Feed Positioning System
Description of Feed Positioning System:
The feed drive circuit consists of a half HP DC servo motor with brakes.This is driven by power MOSFETS.An 'H' configuration is used to enable bidirectional rotation.Hence at a time two diagonally opposite Mosfets are switched ON.The speed of the motor is controlled using Pulse width Modulation technique. The PWM is genrated by 8051 based microcontroller.The appropriate software produces the trapezoidal speed profile for achieving high accuracy of 1.054 arc min.
The microcontroller also facilitates communication with control room using RS485 protocol.
The feed rotation is restricted to 275 deg. as feed cable wrap hose carries the front end,RF Control and Monitor cables. End limit switches are provided at -15 deg. and 285 deg. along with stoppers for ensuring safety from over travel of feed turret.
A CAL switch is provided at 275 deg.for calibration of the incremental encoder.There is a further facility to check the encoder value using the internal marker UA0.
A current limit circuitry is provided to limit armature current to 12 amp. which is the maximum rating of the drive motor.
The NVRAM IC DS1220Y is provided to retain current position in case of power failure and thereby avoiding frequent recalibration.
Manual rotation is facilitated from antenna base .LCD is provided in the electronics unit at the base (to be implemented) for ease and accurate rotation of feed from the antenna base.
The block diagram for FPS system is as shown in fig: