View Full Version : Building a robotarm and use Skeleton...

XSi 4.0
08 August 2004, 01:34 PM

Im about to build a robot arm with 3 main joints that is going to be attatched to a botton joint that only rotates around the Y-axis.

I have tried many different things, read the manuals and tutorials, but i can't find an easy way to do this.

The thing is that i want to be able to drag the "claws" at the end of the arm (effector) so the rest of the arm follows, but not rotating or twisting more than its own local x-axis. And the the first bone should only rotate around the y-axis.

I've tried with rotations limits, but it wont work quite right.

What i thought to be easy, but i can't solve it. Im used to other bone systems where you can disable and enable the x,y,z axis from pitching, and rotating on there own local axis, but in Xsi i can't find it that easy.

Do someone please have any ideas how to do (certainly there is) or tell me what to do, am i blind or did i walk into a corner with the Skeleton, is there other ways to do it?

Please help

XSi 4.0
08 August 2004, 10:26 PM
Did anyone understand what i meant with my post?

Im not that good in english.

I refrase my question:

I want to do a robot arm, like the ones in the industry, that can't move in all axis because there joints are like a hinge joint. And this arm is also attatched to a base via an axle that can rotate in the Y axis.

What is the best way to do an arm like this, i want to be able to use IK for is because it would make it much easier to animate it along path depending on what the arm is doing whe finnished.

When i tried the bones system, i used 2D chain, and tried evrything, and then i tried 3D chain and clicked on evrey button and slider. But it either bugs out after a while, or it is twisting the bones and they are moving in all directions.

What im after is that the bones to just pivot on there Z axis (example) and when im draging the effector (top of the last bone) around the Y- axis (top View) i want the whole hirachy to pivot around the Root bone or Null.

Please help.

(Novis on animations in Xsi)

08 August 2004, 07:25 PM
For a simple robot like the one you mention I would skip the IK, since a more "robotic" movement would be more fitting in this case, in my opinion....
just set up a proper heriarchical (I hope that's spelled OK) chain
like the one above and animate away... just be careful where you place your rotating axes on each peice
P.S. My example was done with Cinema 4D, but I guess it is as easy to do on XSI

this is a quick example i threw toghether in a couple of minutes...

click here (Soreson 3 quicktime movie) (

08 August 2004, 07:43 PM
Take a look at the attached file , is that what you want?

XSi 4.0
08 August 2004, 12:33 AM
Thanks for the reply's. I actually succeded with my skeleton/Ik just a hour ago. :)

jorgevaldes (

Thats the thing that i wanted the IK to control the whole arm with stiffness and rotationlimits.
But your way you discribed was my secondary solution, if i hadn't succed with my IK.

Sheep Factory (

I looked at your scene file, and that was the dilemma, for my purpose, that the base did't rotate along the Y axis when i translated around the effector/last bone.

But here's what i did. I made a Pseudo root on the 2nd bone(bone1) and the i set the rotations limits for the root bone (bone) to be at 90/90 degrees on Z and 0/0 and -180/180 on the Y axis. ( so the bone was standing and poited at the sky) and then i created a Implicit cube and constraind the effector to it(orientation), then i contrained the root bone to the same cube(direction) and the some minor rotation limmits for the rest of the bones, so they did't go throug each other.

And then it worked just the way as i wanted it to. :)

But i have a new problem to solve. On my robo arm i also have some hoses(for compressed air to control the, not yet built, cylinders).

I want to use Softbody on them so they behaive like rubber, but much stiffer. And first i could not attatch them on the arm, just at the centre of the hoses. But i wanted them to be attatched in both ends like they where screwed in or something.

Then i came up with a primitive way. I added the joint objects on the robot arm as Obstacle for the hoses, and then i pressed in the hoses through the polygons on the joint objects and set the friction to 1 on both hoses and the joint objects and also push lenght to 0,5.
Then i started the simulations and animation, and the hoses where stuck in the joint objects and still affected by the gravity and softbody. So it looked OK in the movements, but not the way they are attached. But it worked.

Enough nonsense.

My question now is, how do i attach a softbody object in both ends to another object?

Lets say i you want to have a bubblegum streatched between 2 trees and then have it to be effected by winds, gravity and so on.

I tried clusters and various contraints, but it did't do it.

Again thanks for your answers.


(sorry for all faults in the grammar and wrong pelling)

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