Radio telescope installation gets into gear

Edited by the Engineeringtalk editorial team Oct 22, 2004

ZF's precision planetary gear heads are providing the high levels of torque and accuracy required for Cambridge University's new radio telescope installation.

ZF's precision planetary gear heads are providing the high levels of torque and accuracy required for Cambridge University's new radio telescope installation, designed to find signs of some of the earliest detectable galaxy clusters.

The installation comprises five pairs of 4m diameter parabolic dish antennas whose movement is controlled by a high reduction ratio servomotor system.

A small servo geared motor drives two close-coupled ZF precision planetary gear heads supplied by Drive Lines Technologies.

In conjunction with the final drive, the system achieves an overall reduction ratio of almost 100,000:1.

Using galaxy clusters in order to learn about the early universe has long been recognised as an important area of astrophysics, but observing these distant objects using either light or X-rays has so far proved problematic.

Since the discovery in the late 1960s of a secondary effect of cosmic microwave background radiation (CMB), astrophysicists have been building instruments to study the effects in order to measure the size of galaxies and calculate their mass.

The most recent of which is Cambridge University's series of ArcMinute MicroKelvin Imagers (AMI) installed at its Lord's Bridge site.

Cambridge University's new AMI installation is a series of ten antenna dishes that enables staff and students to survey the sky searching for signs of some of the earliest galaxy clusters, which are detected as "silhouettes" against the backdrop of the CMB.

The AMI system will combine a large field of view with high sensitivity to faint objects, making it ideal for surveying the sky for the galaxy imprints and measuring the angular sizes of the objects in arc-minutes rather than degree scales The parabolic dish antennas have 4m diameters, each weighs 1.5t and is designed to operate 365 days a year in winds up to 70km/h and survive gusts up to 185km/h.

In order to provide accurate results, the antennas need to remain fixed on a distant point in the sky during bad weather and compensate for the movement of the earth on its axis.

This means that the mounts need to rotate about their primary axis at nominally 15 degrees per hour, 360 degrees per day, requiring a high-ratio slewing-ring-based drive system.

From the servomotor speed range of 70 to 3500rev/min a slewing speed from 15 degrees per hour (for tracking) to a maximum of 12.5 degrees per minute (for positioning) was required.

As well as the slow speed, the level of accuracy required from the drive system is in the order of 20 seconds of arc with an encoder resolution of 10 seconds of arc.

"To achieve the required reduction ratio of around 100,000:1", explains Drive Lines Managing Director, Bob Clark, "the drive system incorporates five stages of planetary gearbox and a final drive pinion to slewing ring ratio of 5.5:1".

"The selected servo motor was geared down via a single stage 3.7:1 box into two close-coupled dual-stage 70:1 ZF PG series planetary gearheads".

"The ZF PG units were the natural choice as they offered backlash levels of less than 2arc-min and a very high degree of torsional rigidity".

The use of grease as a lubricant, rather than oil, enables the PG gearheads to be mounted in any orientation on the equipment and, being sealed for life, allows the antennas to operate in all weathers without the need for maintenance.

Their compact size also makes them the ideal choice for the application, which was, due to the need to constantly reposition the 10 antennas at the site, constrained in terms of both design envelope and weight.

ZF PG gearheads are available in single and two stage variants with a range of ratios between 3:1 and 100:1.

The gearheads are rated for motor input speeds from 3000 to 10,000rev/min as standard, and are suitable for use with a wide range of motor frame sizes.