I presented my project in May and will not be adding new posts. The information contained in this blog provides an overview of how a wireless power system may be built. There are posts describing some of the different ways I approached the problem and the results of those attempts. Interested readers may also find the relevant links I have posted useful for further information.
I did not make much of an attempt to clarify my posts as I was writing them because I primarily used this blog to organize my research for my own use. That being the case, please feel free to email if you have any questions.
Enjoy!
Friday, June 13, 2008
Wednesday, May 21, 2008
Final Implementation
The bulk of my time on this project was spent trying to build a transmitter. My assumption was that if I could simply generate a large enough signal, harnessing it would be the easy part. Unfortunately, I was not able to build an adequate transmitter and with the semester’s end rapidly approaching I chose to concentrate on building a working proof of concept model. In the end, my power source was a Sonicare toothbrush base. I built a demonstration model that consists of:
Receiving Coil
Rectification State
Power Storage Circuit
Mechanical Stage (demonstration)
Receiving Coil
Rectification State
Power Storage Circuit
Mechanical Stage (demonstration)
The receiving coil contains two levels of windings of 24 AWG magnet wire. The lower level (closer to transmitter) has 50 windings and the upper has 10 windings. This coil has an alternating current induced in it that must be rectified. I used a classic four diode H-Bridge configuration to achieve rectification. The power level at this point is quite low, hence the necessity of the power storage circuit.
The circuit I used is a modified version of a circuit designed by Mark Tilden for use in low power robotics. It is called a Type 1 Solar Engine because it was intended for use in solar powered applications. The basic operation of this circuit is to charge a 4700uF capacitor from a low power source. When the voltage across the terminals of the capacitors exceeds a predetermined threshold set by zener diodes the circuit switches from charging to discharging.
In this application, the capacitor charges on the secondary coil and discharges into a small micro-cassette recorder motor. The cycle takes approximately one second creating a clock-like movement as the motor is pulsed. Finally, through a series of gears and a rubber pulley belt, a small image is rotated.
Variations on Final Build
My first challenge was to build an oscillator. The oscillator is an essential feature of the power source. I intended to duplicate the Sonicare system and then make modifications to scale up the power output. Of course, there are numerous options when choosing an oscillator. My first choice was a Colpitts oscillator.
I chose the Colpitts because it easy to assemble from discrete components, inexpensive and simple to troubleshoot should problems arise once the circuit became more complex. A final benefit of the Colpitts is that it can maintain oscillations into the megahertz if necessary.
I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental.
At times it was difficult to initiate oscillation. Professor Joseph Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a soldered circuit board. When the Colpitts worked, the oscillations were stable and I measured 400mVPP on the output.
The next step was be to amplify this signal and drive a transmission coil. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely. I attempted to use multiple amplification stages of different designs. Unfortunately, none of them were effective. It seemed that any loading whatsoever on the output of the Colpitts made it collapse. I decided to abandon the Colpitts.
For the sake of brevity I will not go into as much detail about the subsequent design alterations. My next permutation was to use a 555 timer IC. The out put was very stable but not sinusoidal. I considered adding a low pass filter to the output to correct this. For the sake of simplicity, I replaced the 555 with a Wein-Bridge oscillator
Now that I had a stable oscillator, I focused my attention on an amplification stage. There were many variations. Ideally there would be two stages: voltage gain and subsequent current gain. I attempted this with FETs, BJTs, a combination of both and finally with Op-Amps. I was never able to generate enough gain to drive a significant amount of current into my primary coil.
I chose the Colpitts because it easy to assemble from discrete components, inexpensive and simple to troubleshoot should problems arise once the circuit became more complex. A final benefit of the Colpitts is that it can maintain oscillations into the megahertz if necessary.
I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental.
At times it was difficult to initiate oscillation. Professor Joseph Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a soldered circuit board. When the Colpitts worked, the oscillations were stable and I measured 400mVPP on the output.
The next step was be to amplify this signal and drive a transmission coil. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely. I attempted to use multiple amplification stages of different designs. Unfortunately, none of them were effective. It seemed that any loading whatsoever on the output of the Colpitts made it collapse. I decided to abandon the Colpitts.
For the sake of brevity I will not go into as much detail about the subsequent design alterations. My next permutation was to use a 555 timer IC. The out put was very stable but not sinusoidal. I considered adding a low pass filter to the output to correct this. For the sake of simplicity, I replaced the 555 with a Wein-Bridge oscillator
Now that I had a stable oscillator, I focused my attention on an amplification stage. There were many variations. Ideally there would be two stages: voltage gain and subsequent current gain. I attempted this with FETs, BJTs, a combination of both and finally with Op-Amps. I was never able to generate enough gain to drive a significant amount of current into my primary coil.
