DAC102021

52 n DC DRIVES October 2021 www.drivesncontrols.com Iconic Goonhilly dish gets a DC drive upgrade G oonhilly in Cornwall is the oldest satellite earth station in England and is seen as the birthplace of satellite communications. In 1962, the first antenna (known as Arthur) was built to communicate with the pioneering Telstar satellite. As satellite communications mushroomed, further antennae were added and by 2004 the site hosted a total of 64, making it the world’s largest. Antenna No. 3 (also known as GHY-3 or Guinevere), built in 1972, was an early part of this expansion. But in September 2006, it was decided to scale down activities at Goonhilly, and the antenna stopped operating in 2007. In 2011, it was announced that part of the Goonhilly site was to be sold to create a space science centre. This involved upgrading some of the dishes to make them suitable for deep space communications. A new company, Goonhilly Earth Station, was formed to manage the operations and took over the site in 2014. GHY-3 was one of the antennae to be upgraded. The Grade 2 listed landmark consists of a reinforced concrete tower and a 29.6m-diameter counter-balanced reflector dish that is held in a frame of interconnected and braced metal girders. The antenna’s motor drive system consisted of four 150VA, 61A, 1,750 rpm DC motors set up as: n two motors on the azimuth control driving through gearboxes mechanically linked via a driveshaft and rotating around a 6m ring on geared pinions; and n two further motors on the elevation, also mechanically linked via gearboxes and a line shaft driving a rack-and-pinion. The mechanics of the rack and the motors were serviceable. Because the existing motors were in a reasonable condition, there was no need to replace them. Unfortunately, the electronic controls – consisting of rack-mounted analogue controlgear and interconnecting cables – were found to be inoperative. They had to be removed and replaced. Because the loading of the dish changes considerably over its path of movement, it was essential that the drives provided both accurate speed and current control, and full four-quadrant operation Due to the intricate mechanical linking of the drive mechanism, the Worcester-based electrical engineering firm Start Electrical, which was commissioned to undertake the project, decided to replace the defective motion control equipment with the latest digital DC drive technology. It specified four Sprint Electric PLX30 72A DC drives that provide accurate current and speed control, together with full four- quadrant operation. This level of precision was essential to locate and track a star or satellite. AC drives were rejected because they do not have such a wide speed and accuracy range, and would not have been able to control the movements with the necessary precision. The digital DC drives were paired in a master-slave configuration, and either one can be the master. With the master drive set in speed control and the slave in current control, the motors share the load, even though they are linked mechanically by a line shaft. Feedback was achieved using 28-bit encoders that provide more than 26 million pulses per revolution and allow accuracy to be controlled to 0.001 of a degree. This precision is vital for deep space applications. The DC drives’four-quadrant control ensures that this accuracy is maintained throughout the speed range and under varying load conditions. The four DC drives are controlled by a PLC via Profibus. A 7”touchscreen shows speed and current as well as providing easy access to fault diagnostics. The project was completed in two phases: the motor control unit (MCU), described above; followed by an antenna control unit (ACU), undertaken as a joint project with Leeds University. The flexibility of the MCU allowed the University team to maximise the adaptability of the antenna. The ACU allows satellites and sky objects to be located by grid reference and then tracked accurately, thus expanding the use of the GHY-3 antenna and bringing it back to life. The system is far outperforming the previous control equipment. n One of the world’s oldest radio astronomy dishes, at Goonhilly in Cornwall, has been upgraded to operate as a deep-space antenna. The project involved an extensive upgrade of the dish, including overhauling both its radio equipment and its motor drive system – using DC rather than AC drives. Goonhilly’s Guinevere dish has been given a new lease of life following an upgrade of its motion control technology