MIT researchers have developed a battery-free, self-powered sensor that harvests energy from the environment. Because it requires no batteries that must be charged or replaced, and no special wiring required, the sensor can be embedded in hard-to-reach places, such as the innards of a ship's engine. There, it automatically collects data on the machine's power consumption and operation over time.


Researchers have built a temperature-sensing device that harvests energy from magnetic fields generated in the open air around power lines. One simply clips the sensor around a live wire (perhaps the wire that powers the motor), and it automatically collects and stores energy to monitor the motor's temperature.

“This is ambient electrical energy—energy that I don’t have to go through a specific solder connection to get,” said Steve Leeb, Emanuel E. Landsman Professor of Electrical Engineering and Computer Science (EECS) and professor of mechanical engineering, a member of the Electronics Research Laboratory. “That makes this sensor very easy to install.”

In this feature article, which appears in the January issue of IEEE Sensors Magazine, researchers provide a design guide for energy-harvesting sensors that allows engineers to balance the energy available in the environment with their sensing needs.

The paper draws a roadmap for the key components of a device capable of continuously sensing and controlling energy flow during operation.

This versatile design framework is not limited to sensors that harvest magnetic field energy, but can also be applied to sensors that use other power sources, such as vibration or sunlight. It can be used to build sensor networks for factories, warehouses and commercial spaces that are cheaper to install and maintain.

"We provide an example of a battery-free sensor that does something useful and prove that it is a practical solution. Hopefully, others will also use our framework to design their own sensors."

Joining Monagle and Leeb in writing the paper is electrical engineering and science graduate student Eric Ponce.

John Donnal, an associate professor of weapons and controls engineering at the U.S. Naval Academy who was not involved in the work, studies technology for monitoring ship systems. He said getting power on ships is difficult because there are few sockets and there are strict restrictions on what devices can be plugged in.

Donnell added: "For example, continuous measurement of pump vibrations can provide the crew with real-time information on the health of bearings and supports, but powering the additional sensors often requires so much additional infrastructure that it is not worth the investment. Energy harvesting systems like this can add a variety of diagnostic sensors to the ship, greatly reducing overall maintenance costs."

Researchers had to address three major challenges to develop an effective, battery-free energy-harvesting sensor.

First, the system must be able to cold start, meaning it can start the electronics without an initial voltage. They achieved this using a network of integrated circuits and transistors, enabling the system to store energy until a certain threshold is reached. It only turns on when the system has stored enough energy to be fully operational.

Second, the system must efficiently store and convert the harvested energy without using batteries. While researchers could add batteries to the system, this would increase the system's complexity and potentially pose a fire risk.

"You might not even have the luxury of sending a technician out to replace the battery. Instead, our system is maintenance-free. It harvests energy and runs on its own," Monagle added.

To avoid the use of batteries, they use internal energy storage technology, including a series of capacitors. Capacitors are simpler than batteries and store energy in an electric field between conducting plates. Capacitors can be made from a variety of materials and their functionality can be adapted to suit various operating conditions, safety requirements and available space.

The team carefully designed the capacitor to be large enough to store the energy needed for the device to turn on and start collecting electricity, but small enough so that the charging phase doesn't take too long.

Additionally, because the sensors may not be turned on for weeks or even months to take measurements, they want to ensure that the capacitors can hold enough energy, even if some energy leaks out over time.

Finally, they developed a series of control algorithms that dynamically measure and budget the energy collected, stored and used by the device. The microcontroller is the "brain" of the energy management interface, constantly checking how much energy is stored and inferring whether to turn sensors on or off, take measurements, or shift the harvester into a higher gear to collect more energy for more complex sensing needs.

"Like shifting gears while riding a bicycle, the energy management interface looks at how the harvester is doing, essentially whether it's pedaling too hard or too lightly," Monagle explains, "and then it changes the electronic loads to maximize harvesting power and match the harvesting power to what the sensor is asking for."

self powered sensor

Using this design framework, the researchers built an energy management circuit for an off-the-shelf temperature sensor. The device harvests magnetic field energy and is used to continuously sample temperature data, which is then sent to a smartphone interface via Bluetooth.

The researchers designed the device using ultra-low-power circuits, but quickly discovered that there were strict limits on the voltages these circuits could withstand before collapsing. Harvesting too much electrical energy may cause the device to explode.

To avoid this, their energy harvester operating system in the microcontroller automatically adjusts or reduces the amount of harvested energy when there is too much stored energy. They also found that communication - transmitting data collected by the temperature sensors - was by far the most power-hungry operation. "Ensuring that sensors have enough stored energy to transmit data is a long-term challenge that requires careful design," Monagle said.

In the future, the researchers plan to explore less energy-intensive means of transmitting data, such as using optics or acoustics. They also want to more rigorously model and predict the amount of energy entering a system, or required for sensor measurements, so that devices can efficiently collect more data.

"If you only take the measurements you think you need, you may miss something of real value. With more information, you may learn something about the device's operation that you didn't expect. Our framework allows you to balance these considerations," Leeb said.

"This paper details the internal structure of a practical self-powered sensor node in a real-world scenario," said Jinyeong Moon, assistant professor of electrical and computer engineering in the Florida A&M University-Florida State University College of Engineering. "The overall design guidelines, especially the cold start issue, are very helpful. Engineers planning to design self-powered modules for wireless sensor nodes will greatly benefit from these guidelines and easily check off the tedious checklist traditionally associated with cold start."

This work was supported in part by the Office of Naval Research and the Grainger Foundation.