GB1581540A - Stabilisation systems for maintaining the orientation of vehiclemounted apparatus - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO STABILIZATION SYSTEMS FOR MAINTAINING THE ORIENTATION OF VEHICLE-MOUNTED APPARATUS (71) We, HAWKER SIDDELEY DYNA MICS LIMITED, a British Company of Manor Road, Hatfield, Hertfordshire AL10 9LL, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to the provision of means whereby vehicle-mounted apparatus may be held oriented to a point fixed in space while the vehicle is itself being displaced relative to that point.

In one aspect, the invention aims to provide means for stabilising the orientation of equipment such as radio, radar or optical links used for communication between the vehicle and a fixed receiving station and is particularly concerned with mechanical means for such stabilisation.

To give a better understanding of the invention, the following discussion and description is based on the application of the invention to ship borne or buoy-mounted equipment and especially to the stabilisation of a shipbourne radio antenna, it being clearly understood that this description is by way of explanation and not of limitation, the term "vehicle" being used to cover land vehicles, sea vessels, both surface and underwater and air and spacecraft.

Shipbourne antennae which are used for the purpose of communicating with satellites are required to point the beam centre in a specified direction. In the case of a broad beam or omni-directional antennae, the need arises to protect the receiving system from the effects of multipath propagation.

An antenna platform may be stabilized by means of a servo system which employs gyroscopic sensors. Such systems achieve high pointing accuracy but are very expensive to produce and to maintain. They are therefore confined to applications which demand precise pointing.

Alternatively, an antenna platform may be stabilized by means of a self-levelling pendulous system. These systems rely on the Earth's gravitational force to maintain the centre of gravity of the platform directly below the pivot point. Bearing friction and ship accelerations will try to couple the movement of the platform to the movement of the ship. If a radome is not used, an unbalance due to wind force on the portions of the platform above and below the pivot point will also cause a disturbance. Some form of viscous damper may be needed to ensure the pendulum is adequately damped in its response to a major disturbance. If this damping is between the gimbals, then it will increase the coupling between ship and platform motion. If, however, space rate damping is obtained by mounting a momentum wheel with vertical spin axis on the platform, then the coupling between ship and platform motion will be reduced.

The use of broad beam and omni-directional antennae has created a need for an inexpensive and highly reliable shipbourne stabilization system which is capable of maintaining an antenna within 3 to 5 of its desired position irrespective of sea state and ship manoeuvring.

According to the present invention, there is provided a stabilisation system whereby vehicle-mounted apparatus may be held orientated to a point fixed in space, comprising a pendulum which has a depending mast mounted at or near its upper end for universal movement in a gimbal assembly, the mast carrying at or near its lower end a motor-driven momentum wheel that swings with the mast and thereby also constitutes the pendulum bobweight, and a bracket or pedestal consti tuting an upward extension of the mast above the gimbal assembly upon which bracket or pedestal the apparatus to be stabilised is mounted.

Such a system is the simplest commensurate with the pointing accuracy required.

Systems which are in existence or under development, fail in the following respects: i) Servo systems are very expensive.

ii) Pendulum systems with twin momen tum wheel augmentation are expen sive.

iii) Pendulum systems with no momen tum wheel augmentation have in adequate performance.

An arrangement according to the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figures 1, 2 and 3 are, respectively, side and end elevations and a plan view of a schematic arrangement embodying the invention, and Figures 4, 5 and 6 are similar views of a gimbal assembly of the arrangement of Figures 1 to 3.

Referring to Figures 1 to 3, the external radome geometry takes the form of a substantinally elliptical 'cylinder' 11. This has beeen made possible by the significant difference between the maximum ship roll and pitch angles. The ship angular movements have beeen taken to be 40 and 15 maximum in roll and pitch respectively. To these have been added 5 for ppinting error allowance and 5 for ship mounting error.

Consequently, at the angles of 50 in roll and 25 in pitch, mechanical stops 18, 19 (Figure 4) have been positioned within the gimbal assembly so that, in the unlikely event of motion in excess of anticipated values, no damage is done to the antenna 10.

The system is mounted on the ship (or buoy) at its base 12. A simple base plate is provided which is secured to a mast or to superstructure, as required. The system is carried on an arch-like support structure 13 which is attached to the base plate 12 at the corners of each of its feet 14.

In the example described, the total system mass is estimated to be 60kg excluding the antenna 10. The electrical interface consists of a single salt water proof connector mounted in the base 12.

At the apex of the arch structure 13, there is a gimbal assembly on which the antenna 10 is mounted and which allows the antenna to tilt in all directions. The gimbal assembly 15, shown in more detail in Figures 4 to 6, comprises four bearings 21, 22, a bearing-mounted ring 23, viscous dampers 17 and an antenna attachment bracket 16.

It is important to use bearings with a friction torque of less than, say, 0.005 Nm.

