Geared motor gives precise motion on X-ray scanner

A system comprising a custom designed helical geared motor and a worm gearbox is achieving precise control on an X-ray treatment system. Dean Palmer reports A geared motor customised from standard models is helping a manufacturer of precision X-ray scanning machines achieve the level of control and positioning accuracy needed to scan cancer patients safely. Varian Oncology's innovative machine, Acuity, launched in 2003, integrates planning, simulation and verification software for treating cancer with radiation therapy. It enables high quality digital X-ray images to be captured and analysed, while a respiration motion analysis system is available to allow doctors to see how tumours move as patients breathe and then adjust treatments to account for this motion. The machine's requirements of low backlash, high gear ratio, compact size and irreversibility in both static and dynamic conditions, were met by combining two separate units: a helical geared motor with a worm gearbox. The total gear ratio of 171 was achieved with output shaft backlash of only 10 minutes of arc and a medium level of efficiency, which gives patients safety in the event of a power failure. Acuity consists of a couch and treatment head. Crucial to its performance is the accurate and reproducible patient positioning, which allows more focussed and effective treatment with reduced dosages. The head represents a huge inertia and an out-of-balance load that has to be rotated on a horizontal axis. Lenze's geared motor drives a gear ring with an 8:1 reduction so that head rotation speed is around 1 rev/min. Safety of the geared drive was paramount. In case of power failure, the head cannot fall under gravity and cause injury to the patient, so a worm gearbox was selected, giving irreversibility and self-locking. General dynamic irreversibility occurs when the running efficiency is less than 50% and, similarly, static irreversibility is achieved when the starting efficiency is less than 50%. However, in the latter, the gear ratio required was above the range possible for standard worm gearboxes. Helical worm designs can achieve the high ratio but have higher efficiencies of typically between 60 to 70%, which would not meet the self-locking requirement. To avoid the costs of a special gearbox design, Lenze proposed a combination of a shallow ratio helical gearbox and a 60:1 ratio worm gearbox. The combined efficiency of this assembly is around 40%, so self-locking is achieved. After a joint development effort between Lenze and Varian, the final assembly chosen was a 0.55kW motor into a single stage helical gearbox, integrated into the input of a 60:1 ratio worm gearbox. The total transmission ratio was 171, giving an output speed of 8.4 rev/min and a rated torque of 250Nm. According to Lenze, the gear elements were matched to give an adequate strength throughout the assembly and a starting efficiency suitable for the maximum possible out-of-balance loads. Gearbox backlash was also key. Vibration or backlash in the drive would cause a fuzzy image and reduce the machine's ability to give accurate pinpoint treatment so Lenze's worm gearbox was provided with reduced backlash. Backlash in the helical stage is relatively small and the total backlash for the assembly is around 10 arc mins. The effect of backlash was further reduced by an 8:1 pinion drive fixed onto the head. As the computer imaging is done with the head dynamically traversing around the patient's body, the ability to change direction without significant gear backlash is vital. Varian's design manager, Felix Fernando, commented: "Lenze's high quality gearboxes are an integral part of Acuity and we worked very closely with their engineering team in developing this solution." Pointers * The treatment machine requires precise positioning accuracy, low backlash and self-locking in both static and dynamic conditions * Lenze's customised helical geared motor and worm gearbox assembly achieves a combined efficiency of 40% and a total transmission ratio of 171 with torque of 250Nm * A reduced system backlash of around 10 arc/mins also ensured that the machine head's ability to change direction around the patient's body was not affected significantly