Walking structures are no pipe dream

Tom Shelley reports on some of the mechanical and pneumatic technologies underlying some remarkable works of art

Kinetic sculptures that can move around a beach demonstrate a number of innovative techniques that more mainstream engineers might wish to tap into.
Made of plastic tubing that is normally used as cable conduit, they walk about under wind or compressed air power. Some can even sense the presence of water or soft sand and avoid it – without recourse to any kind of electronics.
The walking mechanisms are unique, and offer a much more energy-efficient way of crossing soft ground than using wheels and tracks. They represent a possible template for future planetary exploration vehicles, but it is some of the component mechanisms that many may find most interesting and potentially transferable.
Theo Jansen originally studied physics at the University of Delft, but soon turned to art, while retaining his interest in science and engineering. In 1990, he published an article in the Dutch newspaper de Volkskrant in which he whimsically suggested making machines out of plastic tubing that would be powered by the wind and move sand from the beach on to the dunes, in order to help protect the Dutch coastline. About the same time, he came under the influence of a book called, ‘The Blind Watchmaker’, written by British zoologist Richard Dawkins about evolution, which led him to devise and run computer programmes on natural selection as applied to shapes, and then to possible walking machines.
The two ideas together gave birth to a series of real-world walking machines, which are pure art, but require engineering to make them work.
“The walls between art and engineering exist only in our minds,” he says.
He has given his machines names, as if they were living animals – saying “I got the plans to make new forms of life” – and which he sees as evolving into a new type of machine.
His biggest achievement is probably coming up with a mechanism that enables an energy-efficient walking motion. This is a non-trivial problem that has foxed engineers for a long time. His solution has 11 links and he established the optimum relative dimensions as a result of long successive analyses on an Atari computer, making use of his ideas of evolution to select only the most promising combinations and greatly reduce computation time.
He bends and flares his tubes, using a hot air gun to gently soften the plastic. One of his discoveries is that, if a tube is bent into a ‘V’ shape, it needs to be stretched slightly at the same time – which, he says, “causes all transverse forces to fall away”. Bearing rings he flares so that their ‘U’ profile is somewhat wider than whatever is running in the groove. He says this causes the bearings to naturally reject sand, otherwise they tend to clog up.
In more recent years, he has experimented with using compressed air stored in recycled plastic drinks bottles, which is employed to expand cylinders and move things. He has conducted numerous experiments before coming up with a pneumatic logic element, which he calls a ‘nerve’, which is brilliantly simple. Whereas he has tried a large number of designs in which sleeves moved, and covered and uncovered holes, he has found that the best solution is to have a cylinder which moves in such a way that a flexible plastic pipe, attached to its side, is moved from a position in which it is slightly bent, and passes air, to one in which it is sharply bent and stops air flow. His pneumatic cylinders are telescopic arrangements of smaller diameter tubes inside larger diameter tubes, sometimes incorporating a large-small-large-small-large type of sequence, and not cylinders with pistons and piston rods. This simplifies construction and does away with the need for piston rod guides.
One of the most remarkable abilities of some of his beach animals is their ability to detect both soft sand and water, and, when they do so, reverse. Higher pneumatic pressures in actuators attached to legs detect the soft sand, which means they are having to work harder. A trailing plastic pipe detects water. Air is drawn in through this continuously. If there is no water, air flows freely; but, if water is detected, there is then an increased pressure drop between the outside ambient air and that inside a cylinder, which then retracts under atmospheric air pressure. One of his creations even incorporates a pedometer, which uses a combination of his pneumatic ‘Nerve’ logic elements to count the numbers of paces it has made, before it is required to reverse its path. He has also devised timers that trigger after a certain amount of air has leaked. In one of his animals, this triggers a hammer driving a stake into the ground to secure it against bad weather.
Propulsion is by propellers or sails – which, in some of his animals, are used to move cranks to drive them directly; in others, they are used to compress air in banks of plastic bottles. He has also constructed walking machines from wooden pallets – which has to be seen to be believed – and, in one case, powder-coated steel sheets, but has subsequently reverted to the plastic tubes.
He observes in his book, ‘The great pretender’, that, if a university laboratory is asked to come up with a robot, they will inevitably start thinking in terms of vision systems and microprocessors, and things driven by electric motors. But all of his mechanisms depend only on mechanics and, in some cases, pneumatics to do quite surprising things.
The simple ‘nerve cell’ logic elements and suction increase water sensors could easily find application in industrial automation and the walking mechanisms could well find relevance in military and undersea applications. One only has to think of the 1903 HG Wells story ‘The land ironclads’, which were tanks with a walking mechanism. But because mechanical feet were impractical at that time, and could not have reacted to encountering unsuitable ground, the first military tanks in World War I had to use tracks borrowed from caterpillar tractors. While there is no doubt that robotic machines are the future, particularly on the battlefield, there is a tendency to try to make them either a bit like humans or base them on established military vehicles. However, better models for many tasks could well be other animals, including centipedes and worms; or, as in Jansen’s experiments, animals that do not exist in nature, but could, if they were optimised to undertake tasks of interest to humans.
A Nasa project at its Jet Propulsion Laboratory in Pasadena is developing a balloon-shaped Martian rover that would be blown around the landscape like a piece of tumbleweed and it is still possible that a wind-powered machine really could be devised that would help maintain the Dutch beaches and sea defences, as Jansen originally suggested.
He has chosen not to patent any of his inventions or devices, although they are inherently protected by copyright. All he asks is that, if anyone is seriously interested in the details of any of the devices, they purchase his book, which is available from www.strandbeest.com for 39.50 Euros. The site also includes videos that prove that these artworks really can walk.
Theo Jansen gave a talk and demonstrated one of his machines at the 2008 SolidWorks World event in San Diego in January where the picture above was taken


* The machines are mostly made of plastic cable conduit tubing and are purely mechanical, although some use pneumatic assistance

* They use a walking mechanism for traversing soft grounds that is much more energy efficient than any other devised

* They also include a number of mechanical and pneumatic innovations worthy of study and of potential usefulness in mainstream engineering

Tom Shelley

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