When no target material is specified, nonmagnetic systems will typically be set up and calibrated using a 6000 series aluminum target. For magnetic systems, 4130 steel is typically used.
Sensor Diameter vs. Target Diameter Relationships
Calibrated range-sensor diameter
The calibrated range is typically the first specification to be considered when selecting an eddy current sensor for position/displacement measurements.
The sensor itself contains a precision-wound wire coil that is excited by a high-frequency AC signal. This produces a toroidal-shaped oscillating electromagnetic field with the coil itself at the center of the toroid. As with all electromagnetic coils, the strength of the field produced is proportional to the distance from the coil.
Kaman displacement sensors use the oscillating electromagnetic field to induce eddy currents in the surface of a conductive “target.” Since the magnitude of the eddy current is proportional to the field strength, the amount of eddy current induced in the target decreases with increasing distance between the sensor and the target. The effective measuring range of a given coil is typically equal to 30 percent to 50 percent of the coil diameter.
Sensor diameter-target diameter
For this discussion, the coil diameter and sensor diameter are interchangeable. The electromagnetic field produced by unshielded sensors extends radially from the sensor to a diameter equal to 2.5 to 3 sensor diameters. For shielded sensors, the field diameter extends 1.5 to 2 sensor diameters. Any electrically conductive material in this field will have eddy currents induced into its surface, thereby affecting sensor output.
Sensors are typically calibrated by the manufacturer using a flat target based on the thumb rules described above. To ensure the
sensor will perform to the manufacturer’s published specifications, the same target diameter thumb rules should be followed in actual application.
Nice discussion, Dan. What type of applications are most appropriate for this Dan? In the applications where these sensors are used, how do they perform when compared to what was used before?
Eddy current sensors are used in about any industry you can mention. Typically they are selected when the measurement needs to be non-contact, and the environment precludes using capacitance or laser based non-contact sensors.
Eddy current advantages include 1) remote electronics - means the probes can be designed to withstand high heat, up to 400F for most standard probes and we have probes that will withstand 1000F continuous 2) dirty environments - we've had sensors inside working combustion engines, we've had them buried in the shoes of thrust bearings looking at oil film thickness and wedge shape, we've also had them in operating liquid O2 and H2 pumps looking at shaft vibration 3) they are unaffected by dust, grease, humidity etc that can get between the probe and the target.
Laser displacement sensors are self contained meaning the sensing head contains the electronics also, this limits the operating temp and increases the required space to install them. They do have the advantage of a small spot size (small target area), and large stand-off (don't need to be that close to the target). Capacitance works best in clean applications as anything that changes the dielectric between the probe and target will change the output. Humidity, dust etc are not friendly.
We have sensors in the cleanest of environments, like semiconductor processing equipment, and in some of the hashest environments, like plastic injection molding, aluminum casting and inside operating nuclear reactors.
In most apps involving non-contact sensing, the decision to go with laser, capacitance, or eddy current tends to make itself based on the performance required, and environment they will be used in.
I would be quite cautious about using one of these sensors to check airframe health for the simple reason that it is often a challenge to mount a sensor so that the displacement measured is only due to the change in the displacement that is intended to measure. Thaat is why a lot of checking is done with strain guages, or at least it used to be. Of course there we have the challenge of accuracy and calibration, but at least we can know fairly well the point being measured.
There are several areas where eddy current displacement sensors do a much better job than load cells, mostly where the motion is beyond the range of a load cell, and where the possibility of over-ranging is great. Over ranging an eddy current device merely results in an off-scale reading, not in sensor destruction. That can be a great value in tests where the actual displacement is not clear prior to the test.
You are quite right to be cautious. As pointed out in the article any conductive material engaging the sensor field will influence the output. Hence the thumb rules of c2.5x-3x the sensor diameter for a clear area around the probe. And yes the output from an eddy current sensor indicates a relative change in position between the probe (coil) and the target. We tend to be over cautious I think when discussing applications with customers, and take the time to go over all the potential problems before we quote. Eddy current sensors are quite good performers, and the more the customer knows about them the better they will applied.
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