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[wpilib] Add class to calculate robot resistance
Co-authored-by: ysthakur <45539777+ysthakur@users.noreply.github.com>
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| // Copyright (c) FIRST and other WPILib contributors. | ||
| // Open Source Software; you can modify and/or share it under the terms of | ||
| // the WPILib BSD license file in the root directory of this project. | ||
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| #include "frc/ResistanceCalculator.h" | ||
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| #include <cmath> | ||
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| using namespace frc; | ||
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| ResistanceCalculator::ResistanceCalculator(int bufferSize, | ||
| double rSquaredThreshold) | ||
| : m_currentBuffer(bufferSize), | ||
| m_voltageBuffer(bufferSize), | ||
| m_bufferSize{bufferSize}, | ||
| m_rSquaredThreshold{rSquaredThreshold} {} | ||
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| units::ohm_t ResistanceCalculator::Calculate(units::ampere_t current, | ||
| units::volt_t voltage) { | ||
| // Update buffers only if drawing current | ||
| if (current != 0_A) { | ||
| if (m_currentBuffer.size() >= static_cast<size_t>(m_bufferSize)) { | ||
| // Pop the last point and remove it from the sums | ||
| units::ampere_t lastCurrent = m_currentBuffer.pop_back(); | ||
| units::volt_t lastVoltage = m_voltageBuffer.pop_back(); | ||
| m_currentVariance.Calculate(lastCurrent, lastCurrent, Operator::kRemove); | ||
| m_voltageVariance.Calculate(lastVoltage, lastVoltage, Operator::kRemove); | ||
| m_covariance.Calculate(lastCurrent, lastVoltage, Operator::kRemove); | ||
| } | ||
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| m_currentBuffer.push_front(current); | ||
| m_voltageBuffer.push_front(voltage); | ||
| m_currentVariance.Calculate(current, current, Operator::kAdd); | ||
| m_voltageVariance.Calculate(voltage, voltage, Operator::kAdd); | ||
| m_covariance.Calculate(current, voltage, Operator::kAdd); | ||
| } | ||
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| // Recalculate resistance | ||
| if (m_currentBuffer.size() < 2) { | ||
| return units::ohm_t{std::nan("")}; | ||
| } | ||
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| auto currentVariance = m_currentVariance.GetCovariance(); | ||
| auto voltageVariance = m_voltageVariance.GetCovariance(); | ||
| auto covariance = m_covariance.GetCovariance(); | ||
| double rSquared = | ||
| covariance * covariance / (currentVariance * voltageVariance); | ||
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| if (rSquared > m_rSquaredThreshold) { | ||
| // Resistance is slope of current vs voltage | ||
| return covariance / currentVariance; | ||
| } else { | ||
| return units::ohm_t{std::nan("")}; | ||
| } | ||
| } |
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wpilibc/src/main/native/include/frc/ResistanceCalculator.h
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,160 @@ | ||
| // Copyright (c) FIRST and other WPILib contributors. | ||
| // Open Source Software; you can modify and/or share it under the terms of | ||
| // the WPILib BSD license file in the root directory of this project. | ||
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| #pragma once | ||
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| #include <type_traits> | ||
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| #include <units/current.h> | ||
| #include <units/impedance.h> | ||
| #include <units/voltage.h> | ||
| #include <wpi/circular_buffer.h> | ||
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| namespace frc { | ||
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| /** | ||
| * Finds the resistance of a channel or the entire robot using a running linear | ||
| * regression over a window. | ||
| * | ||
| * Must be updated with current and voltage periodically using the Calculate() | ||
| * method. | ||
| * | ||
| * To use this for finding the resistance of a channel, use the calculate method | ||
| * with the battery voltage minus the voltage at the motor controller or | ||
| * whatever is plugged in to the PDP at that channel. | ||
| */ | ||
| class ResistanceCalculator { | ||
| public: | ||
| /// Default buffer size. | ||
| static constexpr int kDefaultBufferSize = 250; | ||
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| /// Default R² threshold. | ||
| static constexpr double kDefaultRSquaredThreshold = 0.75; | ||
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| /** | ||
| * Creates a ResistanceCalculator with a default buffer size of 250 and R² | ||
| * threshold of 0.5. | ||
| */ | ||
| ResistanceCalculator() = default; | ||
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| /** | ||
| * Creates a ResistanceCalculator. | ||
| * | ||
| * @param bufferSize The maximum number of points to take the linear | ||
| * regression over. | ||
