Why lpf is not set in front of ekf2?

robin-shaun · April 14, 2020, 7:34am

I find that low pass filter(lpf) is set for gyro for the angular velocity control, but it isn’t set for ekf2. As I know, ekf is mainly used for reducing white noise and lpf is used for reducing high frequency noise (mainly caused by vibration), so I think it’s necessary to set lpf for imu information in front of ekf2.

CarlOlsson · April 14, 2020, 1:33pm

Hi!

The low pass filter on the gyro rate before the angular rate controller is there to reduce high frequency noise. There is no low pass filter on the data before the EKF since we don’t want to reduce the information going into the filter.

robin-shaun · April 16, 2020, 12:34pm

Thanks for your answer
But is it possible for EKF to reduce high frequency noise? If EKF cannot reduce it, will the high frequency noise influence the state estimation?

CarlOlsson · April 17, 2020, 6:31am

High frequency noise is attenuated by the integration of the IMU data both before and after it reaches the EKF. Currently the data is downsampled to 250 Hz before the EKF and to 125 Hz inside the EKF.

robin-shaun · April 20, 2020, 2:53am

Downsampling cannot attenuate high frequency noise. In fact, without lpf, downsampling will cause signal aliasing.

CarlOlsson · April 20, 2020, 7:39am

As I wrote above, the data is integrated before downsampling, so effectively being passed through a low pass filter.

robin-shaun · April 20, 2020, 9:25am

Thank you a lot, and I would be grateful if you could show the code link where the integration and downsampling are realized, and I’d like to study the code

CarlOlsson · April 20, 2020, 9:54am

Gyro integration in the driver

github.com

PX4/Firmware/blob/3177b8d763aeb2b937850d45f38a77b0aeef0e34/src/lib/drivers/gyroscope/PX4Gyroscope.cpp#L132-L132


void PX4Gyroscope::update(hrt_abstime timestamp_sample, float x, float y, float z)

Downsampling inside the EKF to 125 Hz

github.com

PX4/ecl/blob/47624a0f021e2c187cc4763ad829afe2aa4fb11a/EKF/estimator_interface.cpp#L48-L88


// Accumulate imu data and store to buffer at desired rate
void EstimatorInterface::setIMUData(const imuSample &imu_sample)
{
	// TODO: resolve misplaced responsibility
	if (!_initialised) {
		init(imu_sample.time_us);
		_initialised = true;
	}


	const float dt = math::constrain((imu_sample.time_us - _time_last_imu) / 1e6f, 1.0e-4f, 0.02f);


	_time_last_imu = imu_sample.time_us;


	if (_time_last_imu > 0) {
		_dt_imu_avg = 0.8f * _dt_imu_avg + 0.2f * dt;
	}


	_newest_high_rate_imu_sample = imu_sample;


	// Do not change order of computeVibrationMetric and checkIfVehicleAtRest

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robin-shaun · April 20, 2020, 10:06am

Thanks a lot, and I will study it.