Bio-Inspired Sensor Electric Eel Sensors for Robotic Arms

Inspired by the electric eel, a research team has built a sensor that detects metals, plastics, glass, and wood by measuring changes in an electric field. It is currently being tested for noncontact material inspection, gesture control of robotic arms, and the inspection of coatings.

Electric eels hunt in the murky waters of the Amazon. Their eyes are of little use there. So they generate an electric field around their bodies to track down their prey. Researchers in China have now developed a sensor that works on exactly this principle. The researchers hope that robots will be able to use it in the future to detect objects without touching them.

The Sensor Measures without Contact and in the Dark

The team at Xidian University in Xi'an described this “bio-inspired electrostatic field sensor” in early July in the journal *Advanced Materials*. According to the paper, it operates without actively transmitting signals and without mechanical contact. It is also independent of lighting conditions, the paper states. The invention could be useful in a variety of scenarios in the future, wrote the Chinese science newspaper Keji Ribao. These include, among other things, contactless material identification on production lines, touchless control of robots via hand gestures, and non-destructive testing of coatings.

How the Sensor Inspired by the Electric Eel Works

“We are not fully replicating the electric eel’s biological system. Instead, we are borrowing its basic principle of generating an electric field, detecting disturbances, and deriving information from them,” the newspaper quotes team leader Zhang Weiqiang of the School of Aerospace Engineering at Xidian University as saying.

At the heart of the sensor is a fluoropolymer electret, which the researchers charged using a method called corona polarization. An electret can permanently store an electric charge, much like a permanent magnet stores a magnetic field. The science journal compares it to a tiny “electrostatic battery.”

“Once charged, it can retain its electrical charge for a long time and form a relatively stable quasi-static electric field around the sensor. You can think of it as an invisible spiderweb of electric fields surrounding the sensor,” Zhang explained to a reporter from the newspaper.

“When an object approaches, the sensor detects changes in the field, and from these changes it can determine the object’s electrical conductivity, dielectric properties, and geometric shape,” the South China Morning Post in Hong Kong quoted the scientist as saying.

Whether it’s metal, plastic, glass, or a human hand, every object alters the local distribution of the field in its own way. It’s like a stone falling into water and creating its own unique ripples. This creates a measurable potential signal at the sensor’s grounded reference electrode.

“A 50-micrometer change in position can produce a voltage change of about one volt,” Zhang said. That is roughly half the diameter of a human hair. According to the paper published in *Advanced Materials*, the sensor’s signal remains stable even after 10,000 measurement cycles.

The Sensor Detects Metals, Polymers, Wood, and Glass

“Different materials interfere with the electric field in different ways, just as every person has a unique fingerprint,” said the team leader. Inductive sensors respond primarily to metal. Optical methods depend on the surface’s reflective properties. The novel eel-inspired sensor, on the other hand, responds to metals, polymers, glass, and wood. In automated sorting of recycled materials or on a production line, a machine could use it to determine the material of an object without the need for physical contact. The sensor can distinguish between two parts that are identical in color and appearance, provided they are made of different materials.

Experiments: Controlling a Robotic Arm Using Gestures

Another property of the sensor is of interest for controlling robots in hazardous environments. When a hand moves within the sensor’s field of view, each gesture follows a different trajectory. It also leaves behind its own unique disturbance wave. The team collected these signals, trained a neural network on them, and experimented with controlling a robotic arm. In the lab, at least, the closed loop consisting of gesture input, signal recognition, and execution worked. However, they also noted that there is still a long way to go before a practical application is possible.

Potential Applications for Gesture Control 

In the future, however, many exciting applications are conceivable. “An operator doesn’t have to wear any equipment. All they have to do is perform simple swiping, tapping, or grasping gestures in front of the sensor, and the robotic arm will carry out the corresponding action with precision,” said Zhang. A worker could thus instruct a robotic arm to grasp and move an object with a simple hand gesture in the air. Such technology would be valuable in medical isolation areas, in hazardous work environments, or in assistive devices for people with disabilities, writes the Keji Ribao. 

Test Coatings

In the manufacturing industry, the electric eel sensors could also be particularly useful in any application where surfaces need to be inspected as gently as possible. “Coating defects alter the local dielectric properties and leave a characteristic signal signature in the electric field. The sensor can detect these changes with a high degree of sensitivity,” Zhang was quoted as saying.

Small bubbles, scratches, or variations in thickness in coatings reduce a product’s performance, and currently, transparent layers and curved surfaces pose challenges for optical inspection. The team from western China tested its sensor on glass with a conductive coating, on composites made of stainless steel and polyimide, and on UV-cured PMMA coatings. The researchers reported that defects of varying severity produced distinguishable signals.

Sensor Applications in Space Exploration

Such sensors could even be used in space exploration one day, for example, to monitor equipment outside spacecraft. After all, they are “energy-efficient, contactless, and light-independent proximity sensors,” Zhang told the South China Morning Post.

In future experiments, the team plans to evaluate the signal's robustness, the stability of the entire sensor, and its reliability under challenging conditions such as a vacuum, temperature fluctuations, or complex electromagnetic fields.

* Henrik Bork, a longtime China correspondent for the *Süddeutsche Zeitung* and the *Frankfurter Rundschau*, is Managing Director at Asia Waypoint, a consulting firm specializing in China and based in Beijing. “China Market Insider” is a joint project of the Vogel Communications Group, based in Würzburg, and Jigong Vogel Media Advertising in Beijing.