Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


Boundary conditions for rigid bodies

Boundary conditions for rigid bodies

Boundary conditions for rigid bodies


I have transformation matrices generated via alignment of rigid bodies in the loop. These matrices consist of rotation and translation. The alignment was done using ICP algorithm ''PointtoPoint'. Bodies which were aligned were not identical in vertices and faces / point clouds. When I apply generated transformation matrices to two rigid bodies and visualise results in the loop, I see the movement which mimic my needs.

For FEA I use open source software, Febio. When I do apply same transformation matrices, displacement and rotation, to rigid bodies, they do not show positive trend in movement. My system unit is mm. In displacement vector there are values of 50 and 70 mm.

What can be wrong?


RE: Boundary conditions for rigid bodies

So you perform alignment of point clouds in some non-FEA software (probably MatLab) and then you want to apply obtained transformation matrices as boundary conditions (prescribed displacement) in Finite Element Analysis, right ? What is the purpose of this FEA ? I’m afraid that the syntax of generated transformation matrix may not be compatible with FEA software. From what I’ve read Febio has some unusual way of treating rigid body rotations. Check its documentation and analyze how it works in this software.

RE: Boundary conditions for rigid bodies

Yes, I perform alignment of point cloud using pcregistericp of Matlab. This function does not give sequence of rotation ((( but allow to align not equal in vertices and faces bodies. The purpose of FEA is to mimic 4DCT movement of the wrist and calculate stress-strain distribution in soft tissue connecting rigid bodies.

The syntax of FEBio in rotation: (1) rotation unit is radian (2) convert rotation matrix to rotation axis and angle. By multiplying rotation axis x angle will give magnitude of rotation in Rx, Ry and Rz directions used in FEBio.

My rotation matrix generated from alignment is
ans =

0.9980 -0.0462 -0.0429
0.0459 0.9989 -0.0088
0.0432 0.0068 0.9990

Calculate magnitude of rotation in Rx, Ry and Rz directions, respectively, is
ans =

-0.0078 0.0431 -0.0461

Can someone check the math of this vector?


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


White Paper - Effects of AIA Quick Guide to UL 489 or UL 1077
The function of a circuit breaker is to provide overload (thermal) and short-circuit (magnetic) protection to a circuit and its downstream components. A circuit breaker functions like an airbag in a car, protecting circuit components and people by tripping the circuit to interrupt the current flow if it detects a fault condition in the control system. Download Now
White Paper - Guide to Integrate Large-Format Additive
As with any new technology, getting into large-format 3D printing begins with investigation. The first question may be a simple one: what does “large-format” mean? For 3D printers, “large” is a relative term. Many extrusion-based (FFF) 3D printers are referred to as desktop machines, because they fit on table space. Some of these have very respectable build volumes – but when it comes to “large-format,” the machines will need their own dedicated floor space. Large-format 3D printers have significant build volumes and are most often found in professional settings, like manufacturing facilities and R&D centers. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close