Q: What happens inside a spinning gyroscope? What are the practical uses of the spinning mass gyroscope?
A: A gyroscope (or gyro) is a device that is used to define a fixed direction in space or to determine the change in angle or angular rate of turning of its carrying vehicle with respect to a reference frame. The spinning mass gyroscope is based on the principle that the spin axis of a spinning mass will remain in a fixed direction in space unless acted upon by an external influence. This principle follows from NewtonÂs laws of motion, and the spinning mass gyroscope uses it in two ways.
First, in the free gyro, the spinning mass is contained in a housing (or case) attached to the frame of a moving vehicle. The spinning mass, typically a sphere, is contained inside but is not attached to a housing. The spherical mass is rotated electromagnetically and is freely suspended inside the housing by hydrodynamic (gas pressure) or electrostatic (electric field) forces which prevent contact with the housing. As the housing moves with the vehicle, the spinning mass orientation remains fixed. Electromagnetic or optical pickoffs (devices used to convert mechanical motion into a proportional electric signal) are used to measure the relative motion of the case to the spinning mass. This relative motion is proportional to the angle (degrees, arc-seconds, etc.)the vehicle has turned through.
Figure:
Free Gyro
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In the second usage, the ends of the spinning mass are attached to a housing inside the gyro. The spinning mass is now constrained to spin in a fixed direction with respect to this internal housing. The outside of the gyro is connected to the frame of a moving vehicle. When the vehicle turns, the spin axis is also forced to turn. According to the principle of gyroscopic precession, the spinning mass exerts a torque sideways to the turning torque exerted by the vehicle. The spinning mass gyro is configured in such a way as to allow the internal housing containing the spinning mass to rotate (or precess) due to the sideways torque. Usually some type of elastic restraint is utilized to oppose the motion due to the precession. This gyroscope configuration is called a rate gyro.
Figure:
Rate Gyro Credit: Dr. Neil Barbour and The Charles Stark Draper Laboratory, Inc.
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The angle of precession is proportional to the rate (degrees per second, degrees per hour, etc.) that the vehicle is turning, and is measured by electromagnetic pickoffs. The output has to be integrated to find the angle the vehicle has turned through. In higher performing devices, the spring is replaced with a viscous damping fluid, and the precession angle is now proportional to the angle the vehicle has turned through.
Spinning mass gyros are used in several areas, such as the guidance of intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs); low-jitter pointing for the Hubble telescope; navigation of strategic submarines; control and stabilization of satellites (an example is the Control Moment Gyros in the International space Station); and legacy systems (that is, older systems already in use) such as aircraft navigators, tactical missiles, and north finders (systems that find the north direction by detecting the EarthÂs spin rate, in place of magnetic compasses). A new use for a spinning mass gyro is the Gravity Probe B experiment to test EinsteinÂs theory of general relativity. This recently launched Gravity Probe B satellite contains four ultra-precise spherical rotor free gyroscopes that are required to measure angles around 40 milli-arcseconds. To illustrate the size of this angle, if you climbed a slope of 40 milli-arcseconds for 100 miles you would rise only 1 inch (NASA Gravity Probe B Experiment press release, April 2004). These gyros are orders of magnitude more precise than any ever built. However, fewer gyros are spinning mass types these days. In the 1980s, ring laser gyros and fiber-optic gyros, which measure changes in light patterns, began to supersede them. Micromechanical Coriolis vibrating gyros are also being used in large quantities.
Dr. Neil Barbour
AccessScience biographies:
Foucault, Jean Bernard Leon;
Sperry, Elmer Ambrose
Related Websites:
Stanford University and NASAÂs Gravity Probe B
