EP0108478A1

EP0108478A1 - Stabilisation mechanisms

Info

Publication number: EP0108478A1
Application number: EP83305180A
Authority: EP
Inventors: Ronald William Elston
Original assignee: Mcmichael Ltd
Current assignee: Mcmichael Ltd
Priority date: 1982-09-06
Filing date: 1983-09-06
Publication date: 1984-05-16
Also published as: GB8323874D0; GB2127622B; JPS5964902A; NO833167L; GB2127622A

Abstract

A stabilisation mechanism for stabilising an antenna has a pendulum arrangement (25, 26, 27) mounted on rotational bearings (22, 24) to give an effective compensatory movement for (e.g.) pitch and roll of a ship. Under complex sinusoidal movements of the mechanism a secondary pendulum comprising a disc member (30) which has a section removed and which is pivoted for three dimensional movement above its centre of mass is caused to rotate thus moving its centre of mass effectively to its pivot point and moving the centre of mass of the primary pendulum up towards its pivot point.

Shortening the vertical distance of the centre of mass of the pendulum below its pivot point alters the resonant frequency of the pendulum to assist maintenance of sufficient stablisation of the position of the antenna (42).

Description

The present invention relates to'stabilisation mechanisms and more particularly but not exclusively to such mechanisms for stabilising platforms and similar supports.
It will be appreciated that communications between moving vehicles (for example ships) and satellites may only be effectively maintained if the transmitting/ receiving aerial is suitably stabilised in space.
Providing stabilisation for ship based aerials requires compensation for pitch and roll of the ship which may be considered as simple harmonic movements and which are more usually compensated for by use of a passive system such as a simple or compound pendulum, an actively controlled system using axis drive motors to correct sensed angular changes or a gyroscopic system. Combinations of these three systems may also be used.
Of the three systems mentioned above the pendulum system is comparatively less expensive and has lower maintenance costs but is only capable of giving relatively basic stabilisation. Other motions for example complex sinusoidal motions such as may be caused by the bow of a ship riding a wave may result in the pendulum oscillating at a resonant frequency and taking a considerable time to re-settle.
It is an object of the present invention to provide a stabilisation mechanism based on a pendulum system in which the above mentioned disadvantage is substantially reduced.
For the avoidance of doubt the term "fluid medium" as used hereinafter means a non-gaseous medium which is capable of flowing.
According to the present invention in a stabilisation mechanism of the kind having a pendulum capable of movement about a pivot and the centre of mass of the pendulum disposed below the pivot, said pendulum includes a moveable mass arranged, in use, to respond to complex sinusoidal displacement of said pendulum by moving relative to the rest of said pendulum to cause an effective displacement of the centre of mass of the pendulum towards said pivot such that the fundamental frequency of the pendulum is altered to assist maintenance of the stabilisation of the mechanism over a broad operating range.
Preferably said moveable mass is a secondary pendulum which is mounted within the first said pendulum and which is capable of effective three dimensional movement with respect to the rest of said first pendulum.
The secondary pendulum may be an irregular planar member which is pivoted above its centre of mass and which is mounted for rotation within a cage, said cage being mounted for rotation transversely of the direction of rotation of said planar member such that in the static state of the stabilisation mechanism the planar member rests with the centre of mass thereof below its effective pivot point and on complex sinusoidal displacement of the first said pendulum said planar member may rotate effectively to move the centre of mass thereof towards the pivot point.
In an alternative embodiment the moveable mass is a fluid medium (as hereinbefore defined), and the pendulum includes a plurality of compartments spaced around the vertical axis of said pivot, each of said compartments containing some of said fluid medium such that, in use, complex sinusoidal displacement of said pendulum causes a displacement of said fluid medium relative to said compartments to cause the effective displacement of the centre of mass towards said pivot.
The fluid medium may be mercury, water, dry sand or steel spheres for example.
Stabilisation mechanisms in accordance with the invention will now be described by way of example only with reference to the accompanying drawings of which:-

Figure 1 is a partially sectioned perspective view of a first mechanism in accordance with the invention.
Figures 2A and B show a section through a part of the mechanism of Figure 1 under two different conditions.
Figures 3 and 4 are respectively front and side elevations of a prototype of a preferred stabilisation mechanism in accordance with the invention; and
Figure 5 is a front elevation of another mechanism similar to the mechanism of Figures 3 and 4 and showing the mechanism as mounted for use on a ship.

