WO1998046967A2

WO1998046967A2 - Relative rate sensor for control moment gyroscopes

Info

Publication number: WO1998046967A2
Application number: PCT/US1998/004171
Authority: WO
Inventors: Gerald K. Foshage
Original assignee: Honeywell Inc.
Priority date: 1997-03-20
Filing date: 1998-03-04
Publication date: 1998-10-22
Also published as: WO1998046967A3

Abstract

A pair of fiber optic rate sensors are shown, one of which is connected to sense the rate of rotation of the gimbal of a control moment gyro and the other of which is connected to sense the rate of rotation of the frame of the control moment gyro, the relative rate therebetween being determined by a difference circuit receiving the outputs of the fiber optic rate sensors for use in a feedback loop for the control moment gyro.

Description

RELATIVE RATE SENSOR FOR CONTROL MOMENT GYROSCOPES

BACKGROUND OF INVENTION

1. FIELD OF THE INVENTION

This invention relates to relative rate sensors and more particularly to a fiber optic relative rate sensor for use with a control moment gyro. Yet more particularly, the invention relates to a control moment gyro with a fiber optic rate sensing arrangement that can provide feedback signals indicative of the relative rotation rates between the gyro gimbal and the frame in order to obtain high accuracy control which is relatively free from rate ripple, is high in noise rate threshold and is small and light weight.

2. DESCRIPTION OF THE PRIOR ART

Control moment gyros are old in the art. Using them to control the torque on the gimbal frame by controlling the spin rate about the gimbal axis has been accomplished. For example, a satellite in space may have its orientation or rate of rotation about an axis changed with a control moment gyro by fixing the gimbal frame to the satellite, mounting a gimbal on the gimbal frame and spinning a mass on the gimbal. Thereafter, by applying a torque to the gimbal at a command rate, the gimbal exerts a torque on the gimbal frame and thus to the satellite to produce the desired rate of rotation about the axis. In such systems it is has been desirable to provide a closed feedback loop for better control by assuring that the actual rate and the commanded rate are the same and for minimizing sensitivity to variations in the control moment gyro. Such feedback loops employ an electromagnetic generator (or tachometer) to supply a signal indicative of the relative rate of rotation between the gimbal frame and the inner gimbal assembly. Such a tachometer desirably measures the relative rate between the gimbal frame and the inner gimbal assembly. However, this tachometer has several undesirable features including 1) its size and weight (which varies inversely with its quality) which is particularly important in space applications where minimizing size and weight are most desirable, 2) its low rate threshold (small signals can't be discerned from the noise), 3) rate ripple (voltage that varies even without change in gimbal rate) and 4) it must be mounted colinearly with the gimbal axis (a particular difficulty when there are large spaces in a structure about its middle). BRIEF DESCRIPTION OF THE INVENTION

The present invention overcomes the problems in the prior art by utilizing at least two absolute rate sensors such as fiber optic rate sensors that are high in accuracy while small and light weight and which have a much higher rate threshold and very low rate ripple and need not be mounted colinearly with the gimbal axis. One of the rate sensors responds to rotation of the inner gimbal assembly and another responds to the rotation of the gimbal frame. The outputs of the two absolute rate sensors are then subtracted to provide the desired relative rate.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a control moment gyro using the present invention;

Figure 2 is the perspective view of Figure 1 with the fiber optic sensors mounted in a different position; and, Figure 3 is an alternate embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In Figures 1 and 2, a control moment gyro 10 is shown comprising a gimbal frame 20 which may be fixedly attached to an object such as a satellite as is shown by hatched markings 22. Frame 20 is shown with upwardly extending arms 24 near the top of which are bearings 26 to provide rotation for a shaft 28 about a horizontal axis 30.

It will be understood that use of the adjectives "vertical", "upward", "horizontal" etc. herein refer to the depiction in Figure 1 and do not imply any specific direction for an actual embodiment. Shaft 28 supports a gimbal 40 which turns with shaft 28 and is shown as a circular ring but which may have various other shapes as desired. The upper and lower parts of gimbal 40 contain bearings 44 to provide rotation for a shaft 48 about a vertical axis 50.

