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POWER TRANSMISSION Gears, Cogs & Wheels


(Left) A QinetiQ ion thruster suspended from an NPL thrust balance; the beam probe array at the bottom of the picture is mounted via the Mclennan stepper motor/gearbox assembly on the horizontal axis of the chamber. The probe arm is rotated vertically through the ion beam with a range of plasma diagnostics mounted along its length. (Centre) The 3.8m diameter LEEP2 vacuum facility. (Bottom) A QinetiQ T6 gridded ion engine under test.


test equipment includes vacuum chambers up to 3.8m inside diameter and 10m in length where motion controlled beam probe arrays are used to characterise the ion flux and beam divergence of the thrusters. As an intrinsic part of the beam probe array, motion control and mechatronics designer and manufacturer McLennan supplies high vacuum and extreme temperature compatible stepper motors and planetary gearboxes with specialist feedback devices, stepper drives and controls to precisely position the probe arm with its array of plasma diagnostics sensors. The two synchronised motor/gearbox assemblies are mounted on either side of the chamber’s horizontal axis and rotate the probe arm to track a vertical path through the beam. Mclennan supply 10-7 torr vacuum compatible, 80mm


diameter stepper motors from its German distribution partner Phytron, fitted with vacuum prepared high


Vacuum rated gears help test ion thrust propulsion system


Stepper motor and gearbox combinations are helping to simulate the extreme conditions that ion thrusters will experience in next generation spacecraft


propellant to high velocity. Whilst the acceleration levels are small, when operating in space these devices propel and position spacecraft with extreme accuracy and with efficiency levels that are an order of magnitude better than with conventional liquid propulsion systems. QinetiQ’s developments in this technology has opened the possibility to extend commercial satellite mission life and allow exploration spacecraft to venture further with


A


s a leading European player in Electric Propulsion (EP) design and development, QinetiQ manufactures ion thrusters which harness electrical energy to accelerate


enhanced payload capability, greater operational flexibility and reduced cost. One of the many current


EP developments by QinetiQ is in support of the European Space Agency’s BepiColombo interplanetary mission to Mercury where ion thrusters will be used to manoeuvre planetary and magnetospheric orbiters. The aim of the mission is to explore and understand the composition, geophysics, magnetosphere and evolution of Mercury, the least explored planet in the inner Solar System. QinetiQ’s Large European Electric Propulsion test


facilities (LEEP) are located at Farnborough and are used for the development, prototyping, manufacture, testing and qualification of electric propulsion thrusters, systems and associated equipment. The LEEP test facilities includes National Physical Laboratory developed thrust balances that ensure calibration results are traceable to national standards along with cryogenic and vacuum chamber equipment that enables QinetiQ’s team of skilled and experienced engineers and scientists to simulate the extreme conditions that the thrusters will experience in deep space. QinetiQ’s complement of highly specialised


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precision planetary gearboxes. To provide the levels of


redundancy that the


application necessitates, both resolver and optical encoder devices are utilised to combine for position maintenance and angular position feedback. The resolver is mounted on the output shaft side of the gearbox where its close proximity to the payload provides high accuracy rotary position that is unaffected by gearbox backlash over the 170 degree working


range of the probe arm. After initial position calibration, there is no need for a homing routine to set a datum even after power down, as the resolver essentially provides absolute position over a full revolution. The optical encoder is fitted on the rear shaft of the stepper motor to provide multi rotation angular positioning and stall detection. All devices supplied for this demanding application


meet the 10-7 torr vacuum specification along with temperature extremes of –160ºC. The dual output shaft motors and all in-vacuum mounted components are modified with outgassing vents to avoid pockets of trapped gas during the cryogenic vacuum pumping process. The planetary gearboxes use selected materials and special lubrication. The motion control and stepper drive is located outside of the chamber with a special Kapton insulated cable arrangement that runs through a hermetically sealed bulkhead feedthrough. www.mclennan.co.uk


INDUSTRIAL TECHNOLOGY • July 2012


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