Patent Description:
The following discussion of the background to the invention is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention.

Unmanned aerial vehicles (UAV) can hover at a position in the air for applications such as aerial photography, videography, inspection, etc. To extend the time that these UAVs remain operational in the air, the UAVs are tethered so that power can be supplied thereto. A tethered UAV is coupled via a tether to a tether management system. As mentioned above, the tether carries power lines. The tether may also carry communications lines to allow communication between the UAV and the tether management system. The tether management system reels the tether in or out as needed. During operation, the UAV is required to be able to climb, descend, translate, and operate in varying wind conditions. Without mitigation, such movements of the UAV can cause the amount of slack on the tether to vary. For example, when the UAV is piloted away from the tether management system, it may result in a decrease in the amount of slack of the tether to have possibly a whiplash effect on the UAV. Conversely, when the UAV is piloted in the direction towards the tether management system, it may result in an increase in the amount of slack of the tether. As a further example, when the UAV is hovering in a fixed position, wind speed may increase, causing aerodynamic drag on the tether to increase. Increase in aerodynamic drag is likely to result in an increase of tension in the tether. Conversely, wind speed may decrease, causing aerodynamic drag on the tether to decrease. A decrease in aerodynamic drag may result in a decrease of tension in the tether. Such changes in tension in the tether tends to cause jerky movements of the UAV, which affects the quality of photography, videography, etc. To prevent such jerky movements, it is necessary to maintain a minimum load variation on the tether. In other words, it is necessary to maintain a reasonably constant tether tension in a variety of conditions.

To mitigate this problem, different solutions have been proposed. One such solution is disclosed in Chinese Patent <CIT>, entitled "Optical fiber micro cable withdrawing and releasing winch. " <CIT> discloses a torsion spring biased rocker arm pivotably mounted at a first end. Attached to a second end is a pulley. The pulley therefore moves in an arcuate path in this winch. Optical fibre to which an underwater robot is connected is threaded through the pulley. The torsion spring biases the rocker arm to maintain a tension of the optical fibre to within a tension range so that the winch can take up excess slack in the optical fibre. The rocker arm takes up significant space. The length of the optical fibre that the rocker arm can take up is limited due to the small angle of rotation of the rocker arm. The design can reel in and reel out optical fibre to cater only to a small change in tension of the optical fibre.

Another solution is disclosed in <CIT>, entitled "Spooler for Unmanned Aerial Vehicle System. " Walker discloses a spooling apparatus including a feeder/tension sensor. The feeder/tension sensor includes a pivotably rotatable dancer that functions in a manner similar to the rocker arm of the Chinese patent, <CIT>. Therefore, this dancer will also be space consuming. Again, this dancer merely functions to mitigate filament slack; it does not by itself control tension of the filament.

Yet another solution is disclosed in <CIT>, entitled "Constant tension Tether Management System for a Tethered Aircraft. " Whitaker discloses a moveable pulley that is rotatably mounted within a ground station along a tether travel path between a fixed pulley and a UAV. The moveable pulley moves in translation along a linear track. The tether exits the ground station through an exit disposed in the ground station in a direction towards the UAV. In this way, because the moveable pulley freely moves in a vertical direction relative to the ground between a first position and a second position, the moveable pulley will move along the linear track as the tension of the tether changes. A constant-force tensioning spring, coupled to the moveable pulley, and anchored to ground station at another end, biases the moveable pulley towards the first position. A sensor is disposed within the ground station to monitor a position of the moveable pulley to detect its movement along the linear track. The constant force tensioning spring, like the earlier described designs, is also space consuming. Furthermore, the upwardly extending linear track requires a lot of space to implement. <CIT> discloses a device which is mounted aboard a missile (<NUM>), guided through an optical cable (<NUM>) from a control post (<NUM>), and includes a reel on which the cable is wound (<NUM>) and a dynamometer (13e) with a spring. In order to provide the link between a ground control station (<NUM>) and a remotely piloted flying machine (<NUM>) equipped with a television camera, <CIT> discloses an optical fibre (<NUM>), with reserve turns wound on a horizontal, fixed drum (<NUM>) in a magazine (<NUM>) overall. <CIT> discloses a power supply control system of an unmanned aerial vehicle. The power supply control system of the unmanned aerial vehicle comprises a control terminal and a power supply mechanism. <CIT> discloses an automatic cable coiling and uncoiling device used for a mooring unmanned plane aircraft. The device comprises a rotating arm and a cable disc. <CIT> discloses a fabrication method for a tire comprising a circumferential reinforcement, said method comprising a stage during which a thread is wound around a form, the tension of the thread being managed during winding, in which method the thread tension is managed through the length of a compensation loop acted upon by a spring.

