TETHERED AIRCRAFT

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application No.

82/467,626 filed March 6, 2017, the contents of which are herein incorporated. BACKGROUND OF THE INVENTION

[0002] The following invention is directed to a system for controlling the position of a tethered unmanned aerial vehicle (UAV), and more particularly, to control the operation of the tether of the tethered unmanned aerial vehicle by controlling the tension of the tether connected thereto to maintain a desired tether strain. [0003] Unmanned aerial vehicles, have the ability to hover. UAVs, such as multiple rotor helicopters, can be tethered for safety, communications, and long term power. This increases the ability of these crafts to stay aloft. This provides the benefit of being able to maintain a consistent visual monitoring of a specified area. [0004] A tethered UAV is coupled to a ground-based counterpart, including a tether management system, to reel the tether in or out as needed. However, the UAV also requires the freedom to climb, descend, translate, and operate in varying wind speeds, all with minimum load variation on the tether. These aircraft typically rely on the skill of an on-site pilot to maintain constant tether tension in a variety of conditions. Other systems rely on complex structures such as either on board tension sensors, optical sensors or satellite navigation in order to maintain the UAV positioning location, and resulting tether tension relative to the ground base. [0005] These systems are satisfactory, however they are extremely complex so that, traditional methods like those above result in a high cost of manufacture and maintenance as well as a high probability of failure.

[0006] Accordingly, a system and method for overcoming the shortcomings of the prior art is desired.

SUMMARY OF THE INVENTION

[0007] A constant tension tether management system for tethered aircraft has a spool rotatably disposed within a ground station. A first pulley is rotatably mounted within the ground station along a tether travel path. A second pulley is rotatably disposed within the ground station and translatable along the tether travel path. The first pulley is disposed along the tether travel path between the spool and the second pulley.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The present disclosure is better understood by reading the detailed description with reference to the accompanying drawing figures in which the reference numerals denote similar structure and refer to the elements throughout in which:

[0009] FIG. 1 is a schematic diagram of the unmanned aerial vehicle constructed in accordance with the invention;

[0010] FIG. 2 is a schematic diagram demonstrating operation of the invention intended to maintain the position of the aircraft; and

[0011] FIG. 3 is a schematic diagram of a tether management system constructed in accordance with the invention. DESCRIPTION OF THE PREFERRED EMBODIMENT

[0012] Turning now to the drawings, in which similar reference characters denote similar elements throughout the several views, the figures illustrating a tethered unmanned aerial vehicle. Reference is made to FIGs. 1 and 2 wherein a schematic diagram of the invention in accordance with a preferred embodiment thereof is provided. Not part of the system is a tether 106, coupling aircraft 104 to ground station 108.

[0013] More specifically, as seen in FIG. 2, tether 106 attaches to aircraft 104. Because of gravity the natural tendency of the tether 106 is to hang directly below aircraft 104. When outside forces, such as wind act on the tether, force differential impose a strain on tether 106 external forces move UAV 104 from a desired location or caused it to roll. When wind, by way of example, is applied to system 100, aircraft 104 will tend to move down wind away from the desired position, in this embodiment away from normal 500 corresponding to the initial position in FIG. 1 . UAV 104 moves away from normal or roles along an angle ά, as seen in FIG. 2, changing the tension on tether 106 as UAV 104 moves from the desired course. However, it is desired to maintain constant tension on the tether 106, regardless of the altitude or attitude of UAV 104 so as to not interfere with separately controlled flight of UAV 104.

[0014] Reference is now made to FIG. 3 wherein a tether management system, generally indicated as 200, for controlling tether tension is shown. The tether management system 200 is housed within the housing of ground station 108. The tether management system includes a spool 102 rotatably mounted within ground station 108. Tether 1 06 is stored and wound about spool 102. Spool 102 is operatively coupled to a bidirectional motor (not shown), as known in the art, capable of precise movement at sufficient speeds in opposite rotational direction to accommodate for the ascent and descent of the attached UAV 102.

[0015] Tether 106 travels along a travel path from spool 102 to UAV 104. A first pulley 107, acting as a guide pulley, is disposed along the travel path within ground station 108. First pulley 107 is rotatably mounted at a fixed position within ground station 108. As tether 106 is spooled out from, or spooled into, spool 102, tether 1 06 comes in contact with and is guided by first pulley 1 07.

[0016] A second pulley 1 10 is rotatably mounted within ground station 108 along the tether travel path between first pulley 107 and UAV 104, and moves in translation along a linear track 1 16. Second pulley 1 10 is disposed along the travel path, in such a way, that first pulley 107 causes tether 106 to always come in contact with substantially 180° of the engaged surface of second pulley 1 10. Pulley 1 10, in a preferred nonlimiting embodiment, is mounted on a linear track 1 16 and is movable between a first position indicated as the pulley 1 10 in solid line and a second position shown in phantom as position 1 10'.

