Control system for oscillating optical element in scanners

• US 6,206,290 B1
• Filed: 03/06/1998
• Issued: 03/27/2001
• Est. Priority Date: 03/06/1998
• Status: Expired due to Term

• Current Assignee: Symbol Technologies, LLC (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Giebel, James, Mazz, Thomas, DiFazio, Constanzo, Stern, Mik, Goren, David
• Primary Examiner(s): TREMBLAY, MARK STEPHEN
• Application Number:
  US09/035,763
• Time in Patent Office: 1,117 Days
• Field of Search: 235/462.32, 235/462.25, 235/462.36, 235/462.33, 235/462.37
• US Class Current: 235/462.36
• CPC Class Codes: G06K 7/10643 Activating means

Abstract

A miniature light beam scanner includes a moving optical element formed on a substrate with electrodes for driving the optical element and sensing its motion as indicated by a varying capacitance between the optical element and the substrate. Control and driver circuitry responsive to the variation in capacitance drives the optical element to move at a resonant frequency and predetermined amplitude.

Claims

1. A light beam scanner for directing a light beam used to traverse a symbol to be read comprising:

a light source;

a mirror for reflecting light from the light source,

a drive electrode for applying electrostatic force to the mirror to vary its position so that the direction of reflected light is varied;

a pick-up electrode for sensing the position of the mirror; and

feedback circuitry for modifying the applied electrostatic force in response to the sensed mirror position,

wherein the electrostatic force is periodically applied to the mirror by imposing a periodic voltage on the drive electrode and wherein the position of the mirror is sensed by detecting variations in the capacitance between the mirror and the pick-up electrode, and

wherein a carrier signal is applied to the mirror and a voltage is induced on the pick-up electrode, which induced voltage corresponds to the carrier signal, amplitude modulated in response to changes in distance between the mirror and the pick-up electrode.

2. The light beam scanner of claim 1, wherein at least the mirror, drive electrode and pick-up electrode are formed on a single substrate.

3. The light beam scanner of claim 1, further comprising

generator circuitry for providing the carrier signal; and

generator circuitry for providing the periodic voltage imposed on the drive electrode; and

wherein the feedback circuitry controls at least one of the frequency and amplitude of the periodic driving voltage in response to detected variations in capacitance between the mirror and the pick-up electrode.

4. The light beam scanner of claim 1, wherein the mirror is pivotably mounted on the substrate by at least one grounded conductive member.

5. The light beam scanner of claim 1, wherein the mirror is suspended on the substrate by conductive members defining a pivoting axis of the mirror.

6. A miniature light beam scanner formed on a semiconductor substrate for detecting optical properties of a scanned target comprising:

means for directing a light beam in a direction dictated by the position of said means;

means for applying a driving force to vary the position of said directing means and for electrically sensing the position of said directing means; and

means for modifying the driving force in response to the sensed position of said directing means to cause the light beam directing means to oscillate at a resonance frequency and predetermined amplitude,

wherein the means for sensing the position of the directing means comprises one or more electrodes on the semiconductor substrate coupled by an electric field to the means for directing the light beam.

7. The miniature light beam scanner of claim 6, wherein the means for applying the driving force includes a heat actuated structure supporting the directing means.

8. The miniature light beam scanner of claim 6, wherein the means for applying the driving force includes means for applying a periodic electrostatic force to the light beam directing means.

9. The miniature light beam scanner of claim 6, wherein the light beam directing means includes a semiconductor laser.

10. The miniature light beam scanner of claim 6, wherein the light beam directing means includes a mirror whose shape is varied in response to the driving force thereby varying the position of its reflective surface.

11. The miniature light beam scanner of claim 6, wherein the light beam directing means includes a mirror whose position is varied in response to the driving force and wherein the driving force is an electrostatic force.

12. The light beam scanner of claim 11, wherein the means for applying an electrostatic force comprises one or more electrodes on the semiconductor substrate coupled by another electric field to the means for directing the light beam.

