|
The employment of a large telescope with a high lateral resolution imposes a high pointing precision of a generic sky target measurement. The paper describes the design of two motor servo—loops for a azimuth stabilization (single axis) of a gondola. This is the first and the most important step in pointing because it is the only one action that moves the whole flight structure and it has the task of controlling the non linear friction of the suspension bearing in order to isolate the instrumentation from the cruise balloon as much as possible. The servo—system is theoretically capable of 1 arc mm of absolute accuracy through two subsequent steps: a first one (the lower accuracy) is performed with magnetometer sensor and a second one (the highest accuracy) is led by a rate integrating gyro sensor or a high definition CCD camera. The system proposed makes use of two DC torque motors. One for any motion of the platform around the vertical axis and one strategically placed at the interconnecting point balloon—payload (Pivot) for attenuating the friction bearing. Some mechanical non—linearities, localized exactly at this point, impose a time domain design for any settling time control whenever the gondola experiences a new step in azimuth coordinate. The work points out as it is possible to control this settling time of the gondola, by means of simplified equations of motion in the time domain, tested in open loop conditions. A mechanical approach of an active Pivot to avoid the disturbance of balloon rotation on the current tracking of the sky azimuthal target is reported. This is the most important goal in order to achieve the highest precision of the action pointing without slowly cancelling the control of the motor devoted to that task. Some results of a true simulation program tailored on the mechanical approach proposed in parametric form are shown. Therefore for any combination of weight and inertia moment this program, tested on a flight prototype, can be an effective simple tool for designing pointing systems where a forecast of a final accuracy either absolute or as root—mean— square (RMS) is requested. The program can also tell us the amount of power supply consumption according either to peak or average current for some possible flight operations for remote sensing. The work suggests also some possible hardware and software solutions that interface a dedicated computer card and analog I/O signals. Finally some results of the last Italy—Spain flight (1988), in order to test the first step of the pointing precision, according to accuracy of an 8 bit A/D converter, is also shown.
|
