InnovationLpf.hpp
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/****************************************************************************
*
* Copyright (c) 2019 PX4 Development Team. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
*
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
* 3. Neither the name PX4 nor the names of its contributors may be
* used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*
****************************************************************************/
/**
* First order "alpha" IIR digital filter with input saturation
*/
#include <mathlib/mathlib.h>
class InnovationLpf final
{
public:
InnovationLpf() = default;
~InnovationLpf() = default;
void reset(float val = 0.f) { _x = val; }
/**
* Update the filter with a new value and returns the filtered state
* The new value is constained by the limit set in setSpikeLimit
* @param val new input
* @param alpha normalized weight of the new input
* @param spike_limit the amplitude of the saturation at the input of the filter
* @return filtered output
*/
float update(float val, float alpha, float spike_limit)
{
float val_constrained = math::constrain(val, -spike_limit, spike_limit);
float beta = 1.f - alpha;
_x = beta * _x + alpha * val_constrained;
return _x;
}
/**
* Helper function to compute alpha from dt and the inverse of tau
* @param dt sampling time in seconds
* @param tau_inv inverse of the time constant of the filter
* @return alpha, the normalized weight of a new measurement
*/
static float computeAlphaFromDtAndTauInv(float dt, float tau_inv)
{
return math::constrain(dt * tau_inv, 0.f, 1.f);
}
private:
float _x{}; ///< current state of the filter
};