Why make a sensor when you can buy one? It's a question researchers at Rockwell Automation's Advanced Technology Lab grapple with all the time. And all
too often, the answer is they simply can't buy what they really need.
So they make their own. Over the years, they've developed a
sophisticated collection of custom sensors for tasks as varied as torque
sensing and the characterization of
lubricating fluids.
"Our sensors aren't products," says Fred Discenzo, a Ph.D engineer
who manages diagnostics and sensing for the Advanced Technology Lab. "We
develop them in response
to specific research needs or customer requests."
And those research needs and requests have resulted in some out-of-the-box thinking about sensor
design. One of the Lab's sensors, for example, shows how multiple
MEMS sensing elements can be greater than the sum of their parts. Other sensors
leverage well-understood physical effects – such as Faraday rotation or birefringence – to yield diagnostic information
about motors and gearboxes. What's more, the Lab's homegrown sensors often have other advantages
over commercial models, such as high sampling frequencies or low-cost design.
Multi-Element Fluid Sensor
The Lab's
standout sensor isn't really just one sensor at all but a multi-element fluid
sensor that brings together five different MEMS sensing devices in one package.
"It's the most interesting sensor we've developed," Discenzo says.
Its usual sensing lineup consists of a viscosity sensor, a
temperature sensor, a conductivity sensor that can perform impedance
spectroscopy, an electrochemical sensor that measures redox potentials and an
open-circuit potential sensor that helps determine the pH of aqueous fluids or
the acid number of non-aqueous
fluids. Discenzo says a multi-element sensor is "active" in that one or
more of the sensing elements require stimulation by an electrical signal.
Rockwell first developed the sensor as a way to understand the
degradation of lubricating fluids in gearboxes and other industrial power
transmission components. Discenzo and his team worked with customers and wear
researchers to identify 20 different parameters that would indicate a breakdown
of the fluids. These include water content, viscosity and oxidation. "The goal
is to pick up the signs of fluid degradation before metal starts tearing up
metal," Discenzo says.
While it
got its start in oil, the multi-element sensor can be applied to seemingly unrelated
fluid-sensing tasks with a few changes – to the coatings that make up the electrochemical
sensors, to stimulus patterns and to the algorithms that integrate the data
from the five sensing elements. "We really think of this sensor as more of a
platform technology," Discenzo says.
For
example, this sensor platform has been used to analyze engine oil in a Pratt
& Whitney jet engine as well as frying oil in a commercial food plant.
"Both applications had roughly the same temperature and were looking at for
soluble metals and oxidation products in hot oil," he says. The sensor has
likewise been used to analyze hydraulic fluids, industrial greases and jet
fuels.
Much of the
recent interest has come from the food and beverage industry, which want to use
the sensor to look for contaminants, the presence of bacteria and much
more. The sensor can also be configured
to monitor the progress of fermentation processes. Another emerging application
is in the analysis of groundwater, where the sensor looks for contaminants.
Click here for technology overview
Optical Torque Sensor
The Lab has
also developed its own optical torque sensor that measures gearbox or gearmotor
torque at high frequencies. Rather than trying to directly measure the torque
by putting a strain gauge on a shaft, Rockwell's sensor instead looks at
torsional strain through the lens of the photoelastic effect-in which certain
materials exhibit birefringence under an applied stress, in this case a
torsional stress.
The sensor
consists of a "sleeve" made from a photoelastic material, such as polycarbonate
or acrylic. When slipped over the gearmotor shaft and viewed under a polarizing
light, this sleeve exhibits a fringe pattern that corresponds to changes in the
torsional strain of the shaft. The sensing system captures images of this
fringe pattern with a linear CCD array. Discenzo and his colleagues created
neural network software that maps fringe patterns associated with a given
amount of torsional strain to actual shaft torque.
Rockwell
initially developed the sensor for use in a torque-sensing motor coupling. "The
motor had to be closely coupled to a gearbox, so there wasn't any room for a
shaft mounted sensor," Discenzo says.
But even when there is room, it turns out the optical torque
method has some advantages over conventional shaft-mounted torque sensors. One
is speed. "Commercial torque sensors are rated at 500 hertz, and you're lucky
to get 250 hertz out of them," says Discenzo. "We're getting tens of
kilohertz." Another is resolution
with Discenzo reporting that optical sensor can pick up microstrain-level shaft
deformations.
The other
advantages have to do with cost, installation and longevity. Discenzo notes
that the optical torque sensor was assembled from components that cost under
$100. "We've paid $10,000 for lab-grade commercial torque sensors," he says.
As for
installation, "we don't have to bring power and signal wires on and off," says
Discenzo. Finally, the optical sensor may last longer than strain gauges.
"Mechanical torque sensors tend to fail because they're subject to high loads
and over-torque conditions," says Discenzo.
Instead of
using the sensor for torque feedback control, which is a common use for
off-the-shelf torque sensors, Rockwell uses its optical sensor for power
transmission diagnostics. "A lot of the work we've done with the sensor is
proprietary," Discenzo says. He did add, however, that the sensor has been used
to analyze not only industrial gearboxes but gearboxes for military helicopter
tail rotors and wind turbines.
Click here for technology overview
Embedded Motor Current
Sensor
Rockwell researchers likewise turned to optical technologies to
create a compact, high-frequency current sensor that can be embedded in motors, starters or related electric
devices.
This sensor
consists of a fiber optic waveguide coiled around one of the motor's electrical
conductors-usually a power supply wire-and a light source that sends a beam of
polarized light through the waveguide. Working at megahertz frequencies, the
sensor measures the difference in polarization angles at the beginning and the
end of the waveguide. As Discenzo points out, the change in polarization angle
corresponds to the electromagnetic force generated by the conductor, thereby
providing insight into current. "We're just making use of the Faraday Effect,"
he says.
Rockwell researchers developed the sensor because it wanted
something fast and compact enough to embed in motor housings. "The commercial
current sensors we looked at were much larger," Discenzo says. For example, a
comparable 2,000 amp commercial current sensors that occupy about ten cubic
inches for one phase. Rockwell's electro-optical sensor fits all three phases
in a package that's "about
the size of computer mouse," says Discenzo.
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