GB2175692A

GB2175692A - Rotatable body with non- uniform mass distribution

Info

Publication number: GB2175692A
Application number: GB08513224A
Authority: GB
Inventors: John David Nuttall; David Charles Witt; Arthur Sydney Huxley
Original assignee: Ferranti PLC; UK Secretary of State for Defence
Current assignee: Ferranti International PLC; UK Secretary of State for Defence
Priority date: 1985-05-24
Filing date: 1985-05-24
Publication date: 1986-12-03
Also published as: GB2175692B; FR2582401A1

Abstract

A rotatable body (10) has the density of at least one region (12, 13) of its surface increased by the implantation in that region of ions of a suitable elemental material. The effect is to produce a small change in the mass distribution of the body. The description relates to the provision of a small change in the mass distribution of the spherical rotor of an electrostatically- suspended gyroscope made from beryllium and implanted with silver ions. <IMAGE>

Description

SPECIFICATION Rotatable body with non-uniform mass distribution This invention relates to a rotatable body with a non-uniform mass distribution and to a method of making such a body. It relates primarily, though not exclusively, to the production of a mass unbalanced rotor for an electrostatically-suspended gyro. The technique could be used equally for the accurate balancing of a rotatable body.

In many instances the deliberate balancing or unbalancing of a rotatable body may be achieved without too much difficulty. Commonly-used techniques involve adding material to, or removing material from, the body at certain selected points. For high precision purposes it is usual to remove material, such as by drilling material from the rim of a gyroscope rotor. However, the accuracy of this technique depends upon many factors, including the size of the rotor and upon the accuracy with which the drilling operation is controlled.

A particular problem exists with the rotor of an electrostatically-suspended gyroscope. In such a gyroscope the rotor is a hollow or solid sphere which is supported by electric fields. The sphere is located inside an evacuated enclosure, with a very small gap between the rotor and the enclosure. The enclosure is of an insulating material, but there are a number of pairs of electrodes on the inside of the enclosure. It is the electric field produced between these electrodes and the rotor which enables the rotor to be suspended. The rotor is caused to rotate by magnetic fields generated by coils, usually external to the vacuum enclosure. Because of the electrostatic suspension and the evacuated enclosure, the rotor is subject to very little drag, and once it has reached the desired spin speed, the spinfield can be switched off.Subsequently the rotor's spin-axis tends to stay fixed in inertial space, even if the enclosure is rotated. Thus, to use the instrument to measure rotation, it is necessary to have a means of determining the angle between the spin-axis and the axis of the enclosure.

One technique for doing this requires a small change in the mass distribution of the rotor, resulting in the separation of its centre of mass and its geometric centre. Provided that the system controlling the suspension reacts slowly relative to the period of the rotor's rotation, the rotor will rotate about its mass centre, not its geometric centre, causing the gap between the rotor and any one point on the enclosure to vary sinusoidally, at rotor spin frequency, and with an amplitude and phase depending on the angle between the spin-axis and the enclosure axis. If there is a suitable circuit arrangement in the suspension system, this frequency can be detected for each electrode pair in the enclosure, and the spin axis angle deduced.

For this technique to be successful it is necessary for the rotor's spin axis to be stable, as seen by a hypothetical observer on the rotor. In general, when such a rotor is first spun, it will not be about a stable axis. In fact, the only truely stable axis is the axis of greatest inertia. It is preferable that the rotor's mass distribution should be such that the axis of greatest inertia is perpendicular to the line joining the geometric and mass centres, and such that it is possible, with external fields, to move the spin axis to coincide with that of greatest inertia.

Several techniques are known for producing this change in the mass distribution. One involves forming one or more flat areas on the surface of the rotor, whilst another involves forming a thin coating on parts of the surface by plating or other techniques. These techniques are difficult to use, particularly because for a rotor of typical size very high dimensional accuracy is required. In addition, rotors are usually made from beryllium because of its high stiffness-to-density ratio. However, beryllium is a hazardous material to machine, and this adds to the problems. Yet another technique involves forming a billet of beryllium with one or more wires of a heavier material embedded in it. The billet is extruded into a rod and the spherical rotor is machined from a piece of that rod. This is clearly a complex and costly technique.

It is an object of the invention to provide a rotatable body in which the mass distribution of the body has been adjusted by a technique which does not suffer from the problems mentioned above.

Another object of the invention is to provide a method of producing such a rotatable body.

