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Rockwell's MEMS Sensors Meet Special Needs
Joseph Ogando, Senior Editor -- Design News, December 23, 2008
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.
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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.
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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.
