Kinematics, design, programming and control of a robotic platform for satellite tracking and other applications.

摘要

In order to avoid the keyhole problems associated with present antenna mounting systems and to meet the requirements of acquisition and tracking for high gain and narrow beamwidth antennas, a novel antenna mount system is developed. The antenna is mounted on a microprocessor controlled robotic platform with six degrees of freedom. The mechanism is based on the principles of Stewart platform, which employes six variable length actuators constrained between a fixed base and a movable platform. This antenna mount is especially suitable for high gain antennas using high frequency band widths for marine satellite communications. The kinematics and geometry of the parallel link mechanism has been studied in detail. The kinematic analysis for the parallel manipulator consists of developing a set of kinematic equations for the six linear actuators in terms of the "world coordinates" (Ф,θ,α,x,y,z). These kinematic equations are then solved for the "machine coordinates'" (L1 ,L2,L3,L4,L5,L6), which are the six actuator lengths. A computer simulation has been done to model the motion of the platform. The simulation has simplified the task of examining various mechanism configurations, the range of motion and the mechanism mechanical constraints. A six motor controller board based on the motion controller microprocessors has been developed. Limited unipolar pulse width modulated servo drives are used to drive the permanent magnet DC motors of the linear actuators. A desktop computer is used as the host processor to generate the command data. The motion control microprocessors generate the velocity and acceleration profiles and drive the six axes simultaneously. The main control programme residing in the host processor schedules the overall operation. The orbital satellite bearings are calculated on a minicomputer and downloaded to the control desktop computer. A prototype antenna mount based on the descriptions above was designed, constructed and tested for tracking a high pass and low pass of an orbiting weather satellite, (the NOAA-7). A microwave antenna mounted on the platform was found to be capable of tracking an orbital satellite throughout the visible hemisphere, although the reception tracking has not been inplemented. This antenna mount offers a light weight, structurally strong and fast dynamic response tracking system for land and marine applications. This parallel robotic mechanism has potential for further applications such as a prepackaged portable communications system which can be deployed rapidly at remote construction sites or during civil defence emergencies. In addition it may be employed as a stabilized platform which can be used for marine VTOL aircraft recovery during rough weather. The application of this antenna mount in a low cost marine satellite communications system is emphasized since the fast dynamic response of the system will eliminate the need for an expensive stabilized platform.