本文详细剖析了PX4飞行控制系统中RC通道的工作原理，重点介绍了RC_Channel、RC_Channels、SRV_Channel及SRV_Channels四个核心类的功能与实现细节。
前言
这部分是之前在折腾ROVER的时候梳理了一遍，但是没有形成记录，现在从新从对象的角度来分析一遍，包括映射、转换、数据结构...等方面进行梳理；
数据结构
首先分析的入口是从Rover::read_radio()的函数进行，我们就从这里为入口并作为本次分析的主线来进行阐述：
void Rover::read_radio()
{
//1如果没有新的RC信号，执行失控保护
if (!hal.rcin->new_input()) {
control_failsafe(channel_throttle->get_radio_in());
return;
}
failsafe.last_valid_rc_ms = AP_HAL::millis();
//获取所有RC通道,并根据初始设定的方式[angle/range]按设置范围归一化保存至control_in中
RC_Channels::set_pwm_all();
control_failsafe(channel_throttle->get_radio_in());
//2处理伺服辅助功能
SRV_Channels::set_output_scaled(SRV_Channel::k_throttle,channel_throttle->get_control_in());
// Check if the throttle value is above 50% and we need to nudge
// Make sure its above 50% in the direction we are travelling
....
...
..
//3开启舵机平滑
if (g.skid_steer_in) {
// convert the two radio_in values from skid steering values
float motor1 = channel_steer->norm_input();
float motor2 = channel_throttle->norm_input();
float steering_scaled = motor1 - motor2;
float throttle_scaled = 0.5f*(motor1 + motor2);
int16_t steer = channel_steer->get_radio_trim();
int16_t thr = channel_throttle->get_radio_trim();
channel_steer->set_pwm(steer);
channel_throttle->set_pwm(thr);
....
...
..
//4舵机解锁检测
rudder_arm_disarm_check();
}
这里涉及到的几个对象单独罗列出来进行分析：
RC_Channel
RC_Channels
SRV_Channel
SRV_Channels
RC_Channels
首先是RC_Channels，从下面的定义来看内部定义了很多静态函数，并定义了定义了一组RC_Channel的数组以及指针，RC_Channels::set_pwm_all(void)的主要作用直接读取hal.rcin的值按range/angle转换并保存至control_in中；这里先不深入研究。
/*
class RC_Channels. Hold the full set of RC_Channel objects
*/
class RC_Channels {
public:
friend class SRV_Channels; #申明SRV_Channels是其友元类
// constructor
RC_Channels(void); #构造函数，并对channels[i].ch_in赋值通道号
static const struct AP_Param::GroupInfo var_info[];
static RC_Channel *rc_channel(uint8_t chan) { #返回RC_Channles[chan]指针
return (chan < NUM_RC_CHANNELS)?&channels[chan]:nullptr;
}
static void set_pwm_all(void);
private:
// this static arrangement is to avoid static pointers in AP_Param tables
static RC_Channel *channels; #指向obj_channels[NUM_RC_CHANNELS]的首地址
RC_Channel obj_channels[NUM_RC_CHANNELS];
};
总的来说RC_Channels就是一个定义了RC_Channel数组长度[16]且具备操作所有通道的输出的类;
RC_Channel
接下来就是RC_Channel，主要是抽象出了单个通道的数据结构，下面先从成员变量进行分析；
/// @class RC_Channel
/// @brief Object managing one RC channel
class RC_Channel {
public:
friend class SRV_Channels;
friend class RC_Channels;
// Constructor
RC_Channel(void);
// used to get min/max/trim limit value based on _reverse
enum LimitValue {
RC_CHANNEL_LIMIT_TRIM,
RC_CHANNEL_LIMIT_MIN,
RC_CHANNEL_LIMIT_MAX
};
// startup
void load_eeprom(void);
void save_eeprom(void);
void save_trim(void);
// setup the control preferences，设置获取通道数值范围/反相/死区的方法
void set_range(uint16_t high);
void set_angle(uint16_t angle);
bool get_reverse(void) const;
void set_default_dead_zone(int16_t dzone);
uint16_t get_dead_zone(void) const { return dead_zone; }
// get the center stick position expressed as a control_in value
int16_t get_control_mid() const;
// read input from hal.rcin - create a control_in value
void set_pwm(int16_t pwm);
void set_pwm_no_deadzone(int16_t pwm);
// calculate an angle given dead_zone and trim. This is used by the quadplane code
// for hover throttle
int16_t pwm_to_angle_dz_trim(uint16_t dead_zone, uint16_t trim);
/*
return a normalised input for a channel, in range -1 to 1,
centered around the channel trim. Ignore deadzone.
