I want to make a humanoid robot with my pi, but am unsure of what servos I would need. I did some research and found out they a rated in kg/cm, so I calculated the weights they would be lifting, assuming a distance of 2 ft(about 61 cm). I used the following equations for upper and lower arm servos-
material in upper arm(cm^2) + material in lower arm(cm^2) * material weight(g) / 60.96cm
material in lower arm(cm^2) *material weight(g) / 30.48cm
Neither of these take into account the weight of servos or any attachments I put on the end.
Can you tell me if I did these calculations right, and if I need to do something different since the upper and lower arms rotate up/down independently, which can change the distance between the shoulder servo and the end of the lower arm? Currently, the equation assumes the longest possible distance from the shoulder servo.
I ran the numbers for the equation shown, and came out with 12.5 kg/cm servo for upper arm, and a 5.5 kg/cm servo for lower arm, but I will probably buy slightly large ones to account for attachments I may add. Is there a website where I can buy servos slightly over these ratings if they are correct, and if they are incorrect, where I could get ones for the calculated weight? Amazon seems to only have 20 and 25 kg/cm servos.
Also- Just an unrelated question, is it okay to use one ground pin for everything I hook up to the pi, or will that hurt it in some way? (I have a pi4) Just checking before I break something.