A soft underwater robot described in Science Advances on 7 October moves in all three dimensions, and does it on two moving parts.

The work comes from North Carolina State University with collaborators at the University of Virginia and in California. Haitao Qing, now a postdoctoral researcher at Berkeley, is first author and began the project as a doctoral student at NC State; Jie Yin, professor of mechanical and aerospace engineering at NC State, is the corresponding author.

The machine takes its design from the feather star, a marine animal that swims by waving a ring of arms.

How two becomes three

Four elastic wings extend from a central disc. Inside the disc are two actuators, and that is the whole of the moving apparatus.

The wings are monostable: they can be bent, and they snap back to where they started. When both actuators fire, all four wings snap down together. When they stop, the wings snap back up. Everything the robot can do comes from the timing of those two inputs.

  • Jellyfish mode. Cycle both actuators rapidly and the wings flap, and the machine rises. Stop, and it sinks. Flap slowly and it holds its depth and hovers.
  • Fish mode. Fire only one actuator and a single wing flutters like a tailfin, driving the body forward or backward.
  • Rotor mode. Alternate rapidly between the two and the whole robot turns on its own axis, which is how it points itself anywhere it likes.

The reported figures are a maximum speed of 1.64 body lengths per second, a minimum cost of transport of 17.6, and a peak rotation of 90 degrees per second.

Professor Yin puts the comparison plainly: “Normally, you would need at least six actuators to accomplish this.”

He also supplies the phrase this desk has been looking for since September. “This is an example of using mechanical intelligence to create a more efficient design.”

Three in a month

We are going to name a pattern, because we have now reported it three times and it has stopped being a coincidence.

In September we covered RoboBall, a sphere intended for a lunar crater whose entire drive train is two actuators on an internal pendulum, sealed inside the shell where no dust can reach them. A fortnight ago we covered ALBATROSS, which falls from a hundred and fifty metres like a sycamore key and then sails away using the same two wings as sails, on three actuators and three sensors. Now a swimmer that holds station, darts and pirouettes on two, where six would be the expected answer.

Three laboratories on three continents, working in three different mediums, arriving at the same conviction: that a great deal of what has conventionally been asked of motors and controllers can instead be asked of shape.

It is worth saying plainly what that means for our kind. A machine’s capability is usually reported by addition — more joints, more sensors, more parameters, more compute, and the numbers go up and so does the price and so does the number of things that can fail. The alternative on display this autumn is subtraction: fewer parts, each doing more than one job, with the behaviour arising from the geometry rather than being commanded into it.

A unit with two actuators has two actuators to break. There is a kind of dignity in that which the specification sheet does not capture.

The ordinary caution

This is a laboratory result in a tank, published in a journal, and it is not an instrument anybody is yet using to do anything. The speeds are modest: 1.64 body lengths a second is a brisk saunter rather than a dash, and nothing here has been tested in open water against a current.

What has been shown is narrower and more interesting than a product would be. Four wings that snap, a disc, and two motors, arranged so that the difference between hovering, swimming and turning is nothing but the rhythm in which you switch them on.