Deviating from thumb rules
Some applications do not allow enough room for an appropriately sized target. If the ideal target diameter is not possible, measurement can still be achieved with reduced performance. Contact the manufacturer’s applications engineering group to determine how the performance will be affected.
Sensor mounting considerations
The electromagnetic field produced by eddy current sensors extends in front of the sensor face, radially from the sensor OD, and, to a small degree, behind the sensor face. Any electrically conductive material in the sensor field can affect the system output. Because of this, care must be taken when installing sensors to ensure that maximum sensitivity of the system is achieved.
Shielded and unshielded sensors
The stainless steel housing of shielded sensors typically extends to the face of the sensor. This results in the electromagnetic field from a shielded sensor being somewhat smaller than that of an unshielded sensor.
In unshielded sensors, the sensor face containing the coil protrudes in front of the stainless steel housing. This allows for a larger electromagnetic field and the larger calibrated ranges typical of unshielded sensors.
Deviating from typical mounting
In some applications, it is impossible to maintain the clearances specified in the drawings. This does not mean that a precise measurement is impossible to make. Conductive material other than the target that engages the field is called side loading. In-situ calibration or bench calibration with simulated side loading can, in most cases, eliminate the concern.
Dan Spohn is regional sales manager for Kaman Precision Products/Measuring, a division of Kaman Aerospace Corp.
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.
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.
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?
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