Comment by imtringued
11 hours ago
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!
> 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.