"Flying Batteries" to make micro drones take off

Scientists at the University of California San Diego (UC San Diego) have developed a technique to operate high-voltage actuators of a micro drone together with a novel solid-state battery, achieving potentially long flight times, the researchers write. The piezoelectric actuators developed can also store voltage and release it back to the batteries.

The UC San Diego technology addresses several problems of micro drones. The drones are small and light, but still have to carry quite heavy batteries. This shortens the flight time considerably. Lithium-ion batteries are usually used, which have a high energy density but cannot supply the required voltage for the piezoelectric high-voltage actuators used in micro drones. These require voltages of several dozen to several hundred volts. However, a lithium cell only supplies around 4 volts.

Converting the voltage to a higher one is difficult in compact dimensions. The circuits require large inductors or several capacitors and therefore a volume and mass equal to or greater than that of batteries. Such a system is therefore unsuitable for micro drones.

Cell switching for solid-state batteries

The research team at UC San Diego is therefore relying on a combination of different technologies. For example, the electrical engineers are using a new type of solid-state battery from the French electronics laboratory CEA-Leti. The battery essentially consists of a thin-film material stack made of lithium cobalt oxide and lithium phosphorus oxnitride. It is produced using semiconductor technology and can be broken down into tiny cells. A single cell can store a charge of 20 microamperes. The cell is only 0.33 mm³ in size and weighs 0.8 g. Extrapolated to one liter, this corresponds to 60 Ah. By comparison, lithium-ion batteries deliver 100 Ah in a volume of one liter, but are also around 1000 times larger. The technology is currently still under development.

As battery technology allows the battery to be split into many tiny cells, the researchers have developed a circuit that rearranges the connections of many small cells, i.e. can switch between parallel and serial switching. The underlying circuit chip is only 2 mm² in size.

The battery cells are initially connected in parallel. The voltage is not sufficient to operate the piezoelectric actuators of a micro drone. The circuit rearranges the connections between the cells. They are gradually connected in series. This happens within a few hundredths of a second. Once the voltage is high enough and the actuator is fully charged, the wings of the micro drone move downwards. Now the reverse process begins: the cells are once again connected individually in parallel until the wings snap back. The continuous repetition causes the wings to flap, which can make a microdrone fly.

Actuators as energy storage

The principle also has a positive side effect: the actuator acts like a capacitor and stores energy. Some of this energy is fed back into the battery, similar to recuperation in electric cars.

However, the researchers are still struggling with one problem: the internal resistance of the batteries. "The challenge is that the more batteries are stacked, the higher the series resistance and therefore the lower the frequency at which the system can be operated," says Patrick Mercier, Professor of Electrical and Computer Engineering at UC San Diego.

However, Mercier and his colleagues are confident that they will also be able to get this problem under control and make micro drones fly with their system.

(olb)