Novel actuators copy structures from fish

By imitating the way that fish fins are constructed, it has been possible to develop novel grippers and actuators – as well as working models of a wing-propelled, smart structured airship and a novel underwater inspection vessel. In fish, the double ribs that support fins are of very subtle construction, but in industrial developments these can be constructed either from two ribs connected by rigid links or by extruding profiles out of semi-rigid elastomer. The basic idea has been given the name ‘Fin Ray Effect’ and comes from Leif Kniese of EvoLogics, a Berlin-based company specialising on advanced technologies inspired by nature. The development into real world products that are suitable for factory automation has been led by Festo. Explaining how he came up with the idea for the wings – standing in front of a robot ‘Aqua-ray’ fish in a water tank on the Festo stand at the Hanover Fair – Kniese said: “We use the muscles with water hydraulics, then try to imitate something that looks beautiful when it flies. The penguin and manta ray ‘fly’ under water. It’s a very efficient way of moving. Under water, the machine can fly and glide by changing its buoyancy. Because of the structure of the fins, it is possible to make these with very small contours. We have the same concept in the ‘Air_ray’.” At that moment, the Air_ray was released from its mooring clamps, flew itself round the hall by moving its wings, and then glided back into the clamps – which were themselves inspired by the Fin Ray Effect, holding the very fragile airship firmly yet securely. The fins can be moved in two ways. In one, pushing on one side of the fin near its base inherently causes the structure to bend towards that side. In the other, pulling the fin on one side causes it to bend. On its stand, Festo demonstrated this in the form of a distributor chute, pulled on either side by the company’s ‘DMSP fluidic muscles’ to direct blue, grey and white 16mm balls falling off a conveyor belt into different bins at three balls per second. The Aqua-ray is 62cm long, and 96cm wide and weighs 10kg. The torso is made of glass-reinforced plastic, while the wings and tail are made of ‘Curv’ – a polypropylene sheet with mechanical properties comparable to some glass reinforced composites, made by Propex Fabrics in Gronau, Germany. The skin is polyamide. A Torcman brushless motor powers a 400 litres/minute vane cell pump, which drives the three pairs of antagonistically acting Festo fluidic muscles. Their force of contraction is transferred by artificial tendons of ‘Dyneema’ cord via spools and sheaths, to the wings and tail – which in turn transform the tendon travel of 55mm into a vertical wing amplitude of more than 550mm. Using water as a working fluid, rather than air – as used in the robotic fish that the company showed on its stand last year – assists cooling. Power is from a 24V, 10Ah battery. Maximum speed is about 1.8 km/h. The device spends much of its time gliding, but minimum flight duration under full load is 65 minutes. Much more than a demonstration toy, it is intended for use in marine research without disturbing the natural environment. Its closed contour and lack of rotating parts such as propellers make it suitable for the underwater inspection of pipelines, cables, or the sea floor without it running the risk of becoming fouled by seaweed or cables. It has a low frontal area yet a large horizontal surface making it an idea carrier for sonar arrays. The demonstration fish is radio controlled. In the air The Air-ray is made of PET foil coated with vapour deposited aluminium. It has a 4.2m wing span, is 2.8m long and its helium filled volume is 1.6 cubic metres, which dictates its maximum mass of 1.6kg. Its propulsion mechanism is also by flapping its wings. The wing module has two alternating pressure and tension flanks flexible connected by ribs. When one flank is subjected to pressure, the geometrical structure automatically bends in the direction opposed to the force applied, again making use of the Fin Ray Effect. A servo drive unit pulls on the two flanks longitudinally. The structure is supplemented by a torsionally resistant central spar developed by Rainer Mugrauer at Effect Technik in Schlaitdorf. Mounted at its exterior end is a servo drive that allows the flapping wing to rotate about its transverse axis, so that the Air_ray can fly backwards as well as forwards and perform bird-like manoeuvres. Power supply is by two lithium polymer cells rated at 8V, 1500mAh. It is expected to find application as a camera platform for UAV inspections of the internals of large buildings. Its lack of external moving devices and compliant surface make it incapable of causing damage. There has been much development effort into devising smart deformable structures for UAV aircraft wings, particularly in the US, but also in the UK, in order to improve manoeuvrability. US and UK efforts have, as far as we know, all been based on inserting piezoelectric actuators in the wing skin. The only problem with this idea is that these devices use fearsome amounts of electric power, as was confirmed to us by Daniel Schmidt of the European Center of Adaptive Systems, exhibiting in another hall at the Fair. The Fin Ray Effect is a much simpler and more energy efficient way of achieving the same goal. Festo is seriously interested in using it in actuators. The Fin Ray Effect mooring clamps for the Air_ray are the best way of gripping soft objects we have seen. Because they unroll onto the object to be gripped, they also offer a good means of gripping and then releasing difficult to handle objects such as sticky cakes in the food industry. EvoLogics additionally sees potential for its patented Fin Ray Effect technology in adaptive vehicle chassis frames, light carrier structures and smart seats. Pointers * The Fin Ray Effect is a very simple way of making a deformable smart structure * Pulling by small amounts on either side produces a large lateral movement at the tip * It has already been demonstrated as being suitable for use as actuators and grippers in factory automation equipment as well as smart wing structures for unmanned underwater and aerial vehicles