Target Material

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1 Perfect target material characteristics
From an electrical perspective, the perfect target material needs to have the highest possible conductivity. This will produce the highest eddy current flow, and hence the maximum change in sensor response based on target movement.
From a mechanical perspective, the target material should be physically stable and exhibit uniform movement without warping or tilting, which could be incorrectly measured as a position shift. Its thermal expansion coefficient should be as low as possible so that any temperature change does not change the physical size of the target.
2 Aluminum targets
Aluminum is an excellent target material for many reasons – it is reasonably inexpensive, light, strong, easy to machine, and also resistant to corrosion. Its high conductivity results in a skin depth only 25% larger than copper. It has a slightly lower temp-co than copper – 4200 ppm/°C vs. copper’s 4300 ppm/°C.
Anodized finishes, which are typically only a few microns thick, do not affect the performance of aluminum targets as long as the overall thickness of the aluminum is sufficient for the sensor frequency.
3 Copper targets
Copper is an excellent target material, with conductivity that is 95% of silver. It is it is heavier than and not as strong as aluminum, so from a mechanical perspective it is often not an optimum target material. Note that some copper alloys may have lower conductivity than aluminum.
Constructing a target as a copper region on a PCB is a good technique. Commonly available PCB fabrication can reliably produce features finer than 5 mils (0.125 mm) with 1 oz. copper on FR4. The common plating thickness of 1 oz. copper is 37 µm thick, which is one skin depth for a sensor frequency of 3.1 MHz. To obtain a larger response, the sensor frequency should be above 6 MHz with 1 oz copper. Thinner copper platings perform better with a correspondingly higher sensor frequency.
Duplicating the target design onto several layers will provide some improvement in response.
FR4 is an excellent substrate for the PCB target – it is strong, dimensionally stable, light, and has a low loss tangent at the frequencies used by LDC sensors.
If the sensor is a spiral trace on a PCB, then the thermal expansion of the PCB target has the added benefit of matching the thermal expansion of the sensor.
4 Steel & Magnetic targets
With the lower conductivity of steel (typically <10% of copper), the amount of eddy currents generated are reduced and the inductance shift of the target will be reduced, but depending on the application requirements, steel can still an effective target.
Magnetic steels, such as Stainless Steel 416, produce a significantly different inductance response across sensor frequency. This is due to the fact that at low frequencies, the magnetic field lines permeate the steel, which results in an increase in the sensor inductance. However, at higher frequencies, the skin depth becomes small enough that the eddy current generation on the surface of the steel blocks the magnetic field lines from entering, and so at these higher frequencies the inductance drops in a manner consistent with other conductive materials. It is important to note that there is a frequency at which the magnetic permeability and eddy currents balance and there is no noticeable inductance shift. This frequency is not stable and can shift due to outside influences, such as temperature. As a result, when using magnetic steel targets, either use a very low sensor frequency or a frequency above 1 MHz.