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Load Cell - standingThe Omega LC302-100 is a 0.75" diameter stainless steel compression load cell, with 0-100lb capacity. Each pedal on the apparatus is equipped with four such load cells, one near each corner, which allows one to measure the total weight on the pedal as well as the center of pressure. (See remarks below regarding limitations.) The cells have an absolute maximum rating as to the amount of force that
may be applied before being damaged. As such, people must step on and
off of a pedal with care.
LimitationsThe load cell reading is true only when the force applied is perpendicular to the surface area of the load button. Hence, measurements are accurate only when the pedal is in a horizontal position. InterfacingA load cell is interfaced to a strain gage conditioner module. Each module needs to be calibrated with a uniquely associated load cell. For testing an individual load cell on a work bench use this interface box to be able to connect the cell to a strain gage conditioner module. The layout for this box may be found in this
Corel DRAW 9 file. Strain Gage Module CalibrationThe
lab has two 50-lb (+/- 2.3 g), Class F, Type II Troemner grip
handle weights. The idea was to stack the weights onto a load cell
and calibrate the strain gage conditioner to read 10V/100lb. However,
in the absence of a jig to support the weights steadily and squarely on
top of a load cell sway of the weights introduce significant calibration
errors. Hence, the following alternate calibration procedure is used. The calibration procedure relies on the full-scale factory calibration value of a given load cell and does not require that the transducer be connected to the strain gage conditioner.
Center of Pressure (COP)The figure here shows the left pedal assembly and the positions of the four load cells; the right pedal is of a similar arrangement. The load cells are actually sandwiched between the plate and the pedal. The origin of the center of pressure (COP) is defined to be at the centroid
of the bounding box formed by the cells. If Fi is the force measured by the i-th load cell, then the COP (in mm) is calculated as follows: Note that the bias on each load cell due to the weight of the pedal (i.e.,
the baseline reading) must first be subtracted from the Fi before
using these equations. Miscellaneous This sign
was produced with this Corel DRAW
9 file. Last Modified: 2006-08-23 Ross Wagner |