DefaultFriction.cpp

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/*+-------------------------------------------------------------------------+
  |                       MultiVehicle simulator (libmvsim)                 |
  |                                                                         |
  | Copyright (C) 2014-2023  Jose Luis Blanco Claraco                       |
  | Copyright (C) 2017  Borys Tymchenko (Odessa Polytechnic University)     |
  | Distributed under 3-clause BSD License                                  |
  |   See COPYING                                                           |
  +-------------------------------------------------------------------------+ */

#include <mvsim/FrictionModels/DefaultFriction.h>
#include <mvsim/VehicleBase.h>
#include <mvsim/World.h>

#include <rapidxml.hpp>

#include "xml_utils.h"

using namespace mvsim;

DefaultFriction::DefaultFriction(
        VehicleBase& my_vehicle, const rapidxml::xml_node<char>* node)
        : FrictionBase(my_vehicle), mu_(0.8), C_damping_(1.0)
{
        // Sanity: we can tolerate node==nullptr (=> means use default params).
        if (node && 0 != strcmp(node->name(), "friction"))
                throw std::runtime_error(
                        "<friction>...</friction> XML node was expected!!");

        // Parse XML params:
        if (node)
                parse_xmlnode_children_as_param(
                        *node, params_, world_->user_defined_variables());
}

// See docs in base class.
mrpt::math::TVector2D DefaultFriction::evaluate_friction(
        const FrictionBase::TFrictionInput& input) const
{
        // Rotate wheel velocity vector from veh. frame => wheel frame
        const mrpt::poses::CPose2D wRot(0, 0, input.wheel.yaw);

        // Velocity of the wheel cog in the frame of the wheel itself:
        const mrpt::math::TVector2D vel_w =
                wRot.inverseComposePoint(input.wheelCogLocalVel);

        // Action/Reaction, slippage, etc:
        // --------------------------------------
        const double mu = mu_;
        const double gravity = myVehicle_.parent()->get_gravity();
        const double partial_mass = input.weight / gravity + input.wheel.mass;
        const double max_friction = mu * partial_mass * gravity;

        // 1) Lateral friction (decoupled sub-problem)
        // --------------------------------------------
        double wheel_lat_friction = 0.0;  // direction: +y local wrt the wheel
        {
                // Impulse required to step the lateral slippage:
                wheel_lat_friction = -vel_w.y * partial_mass / input.context.dt;

                wheel_lat_friction =
                        b2Clamp(wheel_lat_friction, -max_friction, max_friction);
        }

        // 2) Longitudinal friction (decoupled sub-problem)
        // -------------------------------------------------
        double wheel_long_friction = 0.0;  // direction: +x local wrt the wheel

        // (eq. 1)==> desired impulse in wheel spinning speed.
        // wheel_C_lon_vel = vel_w.x - input.wheel.w * 0.5*input.wheel.diameter

        // It should be = 0 for no slippage (nonholonomic constraint): find out
        // required wheel \omega:case '4':
        const double R = 0.5 * input.wheel.diameter;  // Wheel radius
        const double lon_constraint_desired_wheel_w = vel_w.x / R;

        const double desired_wheel_w_impulse =
                (lon_constraint_desired_wheel_w - input.wheel.getW());

        const double desired_wheel_alpha =
                desired_wheel_w_impulse / input.context.dt;

        // (eq. 3)==> Find out F_r
        // Iyy_w * \Delta\omega_w = dt*\tau-  R*dt*Fri    -C_damp * \omega_w * dt
        // "Damping" / internal friction of the wheel's shaft, etc.
        const double C_damping = C_damping_;
        // const mrpt::math::TPoint2D wheel_damping(- C_damping *
        // input.wheel_speed.x, 0.0);

        const double I_yy = input.wheel.Iyy;
        double F_friction_lon = (input.motorTorque - I_yy * desired_wheel_alpha -
                                                         C_damping * input.wheel.getW()) /
                                                        R;

        // Slippage: The friction with the ground is not infinite:
        F_friction_lon = b2Clamp(F_friction_lon, -max_friction, max_friction);

        // Recalc wheel ang. velocity impulse with this reduced force:
        const double actual_wheel_alpha = (input.motorTorque - R * F_friction_lon -
                                                                           C_damping * input.wheel.getW()) /
                                                                          I_yy;

        // Apply impulse to wheel's spinning:
        input.wheel.setW(
                input.wheel.getW() + actual_wheel_alpha * input.context.dt);

        wheel_long_friction = F_friction_lon;

        // Resultant force: In local (x,y) coordinates (Newtons) wrt the Wheel
        // -----------------------------------------------------------------------
        const mrpt::math::TPoint2D result_force_wrt_wheel(
                wheel_long_friction, wheel_lat_friction);

        // Rotate to put: Wheel frame ==> vehicle local framework:
        mrpt::math::TVector2D res;
        wRot.composePoint(result_force_wrt_wheel, res);
        return res;
}