Referring to Figure 1 the mechanism which is arranged to carry a platform (not shown) on which a ground based aerial (not shown) may be mounted has a fixed support 1 carrying a gimbal 13 which is attached by struts 2 and 12 to a pendulum 11.
Thus the effective pivot point of the pendulum 11 is at the intersection of the struts 2 and 12. The pendulum 11 is in the form of a hollow ring which is divided into a number of compartments 14 by dividing walls 3.
It will be appreciated that the pendulum 11 must be mounted such that its centre of mass is below the effective pivot point of the gimbal 13 to prevent the platform (not shown) becoming unstable.
Each of the compartments 14 is partially filled with a fluid 15 such as water or mercury for example.
It will be appreciated that pitch and roll of a ship for example are substantially simple harmonic movements. Accordingly it will be appreciated that the pendulum 11 will adopt a substantially simple harmonic compensating motion for pitch and roll and the fluid 15 will remain substantially stable in its location within the compartments 14, and this stable state is shown in Figure 2A to which reference is also now made.
In the static state the effective centre of mass of the fluid 14 and the pendulum 11 is vertically displaced a stable distance (1) below the centre of the ; pivot by virtue of the respective centres of mass of the fluid 15 within each compartment 14 being stable.
As thus far described the mechanism compensates for "pitch" and "roll" in the manner of a conventional pendulum stabilisation system. The advantage of the present mechanism is in its response to a complex sinusoidal motion of the pendulum 11 such as might occur when a ship rides a large wave.
If a conventional pendulum system is subjected to such a motion it will tend to swing at a resonant frequency ω which depends upon the mass of the pendulum (M), gravitational force (g), the vertical distance of the centre of mass below the pivot point (1) and the moment of inertia in the direction of movement about the pivot point (I). The relationship if known to be of the form
Referring also to Figure 2b it will be appreciated that on rapid acceleration of the pendulum 11, the_. fluid 15 will move in its respective compartment 14 such that the respective centres of mass of each of the compartments and hence the effective centre of mass of the pendulum 11 move upwards with respect to the pivot point of the gimbal 13.
It is here noted that in the above relationship I = _Mk ² where k is the radius of gyration of the pendulum. Whilst in a simple pendulum in which all of the mass is concentrated at a single point k is equal to 1 (thereby reducing the relationship to ω² = g ), where this is not so, k may be defined as a representative measure of the distribution of mass. In the kind of mechanism described hereinafter k is found to be two orders of magnitude greater than 1 and the reduction in the value of 1 has an insignificant effect on the value of k.
Thus the value of 1 is reduced in the above relationship and the resonant frequency is accordingly lower. Therefore the mechanism deviates from a stable state less readily than a conventional pendulum stabilisation mechanism.
Whilst as herein described the pendulum 11 is shown as annular, any other desired shape of the pendulum may be adopted provided that the fluid 15 is restricted within compartments 14 around the pendulum. Similarly the pendulum need not comprise a continuous series of compartments 14 around the pivot and could be provided by a number of compartments individually attached to the pivot.
It will also be realised that the contents of the compartments 14 need not be liquids provided that the contents have sufficient mass and capability of flowing to effect movement of the effective centre of mass. Thus the fluid 15 may also be dry sand or glass or metal spheres for example.
Referring now to Figures 3 and 4 the stabilisation mechanism shown has another method of reducing the effective length of the centre of mass below the pivot point which consists of a secondary pendulum within the primary.
The apparatus shown is a prototype pendulum constructed to prove the efficacy of such a stabilisation mechanism and comprises a support 20 attached to a base plate 21, the support 20 being connected by a rotational low-friction bearing 22 to a 'U' shaped support member 23 which in turn has rotational low-friction bearings 24 supporting the pendulum per se. Thus the pendulum has two degrees of freedom and the mass thereof may move to any point on a sphere surrounding its effective pivot point which is provided by the bearings 24 being transverse of the bearing 22, the pivot point being at the intersection of centre lines through the bearings.
The pendulum per se comprises a bob weight 25 on a shaft 26 connected by way of a supporting cage 27 (which is free to move with respect to the support 20 by its mounting in the bearings 24) to a shaft 28 which has a counterweight 29 thereon to represent for example the weight of an antenna mounted on the mechanism.
