I was wondering what makes a servo strong. I was building a humanoid robotic arm and I had a Problem with the wait. The elbow servos couldn’t pick up the hand and the forearm and the shoulder servo couldn’t pick the arm up. I had 5 15kg servo in the for arm for the finger movement. so my question is what give the servo it’s strength is the the gears or the current is applied to it? I know there are some advance servo out there like the DYNAMIXEL Series robot Servos. what type servo or motor do Industrial robot and Exoskeleton use? also what type of servo does asimo the robot by honda uses. And if i wanted to make mo own servo from scratch where should i begin.
First let’s define something what you need here is called in engineering Torque ( which is forcedistance ; T=FD ) . it appears when you try to open the door ( you have to set a constant torque needed to open it ) by pushing it near the hinge ( this is hard , because distance is small between the force you are applying and the hinge ) or by pushing the same door but near the end far from the hinge ( now the distance is bigger so less force is needed ) .
Servo motor is just a DC motor with gear set and controller unit to make the feedback control system (PID ).
the normal DC motor’s torque has a relation with input Current , but you have a limit for this ( due to wiring and some other constrains in the motor) . to overcome this you put with the motor Gearing System which reduce the speed and increase the torque ( you always has the power constrain which is factor of speed and torque ; you can’t raise both directly ) .
Industrial robots use in most of the time another types of motors called Stepper motor ( it gives higher torques to weight than servo motor in general but there are some ) , their problem they are not so precise so they need electric board called a Driver to make micro steps but this system is higher in price for sure . also its weight is not good with humanoid robot .
stepper motor : 
there is also some industrial servo motors but also very big and weight a lot :
make your own servo motor from scratch , these are good links to start with : http://www.instructables.com/id/DIY-Servo-Motor/
http://www.instructables.com/id/Make-your-own-low-cost-servo/
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