Rapid prototyping using dowels and elastic cords. Using these materials, Tom can design and build fairly elaborate constructions without needing to take accurate measurements. He can make adjustments on the fly and balance the structure after it is made. Limitations of this method: prestress is too low, and prestress is difficult to adjust.
Context: Tom Flemons Archive
(Dec 8, 2014 Tom replies to someone who asks how to get started with building tensegrity structures) Your best bet for building basic models is to obtain a copy of Introduction to Tensegrity by Anthony Pugh. It is still the most comprehensive manual to build models even though it is almost 40 years old. The easiest method I have found to build models is to use 5/16″ dowels (8 mm) 15-20 cm in length and elastic cord approx 2.2 mm dia. Elastic cord is cut shorter than the length of the dowel such that there is some tension and either knotted at either end for simple lengths or tied in a loop depending on what type of tensegrity you are building. Accuracy in measuring is important. Also lightly melt the ends after you have cut them with a candle or lighter to prevent fraying before you tie them. The elastics are fit into slots cut 1.25 cm into each end of the dowel (cut in the same plane) with a band saw or a thin kerf hand saw. It is important that the elastics can not slide freely in the slots – there must be some friction so the slot is necessarily narrower than the elastic cord.
Unfortunately I can not impart to you in an email how to build the more complicated models such as the leg, arm and the spine. I spent years figuring these out so it’s very difficult to explain the method. I still find the leg hard to build myself. It is not only the assembly that is difficult – it is also very hard to adjust all of the tension lines to balance the entire structure. A simpler structure like the 6 strut expanded octahedron is much easier to understand and build. Instructions can be found in Pugh’s book.
Rapid prototyping using dowels and elastic cords
(April 11, 2015) Over the last 30 years I have invented and perfected a means to rapidly prototype tensegrity forms. It involves using very simple materials – dowels and elastic cord. Using these materials I can design and build fairly elaborate constructions without needing to take accurate measurements or spending hours trying to figure out what goes where and how to keep it all together before it explodes on me. I can make adjustments on the fly and balance the structure after it is made. Frankly I think this could be my contribution to furthering the field of tensegrity exploration. Everyone could benefit from having a hands on experience building actual complex tensegrities.
(June 10, 2016) When I build with elastic cord I don’t have to know exactly how long the tension members will be. I fix the length of the struts and cut the elastics so that the proper lengths will fall somewhere within the stretching property of the elastic material. If I guess right the structure is relatively stiff. If I guess wrong I can’t finish building it because the tension is too high, or it is floppy because the tension is too loose.
(Nov 27, 2015 Tom responds to Dorothea’s student building a tensegrity model by hooking bungee cords to eyelets screwed into the ends of struts) Eyelets will work if the models are larger to match bungee cords, but a much better system involves cutting two slots in each (5/16″) dowel end 1/2″ in depth with a bandsaw (in the same plane) and using elastic cord (2.2- 2.5 mm) purchased at a fabric store to make lengths and loops. Cut so that there is some amount of stretch but not too much when assembling. Works well for struts 5-12″ in length.
Detailed instructions for this method of rapid prototyping
(Feb 21, 2017) My method of building tensegrities employs slotted wooden dowels that accept appropriately sized elastic cords such that a friction fit allows for little or no slippage at the nodes. Using elastic allows me to make approximate (good enough) guesses as to tension lengths (which means I end up with taut but flexible models) and because there is a friction fit I can compile multiple tension vectors into one loop or length greatly facilitating ease of assembly. Because tensegrities are modular and periodic in nature (even asymmetrical structures are modified from basic geometrical forms) it is relatively easy to assemble complex structures quite rapidly once the system is understood. However I have never written detailed instructions on how to go about this. I learned through trial and error over the course of decades. It’s not exactly rocket science though. With a good foundation in geometry and an understanding of tensegrity principles it shouldn’t be too difficult to use this system to great effect. There was a company that manufactured simple tensegrity kits but they went out of business about 15 years ago. I believe it’s still possible to find their kits for sale on ebay occasionally. They were know as Design Science Toys and the kits were labeled Tensegritoy. I actually bought out his last stock around 2002.