There is therefore a need for a tether management system which addresses, at least in part, one or more of the forgoing problems.

According to an aspect of the present disclosure, there is provided a tether management system according to claim <NUM>. The tether management system includes a spool, a tension sensor, a controller and a moveable pulley. The spool is rotatable to reel in and reel out a tether. The tension sensor measures a tension of the tether. The controller receives a desired tension of the tether and controls the rotation of the spool to reel in and reel out the tether based on a difference between a measured tension from the tension sensor and the desired tension of the tether. The moveable pulley is disposed to engage tether between the spool and the tension sensor. The moveable pulley being able to be biased and moveable along a linear path to adjust a tension of the tether when the tension of the tether deviates from the desired tension.

In some embodiments, the tether management system further includes an elongated compression spring for biasing the moveable pulley along the linear path.

In some embodiments, the tether management system further includes a first rod over which the elongated compression spring is sleeved.

In some embodiments, the tether management system further includes a second rod at least substantially parallel to the first rod, and a pulley support for supporting the moveable pulley. The pulley support includes two bushings through which the first rod and the second rod penetrate to allow the pulley support to be slidable thereon.

In some embodiments, the tether management system further includes a first end block at a first end of the first rod and the second rod, and a second end block at a second end of the first rod and the second rod. The elongated compression spring has a free length that is substantially equal to a length of the first rod.

In some embodiments, the elongated compression spring, when biased, has a force of about twice the tension of the tether over a range of operating tension of the tether.

In some embodiments, the tether management system further includes a limit stop disposed adjacent an intermediate position between the first end block and the second end block to limit movement of the pulley support to be along the first rod and the second rod between the second end block and the limit stop.

In some embodiments, the thickness of the spool is in the range of <NUM>-<NUM> times the thickness of the tether.

In some embodiments, the tether management system further includes a housing having a base, a roof opposite the base and a sidewall between the base and the roof. The spool is disposed such that a radial plane thereof is at least substantially parallel to the base of the housing.

In some embodiments, the tether management system further includes a fixed pulley disposed to engage tether between the moveable pulley and the tension sensor, and a tension pulley for guiding the tether along an exit section of a path of the tether extending through the roof of the housing. The tension sensor is coupled to the tension pulley to measure a tension of the tether.

In some embodiments, the exit section is transverse to the linear path of the moveable pulley.

In some embodiments, the tether management system further includes a bracket to which the tension pulley and the tension sensor are mounted, and a fairlead mounted to the bracket through which the tether exits the housing. The fairlead includes at least one roller that is able to urge the tether against the tension pulley.

In some embodiments, the controller is adapted to determine a speed for rotating the spool based on a difference between the measured tension of the tether and the desired tension.

In some embodiments, the controller is adapted to further limit the speed for rotating the spool to a cap value corresponding to a length of tether left on the spool when less than a predetermined length of tether is left on the spool.

In some embodiments, the tether management system further includes a sensor for determining if an end of spool is near. The controller is adapted to set the speed to zero when the sensor indicates an end of the spool is near.

In some embodiments, the tether management system further includes a guard disposed adjacent the spool to prevent tether slippage.

According to another aspect of the present disclosure, there is provided a method for managing tether on a spool according to claim <NUM>. The method includes receiving a desired tension for deployed tether and measuring a tension of the deployed tether. The method also includes rotating the spool to reel in and reel out the tether based on a difference between the measured tension and the desired tension. The method further includes engaging the tether with a moveable pulley and biasing the moveable pulley to be moveable along a linear path to adjust a tension of the tether when the tension of the tether deviates from the desired tension.

