EP0171428A1 - Vibreur de miroir basculant pour gyroscope a laser en anneau - Google Patents

Vibreur de miroir basculant pour gyroscope a laser en anneau

Info

Publication number
EP0171428A1
EP0171428A1 EP85901167A EP85901167A EP0171428A1 EP 0171428 A1 EP0171428 A1 EP 0171428A1 EP 85901167 A EP85901167 A EP 85901167A EP 85901167 A EP85901167 A EP 85901167A EP 0171428 A1 EP0171428 A1 EP 0171428A1
Authority
EP
European Patent Office
Prior art keywords
gyro
mirrors
beams
ring laser
mirror
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP85901167A
Other languages
German (de)
English (en)
Inventor
Daryl C. Stjern
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sundstrand Optical Technologies Inc
Original Assignee
Sundstrand Optical Technologies Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Sundstrand Optical Technologies Inc filed Critical Sundstrand Optical Technologies Inc
Publication of EP0171428A1 publication Critical patent/EP0171428A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
    • G01C19/00—Gyroscopes; Turn-sensitive devices using vibrating masses; Turn-sensitive devices without moving masses; Measuring angular rate using gyroscopic effects
    • G01C19/58—Turn-sensitive devices without moving masses
    • G01C19/64—Gyrometers using the Sagnac effect, i.e. rotation-induced shifts between counter-rotating electromagnetic beams
    • G01C19/66—Ring laser gyrometers
    • G01C19/68—Lock-in prevention
    • G01C19/70—Lock-in prevention by mechanical means

