September 21, 2010

Quadcopters for Personal Remote Telepresence

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Valkyrie Ice wrote an interesting article on Quadcopters for eventual usage as personal remote telepresence over at hplusmagazine.

Advances being made in battery and fuel cell and solar power technology could enable a cell-powered drone to run for days. Advances with noise-canceling “stealth” technology should enable the elimination of the “buzz” from the fans on a drone. Inside of five years, a small, silent, RTU could be feasible. Properly constructed, it could be rugged, cheap, and safe enough to use in any human environment. Drones have already been enabled can with grippers, or other robotic limbs.

The University of Pennsylvania grasp lab is where some of the most impressive UAV quadcopter performance is being developed.

UPenn’s GRASP lab released a video of their quadrotor performing its own autonomous flight using onboard sensors and the PixHawk at ETH can also perform autonomous flight.

IEEE Spectrum covers quadcopters, hexacopter and octocopter UAVs.

According to a recent Robots Podcast interview with Joshua Portlock, manager of the CyberQuad project at Australia's Cyber Technology, what happened is a classical case of an enabling technology being driven by the consumer market. Fast, precise and affordable accelerometers are a key technology for Quadcopters. Their development was initially driven by their use for airbags in cars, and now increasingly by their use in consumer devices such as mobile phones. Accelerometers are key because unlike standard helicopters, which use complex mechanics to allow stable flight, Quadrotors use fast onboard motor control to take care of stability. This mechanical simplicity is also their main attraction: Quadrotors can navigate in three dimensions using only four moving parts. And the high reliability of brushless motors makes them a simpler, more reliable alternative to many traditional flying platforms. Hexacopters pack more rotors into a given size to provide more power.

MikroKopter - HexaKopter from Holger Buss on Vimeo.

Quad vs Hexa vs Octo -copter Advantages-Disadvantages

General rules (from Martin Seven) are that more engines means more power and more lift. That means more batteries. That means more time in the air. Brushless electrics like to run slow (lower RPMs), so bigger means more efficient. If efficiency is your goal, coaxial = evil (radial is more efficient).

The breakdown
Tricopters: cheap, easy to build, least stable, not as robust (tail servo and mechanics), low lifting power and flight time (because the motors have to run faster to hold it all in the air). No engine out capability. Quadcopters: mechanically simpler than tris. While they weigh almost the same they have about 1/3 more lift, they are usually more stable (no servo issues) and are capable of staying airborne for a little while longer (they can either lift larger batteries or fly more economically because the weight is spread across 4 motors and not just 3). Still no engine out capability. If it fails, it goes down. Hexacopters: All the good things that quads have, plus more power and more lifting capability. As a bonus they add limited engine out capability - a hexacopter can lose any single engine and still land (it will lose yaw control though), and if it loses one or both engines on the neutral torque bar it could even continue flying unaffected. Downside is that they are larger and a little pricier, especially if you're running high-grade motors like AXI. Octocopters and heavier: All the good things from hexacopters, plus true engine out ability. Loses any single one and still flies fine. This is what you fly if you need horsepower and reliability in one package. This is what you strap that $1300 Canon 7D under. Even more expensive though. Also heavy craft are really power hungry and unless you have some serious chargers at hand they require a lot of work on the ground before a flight can be made (charging say 5 packs for 25 minutes in the air).

Re more motors and velocity/maneuverability - that depends. It depends on the weight-to-lift ratio. If your machine weighs say 2 kg and each of your 4 motors gives out 625 g of thrust (4*.625 = 2.5 kg), then you have a ratio of 2.5/2 = 1.25, which is not really good. However, if your quad weighs 2 kg and each motor maxes out at 1 kg of thrust (4 kg total), then you're looking at a lift-to-weight ratio of 4:1, which means plenty of power and speed for acrobatics.

If you have more motors you have to consider the battery ratings: a 5000 mAh 20C battery has a max current of 5*20 = 100 A. If you have eight motors and each can draw up to 17 A (136 A total), then you either need to use a stronger battery (or more of them in parallel) or limit the engines in software using a current monitor.
Lithium Polymer batteries currently have up to 200 watt hours per kilogram. In future, Lithium-metal batteries approach the energy density of fuel cells without the plumbing needed for these devices; in theory, the maximum energy density is more than 5,000 watt-hours per kilogram, or more than 10 times that of today's lithium-ion batteries.

1000 watt hours per kilogram lithium batteries are expected in mid-2011

So 5-25 times better performance than lithium polymer is possible.

Long Term Affordable UAVs could run for years

Boeing landed a $89m DARPA contract to build the "Vulture", a huge unmanned solar-powered plane intended to cruise the stratosphere for five years without landing

Nearer term is to leverage beamed power (lasermotive) for remote recharging of a continuously flying UAV.

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