Merge pull request #620 from peternewell/wheel-slip

Adjust kinietic friction calculations

Author: peternewell

Source/Orts.Simulation/Simulation/RollingStocks/MSTSLocomotive.cs

@@ -42,7 +42,7 @@
 //#define ALLOW_ORTS_SPECIFIC_ENG_PARAMETERS
 
 // Debug for Advanced Adhesion Model
-//#define DEBUG_ADHESION
+// #define DEBUG_ADHESION
 
 using Microsoft.Xna.Framework;
 using Microsoft.Xna.Framework.Graphics;
@@ -215,10 +215,18 @@ public float CurrentLocomotiveSteamHeatBoilerWaterCapacityL
 
         // Adhesion parameters
         float BaseFrictionCoefficientFactor;  // Factor used to adjust Curtius formula depending upon weather conditions
+        float SlipFrictionCoefficientFactor;
         public float SteamStaticWheelForce;
         public float SteamTangentialWheelForce;
         public float SteamDrvWheelWeightLbs;  // Weight on each drive axle
         public float PreviousThrottleSetting = 0.0f;  // Holds the value of the previous throttle setting for calculating the correct antislip speed
+        float WheelSlipTimeS;
+        float WheelStopSlipTimeS;
+        float CurrentWheelSlipAdhesionMultiplier;
+        float DebugTimer; // Used for debugging adhesion coefficient
+        bool DebugSpeedReached = false; // Used for debugging adhesion coefficient
+        float DebugSpeedIncrement = 1; // Used for debugging adhesion coefficient
+        float DebugSpeed = 1; // Used for debugging adhesion coefficient
 
