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With 2DOF, you can actually use fairly weak servos. The reason is that the force goes straight into the mounts and axles of the servos, rather than actually in the motor radial direction. Once you add a third degree of freedom, that will have to lift some part of the robot, and the leg.It's not so bad. Simplified math that works OK: If you assume that the other legs are already holding the main body, then the middle (knee) joint only needs to lift the weight of the leg outside the joint, times the horizontal distance between the servo spline (center of rotation) and the center of gravity of that leg.For the other three servos to keep the body still, they need to lift one-third of the weight of the entire robot, times the distance between the center of rotation of the joint and the center of gravity of "their third" (which can be approximated as half of the distance between the center of rotation and the center of the robot.)As you know, torque is simply weight, times distance to center of mass.Now, add a 50-100% safety margin, and you're likely good to go!
@jwatte Thanks a lot.I was browsing through internet and I found the following site on hexabot's mechanics. http://www.robotshop.com/robot-leg-tutorial.htmlIn that site, it says that the servomotor near to the ground requires more torque compared to the middle servomotor!
Quote from: saba_rish91 on January 13, 2013, 09:46:18 AM@jwatte Thanks a lot.I was browsing through internet and I found the following site on hexabot's mechanics. http://www.robotshop.com/robot-leg-tutorial.htmlIn that site, it says that the servomotor near to the ground requires more torque compared to the middle servomotor!I suppose by "near to the ground" you mean servo A in this diagram. The question is one of statics/dynamics. Most of the force of the servo A is taken by the bearings of the servo, rather than the servo motor itself, because the foot is pointing straight down. This is not the case for the servo B -- it is close to pessimal in this regard.
If the joint at A has a horizontal pivot then the level arm is the distance from the pivot at A the the body of the Bot at point C. Therefore this servo would require the greatest amount of torque on hold the bot's body off the ground.