This page is about the C3D Parameter FORCE_PLATFORM:ZERO. For Visual3D's ability to track different baseline frames for different force platforms see FP_ZEROS.

The FORCE_PLATFORM:ZERO C3D parameter defines the range of frames used to calculate an average baseline value for each ANALOG signal that is then subtracted from every frame. This does not affect the ANALOG signals (as seen in the data tree), this processing takes place when the Force Platform signals are computed.

Note that most of the manufacturers arbitrarily set the range to be the first 10 frames of the trial. If the force platform is loaded during these first 10 frames, the ground reaction force signals will be incorrect; in this case the user should either set the frames to 0,0 so that no baseline is subtracted, or should select frames in which the force platform is unloaded.

In some cases it is not feasible to use the ZERO parameter to define a BASELINE for each signal. All force platform signals have some drift in the amplifiers, so it isn't a good idea to simply set the ZERO parameter to 0,0 so that no baseline is subtracted. The following two examples demonstrate alternatives to using the ZERO parameter to remove a baseline.

In all 3 examples the ZERO parameter should be set to 0,0 so that no inadvertent baseline is removed.

1. Subtracting the mean value from several ranges from each analog signal

Consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.

Given 12 ANALOG signals associated with a type 6 Force Platform:

FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4 For_Each /Iteration_Parameter_Name= SIGNAL_INDEX /Items= FX+FY+FZ ; For_Each /Iteration_Parameter_Name= NUMBER /Items= 1+2+3+4 ; ! Compute the mean value of the airborne phases. Metric_Mean /Signal_types= ANALOG /Signal_Names= ::SIGNAL_INDEX&::NUMBER /Signal_Folder= ORIGINAL /Signal_Components= ALL_COMPONENTS /Event_Sequence= LTO+RHS /Exclude_Events= /Metric_Name= _ /Apply_As_Suffix_To_Signal_Name= TRUE /Generate_Mean_And_STDEV= TRUE /Append_To_Existing_Values= FALSE ; Multiply_Signals_By_Constant /Signal_types= METRIC /Signal_Names= ::SIGNAL_INDEX&::NUMBER&__MEAN /Signal_Folder= PROCESSED /Result_Names= OFFSET /Result_Suffix= /Signal_Components= /Constant= -1 ; Set_Pipeline_Parameter_To_Data /Parameter_Name= BASELINE /Signal_types= METRIC /Signal_Names= OFFSET /Signal_Folder= PROCESSED ; ! NOTE The following command creates an ANALOG PROCESSED signal, not a DERIVED PROCESSED signal. Add_Constant_To_Signals /Signal_types= ANALOG /Signal_Names= ::SIGNAL_INDEX&::NUMBER /Signal_Folder= ORIGINAL /Result_Names= /Result_Suffix= /Signal_Components= /Constant= ::BASELINE ; End_For_Each /Iteration_Parameter_Name= NUMBER ; End_For_Each /Iteration_Parameter_Name= SIGNAL_INDEX ;

Subtracting the mean value from several ranges from each analog signal

Consider a subject running on an instrumented treadmill. The mean value of each ANALOG signal is determined when the subject is airborne (e.g. between LTO and RHS) and a collection of these airborne phases are averaged. This overall mean value is to be subtracted from each ANALOG signal. The option to use Processed Analog signals for the Force Platform calculations should be selected. The FORCE_PLATFORM:ZERO parameter is set to (0,0) because it is to be ignored.

Given 12 ANALOG signals associated with a type 6 Force Platform:

