Tech Briefs Magazine - October 2021 - ST-20

Applications
output is nonlinear, so it must either be
linearized in the sensor electronics or
the user must account for this nonlinearity
in their plant control system.
In addition to non-linearity, other limitations
include iron losses caused by the
fact that the core itself absorbs the magnetic
field. These losses increase with
frequency, to the extent that an inductive
displacement sensor reaches the
limits of its performance at around 100
measurements per second.
Another issue with inductive displacement
sensors is their sensitivity to
extreme temperature variations due to
the high thermal coefficient of expansion
of the ferrite core. This makes temperature
compensation challenging,
normally resulting in high thermal drift.
Eddy Current vs Capacitive
Displacement Sensors
Eddy current and capacitive sensors
detect the distance to an electrically conductive
measurement object based on
changes in the electrical field. Eddy current
sensors measure the distance via
the change in impedance of the sensor
coil. With capacitive sensing, the sensor
and measurement object form the plates
of a capacitor.
Both types can measure in the submicrometer
range. Nevertheless, they
differ considerably with respect to the
operating environment. Eddy current
sensors are ideally suited to harsh industrial
environments that include dirt,
dust, and humidity. Capacitive sensors
on the other hand, require the kinds of
clean surroundings that can be found in
electronics production, laboratories,
and clean rooms.
Eddy Current Displacement Sensors
Although eddy current sensors
employ the same laws of magnetic induction
as inductive displacement and proximity
sensors, their air-core coil construction
enables higher accuracy, measurement
speed, and stability - nonlinearity
and temperature drift are not
problems.
Their advantages include:
* Fast measurements up to 100 kHz.
* High resolution down to 0.5 μm or better.
* High linearity.
* High temperature stability in fluctuating
ambient temperatures, which can
even be improved with embedded
active temperature compensation.
* Measurement using either ferromagnetic
or non-ferromagnetic target
materials.
Figure 1. Temperature compensation can be built into the sensor for highly precise measurement.
(Courtesy of Micro-Epsilon)
* They are not affected by high pressure,
high temperature, dirt, vapor, or oil.
Eddy Current Sensor Operating
Principle
An alternating current in the sensing
coil produces a changing magnetic
field. This field induces a current in the
target - the eddy current. The alternating
eddy current produces its own magnetic
field, which opposes the sensing
coil's field, thus changing the impedance
of the sensing coil. The amount of
impedance change is dependent on the
distance between the target and the
sensing coil in the probe. Current flow
in the sensing coil, which is impedance
dependent, is processed to produce the
output voltage, which is an indication of
the position of the target relative to the
probe.
Figure 2. The gap in hydrostatic bearings needs continuous inspection. The sensor measures
through the oil film onto the opposing bearing surface. (Courtesy of Micro-Epsilon)
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Temperature Compensation
Since several different eddy current
sensor designs are available from
Micro-Epsilon, engineers can select the
Sensor Technology, October 2021

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Tech Briefs Magazine - October 2021

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