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Scott Fusare

Fri Sept 11 2009, 10:36PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

I am not quite sure what you mean Strider? Have you read the papers, they are indeed claiming novelty here. Certainly they think it novel enough to apply for a US patent, which hopefully will not be granted.

They never actually use the phrase "evanescent resonance", unless there is a new paper out there that I am unaware of. What they are claiming is "evanescent" mode coupling of the resonators. At the frequencies and distances we (and they) are talking about "evanescent" is the same as "near field" or "induction field" or "reactive field". They are playing games with semantics, whether it is intentional or not is a matter up for debate.

As for making use of "a good concept" in a "good project" I am all for that but one should not claim novelty or uniqueness were it does not exist. As an example - Look up remote powering of sub-cutaneous implants and you will find that the medical community has used this technique for power transfer for 3-4 decades now.

I realize I am ranting about this. I just find it shameful that the paper published in Science passed peer review. A less prestigious institution would have been (hopefully) taken to task over their re-invention of the wheel. All from the EE department of my country's (arguably) most prestigious engineering school and from a native Croat no less!

As an aside, to anyone who has read the paper, look at the equation presented for coupling and ponder the physical implications of it....

strider

Fri Sept 11 2009, 11:23PM

Registered Member #2341 Joined: Mon Sept 07 2009, 12:16PM
Location:
Posts: 9

I have read all their papers about 50 times and know what they claim and said, and about patents they have already 2, yeah they claim uniqueness cause if you can find any other application that make use of evanescent field with magnetic resonance for wireless power we can say that its a shame for MIT.. by the way MIT concept comes from concepts which were ignored in the past like coupled mode theory, evanescent tunneling over mid range (not small ranges like in optics), near field non radiative region (which was ignored when designing antenna and we always try decrease its region to avoid interference)...So for me they did a very great job

Marko

Sat Sept 12 2009, 12:40PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys,

I really didn't expect this thread to receive so much attention. I thought a single project thread should be for a single person's project, but I don't really have anything against some hijacking myself :P

A larger high Q system was eventually built that delivered 10s of watts at ~70% transfer efficiency over a 20 cm gap.

Scott,
Could you post some more information and pics of this system?

I have read all their papers about 50 times and know what they claim and said, and about patents they have already 2, yeah they claim uniqueness cause if you can find any other application that make use of evanescent field with magnetic resonance for wireless power we can say that its a shame for MIT.. by the way MIT concept comes from concepts which were ignored in the past like coupled mode theory, evanescent tunneling over mid range (not small ranges like in optics), near field non radiative region (which was ignored when designing antenna and we always try decrease its region to avoid interference)...So for me they did a very great job

''Resonant induction, evanescent wave coupling/tunneling", etc, really just describe an air cored transformer. I see no point making it more complex than that.
We add power factor correcting capacitance to these transformers to compensate their extremely low magnetizing inductance, which would otherwise be very prohibitive to drive. The resulting LC circuit can be useful for a self resonant oscillator as well.

Marko

Scott Fusare

Sat Sept 12 2009, 02:04PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Hi Marko,

It's probably obvious that this topic is one I like to go on about.

I hadn't meant to hijack your thread,
please pardon the breech in etiquette. Should we move the discussion elsewhere?

This technique was well suited to a particular design problem I had at work, so I ended up
spending quite a bit of time looking at the MIT group's work as well as what preceded them.

Come Monday I'll dig up some pictures of the larger system but in the interim I can describe it
as best as my memory will allow. For EMC compliance reasons I lowered the operating frequency to 150 kHz.
The inductors were roughly 15 cm in diameter planar wound from medium gauge Litz wire. I measured
the unloaded Q of the resonators at ~330 as I recall. The tank circuit was completed with poly film capacitors.
A 20 watt class E oscillator was series coupled to the tank via a impedance transformer wound on
a ferrite binocular core. The "receiver" side was identical with the addition of a trimmer cap for fine tuning.
High efficiency demanded running with tighter than critical coupling. I don't recall exactly what is was,
I'll consult my notes on Monday.

I agree with your observation on terminology. I don't think I am doing a good job of presenting my case to Strider though. I'll have to post something more detailed.

strider

Sat Sept 12 2009, 02:33PM

Registered Member #2341 Joined: Mon Sept 07 2009, 12:16PM
Location:
Posts: 9

First of all thank you all for this great discussions and I am sorry if my posts cause any misunderstanding as I mean no offense at all...

Marko can you please tell me the diameter of the copper tube you used at the transmitter side..

Also I had established some great analysis papers of my work which is mainly build upon MIT work and I had also some great simulation results (using Comsol 3.5a RF module) to support the theory and results, and as I stated before I am still working on the concept and every new idea and discussion I have will help me a lot as I am always learning new things and I am honored to have it with you guys here..

Dkauf

Sun Oct 11 2009, 03:33AM

Registered Member #2421 Joined: Tue Oct 06 2009, 01:58AM
Location:
Posts: 2

Hey very cool project !
It seems you putï»¿ 2 capacitors at the receiver coil (electrolytic "black" and polyester "blue"). Can you tell me their values ?

Thanks,

Sulaiman

Sun Oct 11 2009, 09:09AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Nice project.

Just a couple of 'warnings'

1) These devices contravene most regulations as they are deliberate radiators of significant rf power

2) Be sure not to have any important rfid devices nearby..e.g. in the back of my UK passport

Scott, if you removed the inner part of your pcb antenna leaving only the outer few turns you would have a much higher Q antenna, something like the photo above.

Scott Fusare

Sun Oct 11 2009, 12:09PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Hi Sulaiman,

Agreed on the regulations, although using the word "radiator" contributes to the misunderstanding of the principal involved. Pardon my being pedantic. The fields generated in the high power, across the room demos (MIT) will make the authorities apoplectic!

As to your comment on the printed inductor - I guess you are right about the inner turns contributing little but loss. I must admit this wasn't well thought out but rather a quick demo for management. However, the frequency being fixed, it seems to me that as you eliminate inner turns you will pass through a maximum Q point and then begin to descend as your inductance drops. I would think that the "few outer turns" would be in this regime. An interesting optimization problem.

Ultimately my application would have needed field shaping magnetics and also been operated above critical coupling for reasonable efficiency, both will kill my loaded Q anyway.

Scott

Marko

Sun Oct 11 2009, 04:57PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Dkauf wrote ...

Hey very cool project !
It seems you putï»¿ 2 capacitors at the receiver coil (electrolytic "black" and polyester "blue"). Can you tell me their values ?

Thanks,

There are no electrolytic capacitors anywhere in the circuit, the round object on the receiver side is a 6uH choke I mentioned before in my post. It's redudant if higher impedance load is chosen (like 3x 12V bulbs in series). The blue capacitor also isn't polyester but metallized polypropylene, chosen for suitable resonant frequency. In this case it's 47nF, if I recall.

Marko

Dkauf

Mon Oct 12 2009, 07:55PM

Registered Member #2421 Joined: Tue Oct 06 2009, 01:58AM
Location:
Posts: 2

Marko wrote ...

Dkauf wrote ...

Hey very cool project !
It seems you putï»¿ 2 capacitors at the receiver coil (electrolytic "black" and polyester "blue"). Can you tell me their values ?

Thanks,

OK Thanks and congrats again, very cool stuff.-

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