====== Assigned Force Functions ======

The model-based items [[Visual3D:Documentation:Pipeline:Model_Based_Data_Commands:GRF_DATA|GRF_DATA]] and [[Visual3D:Documentation:Pipeline:Model_Based_Data_Commands:COP_Path|COP Path]] are signals stored at the [[visual3d:documentation:c3d_signal_types:target_data_type|POINT data rate]]. This is because these signals have the option to be resolved into a Segment Coordinate System and all Segment Coordinate Systems (except the default LAB) are at the POINT rate. There are times, however, when it would be useful to get the Force, CofP, and FreeMoment signals for a given[[Visual3D:Documentation:Kinematics_and_Kinetics:External_Forces:Force_Assignment|force assignment]] at the [[visual3d:documentation:c3d_signal_types:analog_data_type|ANALOG sample rate]].

==== Forces ====

Syntax: **FORCE**(segment)

Example: Select the force assigned to the Left Foot segment (whose [[Visual3D:Documentation:Modeling:Segments:Segment_Default_Names|default segment name]] is LFT).

<code>
Evaluate_Expression
/EXPRESSION=FORCE("LFT")
/RESULT_NAME=SCOTT
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
;
</code>

==== Center of Pressure ====

Syntax: **COFP**(segment)

Example: Select the center of pressure assigned to the Left Foot segment (whose [[Visual3D:Documentation:Modeling:Segments:Segment_Default_Names|default segment name]] is LFT).

<code>
Evaluate_Expression
/EXPRESSION=COFP("LFT")
/RESULT_NAME=SCOTT
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
;
</code>

==== Free Moment ====

Syntax: **FREEMOMENT**(segment)

Example: Select the Free Moment assigned to the Left Foot segment (whose [[Visual3D:Documentation:Modeling:Segments:Segment_Default_Names|default segment name]] is LFT).

<code>
Evaluate_Expression
/EXPRESSION=FREEMOMENT("LFT")
/RESULT_NAME=SCOTT
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
;
</code>

==== Error Messages ====

There are several possible error messages related to the Assigned Forces expressions.

=== Segment Name must be Quoted ===
<code>
Evaluate_Expression
/EXPRESSION=FORCE(JUNK)
/RESULT_TYPE=DERIVED
/RESULT_FOLDER=PROCESSED
/RESULT_NAME=TEST
;

WARNING! No Segment Name was specified. Did you forget the quotes?
</code>

In order to ensure that a string containing the segment name is recognized quotes must be used.

=== Segment Not in Link Model ===
<code>
Evaluate_Expression
/EXPRESSION=COFP("JUNK")
/RESULT_TYPE=DERIVED
/RESULT_FOLDER=PROCESSED
/RESULT_NAME=TEST
;

INFO! Segment Junk was not found
</code>

The segment must be a part of the Link Model in order to be findable.

=== Use Default Segment Names ===
<code>
Evaluate_Expression
/EXPRESSION=COFP("Right Foot")
/RESULT_TYPE=DERIVED
/RESULT_FOLDER=PROCESSED
/RESULT_NAME=TEST
;

WARNING! Evaluate_Expression requires default segment names. For Right Foot use RFT
</code>

Visual3D has a set of [[Visual3D:Documentation:Modeling:Segments:Segment_Default_Names|default segment names]] that must be used in expressions, so that the expressions can be shared between users that have different config files.

=== Can't Use Kinematic Only Segments ===
<code>
Evaluate_Expression
/EXPRESSION=COFP("Right Fore Foot")
/RESULT_TYPE=DERIVED
/RESULT_FOLDER=PROCESSED
/RESULT_NAME=TEST
;

INFO! Segment Right Fore Foot is Kinematic Only, so there is no Force Assignment possible.
</code>

Only kinetic segments can be assigned a force.