For experimental purposes the bob weight 25 and the counterweight 29 are moveable with respect to their distance from the pivot point although in practice-a fixed length mounting would be provided.
The secondary pendulum comprises a disc member 30 from which a segment has been removed which member is rotationally mounted in a cage 32 at the nominal axis of the disc by a bearing 31. The cage 32 is in turn rotationally mounted by bearings 33 in the cage 27. Thus the secondary pendulum is free to move three dimensionally--with respect to the rest of the primary pendulum.
It will be appreciated that in a static state of the pendulum_.(for example simple harmonic pitch and roll motions of the mechanism mountings which are not at the resonant frequency of the pendulum) the first pendulum comprising the bob weight 25, the shaft 26, the cage 27, the shaft 28 and the counterweight 29 will adopt a compensating simple harmonics movement about its axis by virtue of the bearings 22 and 24. Thus the attitude of the pendulum with respect to the earth remains substantially stable.
As along as the movement of the support 20 and the compensatory motion of the first pendulum are counter-aligned and provided that the frequency of the sinusoidal mortions are not close to the resonant frequency of the secondary pendulum, there is only a gravitational effect on the member 30 which will rest with its centre of mass below the effective pivot point provided at the .intersection of the centre lines of the bearings 31 and 33. Thus the member 30 makes a substantially fixed contribution to the length (1) of the centre of mass below the effective pivot point.
However if a motion results in a complex sinusoidal displacement of the support 20 the dynamic effects are transferred by way of the first pendulum to the secondary pendulum. The cage 33 will tend to swing on its bearings 33 to align the member 30 with the direction of the maximum complex motion and the member 30 will rotate under the influence of the dynamic effects.
Rotational motion of the member 30, the peak of which may result in the member turning full circle as indicated by the dashed lines 30', results in its effective centre of mass moving towards the pivot point of the secondary pendulum effectively moving the centre of mass of the entire pendulum towards the pivot point of the primary pendulum.
Thus Ill in the above mentioned equation is reduced, altering the resonant frequency of the mechanism in the manner aforesaid and minimising disturbances of the mechanism from its static state.
It will be noted that as shown the intersection of the bearings 31 and 33 is above the intersection of the bearings 22 and 24. However it will.be appreciated that the location of the respective pivot points of the primary and secondary pendula may be so arranged as to locate the mass of the secondary pendulum with respect to the pivot point to achieve desired dynamic performance.
Whilst as herein described the member 30 is a disc member having a segment thereof removed it will be realised that any planar member pivoted above its centre of mass could be used.
Alternatively the secondary pendulum and its associated bearing mechanism could be provided by use of a sphere mounted for three dimensional movement and having an uneven distribution of its mass. In one example of such an uneven distribution a radially extending bore of material is removed from the sphere making one area appreciably lighter in mass than the rest. Such a bore may be infilled with a core of denser material or the sphere may have its two halves formed of differing material. A partial sphere could also be used. Alternatively a hollow sphere partially filled with a fluid medium would suffice.
Referring now to Figure 5 the stabilisation mechanism is mounted inside a fibre glass construction radome 40 (only part of which is shown) on a platform 41 which is arranged for horizontal rotation by means of a motor drive (not shown) such that an antenna 42 carried by the mechanism may be appropriately directed.
The radome 40 is for mounting on the masthead of a ship and is provided to prevent for example climatic effects or birds landing on the antenna 42 and affecting the balance of the mechanism.
For vertical angular adjustment the antenna 42 is mounted on a connection plate 43 which carries a counterbalance weight 44 at the opposed end. The connection plate 43 includes a channel 45 in which a spindle (not visible) runs to permit adjustment. The angular direction of the antenna may then be fixed by tightening a wingnut 46 on to the end of the spindle to lock the connection plate 43.
The remaining items of the mechanism are similar to the items of Figures 3 and 4 and are similarly designated.
It will be appreciated, however, that for any specified antenna 42 the distance of the bob weight 25 from the pivot point may be fixed. Thus no adjustment need be provided leading to reduced manufacturing costs.
The dimensions of the stabilisation mechanism's components may be varied to suit any particular installation. As an example for a ship whose typical parameters are