归一化通道的数值为[-1,1]
*/
float norm_input();
/*
return a normalised input for a channel, in range -1 to 1,
centered around the channel trim. Take into account the deadzone
归一化通道的数值为[-1,1],增加死区
*/
float norm_input_dz();
uint8_t percent_input();
int16_t pwm_to_range();
int16_t pwm_to_range_dz(uint16_t dead_zone);
// read the input value from hal.rcin for this channel，从硬件抽象层中获取RC信号
uint16_t read() const;
// read input from hal.rcin and set as pwm input for channel
void input();
static const struct AP_Param::GroupInfo var_info[];
// return true if input is within deadzone of trim
bool in_trim_dz();
int16_t get_radio_in() const { return radio_in;}
void set_radio_in(int16_t val) {radio_in = val;}
int16_t get_control_in() const { return control_in;}
void set_control_in(int16_t val) { control_in = val;}
// get control input with zero deadzone
int16_t get_control_in_zero_dz(void);
int16_t get_radio_min() const {return radio_min.get();}
void set_radio_min(int16_t val) { radio_min = val;}
int16_t get_radio_max() const {return radio_max.get();}
void set_radio_max(int16_t val) {radio_max = val;}
int16_t get_radio_trim() const { return radio_trim.get();}
void set_radio_trim(int16_t val) { radio_trim.set(val);}
void save_radio_trim() { radio_trim.save();}
bool min_max_configured() const
{
return radio_min.configured() && radio_max.configured();
}
private:
// pwm is stored here
// RC通道读出的原始值的保存位置
int16_t radio_in;
// value generated from PWM normalised to configured scale
// RC通道原始值经过归一化和配置后的保存位置
int16_t control_in;
AP_Int16 radio_min; #RC通道原始值的最小值
AP_Int16 radio_trim; #RC通道原始值的中位值
AP_Int16 radio_max; #RC通道原始值的最大值
AP_Int8 reversed; ＃RC通道反相标志
AP_Int16 dead_zone; ＃RC通道死区
uint8_t type_in; ＃RC输入值类型是角度还是范围
int16_t high_in;
// the input channel this corresponds to
uint8_t ch_in; ＃RC通道号
int16_t pwm_to_angle();
int16_t pwm_to_angle_dz(uint16_t dead_zone);
};
所有从上面的数据结构就可以看出，RC_Channel抽象的RC通道如上面注释所描述，其成员函数主要是围绕变量的设置...
在初始的通道号分配时，还涉及到RCMapper rcmap，这个主要是通道映射，在其构造函数中，通过const AP_Param::GroupInfo RCMapper::var_info[]对其成员变量进行赋值，根本上还是对通道进行映射；
RCMapper
class RCMapper
{
public:
/// Constructor
///
RCMapper();
/// roll - return input channel number for roll / aileron input
uint8_t roll() const { return _ch_roll; }
/// pitch - return input channel number for pitch / elevator input
uint8_t pitch() const { return _ch_pitch; }
/// throttle - return input channel number for throttle input
uint8_t throttle() const { return _ch_throttle; }
/// yaw - return input channel number for yaw / rudder input
uint8_t yaw() const { return _ch_yaw; }
/// forward - return input channel number for forward input
uint8_t forward() const { return _ch_forward; }
/// lateral - return input channel number for lateral input
uint8_t lateral() const { return _ch_lateral; }
static const struct AP_Param::GroupInfo var_info[];
private:
// channel mappings
AP_Int8 _ch_roll;
AP_Int8 _ch_pitch;
AP_Int8 _ch_yaw;
AP_Int8 _ch_throttle;
AP_Int8 _ch_forward;
AP_Int8 _ch_lateral;
};
const AP_Param::GroupInfo RCMapper::var_info[] = {
// @Param: ROLL
// @DisplayName: Roll channel
// @Description: Roll channel number.
