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[Multirotor] add yaw rate low pass filter #24173

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[Multirotor] add yaw rate low pass filter #24173

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0519289
add low pass filtering on yaw mc rate control
bresch Jan 6, 2025
28bc950
MC rate: rename yaw torque LPF parameter
bresch Jan 9, 2025
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10 changes: 10 additions & 0 deletions src/lib/mathlib/math/filter/AlphaFilter.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -89,6 +89,16 @@ class AlphaFilter
return true;
}

void setCutoffFreq(float cutoff_freq)
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This function could potentially be misleading in the case you call setCutoffFreq(float cutoff_freq) which sets _time_constant but then use update(const T &sample) for the update which assumes setParameters(dt, _time_constant) was already called. I don't have a great idea how to make it clear but see the potential for people to run into this 😬

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Yes, I was wondering about this too. Should we instead have two distinct classes? One where dt needs to be set during the update call and the other one that uses the setParameters to update the alpha.

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Seems overkill no? Would it be a performance hit to just always use the "One where dt needs to be set during the update" and in the cases it's static just pass a constant? Seems way easier compared to the multiple options for basically the same thing.

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It does seem like overkill, but having separate classes could make each individually a bit easier to use, harder to misuse. Having to know which update to call when used a certain way is a leaky abstraction.

{
if (cutoff_freq > FLT_EPSILON) {
_time_constant = 1.f / (M_TWOPI_F * cutoff_freq);

} else {
_time_constant = 0.f;
}
}

/**
* Set filter parameter alpha directly without time abstraction
*
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7 changes: 6 additions & 1 deletion src/modules/mc_rate_control/MulticopterRateControl.cpp
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Expand Up @@ -95,6 +95,8 @@ MulticopterRateControl::parameters_updated()
// manual rate control acro mode rate limits
_acro_rate_max = Vector3f(radians(_param_mc_acro_r_max.get()), radians(_param_mc_acro_p_max.get()),
radians(_param_mc_acro_y_max.get()));

_output_lpf_yaw.setCutoffFreq(_param_mc_yawrate_fc.get());
}

void
Expand Down Expand Up @@ -214,9 +216,12 @@ MulticopterRateControl::Run()
}

// run rate controller
const Vector3f torque_setpoint =
Vector3f torque_setpoint =
_rate_control.update(rates, _rates_setpoint, angular_accel, dt, _maybe_landed || _landed);

// apply low-pass filtering on yaw axis to reduce high frequency torque caused by rotor acceleration
torque_setpoint(2) = _output_lpf_yaw.update(torque_setpoint(2), dt);

// publish rate controller status
rate_ctrl_status_s rate_ctrl_status{};
_rate_control.getRateControlStatus(rate_ctrl_status);
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4 changes: 4 additions & 0 deletions src/modules/mc_rate_control/MulticopterRateControl.hpp
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Expand Up @@ -34,6 +34,7 @@
#pragma once

#include <lib/rate_control/rate_control.hpp>
#include <lib/mathlib/math/filter/AlphaFilter.hpp>
#include <lib/matrix/matrix/math.hpp>
#include <lib/perf/perf_counter.h>
#include <px4_platform_common/defines.h>
Expand Down Expand Up @@ -129,6 +130,8 @@ class MulticopterRateControl : public ModuleBase<MulticopterRateControl>, public
float _energy_integration_time{0.0f};
float _control_energy[4] {};

AlphaFilter<float> _output_lpf_yaw;

DEFINE_PARAMETERS(
(ParamFloat<px4::params::MC_ROLLRATE_P>) _param_mc_rollrate_p,
(ParamFloat<px4::params::MC_ROLLRATE_I>) _param_mc_rollrate_i,
Expand All @@ -150,6 +153,7 @@ class MulticopterRateControl : public ModuleBase<MulticopterRateControl>, public
(ParamFloat<px4::params::MC_YAWRATE_D>) _param_mc_yawrate_d,
(ParamFloat<px4::params::MC_YAWRATE_FF>) _param_mc_yawrate_ff,
(ParamFloat<px4::params::MC_YAWRATE_K>) _param_mc_yawrate_k,
(ParamFloat<px4::params::MC_YAWRATE_FC>) _param_mc_yawrate_fc,

(ParamFloat<px4::params::MC_ACRO_R_MAX>) _param_mc_acro_r_max,
(ParamFloat<px4::params::MC_ACRO_P_MAX>) _param_mc_acro_p_max,
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14 changes: 14 additions & 0 deletions src/modules/mc_rate_control/mc_rate_control_params.c
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Expand Up @@ -292,3 +292,17 @@ PARAM_DEFINE_FLOAT(MC_YAWRATE_K, 1.0f);
* @group Multicopter Rate Control
*/
PARAM_DEFINE_INT32(MC_BAT_SCALE_EN, 0);

/**
* First order low pass filter cutoff frequency for yaw rate control
*
* Reduces vibrations by lowering high frequency torque caused by rotor acceleration.
* 0 disables the filter
*
* @min 0
* @max 10
* @unit Hz
* @decimal 3
* @group Multicopter Rate Control
*/
PARAM_DEFINE_FLOAT(MC_YAWRATE_FC, 2.f);
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I'm not sure if I have a better suggestion, but does this naming make sense?

Something more along the lines of yaw torque filter cutoff or in someway distinguish that it's specifically the output?

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The suggested name from #21434 was MC_YAW_CUTOFF. I like the _CUTOFF prefix but MC_YAW is too inaccurate. Also, it would be problematic if we want to add a yaw filter (not yawrate like now).
What about MC_YAW_TQ_CUTOFF or MC_Y_TRQ_CUTOFF ?

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Changed it to MC_YAW_TQ_CUTOFF in 28bc950

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