         // parameters for Track Sander based upon compressor air and abrasive table for 1/2" sand blasting nozzle @ 50psi
         public float MaxTrackSandBoxCapacityM3 = Me3.FromFt3(40.0f);  // Capacity of sandbox - assume 40.0 cu ft
@@ -2901,43 +2909,60 @@ public virtual void UpdateWaterTroughRefill(float elapsedClockSeconds, float abs
         public virtual void UpdateFrictionCoefficient(float elapsedClockSeconds)
         {
             float BaseuMax = (Curtius_KnifflerA / (MpS.ToKpH(AbsSpeedMpS) + Curtius_KnifflerB) + Curtius_KnifflerC); // Base Curtius - Kniffler equation - u = 0.33, all other values are scaled off this formula
-            float SandingFrictionCoefficientFactor = 0.0f;
-            //Set the friction coeff due to weather
-            if (Simulator.WeatherType == WeatherType.Rain || Simulator.WeatherType == WeatherType.Snow)
+            float SandingFrictionCoefficientFactor = 1.0f;
+
+            //Set the friction coeff due to weather - uses the vlaues set in the precipitation module to determine whether clear, rain or snow.
+
+            // Adjust clear weather for precipitation presence, ie base value between 60% and 80% 
+            // note lowest friction will be for drizzle (light) rain; friction will increase for precipitation higher than drizzle rail
+            if (!Simulator.Paused)
             {
-                if (Train.SlipperySpotDistanceM < 0)
+                var fogBaseFrictionCoefficientFactor = 0.0f;
+                var pricBaseFrictionCoefficientFactor = 0.0f;
+                float pric = Simulator.Weather.PricipitationIntensityPPSPM2 * 1000;
+                // precipitation will calculate a base coefficient value between 60% (light rain) and 80% (heavy rain) - this will be a factor that is used to adjust the base value - assume linear value between upper and lower precipitation values
+                if (pric >= 0.5)
+                    pricBaseFrictionCoefficientFactor = Math.Min((pric - 0.5f) * 0.0078f + 0.6f, 0.8f); // should give a value between 0.6 and 0.8
+                else
+                    pricBaseFrictionCoefficientFactor = 0.6f + 0.8f * (0.5f - pric); // should give a transition value between 1.0 and 0.6 as rain starts
+
+                // Adjust adhesion for impact of fog - default = 20000m = 20km
+                float fog = Simulator.Weather.FogDistance;
+                if (fog < 20000) // as fog thickens then decrease adhesion
                 {
-                    Train.SlipperySpotLengthM = 10 + 40 * (float)Simulator.Random.NextDouble();
-                    Train.SlipperySpotDistanceM = Train.SlipperySpotLengthM + 2000 * (float)Simulator.Random.NextDouble();
+                    fogBaseFrictionCoefficientFactor = Math.Min((fog * 2.75e-4f + 0.6f), 1.0f); // If fog is less then 2km then it will impact friction, decrease adhesion to 60% (same as light rain transition)
                 }
-                if (Train.SlipperySpotDistanceM < Train.SlipperySpotLengthM)
+                else
                 {
-                    BaseFrictionCoefficientFactor = 0.8f;
+                    fogBaseFrictionCoefficientFactor = 1;
                 }
-                if (Simulator.WeatherType == WeatherType.Rain) // Wet weather
+
+                BaseFrictionCoefficientFactor = Math.Min(fogBaseFrictionCoefficientFactor, pricBaseFrictionCoefficientFactor);
+            }
+
+            // Random slippery track
+            if (Train.SlipperySpotDistanceM < 0)
+            {
+                Train.SlipperySpotLengthM = 10 + 40 * (float)Simulator.Random.NextDouble();
+                Train.SlipperySpotDistanceM = Train.SlipperySpotLengthM + 2000 * (float)Simulator.Random.NextDouble();
+            }
+            if (Train.SlipperySpotDistanceM < Train.SlipperySpotLengthM)
+            {
+                if (BaseFrictionCoefficientFactor > 0.6 && BaseFrictionCoefficientFactor < 0.8)
                 {
-                    if (Simulator.Settings.AdhesionProportionalToWeather && AdvancedAdhesionModel && !Simulator.Paused)  // Adjust clear weather for precipitation presence - base friction value will be approximately between 0.15 and 0.2
-                    // ie base value between 0.8 and 1.0 (TODO) 
-                    // note lowest friction will be for drizzle rain; friction will increase for precipitation both higher and lower than drizzle rail
-                    {
-                        float pric = Simulator.Weather.PricipitationIntensityPPSPM2 * 1000;
-                        // precipitation will calculate a value between 0.15 (light rain) and 0.2 (heavy rain) - this will be a factor that is used to adjust the base value - assume linear value between upper and lower precipitation values
-                        if (pric >= 0.5)
-                            BaseFrictionCoefficientFactor = Math.Min((pric * 0.0078f + 0.45f), 0.8f); // should give a minimum value between 0.8 and 1.0
-                        else
-                            BaseFrictionCoefficientFactor = Math.Min((0.4539f + 1.0922f * (0.5f - pric)), 0.8f); // should give a minimum value between 0.8 and 1.0
-                    }
-                    else // if not proportional to precipitation use fixed friction value of 0.8 x friction coefficient of 0.33
-                    {
-                        BaseFrictionCoefficientFactor = 0.8f;
-                    }
+                    BaseFrictionCoefficientFactor = 0.6f;
                 }
-                else     // Snow weather
+                else
                 {
-                    BaseFrictionCoefficientFactor = 0.6f;
+                    BaseFrictionCoefficientFactor = 0.8f;
                 }
+            }
+
+            BaseFrictionCoefficientFactor = MathHelper.Clamp(BaseFrictionCoefficientFactor, 0.5f, 1.0f); 
 
