Sensors模块在PX4中扮演关键角色，整合来自不同传感器的低级数据，进行投票、故障处理、校准，并发布到系统。它监控传感器驱动的输出，更新校准参数，执行一致性检查，并发布状态信息。模块支持start、stop和status命令，使用WorkQueue进行轮询。初始化时注册传感器回调，不断检查新传感器并发布sensor_combined、airspeed和differential_pressure等消息。
PX4模块设计之三十三：Sensors模块
1. Sensors模块简介
Sensors模块汇总了所有的飞控传感器，最终将处理后的传感数据以sensor_combined/airspeed/differential_pressure消息发布到系统中。
### Description
The sensors module is central to the whole system. It takes low-level output from drivers, turns
it into a more usable form, and publishes it for the rest of the system.
The provided functionality includes:
- Read the output from the sensor drivers (`sensor_gyro`, etc.).
If there are multiple of the same type, do voting and failover handling.
Then apply the board rotation and temperature calibration (if enabled). And finally publish the data; one of the
topics is `sensor_combined`, used by many parts of the system.
- Make sure the sensor drivers get the updated calibration parameters (scale & offset) when the parameters change or
on startup. The sensor drivers use the ioctl interface for parameter updates. For this to work properly, the
sensor drivers must already be running when `sensors` is started.
- Do sensor consistency checks and publish the `sensors_status_imu` topic.
### Implementation
It runs in its own thread and polls on the currently selected gyro topic.
sensors <command> [arguments...]
Commands:
start
[-h] Start in HIL mode
stop
status print status info
注：print_usage函数是具体对应实现。
class Sensors : public ModuleBase<Sensors>, public ModuleParams, public px4::ScheduledWorkItemModuleParams, public px4::WorkItem
注：Sensors模块采用了ModuleBase和WorkQueue设计。
2. 模块入口函数
2.1 主入口sensors_main
同样继承了ModuleBase，由ModuleBase的main来完成模块入口。
sensors_main
└──> return Sensors::main(argc, argv)
2.2 自定义子命令custom_command
模块仅支持start/stop/status命令，不支持其他自定义命令。
Sensors::custom_command
└──> return print_usage("unknown command");
2.3 模块状态print_status【重载】
Sensors::print_status
├──> _voted_sensors_update.printStatus();
├──> <CONFIG_SENSORS_VEHICLE_MAGNETOMETER><_vehicle_magnetometer>
│ ├──> PX4_INFO_RAW("\n");
│ └──> _vehicle_magnetometer->PrintStatus();
├──> <CONFIG_SENSORS_VEHICLE_AIR_DATA><_vehicle_air_data>
│ ├──> PX4_INFO_RAW("\n");
│ └──> _vehicle_air_data->PrintStatus();
├──> <CONFIG_SENSORS_VEHICLE_AIRSPEED>
│ ├──> PX4_INFO_RAW("\n");
│ ├──> PX4_INFO_RAW("Airspeed status:\n");
│ └──> _airspeed_validator.print();
├──> <CONFIG_SENSORS_VEHICLE_OPTICAL_FLOW><_vehicle_optical_flow>
│ ├──> PX4_INFO_RAW("\n");
│ └──> _vehicle_optical_flow->PrintStatus();
├──> <CONFIG_SENSORS_VEHICLE_GPS_POSITION><_vehicle_gps_position>
│ ├──> PX4_INFO_RAW("\n");
│ └──> _vehicle_gps_position->PrintStatus();
├──> PX4_INFO_RAW("\n");
├──> _vehicle_acceleration.PrintStatus();
├──> PX4_INFO_RAW("\n");
├──> _vehicle_angular_velocity.PrintStatus();
├──> PX4_INFO_RAW("\n");
├──> <for (auto &i : _vehicle_imu_list)><i != nullptr>
│ ├──> PX4_INFO_RAW("\n");
│ └──> i->PrintStatus();
└──> return 0;
3. Sensors模块重要函数
3.1 task_spawn
这里主要初始化了Sensors对象，后续通过WorkQueue来完成进行轮询。
Sensors::task_spawn
├──> bool hil_enabled = false
├──> [命令行输入参数，解析hil_enabled]
├──> Sensors *instance = new Sensors(hil_enabled)
├──> <instance>
│ ├──> _object.store(instance)
│ ├──> _task_id = task_id_is_work_queue
│ └──> <instance->init()>
│ └──> return PX4_OK
├──> <else>
│ └──> PX4_ERR("alloc failed")
├──> delete instance
├──> _object.store(nullptr)
├──> _task_id = -1
└──> return PX4_ERROR
3.2 instantiate
注：鉴于该模块不采用任务(线程)，所以ModuleBase::run_trampoline无需执行，所以可以不实现。
3.3 init
在task_spawn中调用init进行ScheduleNow初次调用，后续对_vehicle_imu_sub成员变量进行事件回调注册(当有Sensors消息时，会调用SubscriptionCallbackWorkItem::ScheduleNow，再触发Sensors::Run过程)。
Sensors::init
