Target aircraft is an important branch of UAV family. It is mainly used to simulate threat targets such as combat aircraft and missiles, and provide realistic air targets for air defense weapon system test and training. With the progress of technology, modern anti-ship missiles mostly adopt the penetration mode of “ultra-low altitude sea skimming” flight. Accordingly, unmanned target aircraft used to simulate anti-ship missile targets also need to have the ability of ultra-low altitude sea skimming flight. In the 1960s, western developed countries represented by the United States launched the first generation of target aircraft with ultra-low altitude, sea skimming and high altitude flight capability, and then quickly formed a series. With the application of new technologies, some classic models are still widely used, such as stone chicken, fire bee, mirage and other series target aircraft, The relative altitude of its sea skimming flight is less than 5 m. China started late in this field. The flight control system is the “brain” of UAV. The ultra-low altitude sea skimming flight of UAV is mainly realized under the control of the altitude controller of the flight control system. Therefore, aiming at the development of small target flight control system, based on the basic principle of radio altimeter / accelerometer combined control, a simple and practical altitude controller is designed by using Kalman filter signal processing method and embedded system based on ARM7 microprocessor, which can accurately control the target flight altitude, So as to achieve the purpose of ultra-low altitude sea skimming and fixed altitude flight.

1. Flight control system configuration and altitude control principle

The target aircraft flight control system consists of three parts: sensor, flight control computer and actuator. The sensors mainly include vertical gyro, two axis angular rate gyro, three-axis strapdown magnetic heading sensor, radio altimeter, vertical accelerometer, pitot tube, GPS receiver, engine temperature and speed sensor, etc; The flight control computer is the core component of the flight control system. It is an embedded computer based on ARM7. It receives the measurement information from each sensor, solves it according to the control law, and gives the control signal to drive the actuator; The actuator mainly includes lift steering gear, aileron steering gear, rudder circuit controller, engine throttle motor and ECU controller. The composition of flight control system is shown in Figure 1.

Kalman filter processing and ARM7 microprocessor are used to realize the design of target aircraft flight altitude system

The height controller is mainly composed of internal and external feedback control loops. Firstly, the vertical gyro and angular rate gyro measurement signals are used as feedback control signals to form the internal loop of target attitude control (angular motion control system), including transverse and longitudinal control channels, which is the core control loop of the system. Because the attitude control system has attitude angle static error under the interference of constant interference torque (such as vertical wind), which will lead to altitude drift, it can not be directly applied to the stability and control of altitude trajectory. Therefore, in the altitude control system, it is necessary to directly measure the linear motion information such as aircraft altitude and vertical motion speed, and use sensors such as altimeter and linear accelerometer, Based on the attitude control system, an external height control loop is established, and the two together constitute a complete height control system (shaded part in Figure 1). When flying at ultra-low altitude, high-precision altitude sensors should be used, usually radio altimeters. In the actual design of the system, the combined control of radio altimeter and linear accelerometer is adopted, that is, after the specific algorithm comprehensive processing of the measurement information of radio altimeter and vertical linear accelerometer, two feedback control signals of target altitude and vertical velocity are obtained and sent to the longitudinal attitude control channel to form an external loop of altitude control, The altitude control function is completed together with the attitude control inner loop.

2. Height control law design

2.1 basic control law design

In order to reduce the design risk, the classical proportional differential (PD) control is used in the height control law.

The control quantity consists of four parts, including two control quantities of attitude control inner loop and height control outer loop. Attitude control inner loop: deviation from pitch angle △ θ The proportional controller acts as the main control to correct the pitch angle, which is proportional to the pitch angle rate θ The proportional differential controller is mainly used to improve the damping characteristics of the attitude inner loop.

Height control outer loop: the proportional controller proportional to the height difference △ h is used as the main control to correct the height, and the differential controller proportional to the vertical speed is mainly used to improve the damping characteristics of the height outer loop.

2.2 signal processing method and effect analysis

After the basic control law is determined, a main problem that must be solved is how to obtain high-quality feedback control signal. Pitch angle deviation △ θ And pitch angular rate θ, According to conventional practice, vertical gyro and angular rate gyro can provide signals that meet the accuracy requirements respectively. The main problem here is the acquisition and processing of height signal h and vertical velocity signal H. Due to the influence of wave fluctuation and other factors, the radio altimeter signal itself contains a lot of noise, and if the altimeter signal is directly used to obtain the vertical velocity signal, the system will be unstable and can not be used alone; If the vertical velocity signal and height signal are obtained by accelerometer signal integration, there is the problem of error accumulation, and it can not be used alone. However, if the two are used in combination, it can produce a good complementary effect. Therefore, a Kalman complementary filtering algorithm is adopted in this paper, which can not only filter the height signal noise, but also obtain high-quality vertical velocity signal. The design idea is to integrate the vertical acceleration signal to obtain the vertical velocity, and then integrate the vertical velocity to obtain the height. Compare this height with the actual measured height of the altimeter to obtain a deviation. The deviation is added to the vertical acceleration and vertical velocity as the error estimate at the current time, and the filtering operation is continued. The principle of complementary filtering of height signal and vertical acceleration signal is shown in Fig. 3.