-                //add sander - more effective in wet weather, so increases adhesion by more
+            if (Simulator.WeatherType == WeatherType.Rain || Simulator.WeatherType == WeatherType.Snow)
+            {
+                //sander - more effective in wet weather, so increases adhesion by more
                 if (AbsSpeedMpS < SanderSpeedOfMpS && CurrentTrackSandBoxCapacityM3 > 0.0 && MainResPressurePSI > 80.0 && (AbsSpeedMpS > 0))
                 {
                     if (SanderSpeedEffectUpToMpS > 0.0f)
@@ -2946,7 +2971,7 @@ public virtual void UpdateFrictionCoefficient(float elapsedClockSeconds)
                         {
                             SandingFrictionCoefficientFactor = (1.0f - 0.5f / SanderSpeedEffectUpToMpS * AbsSpeedMpS) * 1.75f;
                             BaseFrictionCoefficientFactor *= SandingFrictionCoefficientFactor;
-                            
+
                         }
                     }
                     else
@@ -2959,27 +2984,9 @@ public virtual void UpdateFrictionCoefficient(float elapsedClockSeconds)
                     }
                 }
             }
-            else // Default to Dry (Clear) weather
+            else // dry weather
             {
-
-                if (Simulator.Settings.AdhesionProportionalToWeather && AdvancedAdhesionModel && !Simulator.Paused)  // Adjust clear weather for fog presence
-                {
-                    float fog = Simulator.Weather.FogDistance;
-                    if (fog > 2000)
-                    {
-                        BaseFrictionCoefficientFactor = 1.0f; // if fog is not too thick don't change the friction
-                    }
-                    else
-                    {
-                        BaseFrictionCoefficientFactor = Math.Min((fog * 2.75e-4f + 0.8f), 0.8f); // If fog is less then 2km then it will impact friction, decrease adhesion by up to 20% (same as clear to wet transition)
-                    }                                        
-                }
-                else // if not proportional to fog use fixed friction value approximately equal to 0.33, thus factor will be 1.0 x friction coefficient of 0.33
-                {
-                    BaseFrictionCoefficientFactor = 1.0f;
-                }
-
-                //add sander - not as effective in dry weather
+                //sander - not as effective in dry weather
                 if (AbsSpeedMpS < SanderSpeedOfMpS && CurrentTrackSandBoxCapacityM3 > 0.0 && MainResPressurePSI > 80.0 && (AbsSpeedMpS > 0))
                 {
                     if (SanderSpeedEffectUpToMpS > 0.0f)
@@ -2999,6 +3006,7 @@ public virtual void UpdateFrictionCoefficient(float elapsedClockSeconds)
                         }
                     }
                 }
+
             }
 
             // For wagons use base Curtius-Kniffler adhesion factor - u = 0.33
@@ -3016,13 +3024,54 @@ public virtual void UpdateFrictionCoefficient(float elapsedClockSeconds)
 
