Stuck with calculating torque needed for Robot servo motion

[Robo3]

Hi there,

I was wondering if you could help me with a quick problem. I should probably know this but for some reason I am getting confused between different points of view:

If I wanted to calculate the maximum torque required by a motor shaft to spin a mass at the end of a rod connected to the motor shaft I would use the eqn:

Torque = inertia*angular acceleration

However would there not also be an extra component of torque due to the gravitational weight of the mass?

Thanks in advance

 

[Unrest]

If the axis of rotation is horizontal -

Find distance (r) of the center of mass from the axis of rotation.
The torque required to overcome gravity in the worst position is T = mass * g * r

If the axis isn't horizontal, replace g with an in-the-plane-of-rotation component of mass*g

 

[Robo3]

Thankyou, my rotational axis is horizontal.

Do I need to add the torque required to overcome gravity to the torque required to accelerate the mass in a circular motion though? Surely the speed that the mass rotates at affects the torque required by the motor shaft?

Thanks again

 

[Unrest]

Do I need to add the torque required to overcome gravity to the torque required to accelerate the mass in a circular motion though? Surely the speed that the mass rotates at affects the torque required by the motor shaft?

Just add them for the worst cast (accelerating while lifting with the CoM 90degrees). But that'll tell you more torque than what's needed when the CoM goes over the other side and is being pushed down.

Speed doesn't affect the maximum torque caused by gravity or rotational inertia.