Varian Medical Systems’ ‘Acuity’ X-ray simulation system captures and analyses high quality digital X-ray images. A key element of this system is the simulation head, which represents a substantial inertia and an out-of-balance load that has to be rotated in the horizontal axis. The head is driven by a customised Lenze geared motor, which comprises both helical and worm gear stages to provide irreversibility or ‘self-locking’ in both static and dynamic conditions. Irreversibility ensures that the head does not fall under gravity in the event of a power failure and cause injury to the patient or the operator.
Lenze proposed a combination of a shallow ratio helical gearbox and a 60:1 ratio worm gearbox. The efficiency of this assembly is about 50% when running at full speed and 25% statically, so self-locking is achieved. The total transmission ratio is 122 giving an output speed of 11.5rpm and a rated torque of 220Nm. 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.
The accuracy and repeatability of the head positioning was also deemed vital, as Acuity is capable of computerised tomography. Any backlash and vibration in the drive produces a fuzzy image, reducing accuracy and thus the ability of the operator to pinpoint a tumour and deliver minimal doses of radiation. Backlash in the helical stage is relatively insignificant and the total backlash for the assembly is below 12 arc minutes.
In order to achieve smooth operation, the 0.55kW motor is controlled by a Lenze 8200 vector frequency inverter. This gives S-shaped ramps for both acceleration and deceleration, smooth running better than +/- 0.1Hz and speed holding better than 0.5% without feedback. Compared to the dc motor and drive used in previous Varian systems, Lenze’s ac alternative also helped to reduce costs.