# Thread: Walker Weight Tolerance Calculation

1. ## Walker Weight Tolerance Calculation

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Edit: Look at post #2 for answer: Cire's post
Note: Fleg = F/#legs on ground
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(Caution: I might not know what I'm talking about )

Hi, I wanted to expand on this subject since I hit a wall myself designing with weight and cost restrictions.

I have already bought tons of micro servos, their torque rating is only 1~1.9 kg*cm (different sites different value...)

Now I'm in the process of buying a battery, and prefer a lead acid battery for its cheapness (yea I'm cheap) However the battery is 2lbs....(2x 1lb BURRITO!)

I have 4 legs on my robot, with just 4 servos in the vertical plane.

SO!:

Torque @ optimal conditions: 1~1.9kg*cm
Center Mass: Battery(2lb) + Guns + Sensors = 2lb+
Leg lengths: approx. 18cm

According to my brain,
At 1 cm from servo shaft, 1-1.9kg is ok......more kg = stall
To figure out how much mass x the servo can handle at radius r from shaft, its just a ratio:
[from servo specs]1kg/1cm = x(kg)/r(cm)

however the battery is not mounted at the end of "r", its in the middle of the body......and the servos are on the 4 corners of that body....

so...According to my brain again,
As the legs/servos "try" to hold the body up,............ah...thats all I got....T.T

Whatever you guys can help me figure out Ill make a tutorial on Weight Calculations out of this thread for others to reference.

Last edited by Fanatic; 01-20-2012 at 11:18 PM.

2. ## Re: Walker Weight Tolerance Calculation

The moment (torque) on the servos based on the weight, leg geometry, and how it walks. The static moment can be found by taking the force at each leg, and the distance between that point and the servo in the horizontal. You can find this distance by taking the angle and the length of the leg. I made a little diagram to show some of the forces involved on a quad when it is just standing still. Please note i used only 2 legs for the force in the legs, because when the robot walks it will mainly be supported by 2 legs.

Now, since you are using 1kg*cm microservo's, and you want to use a 1kg battery, the distance D will never be able to be more then 2CM, assuming the battery was the only weight in your entire robot. If you had a really lean rest of your robot, it would still probably weigh at least 2-3kg's, putting your distance of D to be less then 1CM. Also, this doesnt account for any of the dynamic forces involved, which depending on how fast you try to make your robot move can be very high comparatively. If you make it crawl, then the static calculations are probably pretty close.

Now in reality, I hate to break it to you but most people have been using AX-12's for their mech warfare quads, which have 16.5 kg·cm of torque. And most of these mechs still have overheating problems with the servo's because how heavy the mechs are after the camera and gun is added (using LiPo batteries too, not lead acid).

You can probably make a quad using the microservo's, (There is a hexapod out there that uses them) but don't expect it to have any payload capability.

3. ## Re: Walker Weight Tolerance Calculation

Originally Posted by cire
You can probably make a quad using the microservo's, (There is a hexapod out there that uses them) but don't expect it to have any payload capability.
T.T why must you be right....been calculating all night and day, and sure my robot can walk....if its tethered to the wall socket.....
Any battery choice for me is out of the question not to mention combat capability is non-existent.
I wasn't even thinking about dynamics! Haha would have screwed myself hard if I skimp on new servos and stuck close with my static calculations. TY TY TY.

Oh well, I guess I can replace the servos and use the micro ones for.... aesthetics...

Thank you for the awesome info though, will be using it for farther calculations!
+1 REP!....once I figure out how....

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