Shaft 48 supports a rotor 54 having a weighted ring 58 in a plane perpendicular to axis 50. Ring 58 is for providing the proper inertia for rotor 54 and, depending on the construction of rotor 54, may not necessarily be required. Rotor 54 is driven by motive means (not shown) so as to rotate about axis 50 in a direction shown by arrow 60. A controlled motive device or controller 62 operates through a connection shown as dashed line 64 to rotate gimbal 40 about axis 30.

The thus far described apparatus is a control moment gyro and as such, motion of gimbal 40 about axis 30 will produce a torque on the frame 20 and on the object 22 to which the frame is attached. This will produce a rotation of the object to thus change its orientation in a desired fashion as is known in the art.

As mention above, in order to provide better and more accurate control of the torque applied to the object, it is desirable to provide a feedback signal indicative of the relative rate of rotation between the gimbal 40 about axis 30 and the frame 20 about an axis parallel to axis 30. In the present invention, Figure 1 shows a first fiber optic rate sensor 70 is shown mounted on shaft 28 to sense rotation rate of the gimbal 40 about axis 30 and to provide a signal indicative thereof on a line 82. While, for convenience in Figure 1, fiber optic rate sensor 70 has been shown mounted around axis 30, it may be mounted elsewhere to gimbal 40 and still sense rotation of gimbal 40 about axis 30. For example, in Figure 2, a fiber optic sensor 101 is mounted to gimbal 40 on an arm

103 and about an axis 105. Since a fiber optic rate sensor is an absolute rate sensor, that is sensing rate about its sensitive axis regardless of location, this arrangement is quite operable and there is no requirement that rate sensor 70 be mounted on shaft 28 or otherwise about axis 30 as in Figure 1. In Figure 1 , a second fiber optic rate sensor 80 is shown mounted concentricity about axis 30 by a mounting member 84 connected to the right side upright arm 24 of frame 20. Fiber optic rate sensor 80 senses the rate of rotation of the frame 20 about axis 30 or whatever axis parallel thereto about which the frame rotates. Rate sensor 80 supplies a signal indicative of this rotation rate on a line 86. Again while, for convenience in Figure 1 , fiber optic rate sensor 80 has been shown mounted around axis

30, since it is an absolute rate sensor, it may be mounted elsewhere to frame 20 and still sense rotation about the axis frame 20 rotates. For example, in Figure 2, a fiber optic sensor 107 is shown mounted to frame 20 on an arm 109 and about an axis 111. This mounting will operate quite satisfactorily and there is no requirement that sensor 80 be mounted on arm 24 or otherwise about axis 30 as in Figure 1.

In order to obtain the desired relative rate of rotation between the inner gimbal assembly 40 and the frame 20, the output of fiber optic rate sensors 70 and 80 of Figure 1 (or the outputs of fiber optic rate sensors 101 and 107 in Figure 2) may be subtracted as shown in Figures 1 and 2 by a summation box 90 connected to line 82 (shown as +) and line 86 (shown as -). An output indicative of the difference or the relative rate is produce by summation box 90 on a line 92. The signal on line 92, indicative of the relative rate of rotation between the gimbal 40 and the frame 20, is then presented to a feedback circuit 94 and a feedback signal from feedback circuit 94 is presented to control circuit 62 via line 96. Control circuit may contain a drive motor which is operable to provide a motive force by way of a connection shown as dashed line 64 to move the gimbal 40 in the desired direction and create the desired torque on frame 20 for repositioning purposes.

Figure 3 shows and alternate embodiment of the present invention. It is sometimes desirable to, for example, position equipment aboard a spinning satellite so that the equipment remains stationary with respect to another reference system such as earth. Cameras and antennas need to be positioned so that they remain stationary with respect to earth. When the equipment is mounted on a spinning satellite, the requirement may take the form of a control system to cause the equipment to spin at the same speed as the satellite but in the opposite direction. In Figure 3, a first body 150 is shown mounted on a space craft as seen by hatch lines 152. Body 150 is shown in the form of a hollow cylinder mounted about a spin axis 154. As body 150 rotates, for example in the direction shown by arrow 156, the position of body 150 may be obtained from an optical encoder 158 mounted on body 150 about the axis 154. The rate of spin of body 150 about axis 154 is obtained by a fiber optic rate sensor 160 mounted near the edge of body 150 on an axis 164 parallel to but displace from axis 154.

