Engineers Edge (January 1, 2008) — Engineering researchers at Purdue University, working in collaboration with the U.S. Air Force, have developed very small wireless sensors tough enough to survive the extreme conditions within jet engines to detect when critical bearings are beginning to fail and prevent breakdowns.

The devices are classified as "micro electromechanical systems," or MEMS, which are machines and devices that combine electronic and mechanical components on a microscopic scale.
"The MEMS technology is critical because it needs to be small enough that it doesn't interfere with the performance of the bearing itself," said Farshid Sadeghi, a professor of mechanical engineering. "And the other issue is that it needs to be able to withstand extreme heat."

The engine bearings are designed to operate in temperatures at about 300 degrees Celsius, or 572 degrees Fahrenheit. The engineering researchers have demonstrated that the new sensors can detect temperature-induced bearing failure significantly earlier than other types of conventional sensors.
"This kind of advance warning is critical so that you can shut down the engine before it fails," said Dimitrios Peroulis, an assistant professor of electrical and computer engineering.

The sensors could be in use in a few years within fixed and rotary wing aircraft such as fighter jets and helicopters. The MEMS technology also has significant potential for applications within other products and engineered systems.

"Anything that has an engine could benefit through MEMS sensors by keeping track of vital bearings," Peroulis said. "This is going to be the first time that a MEMS component will be made to work in such a harsh environment. It is high temperature, messy, oil is everywhere, and you have high rotational speeds, which subject hardware to extreme stresses."
This work is an extension of Sadeghi's previous engineering research aimed at developing electronic sensors to measure the temperature inside critical bearings in communications satellites.
"This is a major issue for aerospace applications, including bearings in satellite attitude control wheels to keep the satellites in position," Sadeghi said.

The control wheels are supported by two bearings. If mission controllers knew the bearings were beginning to fail on a specific unit, they could turn it off and switch to a backup.

"What happens, however, is that you don't get any indication of a bearing's imminent failure, and all of a sudden the gyro stops, causing the satellite to shoot out of orbit," Sadeghi said. "It can take a lot of effort and fuel to try to bring it back to the proper orbit, and many times these efforts fail."
The Purdue engineering researchers received a grant from the U.S. Air Force in 2006 to extend the work for high-temperature applications in jet engines.
"Current sensor technology can withstand temperatures of up to about 210 degrees Celsius, and the military wants to extend that to about 300 degrees Celsius," Sadeghi said. "At the same time, we will need to further miniaturize the size."

The new MEMS sensors provide early detection and prediction of impending failure by directly monitoring the temperature of engine bearings, whereas conventional sensors work indirectly by monitoring the temperature of engine oil, yielding less specific or local data.

The MEMS devices are designed to not operate on batteries and will transmit temperature data wirelessly.

"This type of system uses a method we call telemetry because the devices transmit signals without wires, and we power the circuitry remotely, eliminating the need for batteries, which do not perform well in high temperatures," Peroulis said.

Power will be provided using a technique called inductive coupling, which uses coils of wire to generate current.

"The major innovation will be the miniaturization and design of the MEMS device, allowing us to install it without disturbing the bearing itself," Peroulis said.

Data from the onboard devices will not only indicate whether a bearing is about to fail but also how long it is likely to last before it fails, Peroulis said.

The research is based at the Birck Nanotechnology Center in Purdue's Discovery Park and at Sadeghi's mechanical engineering laboratory.

Reference and adapted from materials provided by Purdue University.