The Gyro-X Files

An extract from:

Fundamentals of Naval Weapons Systems

Chapter 5: Automatic Tracking Systems

(a public domain document, see full text)


...The reference gyro can be dedicated to a specific weapons system, or all weapons and sensors could be served by a single gyro reference. The usefulness of a gyro as a stable reference is due to its tendency to remain in a fixed plane in space if no force is applied to it, and its tendency to turn at right angles to the direction of an outside force applied to it.

Inertia. Gyroscopic inertia enables a gyro to remain at an apparently fixed orientation in space. This property allows it to be used as a directional reference or a vertical reference because it generally remains in the same plane while the vehicle or platform carrying it undergoes roll, pitch, and yaw. This apparent rigidity in space has been used to resist roll in ships; however, this means of direct stabilization required from one to three rotors weighing up to 120 tons each. Modern methods of stabilizing ships, aircraft, and weapons employ very small gyros as vertical and directional references for some type of autopilot. The autopilot senses movement of the gyro, then calculates error and generates a control signal that causes a hydraulic or electrical device to rotate a Control Surface that supplies the force to correct the orientation of the ship or aircraft in space or to alter its direction of motion. In each case the gyro (called a free gyro) is permitted to remain fixed in space, thereby moving an attached variable resistor or similar device as the platform rotates under it. In some ships and aircraft, an electrical signal is produced and distributed where needed from a single master gyro for use in navigation, weapons, and sensors, or command and control systems.

Precession. A gyro's spin axis has a tendency to turn at right angles to the direction of a force applied to it (figure 5-18). This precession causes the flight path of spin-stabilized gun projectiles to curve in the horizontal plane (chapter 19). As depicted in figure 5-19, when a downward force is applied by the weight on the end of the gyro spindle, a torque (D) results in precession in a counterclockwise direction. Thus, with a force applied in the vertical plane, the gyro precesses in the horizontal plane. A rate gyro or integrating gyro is fixed in one axis, and the torque of precession is converted to an electrical signal that is used to compute displacement or as a means of controlling gain in an antenna or launcher positioning system.

The integrating gyro. Stabilization and data smoothing can be accomplished with the same equipment. Inputs include roll or pitch angle and target position angle error, which are converted to a single output. The major input is the angle error signal that the gyro smooths to prevent the system from attempting to follow the instantaneous changes in the error signal. The position-error voltage derived from the radar is generally not a smoothly changing signal. If the drive system were to attempt to follow this signal, the result would be a series of jumps and a relatively rough positioning of the tracking element. The key to this smoothing process is the fact that the gyro cannot respond instantaneously to the changing input signal. This results in an averaging of the small incremental input changes. Figure 5-21 illustrates this electrical input/output relationship.

The governing equation for a gyro device is:

T = I dA/dt

where

T is torque

is the angular velocity of spin

I is the moment of inertia about the spin axis

d /dt is the angular velocity of precession

The input to the gyro itself is torque caused by the error signal being fed to small coils about the torque axis shaft. When a torque is applied, the response of the gyro is precession, and a signal proportional to the amount of precession is produced by a signal generator mounted on the precession axis shaft. The ability to follow an input signal accurately is governed by the rate of precession.

d /dt = T/I (5-1)

Equation (5-1) illustrates that the rate of precession, and ultimately the response of the gyro, are essentially governed by the spin velocity. The slower the spin, the greater will be the response to a given input torque. To achieve a smoothing function, the rate of spin, , should be relatively high to prevent the gyro from reacting to all the small changes in the input error signal.

The stabilization process results from the fact that the gyro is mounted in the housing of the tracking element. When the weapon platform pitches and rolls, the gyro reacts to this movement and generates a signal that results in positioning the tracking element in the opposite direction to the rotational motion of the weapon platform.

Limitations. A gyro spinning on earth will appear to tilt with the passage of time without any force applied. In reality the gyro is remaining in its original orientation, and the tile due to the rotation of the earth on its axis. Because the gyro does not rotate on frictionless bearings, there is some reaction force that causes the gyro to drift. Both of these phenomena must be taken into account when the gyros are employed.

Velocity feedback. The primary purpose of velocity feedback is to aid in the prevention of a dynamic overshoot. The velocity feedback signal is employed in a degenerative manner; i.e., the feedback voltage is subtracted form the output drive signal of the gyro. This subtraction serves to limit the speed of the output motors at an equilibrium level. This equilibrium level is governed by the ratio of the feedback voltage to the input drive voltage.

Power driving devices. Naval weapons systems generally employ two broad categories of motive devices: electric motors and electrically controlled hydraulic motors. These devices are used to actually move the driven element of the system. The use of two types of motors usually depends upon the physical size of the load to be driven. Smaller loads, such as missile-steering control surfaces and small fire control directors, are driven by electric motors. Electrohydraulic motors are used to position heavy loads such as missile launchers and gun mounts.


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This page updated Spring, 1999 by webloke © Copyright, 1999, S. Cobb