Need help with servo torque

[ANTlabs]

OK, I have a body for a hexapod robot that I want to make and control with a picaxe 28x1. for now, all i want it to do is go forward. The two middle legs will be connected to servo 1 which will move one leg down and the other up. therefore one side ( with the middle leg down ) will be up and the other will be down. the side that is down will move the outer two legs back causing the robot to go diagonally forward. the side that is up will move its outer legs forward to prepare to move back. then the side's up&down status switches and they do each others jobs (back&forward) and then the cycle repeats. I need help finding which servo motors to use to control the middle lifter legs, and the outer movement legs. I haven't taken physics yet, and don't know much about torque. any help is appreciated, Thanks

 

[ANTlabs]

Oh, forgot the most important part! the robot body's weight (don't forget to add in the servo weight!!) I'll get it and post it here later tonight. also, **broken link removed**.

 

[ANTlabs]

It weighs about 5.5 pounds.

 

[crutschow]

The required torque is simply the length of the lever arm times the weight (force) you need to move at the end of the arm. For example, a two foot arm trying to move 5.5 pounds would require a shaft torque of 11 ft.-lbs (159 kg.cm). That is reduced by the gear ratio between the lever arm and the motor.

You probably want to de-rate that at least 50% for design margin (buy a motor at least twice the size your calculations indicate).

 

[ANTlabs]

It's a 21.5 inch pole for servo 1, at my newly found 6.1 pounds, calculations come out to be 10.92 ft. -lbs (151.101 Kg.-cm) wow all of the servos I'e looked at have, like, 3.7 Kg.-cm. where would I find a motor likethis for a low price (besides ebay).

 

[Sceadwian]

How fast does it need to move? I've seen gearheads that attach to standard hobby servos that multiple torque dramatically, but obviously this slows them down as well. 10ftlbs is a (for the scale) respectable amount of torque required. Hobby servo's are designed to move very fast and be very responsive not to produce large torque generally.

 

[Sceadwian]

I was tryign to find that site I was talking about and found this instead.
**broken link removed**
The prices aren't cheap, I think one that's capable of about 12ft lbs was 130 dollars.

You may need to rethink your mechanical linkage to reduce your torque requirements.

 

[ANTlabs]

Ahhh..... I have not a clue what I've gotten myself into. well, maybe you guys can help me further. I wanted the robot to work like **broken link removed**. Is there any way to make it run on one servo with some gearing or beams, or maybe just with a regular motor. Or is there a better design? Anyway, **broken link removed** are some pics. of the robot now.

 

[Sceadwian]

Then make a robot of that mass cause you'll never get one going using standard hobby servos self lifting with arms that long.
You can't use the image tag to link local files.
Why is it so heavy? The basic micro controller modules and componetse required to operate servos shouldn't weight more than 1lb sans servos.

 

[ANTlabs]

sorry, had a wrong link on the clip board, should be fixed now, and look at the pics. they explain allot.

 

[Sceadwian]

Why does it weigh 5 pounds? What's it made out of heavy gauge steel? That is REALLY heavy for such a small size.
Use aluminum or carbon fiber for the frame and use a nylon bag for the 'box' it will weight almost nothing.

 

[ANTlabs]

The frame is made out of wood and so is the box, now that I'm thinking, I should probably make a 4 wheel robot and make a smaller hexapod robot later. It did take me a while to cut the wood and paint the letters and all. Thanks for the help anyway.

 

[Sceadwian]

What'd you make it out of, oak?

 

[ANTlabs]

don't really know, got it from neighbor when he redid his kitchen cabinets.

 

[ANTlabs]

anyone know of a good pair of motors and wheels to drive a 6 pound robot. wheels must have at least 5 1/4 in. diameter.

 

[Sceadwian]

Not enough information.
How fast do you want to go, what kind of terrain will you be driving on? Almost any motor in existence can drive any weight as long as it can overcome it's inertia. A piddly motor isn't gonna handle a 1% slope though. The torque required to go over something like a rock is dramatically higher than what you'll ordinarily encounter. There's a lot of factors to consider.

 

[ANTlabs]

Alright, let's see if I get this all I would love to have to have it go 2 ft./sec. but 4 or 5 ft./sec. is fine Th terrain is uneven tile and thresholds in doorways. max height difference in tile i 1.5 cm. and thresholds are very curved/smooth and rise at max 1 in. room floors are slightly lower than the tile, maybe 0.75 cm. It will just basically be navigating the house.oh, motors could to have some kind f encoder, or be a stepper (not preferred), or any other way of knowing where they are (may use IR beacons, any motor will be considered). Also need tires.

Thanks

 

[ANTlabs]

How do you think think this combo would work: geabox & **broken link removed**.

 

[ANTlabs]

did i mention that wheels need to be a diameter of 6" and a width of no more than 2 1/2".

 

[Sceadwian]

Try posting a new thread in the robotic forum, better place for it, as I generally don't do robotics stuff.