# Field

Model ElementField defines a force and torque acting between two markers, I and J. The force and torque can only be a function of time and the relative displacement between the I and J markers. The six components (three forces and three moments) are defined in the coordinate system of the J marker.

Field

## Description

Both linear and nonlinear relationships are supported. A nonlinear Field is defined in a user-defined subroutine called FIESUB.

## Attribute Summary

Name Property Modifiable by command? Designable?
id Int ()
label String () Yes
i Reference (Marker) Yes Yes
j Reference (Marker) Yes Yes
cratio Double (None) Yes Yes
cmatrix Double ([], count=36) Yes Yes
kmatrix Double ([], count=36) Yes Yes
force Double ([0,0,0,0,0,0], count=6) Yes Yes
length Double ([0,0,0,0,0,0], count=6) Yes Yes
function Function ("FIESUB") Yes
routine Routine ()
active Bool () Yes

## Usage

Field is available as follows:
#1: Linear Field
Field (i=objMarker, j= objMarker, optional_attributes)

#2: Nonlinear Field defined in a compiled DLL
Field (i=objMarker, j= objMarker, function= userString, routine=string, optional_attributes)

#3: Nonlinear Field defined in a Python function
Field (i=objMarker, j= objMarker, function= userString, routine=functionPointer, optional_attributes)

## Attributes

Linear Field
i
Reference to an existing Marker object.
Specifies the ID of the marker at which the force and moment is applied. This is designated as the point of application of the force.
The i attribute is mandatory.
j
Reference to an existing Marker object.
Specifies the ID of the marker at which the reaction force and moment is applied. This is designated as the point of reaction of the force.
The j attribute is mandatory.
kmatrix
6x6 matrix
kmatrix defines a 6x6 stiffness matrix that is used to calculate the spring force when a linear FIELD is to be defined. The matrix need not be symmetric. It must, however, be positive semi-definite. In other words, {x}T[K]{x} ≥ 0 for any deformation x.
The kmatrix attribute is optional.
When not specified it default to a 6x6 zero matrix.
cmatrix
6x6 matrix
cmatrix defines a 6x6 damping matrix that is used to calculate the damping force for a linear FIELD. The matrix need not be symmetric. However, it must be positive semi-definite. In other words, {y}T[C]{y} ≥ 0 for any arbitrary velocity y.
The cmatrix attribute is optional.
When not specified it default to a 6x6 zero matrix.
cmatrix and cratio are mutually exclusive. Use one or the other but not both.
cratio
Double
Defines a damping scale factor. The damping matrix is calculated as C = cratio * kmatrix.
A value of cratio = 0.01 is normally used.
The cratio attribute is optional. When not specified it default to zero.
cratio and cmatrix are mutually exclusive. Only one of the two methods for specifying damping can be used at any one time.
Cratio ≥ 0.0
cmatrix and cratio are exclusive. Use one or the other but not both.
length
List of 6 doubles
These define the free length of the Field element. In other words, the displacement from J to I, as measured in the J coordinate system, when no external forces are acting on the element. Rotations are measured in terms of AX(I,J), AY(I,J), and AZ(I,J). They are assumed to be small angles (< 10 degrees).
The length attribute is optional. When not specified, it defaults to a zero valued array of length six.
force
List of 6 doubles
These define the three translational preload forces and three rotational preload torques in the Field element. The forces and torques are measured in the J coordinate system and they represent the forces and torques acting at I when there is no deformation.
The force attribute is optional. When not specified, it defaults to a zero valued array of length six.
Nonlinear Field defined in a compiled DLL
i
Reference to an existing Marker object.
Specifies the ID of the marker at which the force and moment is applied. This is designated as the point of application of the force.
The i attribute is mandatory.
j
Reference to an existing Marker object.
Specifies the ID of the marker at which the reaction force and moment is applied. This is designated as the point of reaction of the force.
The j attribute is mandatory.
function
String
The list of parameters that are passed from the data file to the user-defined subroutine.
The function attribute is mandatory
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 Field. The second is the name of the function in the shared library that does the computation.
An example is: routine="/staff/altair/myfield.dll::myfiesub"
• "/staff/altair/myfield.dll is the dll
• "myfiesub" is the function within this DLL that performs the calculations
The attribute routine is optional.
When function is specified, but routine is not, routine defaults to FIESUB.
Nonlinear Field defined in a Python function
i
Reference to an existing Marker object.
Specifies the ID of the marker at which the force and moment is applied. This is designated as the point of application of the force.
The i attribute is mandatory.
j
Reference to an existing Marker object.
Specifies the ID of the marker at which the reaction force and moment is applied. This is designated as the point of reaction of the force.
The j attribute is mandatory.
function
String
The list of parameters that are passed from the data file to the user-defined subroutine.
The function attribute is mandatory.
routine
Pointer to a callable function in Python.
An example is: routine=myField
• myField 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 FIESUB.
Optional attributes - Available to all variants
id
Integer
Specifies the element identification number. This number must be unique among all the Field 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 Field object.
This attribute is optional. When not specified, MotionSolve will create a label for you.
active
Bool
Select one from True or False.
• True indicates that the element is active in the model and it affects the behavior of the system.
• False indicates that the element is inactive in the model and it does not affect the behavior of the system. It is almost as if the entity was removed from the model, of course with the exception that can be turned "ON" when desirable.
The attribute active is optional. When not specified, active defaults to True.

## Example

The first example in the XML syntax section is redone with the Python API.
1. Model the structural stiffness of a truss as shown below, with a FORCE_FIELD.
The FORCE_FIELD statement to model this element is:
<FORCE_FIELD
ID            = "1"
I_MARKER_ID   = "100"
J_MARKER_ID   = "200"
CRATIO        = "0.001"
K11 = "1.03e4"    K22 = "5.98e3"    K33 = "5.98e3"
K44 = "8.0e3"     K55 = "8.0e2"     K66 = "7.0e2"
K26 = "-5.98e3"   K62 = "-5.98e3"
K35 = "1.02e3"    K53 = "1.02e3"
/>
The same entity may be defined in the Python API as:
kmat   = [0.0]*36
rows   = [0, 1, 2, 3, 4, 5, 1, 5, 2, 4]
cols   = [0, 1, 2, 3, 4, 5, 5, 1, 4, 2]
values = [1.03e4, 5.98e3, 5.98e3, 8.0e3, 8.0e2, 7.0e2, -5.98e3, -5.98e3, 1.02e3, 1.02e3]

for row, col, value in zip(rows, cols, values): kmat[row*6+col]=value

f0     = [0.0]*6
f0[2]  = 1e3

# Define the truss object
truss = Field (i=mkr100, j=mkr200, cratio=0.001, kmatrix=kmat, force=f0, label="Truss modeled as a Field")