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removed air density as a parameter. Using ideal gas law
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@JMoore5353
JMoore5353 committed Dec 17, 2024
1 parent 0ee7dde commit 4173d59
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Showing 7 changed files with 9 additions and 11 deletions.
1 change: 0 additions & 1 deletion include/mixer.h
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Expand Up @@ -274,7 +274,6 @@ class Mixer : public ParamListenerInterface

// Define parameters for the mixer
float R_; // Motor resistance
float rho_; // Air density
float K_V_; // Motor back-emf constant
float K_Q_; // Motor torque constant
float i_0_; // Motor no-load current
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2 changes: 0 additions & 2 deletions include/param.h
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Expand Up @@ -62,8 +62,6 @@ enum : uint16_t
PARAM_BAUD_RATE = 0,
PARAM_SERIAL_DEVICE,

PARAM_AIR_DENSITY,

PARAM_NUM_MOTORS,
PARAM_MOTOR_RESISTANCE,
PARAM_MOTOR_KV,
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3 changes: 3 additions & 0 deletions include/sensors.h
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Expand Up @@ -177,6 +177,7 @@ class Sensors : public ParamListenerInterface
Sensors(ROSflight & rosflight);

inline const Data & data() const { return data_; }
inline float rho() { return rho_; }
void get_filtered_IMU(turbomath::Vector & accel, turbomath::Vector & gyro, uint64_t & stamp_us);

// function declarations
Expand Down Expand Up @@ -227,6 +228,8 @@ class Sensors : public ParamListenerInterface
float accel_[3] = {0, 0, 0};
float gyro_[3] = {0, 0, 0};

float rho_ = 1.225;

bool calibrating_acc_flag_ = false;
bool calibrating_gyro_flag_ = false;
uint8_t next_sensor_to_update_ = BAROMETER;
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4 changes: 1 addition & 3 deletions src/controller.cpp
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Expand Up @@ -79,7 +79,7 @@ void Controller::calculate_max_thrust()
{
float R = RF_.params_.get_param_float(PARAM_MOTOR_RESISTANCE);
float D = RF_.params_.get_param_float(PARAM_PROP_DIAMETER);
float rho = RF_.params_.get_param_float(PARAM_AIR_DENSITY);
float rho = RF_.sensors_.rho();
float CQ = RF_.params_.get_param_float(PARAM_PROP_CQ);
float CT = RF_.params_.get_param_float(PARAM_PROP_CT);
float KV = RF_.params_.get_param_float(PARAM_MOTOR_KV);
Expand Down Expand Up @@ -210,8 +210,6 @@ void Controller::param_change_callback(uint16_t param_id)
case PARAM_PID_YAW_RATE_D:
case PARAM_MOTOR_RESISTANCE:
case PARAM_PROP_DIAMETER:
case PARAM_AIR_DENSITY:
case PARAM_PROP_CQ:
case PARAM_PROP_CT:
case PARAM_MOTOR_KV:
case PARAM_NO_LOAD_CURRENT:
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6 changes: 3 additions & 3 deletions src/mixer.cpp
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Expand Up @@ -219,7 +219,6 @@ void Mixer::param_change_callback(uint16_t param_id)
init_mixing();
break;
case PARAM_MOTOR_RESISTANCE:
case PARAM_AIR_DENSITY:
case PARAM_MOTOR_KV:
case PARAM_NO_LOAD_CURRENT:
case PARAM_PROP_DIAMETER:
Expand Down Expand Up @@ -325,7 +324,6 @@ void Mixer::init_mixing()
void Mixer::update_parameters()
{
R_ = RF_.params_.get_param_float(PARAM_MOTOR_RESISTANCE);
rho_ = RF_.params_.get_param_float(PARAM_AIR_DENSITY);
K_V_ = RF_.params_.get_param_float(PARAM_MOTOR_KV);
K_Q_ = K_V_;
i_0_ = RF_.params_.get_param_float(PARAM_NO_LOAD_CURRENT);
Expand Down Expand Up @@ -610,6 +608,8 @@ float Mixer::mix_multirotor_with_motor_parameters(Controller::Output commands)
// Mix the inputs
float max_output = 1.0;

float rho = RF_.sensors_.rho();

for (uint8_t i = 0; i < NUM_MIXER_OUTPUTS; i++) {
if ((*mixer_to_use_.output_type)[i] == M) {
// Matrix multiply to mix outputs for Motor type
Expand All @@ -625,7 +625,7 @@ float Mixer::mix_multirotor_with_motor_parameters(Controller::Output commands)

