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RoboRuss
03-23-2007, 02:24 PM
Hi I am making a humanoid robot. The problem I am currently having is that I have created an arm in the lightest construction suitable and my shoulder servo (continuous rotation standard servo from acroname.com) does not have the strength to hold the arm in a stationary position (90 degrees) With a heavier design I ended up stripping the gears and again with 1 less motor on the arm and a very light wood forearm it seems to be an issue still. My current options to do next would be:
1.) Purchase a stronger servo
2.) Design a second motor that slips under the arm acting as a shelf

Maybe there are other options?

billblack
03-26-2007, 11:17 AM
You will most likely have to use a stronger servo. Most hobby servos have plastic gears, so the gears fail when stressed beyond capacity. A stepper motor might well work, if you get a motor with enough torque.
The torque requirements are important since the force required to move a lever from an attachment at the base with no mechanical advantage can be surprising. Try picking up a hammer by the very end of the handle and then move it at different accelerations and velocities in the 3 planes of motion. If you can find something with roughly the same weight as your arm you can get an idea of what it will require to do the job.
Trial and error engineering tends to be expensive because of the trail of broken parts and failed designs it takes to get to a final product. IMHO you'd do yourself a favor by calculating the loads your arm is going to undergo during operation. Two links that may help you make your choice are < http://www.societyofrobots.com/mechanics_statics.shtml#momentarms >
and from the same site < http://www.societyofrobots.com/robot_arm_tutorial.shtml >.
A little time with a pencil and calculator will save you a lot of time, money and effort.
I hope this is of some assistance.
:)

Alex
03-26-2007, 01:12 PM
Great reference site BillBlack, thanks:D