A second body 170 is shown mounted on body 150 by bearings 172 so as to be rotatable about axis 154. Body 170 may carry cameras, antennae or other equipment

(not shown) which is desirably positioned to be stationary with respect to earth. It should be understood that while, for the purpose of explaining the present embodiment, the environment of a satellite with cameras or other equipment to be maintained as stationary with respect earth is used, other bodies and other apparatus could be used.. In order to make body 170 stationary with respect to earth, it is necessary to rotate body 170 at the same rate as body 150 but in the opposite direction as shown by arrow 176. The spin rate of body 170 is controlled by a control circuit 180 which may contain a spin motor connected to body 170 by a drive connection shown by dashed line 182.

A second fiber optic rate sensor 184 is mounted near the edge of body 179 about and axis 186 and senses the rotation of body 170 about axis 154. The outputs of fiber optic rate sensors 160 (shown as +) and 184 (shown as -) indicative of the rotation rates of bodies 150 and 170 are presented by lines 187 and 188 to a summation circuit 190 which operates like the summation circuit 90 in Figures 1 and 2 to subtract the signals and produce a relative rate signal on a line 192 which is presented to a feed back circuit 194. Feedback circuit 194 present drive signals indicative of the relative rotation rate on a line 196 to control circuit 180 to cause body 170 to rotate at the desired rate..

It is thus seen that I have provided the desired relative rate signal with apparatus which is far more accurate, smaller in size and weight and more convenient to mount and use than was obtainable in the prior art. Many changes and modifications will occur to those having skill in the art and I do not wish to be limited to the specific structures shown in connection with describing the preferred embodiment. For example, while the present invention has been shown specifically for use with space craft apparatus such as a control moment gyro or camera positioning apparatus, other space and non-space applications where it is desired to determine the relative rotation of two objects will occur to those having skill in the art. Also, while only two fiber optic rate sensors have been shown, more than two may be used to assure accuracy and to provide redundancy if desired.

Claims

The embodiments of the invention in which an exclusive property or right is claimed are defined as follows: 1. A relative rate of rotation sensor comprising a first absolute rate sensor operable to produce an output indicative of the rotation rate of a first object; a second absolute rate sensor operable to produce an output indicative of the rotation rate of a second object; and, a subtraction circuit connected to receive the outputs from the first and second absolute rate sensors and to produce an output indicative of the difference therebetween.

2. The relative rate sensor of claim 1 wherein the first and second absolute rate sensors are fiber optic rate sensors.

3. A control moment gyro comprising: a gimbal mounted for rotation about a first axis; a frame rotatable about a second axis; a first absolute rate sensor connected to sense the rate of rotation of the gimbal about the first axis and produce an output indicative thereof; a second absolute rate sensor connected to sense the rate of rotation of the frame about the second axis and to produce an output indicative thereof; and difference means connected to the first and second absolute rate sensors to receive the outputs therefrom and to provide a resultant output indicative of the relative rate of rotation between the gimbal and the frame.

4. The gyro of claim 3 wherein the first and second absolute rate sensors are fiber optic rate sensors.

5. The apparatus of claim 4 further including a rotor mounted for rotation in the gimbal.

6. The apparatus of claim 5 further including control means for moving the gimbal about the first axis to produce a desired torque on the frame.

7. The apparatus of claim 6 further including feedback means receiving the output of the difference means and connected to the control means to provide a feedback signal for the control means.

8. The method of determining the relative rate of rotation between first and second objects comprising the steps of:

A. connecting a first absolute rate sensor to the first object to determine the rate of rotation of the first object;

B. connecting a second absolute rate sensor to the second object to determine the rate of rotation of the second object; and C. subtracting the rate of rotation of the first object from the rate of rotation of the second object to obtain the relative rate of rotation between the first and second objects.