Definitions

  • the present invention relates to a mirror dither for a ring laser gyro and more particularly to a tilt mirror dither for eliminating lock-in at low rates of rotation.
  • a ring laser gyro for measuring rotation about an input axis includes two monochromatic beams of light which are caused to travel in opposite directions about a closed loop path extending about the input axis.
  • the path is formed by a cavity which is typically polygonal in shape having mirrors disposed at the corners thereof to reflect the beams along the path.
  • a beat frequency is produced in response to heterodyning of the two beams as with a combining prism, the beat frequency signal in turn producing a fringe pattern which is typically detected by a photodiode. The latter produces a sine wave output whose frequency is proportional to the rate of rotation.
  • Lock-in arises because of imperfections in the lasing cavity, principally in the mirrors, which produce backscatter from one laser beam into the other laser beam.
  • the coupling of the backscatter from one beam into the other beam causes the two beams to oscillate at the same frequency. This results in a dead band, or lock-in region, in which the gyro output does not track the input.
  • the lock-in threshold rate is determined by the amount of backscatter. When the gyro input rate of rotation exceeds the lock-in threshold rate, the beams separate in frequency and begin to produce the output.
  • Various dither techniques have been employed to eliminate lock-in at low rates of rotation.
  • One such technique is to dither or vibrate the entire body of the ring laser gyro about the input axis.
  • Such body dithers disrupt the environment of the ring laser gyro which does not include any moving mechanical parts. Coupling of the body dither motion to motions caused by the environment, such as shock and vibration, may also be a problem.
  • the output of the ring laser gyro must be sampled synchronously with the body dither in order to cancel the effects of the dither from the gyro output.
  • Hutchings require that all of the mirrors be dithered at the same frequency but out of phase by an amount equal to 360° divided by the number of mirrors.
  • the phase difference between the dither applied to each of the mirrors must be precisely controlled so as to maintain the cavity path length constant. Because all of the gyro mirrors must be dithered as taught by Hutchings, problems in maintaining the cavity path length constant have arisen since the greater the number of mirrors, the more difficult it is to maintain precise control of the phase difference therebetween.
  • the combining optics associated with the output mirror of the gyro, must be displaced in order to install the driver for dithering the mirror.
  • the displaced optics are typically unstable moving relatively easily so as to produce errors in the output of the gyro.
  • the laser gyro of the present invention includes a tilt mirror dither to eliminate errors in the output of the gyro caused by lock-in of the beams.
  • the tilt mirror dither includes means for vibrating two of the ring laser gyro mirrors each in a rotational mode about an axis to tilt the mirrors back and forth, the mirrors being vibrated at the same frequency but 180° out of phase to maintain constant the length of the closed loop path traveled by the beams.
  • the rotation axis of each of the mirrors is in a plane perpendicular to the input axis of the gyro resulting in an up-down tilt mirror dither which causes the beams to translate across the surface of the mirrors in a direction parallel to the input axis of the gyro.
  • the rotation axis of each of the mirrors is parallel to the input axis of the gyro, resulting in a side-to-side tilt mirror dither which causes the beams to translate across the surface of the mirrors in a direction perpendicular to the input axis of the gyro.
  • the tilt mirror dither technique of the present invention eliminates lock-in at low rates of rotation by operating on the backscatter.
  • the result is a Doppler shift in the frequency of the backscatter waves, which are biased away from the primary waves of the laser beams so that coupling effects are minimized and lock-in at low rates of rotation is not present.
  • Fig. 1 is a top view of a cross-section of a ring laser gyro
  • Fig. 2a is an end view of a mirror illustrating the orientation of the mirror for the up-down tilt mirror dither;
  • Fig. 2b is an end view of a mirror illustrating the orientation of the side-to-side tilt mirror dither
  • Fig. 3 is a cross-section of the mirror drive for the tilt mirror dither.
  • the ring laser gyro shown in Fig. 1 includes a body 10, which may be made of quartz, having a cavity 12 therein forming a closed loop path.
  • the cavity 12 has a polygonal shape formed by intersecting gain tubes 14-17 and contains a gas or gases suitable for laser operation such as 90% helium and 10% neon at a pressure of 3 torr.
  • a gas discharge is established between a cathode 18 and a pair of anodes 20 and 22, each of which is in communication with the cavity 12, to produce two counter-rotating laser beams.
  • the beams are reflected around the closed loop path by mirrors 24, 26, 28 and 30 positioned at the corners of the cavity.
  • the effective path length for one beam is increased while the effective path length for the other beam is decreased due to Doppler shifting.
  • a beat frequency which is proportional to the rate of rotation is produced in response to heterodyning of the two beams such as by means of a prism associated with the mirror 26.
  • the beat frequency produces a fringe pattern which is detected by a photodiode 34 providing the output of the gyro.
  • the ring laser gyro employs a tilt mirror dither in which each of the mirrors 28 and 30 is vibrated in rotational mode about an axis to tilt the mirror back and forth.
  • the mirrors 28 and 30 include respective drivers 36 and 38 which are controlled to drive the mirrors at the same frequency but 180° out of phase to maintain constant the length of the closed loop path traveled by the beams.
  • each of the dithered mirrors is as shown for mirror 30 in Fig. 2a wherein the Z-axis is parallel to the input axis 32 of the gyro.
  • a pair of piezoelectric transducer elements 40 and 42, mounted on opposite sides of the X-axis, are responsive to a drive voltage which may take the form of a sine wave or the like to vibrate the mirror 30 in a rotational mode about the X-axis such that the mirror tilts up and down through a small angle with respect to the Z-axis.
  • the up-down tilt mirror dither causes the laser beams to translate across the surface of the mirror in a direction parallel to the input axis 32 of the gyro.
  • the tilt dithered mirrors 28 and 30 are driven 180° out of phase so that the mirrors tilt the same amount but in opposite directions with respect to the Z-axis. That is, as the mirror 30 is tilting upward, the mirror 28 is tilting downward, the tilt angles of the mirrors with respect to the Z-axis being equal.
  • the up-down tilt dither causes the plane of the closed loop path traveled by the laser beams to vary, which in turn varies the input axis of the gyro.
  • the resulting variation in the input axis is less than 1 arc second which has a negligible effect on the ring laser gyro operation since the allowable variation is on the order of 5-10 arc seconds.
  • the orientation of each of the dithered mirrors is as shown for the mirror 30 in Fig. 2b wherein the mirror of Fig. 2a is rotated 90° to provide a side-to-side tilt dither.
  • the piezoelectric transducer elements 40 and 42 are mounted on opposite sides of the Z-axis.
  • the piezoelectric transducer elements are responsive to a drive voltage which may take the form of a sine wave or the like to vibrate the mirror 30 in a rotational mode about the Z-axis such that the mirror tilts from side-to-side through a small angle with respect to the X-axis.
  • the side-to-side tilt mirror dither causes the laser beams to translate across the surface of the mirror in a direction perpendicular to the input axis 32 of the gyro.
  • the plane of the closed loop path traveled by the laser beams does not change and there is no variation in the input axis of the gyro.
  • Fig. 3 shows a basic design for the mirror drivers 36 and 38.
  • a mirror for example, mirror 30, is secured to the outer surface 44 of a diaphragm 46.
  • the diaphragm 46 includes an annular rim 48 which is secured to the inner wall 50 of a frame 52.
  • the diaphragm 46 also includes a centrally located leg 54 which extends inwardly towards the frame and engages a leg 56 extending outwardly from the base 58 of the frame.
  • the leg 56 intersects the base 58 at a point on the axis about which rotation of the mirror is to occur, i.e., the X-axis for the up-down tilt and the Z-axis for the side-to-side tilt dither.
  • Two piezoelectric transducer elements 60 and 62 are mounted on one side of the mirror's rotation axis and on the other side of the axis two more piezoelectric transducer elements 64 and 66 are mounted with the elements 60 and 64 being secured on opposite sides of the base from the elements 62 and 66.
  • the transducer ele ⁇ ments are mounted having a crystallographic orientation such that with the application of a positive voltage to the elements 60 and 66 by means of leads 68 and 70 and the application of a negative voltage to the element 64 and 62 by means of leads 72 and 74, the frame and thus the mirror 30 is caused to tilt in one direction.
  • the piezoelectric transducer elements With the application of a voltage of the opposite polarity to the leads 68 and 72 the piezoelectric transducer elements cause the mirror to tilt in a second direction opposite the first.
  • the drive voltage applied to leads 68 and 72 takes the form of a sine wave to vibrate the mirror in a rotational mode providing either the up-down tilt mirror dither or the side-to-side tilt mirror dither.
  • a pick- off electrode is connected to the piezoelectric trans ⁇ ducer element 62 in order to detect the voltage developed on the transducer as the mirror vibrate .
  • This ' voltage represents the actual displacement of the mirror 30 and is used as a feedback signal in the control circuit for the mirror drivers.
  • the control circuit which maintains the 180° phase difference between the mirrors is shown in detail in the copending application Serial No.
  • the laser beam translates a greater distance across the face of the mirrors where only two of the mirrors are dithered than where all of the mirrors are dithered. Further, the phase relationship between the two dithered mirrors and thus the cavity path length are more easily controlled than in gyros wherein all of the mirrors are dithered. Since the dithered mirrors 28 and 36 do not include any combining optics such as are associated with the mirror 26, for the output of the gyro, the shock and temperature stability of the ring laser gyro is also greatly improved over those gyros wherein each of the mirrors is dithered.
  • the mirror dither technique also has several advantages over the body dither biasing technique. Although the mirror dither technique is not purely optical since the mirrors are driven by piezoelectric transducer elements, the displacement of the mirrors is extremely small compared to the displacement imparted to a body dithered laser gyro, the mirror displacement being typically on the order of 20 inches peak to peak.
  • the tilt mirror dither over the body dither is that the output of the ring laser gyro employing the mirror dither may be sampled asynchronously with respect to the mirror dither.
  • any number of the mirrors may be dithered at the same frequency and at a phase difference of 360° divided by the number of dithered mirrors in order to maintain the path length constant.

