<< BACK to the NEWS
Editor's Note: The following story is one of the features in AUVSI's new magazine,
Mission Critical: Sensors. For a look at the entire issue, click here.
Tiny and Everywhere: The Unmanned MEMS Movement
By Ramon Lopez
|Norway’s Northern Research Institute has developed an unmanned fixed-wing aircraft, named CryoWing, which can be used for power line inspection, environmental monitoring (land and sea), aerial mapping and meteorological measurements. The CryoWing is well suited for operations in extremely cold weather. Xsens provides the CryoWing’s heading and attitude control. Photo courtesy of Xsens.
MEMS devices — tiny machines with moving parts — are everywhere these days, and they have wrought a revolution for shrinking sensors that operate unmanned systems.
An acronym for microelectromechanical, the shrunken sensors can be found throughout daily technologies. Arrays of micromirrors, for instance, enabled digital film projectors, and MEMS gyros and accelerometers like those in Nintendo’s Wii controller have changed gaming forever. MEMS accelerometers provide orientation for smartphones and image stabilization for digital cameras. And smartphones speakers incorporate one or more MEMS microphones.
MEMS devices monitor air pressure in car tires, and auto GPS devices won’t work without their MEMS-based inertial navigation system. Airbag crash sensors and side-impact airbags are lifesavers because of MEMS accelerometers, as are MEMS-based stability control systems that activate during hydroplanes or skids. MEMS accelerators control auto parking brakes, and MEMS-based anti-rollover systems are becoming standard fit in automobiles.
Meanwhile, automakers are stepping up efforts to see if a car can monitor driver stress or illness, saving the operator from having an accident. Vehicles with MEMS-based biometric sensors would keep tabs on driver’s pulse and breathing. The steering wheel would sense sweaty palms, a possible prelude to a heart attack or a fainting spell. The driver’s vital health signs would be fed into a car’s safety system that would take action in an emergency. Cars wouldn’t start if a drunk driver gets behind the wheel. Already, some autos have steering sensors that detect drowsy drivers.
Devices, such as seatbelt-based respiration sensors, are getting cheaper and smaller through the magic of MEMS. The technology could also lead to self-driving cars that combine artificial intelligence software, a global positioning system and an array of sensors to navigate through traffic. Taxicabs might shuttle fares without a driver; people with medical conditions and ineligible for a driver’s license would get around with a virtual chauffer.
Digital health feedback systems use MEMS sensors the size of a grain of sand to detect medications and record when they were taken. And one day, electro-responsive fibers in sleepwear and soft electronics in pillows will monitor your blood pressure, sleep patterns and stress levels while you slumber.
Researchers in Europe have developed a vest embedded with sensors that measure the wearer’s muscle tension and stress level. At the core of the vest is wearable electronics consisting of sensors woven into the fabric that register the electrical excitation of the muscle fibers and thin conducting metallic fibers that pass the signals to an electronic analysis system.
Muscle tension changes with their stress level. Though barely perceptible, electrodes register the change. Electrodes affixed to test subjects’ chests induce stress, making clinical test results of very little use. The smart vest was developed for inconspicuous measuring during stress studies. The vest can also contribute to workplace safety, and sports coaches could use it to measure whether athletes have reached their performance limits.
MC10, a startup U.S. company that makes flexible electronics, recently unveiled a new product: a sports skullcap that measures contact sport impacts that could cause severe concussions. The device is thought to incorporate accelerometers wired up with the firm’s stretchable electronics. The device can also support research into combat brain trauma.
The technology could lead to skin patches that monitor whether the wearer is sufficiently hydrated and other adhesive patches that monitor heartbeat, respiration, temperature and blood oxygenation. The skin patches can wirelessly transmit the medical data to a smartphone. One day, an inflatable balloon catheter equipped with sensors will snake through the heart to treat cardiac arrhythmias.
Surgery to treat strokes, hardened arteries or blockages in the bloodstream may be helped by MEMS-based micromotors small enough to be injected into the human bloodstream.
Australian researchers are harnessing piezoelectricity to power microbot motors just a quarter of a millimeter wide. Remote-controlled miniature robots small enough to swim up arteries could save lives by reaching parts of the body, like a stroke-damaged cranial artery, that catheters are unable to reach.
With the right sensors attached to the microbot motor, a surgeon’s view of a patient’s troubled artery can be enhanced and the ability to work remotely also increases the surgeon’s dexterity.
Researchers at Louisiana Tech University are taking a different tack regarding piezoelectricity. They have developed a technology that harvests power from small generators embedded in the soles of shoes. It is based on new voltage regulation circuits that efficiently convert a piezoelectric charge into usable voltage for charging batteries or for directly powering electronics. The technology, for example, could power emergency locators for lost hikers or cell phones.
Energy harvesting is an attractive way to power MEMS sensors and locator devices such as GPS. However, power-harvesting technologies often fall short in terms of output, as many of today’s applications require higher power levels.
This technology breakthrough uses a low-cost polymer transducer that has metalized surfaces for electric contact. Unlike conventional ceramic transducers, the polymer-based generator is soft and robust, matching the properties of regular shoe fillings. The transducer can therefore replace a regular heel on shoes.
