Finding the torque of a robotic arm

[David lopez]

let's say we have a robotic arm, connected to a servo motor. the servo motor is rated at 500 kilogram-centimeter. the robotic arm lifts something upwards.
will the torque remain constant? if so, torque equals force times distance to point of application of force time sine angle, will the angle be increasing in this case? if so, doesn't that mean the force will decrease, to keep the torque constant?

 

[jrmichler]

The servo motor (any servo motor) produces exactly as much torque as it is commanded to produce. The maximum torque is limited by the lesser of the motor or the drive limits.

A servo motor can be commanded to deliver constant torque. In that case, there are three possibilities:
1) Load torque is less than commanded motor torque, in which case it accelerates.
2) Load torque is exactly equal to motor torque, in which case it runs at constant speed. That constant speed can be zero.
3) Load torque is greater than motor torque, in which case it decelerates until it is pulled in reverse.

A free body diagram (FBD, search the term) will tell you the torque at zero acceleration for any position of the robot arm and load. The zero acceleration torque will be a function of the position of the arm and the amount of the load. You will better understand the relationship between torque and arm if you make several FBD's at different arm positions.