FX1,FY1,FZ1,FX2,FY2,FZ2,FX3,FY3,FZ3,FX4,FY4,FZ4 ! Compute the mean values for all ANALOG signals. Metric_Mean /RESULT_METRIC_NAME=_BASE /APPLY_AS_SUFFIX_TO_SIGNAL_NAME=TRUE ! /RESULT_METRIC_FOLDER=PROCESSED /SIGNAL_TYPES=ANALOG ! /SIGNAL_NAMES= ! /SIGNAL_FOLDER=ORIGINAL ! /SIGNAL_COMPONENTS=ALL_COMPONENTS /EVENT_SEQUENCE=LOFF+RON /EXCLUDE_EVENTS= ! /GENERATE_MEAN_AND_STDDEV=TRUE ! /APPEND_TO_EXISTING_VALUES=FALSE ; ! Setup a for each loop over the ANALOG signals For_Each /ITERATION_PARAMETER_NAME=SIGNAL_INDEX /ITEMS=F1X1+F1Y1+F1Z1+F1X2+F1Y2+F1Z2+F1X3+F1Y3+F1Z3+F1X4+F1Y4+F1Z4 ; ! Subtract the mean value from each signal. This step is a little quirky because I chose to use the evaluate_expression to perform this task. Unfortunately the (::) identified gets confused when four colons (e.g. ::::) are used in sequence. This forced me to define Global Parameters. This isn't actually a terrible thing because it was helpful when I was debugging the pipeline. Set_Pipeline_Parameter /PARAMETER_NAME=TEMP /PARAMETER_VALUE=ANALOG::ORIGINAL::&::SIGNAL_INDEX ; Set_Pipeline_Parameter /PARAMETER_NAME=BASE /PARAMETER_VALUE=METRIC::PROCESSED::&::SIGNAL_INDEX&_BASE_MEAN ; Set_Pipeline_Parameter /PARAMETER_NAME=EXPRESSION /PARAMETER_VALUE=::TEMP&-&::BASE ; ! Now we can evaluate the expression. Note: That in Version 3.69 (the time of writing this topic) the evaluate_expression command was quite slow because of the method used for parsing the expression. Evaluate_Expression /EXPRESSION=::EXPRESSION /RESULT_NAME=::SIGNAL_INDEX /RESULT_TYPE=ANALOG /RESULT_FOLDER=PROCESSED ; End_For_Each /ITERATION_PARAMETER_NAME=SIGNAL_INDEX ;

Computing mean values for the force platform analog signals in one trial, and then use these values in other trials

For this example assume that:

1. one of the files has TAG=walk.
2. there are two AMTI force platforms
3. the force platforms are unloaded for the first 50 frames

! Create events to identify Frame 1 and Frame 50 in the file with TAG=walk Select_Active_File /FILE_NAME=walk ! /QUERY= ; Explicit /EVENT_NAME=FRAME1 /FRAME=1 ; Explicit /EVENT_NAME=FRAME50 /FRAME=50 ; ! Compute the mean value of the force platform ANALOG signals Metric_Mean /RESULT_METRIC_NAME=_ /APPLY_AS_SUFFIX_TO_SIGNAL_NAME=TRUE ! /RESULT_METRIC_FOLDER=PROCESSED /SIGNAL_TYPES=ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG+ANALOG /SIGNAL_NAMES=F1X+F1Y+F1Z+F2X+F2Y+F2Z+M1X+M1Y+M1Z+M2X+M2Y+M2Z /SIGNAL_FOLDER=ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL+ORIGINAL /SIGNAL_COMPONENTS=X /EVENT_SEQUENCE=FRAME1+FRAME50 /EXCLUDE_EVENTS= ! /GENERATE_MEAN_AND_STDDEV=TRUE ! /APPEND_TO_EXISTING_VALUES=FALSE ; ! Construct a For_Each loop to process each ANALOG signal independently For_Each /ITERATION_PARAMETER_NAME=SIG /ITEMS=F1X+F1Y+F1Z+M1X+M1Y+M1Z+F2X+F2Y+F2Z+M2X+M2Y+M2Z ; ! Set the active folder to GLOBAL and create a pipeline parameter for appropriate MEAN signal Select_Active_File /FILE_NAME=GLOBAL ! /QUERY= ; Set_Pipeline_Parameter_To_Data_Value /PARAMETER_NAME=TEMP /SIGNAL_TYPES=METRIC /SIGNAL_NAMES=::SIG&__MEAN /SIGNAL_FOLDER=PROCESSED ! /INDEX= ; ! Define a pipeline parameter to create the expression that we will evaluate in the next step. This is a little awkward but necessary because if we tried to use the evaluation expression with pipeline parameters it get's confused (maybe someday we will find a solution to this problem). Set_Pipeline_Parameter /PARAMETER_NAME=EQUATION /PARAMETER_VALUE=ANALOG::ORIGINAL::&::SIG&-&::TEMP ! /PARAMETER_VALUE_SEARCH_FOR= ! /PARAMETER_VALUE_REPLACE_WITH= ! /PARAMETER_VALUE_APPEND= ; ! Make all of the files in the Workspace active and subtract the GLOBAL mean values from the signals. Select_Active_File /FILE_NAME=ALL_FILES ! /QUERY= ; Evaluate_Expression /EXPRESSION=::EQUATION /RESULT_NAME=::SIG /RESULT_TYPE=ANALOG ! /RESULT_FOLDER=PROCESSED ; ! For example, the first time through the loop /EXPRESSION= ANALOG::ORIGINAL::F1X - ::SIG__MEAN End_For_Each /ITERATION_PARAMETER_NAME=SIG ;