Imagine a sensor with wireless communication could operate entirely on an energy harvesting module. Such a sensor would be able to communicate sensory data and have no requirement for any power source and the designer would need not worry about replacing hardware. This is exactly what a team at MIT has done by figuring out how to utilize the Piezoelectric Effect and the backscatter technique to generate power and send and receive data. These two technologies replace the need for a battery.
Figure 1. MIT's battery-less underwater sensor produces power using piezoelectric materials. Image courtesy of MIT.
Remote Sensing for Data Transfer
Since the first transistor electronic circuits have been reduced in size and cost dramatically. A chip that would cost a fortune back in the 60s with five transistors can now be sold for sub-cent prices at a size almost too small to see with the naked eye. Of course, transistor technology is not the only area of electronics to have improved; batteries have been made incredibly small and have high energy densities, sensors are more sensitive than ever, and circuit boards can have as many as 16 layers.
Even with these small sizes, no matter how much we try, electronic devices will always consume power. This need for power results in difficulty when developing for applications where power is a virtue.
One classic example is remote sensing whereby sensory data is needed from a location that is either inaccessible or does not have access to typical infrastructures such as internet and powerlines. In these environments, devices need to either find their own power source or carefully manage a power source that is replaced periodically by an engineer. If the environment permits it, devices can use solar panels and other renewable sources to charge a small rechargeable battery but this can be clumsy, difficult to install and be potentially unreliable.
Another alternative is to use a non-rechargeable battery which can be used in bursts to transmit sensory data. The use of bursts can help conserve energy by having the device sleep for the majority of its operation. However, this still requires user intervention when the battery power is depleted and batteries can be bulky in scenarios where a device needs to operate for more than a few months.
Powering Sensors with Energy Harvesting
One area of electronics, energy harvesting, is gathering interest as it allows for a device to simply “absorb” the natural energy around it to function. While this may sound like magic there are several aspects to energy harvesting that need to be understood.
Firstly, energy harvesting techniques are not able to provide large amounts of power which makes them more ideal for portable electronics and small sensors. Secondly, the “natural energy” sources are typically mechanical (such as vibration and wind) or thermal. While solar and wind are forms of energy harvesting from the environment they are typically not considered when talking about small sensors and remote devices. Implementing energy harvesting can be an interesting task in its own right and it has become popular enough that energy harvesting modules are now available commercially. These allow for connecting to external “micro” power sources such as piezo discs that can properly manage the storage and conversion of the energy obtained from the power source.
A team at MIT has created a new type of sensor that can wirelessly transmit sensory information underwater without the need for a battery.
The operation of the sensor relies on piezoelectric materials and consists of a master transmitter and a slave receiver which holds sensors. When sensory data is required from the slave sensor the master transmitter uses a piezo element to send out vibrations in the water which hit the slave receiver. Upon receiving the vibration, the device can either do one or two things with the vibration; absorb the vibration and store the resultant energy or reflect the signal back.
If the sensor wishes to send back a digital 1 then the sensor configures the piezo element to not absorb the wave but instead reflect the wave back. If the sensor wishes to send back a digital 0 then it absorbs the incoming wave energy but does not send out a wave. The master transmitter can detect the reflected wave pattern and therefore deduce the information that the sensor has sent. At the same time, the sensor is able to power itself with the small pulses of sound and perform sensor operations as needed.
The team has been able to demonstrate this system underwater with the master and two slave devices separated by up to 10 meters and both sensors were able to transmit sensory data at data rates of 3KB per second.
Challenges for Underwater Designs
Fadel Adib, an assistant professor in the MIT Media Lab and Department of Electronic Engineering and Computer Science, said that his inspiration for the sensors came from watching Blue Planet. Stated in the television series about the Earth’s oceans is that 70% of the Earth’s surface is covered in water and that humans have very little knowledge of what truly goes on in the oceans (when compared to land).
An idea to change this would be to have an equivalent of the IoT underwater but getting power to such sensors would be near impossible at depths where there are very little sunlight and no wind. Another major issue of underwater communication is that radio waves such as Bluetooth and Wi-Fi travel very poorly and long wavelengths require a large antenna. This led Adib to explore piezoelectric materials which can both produce and absorb acoustic waves that can transmit energy and convey information which travels very well in water.
Space Applications: the Final Frontier
While this method of underwater acoustic communication and energy harvesting can be highly beneficial to marine studies it may also provide an invaluable solution for space exploration. Places such as Titan which have surface liquid are mysterious and unexplored but sending sensor in such places comes with a range of problems including a lack of power (from the sun) and no method for changing batteries. Battery-less sensors which can communicate with a host from the surface would provide scientists with sensory data for a very long time (potentially of the lifespan of the sensor itself). But until NASA decides to go to another planet with surface liquid MITs underwater sensor system will have to remain on earth and continue to be developed for a potential underwater network.
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