13. The light beam scanner of claim 6, wherein the light beam directing means is a mirror formed on the chip pivotable about at least one axis.

14. A method for controlling a moving mirror which reflects a light beam in a light beam scanner comprising the steps of

applying a periodically varying electrostatic force to the mirror to vary its position;

sensing the position of the mirror; and

modifying the applied electrostatic force in response to the sensed mirror position,

wherein the position of the mirror is sensed by detecting variation in the capacitance between the mirror and a conductive pick-up pad located in proximity to the mirror.

15. The method of claim 14, wherein the electrostatic force is periodically applied to the mirror by imposing a periodic driving voltage on a conductive drive pad located in proximity to the mirror.

16. The method of claim 15, wherein the applied electrostatic force is modified in response to the sensed mirror position by controlling at least one of the frequency and amplitude of the periodic driving voltage in response to the sensed mirror position.

17. The method of claim 14, wherein the position of the mirror is sensed by locating the conductive pick-up pad proximate to the mirror so that the distance between the pad and at least a portion of the mirror varies as the position of the mirror is varied, wherein the variation in capacitance is detected by the following steps:

applying a carrier signal to the mirror; and

detecting an induced voltage on the pick-up pad which corresponds to the carrier signal, amplitude modulated in response to changing distance between the mirror and the pick-up pad.

18. The method of claim 14, wherein the mirror is driven to rotate about an axis by applying a periodically varying electrostatic force to the mirror.

19. The method of claim 18, wherein the mirror is a torsional oscillator driven by the electrostatic force near its resonant frequency.

20. The method of claim 19, wherein the mirror torsionally oscillates at a frequency below 100 Hz.

21. Electromechanical oscillator and control system for light scanning, comprising:

an optical element at least a portion of which is moveable in relation to a supporting substrate;

an electrically activated driver for applying a driving force to the optical element;

a sensor for sensing movement of the optical element;

feedback circuitry for varying the applied driving force, wherein at least a portion of the optical element is driven to oscillate near a resonance frequency at a predetermined amplitude; and

electronic circuitry for imposing a carrier signal on the optical element, wherein the sensor includes at least one electrode in proximity of the optical element in which a voltage is induced corresponding to the carrier signal whose amplitude has been modulated by movement of the optical element.

22. The system of claim 21, wherein the optical element is a reflector mounted for movement about an axis parallel to the principal plane of the mirror for torsional oscillation.

23. The system of claim 21, wherein the optical element is a reflector, cantilever-mounted for oscillatory bending motion.

24. The system of claim 21, wherein the resonance frequency is between 20 and 30 hertz.

25. The system of claim 21, wherein the feedback circuit comprises:

an AM demodulator; and

a scan amplitude error detector for comparing an output signal from the AM demodulator with a reference to detect errors in the amplitude of oscillation of the optical element and for producing an amplitude control signal applied to the electrically activated driver.

26. The system of claim 21, wherein the optical element includes an integral reflector and detector for detecting light reflected from a bar code.

27. A control system for an oscillating optical element in light beam scanner, comprising:

an electrostatic driver for inducing oscillating motion of the optical element relative to a substrate on which the optical element is mounted;

a sensor on the substrate in which a varying voltage signal is induced containing information concerning the amplitude and phase of the movement of the optical element relative to the sensor; and

feedback circuitry for controlling the electrostatic driver responsive to said signal, to drive the optical element at a resonant frequency and predetermined amplitude.

28. The control system of claim 27, wherein the feedback provided for controlling the electrostatic driver is positive feedback.

29. The control system of claim 27, wherein the optical element is a gold-coated island formed in a micro machined mirror die mounted on the substrate, and wherein the electrostatic driver and sensor include electrodes on the substrate.

30. The control system of claim 29, wherein the gold-coated island provides a mirror about 10 square millimeters in area.