Table of Contents

Research & Developments
Stephanie vL Henkel

Nanotractor Tests Amontons’ Law of Friction

Maarten de Boer and Alex Corwin built a nanotractor to experiment with friction at the microscale.
Tribology, the study of friction, lubrication, and wear on surfaces that are in relative motion to one another, has been revivified by recent advances in micro- and nanotechnology. According to Amontons' law, friction force is proportional to force normal (perpendicular) to the surfaces in contact. But does that hold true at the microscale? The nanoscale? It turns out that at low normal forces the law does not apply so neatly as it does in the macro world. Adhesion between the two surfaces seems to contribute an extra force whose effect is multiplied by the large surface-to-volume ratio of micro devices. Researchers led by Sandia National Laboratories' Maarten de Boer have built a "nanotractor," an actuator about 100 µm in width, to learn more about these special cases of friction and to find out whether the collected data can tell them something about the behavior of this and other extremely tiny mechanisms.

The easy part was fabricating parallel plates to measure normal forces; far trickier was figuring out how to produce and measure tangential forces. Standard comb drives provide 10 µN of tangential force. The millinewtons de Boer wanted called for a means of mechanical amplification, so he gave the nano- tractor a force-amplifying plate that spans two frictional clamps. It moves along in 40 nm increments at up to 80,000 Hz, at about 3 mm/s. The force it develops is measured by a special nonlinear load spring that increases the force a bit more with each step. When the maximum tangential force of 2.5 mN is reached, it stalls out.

The nanotractor’s behavior indicates that adhesive forces cause a deviation from Amontons’ law.

To determine the coefficient of static friction, the nano- tractor is first caused to travel ~20 µm pulling the load spring. A large voltage on one clamp then holds it in place. As this voltage is gradually decreased, the frictional force it can sustain is also reduced. At some point, the load spring overcomes that frictional force and the nanotractor slides backward several microns toward zero position until the load spring force decreases and it stops again. Repeating the measurements at lower and lower loads allows the researchers to study Amontons' law over a wide force range.

The results indicate that the friction force is indeed determined by normal forces from 1 mN down to 50 µN. Further decreasing the load, however, served to increase the coefficient of static friction-the anticipated deviation from Amontons' law as a result of adhesive forces. Dynamic friction, after separating out the effects of inertial forces, air damping, and the spring force, was found to be ~80% of the static friction. Adhesion was also seen to play a role in the dynamic friction testing method the researchers developed. In fact, they were able to measure friction when a small tensile (negative) normal load was applied. Adhesion was also seen to play a role in the dynamic friction testing method they devised.

Next, the investigators set out to evaluate the perform- ance of the nanotractor as implied by the friction data. For these tests, the trailing clamp was loaded during the stepping cycle. As expected, a decrease in average step size was observed. Attempts to use the measured coefficients of friction failed, however, because the model predicted much larger step sizes. The problem was found to be a backward slippage of the leading clamp, more heavily loaded than the trailing clamp and therefore assumed to be secure. High-resolution imaging revealed slip as great as 200 nm before the static friction event.

Sliding of this magnitude has serious implications for micro-optical MEMS applications calling for positioning with nanometer accuracy. Of course, identifying the cause of slip will be very helpful as the researchers continue to work with the nanotractor and other nanoactuators. Other members of the research team include Sandia's Alex Corwin, David Luck, Marc Polosky, and Dustin Carr; Bob Ashurst, Univer- sity of California, Berkeley; and Rod Carpick, University of Wisconsin. The project was fund- ed by Sandia's Laboratory Directed Research and Development project, "High Fidelity Friction Models for MEMS," overseen by Dave Reedy. The work has been reported in the Journal of Microelectromechanical Systems, Vol. 13, No. 1, Feb. 2004; and Applied Physics Letters, Vol. 84, no. 13, March 29, 2004.

Contact Maarten P. de Boer, Sandia National Laboratories, Albuquerque, NM; 505-844-9509,

Fire on the Mountain! Run, Boys, Run!

As this issue goes to press, wildfires are clawing their way through thousands of acres in the West. The courageous men and women who battle these blazes apply various strategies-extinguish, contain, or sometimes allow the fire to safely burn itself out. Making the best judgment call requires as much solid data on the enemy's likely behavior as can be collected and disseminated to the firefighters in the field.

The State of the Art
Today's state of the art is based on identifying areas where lightning is apt to strike and the probable results so that crew leaders can be more or less prepared to contend with them. The Predictive Services of the Northwest Interagency Coordination Center in Portland, OR, can forecast lightning activity based on both past and present atmospheric conditions. By monitoring temperature, rainfall, RH, moisture levels in the trees and brush, and other parameters, the prediction unit can also anticipate with remarkable accuracy the behavior of lightning-sparked conflagrations in the regions of interest. These forecasts are good for a week to 10 days out.

Red and white are hot air and smoke from the fire.
Wind speed and direction are also simulated.