Scientists at the University of Pennsylvania think along the same lines, having developed a power-generating backpack. The suspended-load backpack converts mechanical energy from walking into electricity. It incorporates a rigid frame pack. Rather than being rigidly attached to the frame, a sack carrying the load is suspended from the frame by vertically oriented springs. It is this vertical movement of the backpack contents that provides the mechanical energy to drive a small generator mounted on the frame.
Meanwhile, Sandia National Laboratories scientists have developed tiny glitter-sized photovoltaic cells that could revolutionize the way solar energy is collected and used. The tiny cells fastened to clothing could turn a person into a walking solar battery charger. The cells are fabricated using MEMS techniques.
MEMS Goes Unmanned
|The MTi OEM is a board-only version of the Xsen MTi. The housing-less MTi OEM is a small and ultra-light (11-gram) AHRS with the same functionality as the regular MTi. Photo courtesy Xsens.
Nowhere has MEMS penetration been more pronounced than the area of sensors and avionics for unmanned systems.
Founded in 2000, Xsens is a privately held company with headquarters in Enschede, Netherlands, and a U.S. subsidiary in Los Angeles. The founders were interested in measuring the performance of athletes, and a company was born with launch of a measurement unit used for human motions and industrial applications.
Clients include Sony Pictures Imagework, Daimler, Sagem, Siemens, Saab Underwater Systems and Kongsberg Defence & Aerospace.
Xsens is a leading innovator in 3-D motion tracking technology and products based upon MEMS inertial sensor technology. Since its inception in 2000, several thousands of motion sensors and motion capture solutions have successfully been deployed in areas such as 3-D character animation, rehabilitation and sports science, and robot and camera stabilization.
Xsens officials have found new uses for MEMS sensors initially designed for rollover detection and impact detection in cars and MEMS gyroscopes used in smartphones and game controllers.
It is a market leader in MEMS inertial measurement units (IMUs), attitude and heading reference systems (AHRS) and inertial navigation systems (INS). Xsens’ IMU consists of 3-D gyroscopes, 3-D accelerometers and a 3-D magnetometer. The AHRS adds filtering to that, estimating 3-D orientation based on the IMU sensor data. An INS additionally uses the accelerometers to find velocity and position, using GPS as a reference.
Xsens offers an alternative to bulky and heavy fiber optic IMUs and ring-laser gyros, shrinking similar tracking performance in a significantly smaller package. Xsens is able to offer high performance in a package that is tens of times smaller than the traditional IMUs and INS used for sonar and unmanned aircraft, according to company officials.
Marcel van Hak, Xsen’s product manager for industrial applications, says his product line wouldn’t exist if not for MEMS technology. Using MEMS subcomponents allows Xsens to produce IMUs, AHRS and INS that average 2 inches in length, 1.5 inches in width and 1 inch in height. A traditional IMU, for example, snugly fits into a 4-inch cube.
He said Xsens uses the same MEMS hardware used by the automotive industry, such as smart seatbelts, but for a different application: stabilization and control of unmanned systems, whether air, maritime or ground vehicles. Xsens also applies the technology for camera systems or platform systems that need to be stabilized.
Xsens, says van Hak, provides systems for the smaller unmanned aircraft, weighing between 3 and 300 pounds. The firm is aboard unmanned aerial systems made by Delft Dynamics and Area-I’s PTERA (Prototype Technology Evaluation Research Aircraft). He said his equipment is also on several robotic underwater vehicles.
Xsens makes systems that keep telecommunications satellites and roving vehicles, whether trucks or maritime vessels, connected. He said half of the firm’s earnings come from that application.
The Dutch company’s current MTi product portfolio includes the MTi-10 IMU, the MTi-20 VRU (Vertical Reference Unit) and the MTi-30 AHRS. The MTi 100-series includes the MTi-100 IMU, MTi-200 VRU and MTi-300 AHRS.
The MTi-G-700 GPS/INS is the successor of the MTi-G introduced in 2007. Deliveries of the MTi-G-700 GPS/INS started in December 2012. The MTi-100 series can serve as a cost-effective replacement unit for high-grade IMUs, making the end product more economically viable.
The MTi-G-700 GPS/INS is now being used to navigate an unnamed European target drone, replacing fiber optic gyros in test aircraft. Xsens established that the unit can cope with very high accelerations during launch and cornering. With similar performance to the fiber optic gyro it replaced, the unit is 15 to 20 percent lower in cost, produces a weight savings and provides more room for payload, says van Hak.
He said the MTi-G-700 GPS/INS will work with other target drones and unmanned air systems. “We are searching for additional customers. We are in discussions with three other customers who are actively considering the MTi-G-700 GPS/INS for their target drones.
“We have integrated the Xsens MTi-G AHRS sensor with a range of products designed for installation on land, sea and air platforms, including tactical and rotary wing aircraft,” says Paul Wynns, aircraft systems program manager at Argon ST, a wholly owned subsidiary of Boeing. “We value the Xsens MTi product line for its ease of integration, reliability and accuracy, along with its small size and rugged packaging.”
