<!-- GENERATED by tools/generators/examplesDocsEmitter.py from python/ - do not edit -->
(testmodels-kinematictreeuserfunctiontest)=
# kinematicTreeUserFunctionTest.py

You can view and download this file on Github: [kinematicTreeUserFunctionTest.py](https://github.com/jgerstmayr/EXUDYN/tree/master/python/TestModels/kinematicTreeUserFunctionTest.py)

```{code-block} python
:linenos:

#%%+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
# This is an EXUDYN example
#
# Details:  The forceUserFunction of ObjectKinematicTree, which receives the joint coordinates q and velocities q_t
#           (#2845): two equal trees of a revolute and a prismatic joint under gravity, the first damped by the D control
#           of the tree (jointDControlVector), the second by a user function that returns the same forces from q_t;
#           both must move alike.
#
# Author:   Johannes Gerstmayr
# Date:     2026-10-04
#
# Copyright:This file is part of Exudyn. Exudyn is free software. You can redistribute it and/or modify it under the terms of the Exudyn license. See 'LICENSE.txt' for more details.
#
#+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

import exudyn as exu
from exudyn.utilities import * #includes itemInterface and rigidBodyUtilities
import numpy as np

testIsActive = exu.sys.get('testIsActive', False)

damping = [0.8, 5.]

def Tree(mbs, userFunction=0):
    """a revolute joint about z with a link along x, then a prismatic joint along x, under gravity in -y"""
    nKT = mbs.AddNode(NodeGenericODE2(referenceCoordinates=[0, 0.2], initialCoordinates=[0.3, 0], initialCoordinates_t=[0, 0.1],
                                      numberOfODE2Coordinates=2))
    useControl = userFunction == 0
    oKT = mbs.AddObject(ObjectKinematicTree(nodeNumber=nKT, linkParents=[-1, 0],
                                            jointTypes=[exu.JointType.RevoluteZ, exu.JointType.PrismaticX],
                                            jointTransformations=exu.Matrix3DList([np.eye(3)]*2),
                                            jointOffsets=exu.Vector3DList([[0, 0, 0], [0.5, 0, 0]]),
                                            linkInertiasCOM=exu.Matrix3DList([np.eye(3)*0.01]*2),
                                            linkCOMs=exu.Vector3DList([[0.25, 0, 0], [0.1, 0, 0]]),
                                            linkMasses=[1, 2], gravity=[0, -9.81, 0],
                                            jointPositionOffsetVector=[0.1, 0.2], jointPControlVector=[50, 200],
                                            jointVelocityOffsetVector=[0, 0] if useControl else [],
                                            jointDControlVector=damping if useControl else [],
                                            forceUserFunction=userFunction))
    return nKT

#the D control as a user function: the tree subtracts what it returns, as it subtracts D*(0 - q_t)
def UFforce(mbs, t, itemNumber, q, q_t):
    return [-damping[0]*q_t[0], -damping[1]*q_t[1]]

SC = exu.SystemContainer()
mbs = SC.AddSystem()
nControl = Tree(mbs)
nUser = Tree(mbs, UFforce)
mbs.Assemble()

simulationSettings = exu.SimulationSettings()
simulationSettings.timeIntegration.numberOfSteps = 500
simulationSettings.timeIntegration.endTime = 1
simulationSettings.solution.file.write = False
mbs.SolveDynamic(simulationSettings)

qControl = mbs.GetNodeOutput(nControl, exu.OutputVariableType.Coordinates)
qUser = mbs.GetNodeOutput(nUser, exu.OutputVariableType.Coordinates)
exu.Print('q control=', qControl, ', q user function=', qUser)
result = np.sum(qControl) + (1 if np.linalg.norm(qControl - qUser) > 1e-10 else 0) #the second part must be zero

exu.Print('kinematicTreeUserFunctionTest result=', result)
exu.sys['testResult'] = result
```