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(sec-item-objectjointsliding)=
# ObjectJointSliding

A specialized 3D sliding joint between a list of beam elements (updated marker1) and a position-based marker (marker0); the data coordinate x[0] provides the current index in slidingMarkerNumbers, and x[1] the local position in the cable element at the beginning of the timestep.

## Interface

- Python names: `ObjectJointSliding` or `SlidingJoint`, and `VSlidingJoint` for its visualization
- Nodes it takes: [](#sec-item-nodegenericdata)

## Parameters

The parameters of the item; in a dictionary, its type is 'JointSliding':

| name | type | size | default | description |
|---|---|---|---|---|
| **name** | String |  | '' | constraints's unique name |
| **markerNumbers** | ArrayMarkerIndex |  | [ invalid (-1), invalid (-1) ] | (symbol: $[m0,m1]\tp$) marker m0: position or rigid body marker of mass point or rigid body; marker m1: updated marker to Cable2D element, where the sliding joint currently is attached to; must be initialized with an appropriate (global) marker number according to the starting position of the sliding object; this marker changes with time (PostNewtonStep) |
| **slidingMarkerNumbers** | ArrayMarkerIndex |  | [] | (symbol: $[m_{s0}, \ldots, m_{sn}]\tp$) these markers are used to update marker m1, if the sliding position exceeds the current cable's range; the markers must be sorted such that marker $m_{si}$ at x=cable(i).length is equal to marker(i+1) at x=0 of cable(i+1) |
| **slidingMarkerOffsets** | Vector |  | [] | (symbol: $[d_{s0}, \ldots, d_{sn}]$) this list contains the offsets of every sliding object (given by slidingMarkerNumbers) w.r.t. to the initial position (0): marker m0: offset=0, marker m1: offset=Length(cable0), marker m2: offset=Length(cable0)+Length(cable1), ... |
| **nodeNumber** | NodeIndex |  | invalid (-1) | (symbol: $n_{GD}$) node number of a NodeGenericData for 1 dataCoordinate showing the according marker number which is currently active and the start-of-step (global) sliding position |
| **constrainRotations** | ArrayIndex | 3 | [1,1,1] | flags for constrained rotation about x, y and z-axis: if flag=1, add constraint on rotation of marker m0 relative to respective axis; flag=0: sliding body can rotate freely about this axis; for ANCFCable, rotation about x-axis cannot be constrained |
| **constrainTranslations** | ArrayIndex | 3 | [1,1,1] | flags for constrained translation in x, y and z-direction: if flag=1, add constraint on translation of marker m0 relative to respective axis; flag=0: sliding body can translate freely about this axis; along x-axis this should be usually 0, except for driven motion |
| **axialForce** | Real |  | 0 | (symbol: $f_\mathrm{ax}$) ONLY APPLIES if useClassicalFormulation==True; axialForce represents an additional sliding force acting between beam and marker m0 body in axial (beam) direction; this force can be used to drive a body on a beam, but can only be changed with user functions. |
| **activeConnector** | Bool |  | True | flag, which determines, if the connector is active; used to deactivate (temporarily) a connector or constraint |
| **visualization** | VObjectJointSliding |  |  | parameters for visualization of item |

## Visualization parameters

The parameters of `VObjectJointSliding`, given as `visualization`:

| name | type | size | default | description |
|---|---|---|---|---|
| **show** | Bool |  | True | set true, if item is shown in visualization and false if it is not shown |
| **drawSize** | float |  | -1. | drawing size = radius of revolute joint; size == -1.f means that default connector size is used |
| **color** | Float4 | 4 | [-1.,-1.,-1.,-1.] | RGBA connector color; if R==-1, use default color |

## Drawing

Drawn as [all joints](#sec-drawing-objectsjoint); a sphere of diameter `drawSize` at each marker.

