Browse Prior Art Database

TRACKING PHASE LOCKED OSCILLATOR

IP.com Disclosure Number: IPCOM000005408D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 2 page(s) / 116K

Publishing Venue

Motorola

Related People

Michael D. Kotzin: AUTHOR [+2]

Abstract

A vibrating bar rate gyro contains a bar of a square cross section and length many times larger than its width. It is suspended at its nodes and caused to vibrate at its resonant frequency in a free- free mode in a plane parallel to one of the longitudinal faces. When rotated about its longitudinal axis, a vibration in a plane orthogonal to the driven plane, but at substantially the same frequency, is induced. The magnitude of the induced motion is a measure of the rotation rate. The phase of the induced motion is at 90 degrees to the driven motion. In order for the best sensitivity and accuracy to be obtained, the bar must be driven very accurately at its free resonance frequency, which varies with changes in the environment. The detection of the induced motion, which can be extremely small compared with the driven motion, must be precisely at 90 degrees in phase to the driven motion. The condition of precise oscillation at the free resonant frequency occurs only when the signal from the driven motion sensor (described below) is precisely at 90 degrees to the driving signal applied to the driving transducer, so that the signal suffers a full 360 degrees phase shift through the entire feedback loop.

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m MOTOROLA Teclinical Disclosure Bulletin Vol. 1 No. 1 August 1980

TRACKING PHASE LOCKED OSCILLATOR By Michael D. Kotzin and Anthony Van Den Heuvel

STATEMENT OF PROBLEM

  A vibrating bar rate gyro contains a bar of a square cross section and length many times larger than its width. It is suspended at its nodes and caused to vibrate at its resonant frequency in a free- free mode in a plane parallel to one of the longitudinal faces. When rotated about its longitudinal axis, a vibration in a plane orthogonal to the driven plane, but at substantially the same frequency, is induced. The magnitude of the induced motion is a measure of the rotation rate. The phase of the induced motion is at 90 degrees to the driven motion. In order for the best sensitivity and accuracy to be obtained, the bar must be driven very accurately at its free resonance frequency, which varies with changes in the environment. The detection of the induced motion, which can be extremely small compared with the driven motion, must be precisely at 90 degrees in phase to the driven motion. The condition of precise oscillation at the free resonant frequency occurs only when the signal from the driven motion sensor (described below) is precisely at 90 degrees to the driving signal applied to the driving transducer, so that the signal suffers a full 360 degrees phase shift through the entire feedback loop.

   In the prior art, a conventional oscillator circuit is used to drive the bar at its resonant frequency. In this circuit, the output from a sensor attached to the bar, which senses motion predominantly in the driven plane, is amplified, phase shifted 90 degrees, and used as input to a second transducer attached to the bar which causes the bar to oscillate. The precise frequency at which this circuit will cause the bar to oscillate is determined by both the natural resonance of the bar, as well as the exact value of the phase shift experienced by the sense signal in its path from the driven motion sensor to the driving transducer. However, with conventional circuitry and elements, this phase shift which should be held exactly at 90 degrees, varies with ambient conditions in a manner unrelated to the natural free resonant frequency of the bar. Consequently, the frequency of oscillation is not held at the optimum value under varying am- bient conditions. Similarly, any 90 degrees phase shifted reference signal, derived using similar circuit elements, to be used to demodulate the signal induced in an orthogonal sensor due to rotation of the assembly, will deviate from a precise 90 degrees value.

SOLUTION

  The aforementioned problems and short comings of the conventional vibrating bar oscillator is over- come by a tracking phase locked oscillator which includes a phase locked loop coupled to the output of the bar resonant element sense transducer and a digital module adder (i.e., an exclusive OR gate) which combines the limited o...