According to the present invention there is provided a rotatable body with non-uniform mass distribution in which the density of at least one region of its surface has been increased by the implantation in that region of ions of a suitable elemental material so as to produce a desired change in the mass distribution of the body.

Also according to the invention there is provided a method of manufacture of a rotatable body with non-uniform mass distribution which includes increasing the density of at least one region of the surface of the body by implanting in that region of the surface ions of a suitable elemental material so as to produce a desired change in the mass distribution of the body.

The invention will now be described with reference to the accompanying drawings, which relate to a rotor for an electrostaticallysuspended gyroscope. In the drawings Figs. 1 and 2 are orthogonal elevation views of a rotor showing the implanted areas.

Referring now to Figs. 1 and 2 the rotor 10 of an electrostatically-suspended gyroscope comprises a solid or hollow sphere of beryllium supported within an evacuated enclosure (not shown) by electric fields. The enclosure includes a number of electrodes for producing the electric fields and for detecting movements of the sphere due to its unbalance.

Coils provide magnetic fields which cause the rotor to rotate about its spin axis 11.

The change in mass distribution which causes the separation between the centre of mass of the sphere and its geometric centre is produced by implanting silver ions in clearly-defined regions of its surface. These regions, denoted 12 and 13, are shaded in Figs. 1 and 2. The particular pattern of implantation depends upon the desired relationship between the centre of mass, the geometric centre and the principal inertia axes of the body.

The process of ion implantation involves directing a high-energy beam of the ions at the surface of the body. The ions penetrate a very small distance into the surface of the body and are retained there, increasing the density of that part of the surface. Only a very small increase in density is possible by this means, because the incident ion beam is liable to erode the surface, sputtering surface atoms from the body. Previously-implanted ions are also vulnerable to sputtering, and this means that there is a limit to the number of ions that may be implanted. To obtain the highest change in density obtainabie by this technique it is necessary to consider the mass of the implanted ions and the number which may be implanted per unit area before the sputtering limit is reached.This is the reason for the choice of silver ions for implantation into a beryllium body.

Ion implantation as a process has been known for a considerable time and is used for doping semiconductor materials. It is carried out in an evacuated chamber and requires the production of a beam of electrons from an electron source. The electron beam is used to bombard a gas or vapour containing the elemental material to be ionised and the resulting ions are accelerated and used to bombard the target, in this case the rotatable body. Implantation of a body over an area requires the body to be movable relative to the beam. In certain equipment it is possible to scan the ion beam electrostatically in at least one direction. Alternatively the target body may be moved or both techniques used. In the case of the sphere illustrated in Figs. 1 and 2 the preferred method is to scan the beam in one plane and to rotate the body in an orthogonal plane. Some form of mask may be used to define the areas to be exposed to the ion beam, unless the control of the position of the point of impact of the beam on the body is sufficiently accurate to make this unnecessary.

Whilst the above description has been concerned with the provision of rotor for an electrostatically-suspended gyroscope, the same invention may be used to affect the mass distribution of other rotatable bodies.

The body need not be intended for electrostatic support and may be of any shape. Equally, the same invention may be used to provide for perfect balance rather than a deliberate unbalance. Clearly the shifts possible between the centre of mass and the body and its geometric centre are essentially very small, as are the modifications to the principal axes of inertia, and it is this aspect which will limit the applications of the invention.

Claims

1. A rotatable body with non-uniform mass distribution in which the density of at least one region of its surface has been increased by the implantation in that region of ions of a suitable elemental material so as to produce a desired change in the mass distribution of the body.

2. A body as claimed in Claim 1 which forms the spherical rotor of an electrostatically-suspended gyroscope.

3. A body as claimed in Claim 2 in which the body is made from beryllium.

4. A body as claimed in either of Claims 2 or 3 in which the elemental material is silver.

5. A method of manufacture of a rotatable body with non-uniform mass distribution which includes increasing the density of at least one region of the surface of the body by implantating in that region of the surface ions of a suitable elemental material so as to produce a desired change in the mass distribution of the body.

6. A method as claimed in Claim 5 which includes the steps of directing a beam of ions of the said elemental material at the surface of the body, and producing relative movement between the area of impact of the beam and the body such that ion implantation occurs over the required region of the surface of the body.

7. A rotatable body with non-uniform mass distribution substantially as herein described with reference to the accompanying drawings.

8. A method of manufacture of a rotatable body with non-uniform mass distribution substantially as herein described with reference to the accompanying drawings.