Program Listing for File HuntCrossley.h

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/**
 * @file
 * @brief Hunt-Crossley contact force model.
 */

#pragma once

#include "Karana/Collision/ContactForce.h"
#include "Karana/Frame/ChainedFrameToFrame.h"

namespace Karana::Collision {

    /**
     * @class HuntCrossley
     * @brief ContactForceBase implementation using a Hunt-Crossley model
     *
     * See \sref{collision_dynamics_sec} for more discussion on
     * contact and collision dynamics.
     */
    class HuntCrossley : public ContactForceBase {
      public:
        /// Use the constructor from ContactForceBase
        using ContactForceBase::ContactForceBase;

        /**
         * @brief Create an instance of the HuntCrossley contact force model.
         *
         * @param name The name for the HuntCrossley instance.
         * @return A ks_ptr to the newly created HuntCrossley contact force model.
         */
        static kc::ks_ptr<HuntCrossley> create(std::string_view name);

        /**
         * @brief Compute the contact force for the associated contact.
         *
         * @param contact The contact to compute the force for.
         * @param nd_1 The contact node on the first body.
         * @param nd_2 The contact node on the second body.
         * @returns The contact force.
         */
        km::SpatialVector computeForce(const FrameContact &contact,
                                       const kc::ks_ptr<kd::Node> &nd_1,
                                       const kc::ks_ptr<kd::Node> &nd_2) final;

        /**
         * @class HuntCrossleyParams
         * @brief Structure that holds the HuntCrossley contact force parameters.
         */
        struct HuntCrossleyParams {
            /// Normal penetration stiffness coefficient
            double kp = km::notReadyNaN;

            /// Normal penetration restitution/damping coefficient
            double kc = km::notReadyNaN;

            /// Exponent
            double n = km::notReadyNaN;

            /// Friction coefficient
            double mu = km::notReadyNaN;

            /**
             * @brief Tolerance at which friction is linearly interpolated between the real value
             * and 0.
             *
             * To avoid jittering for cases like rolling without friction, near zero, the friction
             * force uses a linear interpolation between the full friction force and zero. This
             * avoids rapid, large changes in the friction force as the velocity changes sign, and
             * instead, smoothly, linearly interpolates between these large forces over a region of
             * linear_region_tol * 2.
             */
            double linear_region_tol = km::notReadyNaN;
        };

        /**
         * @class HuntCrossleyScratch
         * @brief Structure that holds the HuntCrossley scratch data
         */
        struct HuntCrossleyScratch {
            /// Normal penetration
            double penetration = km::notReadyNaN;

            /// contact linear velocity
            km::Vec3 linvel;

            /// contact force
            km::Vec3 f;
        };

        /// Parameters used for the contact model.
        HuntCrossleyParams params;

        /// Scratch values
        HuntCrossleyScratch scratch;

        /**
         * @brief Used to ensure the HuntCrossley contact force model is ready.
         *
         * @return true if the HuntCrossley is ready, false otherwise.
         */
        bool isReady() const final;

        std::string
        dumpString(std::string_view prefix,
                   const Karana::Core::Base::DumpOptions *options = nullptr) const override;

      private:
        /** Return the magnitude of the normal force based on the
            Hunt-Crossley model */
        /**
         * @brief Return the magnitude of the normal force based on the Hunt-Crossley model
         *
         * @param fext_dir The external force direction.
         * @return The normal force from the Hunt-Crossley contact model.
         */
        double _getHuntCrossleyNormalForce(const Karana::Math::Vec3 &fext_dir) const;

        /**
         * @brief Return the tangential force based on the Coulomb friction model.
         *
         * @param fext_dir The external force direction.
         * @param normal_fmag The magnitude of the normal force.
         */
        Karana::Math::Vec3 _getCoulombFrictionTangentForce(const Karana::Math::Vec3 &fext_dir,
                                                           double normal_fmag) const;
    };
} // namespace Karana::Collision