Why Won’t Amazon’s Drone Tether Swing?

The video of Amazon landing a delivery drone on a happy father’s front lawn has made the rounds, but Amazon is hedging its technology footprint and pursuing tether delivery by drone as well. This patent application is about the flawless final step of delivery, dropping the item at its destination. Apparently, it’s not as easy as it sounds.

As the drone is lowering its item, it can encounter unexpected gusts of wind and the item will be swinging uncontrollably in no time at all. Amazon’s “tether compensation mechanism” works in tandem with the drone’s own corrective movements to zero out the sway and deliver the item with pin-point precision.

Why not land the drone and unclip the item? Amazon says that “[i]t may be more energy efficient to maintain the UAV 100 at a minimum height above the landing surface when delivering the item 140 […] Further, it may be safer for the UAV 100 and/or individuals near the delivery zone 170 to maintain the UAV 100 at a minimum height above the landing surface when delivering the item 140.”

As far as ‘energy efficiency’, the energy needed to fly the extra weight versus landing and taking off is minuscule at best, but their statement about safety is pretty convincing. Between a box dangling from the sky and spinning propellers in my front yard, I’d take a dangling box. It’s strange they don’t mention that if the item is fragile, carefully lowering the payload is the better way to go.

As the tether is slowly unspooled by the winch, it passes through two rollers in a carriage (the “tether compensation mechanism”). That carriage is attached to arms that can push/pull the carriage in either the x or y axis, compensating for undesired movement of the tether. “[T]ether compensation mechanism 150B is designed to provide at least some movement in at least the “X” and “Y” directions based on control signals from the tether response controller 283. To that end, the extension arms 252B and 252C may be embodied as an adjustable-length telescoping arms and electromechanical systems (e.g., motors, servos, solenoids, etc.) capable of adjusting the angles of the extension arms 252B and 252C at the pivots points […] and counterbalance against or compensate for movement or sway detected in the tether 132.”

Needless to say, there are optical and torque sensors sending signals to the system at all times. As an added layer of safety, the system has a tension detector and an item jettison feature. Why? How will the drone know if it’s encountering wind resistance or an unexpected tree branch? It will continue to pull until the winch rips out of the drone: “if disconnected, the winch 130 may fall from the UAV 100 along with the item 140. In this sense, the tension detector 290 offers some protection against the tether 132 or the item 140 being unexpectedly pulled or caught in a tree, power line, etc.” In other words, if an item gets tangled up in something, the system will jettison the item. Don’t be surprised if boxes start falling from trees.


The first claim is pretty air-tight:

  1. An Unmanned Aerial Vehicle (UAV), comprising:
  • a winch secured to the UAV, the winch including a tether to lower an item from the UAV for delivery;
  • a tether compensation mechanism secured to the UAV and configured to contact the tether as it extends from the winch;
  • and a flight controller configured to:
  • control a flight path of the UAV;
  • and direct the tether compensation mechanism to clamp the tether based on the flight path of the UAV.



US Patent Application No: 20170197718

Filed (USA): Mar. 27, 2015

Published: July 13, 2017

Applicant: Amazon Technologies, Inc.