// Ch. 4, setting equation for torque produced by a propeller equal to Eq. 4.19
// Note that we assume constant advance ratio, leading to constant torque and thrust constants.
float V_in = rho_ * pow(D_, 5.0) / (4.0 * pow(M_PI, 2.0)) * omega_squared * C_Q_ * R_ / K_Q_
float V_in = rho * pow(D_, 5.0) / (4.0 * pow(M_PI, 2.0)) * omega_squared * C_Q_ * R_ / K_Q_
+ R_ * i_0_ + K_V_ * sqrt(omega_squared);

// Convert desired V_in setting to a throttle setting
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2 changes: 0 additions & 2 deletions src/param.cpp
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Expand Up @@ -108,8 +108,6 @@ void Params::set_defaults(void)
init_param_int(PARAM_BAUD_RATE, "BAUD_RATE", 921600); // Baud rate of MAVlink communication with companion computer | 9600 | 921600
init_param_int(PARAM_SERIAL_DEVICE, "SERIAL_DEVICE", 0); // Serial Port (for supported devices) | 0 | 3

init_param_float(PARAM_AIR_DENSITY, "AIR_DENSITY", 1.225f); // Density of the air (kg/m^3) | 0 | 1000.0

init_param_int(PARAM_NUM_MOTORS, "NUM_MOTORS", 4); // Number of vertical-facing motors on the vehicle | 1 | 8
init_param_float(PARAM_MOTOR_RESISTANCE, "MOTOR_RESISTANCE", 0.042f); // Electrical resistance of the motor windings (ohms) | 0 | 1000.0
init_param_float(PARAM_MOTOR_KV, "MOTOR_KV", 0.01706f); // Back emf constant of the motor in SI units (V/rad/s) | 0 | 1000.0
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2 changes: 2 additions & 0 deletions src/sensors.cpp
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Expand Up @@ -174,6 +174,8 @@ got_flags Sensors::run()
got.baro = true;
rf_.board_.baro_read(&data_.baro_pressure, &data_.baro_temperature);
correct_baro();

rho_ = (0.0289644 * data_.baro_pressure) / (8.31432 * data_.baro_temperature);
}
}

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5 comments on commit 4173d59

@avtoku
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@avtoku avtoku commented on 4173d59 Dec 17, 2024

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The baro sensor temperature measures the sensor chip's internal temperature and not the outside air temperature.
Rather than rho_ = (0.0289644 * data_.baro_pressure) / (8.31432 * data_.baro_temperature);
Assume we are in a standard atmosphere:
rho_ = 1.225 * pow( data_.baro_pressure/101325.0, 0.809736894596450 );

@JMoore5353
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@JMoore5353 JMoore5353 commented on 4173d59 Dec 18, 2024 •
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Sounds good, thanks. I was going to ask about that. Do you have a reference for that equation? $\rho = \rho_{STP} * (P / P_0)^{x}$ --> I'm not sure where the exponent comes from.

@avtoku
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@avtoku avtoku commented on 4173d59 Dec 18, 2024

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The exponent comes from the hydrostatic model of the atmosphere. In terms of those things, the exponent = 1-RGamma/g, where R = 287.052874 J/kg-K, Gamma = 6.5e-3 K/m, and g = 9.80665 m/s^2
1-RGamma/g = 0.809736894_760188_. The last few digits are different from the one in my equation above because of rounding.

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@avtoku avtoku commented on 4173d59 Dec 18, 2024 •
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Here is a reference. for the pressure vs. altitude: http://mae-nas.eng.usu.edu/MAE_6530_Web/New_Course/Section4/[Section4.2](http://mae-nas.eng.usu.edu/MAE_6530_Web/New_Course/Section4/Section4.2.pdf).pdf With a little more algebra and you get the relationship for rho vs. pressure.

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Thanks!

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