3AC

3AC - Automated Antenna Attitude Control

This module, implemented through the use of a microcontroler, provides a dedicated control system for the GEM-P antenna (Vertex Cassegrain system with ≈5 Tons). Its main tasks are the accurate control of the azimuth table gearbox and receiver ambient parameters readout. Althought focused on GEM-P antenna, it is generic enough for adaptation to other contexts. The system properties:

• Accurate positioning in azimuth and elevation within 1 minute of arc in survey mode.

• Antenna rotation speed control - constant speed (1 rpm for ex.) in scanning mode

• Automatic procedures for antenna calibration

• Friendly User interface for remote configuration and management of the system

• Monitoring of system parameters (LNA’s temperature and control of the noise injector for the receiver calibration)

• Respect security and temporal requirements

The solution:

The hardware is divided in three parts:

• The Microcontroller Module with an Operating System with implemented control routines. This module has CAN, I2C and serial interfaces, Real Time Clock and can operate from 8 to 35 Volt. HW used : 16-bit Microchip PIC18458 m-controller with 32 Kbyte Flash, 256 bytes EEPROM. 27 I/O ports (4 for encoders, 8 for motors and index, 8 analog inputs with 10 bit A/D conversion, 7 digital i/O ports for other uses).

• Encoders Interface to adapt the signals that can be transmitted at large distances from m-controller module to encoders (SSI interface) and vice-versa, filtering environmental noise and protect the system from eventual electrical discharges. HW used : ACURO 17-bits encoders.

• Motors & index interface, to action azimuth and elevation motors, and the references for antenna calibration and monitoring purposes. Azimuth table HW used : steel alloy gearbox custom made by A.BRITO. The rotating bearing was provided by INA (wind mill adapted). The motor is a 3-phase, 1500 rpm, with a relation of 1/100 and a further gear/pinon transmission of 1/10. The VFDs (from SEW) allow full control at nominal, constant 0.3 rpm. For elevation, the original elevation motor and gear are used.

• Operating System: Hw configuration, loop control, user interfaces with text commands and remote access enabled by SSH. Modular portability to other CPUs. Programmed in C, for PIC18xxx architectures, allowing easy addition of new commands and peripherals.

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Conclusions:

3AC is reliable and worked without fails by long periods of time.

The use of commercial VFD’s guarantees easy (and cheap) adaptation to other technologies and industrial control products. The encoders (17 bit) permit accurate and reliable measures.

A simple and reliable control system that can be adapted for other scientific and industrial purposes, high precision large load cranes in (civil metallic structures), telecommunication antennas, CAM systems, etc.

Exists interests by other national teams using antennas as radio telescopes, to upgrade its actual control systems to 3AC.

Acknowledgments :

The implementation was developed by David Macário trough collaboration and funding by TELESAL program and associated company - BRAXEN .