Power Electronics for Robotics 97 Percent Efficiency: Humanoid Charges Inductively through the Feet

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Fraunhofer IISB has developed a wireless charging system for robots that is directly integrated into the feet. Through an intelligent induction field, the energy storage is recharged highly efficiently during operation.

Mobile and humanoid robots currently draw their energy mostly from batteries, which are depleted after just a few hours of operation. A traditional charging process via conductive plug contacts is, however, prone to corrosion and mechanical wear in industrial environments. Alternatives like battery swapping stations force the robot to interrupt its primary task and move to designated areas. For this reason, Fraunhofer IISB is focusing on inductive power transfer. Dr. Bernd Eckard, who heads the Power Electronics department at IISB in Erlangen (Germany), presented his solution during a lecture at the Humanoid Days at the University of Erlangen.

Robotics

Challenges and Opportunities of Humanoid Robotics

"Without efficient energy supply and power conversion, no robot will move. Even if it has the best actuators and the latest software, it still needs energy," says Dr. Eckardt. The concept of his research institute integrates the receiver coils directly into the soles of the robot's feet. In the floor of the workspace, the developers install an array of transmitter coils in parallel. This field operates on the same principle as a modern multi-zone induction cooktop. The system detects the exact position of the robot and activates only the coils where the robot's foot is actually placed.

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Coils in the Foot Transfer up to 14 Kilowatts

A single receiver coil measures approximately twelve centimeters in diameter. This corresponds exactly to the width of a typical robot foot, making the components easy to integrate mechanically. Despite these compact dimensions, a single coil transfers a power of 3.6 kilowatts. If engineers install two coils per foot, a robot could theoretically draw up to 14 kilowatts of charging power through both legs.

Technically, the contactless power transfer is based on a so-called CLLC resonant converter. Unlike the inductive charging of electric cars, where a large air gap must be bridged, a robot stands directly on the floor. This virtually eliminates the gap in the Z-axis. As a result, the system achieves high coupling factors between 0.5 and 0.6. This strong magnetic coupling reduces the need for copper in the windings, making the coils lighter and more cost-effective to manufacture.

High Efficiency Prevents Thermal Issues

The optimized coupling and the use of resonance converters directly impact overall efficiency. The energy transfer from the ground to the robot's direct current circuit operates with an efficiency of 97 percent. This means the inductive system achieves the same efficiency as a conventional wired power supply.

This high efficiency prevents large amounts of power loss in the form of heat. Therefore, the system does not need to be cooled. For inductively charging smartphones, efficiency often ranges only between 80 to 85 percent, which is why these devices become noticeably hotter.

New Freedoms in Robot Design

The ability to continuously charge the robot during its work or at fixed workstations changes the fundamental system architecture. Designers can completely redefine the internal energy storage. Eckardt assesses the potential for mechanical design: "If you supply energy externally during the entire operation of the robot, we can theoretically do away with the onboard energy storage entirely, or at least design it to be very small, which in our view also saves significant weight."

A reduced battery mass not only lowers material costs but also decreases the energy the robot needs to expend just to move its own weight. Additionally, the charging system operates fully interoperably. A manufacturing company can install induction strips or so-called E-roads along main traffic routes. This infrastructure then flexibly and wear-free supplies both humanoid robots and conventional driverless transport systems with energy.