PX4 系统架构概述

PX4 consists of two main layers: the flight stack is an estimation and flight control system, and the middleware is a general robotics layer that can support any type of autonomous robot, providing internal/external communications and hardware integration.

All PX4 airframes share a single codebase (this includes other robotic systems like boats, rovers, submarines etc.). The complete system design is reactive, which means that:

• 所有的功能都可以被分割成若干可替换、可重复使用的部件。
• 通过异步消息传递进行通信。
• 系统可以应对不同的工作负载。

顶层软件架构

下面的架构图对 PX4 的各个积木模块以及各模块之间的联系进行了一个详细的概述。 图的上半部分包括了中间件模块，而下半部分展示的则是飞行控制栈的组件。

The source code is split into self-contained modules/programs (shown in monospace in the diagram). 通常来说一个图中的积木块对应一个功能模块。

TIP

At runtime, you can inspect which modules are executed with the top command in shell, and each module can be started/stopped individually via <module_name> start/stop. While top command is specific to NuttX shell, the other commands can be used in the SITL shell (pxh>) as well. For more information about each of these modules see the Modules & Commands Reference.

The arrows show the information flow for the most important connections between the modules. 使用 发布-订阅 消息总线这个方案意味着：

Modules communicate with each other through a publish-subscribe message bus named uORB. 它包括了为固定翼、旋翼和 VTOL 无人机设计的控制器，以及相应的姿态、位置估计器。

• 系统是 “响应式” 的 — 系统异步运行，新数据抵达时系统立即进行更新。
• 系统所有的活动和通信都是完全并行的。
• 系统组件在任何地方都可以在保证线程安全的情况下使用数据。

INFO

This architecture allows every single one of these blocks to be rapidly and easily replaced, even at runtime.

飞行控制栈

The flight stack is a collection of guidance, navigation and control algorithms for autonomous drones. It includes controllers for fixed-wing, multirotor and VTOL airframes as well as estimators for attitude and position.

The following diagram shows an overview of the building blocks of the flight stack. It contains the full pipeline from sensors, RC input and autonomous flight control (Navigator), down to the motor or servo control (Actuators).

An estimator takes one or more sensor inputs, combines them, and computes a vehicle state (for example the attitude from IMU sensor data).

A controller is a component that takes a setpoint and a measurement or estimated state (process variable) as input. Its goal is to adjust the value of the process variable such that it matches the setpoint. The output is a correction to eventually reach that setpoint. For example the position controller takes position setpoints as inputs, the process variable is the currently estimated position, and the output is an attitude and thrust setpoint that move the vehicle towards the desired position.

A mixer takes force commands (such as "turn right") and translates them into individual motor commands, while ensuring that some limits are not exceeded. This translation is specific for a vehicle type and depends on various factors, such as the motor arrangements with respect to the center of gravity, or the vehicle's rotational inertia.

中间件

The middleware consists primarily of device drivers for embedded sensors, communication with the external world (companion computer, GCS, etc.) and the uORB publish-subscribe message bus.

In addition, the middleware includes a simulation layer that allows PX4 flight code to run on a desktop operating system and control a computer modeled vehicle in a simulated "world".

更新速率

Since the modules wait for message updates, typically the drivers define how fast a module updates. Most of the IMU drivers sample the data at 1kHz, integrate it and publish with 250Hz. Other parts of the system, such as the navigator, don't need such a high update rate, and thus run considerably slower.

The message update rates can be inspected in real-time on the system by running uorb top.

运行时的环境

PX4 runs on various operating systems that provide a POSIX-API (such as Linux, macOS, NuttX or QuRT). It should also have some form of real-time scheduling (e.g. FIFO).

The inter-module communication (using uORB) is based on shared memory. The whole PX4 middleware runs in a single address space, i.e. memory is shared between all modules.

INFO

The system is designed such that with minimal effort it would be possible to run each module in separate address space (parts that would need to be changed include uORB, parameter interface, dataman and perf).

There are 2 different ways that a module can be executed:

• Tasks: The module runs in its own task with its own stack and process priority.

• Work queue tasks: The module runs on a shared work queue, sharing the same stack and work queue thread priority as other modules on the queue.

  • All the tasks must behave co-operatively as they cannot interrupt each other.
  • Multiple work queue tasks can run on a queue, and there can be multiple queues.
  • A work queue task is scheduled by specifying a fixed time in the future, or via uORB topic update callback.

  The advantage of running modules on a work queue is that it uses less RAM, and potentially results in fewer task switches. The disadvantages are that work queue tasks are not allowed to sleep or poll on a message, or do blocking IO (such as reading from a file). Long-running tasks (doing heavy computation) should potentially also run in a separate task or at least a separate work queue.

INFO

Tasks running on a work queue do not show up in top (only the work queues themselves can be seen - e.g. as wq:lp_default). Use work_queue status to display all active work queue items.

后台任务

px4_task_spawn_cmd() is used to launch new tasks (NuttX) or threads (POSIX - Linux/macOS) that run independently from the calling (parent) task:

independent_task = px4_task_spawn_cmd(
    "commander",                    // Process name
    SCHED_DEFAULT,                  // Scheduling type (RR or FIFO)
    SCHED_PRIORITY_DEFAULT + 40,    // Scheduling priority
    3600,                           // Stack size of the new task or thread
    commander_thread_main,          // Task (or thread) main function
    (char * const *)&argv[0]        // Void pointer to pass to the new task
                                    // (here the commandline arguments).
    );

操作系统相关的信息

NuttX

NuttX is the primary RTOS for running PX4 on a flight-control board. It is open source (BSD license), light-weight, efficient and very stable.

Modules are executed as tasks: they have their own file descriptor lists, but they share a single address space. A task can still start one or more threads that share the file descriptor list.

Each task/thread has a fixed-size stack, and there is a periodic task which checks that all stacks have enough free space left (based on stack coloring).

Linux/MacOS

On Linux or macOS, PX4 runs in a single process, and the modules run in their own threads (there is no distinction between tasks and threads as on NuttX).