Convert an electronic personal weighing scale to measure center of pressure (Posturography)

Warning: This application note illustrates a typical strain gage application and is not for actual medical use.

One use of Center of Pressure measurement is in the study of standing posture - Posturography. By measuring the position of the Center of Pressure at regular intervals, a plot called a spaghetti diagram can be generated. By calculating the mean and standard deviation of the center of pressure, an indication of sway is derived¹.

In the paper Context Acquisition Based on Load Sensing², an algorithm is given to measure the center of pressure using four load cells. If the total force exerted at x,y by the load is F_L and assuming static forces, the sum of all 4 load cells F₁, F₂, F₃, and F₄ is equal to F_L as given by eq 1

Similarly, the no load force F_0L is the sum of the forces on each load cell F₀₁, F₀₂, F₀₃, and F₀₄ - eq 2. The position of the load can be calculated by eq 3 and 4 using the load and no load forces.

An electronic Personal Weighing Scale (we used the Soehnle 62867 Hawaii Monotronic Scale) commonly uses 4 half bridge strain gages to measure weight. By using these 4 gages, the strain application adaptor and the EMANT300 USB DAQ module, we are able to measure the center of pressure in a low cost way.

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