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1. (WO2019007835) PERCHED AIR VEHICLE THAT PANS
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PERCHED AIR VEHICLE THAT PANS

FIELD OF THE INVENTION

The present invention relates to a hovering air vehicle (HAV) that can pan whilst perched.

BACKGROUND ART

Market demand is increasing for small, unmanned, hovering air vehicles (HAVs). Most small HAVs are battery powered and are multi-rotor with an on-board camera. Battery powered HAVs are limited to an endurance of 15-40 minutes in the air. Several applications such as remote surveillance require persistent video over many hours. In principle, this is achieved by "perching" the HAV (causing it to alight or rest on something which allows a reasonable field of view) and pointing the camera where required, but existing solutions have major limitations.

Many small HAVs have gimballed payload cameras that are located underneath the central pod of the HAV. The gimbal has 1, 2, 3 or more axes of rotation. Having a gimballed camera under the main body of the HAV is poor for achieving a large field of regard when perched. The camera is close to the ground and can only look straight ahead or slightly upwards. The camera field of regard is blocked by the undercarriage at many pan angles of the camera (pan is defined as rotation about an approximately vertical axis).

In general, a gimbal becomes much heavier with (a) more rotational axes of movement, and (b) more cameras. Therefore, as HAVs get smaller, their gimbals tend to have fewer axes; many of the smallest HAVs have no gimbal. Some small HAVs have gimballed cameras located at the front of the main body with just one tilt axis of movement (tilt is defined as rotation about an approximately horizontal axis) and with two cameras: a visible spectrum camera and an infra-red (IR) camera. Panning with a 1-axis tilt gimbal whilst perched is not possible unless the HAV re-perches by taking off, yawing round in flight and landing again with the camera pointing in another pan direction. This existing re-perch solution has many drawbacks: each take-off and landing cycle may end in a crash; it uses up scarce battery energy; it does not permit smooth panning to track a moving object; there is more chance of the HAV being noticed due to higher visual and audio signatures during the re-perch.

Another solution requires a second gimbal located on top of the HAV's main body with at least 2 motorised gimbal axes (pan and tilt) and often 2 cameras. This means that the small HAV must be able to carry significant extra payload weight for the duplicate gimbal motors and camera(s) on top and the electronics to drive them.

A perched panning solution is required that: permits a limited pan rotation angle of the cameras, typically in the range of 90 to 450 degrees of which at least 360 degrees is desirable or, preferably, an infinite pan rotation; permits camera tilt rotation of around 90 degrees; does not have extra weight from duplicate cameras, motors, wiring and electronics; does not self-obstruct the camera view; can have both visible and IR cameras; has smooth pan and tilt rotation; does not take risks with re-perching the HAV and has a low visual/audio signature when panning.

SUMMARY OF THE INVENTION

A significant advantage of the present invention is the additional use of the HAV's propulsion rotors to provide a controllable panning motion without the additional weight, complexity and reduced robustness of adding an additional panning actuator into the undercarriage.

The present invention provides a hovering air vehicle (HAV) comprising a main body with a plurality of actuators each with a rotor, at least one payload unit on a gimbal, the main body is connected by a rotating element to an undercarriage, the HAV is operable when perched such that when at least one of the rotors is spun up, the main body of the HAV pans.

There may be a braking element on the pan axis such that the HAV main body will not normally auto-rotate. There can be a control system for manual or algorithmic control of panning. The main body may be angled upwards relative to the undercarriage to bring more of the gimbal tilt axis range to bear. The panning axis can be of infinite rotation. The panning process may have low visual and audio signature. The panning motion may have minimal effect on the quality of moving imagery. The panning motion can be coordinated with the motion of one or more gimbal axes.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example and with reference to the accompanying figures, in which;

Figure 1 is a schematic side view of a representative hovering air vehicle-

Figure 2 is a schematic plan view of a representative hovering air vehicle;

Figure 3 is a schematic of a simple control system;

