FEATURE MOTION CONTROL Figure 3:
Creating a solution with the collapsible support block system and precision manufactured long screw length means stroke lengths of 10.8m and over are achievable. A system with a stroke length of two to three metres would have a maximum speed of around 3/4000rpm. Normally with a longer system the rotational speed would have to be lowered considerably to avoid whipping, but Thomson has engineered a solution using these additional supports to maintain stroke lengths over 10m at 3/4000rpm. This results in stiffness of the screw system, long stroke length and high transportation speeds.
MEETING APPLICATION DEMANDS Screw drive systems with long stroke lengths are used in a wide range of industries to provide precise linear motion control. Welding systems are one example, where very accurate positioning of a welding nozzle is
required. In applications where high quality materials are being used, such as titanium, the welding is carried out in a vacuum to avoid oxidisation of the metal. Parts such as long tubes in these applications require long screws to give the linear reach required. Another area which can benefit is
within automotive production lines, such as moving robot-welding arms between stations. Although speed may not be a critical factor for transporting robot arms, long length and very accurate positioning are required. There are
Figure 4:
Thomson’s linear rodless actuator slides provide guided linear motion through a belt drive and saddle that rides along the top of the actuator to carry the load
Enclosed screws are protected against the ingress of dust and debris in Thomson linear drive systems
also many applications which require the transfer of material between assembly stations, and often these must deliver exact positioning for components moved between pick and place stations. Again, a screw drive provides a good solution here. The manufacture of optical cable is a
high-speed application which cannot be stopped without jeopardising the quality of the fibres being produced. The cables are spooled onto large reels and, when one reel is full, it needs to be replaced as quickly as possible. Precision and speed are vital to the process efficiency and long screw drives can offer both in this
application, along with the capability
of handling the heavy load of the reels. Any application requiring the
movement of heavy equipment in the vertical plane benefits from the rigidity and failsafe functionality of a linear screw. In the aircraft industry, for example, high precision cameras are moved up and down and the screws carry the heavy weight securely and precisely. In such applications, special ball guide systems with large diameter balls are used to take up the dynamic load moment.
BIOMEDICAL EQUIPMENT BENEFITS FROM
MOTION CONTROL TECHNOLOGY A customised positioning system has been supplied by LG Motion for the bioengineering innovator MICA BioSystems. The company’s ‘mechanical conditioning bioreactor’ was developed to support a patented technology invented at Keele’s Institute of Science and Technology in Medicine (ISTM). This benchtop machine aims to control cell behaviour by applying time varying magnetic forces to magnetic nanoparticles targeted to cell membrane receptors. The particles are oscillated by a motion controlled magnet array to vary the magnetic force and exposure factors applied. The MICA (Magnetic Ion Channel Activation) Bioreactor features an incubator with provision
for cell growth chambers or multi-well plates to be loaded and located in its upper area. The magnetic array is mounted beneath on an LG motion motorised linear positioning stage. With its microstepping drive and integral indexer housed in a separate desktop console, researchers can initiate and adjust the stage speed and position using GUI software from a standard PC to set the oscillation rate, amplitude and duration parameters of the magnetic array in order to produce the exact magnetic stress profile required for the application. The basic specification covered a single-axis positioning stage with travel range of 50mm and
positional repeatability to within 10 microns, with support frame and control electronics. The vertical orientation of the stage and the relatively heavy overhung payload of the various
interchangeable magnet arrays meant a robust design was needed; and a dual linear bearing arrangement based on linear motion guides was preferred along with a rolled quality ballscrew that would economically provide the precision required. The microstepping motor driven stage includes adjustable over travel limit switches to protect the magnets and the incubator from collision whilst a home sensor maintains a datum position on initialisation as part of the routine to set up an oscillation profile. A two metre cable connects the bioreactor to the control cabinet.
LG Motion T: 01256 365600
www.lg-motion.co.uk Enter 214
IMPROVEMENTS Many existing applications work with screw systems of up to five or six metres where the screw is left completely open. With these, however, the system cannot operate at the desired speed; and it is also difficult to maintain as the open screw attracts dust and debris. In such applications, the additional supports provided by the stacking bearing block configuration mean the screw can be operated at a much higher speed. Cleaning and reliability issues can be resolved using a covered, sealed system which protects the screw and offers significant reductions in maintenance requirements. The enclosed screw, as shown in
Figure 3, is protected from ingress of dust and debris and, without regular cleaning, can maintain optimum performance and reliability. In such a system the carriage can be equipped with drilled channels and connected with a grease nipple. This enables lubrication from a single point without having to open the casing, ensuring rapid and comprehensive maintenance is achieved. As the unit never has to be opened, very limited amounts of dust or water can penetrate the system and it is protected even in the dirtiest of environments.
Thomson Industries
www.thomsonlinear.com Enter 213
20 FEBRUARY 2015 | DESIGN SOLUTIONS / DESIGNSOLUTIONS
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