The principle of operation, as the name implies, depends upon 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 and the probe coil is encapsulated within the tip. The coil is supplied with a constant RF current from a separate Eddy Probe Driver connected via a cable, which sets up an electromagnetic field between the tip and the observed surface.

Any electrically conductive material within this electromagnetic field, i.e. the target material, will have eddy currents induced in its surface. The energy absorbed from the electromagnetic field to produce these eddy currents will vary the strength of the field and hence the energising current, in proportion to the probe-target distance. Such changes are sensed in the driver where they are converted to a varying voltage signal.

The whole probe, extension cable and driver system relies 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 the API670 standard (see later discussion). Three main variants, straight mount, reverse mount and disc type probes 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 metal plate 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.

System overview

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 overview T ypical straight probe with driver

STRAIGHT MOUNT PROBES
Probe Type	Range	Tip Diameter	Data Sheet
PRS 02	2.5 mm	5 mm	L156
PRS 04	4 mm	8 mm	L157
PRS 08	8 mm	20 mm	L158
AECP 02	2.5mm	-	DS1150
AECP 04	2.5 or 4mm	-	DS1151
REVERSE MOUNT PROBES
Probe Type	Range	Tip Diameter	Data Sheet
PRR 04	4 mm	8 mm	L159
AECPR 04	2.5 or 4mm	-	DS1152
DISC PROBES
Probe Type	Range	Tip Diameter	Data Sheet
PRD 04	4 mm	8 mm	L161
PRD 08	8 mm	20 mm	L162
PRD 12	12 mm	25 mm	L163
PRD 18	18 mm	40 mm	L164
PRD 25	25 mm	50 mm	L165
SYSTEM PRODUCTS
EXTENSION CABLES  Available in various ‘tuned’ lengths up to 18m. With or without stainless steel over braid armouring.  Various connectors, oversheaths and sealing glands supplied fitted.
DRIVER UNITS Compact design either baseplate or DIN rail mounted. Four connections per box required - 24V, 0V and Signal
PROBE HOLDER Replace probes without calibration.  In aluminium alloy or stainless steel, internal adjustment of probe depth. Fully Sealed. click here to view data sheet
DRIVER HOUSING Steel enclosure with removable gland plates and lockable door.  Sealed to IP65. Capacity for 1, 2, 4 or 6 drivers. Fully wired with terminals fitted. DATA SHEETS DH1/10 DH2/20 DH4/34 DH6/60