Design of flight control computer for small UAV

0 Introduction

    The application of small drones in modern military and civilian fields has become more and more extensive. After the early remote control flight, its navigation control mode has now developed into autonomous flight and intelligent flight. The change in navigation mode has put forward higher requirements on the accuracy of flight control computers; as the complexity of small drones' tasks increases, the requirements for the flight control computer's computing speed are also higher; and the requirements for miniaturization have also put forward high requirements on the power consumption and volume of flight control computers. High precision not only requires the computer to have high control accuracy, but also requires the ability to run complex control algorithms. Miniaturization requires the drone to be small in size and good in maneuverability, which in turn requires the control computer to be as small as possible.

1 Overview

    At present, with the rapid development of microelectronics technology, many integrated circuits with powerful functions and greatly reduced power consumption and volume have appeared on the market. With the advancement of chip manufacturing technology, CPU chips such as ARM and DSP that integrate many peripheral circuits, as well as large-scale logic devices such as EPLD and FPGA, have also appeared. Their performance is enhanced compared with previous chips, and the external circuits are also greatly simplified. Among them, the digital signal processor (DSP) has the advantages of complete functions, fast speed, and convenient development with its powerful instruction system, interface function and friendly development environment. It can effectively solve the contradiction between high speed and miniature in small flight control computers, making it possible to design a new flight control computer that integrates high speed, high precision and miniaturization for micro and small drones.

    Among many processor chips, the most suitable chip for the CPU of small flight control computers is TI's TMS320LF2407. Its computing speed and numerous peripheral interface circuits are very suitable for completing the real-time control function of small drones. It adopts Harvard structure and multi-stage pipeline operation, and reads data and instructions at the same time. The on-chip resources include 16 10-bit A/D converters with automatic sorting function, which ensures that up to 16 channels are converted during the same conversion period without increasing CPU overhead; 40 general input/output channels that can be programmed or reused individually; 5 external interrupts; integrated serial communication interface (SCI), which enables it to have the ability to communicate asynchronously (RS 485) with other controllers in the system; 16-bit synchronous serial peripheral interface (SPI) can be conveniently used to communicate with other peripheral devices; and also provides watchdog timer module (WDT) and CAN communication module. The rich on-chip resources of TMS320LF2407, combined with LATTICE's EPLD and serial port expansion chip 28C94, enable the flight controller to achieve more complex flight control and flight management functions, while also meeting the requirements of small size and low power consumption of small drones. The CPU provides 192 KWord of expandable external storage space, which can load larger applications and further enhance the control ability of external circuits. The system development language is BC 3.1, which can easily develop applications and make them portable. The integrated development tool CC2000 can easily and friendly debug the system.

2 System Hardware Design

2.1 Composition Modules

    The flight control system collects the flight status data measured by each sensor in real time, receives the control commands and data transmitted by the radio measurement and control terminal from the uplink channel of the ground measurement and control station, and outputs the control instructions to the actuator after calculation and processing, so as to realize the control of various flight modes in the UAV and the management and control of the mission equipment; at the same time, the status data of the UAV and the working status parameters of the engine, airborne power system, and mission equipment are transmitted to the airborne radio data terminal in real time, and sent back to the ground measurement and control station through the radio downlink channel. According to the functional division, the hardware of the flight control system includes: main control module, signal conditioning and interface module, data acquisition module, and servo drive module. The specific hardware composition principle is shown in Figure 1. [page]



2.2 Module Functions

    The various functional modules are combined to form the core of the flight control system, and the main control module is the core of the flight control system. It is combined with the signal conditioning module, interface module and servo drive module. It can meet the flight control and flight management functional requirements of a series of small UAVs by simply modifying the software and the peripheral circuit, thereby achieving one-time development and multi-model use, and reducing the purpose of system development cost. The system mainly completes the following functions:

    (1) Complete the high-precision acquisition of multiple analog signals, including gyro signals, heading signals, rudder deflection signals, engine speed, cylinder temperature signals, dynamic and static pressure sensor signals, power supply voltage signals, etc. Due to the limited accuracy and number of channels of the CPU's own A/D, another data acquisition circuit is used, and its chip selection and control signals are generated by the decoding circuit in the EPLD.

    (2) Output switch signals, analog signals and PWM pulse signals can adapt to the control requirements of different actuators (such as rudders, aileron servos, elevator servos, air ducts and throttle servos, etc.).

    (3) Use multiple communication channels to realize communication with the airborne data terminal, GPS signal, digital sensor and related mission equipment. Since the serial port configured by the CPU's own SCI channel cannot meet the system requirements, the design uses a multi-serial port expansion chip 28C94 to expand 8 serial ports.

3 System software design

    The system software design is divided into two parts, namely the program design of the logic circuit chip EPLD decoding circuit and the application program design of the flight control system.

3.1 Logic circuit program design

    EPLD is used to form a digital logic control circuit, complete decoding and isolation, and provide chip select signals and read/write control signals for A/D, D/A, and 28C94. The design of the software adopts a mixed design method of schematic input and VERILOG HDL language programming, following the process of design input → design implementation → design verification → device programming. The system uses two ispLSI1048 chips, which are used to realize the control of A/D, D/A and the control of the serial port expansion chip 28C94 respectively.
3.2 System application design

    Since C language can not only write application programs and system programs, but also directly control computer hardware like assembly language, the written programs are highly portable. Since the system designed with DSP as the core involves a large number of operations on peripheral ports and the subsequent program transplantation work is considered, the application program of the flight control system is designed using BC 3.1 to realize flight control and flight management functions respectively.

    The software is divided into four modules according to the function: time management module, data acquisition and processing module, communication module, and control law solution module. The time management module is used to control the UAV in real time within milliseconds; the data acquisition module collects the flight status, attitude parameters and flight parameters of the UAV, telemetry codes the flight status and flight parameters and transmits them to the airborne data terminal through the serial interface, and sends them to the ground control station through the wireless data channel for flight monitoring; the attitude parameters are sent to the control law solution module through the internal interface of the software for solution, and the results are sent to the airborne servo system through the D/A channel to control the operation of the servo to achieve the purpose of adjusting the flight attitude of the aircraft; the communication module completes the data exchange function between the flight control computer and other airborne peripherals.

4 Conclusion

    Taking advantage of the high-speed DSP control chip in control law calculation and data processing and its rich external resources, the small airborne flight control computer is designed with the large-scale programmable logic device CPLD and the serial interface expansion chip 28C94. The small UAV flight control system designed with it as the core has the characteristics of full functions, small size, light weight and low power consumption, which well meets the requirements of small UAV for high precision, miniaturization and low cost of flight control computer. The design has been successfully applied to a certain verification UAV system.