Market demand
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During the "10th Five-Year Plan" period, China will become the world's largest passenger car production base, but the current luxury passenger cars are not equipped with an automatic antenna tracking system that can receive and receive satellite TV programs, and can only watch DVDs. At large international sports events, athletes hope to watch real-time TV broadcasts while on the road, so it is an urgent project to receive direct satellite TV programs on the moving cars.
Trains, ships, and airplanes are also a big market for use, but they use different working environments. The ship swaying side to side, the front and rear elevation angles are large, the trains have to cross the caves, and the aircraft needs to be balanced to establish a stable platform.
In order to meet these needs of the market, it is necessary to transplant high-tech in attitude measurement, make full use of missile guidance and satellite attitude measurement technology, and develop an automatic tracking system for receiving satellite TV antennas. Because the satellite TV live broadcast program is clearly received on the mobile, the antenna gain requirement is high, so the beam of the antenna is very narrow. In order to improve the accuracy, the satellite is tracked in real time, so that the central axis of the antenna is always aimed at the satellite. Attitude measurement techniques must be used.
The structure and working principle of the antenna automatic tracking system
The antenna automatic tracking system mainly consists of: gyro sensor, electronic compass, 16bit CPU MCU (because the gyro sensor is a digital 16-bit serial output, so the microcontroller also selects a 16-bit CPU, so that the data can be read in one time from the data bus) , servo system (including servo drive and servo motor), 1.2 m transverse bias feed satellite TV receiving antenna, satellite TV receiver, voltage / frequency (V / F) conversion module, GPS receiver module and GPS antenna, etc. Shown on the right.
1. The gyro sensor is mainly used to measure the heading angle of the car. Considering the performance price ratio, the all-solid-state fiber optic gyro sensor produced in Russia or the micro-electromechanical system (MEMS) inertial gyro sensor produced in the US is selected. Small size, light weight, low power consumption, fast start-up, etc.; random drift is small (<1°/h), dynamic range is ±60°/s, scale factor error is <2%; car output through gyro sensor By integrating the angular rate of the heading angle change, the heading angle change amount Δθ during the running of the vehicle can be obtained. You can also use the methods given in the manual and the manual to program the data collected and send it to the CPU in the microcontroller for processing.
2. The electronic compass is mainly used to measure the variation of the pitch angle and the roll angle of the satellite TV antenna. The electronic compass produced by Honeywell can be used. It is provided in the NMEA format and provides the pitch angle and roll through the RS232 serial port. The output of the angular and aerodynamic variations. It has fast response, up to 20Hz, response time of 0.1s, heading accuracy of ±0.5°, resolution of 0.1°, tilt angle (pitch, roll) of ±40° and accuracy of ±0.1°. Work long hours in harsh environments.
3. The servo system includes servo drive and servo motor. Considering the long-term maintenance-free work under the vehicle condition, the British CT company's miniature AC brushless servo motor and servo drive system are selected. The drive is controlled by the microprocessor to control all functions. There are ASIC chips that generate tunable pulse-width PWM outputs for inverters. The ASIC outputs are used to control IGBT inverters. These are application modules that are packaged together and can perform single-axis or multi-axis control at the same time. Fully digital can eliminate the drift of analog control and can withstand the impact of load at startup. If DC servo drive and DC servo motor are selected, although it can provide good synchronization between linear control, angular speed control and tension rate, DC brush motor brushes (even long-life brushes) must be replaced frequently for after-sales service. Maintenance, backup brings a lot of inconvenience.
4. GPS receiver module, which is used to measure the current latitude and longitude positioning coordinates of the car. Because the gyro sensor will have zero drift and random drift for a long time, this will cause cumulative error, which will affect the tracking accuracy of the antenna, so every second or two. After an hour, the current latitude and longitude positioning coordinates of the car and the automatic gain control (AGC) signal output by the satellite TV are used to correct the position of the primary antenna and correct its error to realign the satellite. However, when the car is driving in a tunnel, a high-rise building, a viaduct, or a jungle, the GPS positioning will be invalid due to occlusion, and a larger error will be introduced. At this time, Dead Reckoning is used for self-discipline navigation. With the current latitude and longitude positioning coordinates of the car given by GPS, and using the coordinates of the synchronous satellite orbit, the heading angle and the elevation angle that the satellite TV antenna should maintain can be derived by calculating the formula.
When the location is in place, the occlusion will invalidate the GPS positioning and introduce a larger error. At this time, Dead Reckoning is used for autonomous navigation. With the current latitude and longitude positioning coordinates of the car given by GPS, and using the coordinates of the synchronous satellite orbit, the heading angle and the elevation angle that the satellite TV antenna should maintain can be derived by calculating the formula.
Heading
Pitch angle
Where B is the current latitude of the car, L is the current longitude of the car, and δ is the longitude of the satellite.
The heading angle thus given ranges from 0 to 360°, and the pitch angle ranges from 0 to 90°. The detailed calculation method will not be described here.
5. The vehicle-mounted satellite TV antenna selects a single-biased parabolic antenna because the feed of the antenna and its supporting occlusion are the smallest, and the focal length-to-diameter ratio is f/D, so that the longitudinal dimension of the antenna becomes larger and the short diameter is reduced. It also reduces the antenna sidelobe level and improves the polarization level of the feed. If the lateral installation on the roof of the car is used to reduce the overall height, the folding design can reduce the drag and facilitate the car.
The initialization condition of the whole system is that the output of the electronic compass data is the highest level interrupt, and the maximum response frequency 20Hz serial output pitch angle and roll angle change amount are selected to keep the head angle change measured by the gyro sensor synchronized. After being processed by the CPU in the single-chip microcomputer, the servo system is driven at a frequency of 20 Hz, and the rotation direction of the antenna is controlled so that the central axis of the antenna is always aligned with the satellite to realize real-time tracking.
The AGC analog signal output by the satellite TV receiver is converted into a frequency by the V/F module and sent to the MCU. The MCU with a 50ms timer interrupt starts counting the frequency of the V/F output. When the signal is maximum, it indicates that the satellite is recaptured.
Conclusion
The vehicle antenna automatic tracking system designed by the attitude measurement technology can change the tracking working platform of the antenna, and can be applied to different carriers. It is not only suitable for receiving satellite TV, but also for satellite communication and multimedia services. The market prospect is very broad.
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