Xsens is not alone in supplying MEMS-based sensors to the unmanned systems industry.
MicroStrain is a Vermont business specializing in combining microsensors with embedded processors to autonomously track operational usage and to navigate and control unmanned systems. It has the 3DM-GX3-45 GPS/INS for vehicle tracking, camera pointing, antenna pointing, and unmanned aerial and micro vehicle navigation and the 3DM-GX3-35 AHRS with GPS. MicroStrain also offers the 3DM-GX3-15 IMU and Vertical Gyro. The 3DM-GX3-15 is a miniature IMU that utilizes MEMS sensor technology and combines a triaxial accelerometer and a triaxial gyro to maintain the inertial performance of the original GX3-25. Applications include unmanned vehicle navigation and robotic control.
Two other players in the field are De Leon Springs, Fla.-based Sparton with its AHRS-8 MEMS-based attitude heading reference system. Dallas-based Tronics has introduced a high-performance angular rate sensor (gyrometer) for demanding applications such as platform stabilization. The product is based on Tronics’ long-standing expertise in high-end inertial sensors using MEMS-on-SOI and high-vacuum wafer-level packaging technologies.
Trends in Manufacturing
|Northrop Grumman supplies the fiber optic, gyrocompassing LCR-100 AHRS for Embraer Legacy 500 and Legacy 450 aircraft. The LCR-100 AHRS provides navigation information regarding the aircraft’s position, heading and attitude. Photo courtesy Northrop Grumman.
MEMS have revolutionized every market in which they play, but the trend for the still-nascent mini technology is just beginning. Analysts predict rapid growth for the types of MEMS now in widespread use and in the making.
MEMS devices, especially motion sensors like accelerometers, have changed consumer electronics forever and, more recently, have enabled an emerging market for facial recognition, motion-controlled apps, location-based services, augmented reality and pressure-based altimeters.
The growing use of disposable medical devices and respiratory monitoring is due to MEMS technology. The most common medical pressure sensor is the disposable catheter to monitor blood pressure. Another type if disposable, low-cost MEMS pressure sensor is the infusion pump used to introduce fluids, medication and nutrients into a patient’s circulatory system. MEMS pressure sensors are used in respiratory monitoring, such as the Continuous Positive Air Pressure device, used to treat sleep apnea, and oxygen therapy machines.
MEMS devices will proliferate as cheaper manufacturing techniques for the micro machines are developed. Massachusetts Institute of Technology researchers have found a way to manufacture them by stamping them on plastic film, opening up the possibility of coating large areas with tiny sensors.
That should significantly reduce their cost, but it also opens up the possibility of large sheets of sensors that could, say, cover the wings of an airplane to gauge their structural integrity. The printed devices are also flexible, so they could be used to make sensors with irregular shapes.
And since the stamping process dispenses with the harsh chemicals and high temperatures ordinarily required for the fabrication of MEMS, it could allow them to incorporate a wider range of materials.
Conventional MEMS are built through the same process used to manufacture computer chips, which is called photolithography: different layers of material are chemically deposited on a substrate — usually a wafer of some semiconducting material — and etched away to form functional patterns.
Photolithography requires sophisticated facilities that can cost billions of dollars, so MEMS manufacturing has high initial capital costs. And since a semiconductor wafer is at most 12 inches across, arranging today’s MEMS into large arrays requires cutting them out and bonding them to some other surface.
Besides serving as sensors to gauge the structural integrity of aircraft and bridges, sheets of cheap MEMS could also change the physical texture of the surfaces they’re applied to, altering the airflow over a plane’s wing, or modifying the reflective properties of a building’s walls or windows.
How they did it: The MIT process begins with a grooved sheet of a rubbery plastic, which is coated with the electrically conductive material indium tin oxide. The researchers use what they call a “transfer pad” to press a thin film of metal against the grooved plastic. Between the metal film and the pad is a layer of organic molecules that weaken the metal’s adhesion to the pad. If the researchers pull the pad away fast enough, the metal remains stuck to the plastic.
Once the transfer pad has been ripped away, the metal film is left spanning the grooves in the plastic like a bridge across a series of ravines. Applying a voltage between the indium-tin-oxide coating and the film can cause it to bend downward, into the groove in the plastic: The film becomes an “actuator” — the moving part in a MEMS device.
Varying the voltage would cause the film to vibrate, like the diaphragm of a loudspeaker. Selectively bending different parts of the film would cause them to reflect light in different ways, and dramatically bending the film could turn a smooth surface into a rough one. Similarly, if pressure is applied to the metal film, it will generate an electric signal that the researchers can detect. The film is so thin that it should be able to register the pressure of sound waves.
The researchers are working on better ways to bond the metal films to the plastic substrate, so that they don’t have to rely on tearing the transfer pad away quickly to get the film to stick. They’re also developing prototypes of some of the applications they envision for the technology.
Ramon Lopez is an aviation, aerospace and defense journalist who previously served as editor-in-chief of Air Safety Week, editor of AUVSI’s
Unmanned Systems and Washington Correspondent for Flight International, Jane’s Defence Weekly and International Defense Review.