## Output variables

Available as `OutputVariableType` in sensors, `Get...Output()` and other functions:

```{tabularcolumns} \Y{0.25}\Y{0.25}\Y{0.5}
```

| output variable | symbol | description |
|---|---|---|
| Position |  | position vector of joint given by marker0 |
| Velocity |  | velocity vector of joint given by marker0 |
| SlidingCoordinate |  | global sliding coordinate along all elements; the maximum sliding coordinate is equivalent to the reference lengths of all sliding elements |
| Force |  | joint force vector (3D) |

(description-objectjointsliding)=
## Detailed description

### Definition of quantities

| intermediate variables | symbol | description |
|---|---|---|
| data node | $\xv=[x_{data0},\,x_{data1}]\tp$ | coordinates of node with node number $n_{GD}$ |
| data coordinate 0 | $x_{data0}$ | the current index in slidingMarkerNumbers |
| data coordinate 1 | $x_{data1}$ | the global sliding coordinate (ranging from 0 to the total length of all sliding elements) at **start-of-step** - beginning of the timestep |
| marker m0 position | $\LU{0}{\pv}_{m0}$ | current global position which is provided by marker m0 |
| marker m0 velocity | $\LU{0}{\vv}_{m0}$ | current global velocity which is provided by marker m0 |
| marker m0 orientation | $\LU{0,m0}{\Rot}$ | current rotation matrix provided by marker m0 (assumed to be rigid body) |
| marker m0 angular velocity | $\LU{0}{\tomega}_{m0}$ | current angular velocity vector provided by marker m0 (assumed to be rigid body) |
| sliding position | $\LUR{0}{\rv}{ANCF} = \Sm(s_{el})\qv_{ANCF,m1}$ | current global position at the ANCF cable element, evaluated at local sliding position $s_{el}$ |
| sliding position slope | $\LURU{0}{\rv}{ANCF}{\prime} = \Sm^\prime(s_{el})\qv_{ANCF,m1} = [r^\prime_0,\,r^\prime_1,\,r^\prime_2]\tp$ | current global slope vector of the ANCF cable element, evaluated at local sliding position $s_{el}$ |
| sliding velocity | $\LUR{0}{\vv}{ANCF} = \Sm(s_{el})\dot\qv_{ANCF,m1}$ | current global velocity at the ANCF cable element, evaluated at local sliding position $s_{el}$ ($s_{el}$ not differentiated!!!) |
| sliding velocity slope | $\LURU{0}{\vv}{ANCF}{\prime} = \Sm^\prime(s_{el})\dot\qv_{ANCF,m1}$ | current global slope velocity vector of the ANCF cable element, evaluated at local sliding position $s_{el}$ |
| algebraic coordinates | $\zv=[\lambda_0,\,\ldots,\,\lambda_5,\, s]\tp$ | algebraic coordinates composed of 3 Lagrange multipliers for forces $\lambda_{0..2}$, 3 multipliers for torques $\lambda_{3..5}$ and the current sliding coordinate $s$, which is local in the current cable element. |
| local sliding coordinate | $s$ | local incremental sliding coordinate $s$: the (algebraic) sliding coordinate **relative to the start-of-step value**. Thus, $s$ only contains small local increments. |

| output variables | symbol | formula |
|---|---|---|
| Position | $\LU{0}{\pv}_{m0}$ | current global position of position marker $m0$ |
| Velocity | $\LU{0}{\vv}_{m0}$ | current global velocity of position marker $m0$ |
| SlidingCoordinate | $s_g = s + x_{data1}$ | current value of the global sliding coordinate |
| Force | $\fv$ | see below |

### Geometric relations

Assume we have given the sliding coordinate $s$ (e.g., as a guess of the Newton method or beginning of the time step).
The element sliding coordinate (in the local coordinates of the current sliding element) is computed as

$$
s_{el} = s + x_{data1} - d_{m1} = s_g - d_{m1}.
$$

The vector (=difference; error) between the marker $m0$ and the marker $m1$ (=$\rv_{ANCF}$) positions reads

$$
\LU{0}{\Delta\pv} = \LUR{0}{\rv}{ANCF} - \LU{0}{\pv}_{m0}
$$

The vector (=difference; error) between the marker $m0$ and the marker $m1$ velocities reads

$$
\LU{0}{\Delta\vv} = \LUR{0}{\dot\rv}{ANCF} - \LU{0}{\vv}_{m0}
$$

### Connector constraint equations (useClassicalFormulation=True)

The 3D sliding joint is implemented having 7 equations, using the special algebraic coordinates $\zv$.
The algebraic equations read

$$
\begin{aligned}
\LU{0}{\Delta\pv} \!&=\! \Null, \quad \mbox{... three index 3 eqs $\ra$ sliding body stays on cable}\\
\vr{\lambda_1}{\lambda_2}{\lambda_3} \cdot  \LURU{0}{\rv}{ANCF}{\prime} - |\LURU{0}{\rv}{ANCF}{\prime}| \cdot f_\mathrm{ax} \!&=\! 0, \quad \mbox{... three index 1 equ.
$\ra$ force in sliding dir.=$f_\mathrm{ax}$}  \\
\end{aligned}
$$