             }
 
-            if (WheelSlip && ThrottlePercent > 0.2f && !BrakeSkid)   // Test to see if loco wheel is slipping, then coeff of friction will be decreased below static value. Sanding will override this somewhat
+            // When wheel slips or skids, then dynamic (kinetic) coeff of friction will be decreased below static value. Sanding will override this somewhat.
+            // The transition between static and dynamic friction appears to decrease at an exponential rate until it reaches a steady state dynamic value.
+            // 
+
+
+            // Test to see if loco wheel is slipping or skidding due to brake application
+            if ((EngineType == EngineTypes.Steam && SteamEngineType != MSTSSteamLocomotive.SteamEngineTypes.Geared) && WheelSlip && ((ThrottlePercent > 0.2f && !BrakeSkid) || (ThrottlePercent < 0.1f && BrakeSkid)))   
             {
-                BaseFrictionCoefficientFactor = 0.15f * SandingFrictionCoefficientFactor;  // Descrease friction to take into account dynamic (kinetic) friction U = 0.0525
+
+                WheelStopSlipTimeS = 0; // Reset stop slip time if wheel slip starts
+
+                // Exponential curve is used to transition between static friction and dynamic friction when wheel slips
+                // Exponential constant calculated between two points, using this tool - https://mathcracker.com/exponential-function-calculator#results
+                // Google search suggests that Steel on steel has a static coeff = 0.74, and a dynamic coeff = 0.57. Hence reduction = 0.77.
+                // Constant points facilitate a decrease from 1 to 0.7 in 3 seconds - P1 = (0, 1), P2 = (5, 0.77). Hence exp constant = −0.0523
+                var expAdhesion = -0.0523;
+                WheelSlipTimeS += elapsedClockSeconds;
+                WheelSlipTimeS = MathHelper.Clamp(WheelSlipTimeS, 0.0f, 5.0f); // Ensure that time to transition between the two friction cases is maintained - currently set to 3 secs
+
+                float adhesionMultiplier = (float) Math.Exp(expAdhesion * WheelSlipTimeS);
+                CurrentWheelSlipAdhesionMultiplier = adhesionMultiplier;
+
+                BaseFrictionCoefficientFactor *= adhesionMultiplier;  // Descrease friction to take into account dynamic (kinetic) friction, typically kinetic friction is approximately 50% of static friction.
+                SlipFrictionCoefficientFactor = BaseFrictionCoefficientFactor;
+
+                BaseFrictionCoefficientFactor = MathHelper.Clamp(BaseFrictionCoefficientFactor, 0.05f, 1.0f); // Ensure friction coefficient never exceeds a "reasonable" value
             }
-            else if (WheelSlip && ThrottlePercent < 0.1f && BrakeSkid) // Test to see if loco wheel is skidding due to brake application
+            else
             {
-                BaseFrictionCoefficientFactor = 0.15f * SandingFrictionCoefficientFactor;  // Descrease friction to take into account dynamic (kinetic) friction U = 0.0525
+                WheelSlipTimeS = 0; // Reset slip time if wheel slip stops
+
+                if ((EngineType == EngineTypes.Steam && SteamEngineType != MSTSSteamLocomotive.SteamEngineTypes.Geared) && SlipFrictionCoefficientFactor < BaseFrictionCoefficientFactor && SlipFrictionCoefficientFactor != 0) // Once these two are equal then assume that wheels have stopped slipping.
+                {
+                    //                    Trace.TraceInformation("SlipFriction {0} Base {1}", SlipFrictionCoefficientFactor, BaseFrictionCoefficientFactor);
+                    // Exponential curve is used to transition between dynamic friction and static friction when wheel stops slipping
+                    // Constant points facilitate an increase from 0.7 to 1 in 3 seconds - P1 = (5, 0.77), P2 = (0, 1). Hence exp constant = 0.0523
+                    var expAdhesion = 0.0523;
+                    WheelStopSlipTimeS += elapsedClockSeconds;
+                    WheelStopSlipTimeS = MathHelper.Clamp(WheelStopSlipTimeS, 0.0f, 5.0f); // Ensure that time to transition between the two friction cases is maintained - currently set to 3 secs
+
+                    float adhesionMultiplier = CurrentWheelSlipAdhesionMultiplier * (float)Math.Exp(expAdhesion * WheelStopSlipTimeS);
+
+//                    Trace.TraceInformation("adhesion {0} StopTime {1} Base {2} Current {3}", adhesionMultiplier, WheelStopSlipTimeS, BaseFrictionCoefficientFactor, CurrentWheelSlipAdhesionMultiplier);
+
+                    BaseFrictionCoefficientFactor *= adhesionMultiplier;  // Descrease friction to take into account dynamic (kinetic) friction, typically kinetic friction is approximately 50% of static friction.
+                    SlipFrictionCoefficientFactor = BaseFrictionCoefficientFactor;
+                    BaseFrictionCoefficientFactor = MathHelper.Clamp(BaseFrictionCoefficientFactor, 0.05f, 1.0f); // Ensure friction coefficient never exceeds a "reasonable" value
+                }
             }
 
             var AdhesionMultiplier = Simulator.Settings.AdhesionFactor / 100.0f; // Convert to a factor where 100% = no change to adhesion