Novel linear motor offers alternative to pneumatics

A linear actuator has been developed that looks and operates like a pneumatic actuator, but offers the reliability of a linear motor with the accuracy of a ballscrew. Dean Palmer reports

A company has developed a unique design of linear actuator that uses its own unique design of tubular linear motor, on-board non-contact, position sensing electronics and a novel long life polymer bearing. The project is also a shining example of how two engineering companies, by working closely together and collaborating on technical issues, resulted in a new product that not only met the original design specification, but was also cost effective to manufacture and offers customers a very real technical advantage from buying the technology. The 'ServoTube', developed by Copley Motion Systems based in Basildon, comes with DIN/ISO 6431 mountings and so is compatible with pneumatic actuators and industry standard accessories, so if design engineers need to install it or replace some existing pneumatics, it really couldn't be more straightforward. At first glance, the device even looks and operates like a double-acting pneumatic actuator. The design consists of a sealed, stainless steel thrust rod which encloses rare earth magnets. This patented configuration ensures a high density, uniform magnetic field. A rugged, IP67 rated enclosure houses the motor coils and position sensing electronics and the forcer field interacts with the thrust rod magnets for around 50-105N of continuous force. The unique magnetic design of ServoTube was patented back in the mid-1980s. Michael Cowan, managing director of Copley Motion Systems explained: "Copley was the inventor and manufacturer of the tubular linear motor. There have been several iterations of the company since then and we are now part of US parent company Copley Controls. "The design of the original actuator was based on our linear motor, an external linear bearing and guide and encoder technology. But we realised that the key strength of our design compared to other competing U-channel and flat bed type linear motor actuators, was the simplicity of our tubular motor. The external bearing and encoder we used simply added cost and complexity to our solution. So, in 2001, we decided to develop a simple, non-contact, direct thrust actuator to try to open up new application areas," explained Cowan. Other design objectives, said Cowan, were to make it high performance (ie. better than ballscrews) but quieter and with an integral rather than external bearing. Copley's sales and marketing director Bill Luckin took over the story: "We turned the whole design of our existing linear motor on its head really. By fixing the magnet track we had made the motor look and behave like other linear motors on the market. With ServoTube, the coils are no longer fixed." But the company had to overcome two problems. As Luckin explained: "With the coil no longer fixed, we needed a method of measuring the position of this relative to the magnetic thrust rod. With the original design, we used a Renishaw encoder and reed head to measure the position and send the data to an amplifier. But to achieve the required rigidity and alignment for this type of position sensing, the original design used a monorail guide (supplied by THK, INA, NSK or STAR. The new actuator uses Hall Effect sensors built into a side-mounted electronics module and terminal block. The sensing electronics includes a patent-pending, low cost PCB design that accurately determines the position of the thrust rod relative to the magnetic coil. The patented part of this design is really the signal conditioning electronics which corrects for errors that normally occur as a result of misalignment. The accuracy of the sensor is +/- 250 microns over one metre of stroke. And repeatability is better than 12 microns! 18 months ago, the company finally got the green light to embark on ServoTube, although the product wasn't officially launched until mid-2004. According to Cowan, Copley had to invest heavily in new automated manufacturing technology for the plant in Basildon and had to employ design-for-manufacture (DFM) methodology on the new actuator. "We've spent around £120,000 on tooling costs alone." Cowan added that there were three design objectives. The first was to remove the need for an external scale and an external encoder and to package the device in such a way as to still make it easy to use as a pneumatic actuator, so that it could be targeted at high speed, medium resolution applications, such as food packaging and bottle filling. He explained further: "For food packaging, the trend today is towards custom labelling and packaging of foods. Batch sizes are smaller and instead of production line changeovers occurring one a week they happen once or twice a day. So linear motion in these plants tends to rely on pneumatic actuators." The problem, said Cowan, is that pneumatics is not flexible enough for some end users. If ballscrews are used, these can be very slow compared to pneumatics. Linear motor technology, on the other hand, is accurate, high speed and reliable but tends not to be used in the food and beverage sector because of the dirty environment and the regulations and standards regarding direct contact with food and spillages. So ServoTube was designed to cope with these types of environment. As Cowan stated: "We had to make the actuator look and feel like a pneumatic actuator and make it easy to install and replace existing pneumatics. We