Introduction

Within the Experimenting with magnetic components challenge I am conducting experiments to develop an intelligent coin discriminator using inductive sensitivity.

In this first experiment I want to see how the proximity of different euro coins affects the impedance of an inductor that is generating an alternating magnetic field. It is assumed that when a metallic coin approaches a variable magnetic field, currents are induced in the coin, called eddy currents. The eddy currents induced inside the coin will modify the electrical impedance of the inductor which will result in variations of the amplitude and phase of the signal. This mechanism looks like a transformer, where the coil is the primary core and the eddy current is the secondary core. The inductive coupling between both cores depends on the distance and the shape. Therefore, the resistance and inductance of the secondary core (eddy current), is shown as a distance-dependent resistive and an inductive component on the primary side (the coil).

Mostly amplitude variations are related to the coin conductivity whereas frequency variations relate to its magnetic permeability. In the absence of variations in distance and angle of position of the coin, the two parameters can help us to discriminate the type of metallic material in front of the coil.

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Smart Coin Sorter by Inductive Sensing. #01 How Coins Change Coil Impedance.

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The system will be composed of a simple coil that will perform the excitation and measurement functions. I will study the dependence of the resistance and inductance of the coil with the distance between the coil and the coin.

The euro coin series

For the different experiments I will observe the response of the system to the proximity of the different currencies of the complete series of euro coins.

The euro coin series comprises eight different denominations: 1, 2, 5, 10, 20 and 50 cent, €1 and €2. The euro coins have a common side and a national side. The national side indicates the issuing country.

Magnetics properties

• €1 and €2 coins: Their inner part is slightly magnetic. The outer part has no magnetic properties.
• 10, 20 and 50 cent coins: They have no magnetic properties.
• 1, 2 and 5 cent coins: They are highly magnetic.

Impedance measurement. Analog Discovery 2 Impedance Analyzer

During the experiments I will use the Analog Discovery 2 Impedance Analyzer AdapterAnalog Discovery 2 Impedance Analyzer Adapter adapter for impedance measurement. The Impedance Analyzer Adapter helps you measure complex electrical impedance as a function of the test frequency. The impedance analyzer takes sensitive measurements of both current and voltage are applied to the device under test (DUT) while the measurement frequency is varied.

Schematics:  https://digilent.com/reference/_media/reference/instrumentation/analog_discovery_impedance_analyzer_sch.pdf

Inductor RLB1112V4 Series 400 Volt Radial Inductor

In these first experiments I use an unshielded bourns radial inductor. The RLB1112V4 Series 400 Volt Radial Inductor.

It is a radial lead through-hole power inductor with ferrite core made with enameled copper.

https://www.avnet.com/shop/emea/products/bourns/rlb1112v4-102j-3074457345642083414/

I have not found the reference in the farnell store but I did find another Bourns inductor of the same series the RLB1112V4 RLB1112V4

Datasheet: https://www.bourns.com/docs/Product-Datasheets/rlb1112v4.pdf

Features

• 400 VDC rated
• Shrink tubing protected winding
• Fixed lead spacing
• RoHS compliant

Applications

• DC/DC converters
• Power supplies

EXPERIMENTS

#E01 Impedance measurements for inductor RLB1112V4 Series 400 Volt Radial Inductor. Coins @ 1mm

In this experiment I measure the change in the impedance of the inductor in the presence of the euro series coins placed horizontally on the coil at a distance of 1 mm.

I have used Lego to make a construction that allows to position the coin and keep it fixed during the experiment at the desired distance.

Impedance (symbol Z) is a measure of the overall opposition of a circuit to current, how much the circuit impedes the flow of charge. It's like resistance, but it also takes into account the effects of capacitance and inductance. Impedance is measured in ohms (ohms).

The effects of capacitance and inductance vary with the frequency of current passing through the circuit and this means that impedance varies with frequency. However the effect of resistance is constant regardless of frequency.

The capacitance and inductance cause a phase shift between the current and voltage which means that the resistance and reactance cannot be simply added up to give impedance. The Phase Shift is how far the function is shifted horizontally from the usual position. In our case it means that the current and voltage are out of step with each other.

As a reference, I study the response at the same frequencies that have been used in the inductor data sheet to indicate its electrical characteristics.

From the RLB1112V4 Series 400 Volt Radial Inductor datasheet

• @ 1 kHz / 1V / 21 ºC / Averaging 500ms / 1 mm separation

Reactance (symbol X) is a measure of the opposition of capacitance and inductance to current. Reactance varies with the frequency of the electrical signal. Reactance is measured in ohms (ohm).

Inductive reactance, XL is small at low frequencies and large at high frequencies. For steady DC (frequency zero), XL is zero (no opposition), which means that inductors pass DC but block high frequency AC.

Capacitive reactance (Xc) is large at low frequencies and small at high frequencies. For steady DC which is zero frequency (f = 0Hz), Xc is infinite (total opposition), which means that capacitors pass AC but block DC.

Coin discrimination based on phase and impedance change.

• @ 252 kHz / 1V / 21 ºC / Averaging 500ms / 1 mm separation

Coin discrimination based on phase and impedance change.

