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

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.

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.

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.

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

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.

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:

Source: 12VAC, 10kHz (connected to primary coil).

Voltage induced on secondary: 100mVAC

Test #3:

Source: 12VAC, 100Hz (connected to primary coil).

Voltage induced on secondary: Negligible

Test #4:

Source: 12VAC, 10MHz (connected to primary coil.

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.

Considerations: Must be low power to reduce chance of interference. Need to ensure will not induce currents in device circuitry. Or, don’t worry about using in a device; just concentrate on charging wirelessly for now. Companies currently developing this concept: NTT DoCoMo (in Japan with Panasonic), SlashPower LTD (www.slashpower.com).