Login

1. Accuracy of feed positioning system = 2 arc min. from the dish vertex
2. Resolution = 1 arc min.
3. Drive specifications :
   1. Half Hp DC servo motor
      1. Speed : 2000 RPM
      2. Torque : 2.7 N-m
      3. Operating Voltage : 55 Volts
      4. Weight : 11kg
      5. Inertia : 2.2 kg sqm X 10 ˆ (-3)
      6. Thermal time constant: 23 min.
      7. Electrical time constant : 2 milliseconds
      8. Mechanical Time constant : 14.7 millisecond.
      9. Torque constant : 0.24 N-m/A
      10. Inductance : 1.1 mH
      11. Armature Resistance : 0.62 ohm
      12. Peak current : 100 amp.
      13. Voltage constant : 25V per 1000 rpm.
   2. Tachometer :
      1. 4V/1000 rpm
      2. Ripple : 1.5% at 400 rpm
      3. Inertia : 8.4 X 10 ˆ (-5) kg-sqm
      4. Weight : 0.32 Kg
   3. Brake :
      1. Holding torque : 13 N-m
      2. Rated voltage to be applied for releasing brakes :24 VDC
      3. Coil current : 1 Amp
4. Feed Drive Controller
   1. Encoder : Incremental encoder with one arc min. resolution
   2. Speed profile : Trapezoidal
   3. Speed control : PWM technique at 10 KHz.
   4. Power drive : 'H' Configuration using IRF150 MOSFETS.
   5. PWM Generation : 8051FA microcontroller with non-volatile memory to retain current position and enabling Liquid crystal display. Microcontroller communicates with Antenna Base Computer using RS-485 protocol with an asynchronous link.

As told earlier,telescope operates at frequecies 150 MHz,233 MHz,327 MHz, 610 MHz,a420 MHz. The four feeds are mounted on four faces of the rotating turret at 90 deg apart.

• 0 deg ==> 233/610 MHz
• 90 deg ==> 150 MHz
• 180 deg ==> 1420 MHz
• 270 deg ==> 325 MHz

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 generated 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 the 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 :

The feed drive motor is coupled to the feed turret through two stages of gear reduction,first is a worm gear with 300:1 reduction and the second one is primary bull gear with 5:1 reduction, Thus total gear reduction is 1500:1. The incremental encoder is used for marking the position. As shown in the figure the encoder is connected to the output shaft of the worm gear reduction .This means that for one complete rotation of the turret,the encoder rotates for 5 times. Encoder gives the direction and count signals which act as a position feedback to 8051FA microcontroller.The CPU card is based on 8051FA microcontroller. 8051 has inbuilt serial port through which it communicates to the antenna base computer through RS485 link.Microcontroller 8051FA gives the command signals like direction, RGSO and PWM to the drive card which drives the MOSFET bridge to rotate the motor in the proper direction and set speed and thus feed is focussed with an accuracy of one arc minute pointing.

Due to the above bull gear and gear box arrangement of the feed ,when feed completes one rotation (rotation through 360 degrees) ,encoder goes through five rotations. The encoder generates 4096 pulses during one complete rotation.thus a complete rotation by feed system will generate 20480 encoder pulses .One pulse from the encoder thus corresponds to 0.0175-degree rotation of the feed(i.e approximately one arc minute). These encoder pulses are counted by the microcontroller to determine the angular displacement of the feed system .

There are various commands to be issued from user to control the feed rotation in the desired manner. Run commands are used to rotate the feed while set commands are used to set the FPS parameters for run commands and read commands are used to read the set FPS parameters. RUN COMMANDS are used to rotate the feed in desired direction,by desired count and in desired manner by setting various FPS parameters. Following are RUN commands used to rotate the feed:

Run to Calibrate:
This is always the first command to be issued .It forms the part of the calibration process. When issued it causes the feed to move towards the 270 deg limit switch .When the feed hits the 270 deg limit switch ,the rotation is stopped and the current encoder count is initialized to 17000.