In this case the discrimination of the coins becomes somewhat more difficult than at the 1 kHz frequency.

• @ 1.1 MHz / 1V / 21 ºC / Averaging 500ms / 1 mm separation

Coin discrimination based on phase and impedance change.

This is near the self resonant frequency for the RLB111V4

How does the proximity of the coin affect the self-resonant frequency?

• Impedance and phase in the neighborhood of the self resonant frequency for the RLB111V4 SRF(MHz) 1.397 MHz @ 20ºC
• Shift of the self-resonant frequency when approaching a 1 cent euro coin. SRF shifts from 1.397 MHz to 1.4857 MHz
• Shift of the self-resonant frequency when approaching a 2 cent euro coin. SRF shifts from 1.397 MHz to 1.448 MHz
• Shift of the SRF for the entire series of euro coins. Impedance and phase in the neighborhood of the self resonant frequency for the RLB111V4.

#E02 Impedance measurements for inductor RLB1112V4 Series 400 Volt Radial Inductor. Coins @ 5mm

For the second experiment I increase the distance of the coins to 5 mm

Again I take measurements, looking mainly at the characteristics that allow me to discriminate the coins.

• @ 1 kHz / 1V / 21 ºC / Averaging 500ms / 5 mm separation

And I compare with the one obtained for 1mm of separation. It is clear that the distance affects the impedance quite a lot and that the coin discriminator should limit both the angular position and the distance to the inductor.

#E03 Impedance measurements for LC tank with inductor RLB1112V4 and several Capacitors

Generating an alternating magnetic field with just the coil consumes a large amount of energy. This consumption can be reduced by adding a parallel capacitor, turning the circuit into a resonator. In this way, power consumption is reduced to inductor losses and eddy currents only.

Introduction to LC Tanks

In the two previous experiments I have made impedance measurements with just the inductor and a series resistor to be able to measure the circulating current. In this experiment I am going to use different configurations with capacitors parallel to the coil.

This is named a parallel LC circuit, also called a resonant circuit, tank circuit, or tuned circuit. It is an electric circuit consisting of an inductor, represented by the letter L, and a capacitor, represented by the letter C, connected together.

If an inductor is connected across a charged capacitor, the voltage across the capacitor will conduct a current through the inductor, creating a magnetic field around it. The voltage across the capacitor drops to zero as the current flow consumes the load. At this point, the energy stored in the coil's magnetic field induces a voltage across the coil, because the inductors oppose current changes. This induced voltage causes a current to begin to recharge the capacitor with a voltage of opposite polarity to its original charge.

The EMF driving the current is caused by a decrease in the magnetic field, so the energy required to charge the capacitor is drawn from the magnetic field. When the magnetic field has completely dissipated, the current will stop and the charge will be stored back in the capacitor, with the opposite polarity as before. Then the cycle will start again, with the current flowing in the opposite direction through the inductor.

There is a resonance effect when an LC circuit is driven from an external source at an angular frequency ω0 at which the inductive and capacitive reactances are equal in magnitude. The frequency at which this equality holds for the particular circuit is called the resonant frequency. The resonant frequency of the LC circuit is

where L is the inductance in henries, and C is the capacitance in farads. The angular frequency ω0 has units of radians per second.

I carry out different tests with a series of capacitors and study the frequency response of the different LC tanks, keeping the inductor fixed.

Components used in the experiment

Series of capacitors used in the experiment: 1 nF, 2.2 nF, 3.9 nF, 5.6 nF, 33 nF, 68 nF, 220 nF and 1000 nF

For the experiment I put the coil and the capacitor to be tested on the impedance analyzer board.

In the picture the RLB1112V4 radial inductor parallel to a 2.2 nF polyester film capacitor

Impedance response to the frequency of the different LC tanks.

As the capacitor capacity increases, the resonant frequency of the LC tank decreases.

I take data from the resonant frequencies and compare them with the theoretical resonant frequencies assuming an ideal inductance of 1 mH.

LC tank C= 220 nF / Resonant Frequency Shift bringing a coin closer to 1mm in front of the coil. For the entire series of euro coins.

Finally, I study the change in the resonant frequency when bringing coins to 1 mm of the inductor of an LC tank composed of the 1 mH inductor and a 220 nF capacitor.

#E04 Coin discrimination with LC Tank L=1mH C=56nF @ Resonant Freq

For the next experiment I build an LC tank with a 56 nF capacitor and a 1 mH parallel inductor. The theoretical resonant frequency is 21.27 kHz

Components

• RLB1112V4 Series 400 Volt Radial Inductor L = 1mH
• Polyester Film Capacitor C = 56 nF

Theoretical Resonant Frequency L= 1mH C= 56nF 21.27kHz. Measured =

Impedance measurement of the system

• @ 21.32 kHz / 1V / 20 ºC / Averaging 500ms / 1 mm separation

Coin discrimination based on phase and impedance change.

Conclusion

I have reviewed concepts that I barely remembered about electromagnetism and analog electronics and now I understand better.

I have verified that the impedance of the system varies differently when approaching different euro coins to the inductor when it is producing an alternating magnetic field and I will be able to use this differentiation to feed the classifier that will allow to discriminate the coins.