knew we couldn't compete on cost as our complete system typically costs ten times that of a pneumatic actuator. However, we knew we could focus engineers' minds on cost reductions in other areas such as reduced compressed air costs, less production downtime with a more reliable actuator and less noise." ServoTube's motor is also three-phase and so is compatible with standard drives amplifiers and controls and the position feedback conforms to the usual standards for encoders. There was another crucial design problem that needed to be solved. The thrust rod would need to be supported somehow. Cowan explained: "Cost was everything to us, so if we felt that if we could combine the insulation layer around the motor coil with a polymer bearing sleeve then we could achieve this." The problem though, said Cowan, was locating an injection moulding company that could actually manufacture a polymer bearing with the required design features. The bearing had to be 285mm in length, which presented a problem in itself as most bearings would be a maximum of three times the nominal inside of the sleeve. This was more than five times longer. Also, the bearing had to be just 1mm thick and have two, perfectly flat inner and outer surfaces. Copley approached five injection moulders who all came back with the same answer, that the bearing simply couldn't be made with existing tooling. Next, during a routine visit by a sales engineer from one of Copley's suppliers, Igus, Cowan invited the company to help out with the bearing problem. Igus already supplied bushes and cable management systems to Copley and so some relationship existed. Matthew Aldridge, product manager at Igus, explained: "After several technical discussions between the two companies, we realised that we might be able to solve the problem using one of our polymers, Iglidur." Although Igus normally only supplies components that are injection moulded at a single point, the company investigated ways of injection moulding the complete 285mm bearing sleeve at its R&D facility in Germany. "The real issue, aside from the thin diameter and overall length, was that Copley wanted a flange at both ends of the bearing, so we couldn't use our standard tooling. The idea was for economic reasons. Copley wanted to be able, for certain sizes of motor, to cut the bearing sleeve in half and use them on smaller motors. "The maximum temperature required for ServoTube was 100°C, so we had to use Iglidur L100," added Aldridge. This material was originally developed by Igus as a low cost version of its Iglidur W300 polymer, which was already being used as a bush in many pneumatics applications. After the most suitable material had been chosen, Igus then set about designing new injection mould tooling for the new bearing sleeves. Aldridge continued: "The tooling we developed enabled us to mould at six separate points around the top of the bearing rim. The polymer is injected down the bearing rather than around it. By moulding it in this way, the grain is parallel to the direction of motion, similar to the grain of a piece of wood." While Aldridge chose not to reveal the actual design of the tooling and injection moulding machine in detail, he did say that the mould tool had to be modified so that it split in half, enabling the bearing sleeves to drop out after the injection process was complete. The machine is three metres high. Copley paid around £13,000 for the tooling. There was one small problem with the tooling which was quickly rectified by Igus. This, explained Aldridge, arose because the ServoTube's motor coil needed to be precisely fitted to the bearing with almost zero tolerance. "The first production batch of bearings had a problem on one end. The coils were not fitting on this end and were too tight. It turned out that, as the injection mould tool was pulled away from the bearing, the polymer was distorting [swelling] by 10 or 20 microns, enough to prevent the coils fitting the sleeve. We solved this by simply letting the mould tool cool down by increasing the dwell time. By keeping the bearing integral to the actuator, Copley has managed to produce a linear motor/actuator that is directly compatible with all other pneumatic actuators. At an exhibition in Italy last year, Cowan said that visitors to Copley's stand were "amazed" at the company's demonstration of ServoTube because they thought it was double-acting pneumatics technology. Potential applications for ServoTube include coordinated cutting, pick-and-place, tensioners, point-to-point positioning, coating, press fitting, dual forcer positioning, bottle filling and laser cutting. And, because it has an IP67 rating, it can be used in frequent washdown environments. ServoTube's housing has small grooves in it (for dissipating heat and mounting), but Cowan said that the company was also developing a "completely smooth version" of the product for use in applications where the actuator comes into direct contact with food. Copley has so far sold more than 200 ServoTube units to all kinds of customers, including a leading US manufacturer of food processing equipment and a rather novel application in which the actuator is being used to control air flow slats controlled by a foot pedal on a church organ. Apparently, the device had the desired low noise and high actuation speed required for the application. ServoTube also won the 2004 Gold Manus Award, a competition aimed at UK and Irish engineering companies, to design the most innovative polymer bearing, which is judged by Cranfield University.