Convincing design engineers that electricaly driven linear motors can replace compressed air pistons

Compressed air is relatively noisy, expensive and inefficient according to Hugh-Peter Kelly, managing director of Direct Thrust Designs. According to most other people, it is a necessary evil. What Kelly was comparing compressed air with is its use in driving air pistons as against his company's alternative of the ElectroPiston, an electrically driven linear motor. However, uptake for the ElectroPiston has been frustratingly slow for Kelly since its introduction of the latest model last year with much of the frustration coming from the advantages he perceives his invention has over the traditional alternatives. Kelly does appreciate where some of this reticence comes from. "If people have an automation machine for churning out 10,000 biscuits an hour it has got to work. There was that case at Gatwick in the spring when the electric motor failed on the luggage line and it caused chaos for days – flights cancelled and holidays ruined. That is why engineers are so conservative, they need to be sure before they change. As an engineer I am sympathetic to that. But when I can go in and demonstrate that something looks, sounds and feels better, and they can't invest £120 on a sample piston, that is frustrating." The product itself was outlined in Eureka a year ago, at a time when the new iteration of the product was completing its development phase. Although the principle had been deployed in the previous generation that had principally been used by a single client in mainland Europe, the new version was aimed at a broader market. Linear motors are typically used in high precision applications on account of their higher capital cost. Kelly invented the tubular linear motor in the 1980s and was initially disappointed that they didn't conquer the world. In retrospect the price issue prevented this from happening – these motors required a positional feedback signal, linear encoders and control electronics and the software to control it. "What I did was take the tubular linear motor and reshape it so that it can be used to go backwards and forwards, but with no software, no electronics and no encoder," said Kelly. "I wanted to come out with a product that could rival pneumatic pistons. And that is what we have done." The result is the ElectroPiston and a key aspect is its simplicity. You put electricity in and it goes. The motor housing comes in three cross-sectional sizes and can be cut to any length. The housing contains a single electric coil or two coils for more powerful motors. The piston rod has permanent magnets inside it and the magnetic field works against the electrical coil to create thrust – and that thrust is constant over the length of the stroke. Kelly said: "The tube with its special sequence of permanent magnets, and the design of that in conjunction with the coils in the housing ensure this perfectly even thrust." This thrust is completely proportional to the current. Also, there are two sensors at each end that give out an analogue signal to show the position the piston is in, which allows determination of what action comes next – changing direction for example. Timing can also be adjusted. So without having any control electronics built into the unit, the ElectroPistons can be set for any form of repetitive motion. "It is incredibly sensitive," added Kelly. Trusted bearings "The choice of bearings, the rods, the bearing material is just as important as the motor. If you get that wrong then it won't work," said Kelly. "Importantly for us we decided to run with igus bearings right from the start. They have been fantastically supportive to us and we have had endless meetings, for example, to make sure we have the right grade of 'DryLin' bearings." Having selected the bearing Direct Thrust life tested a piston to 100million cycles, which took nearly three months even with the accelerated mode of operating. This accelerated wear check, which the bearing passed unscathed, equated to one cycle per second for 16 hours a day, five days a week for seven years. "People need something that will go backwards and forwards all day every day and be sure that it is going to last," added Kelly. "We probably would have got away with using lower spec bearing technology but not with the same degree of confidence. When you go and see potential customers you can say it has got igus bearings inside it and that seems to put people's minds at rest. To some extent, you want to be able to delegate some of the responsibility to others. So if you have got a reputable, well-trusted industrial partner, then you can relax and know that the bearings will do its job." For similar reasons the guide rods have also been bought from igus and are made of a specially hardened aluminium. Being aluminium they are light, so that reduces the 'to and fro inertia', which does matter in very fast moving systems. The consequence is a system that requires no maintenance, unlike its pneumatic counterparts. But still the assurance of these maintenance-free igus components does not appear to be enough to cause engineers to cast aside the compressed air that they are so familiar with. "People just don't know about it because it is new," claimed Kelly. "I think that is part of the problem - getting people to realise that there is an electric piston that can replace air pistons." The ElectroPistons have gained recognition as finalists in both the British Engineering Excellence Awards and the IET Awards, but designers remain interested rather than committed. To address this issue Direct Thrust approached a supplier of compressed air systems to put conservative figures on the installation of a new system, and these are outlined, along with the equivalent costs of an ElectroPiston system, demonstrating that the overall cost for the latter system is approximately a fifth of that for a traditional one over a five year period. It accounts for investment in the compressed air infrastructure – the compressor, valves, control and distribution lines etc – and this may already be in place in many industrial environments, but even then Kelly argues that the cost of running and maintaining pneumatic systems is not often taken into account. Indeed, said Kelly, some design engineers only look to the capital cost as that is what their customers are most interested in. "It is purely about what goes out the door and what profit they can make from it, regardless of what that means for the person who has bought it," he said. "And that is counting against us. The cost of the compressed air and possibly maintaining it is not their problem. Compressed air does cost money and there are safety implications as there is a heavy onus on manufacturers. So people need to take account of the whole equation. Over time this system saves money, both for the installer and the user. But feet are stuck in concrete – it is unbelievably hard to get people to change."