We are using a Hercules (model #F4-4R, 100 psi) pressure sensor to measure the pressure between the foot and shoe. An interface circuit converts the capacitance change into voltage. Over the pressure range from 0 to 1300 kPa, the capacitance changes from 275 to 580 pF. A 555 timer circuit converts the capacitance into a frequency range from 30 to 63 kHz. A phase-locked loop (PLL) converts this frequency to voltage from 0 to 5 V, which is then filtered using a first-order, low-pass filter, having a corner frequency of 20 Hz to reduce the ripple to 10 mV. The sensor's hysteresis is about 8 percent at 40 degrees Celsius (C) and 12 percent at 20 degrees C. The sensor has a maximal nonlinearity of 8 percent and a worst-case nonrepeatibility of 7 percent. Its temperature coefficient is -0.147 percent per degree C. Its spatial sensitivity decreases nonlinearly from 1 to 0.17 from the center towards the periphery. The sensitivity of the system is 2.77 mV/kPa and the temperature drift is +0.53 percent per degree C. We monitor the pressure at 7 locations under each foot (the rear and the front heel, great toe, and 4 of the 5 metatarsal heads). A portable data-acquisition system permits continuous monitoring for 7 minutes. Test results for pressure distribution for normal walk and run are presented. Results are useful when studying normal and abnormal gait, and for possibly providing feedback (sensory substitution) to diabetic patients with insensate feet in order to help them dynamically adjust pressure distribution under their feet.
Three different electronic circuits for characterizing the commercially available Orthoflex Hercules capacitive pressure sensors over the pressure range of 0-2 MPa are discussed. A portable microprocessor-based system is used to measure the pressure between the foot and the shoe. Two of the circuits, the frequency-to-voltage converter and the phase-locked-loop circuits, have the advantages of simplicity of design and higher bit resolution over counter-type circuits, but they have higher component counts, resulting in greater current consumption and less pressure monitoring time. The counter circuit, though more complex in design, does not have a voltage ripple at the output stage, which compensates for the error due to its poorer bit resolution
Willis J. Tompkins合作论文数Department of Biomedical Engineering, College of Engineering, University of Wisconsin-Madison;Department of Electrical and Computer Engineering, College of Engineering, University of Wisconsin-Madison2
John Webster合作论文数Department of Biomedical Engineering, College of Engineering, University of Wisconsin2
Jacqueline J Wertsch合作论文数Milwaukee1
