Issue
132 July 2001
Inductive Sensors
by
George Novacek
Start
•
LVDT Revisited • The
Works • Electrical
Interfaces • Mechanical
Interfaces • The
Future • Sources &
PDF
THE
FUTURE
This is still
a new technology, so you’ll have to wait a while
before it matures enough and high production volume brings
down the price. It’s still expensive, depending on
quantity and special requirements, but it’s in line
with the competing technologies.
Nothing is perfect,
so let’s be constructively critical and see what’s
wrong with this device. Like everything else, it has limitations.
The first one is temperature, although I hasten to add
it is specified for a respectable –40°C to 125°C.
But it may not be good enough for some commercial and
automotive applications. The second limitation is its
relative complexity, which affects reliability.
These two characteristics
don’t allow it to be a contender for work in the
environments in which the simple coil in ’VDTs and
proximity sensors reigns supreme. Consequently, the sensor
competes in a crowded field against numerous other technologies,
such as optical encoding and Hall effect, primarily by
price because it doesn’t have a major feature that
sets it apart from the pack.
No less important
today is the BIT ability, the ability of a built-in test
to run diagnostics on the sensor to determine whether
or not it’s operating properly. In safety-critical
applications, it’s imperative that the electronic
controller can determine whether or not the signal received
from the position (or any other) sensor is valid. With
a detection coil, for instance, you can monitor its voltage
and current. Unfortunately, CSEM doesn’t appear to
address this issue in its literature. I haven’t found
sufficient detail to infer what happens if a part of the
sensor fails.
The traditional
way to verify a signal is through dual redundancy (i.e.,
by using two sensors and comparing their outputs). This
isn’t cost-effective, and therefore isn’t desirable
in modern systems. Even with two such sensors, you could
conclude only that the output is invalid, but you couldn’t
say which one. For that you would need three sensors and
the cost would be hard to justify.
As we continue
to automate our lives, we need more and a wider spectrum
of sophisticated sensors. In this article, I looked at
one method of position sensing. Check out the Resource
section if you want to look for more methods.
