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Southern Manufacturing & Electronics

Farnborough, Hants(GU14 6XL)

11/02/2020 - 13/02/2020

Southern Manufacturing and Electronics is the most comprehensive annual industrial exhibition in the (more)

Vac to the future

Vac to the future Since its commissioning in 2007, the Diamond Light Source particle accelerator in Oxfordshire has been used for projects as diverse as analysing the effects of strain on aircraft wings, studying the behaviour of the HIV virus, and even reading ancient letters without opening them. The 45,000m2 synchrotron facility works by accelerating electrons to 3GeV, generating beams of synchrotron light, up to ten billion times brighter than the sun, in order to understand molecular structures. To prevent electrons being lost in collision with air molecules, the whole process is undertaken in a vacuum, around one billion times lower than atmospheric pressure. Clearly, creating scientific instrumentation for such conditions requires great specialist knowledge, and Widnes-based Instrument Design Technology (IDT) supplies the world's leading synchrotron facilities.

Diamond specified that the that the Double Crystal Monochromator for its new X-Ray Spectroscopy Beamline B18 should drive the crucial Bragg rotational axis with a DC motor rather than the usual stepper motor. IDT managing director Paul Murray explains: "The goal was to achieve higher rotation speeds, with a lower motor temperature and smoother drive train than the stepper motor we had used previously. Stepper motors are inherently noisy, and often sources of vibration. Eliminating this issue would immediately improve results from the DCM - but the new motor would have to work flawlessly in a vacuum of 10-8 Torr." So IDT enlisted the help of Paul Williams, senior sales engineer at Maxon Motor UK.

With experienced engineers handling the project at every stage, and a long history of customising high-performance motors for applications as demanding as space and surgical robotics, Maxon's teams in Britain and Switzerland were eager to rise to the challenge of creating a bespoke solution. Williams says: "Because the vacuum in the synchrotron must not be compromised, each individual aspect of the motor and its construction had to be analysed for possible outgassing. The challenge for us was effectively to create a brushless motor with virtually no glues or plastics, an incredibly high temperature tolerance, and excellent performance."

The starting point for the custom motor was Maxon's EC22 Heavy Duty. Although originally developed for sub-sea oil applications, the 22mm brushless motor's laser-welded stainless steel construction and broad temperature range already addressed many of the needs of high vacuum applications - and being a brushless DC motor meant it would immediately be more efficient, quiet and responsive than the previous stepper motor.

Nonetheless, further challenges lay ahead. Williams continues: "It really was a painstaking process, effectively taking the motor apart and putting it back together, piece by piece. For example, we had to create an entirely new way of encapsulating the magnets, in order to attach them without epoxies. Without Maxon's experience in micro laser welding, I don't think it would have been possible."

When customising the EC22 HD for use in the synchrotron, Maxon accounted for a number of factors. First among these was temperature management. Motors in a vacuum cannot dissipate heat through convection in the normal way, so can be prone to overheating. It is therefore important to choose a motor with high temperature tolerance, stay well within its rated performance in air, and, if possible, position other components in such a way as to spread heat by conduction. A high ratio gearbox was also vital. High vacuums of 10-7 Torr and greater can pull gaseous compounds from materials such as plastics and glues, compromising performance and contaminating the vacuum - a problem known as outgassing.

Each component in the motor was individually tested, and upgraded as necessary. For example, standard PVC cable coating was replaced with a more inert, Kapton version. It was therefore important that the EC22 HD was already substantially made from stainless steel, rather than plastics. Because of the potential for outgassing, the usual glues and epoxies could not be used, and the motor was put together using extensive micro laser-welding. Other areas required a still more innovative solution, such as attaching the motor's magnets to the shaft by encapsulating them within a specially developed, laser-welded sleeve.

From its work with space and aerospace projects, Maxon knew how standard, light greases used on motor bearings are affected in low pressure conditions. It was therefore important to specify an extremely inert lubricant which was also thick enough to avoid giving off vapour under high vacuum - and to account for the effect of this change on the motor's operating characteristics and anticipated life cycle. To finally remove any remaining outgassing threat, motors for controlled vacuum conditions are baked at 120ºC for 24 hours. Already purpose-built for temperatures in excess of 200º C, the EC22 HD proved an ideal starting point.

In traditional brushed motors, contact is maintained with a patina layer on the commutator. The absence of oxygen and humidity can therefore impair performance and increase wear. Most vacuum applications therefore use brushless motors, except where this is impractical for control reasons, or the vacuum is low enough to allow it. For example, NASA's Opportunity rover on the surface of Mars has been able to run happily for eight years with Maxon graphite and precious metal brushed motors, because of the 6mbar atmosphere.

The Double Crystal Monochromator, incorporating the special DC motor, is now in active service in Diamond Light Source's Beamline B18, playing its role in key experiments on a daily basis. As an example, a group of  Italian scientists used Beamline B18 to study dust in snowflakes that fell some 800,000 years ago - about the time of the first hominid life on Earth. Ice cores drilled from the Antarctic are made up of layers of frozen snow, dating back hundreds of thousands of years... and minute dust particles trapped as the snow fell hold key information about the earth's climate, atmosphere and volcanic activity at the time. Using X-Ray Absorption Spectroscopy, the scientists were able to study the mineral composition of the dust, determining its origin, and unlocking clues to the changes in global climate patterns over hundreds of millennia.
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