Datasheet

Target Metal

Surface

Conductance

of Metal

Ls + L(d)

Rs + R(d)

Distance d

Eddy

Currents

Metal

Target

LDC1000

SNOSCX2 –SEPTEMBER 2013

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THEORY OF OPERATION

Inductive Sensing

An AC current flowing through a coil will generate an AC magnetic field. If a conductive material, such as a metal

target, is brought into the vicinity of the coil, this magnetic field will induce circulating currents (eddy currents) on

the surface of the target. These eddy currents are a function of the distance, size, and composition of the target.

These eddy currents then generate their own magnetic field, which opposes the original field generated by the

coil. This mechanism is best compared to 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 distance and shape. Hence the

resistance and inductance of the secondary core (Eddy current), shows up as a distant dependent resistive and

inductive component on the primary side (coil). The figures below show a simplified circuit model.

Figure 7. Inductor with a Metal Target

Eddy currents generated on the surface of the target can be modeled as a transformer as shown in Figure 8. The

coupling between the primary and secondary coils is a function of the distance and conductor’s characteristics. In

Figure 8, the inductance Ls is the coil’s inductance, and Rs is the coil’s parasitic series resistance. The

inductance L(d), which is a function of distance d, is the coupled inductance of the metal target. Likewise, R(d) is

the parasitic resistance of the eddy currents.

Figure 8. Metal Target Modeled as L and R with Circulating Eddy Currents

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