// @Range: 1 8
// @Increment: 1
// @User: Advanced
// @RebootRequired: True
AP_GROUPINFO("ROLL", 0, RCMapper, _ch_roll, 1),
AP_GROUPINFO("PITCH", 1, RCMapper, _ch_pitch, 2),
AP_GROUPINFO("THROTTLE", 2, RCMapper, _ch_throttle, 3),
AP_GROUPINFO("YAW", 3, RCMapper, _ch_yaw, 4),
AP_GROUPINFO_FRAME("FORWARD", 4, RCMapper, _ch_forward, 6, AP_PARAM_FRAME_SUB),
AP_GROUPINFO_FRAME("LATERAL", 5, RCMapper, _ch_lateral, 7, AP_PARAM_FRAME_SUB),
};
最终在使用的时候：
1.只需要定义一个RC_Channel的指针;
2.然后通过RC_Channels::rc_channel()返回一个实际通道指针;
3.实际通道指针的映射存在于RCMapper::var_info[]；
关于使用上面三点了解了也就基本能使用了，接下来深入一点就是这个ＰＷＭ的值究竟是怎么来的，从RC_Channel类中不难发现：
void
RC_Channel::input()
{
radio_in = hal.rcin->read(ch_in);
}
uint16_t
RC_Channel::read() const
{
return hal.rcin->read(ch_in);
}
上述两个方法的中调用都涉及到了hal.rcin，从RCInput可以找到相关实现，最终可以找到是通过orb_copy(ORB_ID(input_rc), _rc_sub, &_rcin);来获取的到外部输入的RC 信号的；
void PX4RCInput::_timer_tick(void)
{
perf_begin(_perf_rcin);
bool rc_updated = false;
if (orb_check(_rc_sub, &rc_updated) == 0 && rc_updated) {
pthread_mutex_lock(&rcin_mutex);
orb_copy(ORB_ID(input_rc), _rc_sub, &_rcin);
pthread_mutex_unlock(&rcin_mutex);
}
// note, we rely on the vehicle code checking new_input()
// and a timeout for the last valid input to handle failsafe
perf_end(_perf_rcin);
}
通过全局搜索我们可以在PX4Firmware/src/driver/px4_fmu/fmu.cpp中找到下面的函数，传输的_rc_in则是通过void
PX4FMU::fill_rc_in（）来进行填充的，也就是我们拿到的原始包含pwm的值；
void
PX4FMU::cycle_trampoline(void *arg)
{
PX4FMU *dev = reinterpret_cast<PX4FMU *>(arg);
dev->cycle();
}
而cycle中就有通过UORB发布input_rc的话题，同时调用了work_queue进行循环，仔细一看就会发现这个函数里面还有对安全开关的操作；
// slave safety from IO
...
..
/* check arming state */
...
..
if (rc_updated) {
/* lazily advertise on first publication */
if (_to_input_rc == nullptr) {
_to_input_rc = orb_advertise(ORB_ID(input_rc), &_rc_in);
} else {
orb_publish(ORB_ID(input_rc), _to_input_rc, &_rc_in);
}
}
work_queue(HPWORK, &_work, (worker_t)&PX4FMU::cycle_trampoline, this,
USEC2TICK(SCHEDULE_INTERVAL - main_out_latency));
到这里基本上能明白的是px4_fmu也是公共继承了CDev的类和之前的I2C、SPI一样都是属于字符型驱动；相同情况px4_io也是同样的是公共继承了CDev的字符类驱动；从nsh的终端上“ls dev”也可以看出，px对fmu和io的抽象，按照官网说明那样fmu和io之间是通过串口进行通信的，而在fmu的work_queue中还没有发现相关源码。这里需要在整理；
SRV_Channel
SRV_Channel是对输出通道的抽象出的数据结构，枚举了所有PWM通道；实现了单个SRV_Channel的从信号的输入到输出的计算，并且附加了对通道数值的类型和限幅操作；
/*
class SRV_Channel. The class SRV_Channels contains an array of
SRV_Channel objects. This is done to fit within the AP_Param limit
of 64 parameters per object.
