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Help with Unresponsive servos

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Posted: 2012-01-25 12:21 
I used 3mm thick Acrylic (I think it's brand same is Perspex) on my spider robot and it seems to do fine (although it's a bit wobbly). I hope this was helpful. How thick is the aluminium you're using?

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Posted: 2012-01-25 13:00 
Acrylic is brittle. Polycarbonate is tough. 3mm (0.125") thickness of either is roughly the same weight as 1.5mm (0.063") aluminum. 1.5mm aluminum is by far the lightest / strongest / affordable solution. Perhaps you could do other things to improve the bot. For example make the tibia much shorter.

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Posted: 2012-01-25 13:34 
Robot Dude wrote:
For example make the tibia much shorter.
Making the femur shorter is a better suggestion.. :wink:

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Posted: 2012-01-25 13:35 
Doh! slaps forehead. That's what I meant to say. lol

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Posted: 2012-01-25 14:56 
Im using Aluminium 1.5mm. Im thinking about changing the middle body with something lighter like balsa wood, and trying to avoid making major changes in the body shape like the legs length or servos.


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Posted: 2012-01-25 19:48 
Or use springs maybe?


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Posted: 2012-01-25 20:10 
The
Hitec HS-225BB servos are pretty small. How long are your tibia and femur parts? Was this discussed?

Alan KM6VV

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Posted: 2012-01-26 10:57 
The femur is 75mm and the tibia is about 88mm all measured from center of the servos. i think the vertical servos connected to the waist need to be changed with some thing stronger, since they are the ones lifting the body up and down. i dont think lightening the middle body will solve the problem, especially when walking; 2-3 servos cant handle the whole weight. Any recommendations ?


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Posted: 2012-01-26 11:24 
To help put things into perspective we use a 2.25" femur with 485 (minimum) 645 (much better). Yours is 3" with less powerful servos. Because the torque is rotational the longer the segments the less torque the servo has. Your 225 servos can indeed be used to make a quadruped, but scale it down proportionally. I think you are probably using the best material. You're just trying to make long Phoenix style legs which can't work.

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Posted: 2012-01-26 12:34 
then ill have to buy 4 new 645 servos and cut 4 more frames for them. are you sure they'll be strong enough to hold the body while gaiting ?


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Posted: 2012-01-26 12:43 
I have only provided input concerning the legs. I have no experience with rotating the entire leg from the chassis.

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Posted: 2012-01-26 16:06 
anyone else can help choose which servo to buy and how many should be changed ?, ill stick to my paper and pen for now and apply what i learned in physics :P


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Posted: 2012-01-26 18:01 
Hi,

You don't necessarily need to buy new servos. But to make your quad walk I would suggest to remove the 4. DOF (the servos inside the body), that would also save some weight. Then make a new upper and lower body section for supporting the coxa servo on both sides. Decrease the femur length as much as possible, maybe 50-40 mm length, its hard to say without not knowing the exact weight of the robot. Also, analyze every part/bracket to see if you can decrease the weight, e.g. drill some holes on the tibia, reduce the length on every screw that holds the servos. Use an accurate weight scale for measuring the main parts before and after doing the modifications for reducing the weight.

Good luck! :)

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Posted: 2012-01-26 18:38 
hmm, drilling wholes would be a good idea to reduce weight, but unfortunatly, i cant remove the servos in the middle of the body since they're the whole idea of why im making this robot x) , thanks for the tips though :)


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Posted: 2012-01-27 06:19 
I'm starting to think of making another design with 3DOF and same 225 servos, ill keep the current one and work on it later. I dont want to go more off topic so ill make a thread for it in the project section as soon as finish the drawings in autocad. Thank you all for the valueable help you gave me in this thread, ill see you later on the other one 8)


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