THE EDDY CURRENT PROXIMITY PROBE

The principle of operation,as the name implies,dependsupon the eddy currents set up in the surface of the target material - shaft,collar,etc.adjacent to the probe tip.The Eddy probe tip is made of a dielectric material andthe probe coil is encapsulated within the tip.The coil issupplied with a constant RF current from a separate EddyProbe Driver connected via a cable,which sets up an electromagnetic field between the tip and the observedsurface.Any electrically conductive material within thiselectromagnetic field,i.e.the target material,will have eddy currents induced in its surface.The energyabsorbed from the electromagnetic field to produce these eddy currents will vary the strength of the field andhence the energising current,in proportion to the probe- target distance.Such changes are sensed in the driverwhere they are converted to a varying voltage signal.The whole probe,extension cable and driver systemrelies for its operation on being a tuned circuit and as such is dependent on the system’s natural frequency. Thus each system is set up for a fixed electrical/cable length.Eddy probe systems are usually supplied with 2,5, 9 or 14 metre total cable lengths.The probe types available are generally according to theAPI670 standard (see later discussion).Three main variants,straight mount,reverse mount and disc typeprobes make up the Sensonics range.The main difference between the straight and reverse mount is the location of the thread on the probe body and the fixing nut.Reverse mount tend to be used exclusively with probe holders, while straight mount are the more common and are used on simple bracketry or mounting threads where adjustment to the target is achieved through use of the thread on the probe body in conjunction with a moveable lock nut.The maximum measurement range available on this type of probe is typically 8mm.The disc probe mounts the encapsulated coil on a metalplate with fixed mounting holes,making a very low profile assembly with a side exit cable.Larger coils can be mounted on this plate;for example,the 50mm diameter tip probe can provide a measurement range of beyond 25mm.However,care must be taken to ensure the target area is sufficient to obtain the required linear response. Note the relationship opposite – between linear range, probe tip and target area. In rotating plant,the variations in shaft/bearing distance created by vibration,eccentricity,ovality etc.can thus be measured by probes mounted radially to the shaft.When the target is stationary the measured voltage can be used to set the probe/target static distance.Shaft speed can also be measured by placing the probe viewing a machined slot or a toothed wheel.
Eddy current probe empirical relationships System overviewTypical straight probe with driver

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