In some embodiments, the method further includes determining a speed for rotating the spool based on a difference between the measured tension and the desired tension, and wherein rotating the spool includes rotating the spool at the determined speed.

In some embodiments, the method further includes limiting the speed for rotating the spool to a cap value corresponding to a length of tether left on the spool when less than a predetermined length of tether is left on the spool.

In some embodiments, the method further includes setting the speed to zero when an end of spool is detected to be near.

In some embodiments, biasing the moveable pulley includes biasing the moveable pulley with an elongated compression spring having a force that is about twice the tension of the tether over a range of desired operating tension of the tether.

Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

The invention will be better understood with reference to the drawings, in which:.

Throughout this document, unless otherwise indicated to the contrary, the terms "comprising", "consisting of", "having" and the like, are to be construed as non-exhaustive, or in other words, as meaning "including, but not limited to.

Furthermore, throughout the specification, unless the context requires otherwise, the word "include" or variations such as "includes" or "including" will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout the description, it is to be appreciated that the term 'controller' and its plural form include microcontrollers, microprocessors, programmable integrated circuit chips such as application specific integrated circuit chip (ASIC), computer servers, electronic devices, and/or combination thereof capable of processing one or more input electronic signals to produce one or more output electronic signals. The controller includes one or more input modules and one or more output modules for processing of electronic signals.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by a skilled person to which the subject matter herein belongs.

As shown in the drawings for purposes of illustration, the invention may be embodied in a novel and compact tether management system. Existing tether management systems tend to be bulky. Referring to <FIG>, a tether management system embodying the invention generally includes a spool, a tension sensor, a controller and a moveable pulley. The spool is rotatable to reel in and reel out a tether. The tension sensor is disposed along a path of the tether to measure the tension of the tether. The controller receives a desired tension of the tether from a user and controls the rotation of the spool to reel in and reel out tether based on a difference between the measured tension of the tether and the desired tension. The moveable pulley is disposed along the path of the tether between the spool and the tension sensor. The moveable pulley can be biased and moveable along a linear path to adjust a tension of the tether when the tension of the tether deviates from the desired tension.

Specifically, <FIG> show the above tether management system <NUM> for reeling in and out a tether <NUM> connected to an unmanned aerial vehicle (UAV) <NUM> (<FIG>). The tether management system <NUM> includes a spool <NUM> having a cylindrical drum <NUM> flanked on both ends by respective flange members <NUM>, <NUM> fixedly attached to the drum <NUM>. The flange members <NUM>, <NUM> retain the tether <NUM> on the spool <NUM>. The flange members <NUM>, <NUM> may be of a diameter that allows the spool <NUM> to hold a length of, for example, <NUM> of tether <NUM>. Each flange member <NUM>, <NUM> has a rim <NUM> and spokes <NUM> radially extending from a central portion thereof to the rim <NUM>. The rim <NUM> and the spokes <NUM> define openings <NUM> in the flange members <NUM>, <NUM>. During use, the tether <NUM> carries power lines supplying power to the UAV <NUM>, amongst others. The tether <NUM> may thus generate a considerable amount of heat, especially after prolonged use, and the openings <NUM> in the flange members <NUM>, <NUM> facilitate dissipation of this heat that is generated. One or more fans (not shown) may be further employed to blow air directly at the wound tether <NUM> to further aid in cooling it. Such a design of the spool <NUM> is simple and yet reliable. The thickness of the cylindrical drum <NUM> may be in the range of <NUM> to <NUM> times, preferably <NUM> times, the thickness of the tether <NUM> to be wound around the drum <NUM>. This thickness of the drum <NUM> allows the tether <NUM> to be neatly spooled without the need for a level winding mechanism. It is found that this thickness of the drum <NUM> is optimized considering factors like the cooling ability, the diameter of the spool <NUM> required for holding a given length of tether <NUM> and the necessity to prevent tether entanglement.

