Files Formats for the APAS System

Yes indeed the total number of samples divided by the number of channels will give the number of datapoints per channel.

The ENVREC contains all the information about the raw channels. The conversion of A/D units to user units is performed in two steps. First there is a conversion of A/D units to volts. This involves properties of the A/D board itself such as the #bits, voltage range, gain, etc and is performed as follows:

float SOFTCHAN_AD::Raw2Volts(int iVal)
/*AD->User Units*/
{  
   return (iVal - chanInfo.iADZeroV) * chanInfo.fAD2Volts/chanInfo.fGain;
}

The 2nd step is to convert volts to user units which is hardware independent. The conversion of raw to user is calculated as follows:

float SOFTCHAN_AD::Raw2User(int iVal)
/*AD->User Units*/
{
    return (Raw2Volts(iVal)-chanInfo.fOff)*chanInfo.fConv;
}

I'm sure some of the structure of the ANA files is somewhat confusing. This is largely because the structure of the file has evolved over the last 20 some years. New capability was added while always maintaining backwards compatibility. Clearly if one were setting out today to design the structure of the file it would be done differently.

The other pieces of information in the ENVREC that are important for the force plates are:

1) at WORD offset=267 is the 1st Plate channel. It need not be that the 1st actual channel collected belongs to a force plate. It is assumed that the force plate channels are consecutive from the 1st plate channel. The order of the channels depends on whether the plate is a 6 channel plate such as AMTI in which case the order is assumed to be:

            Fx|Fy|Fz| Mx|My|Mz

If it is an 8 channel plate such as Kistler then the order is:

            Fx12|Fx34|Fy14|Fy23|Fz1|Fz2|Fz3|Fz4

Additional plates directly follow with the same assumed channel order. There can be only 1 type of plate 6 or 8 channel at any one time.

2) at WORD offset=283 Plate dimension units

   at WORD offset=296 Plate Type 0=Kistler 1=AMTI 2=Bertec

  at WORD offset=310 #Plates on system

   at WORD offset=311 for 2 plates 6 element dimension description

         1="X" Distance centerline to transducers

         2="Y" Distance centerline to transducers

         3=Depth beneath plate surface for the transducers [always positive]

         4="X" length

         5="Y" length

   at WORD offset=335 orientation of 2nd plate relative to 1st.

 

The only item of interest in the EXTENVREC is

   at WORD offset=103 Force units for the plate. 1=Nt 2=Kg 3=Lb

 

Here is code for calculating Fxyz, Mxyz for a Kistler plate. GetVal(channel, sample#) returns the channel value for specified sample# in user units.

 

/***************************************************************/
float KISTLER::RtnFx(LONG lIndx)
//  Override Base class member
{
 float xRet=0.;
 for (int i=0;i<2;i++)
  xRet+=m_sc.GetVal(iChan1st+i,lIndx);
 return(xRet);
}
/***************************************************************/
float KISTLER::RtnFy(LONG lIndx)
//  Override Base class member
{
 float xRet=0.;
 for (int i=0;i<2;i++)
  xRet+=m_sc.GetVal(iChan1st+i+2,lIndx);
 return(xRet);
}
/***************************************************************/
float KISTLER::RtnFz(LONG lIndx)
//  Override Base class member
{
 float xRet=0.;
 for (int i=0;i<4;i++)
  xRet+=m_sc.GetVal(iChan1st+i+4,lIndx);
 return(xRet);
}
/***************************************************************/
float KISTLER::RtnMx(LONG lIndx)
//  Override Base class member
{
 return(plateDims.fB*
   (m_sc.GetVal(iChan1st+4,lIndx)
  + m_sc.GetVal(iChan1st+5,lIndx)
  - m_sc.GetVal(iChan1st+6,lIndx)
  - m_sc.GetVal(iChan1st+7,lIndx)));
}
/***************************************************************/
float KISTLER::RtnMy(LONG lIndx)
//  Override Base class member
{
 return(plateDims.fA*
  (-m_sc.GetVal(iChan1st+4,lIndx)
  + m_sc.GetVal(iChan1st+5,lIndx)
  + m_sc.GetVal(iChan1st+6,lIndx)
  - m_sc.GetVal(iChan1st+7,lIndx)));
}
/***************************************************************/
float KISTLER::RtnMz(LONG lIndx)
//  Override Base class member
{
 return(plateDims.fB*
  (-m_sc.GetVal(iChan1st,lIndx)
  + m_sc.GetVal(iChan1st+1,lIndx)) + plateDims.fA*
   (m_sc.GetVal(iChan1st+2,lIndx)
  - m_sc.GetVal(iChan1st+3,lIndx)));
}

 

From Fxyz & Mxyz the center of pressure and the free moment can be calculated as follows:

 

/***************************************************************/
float FORCE_PLATE::RtnAx(LONG lIndx)
//  Returns COP-X value for plate
// lIndx- sample frame [offset one]
{
 float xRet,fz=RtnFz(lIndx);
 if (fz>fThresh)
 {
  xRet=(RtnFx(lIndx)*plateDims.fAz-RtnMy(lIndx))/fz;
  if (xRet<(-plateDims.fLx/2.) || xRet>plateDims.fLx/2.) file://Off Plate?
   xRet=0.;
 }
 else
  xRet=0.;
 return(xRet);
}
/***************************************************************/
float FORCE_PLATE::RtnAy(LONG lIndx)
//  Returns COP-Y value for plate
// lIndx- sample frame [offset one]
{
 float xRet=0.,fz=RtnFz(lIndx);
 if (fz>fThresh)
 {
  xRet=(RtnFy(lIndx)*plateDims.fAz+RtnMx(lIndx))/fz;
  if (xRet<(-plateDims.fLy/2.) || xRet>plateDims.fLy/2.) // Off Plate?
   xRet=0.;
 }
 return(xRet);
}
/***************************************************************/
float FORCE_PLATE::RtnMzfree(LONG lIndx)
//  Returns Moment-Zfree value for plate
// lIndx- sample frame [offset one]
{
 float xRet,fz=RtnFz(lIndx);
 if (fz>fThresh)
  xRet=RtnMz(lIndx) - RtnFy(lIndx)*RtnAx(lIndx)
   + RtnFx(lIndx)*RtnAy(lIndx);
 else
  xRet=0.;
 return(xRet);
}

I hope this helps. I am leaving Fri 2/18 for a week so fire away before I leave with more questions.

 

Best, Jeremy