The future for vibration energy harvesting and wireless sensors looks bright, according to a leading academic in the field.
Vibration-powered wireless monitoring technology has the potential to enable maintenance free, autonomous measurement of the behaviour of structural elements of infrastructure.
Backers are hoping the technology will reduce costs of obtaining masses of data, useful for managing capacity, maintenance and safety.
The potential is vast: cars, jet engines, bridges, sewers are right now expelling untold amounts of kinetic energy.
And the process to collect this energy seems simple at first glance: cantilevers, transducers, capacitors and wireless transmitters.
“What’s changed is the convergence of technology,” says Ashwin Seshia, head of sensors research for Cambridge University’s department of engineering’s Centre for Smart Infrastructure and Construction (CSIC). “Particularly the convergence of low power sensors and wireless technology to collect data, aggregate and make decisions for large-length time scales for monitoring city-scale infrastructure.”
CSIC was founded in 2011 to attract grants, push research, and importantly, apply the findings to industry.
“We’ve been particularly targeting transportation, or sensor nodes in closed locations, where access to sunlight isn’t available, in London Underground tunnels for instance, or where there are large temperature gradients, on a bridge,” says Seshia.
“For instance on a bridge, there are a lot of monitoring locations of interest: you have vibration induced by traffic, response to wind, different types of loading. This can be quite substantial, in many cases it’s the most plentiful source of ambient energy around that you can scavenge from.”
Explaining how the harvester works, Seshia says it must be tuned to vibrations, which differ depending on the source.
“This is typically limited to certain types of monitoring scenario, for example a pump driven by a fixed frequency motor, in that case the structural vibrations are stuck at a certain frequency. Or if the structural frequencies of a bridge don’t change much, you can optimise for that.”
But conventional resonant-approaches to scavenging kinetic energy can get conventional results. To generate more power, Dr Seshia has been investigating “parametric resonance”.
One way to imagine parametric resonance is playing on a swing in the park. Rocking back and forth will drive the natural harmonic oscillation. But the swing can also be parametrically driven by alternately standing and squatting at key points in the swing arc. This changes moment of inertia of the swing and hence the resonance frequency, and the person on the swing can quickly reach larger amplitudes, so harnessing parametric resonances ought to be able to generate more energy.
Research is going on to reduce the size of the devices from about 200,000mm3 to a micro electromechanical system (MEMS) scale: about the size of a computer microprocessor, or 1,000mm3. The research could allow for huge gains in the efficiency of vibration energy generation.
“At the MEMs scale at the moment we have a device that generates about 15 microWatts for a 100mm2 chip, with designs established last year. There’s work to increase that to about 1 milliWatt peak (1,000 microWatts).”
At this rate of size/power the dream to create a fully self-sustaining sensor system becomes a reality. Seshia says the functions requiring the highest energy usages, or about 10 milliWatts, would be sending information and local computing.
At the MEMs scale at the moment we have a device that generates about 15 microWatts for a 100mm2 chip, with designs established last year. There’s work to increase that to about 1 milliWatt peak
Once the energy is harvested it must be stored. This can be a challenge for batteries as, ideally, sensors would be installed for years or decades at a time.
“You do run out of competing requirements in terms of sensing requirements and available energy to use…. potentially you do want them deployed for a long time, that’s when batteries become very expensive. And if you’ve got monitoring in areas that are remote, hard to access, in a harsh environment, you want to have a ‘fit and forget solution’.”
Despite the challenges, the CSIC is already capitalising on what is a rapid growth industry. The programme has created a handful of patents and spin-off companies.
“There are very active academic communities, organising a lot of events, networking events, from small scale workshops to international conferences, and we share a lot of our ideas very regularly,” Seshia says.
Spin-off Perpetuum, from the University of Southampton, has installed sensors underneath Southeastern Railway carriages to monitor bearing maintenance. There is another company being set up, but it’s ‘hush hush’ right now, says Seshia.
“The company’s still in what we call ‘stealth mode’ so I can’t provide details of what it’s doing. It should be launched in a few months’ time with a website and the detail that’s publicly available at that point.
Road vehicle possibilities
But he can reveal that the company is looking into road vehicles: notorious consumers and distributors of energy.
“In areas of very high temperature, with plenty of vibration available, no access to sunlight, no large temperature gradients, energy harvesting is of interest.
Dr Seshia says the research could play a part in the development in automated vehicles which will require more data and more sensors.
The company has also talked with aerospace companies interested in applying sensors to jet engines.
“These are currently wired up, but every kilogramme of cabling you add on an aircraft is adding litres of fuel. There are plenty of vibrations available, thousands of Gs’ force, so we could be looking at these sorts of systems that are entirely self-powered and operated in an environment where it’s hard to get batteries to work because of the temperature conditions.”
On how much the devices cost, Seshia offers a tight-lipped “no comment”. But it will ultimately be market-driven, with the customer likely to pay for data, rather than electronics.
“Perpetuum doesn’t sell its harvester as a separate component, they sell the data as part of an integrated monitoring solution. They deploy it and then sell the data to the railway companies,” he explains.
Seshia says he did not expect to be dealing with whole of life asset management when he started working in this area, but it’s increasingly the key to unlocking its potential.
“At this time, we’re looking at reliability and lifetime in a lot more detail, with more deployments, so it’s an interesting area where there are questions of design to be answered.”
The academic says energy harvesting requires a multidisciplinary approach, especially between laboratory researchers, civil engineers and infrastructure asset managers.
“It is exciting to be able to look at designing objects, integrating new physics with trying to meet practical applications.
“As well, getting to issues of scale of manufacturing and how to integrate these sorts of systems and look at issues around long-term deployment, reliability, modelling, it is a field that’s very interdisciplinary.”