A through beam type photoelectric sensor (not shown) is used to detect that only about <NUM> of the <NUM> of tether is left on the spool <NUM>, i.e. the end of the spool <NUM> is near. This photoelectric sensor has a light emitting element that is disposed on one side of the spool <NUM> and a light receiving element that is disposed on the other side of the spool <NUM>. When the length of the tether <NUM> wound on the spool is more than <NUM>, light emitted by the light emitting element is blocked by the tether <NUM> and cannot reach the light receiving element. However, when the tether <NUM> is reeled out and only about <NUM> of the tether is left on the spool, light from the light emitting element impinges on the light receiving element for it to indicate that condition. Those skilled in the art will also recognize that other types of sensors can also be used for detecting the end of the spool <NUM>. These sensors include, but are not limited to, retroreflective photoelectric sensors, diffused photoelectric sensors and limit switches.

The spool <NUM> is connected to a shaft of a motor which is operated to turn the spool <NUM> in both clockwise and anti-clockwise directions. The motor is mounted in a cylindrical housing <NUM> with a proximal end thereof adjacent the spool <NUM>. Connected to a distal end of the cylindrical housing <NUM> is a first cantilevered beam <NUM> and a second cantilevered beam <NUM> extending in opposite directions to each other to a first sidewall <NUM> and a second sidewall (not shown) of a housing <NUM>. In this manner, the spool <NUM> is suspended horizontally with respect to a base <NUM> of the housing <NUM>. In other words, the spool <NUM> is disposed such that a radial plane thereof is, more or less, parallel to the base <NUM> of the housing <NUM>. Optionally, a guard <NUM> may be provided to cover a circumferential section of the spool <NUM> for preventing tether slippage. For example, the guard <NUM> may cover about <NUM>% of the circumferential section of the spool <NUM> leaving a small section uncovered for the tether <NUM> to be reeled in and out of the spool <NUM>. The guard <NUM> may be supported in place by the cylindrical housing <NUM> or mounted on the base <NUM> or a sidewall of the housing <NUM>.

The tether management system <NUM> further includes a tension assembly <NUM> mounted to an inner wall (not shown) of the housing <NUM>. This tension assembly <NUM>, shown in <FIG>, includes a bracket <NUM> that is mounted to the inner wall. The bracket <NUM> supports a first guide pulley <NUM>, a tension pulley <NUM> and a fairlead <NUM>. The fairlead <NUM> includes two pairs of rollers <NUM> transverse to each other. One of these rollers <NUM> urges the tether <NUM> against the tension pulley <NUM>. Connected to the tension pulley <NUM> is a tension sensor <NUM> for measuring the tension of the tether <NUM>. Force exerted by the tether <NUM> on the tension pulley <NUM> will be detected and measured by the tension sensor <NUM>. A roof (not shown) of the housing <NUM> includes an opening through which the fairlead <NUM> is exposed.

The tether management system <NUM> also includes a compensator system <NUM> that engages the tether <NUM> between the spool <NUM> and the tension assembly <NUM>. The compensator system <NUM> includes a moveable pulley <NUM> through which tether <NUM> that is reeled out of the spool <NUM> is threaded. The moveable pulley <NUM> can be biased and moveable along a linear path, in a direction indicated by doubleheaded arrow X, to adjust the tension of the tether <NUM> during use. The moveable pulley <NUM> is supported on a pulley block <NUM>. This pulley block <NUM> includes two bushings <NUM> having ball bearings (not shown) therein. Two elongated guide rods <NUM>, <NUM>, at least substantially parallel to each other, are inserted through the two bushings <NUM> of the pulley block <NUM> to allow the pulley block <NUM> to slide along them. Two end blocks <NUM>, <NUM> are connected to the ends of the two guide rods <NUM>, <NUM>. One end block <NUM> is disposed on the sidewall <NUM> of the housing <NUM> proximal to the spool <NUM>. The other end block <NUM> is disposed on a pillar <NUM> of the housing <NUM>, distal from the spool <NUM>. Both guide rods <NUM>, <NUM> are disposed at about the same height from the base <NUM> of the housing <NUM> to define the linear path on which the pulley block <NUM>, and thus the moveable pulley <NUM>, can slide along. Sleeved over an inner rod <NUM> of the two rods <NUM>, <NUM> is an elongated helical compression spring <NUM> that has a free length close to or the same as the length of the inner rod <NUM>. In an uncompressed state of this compression spring <NUM> when the length of the spring <NUM> is at its free length, the pulley block <NUM>, and along with it the moveable pulley <NUM>, is pushed towards the second end block <NUM> that is distal from the spool <NUM>. This position of the moveable pulley <NUM> is shown as Position <NUM> in <FIG>. The compression spring <NUM> may be of a unitary design or it may include two or more sections joined via any connectors or adapters, or simply sleeved over the rod abutting one another.