1) Roll period five seconds (frequency 0.2Hz)
2) Pitch period six seconds (frequency 0.18Hz)
3) Yaw period thirty seconds (frequency 0.03Hz) and
4) Heave period ten seconds (frequency O.lHz) the bob weight 25 could be of steel and comprise a double cone the maximum diameter of which is approximately 120 millimetres and the weight of which is approximately three kilograms. The bob weight 25 is suspended approximately 100 millimetres below the effective pivot point.

The disc member 30 has a weight of approximately one kilogram and comprises a 100 millimetre diameter disc truncated approximately twenty-five millimetres from the centre thereof. The disc member 30 may be suitably manufactured from steel for example and would have a thickness of some twenty millimetres.
Using the above combination of primary and secondary pendula as a stabilisation mechanism for an antenna of the kind known as a "short back fire" antenna which has a diameter of 260 millimetres and a depth of approximately 150 millimetres as the antenna 42 and providing a counter balance 44 such that the combined weights of the antenna 42 and the counterbalance 44 amounts to approximately three and one half kilograms it is found that in the static state of the mechanism the distance of the centre of mass below the pivot point is approximately One millimetre and the static state resonant frequency of the primary pendulum is approximately 0.1 Herz. The secondary pendulum has a resonant frequency of 1.3 to 1.5Hz.
If a complex sinusoidal motion of the support 20 causes the disc member 30 to rotate the effect of such an action is to raise the centre of mass of the primary pendulum to about 0.1 millimetre below the pivot point and the resonant frequency of the mechanism is reduced to less than 0.03 Hz.
It will be noted that the specified heave period gives rise to a simple harmonic movement which approximates to the 'static' resonant frequency of 0.1 Herz. However, if the primary pendulum commences resonating at this frequency the secondary pendulum will tend to move and will reduce the resonant frequency of the mechanism.
As an alternative to a passive stabilisation mechanism the invention may be actively implemented by providing sensors (not shown) which detect complex sinusoidal movement of the pendulum arrangement and control a mechanical shift of the centre of mass. In one implementation of such a mechanism the shaft 26 is equipped with a collar at its suspension point from the cage 27 and a motor is controlled in response to the sensor to raise the shaft and bob weight 25 to effect the required change in the vertical distance (1) of the centre of mass below the effective pivot point.
An alternative active system uses a fluid medium in a container in place of the bob weight 25 and a further normally empty container mounted towards the pivot point and a pumping arrangement controlled by the sensor system to transfer the fluid medium between the two containers.
In addition to use of the mechism for stabilising platforms for ship to satellite communications, the mechanism may find use in ship-to-ship communications (for example where "line of sight" systems such as microwave communication is employed) and in communication between land based vehicles and satellites for example.

Claims

1. A stabilisation mechanism of the kind having a pendulum capable of movement about a pivot and the centre of mass of the pendulum disposed below the pivot characterised in that said pendulum includes a moveable mass (15,30) arranged, in use, to respond to complex sinusoidal displacement of said pendulum by moving relative to the rest of said pendulum to cause an effective displacement of the centre of mass of the pendulum towards said pivot such that the fundamental frequency of the pendulum is altered to assist maintenance of the stabilisation of the mechanism over a broad operating range.

2. A stabilisation mechanism as claimed in Claim 1 further characterised in that said moveable mass is a secondary pendulum (30) mounted within the first said pendulum, said second pendulum being capable of effective three-dimensional movement with respect to the rest of said first pendulum.

3. A stabilisation mechanism as claimed in Claim 2 further characterised in that said secondary pendulum (30) comprises an irregular planar member which is pivoted above its centre of mass and which is mounted for rotation within a cage (32), said cage being mounted for rotation transversely of the direction of rotation of said planar member (30) such that in the static state of the stabilisation mechanism the planar member (30) rests with the centre of mass thereof below its effective pivot point and on complex sinusoidal displacement of the first said pendulum said planar member may rotate effectively to move the centre of mass thereof towards the pivot point.

4. A stabilisation mechanism as claimed in Claim 3 further characterised in that said irregular planar member (30) is a disc shaped member of which a segment is missing.

5. A stabilisation mechanism as claimed inClaim 2 further characterised in that said secondary pendulum (30) comprises a sphere, the mass of which is unequally distributed about its centre of rotation, said sphere being mounted for movement about its centre of rotation and being arranged such that in the static state of the mechanism the centre of mass thereof is below its effective pivot point and on complex sinusoidal displacement of the mechanism said sphere rotates effectively to move the centre of mass thereof to its pivot point.

6. A stabilisation mechanism as claimed in Claim 5 further characterised in that the inequality in the distribution of mass is achieved by removing a substantially radially extending bore of material from a sphere of uniform mass.

7. A stabilisation mechanism as claimed in Claim 6 further characterised in that the inequality is enhanced by insertion of a core of material having a differing density in the removed bore.

8. A stabilisation mechanism as claimed in Claim 1 further characterised in that said moveable mass is a fluid medium (15) (as hereinbefore defined) and the pendulum includes a plurality of compartments (14) spaced around the vertical axis of said pivot, each of said compartments (14) containing some of said fluid medium (15) such that, in use, complex sinusoidal displacement of said pendulum causes a displacement of said fluid medium (15), relative to said compartments (14) to cause the effective displacement of the centre of mass towards said pivot.

9. A stabilisation mechanism as claimed in Claim 1 further characterised in that said moveable mass is a liquid (15) and sensor means responsive to complex sinusoidal displacement of the pendulum controls pump means arranged to transfer the liquid against gravitational effects to effect displacement of the centre of mass of the pendulum towards the pivot point.

10. A stabilisation mechanism as claimed in Claim 1 further characterised in that said moveable mass is a weight (25) and sensor means responsive to complex sinusoidal displacement of the pendulum controls mechanical means to move said weight (25) against gravitational effects to effect displacement of the centre of mass of the pendulum towards the pivot point.