Also known as a ‘high definition multileaf collimator’, Varian Medical Systems’ HD120 MLC computerised radiation therapy machine features 120 individually adjustable tungsten leaves, which can be positioned to form an aperture, perfectly shaped to direct the radiation in a pattern that exactly matches the shape of the tumour. Furthermore, the radiation can be delivered from different angles to match the contours of the tumour. With this level of control, the radiation intensity can be adjusted to deliver a higher dose to more aggressive areas of the tumour and a lower dose around sensitive healthy tissue.
The shape of the aperture is formed by moving the leaves quickly and precisely to pre-programmed positions, in response to computer generated signals. The task for Varian was to seek a suitable type of leaf drive motor for this demanding application – one that was not only very compact, but also capable of moving the leaves reliably and accurately into their pre-programmed positions.
In the end, the company chose maxon’s brushless dc (electronically commutated) motors with diameters of 8, 10 and 13mm. Some 120 motors are required for each 120-leaf collimator array, each equipped with encoder feedback and driving via a planetary gearhead. This combination of motor, gearbox and encoder takes the length of the individual drive systems to between 17.0 and 24.6mm; even so, all 120 motors are contained within an area measuring just 40cm by 40cm.
Despite their compact design, the motors are remarkably powerful – thanks largely to their high-quality neodymium magnets. Moreover, the ironless windings ensure that there is no magnetic detent, so their movements can be controlled with great accuracy.
High torque is also needed to overcome friction within the system of synchronously adjusting leaves. The maxon drive systems used on this application draw between 0.5 and 1.5W, yet they are capable of producing a nominal torque of up to 1.61mNm. When driving via the gearheads, however, this output rises to 200mNm. maxon motor UK chief executive, Angelo Guttoriello summarises:
“The properties of our motors and associated components ticked every box in the designers’ wish list. Needless to say, they also wanted a long service life and reliable operation. In applications like theirs, where component failure can cost lives, the reliability of our products is highly valued.”