| * @param rSquaredThreshold The minimum R² value (0 to 1) considered | ||
| * significant enough to return the regression slope instead of NaN. A | ||
| * lower threshold allows resistance to be returned even with noisier | ||
| * data. | ||
| */ | ||
| ResistanceCalculator(int bufferSize, double rSquaredThreshold); | ||
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| ResistanceCalculator(ResistanceCalculator&&) = default; | ||
| ResistanceCalculator& operator=(ResistanceCalculator&&) = default; | ||
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| /** | ||
| * Update the buffers with new (current, voltage) points, and remove old | ||
| * points if necessary. | ||
| * | ||
| * @param current The current current | ||
| * @param voltage The current voltage | ||
| * @return The current resistance in ohms | ||
| */ | ||
| units::ohm_t Calculate(units::ampere_t current, units::volt_t voltage); | ||
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| private: | ||
| /** | ||
| * Operator to apply to OnlineCovariance<>::Calculate() inputs. | ||
| */ | ||
| enum class Operator { | ||
| /// Add point. | ||
| kAdd, | ||
| /// Remove point. | ||
| kRemove | ||
| }; | ||
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| template <typename X, typename Y> | ||
| class OnlineCovariance { | ||
| public: | ||
| /** | ||
| * The previously calculated covariance. | ||
| */ | ||
| decltype(auto) GetCovariance() const { return m_cov / (m_n - 1); } | ||
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| /** | ||
| * Calculate the covariance based on a new point that may be removed or | ||
| * added. | ||
| * | ||
| * @param x The x value of the point. | ||
| * @param y The y value of the point. | ||
| * @param op Operator to apply with the point. | ||
| * @return The new sample covariance. | ||
| */ | ||
| decltype(auto) Calculate(X x, Y y, Operator op) { | ||
| // From | ||
| // https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Covariance | ||
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| X dx = x - m_xMean; | ||
| Y dy = y - m_yMean; | ||
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| if (op == Operator::kAdd) { | ||
| ++m_n; | ||
| m_xMean += dx / m_n; | ||
| m_yMean += dy / m_n; | ||
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| // This is supposed to be (y - yMean) and not dy | ||
| m_cov += dx * (y - m_yMean); | ||
| } else if (op == Operator::kRemove) { | ||
| --m_n; | ||
| m_xMean -= dx / m_n; | ||
| m_yMean -= dy / m_n; | ||
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| // This is supposed to be (y - yMean) and not dy | ||
| m_cov -= dx * (y - m_yMean); | ||
| } | ||
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| // Correction for sample variance | ||
| return m_cov / (m_n - 1); | ||
| } | ||
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| private: | ||
| /// Number of points covariance is calculated over. | ||
| int m_n = 0; | ||
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| /// Current mean of x values. | ||
| X m_xMean{0.0}; | ||
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| /// Current mean of y values. | ||
| Y m_yMean{0.0}; | ||
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| /// Current approximated population covariance. | ||
| decltype(std::declval<X>() * std::declval<Y>()) m_cov{0.0}; | ||
| }; | ||
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| /// Buffers holding the current values that will eventually need to be | ||
| /// subtracted from the sum when they leave the window. | ||
| wpi::circular_buffer<units::ampere_t> m_currentBuffer{kDefaultBufferSize}; | ||
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| /// Buffer holding the voltage values that will eventually need to be | ||
| /// subtracted from the sum when they leave the window. | ||
| wpi::circular_buffer<units::volt_t> m_voltageBuffer{kDefaultBufferSize}; | ||
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| /// The maximum number of points to take the linear regression over. | ||
| int m_bufferSize = kDefaultBufferSize; | ||
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| /// The minimum R² value considered significant enough to return the | ||
| /// regression slope instead of NaN. | ||
| double m_rSquaredThreshold = kDefaultRSquaredThreshold; | ||
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| /// Used for approximating current variance. | ||
| OnlineCovariance<units::ampere_t, units::ampere_t> m_currentVariance; | ||
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| /// Used for approximating voltage variance. | ||
| OnlineCovariance<units::volt_t, units::volt_t> m_voltageVariance; | ||
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| /// Used for approximating covariance of current and voltage. | ||
| OnlineCovariance<units::ampere_t, units::volt_t> m_covariance; | ||
| }; | ||
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| } // namespace frc |
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