Coming Soon
But improvements are in the works. Re searchers at four universities and the National Center for Atmospheric Research (NCAR) are working on a way to collect and deliver real-time data that will reliably predict wildfire behavior and progression. The information will be relayed to firefighters at the scene, providing a basis for deciding the best course of action and at the same time warning the crews about changing conditions that could bring injury or even death. Recent advances in computer speed and power, high-speed information networks, satellite and sensor monitoring, mathematical theory, and meteorology are making this possible. The Data Dynamic Simulation for Disaster Management project team is headed by UC-Denver mathematician Jan Mandel, who will work with a coupled weather and wildfire computer model developed at NCAR to build a software system that will determine wildfire spread scenarios and probabilities. Acting on data from the actual fire scene, such as temperature, wind direction and speed, and the amount of moisture in combustible materials in the area, a supercomputer will use the model to continuously send maps and predicted fire locations over wireless links to the front lines-which could be hundreds of miles away-in real time, to be downloaded onto palm pilots and other wireless devices carried by the fire crews.

Minute-by-minute predictions of where weather, terrain, fuels, and winds created by the fire could combine to create or intensify a conflagration will help fire managers decide upon the most effective response. For example, more firefighters and equipment might be brought in, or the fire could be allowed to spread harmlessly under controlled conditions for hazardous fuel reduction and natural resource benefits.

The technology is scheduled to be tested on a real wildfire in four years. In addition to NCAR, the consortium consists of researchers at the University of Colorado at Denver, the University of Kentucky, Texas A&M Univer- sity, and the Rochester Institute of Technology. The project is funded by a $2 million grant from the National Science Foundation's division of atmospheric sciences.

Contact Janice Coen, National Center for Atmospheric Research, Boulder, CO; 303-497-8986,

Safety Now
Guardian Mobility Corp. has developed satellite communication devices that locate and report on people, machinery, or other assets in remote areas. The Guardian Sentinel GPS, about the size of a paperback book, can be quickly attached to a backpack, vehicle, or piece
The Guardian Sentinel GPS, easily attached to a backpack, transmits a firefighter’s location.
of machinery and is currently being implemented by wildland firefighting crews. Location data acquired from the Global Positioning System, as well as other data transmissions, are sent to a network of low Earth orbit satellites and then relayed to computer servers at the company's headquarters and/or forwarded to firefighting control centers. Transmission events can also be sent directly to the recipient via email, cell phone, or fax. The data identify the exact location of individuals or machinery equipped with the Sentinel. By adding sensors, the devices can improve situational awareness by monitoring water quality, fuel supply tank levels, and intrusion alarms in their immediate environment. The device also reports on unattended facilities and assets in the field.

The Sentinel GPS has received Industry Canada Technical Acceptance Certification for operation as radio communications devices in Canada. It is currently being tested under FCC rules for operation in the U.S. The first shipments of the product are going out to the U.S. Department of Agriculture, British Columbia Forest Service, Parks Canada, Saskatchewan Environment, and more than a dozen other agencies.

Contact Jean Carr, Guardian Mobility Corp., Ottawa, Canada; 613-225-8885, x-359,

NASA Puts Anderson Loop into the Public Domain

The Anderson loop measurement circuit topology, an alternative to the Wheat-stone bridge, has been placed in the public domain by NASA. This makes the worldwide circuit topology choice between Wheatstone's bridge and Anderson's loop now a technical decision uninfluenced by patent licensing concerns.

Compared to bridge-based signal conditioning designs, the Anderson loop yields inherently linear static and dynamic outputs uninfluenced by even random lead wire resistance variations. And the loop topology develops twice the output level (6 dB) for the same power dissipation in sensing elements, significantly improving measurement system SNRs in noisy environments.

The original Anderson loop patent was granted to NASA in December 1994 and has been returned to the inventor, who has retired from the agency.

Contact Karl Anderson, Valid Measurements, Lutz, FL; 813-920-3052,


Edgar Allen Poe’s “Premature Burial” is one scary short story. The title says it all—a hapless soul is buried alive because he exhibits no conventional signs of life. Such a mischance was generally feared in the 18th and 19th centuries, and with good reason. People did recover consciousness during—and after—their own funerals. Poe’s yarn predates our own era of sophisticated medical technology, of course. But it’s a good idea to stay nervous. Anesthesia has been known (and reported) to fail on the operating table, leaving the patient awake, not necessarily in pain (the analgesic effects of the anesthesia keep working), but in a state of paralyzed panic. Imagine! A preventive has been proposed: a bi-spectral index (BIS) monitor that could indicate a level of alertness which would prompt the anesthesiologist to adjust the dosage of medication. BIS monitors use a modified electroencephalogram to quantify the way drugs that act on the central nervous system affect the level of consciousness. The monitors exist, but have not found favor with the Society of Anesthesiology for reasons that have not been made public. We understand that of the estimated 20 million surgical procedures carried out annually, only a trifling 0.1% or 0.2% are performed on the accidentally wakeful. That is a very small percentage, but of a very large number. You do the math. We would rather think about something else entirely. Hurrah! NASA has given its preliminary nod to a robotic mission intended to save the Hubble Space Telescope! The rescue operation will probably be led by Dextre, a two-armed robot developed by the Canadian Space Agency for work on the ISS. We truly believe that the recent public outcry, most of it not even from astronomers, had considerable influence on NASA’s reconsideration. The final decision will be made in a year, based on proposals received. Send them in!

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