Birth of the Idea: hmmm.. it was actually a pretty circuitous route that led to the machine. The concept evolved from being able to design a vehicle that can divide itself. The initial idea was to fabricate a vehicle that can divide itself so that at one point of time it could be a family car and then divide itself into different parts where each part individually could become a vehicle that a person can drive. After some initial thinking and reading a lot of stuff right from bicycles to advanced cars, it was soon realized that it required a lot of intricacies in terms of designing an independent suspension and transmission mechanism, which is not so difficult if we do not care about user interfaces and efficiencies but coupling them would be difficult. And of course size does matter !!

But then this evolved to designing a smaller scaled down version – but then it is not a problem anymore for simply have four different motors for the different parts !!!

So far so good.. But during this time I was also reading about transmission of power for the different parts of the car and here I found stuff about wireless power, which could remove all the different wires required for signaling and similar things. But this was just the starting point…

The more I thought about it.. the more different applications I could think for it.. so finally it became the main topic itself… designing a wiressly powered car.. and ofcourse it was just a small piece of a bigger story… In most of the fabrication machines, the tools move on a spindle and generally have wires for signal and power. If we could possibly remove all these wires, and replace the motor with small robots then we could have a lot more degrees of freedom and a lot more work done, instead of always trying to orient the object in the right direction !!

So here was I designing a fabrication machine powering mechanism to power small robots which would be

- Re-configurable

- Remotely programmable

- Provide a platform for things to work on

- Different sensors and control systems to automate most of the tasks

- Small in size so as to allow maximum work area

 

Design

______________

Different ways to do wireless power were thought of and read that ranged especially in the frequency of operation and the way power was transmitted. Some of the more prominent ones for which some of the resources are available are:

a) Wireless Power at 2.4 GHz [8]

b) Using the kind of technology used in RF-ID [7]

c) Using principles from the early experiments as done by Tesla at low frequencies [3]

d) Optical Transmission of Power possibly through some kind of optomechanical devices that convert the energy into more useful forms

e) Inductive coupling in the near field through some oscillating circuit

The most intriguing was of course the one at 2.4GHz. So this was the first one that was tried. After doing preliminary calculations for the amount of power that may be required for transmission and after unsuccessful attempts in making a good antenna, the approach was finally abandoned (for later time) for greener pastures…[8] [7] [5].

Another interesting hack that was tried was using a CISCO network LAN card and adding a power amplifier to it to jack up the power being transmitted. Optical transmission was not considered, as it does not prove to be a good means of transmitting power especially when the robots may be busy doing work and may not be in the line of sight for power transfer.

Thus I finally landed up with the inductive coupling method where I had to generate a high electric field and use the power in its near field to wirelessly power devices. One of the possible ways to do it is through a corona discharge.

Corona discharge is a dielectric breakdown of air, similar to a lightning bolt. The formation of corona discharge requires a very high voltage over a short distance, specifically, at least 20kV/cm. Most of the current implementations for generating a corona discharge use vaccum tubes to develop the high voltages required to generate corona discharge. However some initial attempts have been made to use solid-state devices to generate the corona discharge where it was used to design high frequency plasma audio speakers.

The circuit consists of a high voltage system responsible for creating the corona discharge. The circuit has the following block diagram [5] :

The resonant coil, which acts like a parallel resonant circuit, is used to transform small current fluctuations into the large voltages necessary to sustain corona discharge. For handling high voltages different MOSFET devices were tried and ultimately STP3NB80 FET was used based on its characteristics. Among the characteristics that were found to be important were that the transconductance of the device has to greater than 2 [5], and a low gate capacitance.

Also unlike the vacuum tubes which have a very low gate capacitance, a FET requires a gate driver to force charge into and out of the gate in order to turn the FET on and off rapidly. For this TPS2814P dual gate driver was used. The figure below gives a schematic of the circuit diagram used in the project.

The final corona generation circuit using one STP3NB80.