The compensator system <NUM> further includes a limit stop <NUM> sleeved over the outer rod <NUM> and mounted to the sidewall <NUM>. The limit stop <NUM> is disposed adjacent an intermediate position between the first end block <NUM> and the second end block <NUM> to limit movement of the pulley support <NUM> to be along the first rod <NUM> and the second rod <NUM> between the second end block <NUM> and the limit stop <NUM>. Typically, the helical compression spring <NUM> has a working length of about <NUM>%-<NUM>% of its free length. For a selected range of tension of operation of the tether <NUM>, the length of the spring is selected such that the spring force is <NUM> at uncompressed state and about <NUM> (maximum operating load) when it is compressed to be at the limit stop <NUM>. Thus, the working length is about <NUM>% of the maximum length of the compression spring <NUM>.

The tether management system <NUM> further includes a second guide pulley <NUM> mounted to the sidewall <NUM> to be on a same horizontal plane as the moveable pulley <NUM>. The tether management system <NUM> further includes a fixed pulley <NUM> mounted on a columnal structure <NUM> to be on the same horizontal plane as the second guide pulley <NUM> and the moveable pulley <NUM>. The columnal structure <NUM> extends from the base <NUM> of the housing <NUM>. The inner wall (not shown) described above is supported by this columnal structure <NUM>.

During use, the tether <NUM> is threaded out of the spool <NUM>, through the second guide pulley <NUM>, the moveable pulley <NUM> and the fixed pulley <NUM> along a path on a horizontal plane. The tether <NUM> is further threaded through the first guide pulley <NUM> and the tension pulley <NUM> of the tension assembly <NUM> to have its path translated in a vertical plane. The tether <NUM> is further threaded through the fairlead <NUM> to extend out of the housing <NUM> for connection to the UAV <NUM>. In other words, an exit section of the path of the tether <NUM> is transverse to the linear path of the moveable pulley <NUM>.

The tether management system further includes a controller <NUM> that is connected to a touchscreen input and/or a mobile phone (both not shown) via a Bluetooth connection. Through a suitable interface on any of these devices, the controller <NUM> is able to receive, from a user, a desired tension for operating the tether <NUM> and to control the rotation of the spool <NUM> to reel in and reel out the tether <NUM> based on a difference between a measured tension provided by the tension sensor <NUM> and the desired tension. The controller <NUM> is further able to receive an end of spool signal from the photoelectric sensor.