Figure 4 is a schematic of a sensor-based control system.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Figure 1 is a schematic of an HAV 100 with a main body 1, a camera 2 on a gimbal 3, a rotational bearing 4 and an undercarriage 5. The gimbal 3 is one tilt axis. There is one payload unit, a camera 2. The camera 2 with camera axis 30 is rotated by the gimbal 3 from a forward camera axis 30 to a down vertical camera axis 30. The main body 1 has a battery 20 and a printed circuit board (PCB) 21. The PCB 21 has many integrated circuit components mounted on it including: a processor, memory, a magnetometer 22, an Inertial Measurement Unit (IMU) and motor drivers to drive the propulsion motors and the gimbal motor 7 (situated behind the camera). The bearing 4 is mounted between the main body 1 and the undercarriage 5. It is a ball bearing. When the HAV 100 is perched on the ground 6, the undercarriage 5 is stationary (although it may rock on uneven ground). The main body 1 is supported on the bearing 4 and can rotate infinitely about the pan axis. The arms, motors and propellers of the HAV 100 are not shown in Figure 1.

Figure 2 is a schematic of the HAV 100 which is a quad-rotor. The HAV 100 has a main body 1, a camera 2 on a gimbal 3, a bearing 4 (not visible) and an undercarriage 5 (partially visible). The gimbal 3 is driven by the gimbal motor 7. There are four propulsion motors 10 with four propellers 11. Two of the propellers 11a rotate clockwise to produce lift and two propellers lib rotate anti-clockwise to produce lift.

When a propeller 11 is spun up from stationary by its propulsion motor 10, there is a torque reaction that provides the torque to pan the main body 1 about the pan axis. The main body 1 rotational ly accelerates as the propeller 11 is rotationally accelerated by its propulsion motor 10. This means that one or more of the existing motors and propellers that in normal use propel the HAV in flight, are re-used when perched on the ground to pan the main body of the HAV without requiring any further actuators, gearboxes, drive and control electronics, mounting elements or wiring.

Two brakes 40 are friction pads that are mounted on the main body 1 and rub on the undercarriage 5 during panning to create a braking torque against the direction of rotation. The brakes are implemented to provide optimal braking torque such as by specifying down force, coefficient of friction and radius from the pan axis in order to: (a) stop the main body auto-rotating undesirably in reaction to a disturbance such as a wind gust, (b) help control the panning rotation in a smooth and predictable manner, (c) not waste energy from unnecessarily high friction, (d) brake in both rotational directions, (e) prevent a panning rotation caused by a net gravitational moment on the pan axis when the ground is not level. When the propeller 11 has spun up and is at a constant velocity, the main body 1 will have reached a rotational velocity and will then slow down under the frictional braking force of the brakes 40. Brakes 40 that are passive add only a small amount of mass (of the order of 0.5g on a 150g HAV) and add no system complexity such as wiring, electronics and electronics.

Figure 3 shows a simple control system 50. The control 52 of the simple control system 50 is provided to receive a demand input 51 and output one or more demand signals 53 to one or more propulsion motors 10 for panning.

Figure 4 shows a sensor-based control system 60. The control 62 of the sensor-based control system 60 is provided to receive demand input 61, use estimates of pan angle from a sensor 64 (such as a magnetometer 22 with reference to the earth's magnetic field) to output one or more demand signals 63 to one or more propulsion motors 10 to achieve desired pan rotation with a desired velocity and/or acceleration profile. The magnetometer 22 is a MEMS 3-axis magnetometer. The magnetometer 22 will not work well if the HAV 100 is under the influence of a strong magnetic field such as in the presence of a large amount of iron. As an alternative, the IMU can be used to provide pan angle estimates. As a further alternative, image processing of the flow of images from the camera 2 can be used to provide pan angle estimates. A filter such as a Kalman Filter can be used to combine estimates from different sensors to produce a more robust estimate of the pan angle. Pan angle estimates may be absolute or relative. In a further embodiment, the sensor-based control system can integrate control of the roll of the payload units in the gimbal. The scope of this invention includes any control method of converting demand input and sensor measurements into an output which will achieve panning of the HAV and control of the gimbal axis or axes.

