SENSITIVE AND CHEAP: To achieve a thousandfold sensitivity gain, HP’s new MEMS inertial sensor design uses a much larger proof mass than what’s inside traditional MEMS designs. The proof mass [light blue] is a piece of silicon suspended on a spring inside the device. MEMS accelerometers use a variable capacitor to measure motion. A capacitance is formed between any two electrodes [yellow and dark blue] in proximity to each other.
HP changed the “proof mass” [see illustration], which is a piece of silicon suspended on a stiff silicon spring or flexion inside the MEMS device that moves when the sensor moves. Electrodes on the mass, and in an unmoving section of silicon such as the device’s surface, form a capacitor. The capacitance changes when the mass moves, and that change translates to acceleration information.
It turns out that there is a limit to the smallest measurable acceleration signal (called a noise floor), and that limit is set by the thermal vibration of the atoms in the proof mass. In a small proof mass, statistics take over. Taking a simple model as an example, if the mass contains 1000 atoms, 50.1 percent of atoms might jump one way, and 49.9 percent might jump the other way. As a result, the proof mass moves. “We can measure the position of the proof mass accurately enough to see it wiggling from that thermal energy,” says Erickson. If the proof mass is too small, those vibrations are indistinguishable from acceleration.
To solve that, Hartwell says, you need to add mass. With a big enough mass, the actions of the atoms are more likely to cancel one another out. The larger the number of atoms in the proof mass, “the smaller that [thermal] vibration,” he says. The proof mass inside HP’s new device is 1000 times as massive as the proof mass inside most consumer MEMS accelerometers. “So that dramatically improves the sensitivity of the device,” says Hartwell—by a factor of 1000.
Making a component chunkier might seem easy, but in fact, most MEMS production processes can’t do it. The new device has the sensitivity needed for high-end applications, but it won’t carry the old price tag. The sensing device will not be available for individual sale. Rather, HP wants to piggyback the technology on other sensors, so the accelerometer is essentially free.
* HP sensors requires less than 50 milliwatts
* HP envisions 1 trillion sensors in use around the world, creating a central nervous system in the form of a complex, far-flung sensor network that could monitor climate change, help with oil and gas discovery and seismic monitoring, and likely be useful in monitoring the health of the United States’ roughly 600 000 bridges
Age of Abundant Data – Low Hanging Fruit for the Somewhat Better Nanotech/MEMS
* Aggregating information from sensors in cell phones to gather information from the environment.
* cheaper, smaller and more effective sensors
* key aspect is getting the power usage down and getting better at harvesting energy from radio waves and ambient energy
More HP Sensors
For CeNSE to work, “we have to make sensors that are vastly more sensitive than anything else that have ever existed before, while being absolutely dirt cheap so that we can deploy them in very large numbers,” Williams says.
Hartwell is working on the first sensor to go into the field, a motion and vibration detector. More accurately called an accelerometer, Hartwell’s device is sensitive enough to “feel” a heartbeat. The source of that sensitivity is a 5mm-square, three-layer silicon chip. A portion of the center wafer is suspended between the two outer wafers by flexible silicon beams. When the chip moves, the suspended center lags behind due to its inertia. A measurement of that relative motion is used to calculate the speed, direction and distance the chip has moved.
This exquisitely sensitive accelerometer can detect a 10 femtometer change in the position of its center chip. That’s less than one-billionth the width of a human hair. As a result, it can measure changes to acceleration in the micro-gravity range. That’s about 1,000 times more sensitive than accelerometers used in a Wii, an iPhone or an automobile’s airbag system.
IQSL Lab researchers also plan to add sensors for light, temperature, barometric pressure, airflow and humidity.
While Hartwell’s accelerometer gives CeNSE its “feel,” the system’s “taste and smell” are just around the corner. Researchers in the group are using nanomaterials to boost a standard chemical and biological detection technology (Raman spectroscopy) to 100 million times its usual sensitivity rates. As sensitivity rises, sensor size can shrink. That could lead to detectors small enough to clip onto a mobile telephone. With a wave over produce, the sensor might warn consumers of salmonella on spinach leaves or pesticides present in “organic” produce, Hartwell says.
* one sensor can give a lot of false positives
* Many sensors would enable collective detection and confirmation of an effect
* first industry targets are energy, transportation and manufacturing
* use cloud computing to process it