[quanto]

A genuine intellectual question: what's driving the (hardware) cost of these humanoid robots? Dynamixel smart servo motors? Exotic sensors?

I do robotics research but not humanoid. Almost every component used in production came down drastically in price in recent years. Lidar sensors are a few hundred dollars now.

[codekansas]

Vast majority of BOM is actuators, although for low volume if you need to CNC a bunch of stuff that can hike the price as well. You can use exotic sensors but they're not particularly necessary. It's a death by a thousand cuts thing with many of the humanoids you see out there right now - hence our approach, which is to get the hardware cost down as much as we could and then start building the software to make up for it

[MoonGhost]

Not sure Dynamixel or other servo is a good choice for dynamic locomotion. In high end they use large brushless motors with minimum gear. Just look at latest dog and humanoid from Boston Dynamics or Chinese Unitree(?) and nonames. The reason is for dynamics it has to be fast and strong. For sensors they use gyro/accelerometer in the body as main. Advanced models use pressure sensors in feet. Looks like they are needed for precision acrobatics. And rotary encoders in all joints, of course.

[imtringued]

The reason why you want quasi direct drive with low gear reduction has to do with the kinetic energy stored in the gears. It's difficult to get an intuition as a human, because we don't have rapidly spinning disks inside our arms.

The moment your high gear reduction actuator is forming a contact with anything, it will have to either decelerate instantly, push the object away or move into the object by deforming it. If your robot arm is moving at speed and hitting a wall, the deceleration needs to shed all velocity in a few milliseconds. This is fine for the arm itself, but if you have a 100:1 gear reduction ratio, one gear is moving at 100 times the speed of the robot arm and since kinetic energy is 1/2mv^2, the energy stored in the gear is significant. Stop the arm and you'll break off the teeth on your gears!

[MoonGhost]

> Stop the arm and you'll break off the teeth on your gears!

Unless you have some energy dumping mechanism, like connect your servo to arm with flexible connector. Making whole arm or leg with bit flexible plastic will also reduce max load on gear. Precision will suffer, but should be still good enough for walk and house work. The problem with energy is that it should be undone when direction changes. This affects reaction time. And that time should be smaller for small, a... proportional to square root of size, right? That's how long it takes to fall. Not sure, didn't take into account the inertia of solid body.

BTW, heavily geared humanoid robots walk in some more stable way on bent legs. This is obvious on old videos. Like they are afraid to sh*t their pants. Also have problems with balance. Small hobby robots usually in addition to bent legs have huge feet. The point is, with servos it cannot be done much better.

[iancmceachern]

Harmonic drive gearboxes are one thing, they had rust whole market locked up until recently I believe