Specifying Force Platform Parameters

Force platforms may be mounted in any orientation with respect to the 3D reference coordinate system. The problem of measuring the location of the force platforms in the reference coordinate system is easily overcome by placing a marker on each corner of each force plate and then measuring the locations using the calibrated 3D system, taking the height of the centers of the markers above the force plate surfaces into account to record the correct surface z-coordinates within the 3D environment. The 3D coordinates of each force plate in the 3D environment must be stored in the CORNERS parameters in the correct order to document both the location of the force plate and its orientation within the 3D environment.

The internal force plate coordinate system is defined by the force plate manufacturer. Usually the force plate coordinate system z-axis points vertically downwards and the origin is somewhere near the geometrical center of the force plate, just below the force plate surface. This is not always the case; refer to the manufacturer's manual to identify the correct force plate coordinate system for each plate. Corner number 1 as specified in the FORCE_PLATFORM:CORNERS parameter must be in the 1st quadrant of the force plate X-Y plane (positive X and Y), corner 2 in the 2nd quadrant (negative X, positive Y), corner 3 in the 3rd quadrant (negative X, negative Y), and corner 4 in the 4th quadrant (positive X, negative Y). The corner information is used to draw the force plate locations in stick figure displays as well as to compute the transformations from force plate coordinate systems to the reference coordinate system.

The FORCE_PLATFORM:ORIGIN parameter is used to specify the location of the origin of the force platform coordinate system relative to the geometric surface of the force platform for types 1, 2 and 4 force platforms. The origin of the force platform coordinate system is normally determined by the sensors and analog electronics of the force platform system. In a normal analog force plate, the origin is some distance directly below the geometrical center of the force platform surface because the force plate sensors are located below the surface of the plate. In practice the center may be translated a small distance laterally because of minor imperfections in geometry or transducer sensitivities.  As a result the FORCE_PLATFORM:ORIGIN vertical (Z) coordinate will almost always be a negative value.

Digital force plates use standard analog sensors to sense the forces applied but then process the signals internally to remove internal crosstalk or subject motion on a treadmill belt above the force plate, thus generating calculated force and moment signals while potentially compensating for subject motion on the plate surface in a treadmill configuration.  Since the applied force and moment signals are digitally processed and recalculated before being presented to the user, the force and moment signals will normally be referenced to locations determined by the processing, not the physical location of the plate.

Instrumented dual belt treadmills normally generate force and moment signals from two individual force plates, each generating force and moment data referenced to a unique location.  This can result in the collection of force and moment data from two apparently identical force plates, each generating data that has been calculated and referenced separately with a different origin.  As a result, some applications may display the force platform origins incorrectly.

Normally, very little error will result from specifying that the force platform origin is located directly below (in the force platform Z-direction) the geometric center of the plate. However, an experimental determination of the of the X-Y location of the force platform origin may be made by identifying the location on the force platform where a vertically directed force produces zero X and Y moments as measured at the FP outputs. The force platform coordinate system origin's distance below the working surface must be correctly specified to produce accurate center of pressure results for forces that are not normal to the force platform surface.

A simple test of the force plate performance within the 3D measurement volume may be carried out by the use of a rod about one meter long and about which is wrapped two strips of retroreflective material approximately 10mm wide, at locations a short distance from either end.  Collect force plate and 3D data with the lower end of the rod on the force platform and a force applied to the top of the rod, moving the upper end of the rod around while the force is applied in order to create varying angles of force applied to the force plate surface. Then generate stick figures showing the force vector and a segment joining the two rod markers. If the force platform is correctly set up the force vector and the line joining the two markers should coincide for the full range of motion of the stick.