Landscapes

  • Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Optics & Photonics (AREA)
  • Electromagnetism (AREA)
  • Power Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Gyroscopes (AREA)
  • Lasers (AREA)

Abstract

Vibreur de miroir basculant pour un gyroscope à laser en anneau permettant d'éliminer le blocage aux faibles vitesses de rotation. Le vibreur de miroir basculant comprend des organes (36, 38) servant à faire vibrer deux des miroirs du gyroscope à laser en anneau (30, 28), chacun en un mode de rotation autour d'un axe pour faire basculer les miroirs avec un movement de va-et-vient, les miroirs étant mis en vibration à la même fréquence mais avec un déphasage de 180o, afin de maintenir constante la longueur du cheminement des faisceaux laser. Dans un mode de réalisation (Fig. 2A) on décrit un vibreur de miroir basculant verticalement et dans un autre (Fig. 2B) on décrit un vibreur de miroir basculant horizontalement.
EP85901167A 1984-02-08 1985-01-30 Vibreur de miroir basculant pour gyroscope a laser en anneau Withdrawn EP0171428A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US57811384A 1984-02-08 1984-02-08
US578113 1984-02-08

Publications (1)

Publication Number Publication Date
EP0171428A1 true EP0171428A1 (fr) 1986-02-19

Family

ID=24311502

Family Applications (1)

Application Number Title Priority Date Filing Date
EP85901167A Withdrawn EP0171428A1 (fr) 1984-02-08 1985-01-30 Vibreur de miroir basculant pour gyroscope a laser en anneau

Country Status (4)

Country Link
EP (1) EP0171428A1 (fr)
AU (1) AU3997785A (fr)
IT (1) IT8547649A0 (fr)
WO (1) WO1985003568A1 (fr)

Families Citing this family (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4817112A (en) * 1985-05-10 1989-03-28 Honeywell Inc. Low cost ring laser angular rate sensor
GB8615789D0 (en) * 1986-06-27 1986-08-06 British Aerospace Lasers
US5116131A (en) * 1990-04-19 1992-05-26 Litton Systems, Inc. Mirror transducer assembly for ring laser gyroscopes
US5148076A (en) * 1991-04-09 1992-09-15 Honeywell Inc. Apparatus for thermal tuning of path length control drivers
US5469258A (en) * 1993-11-29 1995-11-21 Allied Signal Inc. Ring laser gyroscope with tilting mirrors

Family Cites Families (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
FR2414715A1 (fr) * 1978-01-13 1979-08-10 Sfena Procede d'attenua
US4410274A (en) * 1981-06-08 1983-10-18 The Singer Company Ring laser gyroscope with doppler mirrors and offset actuators

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO8503568A1 *

Also Published As

Publication number Publication date
WO1985003568A1 (fr) 1985-08-15
AU3997785A (en) 1985-08-27
IT8547649A0 (it) 1985-02-07

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Legal Events

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PUAI Public reference made under article 153(3) epc to a published international application that has entered the european phase

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Effective date: 19860109

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Inventor name: STJERN, DARYL, C.