Tuesday, March 18, 2008
Rectification & Voltage Regulation
Rectification Overview: Will implement a full wave rectifier with Shottky Diodes (0.3V drop instead of 0.7v with standard diode). Since output will not be constant over time will need some sort of voltage regulator. Cannot use a transistorized regulator, maybe a shunt regulator?
Basically, when the current through the Zener is sufficient to take it into the breakdown region, it tends to regulate the voltage across it’s terminals.
Wednesday, March 12, 2008
Colpitts & Series Resonance
With the help of Prof. Pozzi I was able to observe clear indication of series resonance using my .003mH coil. At 9.1Mhz this coil and a 1pF capacitor were used. The resonant bandwidth of +-5% Vmax was very narrow with Vmax falling at +- 200kHz. Colpitts Oscillator – I would like to be able to build an oscillator for this project. I chose the Colpitts because it seemed fairly simple.
I was unable to get this circuit to oscillate on the first build. I trimmed the leads on all components to reduce unwanted impedances. I finally was able to observe a 2.1MHz sinusoidal oscillation in the above configuration. I also observed a 8Mhz signal and as high as 13Mhz by adjusting the values of C2 & C3. It is important to note that this circuit is very temperamental. At times it was difficult to initiate oscillation. Prof. Clark pointed out that stray impedances on the breadboard are problematic at high frequencies. He says I would get better results on a circuit board.
When the Colpitts worked, the oscillations were stable and I measured 400mVPP. The next step would be to amplify this signal. I attempted to use a 741 in an inverting configuration with a gain of 2. Unfortunately when the Colpitts was connected to the 741 it stopped working entirely at which point I gave up for the night.
3/17/08
Colpitts testing again - Was able build stable, functioning oscillator. I must’ve misplaced a capacitor during the last trial because the Colpitts worked perfectly this time. I attempted to slow down the oscillator using larger capacitors but was unable to attain kHz oscillations. Not sure why. Also, I added a capacitor in parallel with C3 (see above) with resulted in approximately 75% increase in amplitude at the same frequency. The observed oscillation amplitude was 2Vpp-3.5Vpp with a source of 10VDC-30VDC.
Colpitts Oscillator with loosely wound air-core coil – I connected one end of the transmission coil to the node between L1 and C2. The other end was free. With fo of 8.54MHz and Afo of 2Vpp I observed 1Vpp on the secondary coil. I was unable to observe resonance at this frequency since the capacitance required to match the 0.004mH inductor was 9E-11F which was unavailable to me (and might not exist).
Conclusion – I need to slow the Colpitts to a frequency that is more useful.
Thursday, January 31, 2008
Disassembly of Sonicare Electric Toothbrush
Secondary coil of toothbrush has two levels of coils, the second having a larger diameter than the first. One possibility for this coil design is to have the coil widen as the magnetic field widens - since this device operates in a fixed position when charging. The voltage measured across the the coil when in the charging position was in the hundreds of millivolts. The voltage measured across the battery charging terminals was approximately 2.21V. This would indicate that the secondary coil is not self resonant and requires support reactances for the circuit to achieve resonance. I was unable to disassemble the base, charging unit (transmitter) because it is encased in epoxy. I determined that the base unit oscillates at 80kHz.
Charging cell phone with Sonicare toothbrush –
measurements: Vterm=2.21V, Iterm =0.165mA
Saturday, November 17, 2007
Resonance Review 11/9/07
XL = XC, fres=(2*pi*sqrt(LC))^-1, resonance effected by resistance (shifts fres). Skin effect in inductor needs to be considered. Should use series RLC circuit to reduce impact of resistance. Not sure if a high Q circuit would be beneficial.
At resonance the series resonant circuit appears purely resistive. Below resonance it looks capacitive. Above resonance it appears inductive.
Impedance is at a minimum at resonance in a series resonant circuit.
Source: www.allaboutcircuits.com - 11.13.2007
At resonance the series resonant circuit appears purely resistive. Below resonance it looks capacitive. Above resonance it appears inductive.
Impedance is at a minimum at resonance in a series resonant circuit.
Source: www.allaboutcircuits.com - 11.13.2007
Battery Charging Research 11/9/07
Ideally, would like to be able to charge cell phones and other small computing devices that utilize Li-Ion batteries. I have seen wireless charging designs that include an external plug that is connected to the host device for charging. Initially I intended to devise a system that would be contained within the host device to manage charging. Although it would be far simpler to design an external plug than an internal charging controller. I know that Li-Ion tend to be more difficult to charge than the Ni-Cad and NiMH varieties. On this site:
http://www.seattlerobotics.org/encoder/200210/lithiumion.htm
the author researched charging methods and guidelines for Li-Ion. Bottom line is that Li-Ion require substantial charging controls to monitor temperature & current in addition to voltage. Charging must cease once the cells have topped-off or the cells can be damaged, destroyed or explode. The author had to design and build a custom charger that was able to communicate with Smart Li-Ion packs which use SMBus for communication between charger and battery pack – very complicated. I believe that cell phones have these smart charging controls onboard and the wall adapters are dumb sources.
