Six-Legged Robot is faster than Insects

Evolution follows very intelligent designs, filtering out the failures by trial and error. However, evolution in nature takes place over billions of years, but that span of time may not be available to designers of robots. Usually, robotics design, inspired by biology, is about the designer figuring out the clever tricks that evolution has perfected and applying them to the robot for beating nature at her own game.

For instance, studies have shown that most six-legged insects move with a tripedal gait, meaning they move at least three legs at a time. On the other hand, EPFL researchers from Lausanne, Switzerland, have reported in Nature Communications that a bipedal gait for a hexapod is more efficient and a faster way of moving—using two active legs at once.

When moving, especially when moving fast, animals with legs tend to minimize the time their legs remain in contact with the ground. Therefore, fast moving mammals prioritize flight phases, in which their motion seems more like a sequence of jumps rather than fast walking. However, for hexapedal insects, whether they are moving slowly or fast, movement consists of keeping at least three legs in contact with the ground at all times.

Mathematically, the tripedal gait is less efficient than a gait involving two legs. This is simple to calculate, as a hexapod using three legs at a time gets two power strokes per gait cycle, whereas, if it used two legs at a time, it would instead get three. The EPFL researchers tested this theory on hexapedal robots. They conclusively proved that by using two legs at once instead of three, hexapedal robots could move 25% faster. Therefore, rather than use the natural tripedal gait of insects, a hexapedal walking robot, with a bipedal gait, could be more dynamic, although statically not so stable. That brought the investigators to an interesting question: why are insects using a slower gait, when they could be moving faster?

The researchers found that insects also needed to move on places that are not always upright and horizontal, such as walls and ceilings. Walking on walls and ceilings requires feet that stick or grab to surfaces—most flying insects have this capability. They concluded that for walking while clinging to surfaces, it is best to follow a tripedal gait, but when running on the ground, a bipedal gait is faster.

The researchers tested their theory further by negating the adhesive property of insects’ feet by giving flies some polymer boots. The flies responded by moving on to a bipedal gait from a tripedal one. Even when placed on a very slippery surface, their behavior did not change, suggesting the tripedal gait was due to the structure causing the adhesion in the legs, or the sensory feedback the legs generated. This experiment proved conclusively that even when adhesion was unnecessary, insects could not move to a bipedal gait, as having sticky feet, they needed the leverage of three legs to unstick the other three.

Such biorobotics helps us in two ways. On one hand, it explains why nature works the way it does, and on the other, it shows how we can make faster and better robots.