On-board directional antenna PTZ servo system design

Foreword

In the wireless image transmission communication system, in order to obtain a higher receiving gain, it is necessary to direct the receiving antenna to align the signal transmission source. This paper designs a vehicle-mounted directional antenna PTZ servo system based on the application platform consisting of the command vehicle and the controlled vehicle, using the magnetic compass and GPS orientation and positioning technology. In this system, the directional receiving antenna and the magnetic compass are fixed on the command vehicle head and the GPS receiver antenna is mounted on the command vehicle. By controlling the rotation of the PTZ by the follow-up system, the directional receiving antenna is aligned with the controlled vehicle in real-time to achieve the best image receiving effect.
System Structure Design

The system is mainly composed of a GPS receiver, a magnetic compass, a directional antenna head, and a measurement and control computer with a PIC18F458 microcontroller as the core. The system is divided into three parts: data acquisition, follow-up control, and mechanical transmission. The schematic structure of the system is shown in Figure 1. .

In Fig. 1, 1 is the GPS receiver on the command vehicle, 2 is the magnetic compass on the directional antenna PTZ, 3 is the directional antenna PTZ, 4 is the transmission mechanism, 5 is the DC torque motor, 6 is the power amplifier, 7 is Measurement and control computer (PIC18F458), 8 is the target vehicle's GPS (transmitted via data radio).

The data acquisition and processing part consists of a GPS receiver, a magnetic compass, and a measurement and control computer on the command vehicle. The command vehicle and the controlled vehicle obtain their own coordinate position through the GPS receiver in real time, and the GPS data on the command vehicle is sent to the monitoring computer on the command vehicle through the serial port. The GPS data of the vehicle under control is sent to the monitoring computer on the command vehicle through the data radio station, and then sent to the measurement and control computer through the serial port. The current direction angle of the directional antenna PTZ is obtained from the magnetic compass and transmitted to the measurement and control computer through the serial port. Through the coordinate position of the command vehicle and the controlled vehicle, the azimuth angle between the two is calculated, and compared with the current direction angle obtained by the magnetic compass, the angle data that the gimbal should turn is obtained.

The servo control part of the system uses the digital PID control algorithm downloaded to the measurement control computer. The PID controller is based on the system error (this system is the deviation between the azimuth angle between the command vehicle and the controlled vehicle and the current heading angle of the gimbal), and the control amount is used to control the controlled object by using proportional, integral, and differential calculations. . Through the actual system debugging, determine the parameters of the PID controller.

The mechanical transmission part is mainly composed of a power amplifier, a speed reducer, a direct current torque motor, a directional antenna head, and the like. This part receives the control signal and feedback angle information to complete the rotation of the PTZ.

The onboard directional antenna PTZ servo system is a typical closed loop negative feedback control system. The azimuth angles of the command vehicle and the target vehicle are calculated, and the magnetic compass is acquired to obtain the current direction angle as feedback information. The difference between the two is the control signal of the mechanical part. The control signal passes through the PID digital regulator, D/A converter, power amplifier, The torque motor and reduction gear rotate the PTZ to make the directional antenna point to the target vehicle. When the control signal is reduced to zero, the directional antenna PTZ stops rotating. In this way, the best communication status of the directional receiving antenna in real-time alignment with the moving target can be achieved. The directional antenna PTZ follow-up system control structure is shown in Figure 2.

System hardware design and algorithm implementation

System hardware design

The key part of this system's hardware is the PIC18F458 microcontroller as the core of the measurement and control computer. The PIC18F458 is an MCU-enhanced MCU designed by Microchip, which has a short instruction cycle, high processing power, and high computing power. It also has a rich peripheral module.
The measurement and control computer has a rich external interface. In this system, two serial ports and one D/A output are used. Measurement and control computer configuration block diagram shown in Figure 3.

The external two RS232 serial ports are used for communication between the magnetic compass, the command vehicle monitoring computer (IPC) and the measurement and control computer. The digital control information of the measurement and control computer is used by the D/A converter as the analog rotation signal of the directional antenna PTZ to achieve the follow-up function of the directional antenna PTZ system.

Azimuth calculation

In the follow-up function implementation, the direction of the antenna is determined by the command vehicle and the controlled vehicle, and the azimuth of the directional antenna is calculated based on the GPS data received by the two vehicles. Azimuth refers to the angle through which the axis of rotation of the antenna is taken as the axis, starting from the geographical north and turning clockwise to the direction in which the antenna points. When the directional antenna on the command vehicle is aimed at the target, the transmission of the image is best at this time.
Draw the GPS azimuth graphic according to the earth model, as shown in Figure 4.

In the figure, A is the command vehicle and B is the controlled vehicle. Their coordinates are (λ1, φ1), (λ2, φ2), θ1 is the difference between the longitude of the two cars, θ2 is the difference between the latitudes of the two cars, O is the center of the earth, and O1 is the latitude of the controlled vehicle. The center of the circle, R is the radius of the earth, and r is the radius of the plane of the latitude of the controlled vehicle. Among them, θ1=?λ1-λ2?, θ2=?φ1-φ2?.

Azimuth calculation:

Due to the different orientations of the command vehicle and the accused vehicle, the azimuth angle T is as follows:
(1) When the command vehicle is in the northeast direction (including north) of the target, T=π+∠BAC
(2) When the command vehicle is in the northwest direction (including west) of the target, T=π-∠BAC
(3) When the command vehicle is in the southwest direction (including south) of the target, T=∠BAC
(4) When the command vehicle is in the southeast direction (including east) of the target, T=2π-∠BAC
T∈[0°,360°]

PID control algorithm

In the follow-up control, digital PID technology is adopted, and the control law is realized by executing a control program that is solidified to the measurement control computer. In actual use, the system requires good dynamic performance, short control time, and small overshoot.
The PID controller is composed of a proportional controller, an integral controller, and a differential controller that are linearly combined to control the controlled object. The control expression is: u(k)=K_{P}?e(k)+Ki ∑e(k)+Kd?[(e(k)-e(k-1)]。 The main flow chart of this system is shown in Fig. 5.

Conclusion The onboard directional antenna PTZ servo system implements the follow-up function of the antenna, which is close to the ideal state in which the directional antenna is aligned with the target source in the communication system. After field testing in the field, the system operates quickly and has a small overshoot. The design requirements have achieved satisfactory results and have great reference value in practical applications.