RING LASER GYRO WITH RANDOMIZED MIRROR DITHER

Abstract: A ring laser gyro employing a periodic primary dither to eliminate lock-in at low rates of rotation and a random secondary dither to eliminate residual lock-in. The primary dither is provided by a dither motor (36) coupled to the body (10) of the gyro and responsive to a sine wave drive signal to vibrate the gyro body in a rotational mode. The secondary dither is provided by randomly vibrating two of the laser gyro mirrors (28, 30) in a direction perpendicular to the face of the mirrors and in a complementary manner to maintain constant the length of the path traveled by the beams.

Description: " Ring Laser Gyro with Randomized Mirror Dithef Technical Field The present invention relates to a means for eliminating lock-in in a ring laser gyro and more par- ticυlarly to a ring laser gyro employing a periodic pri¬ mary dither to eliminate lock-in at low rates of rotatio and a random secondary dither to eliminate residual lock-in. Background of the Invention 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 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. As the gyro is rotated, the effective path length for one beam is in¬ creased, while the effective path length for the other beam is decreased, due to Doppler shifting, λ beat fre- quency is produced in response to heterodyning of the tw beams as with a combining prism, the beat frequency sig¬ nal 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. At very low rates of rotation, errors arise due to "lock-in" effects, whereby no frequency difference is observed. 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. At low rates of rotation where the frequency split ting between the two beams is small, the coupling of the backscatter from one beam into the other beam causes the two beams of oscillate at the same frequency * This re¬ sults in a deadband, 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. One known technique used to eliminate lock-in at low rates of rotation employs a dither motor which is responsive to a sinusoidal drive signal to vibrate the body of the ring laser gyro about an input axis of the gyro. Although this dither technique reduces lock-in at low rates of rotation, the lock-in is not completely eliminated. It has been found that with a sinusoidal body dither, residual lock-in causing nonlinearities in the gyro output occurs at the harmonics of the sine wave body dither and also at low rates of rotation when the sine wave drive for the body dither is reversing. Another known body dither technique combines a sinusoidal signal with a random noise signal to provide a modulated drive signal to which the dither motor is res¬ ponsive, to vibrate the gyro body. Although the contri- bution of the random noise signal to the drive signal of the body dither reduces the size of the nonlinearities in the gyro output, the nonlinearities are not eliminated by this single dither technique. Disclosure of the Invention In accordance with the present invention, the disadvantages of prior ring laser gyros as discussed above have been overcome. The ring laser gyro of the present invention employs a primary dither for elimin¬ ating lock-in at low rates of rotation, the primary dither being periodic. The ring laser gyro also employs a secondary dither for eliminating residual lock-in, the secondary dither being random. The primary dither is provided by a dither motor coupled to the body of the gyro and responsive to sinusoidal drive signal to vibrate the gyro body in a rotational mode. The secondary dither is provided by tw of the laser gyro mirrors which are randomly vibrated in a complementary manner. The randomized mirror dither eliminates residual lock-in by operating on backscatter, the result being a Doppler shift in the frequency of the backscatter waves which are biased away from the primary waves to prevent coupling of the beams. Nonlinearities in the gyro output when the input rate is equal to a har monic of the body dither are thereby eliminated. Becaus the mirror dither is random, the integration of the non¬ linearities to zero is enhanced, eliminating residual lock-in caused when the sine wave drive for the body ' dither is reversing as well as lock-in occurring at the harmonics of the body dither. These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the fol¬ lowing description and the drawings. Brief Description of the Drawings Fig. 1 is a plan view of the ring laser gyro o the present invention employing a body dither in com in- ation with a randomized mirror dither; Fig. 2 is a graph illustrating the input/outpu curve of a ring laser gyro and the nonlinearities in the gyro output caused by lock-in. Best Mode for Carrying Out the Invention 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 pair of anodes 20 and 22, each of which is in communi¬ cation with the cavity 12, to produce two counter- rotating laser beams. The beams are reflected around th closed loop path by mirrors 24, 26, 28 and 30 positioned at the corners of the cavity. As the gyro is rotated about an input axis, the effective path length for one beam is increased while the effective path length for th 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. In order to prevent lock-in of the two counter rotating laser beams, the body 10 of the ring laser gyro is vibrated in a rotational mode by a dither motor, gen¬ erally designated 36, which is mounted in a centrally lo cated cylindrical opening of the gyro body. The dither motor 36 includes a central hub 38 having eight radial spoke-like assemblies extending therefrom with the as¬ semblies being alternately coupled at their outer ends t segments 40 and 42. Segments 40 and 42 are similar in configuration} however, segments 42 are bonded to the body 10 of the ring laser gyro whereas the segments 40 are fastened by mounting screws 44 to a mounting plate 4 disposed thereunder. Spring members 48 extend radially outward from the central hub 38 and are fastened to the segments 40, 42 by screws 50. The spring members 48 pro vide an electrical contact for a pair of piezoelectric members 52 and 54 which are bonded to opposite sides of each of the members 48. The piezoelectric members 52 an 54 are connected in parallel by lines 56, the members having a crystal orientation such that when a single voltage is applied between the common connection of line 56 and the dither motor ground, the strain of the member 52 is complementary to the strain of the members 54. In response to the applied voltage, the piezoelectric mem¬ bers and associated spring members coupled between the segments 40 and the hub 38 deflect, causing a slight rotation of the hub. The applied voltage also causes th piezoelectric members and associated spring members coupled between the segments 42 and hub 38 to deflect, resulting in a rotation of the laser gyro through an angle which is approximately twice the angle of rotation of the hub 38. Further details of the dither motor as¬ sembly 36 may be found in the copending. application Serial Mo. 496,606 filed May 20, 1983. The drive signal applied to line 56 of the dither motor is provided by a signal generator 58. The drive signal is sinusoidal so as to impart a sine wave dither to the body of the ring laser gyro. Although the sine wave body dither reduces lock-in at low rates of rotation, residual lock-in caused when the sine wave drive for the body dither is reversing still occurs. Further, it has been found that nonlinearities in the output of the gyro exist when the input rate is a har¬ monic of the body dither frequency, these nonlinearities being illustrated at 60 for the gyro input-output curve shown in Fig. 2. In order to eliminate residual lock-in, the ring laser gyro shown in Fig. 1 employs a secondary dither. The secondary dither is provided by randomly vibrating each of the mirrors 28 and 30 in a comple¬ mentary manner. The mirrors 28 and 30 are vibrated in a direction perpendicular to the mirror's face by res¬ pective drivers 62 and 64 in response to a random drive signal provided by a random noise generator 66. It is noted that the random drive signal for the mirror dither may be a pure random signal or it may be a pseudo-random signal in which case the noise generator 66 is a pseudo¬ random noise generator. The output of the random noise generator 66 is inverted by an inverter 68 before being applied to the mirror driver 62 associated with the mirror 28 so that the mirror 28 is moved the same dis- tance as the mirror 30 but in the opposite direction Vibrating the mirrors 28 and 30 in this complementary manner maintains the length of the closed loop path traveled by the beams constant. Details of the mirror drivers 62 and 64 as well as a control circuit which may be employed to ensure that the path length remains constant may be found in the copending application Serial No. 462,548 filed January 31, 1983. As the mirrors 28 and 30 are vibrated, the laser beams travel back and forth across the surface of the mirrors. This results in scatter center displacement with respect to the translated standing wave field modes, hence satisfying the phase shift requirements of phase modulation. In addition, the displacement between the scatter groups of the mirrors changes with time. It is the vector summation of these scatter groups which gives rise to a magnitude of lock-in. The vibration of the mirrors causes the net scatter vector to be time-modu- lated so as to eliminate lock-in, and the nonlinearities caused thereby, occurring at the harmonics of the body dither frequency. Further, because the vibration of the mirrors is random, the integration of the nonlinearities to zero is enhanced, eliminating residual lock-in occur- ring at low rates of rotation when the sine wave drive for the body dither is reversing, as well as lock-in occurring at the harmonics of the sine wave body dither.

