The Good, The Bad, and The Ugly

The Basics
Inductive 'eddy current' displacement sensors require an electrically conductive target. The coil is excited by a high frequency AC signal. The less resistive the target, the more eddy currents are produced in the target. This improves sensitivity and signal to noise ratio, ultimately increasing resolution.


Materials that have been successful targets for eddy current sensors vary in resistance from 1.59 µOhm-cm for silver (good), to 127 µOhm-cm for Inconel (can be bad). Depending on application requirements, even graphite at 1050 µOhm-cm (usually ugly) has been used for a target.


Magnetic / Non-Magnetic Distinction
As they pertain to eddy current position sensors, all conductive material can be categorized into two groups, magnetic (permeability >1), and non-magnetic (permeability of 1). Note that for eddy current sensors, permeability of up to 1.1 is still considered non-magnetic.


A small diameter sensor (<0.5") set up for a magnetic target will perform poorly (if at all) with a non-magnetic target, (and vice-versa). In most cases electronic bridge components must be changed in order to convert a system set up for magnetic targets to be used with non-magnetic targets. Large diameter sensors (>0.5") typically need only to be recalibrated to the new target material. It is best to check with Kaman's Applications Engineering Group, or Service Group before considering any target changes.


Kaman can provide sensors calibrated to your specific target material. In some instances the exact target (shaft, disc, stage component) should be provided to ensure best possible system performance. When no target material is specified, non-magnetic systems will be set up and calibrated using a 6000 series aluminum target. For magnetic systems, 4130 steel is used.


Various target materials and their resistivity values are listed below. Materials underlined are considered magnet targets. This is by no means a complete list of materials that can be successfully used as targets with eddy current sensors.


Material Resistivity µOhm-cm
Silver 1.59
Copper 1.7
Gold 2.35
Beryllium 4.3
Aluminum 4.5
Molybdenum 5.17
Iridium 5.3
Tungsten 5.51
Cobalt 6.24
Zinc (cast) 6.5
Brass 7.4
Cadmium 7.54
Nickel 7.85
Bronze 9.8
Platinum 10.6
Palladium 10.8
Tin (cast) 10.8
Magnesium 13.6
1030 Steel 14
Carbon Steel 17.5
Lead 22
Cobalt Steel 28
Chrome Steel 29
Tungsten Steel 30
Uranium 37.5
Mu-metal 60
400 Series SS 60
416 Steel 60
Iron (cast) 65
4130 Steel 65
200 Series SS 69
300 Series SS 72
17-4PH SS 100
Titanium 113
Inconel 625 127
Inconel 718 127
Inconel 725 127
Graphite 1050


Kaman's Applications Engineering Group is available to evaluate your application.






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