Preset Run Command:
Using the preset run command the feed can be rotated to the target encoder count .This can used only after calibrating the feed i.e when feed is calibrated and idle.This command will get rejected if The target encoder count entered is less than the minimum angle or more than the maximum angle. The trapezoidal profile followed by preset run command is as in fig.2

The speed begins to ramp up from the starting point until it reaches to the maximum PWM where it remains constant till the turning point difference is reached.When the turning point difference is reached, the speed begins to ramp down till lower RPM reaches.Once the lower RPM is reached , speed remains constant till it reaches the brake count difference,Then the brakes are applied at the target position.

Run To UA0:
As the motor shaft rotates the encoder generates two incremental square wave signals UA1 and UA2.Once every rotation ,it also generates a square wave pulse called UA0.A UA0 pulse occurs after every one rotation.This pulse can be used as the most accurate reference to calibrate the feed position. This command is used to search the UA0 pulse in both direction.This command forms the part of the calibration process.the feed rotates in the specified direction till the UA0 pulse occurs.When UA0 is found the PWM is made zero percent and brakes are applied.

Fine Tune
This command is used to rotate the feed through small amount of counts generally less than 200 counts i.e this command gets executed only when the difference between the current and target encoder count is less than 200 counts.

Set commands as follows:
Null Command: This command is a general purpose command used to communicate with the FPS hardware.
Set Turning Point Turning point: difference is the difference between the current position and target position at which the rpm of the feed starts ramping down.
Set Ramp Down Time Count: Once the turning point is reached, motor rpm has to be reduced with a uniform rate. This rate can be specified by issuing this command .The motor rpm is controlled by pwm register of 8051.The Ramp Down Time Count is nothing but the time difference in milliseconds after which the count in pwm register is incremented by one to decrement the duty cycle and hence the rpm.
Set Lower RPM Limit: This command is used to set the lower limit of the motor rpm when it is ramping down. Once the pulse count lower rpm limit reached , further decrement in rpm is stopped.
Set Brake Count Differance: Brake count difference is the difference between current angle and the target angle at which brakes should be applied.
Set Ramp Up Time count: As feed starts rotating ,it is not wise to increase the rpm to its maximum allowed value .This transition in the count can be made smooth by decrementing count in the pwm register of 8051 microcontroller at regular interval.Set ramp up time count command is used to specify this time.
Set Stop Time Count: This is the rate at which the pwm register count is incremented by one when a stop command is issued by user.
Set Max PWM Count: This command controls the maximum rpm of motor that can be achieved in any run command. Argument to this command is a byte that is nothing but the minimum value in hex in the PWM register.
Set Max Angle: As the feed starts moving towards 270 deg. limit switch angle count goes up.Upper limit on this angle is set up by this command so that software does not allow the feed to cross this limit.
Set Minimum Angle: Similarly ,as feed moves toward -15 deg. limit switch angle count goes down.This command is used to set the lower limit on the minimum angle position that feed can attain.

READ COMMANDS are used to read all FPS parameters that can be set using set commands can by usibg the respective READ commands.

The communication between the Antenna Base Computer and the FPS is asynchronous in the sense that in each burst,a number of packets are sent in given direction.Each frame is 11 bit which consists of 1 start bit,1 stop bit, and 9 data bits.Baud rate is 9600bits/sec ABC uses its serial communication port for transmission and reception of data to various MCMS . LTC485 differential signals are used for communication purpose. The communication protocol for ABC-MCM communication is nothing but the fixed packet structures. The ABC sends the predefined input packet for each command. MCM microcontroller accepts the data send by ABC, decodes it and executes the corresponding command. The MCM responds to ABC by transmitting the output packet structure which gives the command status,FPS status ,motor speed, current encoder count.

The packet send by ABC to MCM has the following structure :

---

The packet send by MCM to ABC consists of three parts:

1. Packet Header

2. First Logical Packet

3. Second Logical Packet

---