Update slip adhesion

Author: peternewell

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

@@ -215,10 +215,14 @@ 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;
 
         // 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
@@ -3016,13 +3020,54 @@ 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 (WheelSlip && ((ThrottlePercent > 0.2f && !BrakeSkid) || (ThrottlePercent < 0.1f && BrakeSkid)))   
             {
-                BaseFrictionCoefficientFactor *= 0.5f;  // Descrease friction to take into account dynamic (kinetic) friction, typically kinetic friction is approximately 50% of static friction.
+
+                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 = (3, 0.77). Hence exp constant = −0.0871
+                var expAdhesion = -0.0871;
+                WheelSlipTimeS += elapsedClockSeconds;
+                WheelSlipTimeS = MathHelper.Clamp(WheelSlipTimeS, 0.0f, 3.0f); // 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 *= 0.5f;  // Descrease friction to take into account dynamic (kinetic) friction, typically kinetic friction is approximately 50% of static friction.
+                WheelSlipTimeS = 0; // Reset slip time if wheel slip stops
+
+                if (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 = (3, 0.77), P2 = (0, 1). Hence exp constant = 0.0871
+                    var expAdhesion = 0.0871;
+                    WheelStopSlipTimeS += elapsedClockSeconds;
+                    WheelStopSlipTimeS = MathHelper.Clamp(WheelStopSlipTimeS, 0.0f, 3.0f); // 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

Source/Orts.Simulation/Simulation/RollingStocks/MSTSSteamLocomotive.cs

@@ -4652,7 +4652,7 @@ protected override void UpdateTractiveForce(float elapsedClockSeconds, float t,
 
             #region - Steam Adhesion Model Input for Steam Locomotives
 
-            // Based upon information presented in "Locomotive Operation - A Technical and Practical Analysis" by G. R. Henderson - https://archive.org/details/locomotiveoperat00hend
+            // Based upon information presented on pg 276 of "Locomotive Operation - A Technical and Practical Analysis" by G. R. Henderson - https://archive.org/details/locomotiveoperat00hend
             // At its simplest slip occurs when the wheel tangential force exceeds the static frictional force
             // Static frictional force = weight on the locomotive driving wheels * frictional co-efficient
             // Tangential force = Effective force (Interia + Piston force) * Tangential factor (sin (crank angle) + (crank radius / connecting rod length) * sin (crank angle) * cos (crank angle))
@@ -4979,29 +4979,36 @@ protected override void UpdateTractiveForce(float elapsedClockSeconds, float t,
                 // If locomotive slip is occuring, set parameters to reduce motive force (pulling power), and set wheel rotational speed for wheel viewers
                 if (IsLocoSlip)
                 {
-
+                    // Based upon information presented in "Locomotive Operation - A Technical and Practical Analysis" by G. R. Henderson - https://archive.org/details/locomotiveoperat00hend
                     // This next section caluclates the turning speed for the wheel if slip occurs. It is based upon the force applied to the wheel and the moment of inertia for the wheel
                     // A Generic wheel profile is used, so results may not be applicable to all locomotive, but should provide a "reasonable" guestimation
                     // Generic wheel assumptions are - 80 inch drive wheels ( 2.032 metre), a pair of drive wheels weighs approx 6,000lbs, axle weighs 1,000 lbs, and has a diameter of 8 inches.
                     // Moment of Inertia (Wheel and axle) = (Mass x Radius) / 2.0
+                    // Reference model assumption
                     float WheelRadiusAssumptM = Me.FromIn(80.0f / 2.0f);
                     float WheelWeightKG = Kg.FromLb(6000.0f);
                     float AxleWeighKG = Kg.FromLb(1000.0f);
                     float AxleRadiusM = Me.FromIn(8.0f / 2.0f);
+
+                    // Take into account the actual lcomotive that we are modelling, and vary the above values by the ratio of drive wheel size.
+                    var wheelSizeRatio = DriverWheelRadiusM / WheelRadiusAssumptM;
+                    WheelRadiusAssumptM *= wheelSizeRatio;
+                    WheelWeightKG *= wheelSizeRatio;
+                    AxleWeighKG *= wheelSizeRatio;
+                    AxleRadiusM *= wheelSizeRatio;
+
                     float WheelMomentInertia = (WheelWeightKG * WheelRadiusAssumptM * WheelRadiusAssumptM) / 2.0f;
                     float AxleMomentInertia = (WheelWeightKG * AxleRadiusM * AxleRadiusM) / 2.0f;
                     float TotalWheelMomentofInertia = WheelMomentInertia + AxleMomentInertia; // Total MoI for generic wheel
 
-                    // The moment of inertia will be adjusted up or down compared to the size of the wheel on the player locomotive compared to the Generic wheel                
-                    TotalWheelMomentofInertia *= DriverWheelRadiusM / WheelRadiusAssumptM;
-
                     // The moment of inertia needs to be increased by the number of wheel sets
                     TotalWheelMomentofInertia *= LocoNumDrvAxles;
 
                     // the inertia of the coupling rods can also be added
                     // Assume rods weigh approx 1500 lbs
                     // // MoI = rod weight x stroke radius (ie stroke / 2)
                     float RodWeightKG = Kg.FromLb(1500.0f);
+                    RodWeightKG *= wheelSizeRatio;
                     // ???? For both compound and simple??????
                     float RodStrokeM = CylinderStrokeM / 2.0f;
                     float RodMomentInertia = RodWeightKG * RodStrokeM * RodStrokeM;
@@ -5010,11 +5017,19 @@ protected override void UpdateTractiveForce(float elapsedClockSeconds, float t,
 
                     // angular acceleration = (sum of forces * wheel radius) / moment of inertia
                     float AngAccRadpS2 = (N.FromLbf(SteamTangentialWheelForce - SteamStaticWheelForce) * DriverWheelRadiusM) / TotalMomentInertia;
+
+ //                   Trace.TraceInformation("AngAcc {0}  Tang {1} Static {2} Radius {3} Inertia {4} WheelSpeed {5}", AngAccRadpS2, N.FromLbf(SteamTangentialWheelForce), N.FromLbf(SteamStaticWheelForce), DriverWheelRadiusM, TotalMomentInertia, FrictionWheelSpeedMpS);
                     // tangential acceleration = angular acceleration * wheel radius
                     // tangential speed = angular acceleration * time
                     PrevFrictionWheelSpeedMpS = FrictionWheelSpeedMpS; // Save current value of wheelspeed
                     // Speed = current velocity + acceleration * time
-                    FrictionWheelSpeedMpS += (AngAccRadpS2 * DriverWheelRadiusM * elapsedClockSeconds);  // increase wheel speed whilever wheel accelerating
+                    var wheelSpeedRadpS = (AngAccRadpS2 * elapsedClockSeconds);
+                    // Convert from radpS to MpS for the wheel size
+                    var revsPs = wheelSpeedRadpS / (2.0f * (float)Math.PI); // Convert rads to revs
+                    var diffWheelSpeedMpS = revsPs * 2.0f * (float)Math.PI * DriverWheelRadiusM;
+                    
+                    // each wheel rev will travel the circumference of the wheel
+                    FrictionWheelSpeedMpS += diffWheelSpeedMpS;  // change wheel speed whilever wheel accelerating/decelerating
                     FrictionWheelSpeedMpS = MathHelper.Clamp(FrictionWheelSpeedMpS, 0.0f, 62.58f);  // Clamp wheel speed at maximum of 140mph (62.58 m/s)
 
                     WheelSlip = true;  // Set wheel slip if locomotive is slipping