Sensors & transducers


TemperaTure flucTuaTions Depending on the supplier, eddy current sensors can operate in ambient temperatures from -40°C to +200°C. However, temperature fluctuations occur during operation. As the temperature of the target material changes, its resistance changes and so the accuracy of the sensor depends on the target temperature. It is therefore important to monitor the temperature of the target and compensate for this. Micro-Epsilon, for example, provides an ‘Active Temperature Compensation’ feature on its eddy current sensors. This actively measures the temperature of the sensor, electronics, cable and controller, adjusting the measurement values accordingly.


calibraTion To improve sensor performance, most eddy current sensors are factory-calibrated by the supplier to a specific target material. Some suppliers, however, offer advanced eddy current measurement systems that are able to calibrate themselves. The eddyNCDT 3300 series from Micro-Epsilon, for example, can store up to four different material types.


mounTing and insTallaTion Eddy current sensors are grouped into shielded and unshielded sensors. With shielded sensors, the field lines run closer together due to a separate casing. These are less sensitive to radial flanking metals. With unshielded sensors, the field lines emerge at the side of the sensor normally causing an extended measuring range. Correct installation is crucial for good signal quality.


TargeT size and geomeTry The relative size of the measuring object to the sensor affects the linearity deviation for eddy current sensors. Ideally, the measuring object size for shielded sensors should be at least 1.5 times the diameter of the sensor and at least three times the coil diameter of the sensor for unshielded versions. From this size, almost all lines of the magnetic field run from the sensor to the target. Therefore, almost all magnetic field lines penetrate the target via the face and so contribute to eddy current generation, where only a small linearity deviation occurs. Eddy current sensors have a relatively large spot


size compared to their measuring range. For example, a 3mm measuring range will typically require a target size of 1.5 times this measuring range, i.e. 8-9mm. However, this depends on whether the sensor is shielded or unshielded. If shielded, the measuring range of the sensor decreases. If unshielded, the sensor will offer a larger measuring range. Target geometry is also an important factor.


Eddy current sensors are often used to measure against rotating or curved shafts, where they measure oil gaps or vibrations. However, almost all suppliers calibrate their sensors against a flat target. If this is the case, users must therefore perform their own linearity adjustments against the target geometry. Most mid-to high-end eddy current sensor suppliers will compensate for curved targets, but it is


Instrumentation Monthly August 2019


important to check the supplier’s datasheet or ask for advice and guidance on sensor selection directly from the supplier.


TilT angle and measuring signal Many non-contact eddy current displacement measuring systems offer excellent linearity and high resolution, but only if they are installed at right angles to the target. As exact right angle mounting of the sensor to the target is often difficult to achieve, the extent of deviation is different from one sensor supplier to another. If the controller is not linearised for tilt angles,


the measured values will deviate marginally from values measured in the right angle position. Hence it is important to know the influence on the measuring signal if the sensor is tilted. Generally, a tilt angle of more than six degrees is


possible with unshielded sensors than with shielded, but should be avoided. In principle, only a special linearised sensor will provide a precise signal. A permanent tilt angle can already be logged in the controller with three-point linearisation. This avoids any influence of this tilt angle on the signal quality.


required TargeT Thickness The principle of eddy current displacement measurement requires a minimum thickness for


stable measurement results. This minimum thickness depends on the target material used and the measurement frequency used. Shorter range sensors tend to use high excitation frequencies, which penetrate less into the target material, resulting in a requirement for a thinner target. For example, the minimum target thickness of an aluminium target is 80 to 100 microns with displacements of 1mm or less. For steel targets, this increases to 400-plus microns. Temperature fluctuations must also be


considered here, as the thickness of the target material will increase as temperature increases. For example, if you are measuring a target where the temperature fluctuates by +/- 25°C, it is recommended that the minimum target thickness be increased by a factor of three.


skin or peneTraTion depTh Electromagnetic fields are attenuated on entering an electrically or magnetically conducting material. The reduction in the field strength and therefore the current density is accompanied by losses that occur in the vicinity of the material surface. The characteristic length at which the current density reduces to the value 1/e or to 37 per cent is known as the skin depth.


Micro-Epsilon www.micro-epsilon.co.uk 51


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