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Comment by grogenaut | Hacker News Reader

One big drawback not called out to electric is you have to power it to have it hold still. Hydraulic can just leave a valve closed which may or may not require power.

Then again just have it "sit". This may not be the best tactically though in small group engagements as "sit" is movement and freeze is dead stop.

I'll let people who know more about that debate it though.

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> you have to power it to have it hold still

Is it common to work around this by supplementing the electric motor with a disc brake?

Depending where you want to be on the tradeoff between complexity vs energy-efficiency, you could use a hydraulically-powered disc brake in order for the brake to not need power except during state transitions.

Thought about this for a bit. I'm betting it's used. The deal with brakes is the ratio of the friction applicable by the brake and the lever distance of the compared to the lever arm it's stopping. I'm watching a movie so I'm not going to do the math right now but I think the size of disc rotor you could get on say a mule's knee joint versus the lever arm length of say a lower leg might cause problems either with the size of the rotor.

My thinking is mountain bike discs are 120mm/4.8"-203mm/8" diameter compared to a 29" wheel.

These are meant to stop a 160lb person who is going down a hill. Well Slow to a stop not lock dead in place as the wheel would break free so it's not the full stopping power of a real.

Anyway my gut reaction is it might be difficult in this use case. I think a 8" rotor on a similar lever arm might have trouble fully locking down that joint and would be very large and flimsy (8" thin sheet of metal) for rugged use. Plausible though.

But hey I failed out of mechanical engineering into computer science for a reason so maybe I shouldn't be trusted :)

Think smaller. Most of the mechanism has to fit inside the dimensions of an arm or elbow.

I think you're missing that this is EXACTLY my point. I'm really doubting a disc that could fit inside an elbow would be able to stop the torque of a few hundred (900/4) lbs on it. Actually one inside the elbow would have a very minimal surface area on the disc in the first place.

Many robotics applications use ballscrew linear actuators. These contain a cage of ball bearings in a spiral around a ball screw, and they move a given distance for each revolution. You can apply a braking force to the ball screw, which will have a large mechanical advantage over trying to brake against the joint itself.

Very true. My cnc uses these. However I couldn't see getting it to walk or hop like these mules. You don't really get impulse with a ballscrew.

To complement other answers, it seems a 3-phase AC induction motor has good torque at w=0. Also, in theory it's power consumption should be 0, but of course there's the non-negligible resistance of the winding and other losses, but the EMF will still be small, so P=VI is small.

> One big drawback not called out to electric is you have to power it to have it hold still.

How so? There are plenty of ways to mechanically lock a joint in place.

Sure there are. They add complexity and weight and failure conditions. There's also a difference between lock and place and lock at an exact position.

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