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Fiber Optics in Sensors and Contr ol Systems

FIGURE 3.26 Inductive proximity sensor. 149

In the circuitry in Fig. 3.26, the oscillator generates an electromag-
netic field that radiates from the sensor’s face. This field is centered
around the axis and detected by the ferrite core to the front of the sen-
sor assembly. When a metal object enters the electromagnetic field,
eddy currents are induced in the surface of the target. This loads in
the oscillator circuit, which reduces its amplitude.
The detector circuit detects the change in the oscillator amplitude
and, depending on its programming, switches ON and OFF at a specific
oscillator amplitude. The sensing circuit returns to its normal state when
the target leaves the sensing area and the oscillator circuit regenerates.
The nominal sensing range of inductive proximity sensors is a
function of the diameter of the sensor and the power that is available
to generate the electromagnetic field. This is subject to a manufactur-
ing tolerance of ±10 percent, as well as a temperature drift tolerance
of ±10 percent. The target size, shape, and material will have an effect
on the sensing range. Smaller targets will reduce the sensing range, as
will targets that are not flat or are made of nonferrous material.
Basically two types of inductive proximity sensors are used: (1)
shielded and (2) nonshielded. The shielded version has a metal cover
around the ferrite core and coil assembly. This focuses the electro-
magnetic field to the front of the sensor and allows it to be imbedded
in metal without influencing the sensing range. The nonshielded sen-
sor can sense on the side as well as in front of a sensor. It requires a
nonmetallic area around the sensor to operate correctly.
Inductive proximity sensors have several benefits:

• High repeatability. Visibility of the environment is not an issue,
since inductive proximity sensors can sense only electromag-
netic fields. Therefore, environments from dirt to sunlight
pose no problem for inductive proximity sensors. Also,
because they are noncontact sensors, nothing wears.