The user can provide demand input to the control system for manual control of pan and tilt by any means such as a joystick or a touch screen whilst viewing real-time video imagery from a payload unit at a manageable latency. A software algorithm for automated tracking of a moving object such as a person, a car or an air vehicle can provide demand input to the control system. A software algorithm can generate the demand input to the control system for an automatic pan-tilt scan pattern as set up by a user. The scope of this invention includes any manual or algorithmic input to the control system.

In a further embodiment, the gimbal 3 incorporates an actuated roll axis for the payload unit(s). A roll axis has a typical range of +/- 30 degrees. The roll -axis is used inflight to stabilise the camera 2 with a horizontal axis with respect to gravity to compensate for HAV roll angle. When perched, the roll axis can be used to compensate for a perch angle on sloping ground. As the HAV is panned, the roll motion of the camera 2 can be smoothly coordinated by the control system to maintain a horizontal axis which improves the perceived quality of the imagery. The accelerometers in an IMU mounted in the same frame as the camera 2 can be used to generate estimates of roll. The payload units and gimbal axes can be used both in flight and, when perched, in conjunction with HAV panning.

On an HAV weighing around 150g, each motor typically generates up to 10 mNm of torque. In testing a quad-rotor prototype of this invention, it was found that the prop rotation method of controlling one propulsion motor and propeller provided enough torque to pan the main body. Ona quad-rotor, propellers that are diagonally opposite have the

same rotation; they are both clockwise or both anti-clockwise. It was found that the method of spinning up two diagonally opposite propellers created a lower level of undesirable vibration artefacts in the video imagery. Most propulsion motors can accelerate the propeller in either direction. Most propellers on small HAVs are fixed. Spinning fixed propellers backwards gives down thrust rather than up-thrust, so there is less chance of the undercarriage shifting position due to becoming less weighted. HAVs with variable pitch propellers can have the pitch minimised to reduce the thrust. HAV designs vary; sometimes particular prop rotation speeds may need to be avoided to avoid natural frequencies of the HAV airframe. The systems integrator would normally experiment with a number of different prop rotation methods on the particular HAV to see which method best achieves the desired pan motion for the least vibration in the imagery, the least audio noise generated and the least use of energy. The scope of this invention includes all arrangements of the main propulsion and methods of driving it for controlling panning as desired.

The undercarriage 5 with the rotating element 4 can be an 'undercarriage attachment' that attaches to the main body 1 so that it can be quickly and easily attached and detached. In a further embodiment, the distance H (referring again to Figure 1) between the main body 1 and the base of the undercarriage 5 is increased to improve the chance of a clear field of regard that is unobstructed by objects such as grass when perched. In an alternative embodiment, the main body 1 is angled upwards such that the camera 2 is raised higher. This angling upwards of the main body increases the upward angular limit of the field of regard which is useful when the HAV is perched in a low location. It is important that the whole system performance is considered in the design. For example, raising the height of the main body may require that the spread of the undercarriage be increased to achieve stability in perching. Also, angling the main body up at the gimbal end may result in the rear propellers grounding during perching, so additional height H will be required. The 'undercarriage attachment/ can be provided so that the user can fit the 'undercarriage attachment' to the main body with a manually adjustable joint permitting the main body 1 to be in a plane parallel to the plane of the base of the undercarriage 5, or angled to it. Such a manually adjustable joint could be a 1-axis hinge joint with high friction. Alternatively, it could be a 2-axis ball and socket with high friction. In this way, if the user knows that he will be landing on a sloping surface of known fall-line, or that he wants the HAV to look more upwards, or more downwards, and/or lean sideways, he can pre-compensate for it by

setting the manually adjustable joint. The adjustable joint could be actuated in one or more axes. The scope of this invention includes all ways of providing relative angling between the plane of the main body 1 and the plane of the base of the undercarriage 5.

