# Vforce

Model ElementVFORCE defines a general force acting between two markers.

Vforce

## Description

The force vectors is defined by its three components with respect to a third marker. The components may be defined using MotionSolve expressions or a user-defined subroutine. They may be a function of any system state and time.

## Attribute Summary

Name Property Modifiable by command? Designable?
id Int ()
label Str ()
i Reference (Marker) Yes Yes
jfloat Reference (Marker) Yes Yes
rm Reference (Marker) Yes Yes
fx Function () Yes Yes
fy Function () Yes Yes
fz Function () Yes Yes
function Function ("VFOSUB") Yes
routine Routine ()
active Bool () Yes

## Usage

Vforce is available as follows:
#1. Force defined in a MotionSolve expression Vforce (i=objMarker, j=objMarker, fx=expressionString, fy=expressionString, fz=expressionString, optional_attributes)
#2. Force defined in a compiled DLLVforce (i=objMarker, j=objMarker, function=userString, routine=string, optional_attributes)
#3. Force defined in a Python scriptVforce (i=objMarker, j=objMarker, function=userString, routine=functionPointer, optional_attributes)

## Attributes

Force defined in a MotionSolve expression
i
Reference to an existing Marker object.
Specifies the marker at which the force is applied. This is designated as the point of application of the force.
j
Reference to an existing floating Marker object.
Specifies the marker at which the reaction force is applied. This is designated as the point of reaction of the force.
fx
String defining a valid MotionSolve expression.
Specifies the MotionSolve expression that defines the force acting along the x-axis of the reference coordinate system (see rm below). Any valid run-time MotionSolve expression can be provided as input.
fy
String defining a valid MotionSolve expression.
Specifies the MotionSolve expression that defines the force acting along the y-axis of the reference coordinate system (see rm below). Any valid run-time MotionSolve expression can be provided as input.
fz
String defining a valid MotionSolve expression.
Specifies the MotionSolve expression that defines the force acting along the z-axis of the reference coordinate system (see rm below). Any valid run-time MotionSolve expression can be provided as input.
Force defined in a compiled DLL
i
Reference to an existing Marker object.
Specifies the marker at which the force is applied. This is designated as the point of application of the force.
j
Reference to an existing floating Marker object.
Specifies the marker at which the reaction force is applied. This is designated as the point of reaction of the force.
function
String defining a valid user function MotionSolve expression.
The list of parameters that are passed from the data file to the user defined subroutine where the Vforce is defined.
routine
String.
Specifies an alternative name for the user subroutine. The name consists of two pieces of information, separated by "∷". The first is the pathname to the shared library containing the function that computes the response of the user-defined Vforce. The second is the name of the function in the shared library that does the computation.
An example is: routine="/staff/Altair/engine.dllmyVforce"
• "/staff/Altair/ engine.dll is the DLL
• "myVforce" is the function within this DLL that performs the calculations
When not specified, routine defaults to VFOSUB.
Force and Torque defined in a Python function
i
Reference to an existing Marker object.
Specifies the marker at which the force is applied. This is designated as the point of application of the force.
j
Reference to an existing floating Marker object.
Specifies the marker at which the reaction force is applied. This is designated as the point of reaction of the force.
function
String defining a valid user function MotionSolve expression.
The list of parameters that are passed from the data file to the user defined subroutine where the Vforce is defined.
routine
Pointer to a callable function in Python.
An example is: routine=myVforce
• myVforce is a Python function or method that can be called from wherever the model resides.
The attribute routine is optional.
When not specified, routine defaults to VFOSUB.
Optional attributes - Available to all variants
id
Integer
Specifies the element identification number. This number must be unique among all the Vforce objects in the model.
This attribute is optional. MotionSolve will automatically create an ID when one is not specified.
Range of values: id > 0
label
String
Specifies the name of the Vforce object.
This attribute is optional. When not specified, MotionSolve will create a label for you.
rm
Reference to an existing Marker object
Specifies the marker in whose coordinate system the torque components are computed. rm can be on any body, including Ground.
The rm attribute is optional.
When not specified, rm defaults to the global coordinate system.

## Example

1. Vforce defined via expressions.
Vfo1 = Vforce (label="nonlinear bushing", i=m1801, jfloat=m1901, rm=m1903,
fx="-1e3*DX(1801,1903,1903) -2*(VX(1801,1903,1903,1903)**3)",
fy="-1e3*DY(1801,1903,1903) -2*(VY(1801,1903,1903,1903)**3)",
fz="-1e3*DZ(1801,1903,1903) -2*(VZ(1801,1903,1903,1903)**3)")
2. Vforce defined in a Python function.
# Define the user subroutine first
def myVfosub (id, time, par, npar, dflag, iflag):
i = par[0]
j = par[1]
k = par[2]
c = par[3]

# Get the state of the bushing
dx = DX(i,j,j)
dy = DY(i,j,j)
dz = DZ(i,j,j)
vx = VX(i,j,j,j)
vy = VY(i,j,j,j)
vz = VZ(i,j,j,j)

# Compute force
fx = -k*dx - c*(vx**3)
fy = -k*dy - c*(vy**3)
fz = -k*dz - c*(vz**3)

return [fx, fy, fz]
# Define a bushing with nonlinear damping
vfo2 = Vforce (label= "nonlinear bushing" , i=m1801, jfloat=m1901, rm=m1903,
function= "user(1801,1903,1000.0, 2.0)" , routine=myVfosub )