Model Element

Description

Param_Unit defines the units for the model being defined. Param_Unit allows you to specify four types of units: Mass, Length, Time, and Force.  The last is obviously a function of the first three and can be automatically calculated for a consistent set of units.  However, many unit systems are not "consistent", so it may be necessary to specify a force unit also.

The table below summarizes the key units types supported in MotionSolve.

Mass Units

Length Units

Force Units

Time Units

Gram

Inch

Ounce_Force

Millisecond

Milligram

Foot

Pound_Force

Second

Kilogram

Mile

Kpound_Force

Minute

Megagram

Millimeter

Dyne

Hour

Pound_Mass

Centimeter

Newton

Microsecond

Slug

Meter

Kilogram_Force

Nanosecond

Ounce_Mass

Kilometer

KNewton

Day

Kpound_Mass

Nanometer

CentiNewton

Tonne

Angstrom

MilliNewton

Microgram

Yard

MicroNewton

Nanogram

MilliInch

MegaNewton

US Ton

MicroInch

Poundal

Slinch

Mils

NanoNewton

Micrometer

A summary of the units supported in MotionSolve

In lieu of specifying the mass, length, and time units, you can alternatively specify a mass multiplicative factor, mass_mult_factor. This factor should ensure that for the set that's chosen, Newton’s second law, F=MA, can still be used.

mass_mult_factor has a default value of 1.0, so that when neither the units nor the mass multiplication factor are defined, the software assumes that the model has been defined in some (unknown) set of consistent units.

Format

<Param_Unit

  {     mass_mult_factor    = "real"

    | [ force_unit          = "name" ]

      [ mass_unit           = "name" ]

      [ length_unit         = "name" ]

      [ time_unit           = "name" ] >

  }

</Param_Unit>

Attributes

mass_mult_factor

Defines the multiplication factor to be used so that Newton’s second law, Force = Mass * Acceleration, is still valid for the set of units used by the user. mass_mult_factor has a default value of 1.0.

force_unit

Defines the units for force.  Select from:

mass_unit

Defines units for mass.  Select from:

length_unit

Defines the units for length.  Select from:

time_unit

Defines the time units.  Select from:

Comments

User units are defined using the mass_mult_factor keyword.

Example

Defining an SI units system:

<Param_Unit

    force_unit          = "NEWTON"

    mass_unit           = "KILOGRAM"

    length_unit         = "METER"

    time_unit           = "SECOND" >

</Param_Unit>

A user defined units system:

Assume the set of units you are working on is as follows:

The mass scale is 10-3 Kg.

The length scale is 10-6 m.

The time scale is 10-3 s.

The force scale is 1 N.

A mass multiplication factor is defined as follows:

mass_mult_factor = mass_scale * length_scale/(force_scale * time_scale*time_scale)

 = 10-3 * 10-6 / (1 * (10-3)2)

 = 10-9 / 10-6

 = 10-3

The Param_Unit blocks is as follows:

<Param_Unit

  mass_mult_factor = "0.001"

</Param_Unit>

Here is the definition of Param_Unit for the following set of units:

<Param_Unit

    force_unit          = "NEWTON"

    mass_unit           = "KILOGRAM"

    length_unit         = "MILLIMETER"

    time_unit           = "SECOND" >

</Param_Unit>

The example below shows the definition of the British units.

<Param_Unit

    force_unit          = "POUNDFORCE"

    mass_unit           = "POUNDMASS"

    length_unit         = "FOOT"

    time_unit           = "SECOND" >

</Param_Unit>

Model Element

Description

UNITS defines the units for the model being defined.  It allows you to specify four types of units: Mass, Length, Time, and Force.  The last is obviously a function of the first three and can be automatically calculated for a consistent set of units.  However, many unit systems are not "consistent", so it may be necessary to specify a force unit also.

Declaration

def UNITS(FORCE="NEWTON", LENGTH="METER", MASS="KILOGRAM", TIME="SECOND", SYSTEM="NONE", UCF=0.0):

Attributes

FORCE

Defines the UNITS for force.  Select from

LENGTH

Defines the UNITS for length.  Select from

MASS

Defines the UNITS for mass.  Select from

TIME

Defines the time UNITS.  Select from

SYSTEM

Instead of specifying each of the above individual attributes FORCE, LENGTH, MASS and TIME, you may simply specify a system of UNITS and MotionSolve will assign relevant units automatically overwriting the individual attributes.  Select from one of the following "CGS", "FPS", "IPS", "MKS", "NONE".

When specified "NONE", MotionSolve would refer to the individual attributes FORCE, LENGTH, MASS and TIME.  In the absence of individual attributes, it defaults to "MKS" system.

UCF

In lieu of specifying the mass, length, and time units, you can alternatively specify a Unit Conversion Factor, UCF.  This factor should ensure that for the set that's chosen, Newton’s second law, F=MA, can still be used.

UCF has a default value of 1.0, so that when neither the units nor the mass multiplication factor are defined, the software assumes that the model has been defined in some (unknown) set of consistent units.

Comments

See Param_Unit

Example

The first example shows how to define a SI unit system.

UNITS(FORCE="NEWTON",MASS="KILOGRAM",LENGTH="METER",TIME="SECOND")

In the second example, let's assume the set of units you are working on is as follows:

The mass scale is 10-3 Kg.

The length scale is 10-6 m.

The time scale is 10-3 s.

The force scale is 1 N.

UCF may be calculated as follows

UCF = (force_scale * time_scale*time_scale)/( mass_scale * length_scale)

      = (1 * (10-3)2)/( 10-3 * 10-6)

       = 10-6 / 10-9

       = 1000

This may be defined as below

UNITS(UCF=1000)

The third example shows how the following units may be set.

UNITS(FORCE="NEWTON",MASS="KILOGRAM",LENGTH="MILLIMETER",TIME="SECOND")

The fourth example shows how to define a British unit system (FPS).

UNITS(SYSTEM="FPS")