Smaller, faster, cheaper – it’s a formula for sensors too. Smaller sensors are related to cheaper sensors, since manufacturing costs are reduced (usually). Smaller can also be related to faster as in the shift from large analog to much smaller digital sensors. One more thing; the very small sensor is much more…discreet. So it’s only natural that sensor designers would turn to micro and nano technology. A team led by Oak Ridge National Laboratory’s (USA) Panos Datskos has found a significant twist to one of the most important new approaches, which involves a microcantilever.
Perhaps you’ve seen cantilevered buildings? That’s where a portion of the building hangs out into open space, supported by the main structure. Another form of cantilever, of a sort, is a favorite analogy for sensor technologists – the diving board. (Walking the plank is a variant, but has unfortunate associations.) As the analogy goes, a microcantilever is like a diving board, when energy is applied to the end of it, like a diver from a diving board, the board springs up and down (oscillates). The heavier the diver, the more force put on the end of the board, the more it vibrates. If you know exactly the spring capacity of the board, you can calculate the weight of the diver. That’s just about what a microcantilever does, calculate the weight of particles that strike it. Of course, this is happening at the level of one or a few atoms, a molecule at most, so that the variations in ‘spring’ have to be measured with the most accurate of lasers. The weight of the particle can be used to determine its identity, making the microcantilever a sensor for almost any kind of gas, or anything that can be found in a gas such as chemical or biological particles in the atmosphere.
Most microcantilever sensors use the approach of measuring the oscillation as a variance from no motion. The ORNL team instead sends an initial pulse of electricity through the microcantilever to give it an immediate oscillation of considerable – but known – magnitude. The measurements of this non-linear oscillation then produce the miniscule readings for the presence of specific atoms (or molecules).
“In the past, people wanted to avoid this high amplitude because of the high distortion associated with that type of response,” said Datskos, a member of the Measurement Science and Systems Engineering Division. “But now we can exploit that response by tuning the system to a very specific frequency that is associated with the specific chemical or compound we want to detect.”
When the target chemical reacts with the microcantilever, it shifts the frequency depending on the weight of the compound, thereby providing the detection.
It is predictable that within a decade micro and nano scale sensor devices will be commonplace, and in most cases the preferred form – especially for surveillance and intelligence gathering systems. The microcantilever sensor will most likely be one of the major types. As in the case of this innovation with the microcantilever, it will be several years of additional trials and experimentation with systems and controls before commercial applications will appear.