Satellite tracking antenna system

• US 4,100,472 A
• Filed: 08/19/1976
• Issued: 07/11/1978
• Est. Priority Date: 07/23/1975
• Status: Expired due to Term

• Current Assignee: Scientific-Atlanta Incorporated (Cisco Systems, Inc.)
• Original Assignee: Scientific-Atlanta Incorporated (Cisco Systems, Inc.)
• Inventors: Mobley, Joseph Graham
• Primary Examiner(s): Dobeck, B.
• Application Number:
  US05/715,731
• Time in Patent Office: 691 Days
• Field of Search: 318/685, 318/696, 318/318
• US Class Current: 318/685
• CPC Class Codes: G01S 3/44 the adjustment being varied periodically or continuously until it is halted automatically when the desired condition is attained
• CPC Class Codes: H01Q 3/08 for varying two co-ordinates of the orientation

Abstract

A satellite tracking antenna system for use on board ship as part of a maritime communications satellite terminal is disclosed. The system has a directional antenna which is continually trained in the direction of the satellite so as to receive signals therefrom and transmit thereto in spite of the continual rolling, pitching and turning movement of the ship as it travels along its course, even in heavy seas. The system includes a platform which is adapted to be mounted at the head of the mast of the ship. A servo control unit receives position and angular rate signals from roll and pitch sensors mounted on the platform, and serves to counteract pitch and roll without the need for gyroscopic stabilization means so that the platform always remains level regardless of the pitching and rolling of the ship. A structure including the antenna, is mounted on the platform for rotation in azimuth and elevation. A heading input unit receives synchro information from the ship'"'"'s gyrocompass by way of a gyrocompass converter which translates this information into digital form. The heading input unit also receives azimuth information regarding the position of the satellite as well as elevation data, also regarding the position of the satellite. Azimuth and elevation control servos operated by control signals from the heading input unit train the antenna in azimuth and elevation so that it points in the direction of the satellite. Yawing or turning is counteracted by the heading input unit which modifies the azimuth control signal in accordance with the data as to the ship'"'"'s heading from the gyrocompass converter. The direction of the pointing of the antenna, to accommodate slow changes in latitude and longitude as the ship moves along its course which causes a change in satellite position relative to the ship, is accommodated by either changing the elevation and azimuth data provided to the heading input unit or by means of an automatic satellite position update unit which changes the heading input data to the azimuth and elevation servos in accordance with changes in the strength of the signal received by the antenna from the satellite. A system for converting angle data from synchro or step repeaters into digital data which may be used in the gyrocompass converter is also described. A system for controlling the speed and direction of a stepper motor in response to control voltages which does not require tachometers or other feedback control is described in connection with the servo control system in the pitch and roll servo units and in the azimuth and elevation servo units.

Claims

1. Apparatus for controlling in response to a control voltage the speed and direction of rotation of a two phase D.C. stepper motor having first and second field windings, said apparatus comprising

means for generating first and second sinusoidal drive voltages which are in quadrature phase relationship, said first voltage being applied to said first winding and said second voltage being applied to said second winding, and

means responsive to said control voltage for varying the frequency of both said first and second drive voltages and reversing the polarity of said first drive voltage, respectively, for varying the speed and direction of rotation of said motor.

2. The invention as set forth in claim 1 wherein said control voltage responsive means includes means for varying the frequency of said control voltages continuously between about a range from zero frequency to a certain frequency above zero frequency as the amplitude of said control voltage varies between a certain voltage and a voltage greater than said certain voltage and for reversing the polarity of said first drive voltage when said control voltage reverses polarity with respect to said certain voltage.

3. The invention as set forth in claim 2 wherein said certain voltage is zero volts.

4. The invention as set forth in claim 1 wherein said generating means comprises a source of reference signals of constant frequency, phase shift means for shifting the phase of said reference signals 90°

, said first drive voltage being provided by said reference signals and said second drive voltage being provided by the output of said phase shift means.

5. The invention as set forth in claim 4 wherein said control voltage responsive means comprises a variable frequency oscillator controlled by said control voltage, and means responsive to the output of said voltage controlled variable frequency oscillator for shifting the frequency and polarity of said first drive voltage and the frequency of said second drive voltage.

