API Contents

Properties for MBS

Python does not require the data type of variables to be declared. Thus a variable can have any value. In the same session, you can do the following:

•	Pressure = 200.0

•	Pressure = "High"

•	Pressure = [1, "High", False]

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.

For instance:

•	A body’s mass must be a double greater than or equal to zero. It cannot be an array of doubles, a string, a negative value or anything else.

•	A body’s inertia is a positive semi-definite, symmetric, 3x3 matrix that is specified with 0, 3 or 6 values. It cannot be anything else.

•	The stiffness, K, and the damping, C, for Bushings are specified as diagonal 3x3 matrices.

o	K and C accept exactly three numeric values as input – no more and no less

o	Each of the numeric values must be greater than or equal to zero

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)

Properties define the user defined values of a MotionSolve object:

•	The mass of a Part

•	The error of an Integrator

•	The stiffness, k, of a SpringDamper

•	And so on...

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.

Property Name	Description
Int	A Python int value. Defaults to 0
Bool	A Python Boolean value. Defaults to False
Double	A Python float value. Defaults to 0.0 Note: For some Doubles, like the Part velocities (vx, vy, vz, wx, wy, wz) a value of 0 is different than no value. These are defaulted to None
Str	A Python string value. Defaults to ""
Enum	A Python Str value that must be one of a specified list of values. For instance, an ARRAY must be one of the following 4 types: "X” “Y” “IC” “U". The type attribute for an ARRAY has the property Enum.
EnumStr	An Enum that takes multiple values. Each of the values is a string. For instance, the INPUT attribute for a Friction object can take multiple values. These values must be from: "PRELOAD”, “NONE”, “ALL”, “REACTION_FORCE_BENDING_MOMENT”, “TORSIONAL_MOMENT"
Reference	Reference to an existing MotionSolve object. For instance, the IGEOM attribute of a CONTACT object must reference one or more existing GRAPHICS objects.
Location	An instance of the Point class. For instance, the QP attribute of a MARKER object must be Point objects. A Point is a list, so you can specify a list of three numbers also.
Angles	Three Double values. Used to specify the properties of the REULER attribute of MARKERs and PARTs.
Ips	0, 3, or 6 Double values to specify a body’s inertia matrix. This property ensures that the inertia matrix is positive semi-definite.
Exact	A Python string to specify a body's fixed exact position for. Example: exact="X:Z:PSI"
Pattern (String)	A Python string to specify up to 10 True False values for. Example: pattern="TFFFTFFFTF"
Function	A Python string that contains a legal MotionSolve function expression. Example1: “DX(21,11)VX(21,11) + DY(21,11)VY(21,11) + DZ(21,11)*VZ(21,11)” Example2: “USER(21,11,4,0e-4)”
Routine	A Python string a DLL/SO name followed by a function name or a callable function pointer. Example: “tires::pacejka2002”. The DLL/SO name is “tires”. The DLL/SO contains a function called “pacejka2002” that is to be executed by MotionSolve.

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
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:

It accepts up to four arguments:

•	I: the origin of a Marker for which the x-component of the acceleration is desired

•	J: the origin of a Marker from which the displacement to I is being measured

•	K: the coordinate system Marker in which the results are expressed

•	L: the reference frame Marker in which the two time derivatives of the displacement vector from J to I are being taken.

The ACCX function is implemented in this interface in the following way:

•	I can be a Marker object or a Marker ID

•	J can be a Marker object, a Marker ID or 0 signifying the global origin

•	K can be a Marker object, a Marker ID or 0 signifying the global coordinate system

•	L can be a Marker object , a Marker ID or 0 signifying the inertial reference frame

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

AX	AY	AZ	ACCM	ACCX
ACCY	ACCZ	ACCXYZ	DM	DX
DY	DZ	DXYZ	FM	FX
FY	FZ	FXYZ	TM	TX
TY	TZ	TXYZ	VM	VR
VX	VY	VZ	VXYZ	WM
WX	WY	WZ	WXYZ	WDTM
WDTX	WDTY	WDTZ	WDTXYZ	PHI
PSI	THETA	YAW	PITCH	ROLL
INCANG	Q	QDOT	QDDOT	UVX
UVY	UVZ	BUSH	SPDP	VTORQ
BEAM_FNC	FIELD_FNC	SFORCE_FNC	VFORCE_FNC	GFORCE_FNC
NFORCE_FNC	JOINT_FNC	JPRIM_FNC	MATE_FNC	MOTION_FNC
COUPLER_FNC	CVCV_FNC	PTCV_FNC	CVSF_FNC	SFSF_FNC
ARYVAL	DIF	DIF1	PINVAL	POUVAL
VARVAL	SENVAL	DVAL	DSVAL	DSVAL1
DSARY	DSARY1	RVAL	RVAL1