It is also desirable to use a double-sealed bearings because, a) the bearing is supplied sealed for life and thus protected during stor age and installation; b) as the bearing is housed adjacent to a damper, a double seal provides greater effectiveness in the retention of damper fluid. Any fluid that leaks past the seal on the damper side will have insufficient pressure head to leak past the second seal.

Although leakage past a seal is unlikely it is considered prudent to use a bearing lubricant that is compatible with the damper fluid. Analysis has shown that viscous damping of 0.1 to 0.4 Nm/rad/sec is required in each axis.

For reasons of economy simplicity submerged drum viscous dampers are used.

This utilises the viscous shear of the damper fluid between a drum rotor 24 and a stator in the form of a cup 25 surrounding the drum. The rotor is secured on the bearing shaft 26 and the stator is part of the gimbal frame 27. To minimise the viscosity change due to temperature, silicone fluid is used.

The frame of the gimbal assembly 15 has a plane bolt-up flange to allow easy attachment of the complete gimbal assembly to the arch-structure 13. The antenna attachment brocket 16 is carried centrally by the ring 23 and consists of an upstanding pillar 28 with a pedestal plate 29 on its upper end to which the antenna is bolted.

Below the ring 23, the pillar 28 is extended downward by a depending mast 30 on the lower end Qf which is mounted a momentum wheel 31 driven by a motor 32. The mast 30 and wheel 31 constitute a pendulum; that is to say, the wheel 31 serves both as a momentum wheel and as a pendulum bob-weight.

The size of the momentum wheel drive motor 32 is dictated by the windage effects and to a lesser extent by bearing friction.

Selection of the motor type is basically between brushed and brushless. The brushed motors are, in general, smaller and more reliable. However, when the electrical supply is AC it is considered that, although power consumption is relatively high, an induction motor is the best solution as it has the advantages of low cost, high reliability, quiet operation, good efficiency and long life. The smallest suitable motor has an output of 25W which is adequate.

The momentum wheel 31 has to meet two conflicting requirements and consequently some compromise is involved.

The wheel acts both as pendulum mass and also needs to have a specific angular momentum capacity.

The arch-like support structure 13 is the primary structural component of the system. It directly supports the antenna and pendulum assembly and it also transfers the radome loads, caused mainly by windage, into the ship structure. Its geometry is largely dictated by the motion of the momentum wheel 31. To leave room for this motion it is designed as an arch or portal frame lying generally in the plane of pitching and narrow in the direction of the plane of roll because of the relatively large roll angle precluding space for structure.

To keep fabrication costs to a minimum, the radome 33 preferably consists of a single skin of glass-fibre-reinforced plastics.

The cover 11 closes the system at the base and transfers windage loads from the radome to the support structure. It incorporates an access panel for maintenance and repair to the lower part of the system.

The length of the pendulum mast 30 is adjustable to facilitate the balancing operation. A coaxial antenna feeder cable and power cable 34 are taken around the gimbal assembly in a large loop so as to minimise their bending and torsional stiffness.

The most important possible system failure to be considered is electrical power loss. If power is removed from the system then the momentum wheel will stop spinning in about 15 minutes. This could cause the angular movements to be larger than designed for, thus causing collision. Protection is therefore provided by a simple fail-safe device which rapidly increases the damping effect of the dampers 17 beyond 40 in roll and 15 in pitch.

The performance of the system has been confirmed in two ways. Mathematical modelling can be considered in two phases analysis and simulation.

An accurate representation of ship motion was derived using a wave motion represented by a Pierson-Moskowitz spectrum. Ship motion spectra were then derived by calculating ship response operators for typical ships. These were approximated by weighted sums of five sine waves.

The frequencies were selected to coincide with the peak, 50% and 10% amplitudes of each of the ship motion spectral density plots.

The simulation description can be divided into three sections: i) generation of the ship motion; ii) description of the modified simula tion iii) analysis of the simulation outputs.

The selected method of generating a sum of five sine waves involved the recording of the five sine waves on separate channels of a video tape recorder. These were played back into an analogue-computer, combined to form the ship's angular position, rate and acceleration, and recorded on another video tape recorder. The resultant signals were checked for accuracy by spectral analysis and for validity by statistical analysis of a sequence of 2000 seconds.

The simulation described previously was modified to take account of the more accurate representation of the ship's motion and to increase the display capability.

The runs performed can be divided into two groups: a) correlation of roll and pitch motion for the system parameters for two locations on each of three ships; b) sensitivity analysis for the most criti cal ship location.

From the results of the correlation analysis the following conclusions can be drawn: i) the amplitude of the high frequency component of the antenna motion de creases with decreasing ship motion frequency.

ii) for small and medium ships the cor relation between roll and pitch motion was small. A larger correla tion was observed for both locations on a large ship.

iii) the optimum antenna location on a given ship is at the point which ex periences minimum acceleration due to the ship's motion.

iv) an accuracy of 5 degrees is achiev able for the system.

The sensitivity analysis demonstrated that a tolerance of + 1 millimetre can be allowed on the effective pendulum length.

