Measuring Roll, Pitch, and Yaw - in the Micro World
By Anna Lynn Spitzer
1.21.03 -- As instruments that monitor - and stabilize - the dynamics of rotating bodies, gyroscopes are well known to be important in such applications as air and space travel and missile defense systems. But gyroscopes are similarly known to be very expensive, bulky, and heavy, which limits the range of applications to which they can be applied.
Micro-electrical-mechanical systems (MEMS) technology, though, is now providing a platform that will dramatically reduce the cost, size, and weight of this technology. Price will drop from a high of $100K to as little as a few hundred dollars. Size will decrease from something approaching a foot-cube to the size of a penny. And the weight will decrease from some 25 pounds to a few hundredths of an ounce.
As a result, MEMS is enabling researchers to experiment with gyroscopes in such diverse applications as rollover detection in automobile accidents, GPS-assisted navigation and navigation in micro-robotics, camcorder stabilization, interactive pointing devices associated with computing systems, tracking in virtual reality, micro-satellite sensing, and implantable devices for inner-ear balance disorders.
"There are two types of gyros: rate and rate-integrating," explains Chris Painter, a Ph.D. student working with assistant professor Andrei Shkel in Mechanical and Aerospace Engineering at UCI.
Suppose you were flying a plane from the U.S. to Australia You'd always want to know your plane's orientation in terms of roll, pitch, and yaw. You have a choice of directly measuring the angles using a rate-integrating gyroscope, or measuring angular rate with a rate gyroscope and integrating to obtain angular position.
"In the macro world, both options are being implemented," says Painter. "Using a rate gyro to obtain the heading is typically cheaper, but is also more problematic because, when you integrate the signal to obtain position, you also integrate errors. In the MEMS world, to minimize integration errors, you either need a high-precision MEMS rate gyro or a rate-integrating gyro that directly outputs position. Unfortunately, at this time, neither of these options is available."
So there's a lot of work still to be done.
Painter is developing a MEMS-based rate-integrating gyroscope. This gyroscope operates on a very old principle. Consider the classic high school physics experiment to demonstrate the rotation of the Earth by watching a swinging bucket of sand as its line of oscillation (the line the bucket is swinging along) appears to rotate as time passes. In reality, the line of oscillation remains fixed in inertial space with the Earth revolving underneath it. If you were to measure the angular difference between where the line of oscillation started and where it is sometime later, this difference would be equal to the angle the Earth has rotated. In like fashion, then, the MEMS gyroscope is a very tiny mass (the bucket) on a very tiny suspension system (the cord) and is made to oscillate back and forth very quickly. If a tiny person sat next to the mass, as the object (airplane, satellite, etc.) that the gyroscope was attached to rotates, the line oscillation would also rotate, allowing the person to measure the angle of rotation of the object.
"I originally came to do UCI to focus on robotics for post-stroke rehabilitation using force-feedback joysticks in the biomechatronics lab," says Painter. "Originally, I was going to obtain an M.S. degree. But during the course of my last year, I took Andrei Shkel's MEMS course. The course got me really excited about MEMS technology - so much so that I decided to switch to studying MEMS inertial systems - and go for a Ph.D.!"
Painter continues, "Building a MEMS-based gyro is a major engineering challenge as we are dealing with fundamental problems of physical phenomena on the micro scale that can be disregarded on the macro scale." But he adds that the work is well worth the effort as they are pioneering a novel technology and have already acquired several patents in the field.
Under Shkel's tutelage, each student in the lab is required to test research concepts by making his or her own devices, with several devices combined on a single chip. To enhance the creativity of the designs, Shkel looked for inspiration to Calit². He acquainted the students of his lab with the institute, encouraged them to review at Calit²'s Web site, and suggested they design logos to be implemented on chip. Images of the results accompany this article.
"MEMS-based gyros," says Painter, "say, because of their small size, offer the possibility of enabling things not previously possible; that's why they're so exciting." For example, he points to gyroscopic prostheses being developed in a collaboration with the UCI Medical School to address a disorder in the vestibular canal in the human ear.
"Vestibular disorders result in a lost of balance control, causing the person affected to be in a constant state of vertigo," says Painter. "A secondary effect is that the person quickly comes to fear something as simple as walking down the street."
The small size of the gyro enables implantation in the ear, and work is underway to develop an interface between the gyro and the neurons in the brain - "so that electrical signals from the gyro can be interpreted, quite literally, by the brain and compensate for loss of balance," explains Painter.
Given this scenario using integrated circuit technology, multiple gyro devices with control electronics could be fabricated on a single chip. Such a chip, for example, could contain three gyros for sensing 3-D orientation, with each smaller than 1 mm in diameter and thinner than 2 microns (about 1/50 the width of a human hair). "Not to mention the fact," enthuses Painter, "that they are low-power and extremely cheap!"
Painter insists that research in this area will affect people broadly, not just in the military domain, which has provided his fellowship.
"I'd love to work at the micro-gyroscope division at the Jet Propulsion Laboratory in Pasadena," says Painter. "I can imagine working on micro-satellites that ride a mothership satellite into orbit, then venture out on their own to take readings before returning to the mothership. For example, they are currently working on micro-satellites for orbit around Mars to serve as navigation aids and communication relays."
For more information on Shkel's lab and Painter's research activities, see mems.eng.uci.edu.