[0017] Tether 106, then exits ground station 108 through an exit 120 disposed in ground station 1 08 in a direction towards UAV 104. In this way, because second pulley 1 1 0 freely moves in a vertical direction relative to the ground between the first position and the second position, second pulley 1 10 will move along track 1 16 as the tension of tether 106 changes. A constant-force tensioning spring 1 12, coupled to pulley 1 1 0, and anchored to ground station 108 at another end, biases second pulley 1 10 towards the first position shown as 1 10. A sensor 1 14 disposed within ground station 108 to monitor a position of second pulley 1 10 detects the movement of second pulley 1 10 along the linear track 1 16. [0018] In a preferred nonlimiting embodiment, second pulley 1 10 includes a slider, such as bearings or a low friction contact disposed within linear track 1 16 to enable the free travel of second pulley 1 10 along track 1 16. As a result, movement of second pulley 1 10 between the first position and at least the second position 1 10' occurs smoothly and with minimal friction. Having a known range of movement and positions, allows for the attachment of the constant-force spring 1 12 as well as a reference point for linear position sensor 1 14 to track.

[0019] During operation, a motor drive (not shown, but known in the art) attached to spool 102 operates at varying speeds, in either one of a first direction to retract tether 106 into ground station 108, or a second direction to extend tether 106 from ground station 1 08 in response to the output of sensor 1 14 which periodically determines the position of second pulley 1 1 0 along linear track 1 16. Sensor 1 14 may be any sensor for measuring a position of an object along a straight line while offering minimal friction; such as a laser, noncontact electrical sensor, an

electromechanical contact sensor or other like type based detector.

[0020] At the same time, constant force tensioning spring 1 1 2 provides a force on second pulley 1 10; biasing second pulley 1 10 in the direction of the first position. Constant force tensioning spring 1 12 acting on movable second pulley 1 10, provides a constant tension to tether 1 06 that is equal to one half of the force provided by constant force tensioning spring 1 1 2. This results from the substantially 180° wrap of tether 106 about second pulley 1 10. The motor applies a torque to spool 102, and therefore a tension to tether 106, until sensor 1 14 indicates to the motor that the linear position of the second pulley 1 10, as detected by sensor 1 14, is substantially in the middle of the travel range along linear track 1 16. In effect, the motor is not directly controlling the tension of tether 106 as tether 106 leaves ground station 108. The motor works to keep pulley 1 10 within the range of linear track 1 1 6, and the constant-force spring 1 12 adds tension to tether 106 through pulley 1 10.

[0021] During operation, when sensor 1 14 detects second pulley 1 10 moving away from the middle of linear track 1 1 6 towards the first position, this indicates a decrease in tension on tether 106 as constants force tensioning spring 1 12 overcomes this lower tension force (force in a feed direction) by tether 1 06. Sensor 1 14 outputs a signal to control the motor indicating this change. System 100 makes use of a proportional integral derivative (PID) loop to control the motor in response to outputs from sensor 1 14. Here, by way of nonlimiting example, a detection that second pulley 1 10 is moving from the midway point along linear track 1 16 in the direction of the first pulley position causes the motor to reel tether 1 06 into ground station 108. This is done until second pulley 1 1 0 returns to substantially the middle position along track 1 16, an equilibrium position as detected by sensor 1 14. Sensor 1 14 then outputs a control signal to the motor and the motor is then stopped.

[0022] Conversely, if sensor 1 14 detects second pulley 1 10 moving away from substantially the middle position along linear track 1 16 towards the second position 1 10' of second pulley 1 10, this indicates that the tension experienced by tether 1 06 is increasing; it is overcoming the force applied by constant-force tensioning spring 1 12. Sensor 1 14 outputs a signal causing the motor to reel tether 1 06 out from ground station 1 08 until the sensor 1 14 indicates that second pulley 1 1 0 has returned to the substantial midpoint along linear track 1 1 6. System 100 makes use of a proportional integral derivative (PID) loop to control the motor in response to outputs from sensor 1 14. The motor is then stopped.

[0023] The linear travel length is determined as a function of the inertia of the spool, the torque of the motor, the ascent and descent rates of the UAV and the constant tension spring rate. By utilizing a constant force spring combined with a relatively long linear travel path, tensioning adjustments may be made in

substantially real time to maintain a constant tension on the tether. The travel length should be long enough to enable the motor to transition from full speed clockwise to full speed counter clockwise (and vice versa) without either introducing slack in the tether, or allowing the translatable pulley to reach either end of its range, which would introduce a sudden increase in tether tension; a jerk motion.

[0024] The constant force tensioning spring does not have a natural frequency like traditional springs with a varying force depending on its position. This ensures stability of the system across a broad range of conditions. This functionality is necessary in an environment in which a sufficiently useful tether management system must be capable of storing a large amount of tether on a single spool because such a spool will have high inertia. The motor will require a significant amount of time to either start rotating, stop rotating or change its direction of rotation.

[0025] It should be noted, that the above embodiment utilized a constant force spring. However, gravity may also be used to maintain a constant tension to the tether. In such an embodiment, weighting of the sliding pulley assembly may be utilized when an appropriately sized constant-force spring is unavailable; for extremely large or small tether management systems. Again, the tension applied to the tether would equal half the weight of the slider pulley assembly due to the 180° wrap angle of the second pulley.

[0026] By utilizing the pulley-spring arrangement described above, a simple yet effective structure and method for maintaining constant tension on a tether, regardless of the attitude of the UAV to which is attached, is provided. The system will reel tether in or out as required by the UAV. This is done even while simplifying and reducing the amount of work an operator must put forth, minimizing required training as well as the time between set up and launch.

[0027] While this invention has been particularly shown and described to reference the preferred embodiments thereof, it would be understood by those skilled in the art that various derivatives and changes in form and detail may be made therein without departing from the spirit and the scope of the invention, by the appended claims.