HopTo’s Hop-1 Robot Lives on the Bounce

HopTo’s first machine, Hop-1, looks like a small quadcopter that grew a pogo stick. That extra leg is the whole argument. Continuous flight means the motors fight gravity every second the machine is aloft. Hop-1 spends most of its trip bouncing on a passive spring, so the rotors only top up energy lost on each landing and fully take over when a gap, a wall, or a sudden climb actually requires flight. HopTo puts hopping energy use at about a quarter of continuous flight. Stay on the ground as much as possible, and only fly when you have to.

A carbon fiber telescoping leg protrudes beneath a four-rotor body. It is held in position by either rubber bands or a spring, which stores the downward force before firing it back up. Contact is brief, lasting a few dozen milliseconds. As soon as that’s over, the robot transforms into a nearly silent ballistic object, sitting there until it receives another tap from the ground. The City University of Hong Kong team (later known as HopTo) released lab versions of this concept, known as the HopCopter, weighing approximately 35 grams and capable of hopping from 0.6 to 1.63 meters. At their biggest leap, they achieved an average vertical speed of 2.38 meters per second and remained in the air for less than 45 milliseconds. That is why they were able to sustain a relatively high hop rate without the use of a motorized knee or latch.

When they ran endurance tests on that lab rig, they discovered that a 250 milliamp hour battery pack could survive over six minutes in hover mode and more than 20 minutes in continuous hops. A larger 650 milliamp hour pack increased the hopping time to more than 50 minutes, albeit it did push the robot somewhat beyond its flight weight limit. They were also running the motors at a rather low duty cycle, around 0.28. The ability to make contact with the ground greatly enhances their agility; they can dump speed or snap around in midair at accelerations much exceeding 14 g. When they come into contact with a wall or an uneven slope, that same leg acts as a bumper.

HopTo split off from that lab in 2024 and is now developing this concept into a first-generation product. They’ve just finished building an engineering prototype, which recently passed a two-meter drop test on the South African shore, and they’re doing a short public survey ahead of launch. They’re talking about employing Hop-1 for a variety of purposes, from searching rough terrain on inspection missions to regions where hovering is too expensive, and they’re even looking into low gravity worlds on the outskirts of the solar system. Small flying robots have traditionally had rather low battery lives. Hop-1, on the other hand, is taking a novel approach, seeing the ground as a free energy source rather than an obstacle.