If the model fails with a Newton relaxation too small. please consider these points

  • This error means that the solver failed to make the force residual lower than the ForceTol defined in the AnyBodyStudy.

  • When you run the FDK it will start by inserting the driver positions for the DOF you have made force dependent. The first thing which happens is that the kinematics is solved. Then it will then start to alter the FDK DOF to get rid of the residuals if that fails you will get a forced error like the one above.

  • If you only have forces i.e. linear translations to be force-dependent then the number of the force error is in Newton if you have it mixed with rotational of the number can also be in Nm.

  • If you increase the ForceTol it simply means that there will be a higher residual force in the constraints you are trying to make zero or reduce to be below the limit you give in the ForceTol.

  • The remaining residual in the FDK of can be seen in the driver function of the FDK DOF. Keep in mind that this error be an artificial reaction in the model which has not cost for the human model, so it is recommended to have it low.

  • There is a practical limit to how low the ForceTol can be, essentially the stiffer structures appearing around the FDK DOF the higher this tolerance needs to be because the kinematic tolerance cannot be infinitely small. The ForceTol cannot be smaller than the force difference arising from moving the FDK DOF with the magnitude of the kinematic tolerance.

  • If the model fails at the first-time step consider altering the initial position of the DOF you have as FDK so that the initial position is closer to where you expect the solution to end. Example: you have a revolute knee joint which should be FDK on one or several of the linear DOF. In the revolute joint, you cannot alter the driver positions these will be zero always. So instead replace the joint with an AnyKinLinear and AnyKinRotational drove by an AnyKinEqSimpleDriver, in this way you can alter the DriverPos value to non-zero values that can be closer to the solution. If the model contains surface to contact, it can be a good idea to ensure a small amount of initial contact.




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