Another important component of the project is the resonant coil. The resonant col behaves like a parallel resonant circuit as show in the figure below. The lower the series resistance (R), the lower the losses, and the more the coil behaves like a parallel resonant circuit with no losses. At the resonant frequency, the inductive and capacitive reactances nearly cancel each other out, and the impedance of the coil looks like a near-open circuit (high AC resistance). Putting a small current across a near-open circuit produces the necessary very high voltages required for corona discharge

But designing such a coil was not easy. Ultimately a 4.5-inch PVC pipe was used to serve as the coil. The turns were hand wound onto it until I was able to achieve a resonant frequency of around 5 MHz. The resonant frequency of the coil was tested using an Agilent Network Analyzer.

Having built the circuit, the initial testing was successful. Infact to test the resonance of the circuit, I would often keep a 22W florescent tube next to it which would automatically start glowing, especially when the circuit was resonating. Though I had a start switch to initiate the resonance, many times due to the low voltage being applied (~70Volt DC), as compared to other circuits an input square wave of MHz was used to initiate oscillation. Once the oscillation has been initiate sustaining the resonance was not a problem.

After constructing the wireless power generator it was the turn of the receiver that could extract the power present in the environment which too proved to be a substantial task. I had the initial thought that deriving continuous power would be difficult so one can charge up the capacitor and use the power from the capacitor intermittently. Different circuits were tried, some of them borrowed from another field of robotics called beam robotics which uses solar cells to charge the capacitors. So I added another inductor with the same resonant frequency followed by a rectifier circuit and a capacitor to reduce ripples. But they didn’t work out. Schematics and pictures of some of them are given below.

After doing some of the circuits, it was realized that something is wrong in terms of the way I am extracting power and did a bit of theoretical calculations to ultimately realize that a single big loop works best and adding more turns decreases the power available!!! (even though it increases the rate of change of flux). However debugging the circuit had some interesting results:

a) Never use a breadboard for high frequency circuits. The connections have a good bit of hidden capacitance which really plays havoc with your circuit

b) Its better to keep the oscilloscope away from all the noise even that generate by the circuit itself. Initially I had my oscilloscope on my desk and it did pick up quick a bit of signals even when it was not connected to anything due to the high electric field being generated. So a lot of the readings were error prone until I moved the oscilloscope away from my work bench

c) Also it helps a lot to coil the two probe leads of the oscilloscope (incase you are using two channels). That does cancel out the inductance in the leads themselves and gives you a better reading.

But All well’s that ends well… so when I finally used a simple large loop attached to a motor… one of those tiny ones salvaged from a toy car.. it was able to deliver more than enough power to power the motor continuously ..infact the motor gets heated due to the high current being passed through it now.

Me and my project

Another aspect of the project that I would be continuing is to design the micro-robot. What kind of a platform would be suitable for doing a variety of tasks as required in machining… a walking robot? a platform? A wheeled robot? What would be a suitable architecture… do one requires an OS for handling all the different scheduling tasks? What’s the kind of processing power that is required Having read quite a bit of literature related to the field [9], now it’s the time for action J

So now I am proceeding to develop a full version of the fabrication machine that would be using this kind of a mechanism. Among the important things that I have to do is to use the corona for actual fabrication purposes possibly like the laser cutter.

References and Bibliography:

1. Kong, Jin Au and Shen, Liang Chi, Electromagnetics and its Applications, Third Edition.

2. US Patent 4,464,544. Klein, Corona-Effect Sound Emitter, 8/1984

3. FCC Regulations – Radio Frequency Devices. http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/cfr/1998/47cfr15.pdf ; Section 15.223: Operation in the band 1.705-10MHz. See Appendix 6

4. http://www-s.ti.com/sc/psheets/slvs132d/slvs132d.pdf -- Gate Driver Data sheets, page 15. See Appendix 4.

5. Alejnikov, Blattner & Joye [2002]. Developing Ion Tweeters using a Corona Discharge

6. Fletcher, Rich [2002]. Low-Cost Electromagnetic Tagging: Design and Implementation. PhD Dissertation.

7. Fletcher, Rich [1997]. A Low-Cost Electromagnetic Tagging Technology for Wireless Identification, Sensing, and Tracking of Objects. SM Thesis.

8. Martinez, J. U. [] Wireless Transmission of Power Sensors in Context Aware Spaces. SM Thesis.

9. Hollar, Austin [2000] Costs Dust. Master’s Thesis

There are a lot lot more documents than I could possibly list here so incase you want to know more… mail me at iinvent@mit.edu