Sensor Sense: Inductive Position-Measurement Sensors
Analog inductive sensors, as
discussed in the
Jul y 2006 Sensor
Sense (
tinyurl.com/2aok9m), return
the relative
distance of the target
from the sensor
face.
But their
limited range and
linearity limit many
of their applications
to detecting odd or
irregularly shaped
targets rather than actual
position.
On the other hand, inductive
linear-position sensors use
analog sensing to return the position
of the target over a distance
of up to 360 mm with a linearity
error of ±0.4 mm and resolution of
360 μm. They can also be used for
rotational angle measurements for
360° applications.
Linear-position sensors perform
this feat using multiple sensing
coils in an array that spans the
sensing distance. A special target
called an attenuator slides along the
sensing range, moving from one
coil to the next buried in the main
body of the sensor. A microprocessor
integrates the output from each
coil, translating the information
into a true target position that is
sent as a 0-to-10-V or 4-to-20-mA
analog signal.
The microprocessor adds several
advantages: Not only does it
evaluate position using the multiple
coils along the length of the sensor,
it compensates for temperature
variations and applies linearization
correction to boost measurement
accuracy. Future connectivity and
programmability options might
include scaling, range setting, limit
detection, and value evaluation as
well as communication interfaces
such as CANopen and RS-232.
Each linear-position sensor
comes with an attenuator. But any
ferrous material will also work as
an attenuator provided it maintains
the specified width across the entire
active region of the sensor. For
example, a specified 8-mm-wide
attenuator centered directly over
one sensing coil partially overlaps
the coils ahead and behind. The
microprocessor reads the value
from all three coils, calculating the
center line of the attenuator based
on the ratio of the coil outputs and
the attenuator’s 8-mm width. A
wider or thinner attenuator would
reduce accuracy and resolution.
The actual distance of the attenuator
above the sensor is immaterial
as long as it stays within
the specified range, typically 1 to
6 mm, with a corresponding reduction
in distance if the attenuator
is nonferrous metal.