No index 2 case exists, because no time derivative exists for $s_{el}$. The jacobian matrices for algebraic and {ref}`ODE2 <ODE2>` coordinates read

if `activeConnector = False`, the algebraic equations are changed to:

$$
\begin{aligned}
\lambda_0 &= 0,   \\
\ldots && ,   \\
\lambda_5 &= 0,   \\
s &= 0
\end{aligned}
$$

In case that `constrainRotations=[0,0,0]`, the Lagrange multipliers for rotations are set

$$
\left[\lambda_3,\lambda_4,\lambda_5\right] = \Null
$$

In case that any flag in `constrainRotations` is equal to 1, the constraints read
for `constrainRotations[0] = 1`:

$$
\LURU{0}{\rv}{ANCF,y}{\mathrm{T}} \LU{0,m0}{\Rot} \vr{0}{0}{1} = 0
$$

for `constrainRotations[1] = 1`:

$$
\LURU{0}{\rv}{ANCF}{\prime\mathrm{T}} \LU{0,m0}{\Rot} \vr{0}{0}{1} = 0
$$

for `constrainRotations[2] = 0`:

$$
\LURU{0}{\rv}{ANCF}{\prime\mathrm{T}} \LU{0,m0}{\Rot} \vr{0}{1}{0} = 0
$$

### Post Newton Step

After the Newton solver has converged, a PostNewtonStep is performed for the element, which
updates the marker $m1$ index if necessary.

$$
\begin{aligned}
s_{el} < 0 \quad \ra \quad x_{data0}\;-\!\!=1 \\
s_{el} > L \quad \ra \quad x_{data0}\;+\!\!=1
\end{aligned}
$$

Furthermore, it is checked, if $x_{data0}$ becomes smaller than zero, which raises a warning and keeps $x_{data0}=0$.
The same results if $x_{data0}\ge sn$, then $x_{data0} = sn$.
Finally, the data coordinate is updated in order to provide the starting value for the next step,

$$
x_{data1} \;+\!\!= s.
$$

(miniexample-objectjointsliding)=
## Mini example


```python
#the shape of 3D ANCF cable elements for a sliding joint: a mass point slides along a clamped, stiff cable
from exudyn.beams import GenerateBeamElementsAlongLine
cable = ObjectANCFCable(massPerLength=1, bendingStiffness=1e4, axialStiffness=1e6)
beamInfo = GenerateBeamElementsAlongLine(mbs, positionStart=[0,0,0], positionEnd=[2,0,0],
                        numberOfElements=4, beamTemplate=cable,
                        groundConstraintsStart=[1,1,1, 1,1,1], groundConstraintsEnd=[1,1,1, 1,1,1])
nodes, elements = beamInfo['nodes'], beamInfo['elements']
nMass = mbs.AddNode(NodePoint(referenceCoordinates=[0.6,0,0]))
mbs.AddObject(ObjectMassPoint(nodeNumber=nMass, mass=1))
mMass = mbs.AddMarker(MarkerNodePosition(nodeNumber=nMass))
mbs.AddLoad(LoadForceVector(markerNumber=mMass, loadVector=[1,0,0]))

cableMarkers = [mbs.AddMarker(MarkerBodyBeamShape(bodyNumber=e)) for e in elements]
offsets = [0.5*i for i in range(4)] #the element length is 0.5
nData = mbs.AddNode(NodeGenericData(numberOfDataCoordinates=2, initialCoordinates=[1, 0.6])) #element 1, sliding coordinate
mbs.AddObject(ObjectJointSliding(markerNumbers=[mMass, cableMarkers[1]], slidingMarkerNumbers=cableMarkers,
                                 slidingMarkerOffsets=offsets, nodeNumber=nData, constrainRotations=[0,0,0]))

mbs.Assemble()
mbs.SolveDynamic()

#the mass slides: x = 0.6 + F/(2m)*t^2 at t=1, the stiff cable deflects little
exu.sys['testResult'] = mbs.GetNodeOutput(nMass, exu.OutputVariableType.Position)[0] #1.1
```


Relevant Examples (Ex) and TestModels (TM) with weblink to github: [`ANCFslidingJoint.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/ANCFslidingJoint.py) (Ex), [`NGsolveFFRFSlidingJoint.py`](https://github.com/jgerstmayr/EXUDYN/blob/master/python/Examples/NGsolveFFRFSlidingJoint.py) (Ex)