Mechanics and Construction > Mechanics and Construction

Building Bipedal Robot


Hello, I'm building a bipedal robot but have encountered a problem (several, really).

While balance is under control (most of the weight will be situated at the lower torso), the next thing I need to add on would be the motors, which brings me to the current problems...

1. Which is stronger, normal gearboxes, planetary boxes, or worm gear systems?

2. Is torque lost past the 1" calculation?

just one quickie, search this forum for biped, there is a huge collection of information already on it . . .

all functional bipeds I have seen use servos, while most hobbyists stick to the small and cheap hobby servos . . .

1) for gearing, strength is determined by gearing ratio not gearing type . . . typically planetary gear boxes have a higher ratio than the others you mentioned. but remember, the higher the gearing, the slower the actuator.

2) torque is defined as force * distance . . . for example, pound-inches or newton-meters . . . Is that what you meant?

To #2. Yes, that was what I meant, I guess I should have been more specific.

To #1. By slower do you mean 3 seconds slower or .03 seconds slower (numbers as examples)?

Also, I feel I should put the robot itself into perspective...

+The robot is about 4-5 feet tall (give or take the way the leg servos are mounted)
+Center of gravity is located at the torso (battery pack, electronics)
+10 motors (the torso and neck are static, hands are for another time, arms only move parallel to the robot, etc
+Made mostly of pvc piping (plus bolts and metal brackets). Skeletal structure is the bare minimum for weight


--- Quote ---To #1. By slower do you mean 3 seconds slower or .03 seconds slower (numbers as examples)?

--- End quote ---
By gearing, what you are doing is changing the torque/rpm ratio. For example, if you have a gearing ratio of 300:1, then your ratio changes to:

(torque*300) / (rpm / 300)

So in effect, your torque multiplies by the ratio, and your speed divides by it.

Adding gearing also reduces efficiency. For example, if you used a worm gear, multiply your output torque and speed by .8. This is from internal losses such as friction and force angles.

I actually did a tutorial on robot gearing awhile back, if your interested:

Hobby servos probably wont work for you at that weight. You will need to add some type of feedback control to each motor. Perhaps use rotary potentiometers at the joints?

I just did some rough math, sounds like your gonna spend $1.5k+, right? :P

I have plans for a biped in the works too, but that pricetag isnt far from the one I calculated for mine . . . bipeds arent just inherently hard, but inherently expensive too . . .

Yes, that price tag sounds about right.


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