*/
class SRV_Channel {
public:
friend class SRV_Channels;
// constructor
SRV_Channel(void);
static const struct AP_Param::GroupInfo var_info[];
typedef enum
{
k_none = 0, ///< disabled
k_manual = 1, ///< manual, just pass-thru the RC in signal
k_flap = 2, ///< flap
k_flap_auto = 3, ///< flap automated
k_aileron = 4, ///< aileron
k_unused1 = 5, ///< unused function
k_mount_pan = 6, ///< mount yaw (pan)
k_mount_tilt = 7, ///< mount pitch (tilt)
k_mount_roll = 8, ///< mount roll
k_mount_open = 9, ///< mount open (deploy) / close (retract)
k_cam_trigger = 10, ///< camera trigger
k_egg_drop = 11, ///< egg drop
k_mount2_pan = 12, ///< mount2 yaw (pan)
k_mount2_tilt = 13, ///< mount2 pitch (tilt)
k_mount2_roll = 14, ///< mount2 roll
k_mount2_open = 15, ///< mount2 open (deploy) / close (retract)
k_dspoiler1 = 16, ///< differential spoiler 1 (left wing)
k_dspoiler2 = 17, ///< differential spoiler 2 (right wing)
k_aileron_with_input = 18, ///< aileron, with rc input
k_elevator = 19, ///< elevator
k_elevator_with_input = 20, ///< elevator, with rc input
k_rudder = 21, ///< secondary rudder channel
k_sprayer_pump = 22, ///< crop sprayer pump channel
k_sprayer_spinner = 23, ///< crop sprayer spinner channel
k_flaperon1 = 24, ///< flaperon, left wing
k_flaperon2 = 25, ///< flaperon, right wing
k_steering = 26, ///< ground steering, used to separate from rudder
k_parachute_release = 27, ///< parachute release
k_gripper = 28, ///< gripper
k_landing_gear_control = 29, ///< landing gear controller
k_engine_run_enable = 30, ///< engine kill switch, used for gas airplanes and helicopters
k_heli_rsc = 31, ///< helicopter RSC output
k_heli_tail_rsc = 32, ///< helicopter tail RSC output
k_motor1 = 33, ///< these allow remapping of copter motors
k_motor2 = 34,
k_motor3 = 35,
k_motor4 = 36,
k_motor5 = 37,
k_motor6 = 38,
k_motor7 = 39,
k_motor8 = 40,
k_motor_tilt = 41, ///< tiltrotor motor tilt control
k_rcin1 = 51, ///< these are for pass-thru from arbitrary rc inputs
k_rcin2 = 52,
k_rcin3 = 53,
k_rcin4 = 54,
k_rcin5 = 55,
k_rcin6 = 56,
k_rcin7 = 57,
k_rcin8 = 58,
k_rcin9 = 59,
k_rcin10 = 60,
k_rcin11 = 61,
k_rcin12 = 62,
k_rcin13 = 63,
k_rcin14 = 64,
k_rcin15 = 65,
k_rcin16 = 66,
k_ignition = 67,
k_choke = 68,
k_starter = 69,
k_throttle = 70,
k_tracker_yaw = 71, ///< antennatracker yaw
k_tracker_pitch = 72, ///< antennatracker pitch
k_throttleLeft = 73,
k_throttleRight = 74,
k_nr_aux_servo_functions ///< This must be the last enum value (only add new values _before_ this one)
} Aux_servo_function_t;
// used to get min/max/trim limit value based on reverse
enum LimitValue {
SRV_CHANNEL_LIMIT_TRIM,
SRV_CHANNEL_LIMIT_MIN,
SRV_CHANNEL_LIMIT_MAX,
SRV_CHANNEL_LIMIT_ZERO_PWM
};
// set the output value as a pwm value 设置当前通道的待输出的PWM值
void set_output_pwm(uint16_t pwm);
// get the output value as a pwm value 获取当前通道输出的PWM值
uint16_t get_output_pwm(void) const { return output_pwm; }
// set angular range of scaled output 设置在转换为PWM信号时，最大输入角度
void set_angle(int16_t angle);
// set range of scaled output. Low is always zero 设置在转换为PWM信号时，最大输入范围
void set_range(uint16_t high);
// return true if the channel is reversed 返回通道是否反相
bool get_reversed(void) const {
return reversed?true:false;
}
