Frog-inspired robot uses flexible elastic limbs to hop across rough surfaces and swim through water

Robotics: Animals have repeatedly influenced the development of robots, with researchers analyzing their movements to address concerns involving propulsion and mobility. A new study now looks to frogs for inspiration, with the use of flexible structures to help a small robot generate powerful movements on land and in water.

A Frog-Inspired Approach to Robotic Movement

Researchers from the University of Michigan and the University of California, Los Angeles created a palm-sized robot to analyze how flexible structures can help small machines generate strong movements without depending on motors.


The study, published Sept. 18 in Science Advances, analyzed how twisting bent elastic rods can create a "snapping motion." The mechanism enables the robot to hop and, with an adaptation, swim.

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How the Elastic Limbs Produce Power

The researchers started by analyzing what happens when the ends of a bent, flexible rod are twisted.

As per the conditions, the rod may gradually change its shape or store energy before rapidly shifting into another form. Determining when this abrupt change would happen was one of the major challenges of the research.

To examine the behavior, the team included mathematical models, computer simulations and robotic experiments. The researchers then utilized their findings to make helical elastic limbs that can store energy and release it in short bursts.

Small motors will be rotated at the ends of two bent rods attached to the robot. As the rods bend and twist, they eventually accumulate elastic energy. Once a main point is reached, the rods snap into another shape and strike the ground, sending the robot forward.

The motor then reverses the limb so that it can reset for another hop.

"Typically, with conventional motors, we get motion relatively steadily, like a wheel turning at the same speed," UCLA mechanical and aerospace engineer M. Khalid Jawed stated to ScienceAlert.

Stored Energy Creates a Powerful Burst

The elastic limbs enable the robot to accumulate energy before releasing it suddenly. This means the small motors do not have to produce the complete burst of movement directly.

The untethered robot has its own battery and control electronics and has a measured mass of 98.2 grams.

During testing, it continued hopping in wood, cloth, acrylic, leather, grass and sand. Its highest measured speed was 3.21 body lengths each second.

Flexible Legs Beat Rigid Legs in Testing

To analyze the effect of the elastic limbs, the researchers made another robot with almost identical components but rigid legs. The two machines were tested using the same core components and testing approach.

The flexible-legged robot recorded an average speed of 2.46 body lengths each second. The rigid-legged version averaged 0.79 body lengths per second.

The difference became especially significant on soft cloth and grass, where the rigid-legged robot nearly stalled.

Researchers Highlight the Prototype's Limitations

Despite the robot's range of movements, the researchers highlighted that it remains a research prototype.

Its movement happens in bursts rather than continuously. After every snap, the elastic limb has to reset, and the timing of both the snapping and resetting must correspond with the robot's movement and its interaction with the surrounding environment.

These characteristics leave many regions for further development, especially because the swimming system has not been optimized.

Possible Applications for Smaller Robots

The researchers believe the approach could be mainly important to small machines.

Tiny robots have limited room for heavy motors and complex transmissions. At the same time, they still require enough power to overcome challenges or get out of soft terrain.

A Robot That Demonstrates a Different Way to Generate Power

The current prototype is not an all-terrain amphibious machine. Its burst-style movement, need to reset the elastic limbs, timing requirements and unoptimized swimming capability remain challenges.

Even so, the research illustrates how a robot's structure itself can lead to powerful movement. Rather than needing a motor to directly produce every burst of power, the elastic limbs can store energy and release it when required.

Source: ScienceAlert