SALTO the Agile Jumping Robot

In the Biomimetic Millisystems Lab at UC Berkeley, Duncan Haldane is responsible for numerous bite-sized bio-inspired robots—robots with hairs, robots with tails, robots with wings, and running robots. Haldane and the other members of the lab look at the most talented and capable animals for inspiration for their robotic designs.

The African Galago or Bushbaby is one such animal. It is a fluffy, cute, talented, and capable little jumping animal weighing only a few kilos. However, this little creature can jump over and clear bushes nearly two meters tall in a single bound. Biologists have discovered that Galagos’ legs are specially structured to amplify the power of their tendons and muscles. Haldane and his team accordingly made Salto, a legged robot weighing only a hundred grams, endowing it with agility and the most impressive jumping skills. Salto features in a paper the new journal Science Robotics.

Jumping is not only about how high one can jump, how frequently you can jump also matters. Haldane and his team have coined the term agility to refer to how far upwards one can go while jumping repeatedly. Technically, they define it as the maximum average vertical velocity achieved while performing repeated jumps. For a Galago, it can jump 1.7 m in height repeatedly every 0.78 s—an agility of 2.2 m/s.

Therefore, to be called agile, the jumper not only has to jump high, he also has to be able to jump frequently. For instance, although EPFL’s Jumper can jump to impressive heights of 1.3 m, it can only do so every four seconds. That means its agility is a measly 0.325 m/s. On the other hand, Minitaur can jump up only to 0.48 m, but it does so every 0.43 s, which gives it a much better agility of 1.1 m/s, despite its lower jumping ability.

That means improving agility involves jumping either higher, or more frequently, or both. Galagos are agile, because of not only their ability to jump high, but also their ability to do so repeatedly. Most jumping robots have low agility, because, they need to spend time winding a spring for storing up enough energy to jump again, and this reduces their jump frequency. The researchers at Berkeley wanted a robot with an agility matching that of the Galagos. With Salto, they have come close, as Salto can jump 1 m every 0.58 seconds, earning an agility of 1.7 m/s.

Just as with many jumping robots, Salto also uses an elastic element, such as a spring, for the starting point. For Salto, the spring, actually a piece of twistable rubber is placed in series between a motor and the environment, making a Series Elastic Actuator (SEA). Apart from protecting the motor, SEAs also allow for force control and power modulation, while allowing the robot to recover some energy passively.

Such springs are available to the Galagos in the form of their tendons and muscles. However, the leg of the Galagos is in such a form that allows it to output nearly 15 times more force than its muscles can by themselves. Haldane has used this same design for Salto as well.