// set MIN/MAX parameters 设置通道的PWM最大最小值
void set_output_min(uint16_t pwm) {
servo_min.set(pwm);
}
void set_output_max(uint16_t pwm) {
servo_max.set(pwm);
}
// get MIN/MAX/TRIM parameters 获取通道的PWM最大最小值
uint16_t get_output_min(void) const {
return servo_min;
}
uint16_t get_output_max(void) const {
return servo_max;
}
uint16_t get_trim(void) const {
return servo_trim;
}
private:
AP_Int16 servo_min;
AP_Int16 servo_max;
AP_Int16 servo_trim;
// reversal, following convention that 1 means reversed, 0 means normal
AP_Int8 reversed;
AP_Int8 function;
// a pending output value as PWM
uint16_t output_pwm;
// true for angle output type
bool type_angle:1;
// set_range() or set_angle() has been called
bool type_setup:1;
// the hal channel number
uint8_t ch_num;
// high point of angle or range output
uint16_t high_out;
// convert a 0..range_max to a pwm 将PWM转换为范围
uint16_t pwm_from_range(int16_t scaled_value) const;
// convert a -angle_max..angle_max to a pwm 将PWM转换为角度
uint16_t pwm_from_angle(int16_t scaled_value) const;
// convert a scaled output to a pwm value 输入output_scaled[范围、角度]转换为PWM并set_output_pwm()；
void calc_pwm(int16_t output_scaled);
// output value based on function， 调用hal.rcout直接输出PWM
void output_ch(void);
// setup output type and range based on function
void aux_servo_function_setup(void);
// return PWM for a given limit value
uint16_t get_limit_pwm(LimitValue limit) const;
// get normalised output from -1 to 1
float get_output_norm(void);
// a bitmask type wide enough for NUM_SERVO_CHANNELS
typedef uint16_t servo_mask_t;
// mask of channels where we have a output_pwm value. Cleared when a
// scaled value is written.
static servo_mask_t have_pwm_mask;
};
SRV_Channels
与SRV_Channel互为友元类，在成员变量上定义了SRV_Channels的类数组，方法上更对是对SRV_Channel的补充，例如设置中位、设置失控保护值....
下面分析一个简单的例子：
首先指定通道function（类中枚举的值）和待写入的value，这个值会被保存在functions[function].output_scaled中；
void SRV_Channels::set_output_scaled(SRV_Channel::Aux_servo_function_t function, int16_t value)；
|
V
SCHED_TASK(set_servos, 50, 1500),//全部PWM输出这里会周期执行，里面会计算来自RC信号/其他填充的信号
|
V
先调用SRV_Channels::calc_pwm();
再执行SRV_Channels::output_ch_all();
|
V
channels[i].output_ch();//最终回归到SRV_Channel 定义的channels[i]数组
|
V
hal.rcout->write(ch_num, output_pwm);//调用AP_HAL执行输出；
/// map a function to a servo channel and output it
void SRV_Channel::output_ch(void)
{
int8_t passthrough_from = -1;
// take care of special function cases，根据通道的编号进行不同处理，1,[51,66]k_rcin1~k_rcin16均为直通信号
switch(function)
{
case k_manual: // manual
passthrough_from = ch_num;
break;
case k_rcin1 ... k_rcin16: // rc pass-thru，转换后的通道编号为0~15
passthrough_from = int8_t(function - k_rcin1);
break;
case k_motor1 ... k_motor8:
// handled by AP_Motors::rc_write()
return;
}
if (passthrough_from != -1) {
// we are doing passthrough from input to output for this channel
RC_Channel *rc = RC_Channels::rc_channel(passthrough_from);
if (rc) {
if (SRV_Channels::passthrough_disabled()) {
output_pwm = rc->get_radio_trim();
} else {
output_pwm = rc->get_radio_in();
}
}
}
hal.rcout->write(ch_num, output_pwm);//直接通过抽象层输出，这里继续往下就要看回AP_HAL_PX下的RCOutput.cpp
}
总结
RC_Channel、RC_Channels、SRV_Channel、SRV_Channels这4个类都是基于hal.rcin和hal.rcout与硬件沟通的；
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