I can still charge other types of batteries using the simple resistor charger but will need to utilize an external plug for charging a Li-Ion powered device.
http://www.seattlerobotics.org/encoder/200210/lithiumion.htm
the author researched charging methods and guidelines for Li-Ion. Bottom line is that Li-Ion require substantial charging controls to monitor temperature & current in addition to voltage. Charging must cease once the cells have topped-off or the cells can be damaged, destroyed or explode. The author had to design and build a custom charger that was able to communicate with Smart Li-Ion packs which use SMBus for communication between charger and battery pack – very complicated. I believe that cell phones have these smart charging controls onboard and the wall adapters are dumb sources.
I can still charge other types of batteries using the simple resistor charger but will need to utilize an external plug for charging a Li-Ion powered device.
Patent Research 11/5/07
Summary of US Patent 3938018 – Inductive Charging – One device.
Primary oscillator @ 10-40kHz. Secondary orientation is fixed, for use with one specific device.
Summary of US Patent 691718 – Increased Efficiency Inductive Charging.
Control module monitors secondary voltage and switches-in a plurality of coils to increase/decrease charging voltage. Theory is that different devices may require different charging voltages which will be controlled by this module located in the secondary circuitry.
Summary of US Patent 5959433 – Inductively Charged Battery Pack.
Not much info about design of coils. Good discussion of battery charging circuit.
Primary oscillator @ 10-40kHz. Secondary orientation is fixed, for use with one specific device.
Summary of US Patent 691718 – Increased Efficiency Inductive Charging.
Control module monitors secondary voltage and switches-in a plurality of coils to increase/decrease charging voltage. Theory is that different devices may require different charging voltages which will be controlled by this module located in the secondary circuitry.
Summary of US Patent 5959433 – Inductively Charged Battery Pack.
Not much info about design of coils. Good discussion of battery charging circuit.
Patent Research 11/2/07
Summary of Patent for “INDUCTIVE TOWER TRANSFER SYSTEM HAVING AREAS WITH HORIZONTAL FIELD”
Location:http://v3.espacenet.com/textdes?DB=EPODOC&IDX=GB2399230&F=0&RPN=GB2399229&DOC=ce973ee58c45ffbf7f38f1b17920c7973e&QPN=GB2399229
Physical Description:
Device utilizes two rectangular, perpendicular coils for the primary. Secondary is one or more conductive plates instead of coils. Secondary is backed by ferrous material. Primary charging field encircles charging area (field is parallel to charging surface), instead of a perpendicular field emanating from charging area. Coil 1 & Coil 2 have AC currents that are 90 degrees out of phase to create a “rotating magnetic dipole”.
Primary contains high permeability core to confine field. When a core of higher permeability is introduced, some filed lines will couple with secondary device.
Control Description:
Authors indicate that as devices are added to the charging area the net inductance is changed. Their goal is to allow up to six devices to charge simultaneously. To compensate, they suggest a capacitor bank which will switch-in capacitances to bring the device back to resonance. They theorize that the inductance will change in quantized levels allowing 6, fixed capacitances to be utilized.
Design Concerns:
1. Intensity of electromagnetic emissions is governed by regulatory limits.
2. Heating of nearby objects by magnetic field.
Location:http://v3.espacenet.com/textdes?DB=EPODOC&IDX=GB2399230&F=0&RPN=GB2399229&DOC=ce973ee58c45ffbf7f38f1b17920c7973e&QPN=GB2399229
Physical Description:
Device utilizes two rectangular, perpendicular coils for the primary. Secondary is one or more conductive plates instead of coils. Secondary is backed by ferrous material. Primary charging field encircles charging area (field is parallel to charging surface), instead of a perpendicular field emanating from charging area. Coil 1 & Coil 2 have AC currents that are 90 degrees out of phase to create a “rotating magnetic dipole”.
Primary contains high permeability core to confine field. When a core of higher permeability is introduced, some filed lines will couple with secondary device.
Control Description:
Authors indicate that as devices are added to the charging area the net inductance is changed. Their goal is to allow up to six devices to charge simultaneously. To compensate, they suggest a capacitor bank which will switch-in capacitances to bring the device back to resonance. They theorize that the inductance will change in quantized levels allowing 6, fixed capacitances to be utilized.