The method for controlling the rotation of the spool to adjust the tension of the tether <NUM> when the tension of the tether <NUM> deviates from the desired tension is next described. <FIG> is a flowchart showing the method <NUM>. The method <NUM> starts in a POWER ON step <NUM> when power to the controller <NUM> is turned on. The method <NUM> next proceeds to a READ DEPLOYED TETHER LENGTH step <NUM>, wherein the controller <NUM> reads data associated with a length of the tethered <NUM> deployed from a non-volatile memory (not shown), for example, an SD card. The method <NUM> next proceeds to a RECEIVE DESIRED TENSION step <NUM>, wherein the controller <NUM> is able to receive from a user via either the touchscreen or mobile phone a desired tension value at which deployed tether <NUM> is to be maintained. The method <NUM> next proceeds to a MEASURED TENSION > DESIRED TENSION? decision step <NUM>, wherein the controller <NUM> determines if the measured tension captured by the tension sensor <NUM> is greater than the desired tension set by the user. If the measured tension is greater than the desired tension as determined in this decision step <NUM>, such as when the UAV is increasing in height or there is an increase in wind speed, the method <NUM> proceeds to a SET MOTOR DIRECTION TO REEL-OUT DIRECTION step <NUM>, wherein a direction to operate the motor is set to a reel-out direction. If however it is determined in the decision step <NUM>, that the measured tension is not greater but lower than or equal to the desired tension, the method <NUM> proceeds to a MEASURED TENSION < DESIRED TENSION? decision step <NUM>, wherein the controller <NUM> determines if the measured tension is lower than the desired tension. If the measured tension is lower than the desired tension as determined in the decision step <NUM>, such as when the UAV is decreasing in height or there is a decrease in wind speed, the method <NUM> proceeds to a SET MOTOR DIRECTION TO REEL-IN DIRECTION step <NUM>, wherein the direction to operate the motor is set to a reel-in direction that is opposite to the reel-out direction. If however it is determined in the decision step <NUM>, that the measured tension is not lower than the desired tension but equal to the desired tension, the method <NUM> proceeds to SET MOTOR DIRECTION TO NIL step <NUM>, wherein the direction to operate the motor is set to zero to indicate that the motor need not be rotated. After the SET MOTOR DIRECTION steps <NUM>, <NUM>, <NUM>, the method <NUM> proceeds to a SET MOTOR SPEED VALUE step <NUM>, wherein the controller <NUM> determines a speed value for driving the motor using a PID control based on the difference between the measured tension and the desired tension. The determined speed value may range, for example, from <NUM> - <NUM>.

The method <NUM> further proceeds to an ADJUST MOTOR SPEED VALUE step <NUM>, wherein the controller <NUM> adjusts the speed value obtained in the SET MOTOR SPEED VALUE step <NUM> based on the length of tether <NUM> deployed. For example, as described above, the spool <NUM> may be able to hold <NUM> of tether. When the length of the deployed tether <NUM> is less than or equal to <NUM>, i.e. about <NUM> or more of tether is left on the spool <NUM>, no adjustment of the motor speed value is necessary. However, when the length of deployed tether goes beyond <NUM>, the motor speed value is adjusted downwards to be capped at cap values decreasing with increasing length. For example, the cap value may be correspondingly lowered to <NUM> for a length of deployed tether <NUM> of <NUM>, and correspondingly lowered to <NUM> for a length of deployed tether <NUM> of <NUM>. As an example, the motor speed value may be determined to be <NUM> in the SET MOTOR SPEED VALUE step <NUM>. But when it is determined that the length of the deployed tether is at <NUM> in the ADJUST MOTOR SPEED VALUE step <NUM>, the motor speed value is lowered from <NUM> to be capped at the cap value of <NUM> corresponding to the <NUM> mark. As a further example, the motor speed value may be determined to be <NUM> in the SET MOTOR SPEED VALUE step <NUM>. But when it is determined that the length of the deployed tether is <NUM> in the ADJUST MOTOR SPEED VALUE step <NUM>, the motor speed value is lowered from <NUM> to be capped at the cap value of <NUM> corresponding to the <NUM> mark. The remaining <NUM> of the tether <NUM> is used as a buffer.

