forces caused by the moving masses are cancelled at the base, thereby substantially reducing overall vibrations.

More specifically, the mirror support consists of a "tuning-fork" configuration with the niirror mounted on one arm and a counterbalance mass mounted on the other arm. The driving motor comprises a magnet and coil structure which drives one or more of the the arms so that the arms move in opposite directions.

In one embodiment, the niirror is mounted to the base of the display device by crossed flexure springs. A counterbalance mass is also connected to the base of the video display device by a spring. The stiffness of both the niirror flexures and the counterbalance mass spring are selected so that the mirror and counterbalance mass have substantially the same resonant frequency.

In this embodiment, a voice-coil electromagnetic motor is used to dive the niirror and the counterbalance mass. The motor comprises a permanent magnet portion and associated magnetic return path mounted on one arm of the tuning fork configuration and a coil mounted on the other arm. When a properly controlled current is applied to the coil, the permanent magnet is alternately attracted and repulsed from the coil. In this fashion, a driving force is applied to both the mirror and the counterbalance mass causing each to oscillate at the frequency of the driving force.

The spring forces which accelerate and deaccelerate the mirror and counterbalance mass are also applied by the flexure springs to the base, and constitute "reaction forces". The geometry of the counterbalance mass and the counterbalance mass pivot point location are both 3

selected so that the reaction force applied to the base by the counterbalance mass substantially cancels the reaction force applied to the base by the mirror.

In addition, the geometry of the electromagnetic motor is selected so that the drive forces applied to the 5 mirror and counterbalance mass are substantially equal to the air resistance forces acting on the mirror and counterbalance mass, with the result that little or no net force is applied to the base due to drive forces.

BRIEF DESCRIPTION OF THE DRAWING 10

FIG. 1 is a schematic view of a typical prior art resonant electromechanical scanner.

FIG. 2 is a partial cross-sectional view of a resonant scanner constructed in accordance with the present 15 invention.

FIG. 3A is a schematic diagram of a miniature display using a scanning mirror which is pivoted near the mirror center.

FIG. 3B is a schematic diagram of a miniature display 20 using a scanning mirror which is pivoted near the mirror end.

FIG. 4 is a perspective view of the resonant scanner shown in FIG. 2.

FIG. 5 is a perspective view of a preferred embodi- 25 ment of a resonant scanner utilizing a drive motor with improved efficiency.

FIG. 6 is a plan view of the preferred drive motor construction showing the mirror assembly overlaid by the counterbalance mass. Spring 42 which connects the 30 mirror assembly to the base has been omitted for clarity.

FIG. 7isa cross-sectional view of the preferred drive motor embodiment taken along the line 6—6 shown in FIG. 6.

FIG. 8 is another cross-sectional view of the pre- 35 ferred drive motor embodiment taken along the line 7—7 shown in FIG. 6.

FIG. 9 is a longitudinal cross section of another embodiment using a sensor flag to sense position of the mirror 30. 40

FIG. 10 is a partial exploded view of a portion of the embodiment shown in FIG. 9 showing the voice coil with sensor flag, the counterbalance mass and the sensor assembly.

FIG. 11 is an electrical schematic diagram of an illus- 45 trative drive circuit.

FIG. 12 is a perspective view of a miniature optical display device utilizing an illustrative embodiment of the inventive scanner unit.

DETAILED DESCRIPTION OF PREFERRED 50 EMBODIMENTS

FIG. 1 is a schematic diagram of a typical prior art resonant electromechanical scanner of the type shown in U.S. Pat. No. 4,632,501. As this device is explained in 55 detail in the latter patent, it will not be fully discussed herein. In FIG. 1, the resonant scanner is used in a scanned image display device of the type covered in copending U.S. patent application entitled Miniature Video Display System, filed on July 27,1987 under Ser. 60 No. 078,295 and assigned to the same assignee as the present invention. As the display device is described in detail in that application, which is hereby incorporated by reference, only a brief description of the operation will be given here. 65

In a scanned image type of display device, a row of light emitting devices 10 (which may illustratively be light-emitting diodes) is electrically excited to selec

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tively emit-light thereby generating an illuminated line. In FIG. 1, the row of LEDs extends perpendicularly into the page.

The light from LED row 10 passes through an optical system schematically illustrated as lens 5, which creates an enlarged virtual image of the LED's. The image is reflected from mirror 30 to an observer's eye 15 as mirror 30 is repetitively oscillated in the direction of arrow 16. By selectively illuminating the LEDs in row 10 as mirror 30 moves, a rectangular raster can be formed which can be observed by the viewer.

The mechanism which moves the mirror is generally termed as a resonant scanner. It consists of a base 20 to which plane mirror 30 is attached by means of a flat spring 34 which extends perpendicularly into the page. Mirror 30 is oscillated by a drive motor consisting of two cylindrical permanent magnets 44 and 120 and two ring coils 46 and 125. In operation, one of coils 46 and 125, for example coil 125, is excited and the corresponding permanent magnet, 120, is either attracted into coil 125 or repulsed depending on the relative magnetic fields produced by coil 125 and magnet 120. The resulting force causes mirror 30 to pivot around the attachment point with spring 34 so that mirror 30 oscillates in the direction of arrow 16.

The remaining coil (coil 46 in the example) is used as a sensing coil to sense the motion of mirror 30. The electrical signals derived from the motion of magnet 44 relative to coil 46 are used by driving circuitry (not shown) to control the current provided to drive coil 125 in a conventional fashion and as described in the aforementioned U.S. Pat. No. 4,632,501.

In practice, the mass and geometry of mirror 30 and the spring constant of spring 34 are chosen so that a resonant mass system is formed at the desired operating frequency. In this manner, a large excursion angle for mirror 30 is produced by a driving force which is much lower than would be required if the mirror were driven in a non-resonant fashion.

The problem with the mirror driving system shown in FIG. 1 is that the spring forces which cause the mirror mass 30 to oscillate are also applied to base 20 and any supporting structures attached to base 20. Although mirror 30 is generally quite small, its motion is typically at a sufficiently high frequency that the forces are large in amplitude. These large amplitude forces are transmitted to base 20 causing it to vibrate in response.

FIG. 2 shows a partial cross-sectional view of the mirror assembly of the present invention. The elements of FIG. 2 which correspond to those of FIG. 1 have been given the same numeral designations. In the FIG. 2 structure, mirror 30 is part of a balanced assembly with two arms. One arm comprises mirror 30, mirror support 36 and driving coil 46. The other arm consists of weight 40 and permanent magnet 44. The mirror arm is attached to base mounting 21 by means of two flexure springs 32 and 34. Springs 32 and 34 are both flat springs which extend into the page. As will be discussed hereinafter, two springs are used in a crossed arrangement to constrain rotation of the mirror assembly to a single axis.

Mirror 30 is directly attached to a mirror support 36 (which may be comprised of a suitable plastic or other material) by means of adhesive or cement. One end of spring 32 is attached to mirror support 36 by means of a screw or rivet or other fastener 35. The other end of spring 32 is attached to base mounting 21 by means of another fastener 33. A second spring, 34, is also at