Design Concerns:
1. Intensity of electromagnetic emissions is governed by regulatory limits.
2. Heating of nearby objects by magnetic field.
Coil Design 10/26/07
Need to increase inductance of primary coil, want physical dimensions to be approximately 6” diameter and .25” high (flat pad for primary). Will obtain small gauge (24-30awg) magnet wire.
Inductance calculator:
http://www.66pacific.com/calculators/coil_calc.aspx
Inductance calculator:
http://www.66pacific.com/calculators/coil_calc.aspx
Estimate: N=1000, D=6”, Length=0.25” => L = 305mH=[k,
Reactance for this inductor @ 10kHz = 19k.
Capacitor to match this reactance would be .8nF for resonance. I think.
Reactance for this inductor @ 10kHz = 19k.
Capacitor to match this reactance would be .8nF for resonance. I think.
Experiment - 741 OpAmp On Primary 10/25/07
Submitted proposal for wireless charger.Boost Converter for secondary side?
DC-DC converter to increase voltage. Maybe do this instead of increasing primary side voltage.
boost converter schematic source: http://www.daycounter.com/LabBook/BoostConverter/Boost-Converter-Equations.phtml
Experiment 10/25/07 – Op Amp used on primary side.
Note: Op Amp rectification not likely since only power source on secondary is the secondary coil itself, unless secondary voltage is increased to 18V+.
Op Amp on primary side resulted in approximately 2Vp-p on secondary.
Equipment:
LM741CN
2 X 10Kohm Resistors
Diode (unknown spec)
Input for V+: 15VAC @ 10 kHz
Measured Inductance of coils: approx. 0.007mH
Theoretical Formula for this coil (flat spiral coil):
Approximate values used:
Coil mean radius (r) = .5 inch = 0.0127 meter
wire coil number of turns (N) = 10
Coil depth (d) = .1 inch = 0.00254 meter
Source: http://www.ajdesigner.com/phpinductor/inductor_equation_n.php
Did not measure voltage at primary coil, oops.
Coils: Both primary and secondary were loosely wound coils made from straight lengths of wire (approx 4 inches) wound to approx. 1 inch diameter.
Conclusion: At this secondary voltage level, a rectifier diode will operate.
Experiment - Initial Test 10/19/07
Experiment: Wireless Charger.
Goal: Demonstrate coil-to-coil induced voltage.
Materials: 2 80mH Inductors
Equipment: 12VAC power supply, Oscilloscope
Configuration: transmit 80mH inductor, receive 24mH (windings undetermined) in fixed positions approximately 2 inches center-to-center, approximately 0.5 inch radii.
Test #1:
Source: 12VAC, 1kHz (connected to primary coil).
Voltage induced on secondary: 50mVAC
Test #2:
Voltage induced on secondary: 100mVAC
Source: 12VAC, 100Hz (connected to primary coil).
Voltage induced on secondary: Negligible
Voltage induced on secondary: 100mVAC (same as @ 10kHz).
Conclusions:
Relationship between frequency on the primary side and the induced voltage in the secondary is due to resonance, this relationship is not linear. Coils are essentially RL circuits, need further analysis from this perspective.
Further Questions:
What is the ideal voltage, frequency, coils distance, geometry of coils relative to each other and of coils themselves (is an elongated solenoid the best shape? Maybe something flatter or secondary normal to primary.) Can secondary coil be minimized? What is ideal turns ratio? Effect of L needs to be determined. Is attenuation between coils linear?
Would need to determine a rectification scheme. Kelly Campbell suggests Op-Amp for low voltage rectification instead of diodes. This will not work because it would require more power on the secondary side than is available.
Estimated Timeline:
I – Initial prototyping.
I.1 [10/07-11/15/07] Design experiments to determine the following:
a. Ideal operating voltage for primary coil.
b. ideal operating frequency for primary coil.
c. ideal windings and turns ratio for primary & secondary.
d. ideal geometry for device; e.g. placement of coils relative to each other and ideal geometry of coils themselves).
e. impact on RL effect of coils on power transmission.
I.2 [11/16/07-12/15/07] Design, construct and test prototype device based upon preliminary findings.
I.3 [1/15/08-1/31/08] Design, construct and test rectification stage.
I.4 [2/1/08-2/15/08] Design, construct and test charging circuit for battery.
II – [2/16/08-4/08] Fine tuning of working device.
II.1 Scale-down secondary coil to fit into portable device.
II.2 Scale-down rectification & charging circuits.
II.3 Streamline primary circuit of charger and build enclosure or incorporate into existing enclosure.
Decided on a project 10/12/07
New idea for senior project: wireless battery charger. Air-core transformer, device (cell phone to demonstrate) charges when in close proximity to charging device (some sort of cradle for proper positioning if necessary). Secondary coil contained within battery pack.
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