The method <NUM> next proceeds to an END OF SPOOL REACHED? decision step <NUM>, wherein the controller <NUM> determines if an end of spool condition, e.g. <NUM> of the tether <NUM> has been deployed leaving only about <NUM> of tether <NUM> left on the spool <NUM>, is reached by reading the photoelectric sensor. If it is determined in this decision step <NUM> that the end of spool is reached, the method <NUM> proceeds to a SET MOTOR SPEED VALUE TO ZERO step <NUM>, wherein the controller <NUM> sets the motor speed value to zero regardless of its value as determined in the steps <NUM>, <NUM>. If it is determined in this step <NUM> that the end of spool has not been reached, no change is made to the motor speed value determined earlier. This is a fail-safe design just in case the controller <NUM> determines the tether length deployed to be still less than <NUM> when it has reached <NUM>. The method <NUM> proceeds to an OPERATE MOTOR step <NUM>, wherein the controller <NUM> operates the motor according with the set direction and motor speed to reel the tether <NUM> in or out to return the tension of the tether <NUM> closer to the desired tension set by the user. The method <NUM> further proceeds to an UPDATE DEPLOYED TETHER LENGTH step <NUM>, wherein the controller <NUM> reads the number of revolutions made by the motor via a motor encoder (not shown), calculates the amount of tether <NUM> reeled in or out and updates the length of tether <NUM> deployed in the non-volatile memory. Each turn of the motor will correspond to a different tether length depending on how much tether is left on the spool <NUM>. To obtain the length of tether deployed for any length of tether <NUM> left on the spool <NUM>, a formula corresponding to a 3rd order best fit curve between two known deployed lengths of <NUM> and <NUM> is used. Although such a formula is not very accurate, the length of tether obtained using the formula is acceptable as the user does not need to know the exact length of the tether. The length of tether deployed is also displayed on an LCD screen (not shown) or the user's mobile phone so that the user is aware of the amount of tether <NUM> available in the spool <NUM> to enable the user to operate the UAV <NUM> accordingly. For example, with the information on how much tether <NUM> is remaining in the spool <NUM>, the user can choose to fly the UAV <NUM> higher, further or both. Without this information, the user will not be aware that tether <NUM> may be running out and their filming or photo taking using the UAV <NUM> might be affected by the sudden jam when the tether <NUM> runs out.

The method <NUM> loops around the steps <NUM> -<NUM> to provide a closed-loop control of the spool <NUM>. With such a control loop, the controller <NUM> is able to reel in and reel out tether <NUM> such that the tension of the tether <NUM> is close to or at the desired tension. However, there is latency in such control. Any change in the tension in the tether <NUM>, if not immediately corrected, may result in jerky movements of the UAV <NUM>. The compensator system <NUM> can mitigate this problem at least to some extent. When the tension of the tether <NUM> is at the desired tension level, the controller <NUM> will keep the motor still. At this tension level, the compression spring <NUM> is compressed such that the force of the spring <NUM> is double the tension experienced by the tether <NUM>. The position of the moveable pulley <NUM> is at Position <NUM> in <FIG>.

When the tension of the tether <NUM> is greater than the desired tension level and before the controller <NUM> is able to reel out the tether <NUM> as described above, the moveable pulley <NUM> is forced to slide along the rods <NUM>, <NUM> in a direction of the first end block <NUM> to move to Position <NUM> as shown in <FIG>. In this position of the moveable pulley <NUM>, the compression spring <NUM> is compressed, such that the force in the spring <NUM> is about twice the new increased tension in the tether <NUM>. This movement of the moveable pulley <NUM> closer to the spool <NUM> reduces the length of the tether <NUM> between the spool <NUM> and the tension pulley <NUM>. The excess tether <NUM> is drawn beyond the tension pulley <NUM> to increase the tether length between the tension pulley <NUM> and the UAV <NUM>. In this manner, the tension in the tether <NUM> is reduced quickly before the controller <NUM> is able to respond. When the controller <NUM> finally operates the motor to reel out more tether <NUM>, the tension in the tether <NUM> will decrease and the compression spring <NUM> will push the moveable pulley <NUM> back towards the steady state Position <NUM> as shown in <FIG>.

When there is a sudden decrease in tension in the tether <NUM> due either to the UAV <NUM> decreasing in height or a decrease in the wind speed, the tension sensor <NUM> will sense the decrease in tension in the tether <NUM>. As described above, the controller <NUM> will operate the motor to reel in the tether <NUM>. However, due to latency in the method <NUM>, there will be a delay in reeling in the tether <NUM>. Before the motor is able to reel in the tether <NUM>, the moveable pulley <NUM> in the compensator system <NUM> will move in a direction towards Position <NUM> as shown in <FIG> to decompress the compression spring <NUM>, such that the force in the compression spring <NUM> is about twice the new decreased tension in the tether <NUM>. This movement of the moveable pulley <NUM> draws more tether <NUM> in to increase the length of the tether <NUM> between the spool <NUM> and the tension pulley <NUM>. The length of the tether <NUM> between the tension pulley <NUM> and the UAV <NUM> is reduced, thus reducing slack and increasing the tension in the tether <NUM>. When the controller <NUM> finally operates the motor to reel in the tether <NUM>, the tension in the tether will increase, and the moveable pulley <NUM> will once again be pushed by the tether <NUM> to move back to its steady state Position <NUM>.

