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FREQUENTLY ASKED QUESTIONS

 

TECHNOLOGY

Q: Why would I use an eddy current displacement sensor over a capacitance displacement sensor?
A: It essentially comes down to environment and performance requirements. Unlike eddy current sensors, capacitance sensors are influenced by anything and everything that comes between the sensor face and the target. Eddy current sensors are well suited for all environments from inside internal combustion engines to semiconductor clean rooms.

 

Q: Why would I use an eddy current displacement sensor over a laser displacement sensor?
A: With laser sensors it comes down to environment. In higher-temperature environments, the eddy current sensor - independent of the electronics (with the exception of the cable) - can withstand higher ambient temperatures than laser sensors that have their electronics integral with the sensing head. Also, vapors and fluids can condense on the laser sensor optics and degrade performance over time. This is not the case with eddy current sensors.

 

TARGET

Q: Does the target need to be electrically conductive?
A: Yes, and the more conductive it is, the better target it is. Nonconductive targets can be sensed if a thin piece of conductive material is attached to it to act as the target.

 

Q: Does the target need to be grounded?
A: No, the oscillating electromagnetic field produced by the coil induces eddy current in the surface of a conductive target whether grounded or isolated.

 

Q: Does it have to be a ferrous target?
A: No, eddy current sensors work equally as well with ferrous and nonferrous targets. Ferrous targets that translate in front of the sensor - such as a rotating shaft - are susceptible to errors from electrical runout.

 

Q: What is electrical runout, and why should I be concerned?
A: Electrical runout is seen in rotating ferrous targets. It shows up as a very repeatable once per rev error on the system output. It is caused by small variations in the permeability and conductivity along the circumference of, for example, a rotating shaft that affects the induced eddy current.

 

Q: How thick should the target be?
A: In general, aluminum targets as thin as 0.020” are acceptable. Steel targets as thin as 0.005” are acceptable. For exact thicknesses for various target materials, contact an applications engineer for assistance.

 

Q: Can thinner targets be used?
A: Yes, but there are concerns that should be reviewed with an applications engineer before proceeding.

 

Q: How big in diameter does the target need to be?
A: An ideal target is a flat, clear area at least 2.5x to 3x the sensor diameter for unshielded sensors, and 1.5x to 2x for shielded sensors. Different sizes and shapes can be used, but performance will vary. Contact an applications engineer for assistance.

 

CALIBRATION

Q: What is offset?
A: Offset is the distance from the face of the sensor to the beginning of the calibrated range.

 

Q: Can I increase the offset without reducing the calibrated range?
A: No, not without negatively affecting system performance. If this is a requirement, consult with an applications engineer before proceeding.

 

Q: Can I calibrate the system myself?
A: Yes, nearly all of Kaman’s displacement sensors have access to the potentiometers to do calibrations. The ability to position the sensor relative to the target accurately and repeatably is paramount for a good calibration.

 

Q: Can I calibrate the system beyond the published range in the spec sheet?
A: Yes. It is relatively easy to calibrate to 1.5x the published range. However, linearity, thermal stability, resolution and repeatability specs will suffer.

 

ENVIRONMENT

Q: Does humidity affect the sensor output?
A: No, not like it would with capacitance sensors. Eddy current sensors are unaffected by oil, grease, water, or anything not conductive that gets between the sensor and the target.

 

Q: Do I need to worry about temperature changes?
A: Yes and no, depending on the performance you want to achieve and the temperature change. Eddy current sensors are sensitive to thermal changes in the environment. Refer to the thermal sensitivity spec in the product brochures, or call an applications engineer for assistance.

 

Q: Do I need to worry about stray electromagnetic fields?
A: Kaman systems are typically tested to comply with EN 61326-1: 2006-Industrial specification related to susceptibility. They have been used in magnetic levitation applications and others where there are strong electromagnetic fields present. The systems can operate with carrier frequencies from 10kHz to 2Mhz, and most systems operate at either 500kHz or 1Mhz with a passband of <50kHz. Though somewhat unusual, there are occasions - if a signal is strong and within the passband of the system (typically the carrier frequency ±50khz - that it could modulate the carrier and, hence, be detected on the output. If this is the case, usually a special system can be built to avoid the frequency that is causing a problem.

 

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