Small motors find new applications

Written by: Paul Fanning | Published:

Design engineers in all industries are faced with the ever-increasing challenge of designing solutions to drive functions that deliver more from a smaller package size. Ordinarily they are looking for inherently more power from a smaller package size, with greater efficiency and longer life, to reduce power consumption in general and extend simple usability in the case of portable handheld devices. This is where the use of small motors comes to the fore.

In today's market of miniaturisation of portable batterypowered devices, the need for ever more efficient small motors is apparent. Quite how small such motors are can be gauged from Faulhaber's tiny brushless motor 0206, which is just 1.9mm diameter. Using rare earth magnet materials and a minute skew wound ironless rotor, this motor can deliver 100,000rpm. Additional motor series have been developed giving motors in 3 and 5mm diameter, with available planetary gearheads in the same diameter and with the option of integrated lead screws for precise linear movement.

"However," says Stewart Goulding of Faulhaber distributor EMS, "design engineers are not only interested in size but also about functionality, with an increasing bias towards system solutions with on-board speed control or full motion control, negating the need for supplementary electronics. Faulhaber has extended its range of four pole brushless DC servomotors with the introduction of the BX4 series with integrated speed control and the world's most compact drive with integrated motion controller. High reliability, high torque, compact slot less design with no cogging torque, and robust construction without the use of adhesives make this new series ideal for demanding applications like robotics, automation, medical devices and aerospace.

Piezomotor technology is also a growing industry and offers design engineers unique characteristics that are not present in conventional DC motors. Piezo technology is simple and reliable, using very few parts and only elementary motion. Using Piezo motors negates the need for many supplementary components simplifying the design and extending lifetime. The astonishing power and precision of the small Piezo motors offers unique advantages when compared to traditional DC motors of the same size.

Piezo LEGS, from Swedish company PiezoMotor is in essence a walking machine constructed in a single piece. The construction consists of four legs with each leg being able to elongate and bend when electrically activated. By electrically pairing the legs and observing the transition and sequence of movement, two legs remain constantly in contact with the secondary interface resulting in propulsion of this part. Piezo LEG is available in different constructions so that a linear or rotary movement can be achieved. As the motor relies on friction and the legs remain in contact with the interface at all times high torques can be achieved without the need for a supplementary gearhead and as a consequence no unwanted backlash is introduced into the system. The result is a motor that delivers a step resolution of 1 Nanometre in the linear format and 25 Nanorads in the rotary format. In addition the friction element ensures that the motor remains self-locking with no power applied making them ideal for handheld battery powered devices.

An intriguing recent application of a small, brushless motor can be seen in Maxon Motor's collaboration with Instrument Design Technology (IDT) to customise a small, brushless motor to perform in the extreme vacuum conditions of the synchrotron within the Diamond Light Source particle accelerator in Oxfordshire.

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 facility works by accelerating electrons to 3 Giga Electron Volts (GeV), generating beams of "synchrotron light", up to 10 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 to the world's leading synchrotron facilities.

Diamond specified 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 are often sources of vibration. Eliminating this issue would immediately improve results from the DC motor – but the new motor would have to work flawlessly in a vacuum of 10-8 Torr."

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. However, the real challenge for us was effectively to create a brushless motor with virtually no glues or plastics, an incredibly high temperature tolerance, and excellent performance."

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.

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 would immediately be more efficient, quiet and responsive than the previous stepper motor.

Nonetheless, further challenges lay ahead. Says Maxon senior engineer Paul Williams: "It really was a painstaking process, effectively taking the motor apart and putting it back together, piece by piece. "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."

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. The Beamline has recently been praised for the quality of data produced.

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