We were scanning through the upcoming ICRA2012 program and noticed an interesting paper titled, "Resonant Wireless Power Transfer to Ground Sensors from a UAV." This certainly piqued our interest -- especially for Travis, who happens to work with wirelessly-powered sensors at his day job. Come to find out... the article is by Dr. Carrick Detweiler, PI of the NIMBUS Lab at the University of Nebraska-Lincoln (our undergrad alma mater!). Furthermore, he just provided a preprint of the paper (PDF) and a video on his website. Score! Their quadrotor delivers power via magnetic resonance (ie. WiTricity-style) to a load on the ground. This same type of technology is being actively researched for lots of applications, including: consumer electronics, transportation (eg. electric vehicle recharging), and remote sensing (this application). Adding it to a UAV adds a bit of flexibility to the system. Anyway, be sure to check out the video below... and we'll also give a brief overview of a few different wireless power + robotics projects over the ages.
Magnetic resonance can be a good method for wireless power transfer at medium distances (ie. a meter or so). Owing to the resonant fields, it is more efficient that inductive coupling, which would require the UAV to be in close proximity to the load (a few centimeters) to get good mutual coupling. Furthermore, the magnetic fields are less susceptible to occlusions or material composition compared to microwave (electromagnetic) or laser-based wireless power transfer.
However, being at least partially familiar with the underlying technology, we know that alignment and (dynamic) frequency tuning are absolutely critical for efficient power transfer using magnetic resonance. Presumably, the quadrotor itself could supply the necessary flexibility in the system to help align the coils -- heck, we've all seen quadrotors perform impressive feats of acrobatics. So kudos to Carrick and Brent for their insights!
In case you were wondering... wireless power and robotics actually have a long history. We thought we'd highlight a few projects for the curious...
In Dr. Detweiler's case, the wireless power transfer is going from drone-to-ground. The reverse direction, from ground-to-drone, has a long history going back to the 1960's. In "The History of Power Transmission by Radio Waves" (PDF), we see microwave electromagnetic radiation and rectifying antennas (rectennas) being used for wireless power transfer:
Left: The special "string" rectenna made for the first microwave-powered helicopter in 1964. The array area of four square feet contained 4480 IN82G point-contact diodes. Maximum DC power was 270 Watts.
Right: Microwave-powered helicopter in flight 60 ft above a transmitting antenna. The helicopter was demonstrated to media in October 1964. A 10-hour sustained flight was achieved in November of that same year.
More recently (1992), projects like MILAX used microwave rectenna-based power transfer systems to supply power to a airplane (UAV) that stayed aloft above a moving vehicle! (photo source)
Now... robot-to-sensor power transfer isn't exactly new either. In fact, it's the basic operating principle behind many RFID technologies (eg. passive long-range RFID) that use electromagnetic radiation to power a small piece of silicon (typically for just identification, but many new variants are coming bundled with sensors). Hizook's own, Travis Deyle, did his PhD thesis on this topic: "Ultra High Frequency (UHF) Radio-Frequency Identification (RFID) for Robot Perception and Mobile Manipulation." We covered its appearance on CNN, where the PR2 holds the power transmit antennas and powers the remote "sensors" (long-range RFID tags) from several meters away. This could easily be extended to a UAV too... [ Can't divulge too much more since it's an active area of Travis' research. We promise to give a more in-depth analysis later! ;-) ]
We already briefly mentioned inductive power transfer... But this topic is special for Hizook. Wirelessly powered robot swarms were the subject of our very first Hizook post. It was also one of Travis' earliest publications, presented back at ICRA 2008 in a paper titled, "Surface Based Wireless Power Transmission and Bidrectional Communication for Autonomous Robot Swarms" (PDF and presentation slides).
This form of wireless power transfer will be immediately familiar to anyone who uses access control badges (eg. in their ID cards or for subway access). This is basically a low-frequency (125kHz) or high-frequency (13.56 MHz) RFID system where the coil under the table is the reader and the robots are the tags!
In this case... the non-resonant power transfer is actually preferable. One oft-omitted fact about resonant power transfer is that the presence of three (or more) coils will naturally de-tune the system. You can try to solve this problems in a few ways: sense the multiple peaks in the system response and transmit at those frequencies only, or try to do time-division multiplexing. Either way, it would add a lot of system complexity to a swarm application and would probably make inductive coupling preferable!
Regardless... Travis was little surprised (disappointed?) that Brent and Carrick didn't cite this work. He did a very similar analysis of the power availability above our coil(s), and it would have proved a very apt comparison. ;-)
No discussion of wireless power transfer is complete without discussing lasers! Between 2005 and 2009 (excluding 2008), NASA held a competition to beam power to a robot on a tether as part of the "Centennial Challenges program" to build a space elevator! In 2009, LaserMotive won the challenge (and $900,000) when their robot made it 900m up the tether. Today, LaserMotive is actively working to build wirelessly powered UAVs using similar light beaming techniques. Of course, there are other laser-powered airplanes too.
Anyway... Kudos to the folks at the NIMBUS Lab for showing us another new trick! Oh, and did we mention that University of Nebraska-Lincoln is our undergrad alma mater? Go Big RED! ;-)
This article was jointly written by Travis Deyle and Robert Powers.