Distributed motion control system delivers precision positioning for Brazil's ground-breaking radio telescope array

2.5 arc-sec resolution motion system will align wide-area array of telescopes to track Sun

Baldor motion components will provide the ultra precise movement components required to control the positioning of Brazil's new radio telescope system, the Brazilian Decimetric Array (BDA).

Developed by Professor Hanumant S Sawant of Brazil's INPE space research institute (Instituto Nacional de Pesquisas Espaciais), the BDA radio telescope operates at frequencies of 1.2-1.7, 2.7 and 5 GHz. Brazilian and international institutes are also collaborating in the project, including the NCRA-TIFR National Centre of Radio Astrophysics (Pune, India), the Indian Institute of Astrophysics (Bangalore, India), and the Department of Radio Astrophysics of the University of Berkeley (CA, USA). BDA is funded by the São Paulo State agency, FAPESP.

Designed primarily to monitor the sun and predict space weather, the BDA - when fully installed - will acquire data from 38, 4-metre diameter, mesh antennas laid out in a T-shaped array along base lines of 2.5 x 1.25 km. All the antennas in the array will track the Sun synchronously and continuously, and the position control system must offer extreme positioning resolution and tracking accuracy to achieve this, because of the large physical spacings involved, and the micro movements needed to rotate the antennas.

Two servo motors are used to control each antenna's tracking movements, to drive the azimuth and altitude axes. The designers of the positioning system on this project, Inteltek Automation (Pune, India), chose a Baldor NextMove-BX motion controller for the task, together with a combination of Baldor drives and brushless AC servo motors. Operating at just 0.6 RPM, the motors drive the loads through a combination of reduction and spur gearing to provide a positioning resolution of just 2.5 arc-seconds, at wind speeds of up to 60 km/hour.

NextMove-BX is well suited to this control task because of its combination of facilities, including dual axis capability, ruggedness for easy mounting underneath the antenna dish, and its built-in Mint motion language.

Each of the 38 NextMove-BX units link to a host PC using a multi-drop RS-485 network. Running custom position control software, the PC transmits new position commands to the remote motion controllers at intervals of typically 60 seconds. The motion controllers' Mint application programs then autonomously provide interpolated movement commands to drive the azimuth and altitude servo motors, which ultimately move the axes at around one thousandth of a revolution per minute following gear reduction ratios of 800:1.

Prototype altitude-azimuth mounted dishes for this project have already been tested in India. The measured accuracy of the positioning and tracking was less than three minutes of arc. During this development period, the style of gear and gear ratios were refined, to minimise backlash in the mechanical system. These modifications will improve the pointing and tracking accuracy even further for current, initial, BDA deployment phase involving an interferometer array of five antennas. Inteltek Automation is additionally currently implementing error correction measures which it is hoped will further improve the accuracy, for BDA's subsequent deployment phase from late 2002 onwards.

The final location for the BDA is in a remote area some 200 km inland from São Paulo with longitude and latitude of 45 degrees west, 22 degrees 41 minutes south. This location has been selected for its minimal levels of electro-magnetic noise. Although monitoring solar activity is a primary goal of the BDA project, the flexibility of the tracking control software will also allow investigations of galactic and extra-galactic objects.