It will be understood by one skilled in the art, that the scope of this invention is not limited to the embodiments disclosed but includes all arrangements that are limited only by the scope of the claims. The ball bearing is one example of a rotating element. Another suitable rotating element might be a plain bearing. The rotating element and the brake might be combined into a single item: a rotating element that also provides braking friction. Two ball bearings, one above the other with a common shaft might be provided to produce a stiffer rotating element to better support the main body 1. With an undercarriage attachment, the rotating element and/or the brakes may be mounted on either the main body or the undercarriage attachment.

Alternatively, brakes 40 that are active can be provided. Active brakes can be provided in a variety of ways: they can lock the pan axis on and off like the handbrake on a car, and they can be applied progressively from zero braking to full axis lock. An on-off lock brake has the advantages of: reducing the energy needed to pan because friction force does not need to be overcome; a consequent reduction in audio signature; further reducing the chance of undesirable pan axis movement in a disturbance. Progressive braking provides the full range of advantages. The scope of this invention includes all forms of braking and control of braking.

The gimbal may have any number of payload unite mounted on it. A payload unit may be passive or active. Passive payload units typically sense the environment and include visual spectrum cameras, IR cameras, multi-spectral cameras and directional microphones. Active payload units include LIDARs, Radars, loud hailers, illuminators and laser designators. A typical payload unit combination is a visual spectrum camera and a thermal IR camera; this payload unit combination provides day-night vision outdoors and vision indoors in all lighting conditions. Another example payload unit combination is an HAV gimbal with a camera payload unit and a laser designator payload unit, permitting the user to designate an object in the field of regard.

The propulsion motors (actuators) are typically of brushless, 3-phase, out-runner type but could be brushed or any other type of electric motor. The actuators can direct drive the propellers or use a gearbox or other gearing method. In HAVs there are two or more propulsion rotors; quad-rotors have four rotors, hex-rotors six, octo-rotors eight etc, The rotors are typically plastic propellers made of nylon or glass reinforced polymer, but could be carbon fibre. The propulsion rotor axes of rotation are generally approximately vertical, defined as being within 15 degrees of a vertical orientation, but the scope of this invention includes HAVs that have arrangements of propulsion rotor axes such that they can fly at different orientations and even upside down. In an HAV with the arrangement of a conventional helicopter there is a main rotor with a vertical axis and an orthogonal tail rotor; either the main rotor or the tail rotor or both rotors can be used to pan the HAV.

In this invention, the visual signature of the panning process is kept low. This is achieved by: obviating the need to take-off and the consequent noticeable movements up and down; panning slowly which is less noticeable than quickly; arranging the panning axis to be at or close to the centre of gravity of the main body to minimise side to side movement during panning.

The electronics for the HAV and the functionality of this invention may comprise any number of PCBs located in one or more locations. If there is a need for electrical and/or optical connections between the main body 1 and the undercarriage 5, such as for providing electrical recharge of the battery through contact with an electrical supply on the ground, then slip rings can be provided with a cable path through the rotating element. Slip-rings permit infinite rotation. Alternatively, a cable routing can be arranged and stops provided which permits any designed pan rotation angle range up to or greater than 360 degrees, but probably less than 450 degrees due to the added complexity and limited additional utility.

The application of this invention is particularly suited to small HAVs of less than 250g take-off weight, but the scope of this invention is applicable to HAVs that are equal to or larger than 250g take-off weight. The scope is particularly suited to very small HAVs of less than 50g in weight, where it is most advantageous to use every actuator for multiple functions so as to reduce weight and reduce the bulk of electronics and wiring. HAVs can perch and orient their payload units using pan and tilt both outdoors and indoors.

Where different variations or alternative arrangements are described above, it should be understood that embodiments of the invention may incorporate such variations and/or alternatives in any suitable combination.