Advantageously, the tether management system <NUM> having a moveable pulley <NUM> moveable along a linear path as described above may be compactly built. The compensator system <NUM> of the tether management system <NUM> can maintain tension in the tether <NUM> within an acceptable range, compensating for the latency in the tension sensor <NUM> and motor feedback loop. The compensator system <NUM> can instantaneously reel out tether <NUM> when more tether <NUM> is required thus preventing the tether <NUM> from experiencing a sudden large tension force. The compensator system <NUM> can also instantaneously reel in the tether <NUM> when there is too much tether slack below the UAV <NUM>, thereby lowering the risk of tether entanglement with objects near the UAV and preventing the tether <NUM> from experiencing a sudden loss of tension. While it reduces the amplitude of change in the tension in the tether <NUM>, it does not completely remove the change in tension in the tether <NUM>. It also retains the polarity of change. In other words, the compensator system <NUM> scales down the change in tension of the tether <NUM> but it does not eliminate it. This is important to allow the tension sensor <NUM> to continue to sense a change in tension so that the controller <NUM> can change the motor's direction and speed as described above. This allows the tether management system <NUM> to perform its function of keeping the tether <NUM> within an acceptable tension range responsively.

Although the present invention is described as implemented in the above described embodiment, it is not to be construed to be limited as such. For example, although the tether management system <NUM> is described for use with a UAV <NUM>, those skilled in the art will recognize that such a system <NUM> can be used in other applications including, but not limited to, tethering of other robots, e.g. an underwater robot, and also in applications for managing a cable between two moving vehicles.

As another example, the compensator system <NUM> may include two compression springs instead of only one <NUM> as described above. A second compression spring (not shown) may be sleeved over the second rod <NUM> for example. In such an embodiment, the limit stop <NUM> will accordingly have to be moved to another location. As yet another example, instead of a compression spring <NUM>, a torsion spring <NUM> such as that shown in <FIG> may be used instead. One end of the torsion spring <NUM> is attached to a side wall <NUM> and the other end is attached to the pulley block <NUM>.

As yet a further example, the compensator system <NUM> may include a second moveable pulley <NUM> that is moveable along a second pair of moveable rods <NUM> disposed side by side with the moveable pulley <NUM> and its corresponding rods <NUM>, <NUM> as shown in <FIG>. Such a design having two or more moveable pulleys in series can provide a larger buffer, and the reel in and reel out speed may also be doubled.

As another example, the double rod <NUM>, <NUM> in the above described embodiment may be replaced with a linear track on which the pulley block <NUM> is slidable. Alternatively, a single rod may be used. In such a case, the pulley block <NUM> may be provided with rollers that rest against the sidewall to prevent the pulley block <NUM> from rotating about the single rod.

As yet another example, the compression spring <NUM> sleeved on the rod design described above may be replaced with a compression spring in a tubular structure. This tubular structure has a groove running along its length that allows the pulley block <NUM> to be coupled to an end of the compression spring and to be slidable thereon.

Claim 1:
A tether management system (<NUM>) comprising:
a spool (<NUM>) rotatable to reel in and reel out a tether (<NUM>);
a tension pulley (<NUM>) engaging the tether (<NUM>);
a tension sensor (<NUM>) coupled to the tension pulley (<NUM>) for measuring a tension of the tether (<NUM>);
a controller (<NUM>) for receiving a desired tension of the tether and for controlling the rotation of the spool (<NUM>) to reel in and reel out the tether based on a difference between a measured tension from the tension sensor (<NUM>) and the desired tension of the tether (<NUM>); characterized in that the tether management system (<NUM>) further comprises:
a moveable pulley (<NUM>) that is not coupled to the tension sensor (<NUM>) and is disposed to engage tether between the spool (<NUM>) and the tension pulley (<NUM>), the moveable pulley (<NUM>) being biased to a steady state position along a linear path when the tether is at the desired tension and moveable along the linear path in either direction from the steady state position to adjust a tension of the tether (<NUM>) when the tension of the tether deviates from the desired tension.