Coin Detection Using Eddy Currents
Frank Lindauer, Momin Garrouch, Mohammad Alhusainan
Advisors: Dr. George Collins, Garrett Durland
Senior Design Website
Objectives
•
•
•
•
•
•
Demonstrate the use of eddy current to identify the denomination of United States
coins based upon measured changes in the magnetic field caused by induced eddy
current.
Design, test, and build a coin ramp and use it to detect and identify different coins.
Program microcontroller to linearize, offset, and scale data received from the photo
interrupter.
Integrate the sensors, LCD screen, and the microcontroller with the circuit.
Build and use an electromagnet to provide a larger, variable magnetic field.
Add a hall effect sensor to measure changes in the magnetic field strength.
Programing Flowchart
Metal Composition Of Coins
Denomination
Cent(Penny)
Nickel
Dime
Quarter
Composition
Copper plated Zinc
2.5% Cu
Balance Zn
Cupro‐Nickel
25% Ni
Balance Cu
Cupro‐Nickel
8.33% Ni
Balance Cu
Cupro‐Nickel
8.33% Ni
Balance Cu
Weight
2.5g
5g
2.268g
5.670g
Diameter
Thickness
0.750 in.
0.059 in.
0.835 in.
0.076 in.
0.705 in.
0.053 in.
0.955 in.
0.068 in.
•
•
•
Proper ramp height and incline angle.
Sensor location within coin path.
Choosing ideal location for magnets/electromagnet.
Ideal coin channel width.
Delivering a minimum magnetic field strength value of 1600 Gauss from
magnet/electromagnet.
Proper wire soldering to aid device mobility and avoid wiring damage to device.
Coin Fun Facts •
•
•
•
Design Constraints
•
Our coin identification system will be designed to work only with U.S. currency.
Minimizing the power consumption of the electronics.
Minimize physical size of electronics and circuitry in anticipation of small available
space in commercial application.
Overall project budget limited to ≤ $450.00, minimize per unit cost of electronics and
circuitry.
•
•
•
•
•
•
•
•
Eddy Currents are closed loops of induced current
circulating in planes perpendicular to the magnetic flux.
By Lenz's Law, the induced eddy currents in turn
produce a magnetic field that is opposite in direction to
the original field.
•
The Hall effect provides information regarding the type of magnetic pole and
magnitude of the magnetic field. The output voltage, called the Hall voltage, (VH) of
the basic Hall Element is directly proportional to the strength of the magnetic field
passing through the semiconductor material (output ∝ H).
•
Microcontroller records voltage change from sensor and
converts it to magnetic field strength.
•
Use the magnetic field strength / voltage
change data as another way to identify
the denomination of each coin.
Electromagnet Construction & Theory
Part
LCD Display (16x2)
Hard wire power supply
Microcontroller Board (Uno)
Coin Ramp
Wires and Connectors
Breadboard
Copper coil
PVC pipe
Welding rods (Electromagnet core)
Photo interrupter
Potentiometer
Hall effect sensor
Total
Cost
$17
$10
$27
$35
$10
$15
$20
$3
Figure A: Standard deviations of timing data by denomination and mint year
•
•
Etch circuit on circuit board.
Mechanically sort coins into different
bins.
Make system compatible with other
foreign coins.
Involve a high proximity sensor to
possibly detect coin composition.
$14
$2
$23
$8$
$184
The amount of flux available in any given magnetic circuit is directly proportional to
the current flowing through it and the number of turns of wire within the coil. This
relationship is called magneto motive Force or m.m.f. and is defined as:
. . . = ∗ [
]
•
Create higher strength magnetic field with electromagnet, then vary the field to find
the ideal field strength for each coin.
Bought commercial electromagnet.
Made multiple electromagnets:
• Large single EM ~ 500 turns
• Two small Ems ~ 75 turns each
Used different core material:
• Solid iron bolts, ¾” & 1” dia.
• Steel welding rods, 1/8” dia.
Used different power supplies:
• Bench top
• 12v battery
Figure C: Timing data by denomination
•
•
•
Recommendations for Continuation:
•
•
•
Data Analysis
These opposing forces work to slow the momentum of
the coin as they roll through the magnetic field.
By timing the coin as it passes through the magnetic
field, we can measure the effect that the induced eddy
current has on the coin and in turn identify the
denomination of the coin.
Budget
Electromagnet failed to deliver high magnetic field.
Potentiometer had minimal variation in the electromagnetic field strength.
Failed to minimize friction applied on the coins due to the width of the acrylic channel.
Some overlapping in coin readings still exist due to the following factors:
• Surface imperfections within the ramp channel.
• Similarities within coin geometry (Dime and Penny)
• Launching system creates variation in coin release speed.
• Fixed magnetic field is not strong enough to affect all coins.
Hall Effect Theory & Application
Around $10 billion in loose change sits idle in American households.
The world’s smallest coin weighed only .002 grams and was from Nepal.
The world’s heaviest coin was from Sweden and weighed 43 pounds 7 1/4 oz.
The life expectancy of a circulating coin is 30 years, while paper money usually only lasts
for 18 months.
Most coins now have a grooved edge because in the past dishonest traders filed down
the edges to remove some of the precious metal.
The first coins were developed independently in Iron Age Anatolia and Archaic Greece,
India and China around 600–700 BC
One and two pence British coins are copper‐plated steel. The five and ten pence British
coins are nickel‐plated steel. The steel‐plated cores of these coins are magnetic and
cause these coins to stick to the magnets in our system.
Eddy Current Theory & Application
•
Successfully induced eddy currents in coins .
Experimentally determined optimum angle for ramp that balanced eddy current force
and gravitational force.
Determined magnitude of fixed magnetic field necessary to affect coins enough to
differentiate them.
Developed algorithm to determine denomination of coin.
What Went Wrong
•
•
•
•
Design Considerations
•
•
•
•
•
What Went Right •
•
•
Conclusion
Figure B: Standard error of mean for each coin Figure D: Nickle timing data by mint year
Throughout the previous semester we struggled to correctly identify the correct coins
passed within the ramp. In this semester, we tried using different techniques to
differentiate between the four coins. Our data analysis clearly proves that we successfully
managed to identify all U.S. coins. Senior design class was a very fruitful experience where
we got the opportunity to apply our knowledge from different classes into one project. We
successfully managed to achieve almost all of the project objectives outlined in the
previous semester.
Acknowledgements
References
http://criterionndt.com/eddy‐current‐testing/eddy‐current‐theory
http://www.innospection.com/pdfs/Eddy%20Current%20Theory.pdf
http://www.electronics‐tutorials.ws/electromagnetism/hall‐effect.html
http://www.usmint.gov/about_the_mint/?action=coin_specifications
http://en.wikipedia.org/wiki/Eddy_current
http://www.electronics‐tutorials.ws/electromagnetism/mag13.gif?74587b
http://www.electronics‐tutorials.ws/electromagnetism/mag35.gif?74587b
We would like to extend our thanks to our Faculty Sponsor, Dr. George Collins, our Industry
Advisor, Mr. Garrett Durland, and our Senior Design Professor, Olivera Notaros for their
advice, guidance, support, and constructive criticism throughout the course of this
project. We would also like to thank Josh Olson for lending us his Arduino Uno board after
our Nano board burned out, the mechanical engineering lab for being generous with their
tools, the graduate students in the Electromagnetics Lab for their willingness to help, and
Professor Mario Marconi for his electromagnet advice and encouragement.