├──> _vehicle_imu_sub[0].registerCallback() // 这里有点奇怪，为什么只是注册一个IMU；假如多个IMU，第0个损坏且数据不发生变化，怎么办？？？
├──> ScheduleNow()
└──> return true
注：最多支持4个IMU设备。
uORB::SubscriptionCallbackWorkItem _vehicle_imu_sub[MAX_SENSOR_COUNT] {
{this, ORB_ID(vehicle_imu), 0},
{this, ORB_ID(vehicle_imu), 1},
{this, ORB_ID(vehicle_imu), 2},
{this, ORB_ID(vehicle_imu), 3}
};
3.4 Run
在没有解锁时进行传感器遍历和添加；当解锁以后就不在添加传感器，仅对当前已经确认支持的传感设备进行数据更新。
Sensors::Run
├──> [优雅退出处理]
├──> <_vcontrol_mode_sub.updated()><_vcontrol_mode_sub.copy(&vcontrol_mode)>
│ └──> _armed = vcontrol_mode.flag_armed
├──> <(!_armed && hrt_elapsed_time(&_last_config_update) > 500_ms) || (_last_config_update == 0)> // keep adding sensors as long as we are not armed
│ ├──> bool updated = false
│ ├──> <CONFIG_SENSORS_VEHICLE_AIR_DATA>
│ │ ├──> const int n_baro = orb_group_count(ORB_ID(sensor_baro))
│ │ └──> <n_baro != _n_baro>
│ │ └──> _n_baro = n_baro; updated = true;
│ ├──> <CONFIG_SENSORS_VEHICLE_GPS_POSITION>
│ │ ├──> const int n_gps = orb_group_count(ORB_ID(sensor_gps))
│ │ └──> <n_gps != _n_gps>
│ │ └──> _n_gps = n_gps; updated = true;
│ ├──> <CONFIG_SENSORS_VEHICLE_MAGNETOMETER>
│ │ ├──> const int n_mag = orb_group_count(ORB_ID(sensor_mag))
│ │ └──> <n_mag != _n_mag>
│ │ └──> _n_mag = n_mag; updated = true;
│ ├──> <CONFIG_SENSORS_VEHICLE_OPTICAL_FLOW>
│ │ ├──> const int n_optical_flow = orb_group_count(ORB_ID(sensor_optical_flow))
│ │ └──> <n_optical_flow != _n_optical_flow>
│ │ └──> _n_optical_flow = n_optical_flow; updated = true;
│ ├──> const int n_accel = orb_group_count(ORB_ID(sensor_accel));
│ ├──> const int n_gyro = orb_group_count(ORB_ID(sensor_gyro));
│ ├──> <(n_accel != _n_accel) || (n_gyro != _n_gyro) || updated>
│ │ ├──> _n_accel = n_accel;
│ │ ├──> _n_gyro = n_gyro;
│ │ └──> parameters_update();
│ ├──> _voted_sensors_update.initializeSensors();
│ ├──> InitializeVehicleIMU();
│ └──> _last_config_update = hrt_absolute_time();
├──> <else><_parameter_update_sub.updated()> // when not adding sensors poll for param updates
│ ├──> _parameter_update_sub.copy(&pupdate);
│ ├──> parameters_update();
│ └──> updateParams();
├──> _voted_sensors_update.sensorsPoll(_sensor_combined);
├──> <_sensor_combined.timestamp != _sensor_combined_prev_timestamp>
│ ├──> _voted_sensors_update.setRelativeTimestamps(_sensor_combined);
│ ├──> _sensor_pub.publish(_sensor_combined); // 发布sensor_combined消息
│ └──> _sensor_combined_prev_timestamp = _sensor_combined.timestamp;
├──> <CONFIG_SENSORS_VEHICLE_AIRSPEED> // check analog airspeed
│ ├──> adc_poll(); //发布differential_pressure消息
│ └──> diff_pres_poll(); //发布airspeed消息
└──> ScheduleDelayed(10_ms); // backup schedule as a watchdog timeout
4. 总结
该模块主要处理多IMU数据，空速计，气压差传感数据，从代码角度：
- 输入
uORB::SubscriptionCallbackWorkItem _vehicle_imu_sub[MAX_SENSOR_COUNT] {{this, ORB_ID(vehicle_imu), 0},{this, ORB_ID(vehicle_imu), 1},{this, ORB_ID(vehicle_imu), 2},{this, ORB_ID(vehicle_imu), 3}};
uORB::SubscriptionInterval _parameter_update_sub{ORB_ID(parameter_update), 1_s};
uORB::Subscription _vcontrol_mode_sub{ORB_ID(vehicle_control_mode)};
uORB::Subscription _diff_pres_sub {ORB_ID(differential_pressure)};
uORB::Subscription _vehicle_air_data_sub{ORB_ID(vehicle_air_data)};
uORB::Subscription _adc_report_sub {ORB_ID(adc_report)};
- 输出
uORB::Publication<sensor_combined_s> _sensor_pub{ORB_ID(sensor_combined)};
uORB::Publication<airspeed_s> _airspeed_pub{ORB_ID(airspeed)};
uORB::PublicationMulti<differential_pressure_s> _diff_pres_pub{ORB_ID(differential_pressure)};
5. 参考资料
【1】PX4开源软件框架简明简介
【2】PX4模块设计之十一：Built-In框架
【3】PX4模块设计之十二：High Resolution Timer设计
【4】PX4模块设计之十三：WorkQueue设计
【5】PX4模块设计之十七：ModuleBase模块
【6】PX4模块设计之三十：Hysteresis类
【7】PX4 modules_main
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