Motion sensing has traditionally relied on accelerometers to measure linear acceleration along three axes and gyroscopes to capture an object's rate of rotation about three axes. Researchers at the Center for Bits and Atoms at MIT (CBA; Cambridge, MA), however, are pushing the technology forward by fabricating a low-cost motion sensor that combines the functionality of an accelerometer with that of a gyroscope to provide six-degree-of-freedom inertial sensing.

Inertial sensing has experienced a great deal of progress in recent years thanks to microelectromechanical systems (MEMS)-based technologies. But MEMS aren't the sole enabling technology, according to Rehmi Post, a post-doc researcher and visiting scientist at the CBA. "We decided to forego the expensive MEMS process that involves a billion-dollar semiconductor fab. [We] tried to do something based more on simple physics, using electrostatic fields to guide the motion of a particle and to use that particle as the proof mass of an accelerometer and a gyroscope," he states.

Drawing from experimental physics, the researchers pursued this idea of using a 'particle trap' to detect motion. The resulting sensor consists of a printed circuit board with a hole drilled into it, around which are electrodes. Using a specific pattern of electrical signals for precise control, the electrodes suspend a particle in a tight orbit in an electric field-generated particle trap. As the sensor experiences acceleration and rotation, the particle's orbit changes, according to the researchers.

"The interesting thing is that for a single device, we should be able to gain all six degrees of freedom from monitoring the motions of a suspended particle," Post says. Each of the six degrees of motion affects the particle differently, he adds.  

Essentially performing the work of six micromechanical sensors, the product is referred to by the group as a microdynamical sensor because its motion is determined by fields rather than a mechanical system. Instead of a complex mechanical linkage such as that found in a MEMS sensor, for example, the microdynamical sensor relies primarily on the electrical field that keeps the particle in position, Post notes.

Further distinguishing itself from conventional MEMS technology, the microdynamical sensor is not limited by a fixed range of sensitivity. "The stiffness of the trap can be tuned to change the response and sensitivity of the sensor dynamically," Post observes. "So, at one moment, it may be sensitive to very small motions on the order of 1 G, but if it senses that there's a trend toward a larger acceleration, the trap can be stiffened, giving it a wider dynamic range. The benefit of that shows up when you're trying to monitor complex motions."

The ability to monitor complex motions could prove to be a valuable asset in medical products. Although the microdynamical sensor could replace existing inertial sensors in medical devices, it also has the potential to advance diagnostics and monitoring applications. It could be used, for example, in the field of gait analysis as well as in a multitude of other specialty areas. Post adds that, because of the possibilities across various industries, the group does not want to pigeonhole the sensor as suitable for one area or specific end use.

In the meantime, the group is exploring a variety of avenues. "We're taking a couple of different approaches to make sensors that are appealing to a variety of commercial processes," Post states. "We aim to make this technology commercially viable and are interested in licensing to parties that have the skills and inclination to bring it to market."