3d Cad: Tetrahedron Tensegrity 3v.

This is my first 3D Cad Tensegrity model, technically described as a 3v Pars Tetra Tensegrity.

This design is actually not from the Bucky Fuller stable or indeed from Kenneth Snelson, instead it is devised by a very clever fellow called Marcelo Pars, who has done extensive work with tensegrity structures. I only recently came across Marcelos’ work which has opened a whole new understanding of the subject.

The design is based on a data-set created by Bob Burkhardt, who has studied in detail the mathematics of tensegrity structures.

3D CAD is an ideal medium to explore these structures enabling a degree of accuracy that physical models cant achieve. This environment also provides additional visualization tools that can dissect the models to further examine the construction and gain insights into the underlying geometric symmetry.

For all new tensegrity structures I would first develop the design in 3d Cad using where possible available data-sets from various sources. Occasionally where no data-set exists I would simply develop the design from visual inspection.

To fully comprehend the structural form it is absolutely imperative to attain a high degree of accuracy working to approximations will not suffice. It is therefore not unusual to have to work to 4 decimal places to achieve a fully constrained accurate model.

Working with datasets is an ideal starting point but in my experience still requires additional micro adjustment to fully constrain the final model.

Normally the process involves starting with building the struts and tendons as individual parts and laying out a rough shape of the form as an assembly and then constraining each of the individual struts to each node point on the tendons.

However it is very easy for this whole thing to become chaotic as you constrain the struts and occasionally the structure may become unstable as the Cad program will take the shortest route to connect the elements. To negate this and provide some control it is always best to ‘ground’ one component prior to connecting another keeping the items as close as possible to each other before constraining.

When the assembly is complete it is not unusual to find that the model exhibits some flexibility which technically should not be there. Its not because the model is not fully constrained its just that dimensionally struts or tendons are not nearly accurate enough to fully define the model. This is where it gets tedious...in order to achieve the final "fixed" result we need to select a tendon part or strut and micro adjust the dimension of this part to finalise the structure. I should note that selection of a "part" in an assembly will apply the adjustment to all the parts with the same properties (which is exactly what we want).

Once completed we can then begin to explore the model for geometric consistencies and symmetry. 

To prove geometric symmetry and relationships it is necessary to build a number of dimensional variations of the same structure and overlay these in one assembly. In the blog entries to follow I have often demonstrated this principle by showing dimensional variations of one model in the context of proving an observation.

For more information please contact me at hught2008@gmail.com
Bob Burkhardt website: http://bobwb.tripod.com/synergetics/photos/pars.html
Marcelo Pars: http://www.tensegriteit.nl/

Updated: 11th February 2012