Hi Radu, Thanks for the reply and the update. This can be very helpful. The problem we found is that the tire frame is defined in ChPac02Tire.cpp is SAE frame so we need to make the slip angle negative and the negative back the lateral forces. By doing this, the error is less than 20N and that error could be result from camber angle or the problem you mentioned.
Thanks again for the answer and fix! Siyuan On Tuesday, February 21, 2023 at 5:11:01 AM UTC-8 Radu Serban wrote: > Siyuan, > > > > The differences you see between the Chrono and Matlab results could be > possibly explained by a delay of 1 step between the calculation of > tire-terrain geometry and calculation of tire forces (the reasons why this > is happening in Chrono are subtle and beyond the scope of this discussion). > Of course, there are many other possible sources for these discrepancies. > > > > I just pushed code to also report the tire forces in a local tire frame. > See > https://github.com/projectchrono/chrono/blob/9c4ac1c4c1d5bd6a2cee16e6f7980e2520982204/src/chrono_vehicle/wheeled_vehicle/tire/ChForceElementTire.h#L42. > > Note that I only implemented this for classes derived from > ChForceElementTire (so Pac89, Pac02, Fiala, and TMeasy) and didn’t test it. > Please let me know if this addresses your needs. > > > > --Radu > > > > *From:* [email protected] <[email protected]> *On > Behalf Of *Siyuan Yu > *Sent:* Sunday, 5 February 2023 22:07 > *To:* ProjectChrono <[email protected]> > *Subject:* [chrono] Calculation of Tire forces in tire frame > > > > Hi Developers, > > > > We would like to ask about the calculations of tire forces in tire frame > of wheeled vehicle. The function ReportTireForce reports the tire forces in > global frame and we want to examine it in local frame. Here are our > calculations: > > > > WheelState state0 = vehicle.GetWheel(0, > VehicleSide::LEFT)->GetState(); > ChVector<> wheel_normal0 = state0.rot.GetYaxis(); > ChVector<> contact_point = ChVector<>(state0.pos.x(), > state0.pos.y(), terrain.GetHeight(state0.pos)); > ChVector<> Z_dir0 = terrain.GetNormal(contact_point); > ChVector<> X_dir0 = Vcross(wheel_normal0, Z_dir0); > X_dir0.Normalize(); > ChVector<> Y_dir0 = Vcross(Z_dir0, X_dir0); > ChMatrix33<> rot0; > rot0.Set_A_axis(X_dir0, Y_dir0, Z_dir0); > ChMatrix33<> rot0inv = rot0.transpose(); > double alpha_fl = vehicle.GetTire(0, > VehicleSide::LEFT)->GetSlipAngle(); > double kappa_fl = vehicle.GetTire(0, > VehicleSide::LEFT)->GetLongitudinalSlip(); > ChVector<> F_fl = rot0inv * (vehicle.GetTire(0, > VehicleSide::LEFT)->ReportTireForce(&terrain).force); > ChVector<> M_fl = rot0inv * (vehicle.GetTire(0, > VehicleSide::LEFT)->ReportTireForce(&terrain).moment); > > > > This calculation meets our expectation to some extent. We reimplement the > exact same Pacejka tire force as used in Project Chrono. And we use the > exact tire force parameters. To make sure it's not the camber angle > problem, we override the camber angle in ChPac02tire.cpp and make it 0 all > the time. Our expectations are that they should behave the exactly the > same. But the calculation shows that there's an offset when we are using > our matlab calculation. > > It is not a big deviation in terms of tire force, but it does matter when > we are using this in yaw rate calculation. > > > > The other one is to use the spindle's rotation to transform the tire > forces. That result huge deviations, so we suspect we got it wrong > somewhere. Here's the code: > > ChQuaternion<> rot0inv = vehicle.GetSpindleRot(0, > VehicleSide::LEFT); > double alpha_fl = vehicle.GetTire(0, > VehicleSide::LEFT)->GetSlipAngle(); > double kappa_fl = vehicle.GetTire(0, > VehicleSide::LEFT)->GetLongitudinalSlip(); > ChVector<> F_fl = rot0inv.Rotate((vehicle.GetTire(0, > VehicleSide::LEFT)->ReportTireForce(&terrain).force)); > ChVector<> M_fl = rot0inv.Rotate(vehicle.GetTire(0, > VehicleSide::LEFT)->ReportTireForce(&terrain).moment); > > > > We are wondering what is the exact correct way to get the tire forces in > tire frame. > > > > Thanks in advance! > > Siyuan > > > > > > > > -- > You received this message because you are subscribed to the Google Groups > "ProjectChrono" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/projectchrono/8f4acfa4-81a7-411c-9490-cff47d94f77bn%40googlegroups.com > > <https://groups.google.com/d/msgid/projectchrono/8f4acfa4-81a7-411c-9490-cff47d94f77bn%40googlegroups.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "ProjectChrono" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/projectchrono/f7659b3b-ad4f-4f8f-8397-b84b807b954cn%40googlegroups.com.