WHAT WE CLAIM IS: 1. A stabilisation system whereby vehicle-mounted apparatus may be held orientated to a point fixed in space, comprising a pendulum which has a depending mast mounted at or near its upper end for universal movement in a gimbal assembly, the mast carrying at or near its lower end a motor-driven momentum wheel that swings with the mast and thereby also constitutes the pendlum bob-weight, and a bracket or pedestal constituting an upward extension of the mast above the gimbal assembly upon which bracket or pedestal the apparatus to be stabilised is mounted.

2. A system according to claim 1, wherein movement-damping means are associated with the gimbal assembly bearings.

3. A system according to claim 2, wherein the damping means are viscous friction damping means.

4. A system according to claim 2 or claim 3, wherein the damping means become stiffer when the pendulum swings

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   The arch-like support structure 13 is the primary structural component of the system. It directly supports the antenna and pendulum assembly and it also transfers the radome loads, caused mainly by windage, into the ship structure. Its geometry is largely dictated by the motion of the momentum wheel 31. To leave room for this motion it is designed as an arch or portal frame lying generally in the plane of pitching and narrow in the direction of the plane of roll because of the relatively large roll angle precluding space for structure.

   To keep fabrication costs to a minimum, the radome 33 preferably consists of a single skin of glass-fibre-reinforced plastics.

   The cover 11 closes the system at the base and transfers windage loads from the radome to the support structure. It incorporates an access panel for maintenance and repair to the lower part of the system.

   The length of the pendulum mast 30 is adjustable to facilitate the balancing operation. A coaxial antenna feeder cable and power cable 34 are taken around the gimbal assembly in a large loop so as to minimise their bending and torsional stiffness.

   The most important possible system failure to be considered is electrical power loss. If power is removed from the system then the momentum wheel will stop spinning in about 15 minutes. This could cause the angular movements to be larger than designed for, thus causing collision. Protection is therefore provided by a simple fail-safe device which rapidly increases the damping effect of the dampers 17 beyond 40 in roll and 15 in pitch.

   The performance of the system has been confirmed in two ways. Mathematical modelling can be considered in two phases analysis and simulation.

   An accurate representation of ship motion was derived using a wave motion represented by a Pierson-Moskowitz spectrum. Ship motion spectra were then derived by calculating ship response operators for typical ships. These were approximated by weighted sums of five sine waves.

   The frequencies were selected to coincide with the peak, 50% and 10% amplitudes of each of the ship motion spectral density plots.

   The simulation description can be divided into three sections:

   i) generation of the ship motion;

   ii) description of the modified simula tion

   iii) analysis of the simulation outputs.

   The selected method of generating a sum of five sine waves involved the recording of the five sine waves on separate channels of a video tape recorder. These were played back into an analogue-computer, combined to form the ship's angular position, rate and acceleration, and recorded on another video tape recorder. The resultant signals were checked for accuracy by spectral analysis and for validity by statistical analysis of a sequence of 2000 seconds.

   The simulation described previously was modified to take account of the more accurate representation of the ship's motion and to increase the display capability.

   The runs performed can be divided into two groups: a) correlation of roll and pitch motion for the system parameters for two locations on each of three ships; b) sensitivity analysis for the most criti cal ship location.

   From the results of the correlation analysis the following conclusions can be drawn:

   i) the amplitude of the high frequency component of the antenna motion de creases with decreasing ship motion frequency.

   ii) for small and medium ships the cor relation between roll and pitch motion was small. A larger correla tion was observed for both locations on a large ship.

   iii) the optimum antenna location on a given ship is at the point which ex periences minimum acceleration due to the ship's motion.

   iv) an accuracy of 5 degrees is achiev able for the system.

   The sensitivity analysis demonstrated that a tolerance of + 1 millimetre can be allowed on the effective pendulum length.

   WHAT WE CLAIM IS: 1. A stabilisation system whereby vehicle-mounted apparatus may be held orientated to a point fixed in space, comprising a pendulum which has a depending mast mounted at or near its upper end for universal movement in a gimbal assembly, the mast carrying at or near its lower end a motor-driven momentum wheel that swings with the mast and thereby also constitutes the pendlum bob-weight, and a bracket or pedestal constituting an upward extension of the mast above the gimbal assembly upon which bracket or pedestal the apparatus to be stabilised is mounted.
2. 2. A system according to claim 1, wherein movement-damping means are associated with the gimbal assembly bearings.
3. 3. A system according to claim 2, wherein the damping means are viscous friction damping means.
4. 4. A system according to claim 2 or claim 3, wherein the damping means become stiffer when the pendulum swings

   beyond preselected angular values of pitch and roll.
5. 5. A system according to any one of the preceding claims, wherein the length of the pendulum mast is adjustable.
6. 6. A system according to any one of the preceding claims, wherein the gimbal assembly is carried on a supporting structure of arch-like or portal frame configuration.
7. 7. An antenna stabilisation system substantially as described with reference to the accompanying drawings.