Then the effect of the filtering algorithm is analyzed. Let the actual height of the towed target be HR, and record a = HR + Na, where Na represents the error of acceleration, then:

As can be seen from the above, the calculated velocity V consists of three parts:

Differential of true height: SHR (s)

In the vertical velocity and altitude signals obtained in this way, the signals of acceleration error and altitude error can be effectively filtered, especially the influence of sea clutter can be effectively removed.

3. Design and implementation of hardware and software of the controller

The altitude controller is a part of the target aircraft flight control system. It takes the flight control computer as the core and forms a closed-loop control system together with various sensors and execution structures. It is mainly composed of flight control computer, vertical gyro, angular rate gyro, radio altimeter, accelerometer, rudder loop controller, steering gear and other equipment.

(1) Flight control computer

The flight control computer includes main board, a / D conversion board, serial data communication interface board, switching value interface board, D / a conversion board, analog input preprocessing board, discrete input / output and photoelectric isolation / drive board, DC / DC power conversion board, etc. The embedded system design based on arm has the advantages of small volume, low power consumption, rich hardware resources, easy development, good real-time and reliability. The motherboard processor adopts 32-bit LPC2124 chip of arm7tdms-s architecture. Its 16 KB internal RAM and 128 KB internal flash memory can well meet the embedded real-time operating system RTOS( μ The operation requirements of C / OS – Ⅱ) ensure that the system can solve the height control law every 20 ms.

(2) Vertical gyro

Tc-3e vertical gyroscope is selected, which is mainly used to measure the aircraft pitch angle and attitude in the altitude control loop. The working range pitch angle is ± 30 °, and the analog voltage signal is output.

(3) Angular rate gyro

The 2-axis angular rate gyroscope is mainly used to measure the aircraft pitch attitude angular velocity in the altitude control loop. The pitch angular velocity output range is ± 60 ° / s, and the analog voltage signal is output.

(4) Radio altimeter

The frequency modulation / continuous wave (FM / CW) system is adopted, the working frequency is 4200 ~ 4400 MHz, the height measurement range is 0 ~ 300 m, the height output form is RS 232 serial port output, the baud rate is 38400 B / s and the update rate is 50 Hz.

(5) Accelerometer

The accelerometer is installed at the center of gravity of the target to measure the vertical acceleration of the towed target. Adxl105aqc accelerometer chip is selected, with accuracy of ± 1 ~ ± 5g, single channel analog output and resolution of 2 mg.

(6) Rudder loop controller

Design of special integrated circuit L292 for DC motor driver based on PWM. It includes position regulator, current regulator, PWM power amplifier, position feedback and current feedback sensor. Position feedback is the main feedback loop, and wdl-25 direct sliding precision conductive plastic potentiometer is adopted; The current feedback adopts standard resistance.

(7) Lifting steering gear

Gear reduction permanent magnet DC torque motor is adopted, with rated torque of 15 nm and rated speed (after deceleration) of 4 ± 1 R / min.

(8) Software design

Based on embedded real-time multitasking operating system μ The application program written under C / OS-II has the characteristics of portability, cutting, multitasking and preemptive task scheduling based on priority. It has good real-time performance and high reliability. The real-time operating system (RTOS) is used because it can decompose the application into multi tasks, simplify the design of application software, and ensure the real-time performance of the control system; Good multitasking design helps to improve the stability and reliability of the system.

4. Hardware in the loop simulation experiment and results

On the self built UAV hardware in the loop simulation platform, the developed small UAV height control system is simulated and verified. The aircraft model adopts six degrees of freedom nonlinear total motion equation, the basic simulation step size is 5 ms, and the simulation calculation accuracy is 64 B (double precision floating point). The simulation system includes the vertical gyro and angular rate gyro on the three-axis turntable, the airspeed sensor connected with the dynamic and static pressure simulator, and the radio altimeter signal and accelerometer signal are simulated by computer, and then connected to the altitude control computer and steering gear to form the simulation loop. The simulation results are shown in Fig. 4 and Fig. 5. The results under two main interference conditions are given in this paper.

The simulation results show that under the influence of altimeter noise and accelerometer measurement error caused by level 4 sea conditions, the system can control the target to enter the 14 m fixed altitude flight state smoothly and keep flying at the predetermined altitude according to the accuracy requirements. Under the interference of 5 m / s vertical wind, the target can quickly return to the predetermined 14 m reference height. It can be seen that the static and dynamic response index of the system can meet the design requirements.

5. Conclusion

Based on the combined control mode of radio altimeter / accelerometer, this paper adopts the mature proportional differential (PD) control principle, the signal processing method of Kalman complementary filter, and the hardware implementation and Implementation Based on arm μ Based on the software development of C / OS-II embedded real-time operating system, an ultra-low altitude sea skimming flight altitude controller for small target aircraft is designed. Its structure is simple and compact and its principle is feasible. The hardware in the loop simulation results show that the system has good static and dynamic response characteristics, and can fully realize the control of the target aircraft’s ultra-low altitude high-precision sea skimming and fixed altitude flight. The system design and software and hardware implementation are successful.

Responsible editor: GT

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