API Contents

Properties for MBS

While such generality is desirable from a programming perspective, it is not desired when defining a language for specifying multibody systems. In the multibody context, all user-defined inputs have very well defined "properties" and any value given to any variable must conform to its properties.

The Python Interface for MotionSolve has introduced the concept of a "Property" to encapsulate this notion.

• Classes have attributes (for instance, PART has mass as an attribute)
• Attributes have values that must conform to the property the attribute has (for instance, mass has a numeric value and mass ≥ 0)

All attributes have a property. Trying to set an attribute value to an incorrect property type raises an error. For example:

>>> Part1.mass = "A bad value"

TypeError: float() argument must be a number

Not all attribute value validation can be handled when the value is set. Some checking needs to ensure required values are specified and there are no conflicts with other property values. For example, the i and j markers for a Joint must be specified and they must by associated with different bodies. This checking is done when the model is validated. Model validation is done after the entire model is defined.

Some property values can be set after a simulation has been started, other properties cannot be changed. You can change the mass of a Part but you cannot change the CM location or orientation. This limitation exists because the CM changes will cause all mass properties to change also.

The properties supported by MotionSolve, along with a brief description of each property, are shown in the table below.

Classes for MBS

Modeling Class Library

These classes are used to define a model. Many of these can be used as commands also. Commands are used to change model or simulation attributes during a simulation.

Model	Accgrav	Array	Beam	Bushing
Contact	Coupler	Curve	Cvcv	Cvsf
Debug	Dcurve	Diff	Dsurface	Dv
Equilibrium	Field	FlexBody	Friction	Gcon
Gear	Gforce	Graphics	Gse	Ic
Integrator	Joint	Jprim	Kinematic	Lse
Marker	Mate	Matrix	Mforce	Motion
Nforce	Output	Part	Pforce	Pinput
PointMass	Poutput	Preferences	Ptcv	Ptdcv
Ptdsf	Ptdsff	Ptsf	Request	Rv
Sensors	Sforce	Sfsf	Spline	SpringDamper
String	Surface	Tfsiso	Transient	Ucon
Units	Variable	Vforce	Vtorque

Table 1. Results Class Library

BodyResult	H3dOutput	RequestResult	ResultObject	RvResult
SimulationResults	Upost	UserMsg

Access Functions

The MSOLVE API supports all the access functions that MotionSolve provides. Access functions are meant to be invoked from within user-subroutines to get system state dependent information.

Unlike the XML interface, where modeling elements were identified with IDs, in the Python interface, you can pass in objects or IDs to these access functions. The interface will accept any combination of objects and IDs as arguments.

For instance, consider the function ACCX (I, J, K, L). It is defined as:

$\text{ACCX (I, J, K, L) = }\frac{{}^{L}d{}^2}{d{t}^2}\left({\overrightarrow{R}}_I-{\overrightarrow{R}}_J\right)·{\widehat{x}}_K$

${\overrightarrow{R}}_I$ is the vector from the global origin to the origin of I

${\overrightarrow{R}}_J$ is the vector from the global origin to the origin of J

${\widehat{x}}_K$ is the unit vector along the x-axos of marker K

The following access functions are supported in the manner described above: