The FORCE_PLATFORM group
Force-plates are used to measuring forces and moments – typically the ground reaction forces and moments produced by human gait although other applications exist. The FORCE_PLATFORM group is used to store information about the type, location, and implementation of the force plates within the data collection environment.
The FORCE_PLATFORM group must be present whenever a C3D file contains analog data from force platforms. It describes the type of force platforms used, their locations within the calibrated 3D data recording volume, the assignment of force plate signals to specific analog channels as well as storing the force plate calibration data required to calibrate the platform or interpret the force platform signals. This is one of the more complex parameter groups to set up but, in general, it is usually only done once for any given data collection configuration. Once it has been setup correctly, it need not be changed unless the force plates change their location within the calibrated 3D data collection volume.
Since many applications use the parameters from this group to determine if force plate data exists in the C3D file it is a good idea to include the parameter FORCE_PLATFORM:USED with a value of zero even if no force plate data is present. This will enable other applications to determine that the C3D file does not contain any force platform data.
The C3D file format allows force platform information to be recorded in any analog channel. There is no requirement that force platform data be ordered in any specific way in the recorded analog data as the FORCE_PLATFORM:CHANNEL parameter is used to specify the correspondence between recorded analog data channels (1, 2, 3 etc) and force platform channels (e.g. Fx, Fy, Mz).
The physical location and orientation of the force platform within the 3D data collection space is defined by the FORCE_PLATFORM:CORNERS parameter. This parameter provides information about the location of the corners of the platform in 3D space and the order in which the corners are specified provides the rotational alignment between the 3D co-ordinate system and the force plate co-ordinate system as well as information to compute the locations of force vectors, center of pressure, etc., in the calibrated 3D space.
Analog data from the force platforms is scaled using the ANALOG:GEN_SCALE, ANALOG:SCALE, and ANALOG:OFFSET parameters that are applied to the raw analog data before its use in the force plate computations. The raw analog data is stored within the C3D file within the range of the range of the recording hardware (the ADC card) e.g., -5 volts to +5 volts, or 0 volts to +10 volts. The ANALOG:OFFSET, SCALE and GEN_SCALE factors are used to convert the recorded raw analog data to force and moment values while the OFFSET is simply the raw data value corresponding to 0 volts of input. The ANALOG:OFFSET value is subtracted from each analog data value before the scale factor for the channel is applied.
Two values must be determined before it is possible to calculate the scale factors for each force plate channel. These are the force plate sensitivity value, and the ADC sensitivity value:
• Each individual force plate output channel has a value associated with it by the manufacturer that expresses the sensitivity of the channel – generally in terms of the amount of force required to produce one volt of output or the moment that must be applied to the plate to produce one volt of output. This information is usually available from the manual supplied by the force plate manufacturer.
• The ADC sensitivity value is expressed in units of volts per bit, where a bit is a raw analog data unit (4095 bits will correspond to full scale for a 12-bit ADC). Note that the ADC sensitivity depends on both the hardware range setting of the ADC as well as any gains that are applied to the signal, in either hardware or software before the data is recorded.
If the force plate sensitivity for a given channel is S, and the ADC sensitivity is A, then the value to enter into the ANALOG:SCALE parameter for that channel is A*S, i.e. the units for the scale factor parameter must be force/bit or moment/bit. If the parameter ANALOG:GEN_SCALE is not set to 1.0, then the value of A must be first divided by the value used in ANALOG:GEN_SCALE.
Alternately, the ANALOG:GEN_SCALE parameter may be set to the value of A, then the ANALOG:SCALE factors can be set to the values of S for each individual channel to provide the desired result. Care must be taken to use consistent units, i.e. if force is being expressed in Newtons, the moments should be in Newton-millimeters (Nmm) or Newton-meters (Nm).
A full discussion of all the factors involved in calculating analog scaling factors can be found if the discussion of the analog SCALE parameters – please refer to this chapter for complete details (including worked examples) of the calculations.
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USED