6. The invention as set forth in claim 5 wherein said source of reference signals is a reference oscillator, said voltage controlled oscillator is a variable frequency oscillator having a center frequency equal to the frequency of said reference signals, and said frequency shifting means is (i) first modulator means responsive to said drive voltage reference signal from said reference oscillator and to the output of said variable frequency oscillator for providing as said first drive voltage a signal from said first modulator having a frequency equal to the difference between said reference oscillator frequency and said voltage controlled oscillator output frequency, and (ii) second modulator means responsive to the output of said phase shift means and to the output of said variable frequency oscillator for providing as said second drive voltage a signal from said second modulator means having a frequency equal to the difference between the frequency of said phase shifted reference signal and said voltage controlled oscillator output frequency.

7. The invention as set forth in claim 5 wherein said frequency shifting means comprises means responsive to said reference signals and to the output of said voltage controlled oscillator for increasing and decreasing the frequency of said reference signals when the control voltage is above or below a certain level, respectively, a first modulator responsive to said reference signals and to said frequency shifted reference signals from said frequency shifting means for generating as said first drive voltage a signal having a frequency corresponding to the difference between the frequency of said reference signals and said frequency shifted reference signals, and a second modulator responsive to said phase shifted reference signals and said frequency shifted reference signals for generating, as said second drive voltage, a signal having a frequency corresponding to the difference between the frequency of said phase shifted reference signals and said frequency shifted reference signals.

8. The invention as set forth in claim 7 wherein said voltage controlled oscillator comprises a capacitor, means responsive to said control voltage for charging said capacitor to voltages of opposite polarity depending upon the polarity of said control voltage and at a rate depending upon the magnitude of said control voltage so as to generate across said capacitor a ramp voltage wave, first detector means responsive to said ramp voltage wave when it changes in polarity in a positive sense for providing first output pulse when said ramp reaches a certain voltage magnitude, said first detector means providing one output of said voltage controlled oscillator, second detector means responsive to said ramp voltage when it changes in polarity in a negative sense for providing a second output pulse when said ramp reaches said certain magnitude, means responsive to said first and second output pulses for discharging said capacitor and terminating said ramp, said first detector means providing said voltage controlled oscillator output when said control voltage is of one polarity and said second detector means providing said voltage controlled oscillator output when said control voltage is of a polarity opposite to said one polarity.

9. The invention as set forth in claim 8 wherein said ramp generating means comprises an operational amplifier having said control voltage input connected thereto and having said capacitor connected in feedback relationship therewith,

said first detector means comprising, an operational amplifier, a source providing said certain voltage, said voltage source and the output of said ramp generating means operational amplifier being connected to different inputs of said first detector means operational amplifier whereby said first detector means operational amplifier provides said first output pulse when said ramp voltage is positive and exceeds said certain voltage,

said second detector means comprising an inverter connected to the output of said ramp generating means operational amplifier, another operational amplifier, said inverter and said certain voltage source being connected to different inputs of said second detector means operational amplifier whereby said second detector means operational amplifier provides said second pulses when said ramp voltage is negative and exceeds said certain voltage, and

said capacitor discharge means comprising a switching transistor connected across said capacitor, said transistor having a control electrode connected to said first detector means operational amplifier output and to said second detector means operational amplifier output.

10. The invention as set forth in claim 7 wherein said reference signal oscillator provides a train of pulses at a repetition rate much higher than the frequency of said drive voltages, said voltage controlled oscillator including means for providing first pulses at a rate corresponding to the magnitude of said control voltage when said control voltage is greater than a certain level and second pulses at a rate corresponding to the magnitude of said control voltage when said control voltage is below said certain level, combinational logic means responsive to said first pulses and to said second pulses for respectively adding and subtracting pulses from said reference signal oscillator train of pulses whereby to increase and decrease the repetition rate of said reference signal oscillator pulses, first and second counters respectively connected to the output of said reference signal oscillator and said combinational logic means for dividing the repetition rate thereof to the frequency to said drive voltage, filter means connected to said first counter for providing a reference sinusoidal wave being connected to said phase shift means, said reference sinusoidal wave and the sinusoidal wave from said phase shift means being connected to different ones of said modulators, said second counter output being connected to both of said modulators.