Claims: Claires 1. In a ring laser gyro having a body with a 2 polygonal-shaped cavity therein forming a closed loop path through which two counter-rotating beams travel and 4 having a mirror disposed at each of the corners of the cavity for reflecting the beams about the path, means fo 6 eliminating errors in the output of the gyro caused by lock-in of the beams comprising: 8 a primary dither means for eliminating lock-in at low rates of rotation, said primary dither means bein .0 periodic; and means for randomly vibrating two of said mir- 12 rors to eliminate residual lock-in, said mirrors being vibrated in a complementary manner to maintain the lengt 14 of the closed loop path constant. 2. The ring laser gyro of claim 1 wherein sai 2 primary dither means includes means for vibrating the body of the ring laser gyro in a rotational mode. 3. The ring laser gyro of claim 1 wherein eac of said mirrors is vibrated in a direction perpendicular to the face of the mirror. 4. In a ring laser gyro having a body with a polygonal-shaped cavity therein forming a closed loop path through which two counter-rotating beams travel and having a mirror disposed at each of the corners of the cavity for reflecting the beams about the path, means fo eliminating errors in the output of the gyro caused by lock-in of the beams, comprising: means for generating a sine wave drive signal means responsive to said sine wave drive sign for vibrating the body of said ring laser gyro in a rot tional mode; means for generating a random noise drive sig nal and a complement of the random noise drive signal; means for vibrating one of said mirrors in response to said random noise drive signal; and means for vibrating another of said mirrors i response to the complement of said random noise drive signal. 5. The ring laser gyro of claim 4 wherein sa random noise drive signal generating means generates a pure random noise drive signal. 6. The ring laser gyro of claim 4 wherein sa random noise drive signal generating means is a pseudo-random noise generator.

Inventors: STJERN DARYL C, STJERN, DARYL, C.
Applicants: SUNDSTRAND OPTICAL TECH, SUNDSTRAND OPTICAL TECHNOLOGIES, INC.
Application year: 1985
Main classification: G01C19
IPC codes: G01C19