Underactuated robotics are so cool. In the hobby BattleBots community, there is a design where the whole robot only has a single motor, but it's able to move in any direction and damage the opponent, all just by controlling the single motor's speed.

Most robots have at least three motors, two drive sides and a weapon motor. The trick to getting away with one is integrating an impactor into the chassis of your bot, using the single wheel to spin up to ~3k RPM, then by tracking the rotation angle of the bot as it spins, you speed up a little while the wheel is pointing where you want to go, and slow it down a little when it's pointing the other direction.

▲ Onavo 3 hours ago | parent [-]

Well not just battlebots, real life 6th gen fighters now all lack the vertical stabilizer so they have to rely on very sensitive real-time software control of the elevator/spoilerons to do banks and yaws. Not sure if it counts as under-actuated but they are tremendously difficult to steer without computer control.

▲ schiffern 2 hours ago | parent | next [-]

  >Not sure if it counts as under-actuated

Generally these aircraft still have three+ actuators, they just replace the rudder with differential spoiler or split elevons. Three actuators, three axes. Same number, so it's not under-actuated.

You're referencing the fact that it's dynamically unstable, and thus needs to be stabilized with a computer. That's a different thing.

You can also have aircraft which are under-actuated yet are dynamically stable. One example is a parafoil, which achieves controlled flight using only two actuators (pull lines).

▲ jmspring 27 minutes ago | parent | prev | next [-]

Is there a way to track what's going on in the battle bots community? It was pretty cliche, but battle bots was fun to watch in the 90s/2000s.

▲ jmaw 3 hours ago | parent | prev | next [-]

I'm not super familiar with heavy equipment like dozers and such, but some of them may also have a single motor to pressurize hydraulic fluid and valves to articulate the various pistons. So may be able to make the argument there too

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nomel 2 hours ago | parent | next [-]

In that case, the valves/pistons pairs are all independent actuators. The hydraulic motor is more a constant power source for all of them.

	▲	muti an hour ago \| parent [-]
		Yeah, the analogy for drones is hydraulic piston -> electric motor and hydraulic pump -> battery

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bluGill 2 hours ago | parent | prev [-]

That is common and you can't run two different cylinders at the same time. Depends on how the pump and valves are plumbed of course. Sometimes you can run more than one but you split power.

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HPsquared 2 hours ago | parent [-]

That sounds like it could be solved with more valves.

	▲	solyin an hour ago \| parent [-]
		Or more power ;)

▲ KolmogorovComp 2 hours ago | parent | prev [-]

> but they are tremendously difficult to steer without computer control.

they are not humanly pilotable without computers. they are inherently unstable.