Citation
Abstract
Pointing control for deep-space lasercom is expected to be challenging because, for the apertures and wavelengths contemplated (of order 20 cm and 1 mm, respectively), the width of the beam transmitting data to Earth will be of order a few microradians. To address this challenge, JPL and others have been developing a vibration isolation system in which the lasercom telescope is nearly free-floating next to the spacecraft, being physically connected to the spacecraft only by a set of flexible wires and fibers referred to as an umbilical. The telescope’s position relative to the spacecraft is sensed by noncontact sensors and the telescope is controlled by noncontact (voice coil) actuators. The telescope pointing error, relative to Earth, is also sensed by a pointing detector in the telescope, which images an Earth-based laser beacon. The telescope moves in six degrees of freedom, of which two (pitch and yaw) are the pointing of the telescope, and are of principal importance. This article describes a controller for controlling the telescope, and a simplified method of analyzing the closed-loop behavior of the system. Several mechanisms for cross-coupling between the degrees of freedom are present, including off-diagonal elements in the umbilical spring constant matrix, and the telescope having significant products of inertia; as a result, the dynamics of the closed-loop system are described by a full 6 × 6 transfer matrix. Approximations that take into account only one or two cross-coupling mechanisms at a time, however, and which result in block-diagonal models for the system, provide excellent agreement with the full model. These approximations provide insight useful for designing the controller, and numerical models indicate that a controller designed using these approximations provides performance that meets pointing requirements.
Details
- Volume
- 42-200
- Published
- February 15, 2015
- Pages
- 1–16
- File Size
- 968.3 KB