Regardless here is my suggestion for you: You will need access to a bandsaw. Cut 5/16″ dowels to useful lengths – 5-8 inches or larger. Slot a 1/2″ cut into each end in the same plane. I use a jig to insure a precise depth and bifurcation. The kerf on a 3/8″ bandsaw blade is about .041″ Purchase at a fabric store or online spools of elastic cord 2.2 mm dia. Try to find high quality elastic cord – it pays off. This diameter will fit the slot tightly but won’t split the wood. Analyze the tension net of the structure you are trying to make and figure out how to economize on the number of tension members you will need. For example an expanded octahedron tensegrity requires six dowels arranged in three parallel pairs oriented at 90 degrees from each other. They are connected by 24 lengths of line but a little thought will suggest that six knotted (square knot) loops – one for each dowel will suffice as each loop wraps each dowel and substitutes for 4 lines each. I cut the elastic approximately 2/3 the length that a fixed line would cover (and multiply by the number of nodes it crosses).
I also suggest you get a hold of Anthony Pugh’s 1976 classic ‘Introduction to Tensegrity‘ which gives a very detailed taxonomy of tensegrity forms. (perhaps you know it?) Unfortunately I didn’t find this book until well into the morass of building tensegrities – it had a very limited academic publication initially but you can now get it online. In it he discusses the basic design strategies – employing either loops or lengths (in my system knotted with a simple overhand loop at either end) of line. Whether loops or lengths they are meant to cross several nodes. He explains very well how to assemble complex structures at least for spherical tensegrities.
Improving on this method of rapid prototyping
(April 11, 2015) But there are limitations to this method and I recognize the next step will be designing a small 3-D printed mechanism that will allow me or anyone to assemble tensegrity constructions quickly and tension them after assembly using more robust materials than sticks and elastic.
Attachment method must be secure, adjustable, convenient, strong, and avoid congestion
(June 28, 2017) The attachment method must be secure enough that you can assemble a Tensegrity without it coming apart but adjustable so you can make changes afterwards. This usually necessitates guessing fairly accurately the lengths of the tension members beforehand – even so it’s almost impossible to get it right the first time so it requires going over the entire structure and tediously re-tensioning every line. Depending on the method used this can take hours and be very frustrating. Add to this that any one tensegrity is usually just one of a long string of prototypes… necessitating building dozens if not hundreds before something starts to work. When I was designing my leg foot combination I lost track of how many versions there were. If I hadn’t developed a system using elastic cord caught in slots in the ends of dowels I wouldn’t have been able to make any progress over the years. At this point I have a literally built thousands of tensegrities using this method. Granted most of them were toys, but even so there were hundreds of iterations of biotensegrity models. (I have large boxes stored in my workshop filled to the brim with well worn out models). My system works so well because I don’t have to measure the exact length of the tension members. I just have to get it tight enough and the elasticity takes care of any inaccuracies. This is great for prototyping but is pretty much useless in testing out actual purpose driven designs. For example, I’ve designed a number of possible prosthetic legs including a knee joint and a foot. They look plausible but they are nowhere near strong enough to bear weight. I could thicken the elastic cords but then it starts to get congested at the nodes and I have to beef up the strength of the struts which adds weight and usually means thickening the dowels which creates more congestion…
The need for robust prototypes
(April 11, 2015) I’ve been building things since I was a boy so I have a fairly comprehensive set of hands-on skills. Among other things, I have built houses, done renos, and have designed and built a number of sail boats. (I’m building a trimaran now). What really interests me at present is solving the puzzle of how to build articulating tensegrity constructions roughly based on vertebrate anatomy, that can be used to create functioning robots, exoskeletons, and prosthetic devices. I’m limited in my research by the lack of computer skills but more importantly a lack of good assembly tools and systems that would allow me to build robust prototypes.
Related discussion in Adjusting Prestress; High Prestress