Dynamic load for cylindrical robot

[kennykck]

total_torque about joint 0 = static and dynamic torque from link 1 + static and dynamic torque from joint 1 + static and dynamic torque from link 2 + static and dynamic torque from joint 2 + . . .

so basically you use the mr^2 * alpha equation for each point mass individually, then at the end you sum them all up . . .
In my robot arm tutorial, you will see how I divided up each part of the arm for calculation.

Hmm... refering to equation above, the mr^2 * alpha equation u ask me to use, is it representing the dynamic torque only or  static and dynamic torque for each link/joint?


U mentioned this formula Ttotal=aA*mA*rA^2 + aB*mB*rB^2 + aC*mC*rC^2 for my rotational joint. In the case of cylindrical robot, the vertical and horizontal motion, do they have any influence onto this Ttotal for rotational joint? Meaning do they increase the value of Ttotal if they move too at the same time?


The picture shows the way motor attached to the robot to drive the belt/pulley. I thought since the motor(vertical joint) rotates in perpendicular direction to the motor(rotational joint), the effect does not stack up and therefore does not increase the Ttotal of the rotational joint.


Note: I'm searching for the maximum torque requirement for each joint. So i want to know whether movement of other two linear joints will in any case increase the torque requirement for rotational joint.

[Admin]

We are going in circles here

Tell you what, go ahead and do the calculation, telling me what formulas you used, and I will tell you if its correct.

At some point you just need to think and do it . . . I already gave you all the answers

[kennykck]

ok this is the way of how i find my maximum torque requirement for each joint (motor):


For horizontal arm, I use T=m[horizontal arm+load]*r[pulley]*a        ;   from the formula T=F*r and F=m*a

For vertical arm moving upwards, I use T=m[horizontal arm+load]*r[pulley]*(a+g)     ; added gravity acceleration

For rotational about z axis, I use T=m[whole robot]*r[distance from point of mass]^2*angular acceleration


So i calculated the dynamic load for each joint separately. My supervisor asked me to calculate the dynamic torque when all joints move at the same time, primarily due to that when all joints moving together, the load is higher than the value i had calculated through separate calculation.

So is my original calculation correct? 

[Admin]

Again, you need to draw a proper FBD. You need to label torques, forces, axes, and distances on your drawing. My robot arm tutorial gives examples.

(sorry for being rough on you, but an FBD is the very first thing I do before writing down a single equation, it makes it much easier just trust me here . . .)

Anyway, looking at your equations, the horizontal arm motor should be:
T=m[load]*r[pulley]*a

And about the z axis, as I said again and again, you have to consider it as separate point masses

The upwards equation looks right.

Looking at this, you should also consider efficiency and friction effects. You can't calculate either one, only determine it after its all built. For dynamic torque, you should add about ~20% more required torque to account for these factors.

[kennykck]

so it seems that my dynamic load for horizontal, vertical, and rotation motion is alright (Well, after including friction and points of mass into those equation)

The main problem that i faced here is if the robot arm moves upwards, extend outwards, and rotates all at the same time; the dynamic load is still calculated separately for each joint like what i did, is it?

[Admin]

Well, calculating all that gets really complicated. Just calculate for worst case (the arm fully extended), and it will work.