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PEDIATRIC STEP MONITOR Abstract The downward trend of physical activity in children is of great concern to public health because it is directly linked to the growing epidemics of obesity and osteoporosis. Research has shown that the intensity of weight-bearing activities is the most crucial factor in developing bone mass and strength, thus the Pediatric Step Monitor is a tool that measures the magnitude and duration of forces exerted on the soles of children’s feet. The device will aid current bone health studies in detecting trends, evaluating programs, and quantifying the correlation between load-bearing activity and bone development. Authors Linda L. Lamptey, EE ’05 Jennifer W. Sin, EE ’05 Michael C. Wong, EE ’05 Advisors Dr. Jay N. Zemel Dr. Babette Zemel Special Thanks Dr. Mitch Thompson, MSI Overview System Design The Pediatric Step Monitor consists of a piezoelectric polyvinylidene fluoride sensor, a signal conditioning circuit, and a digital subsystem. The signal generated by the sensor is proportional to dF/dt, or the time rate of change in force applied to it. Since researchers are interested in actual force magnitudes and energy expenditure values, the sensor output is processed according to the following equations, in order to obtain the desired quantities. dF dv F dt m a m dt d t dv 1 dF 1 dF dt v dt d d m t dt m t dt 1 2 1 dF Kinetic Energy m v dt d 2 2m t dt April 21, 2005 11 AM, 1:30, 2:00, 2:30 PM RCA Lab The purposes of the circuit are to • transform the high input impedance from the sensor to a low output impedance for the ADC • amplify the sensor output voltage and scale it into a desirable range of 0-5Vpp with 2.5VDC offset • cancel common mode noise signals using the differential nature of the input signal Up step Department of Electrical and Systems Engineering Down step Measurement Specialties, Inc. DT1-028K Piezoelectric PVDF Sensor Analog Devices AD623 Instrumentation Amplifier Microchip PIC16F88 Microcontroller The DT1-028K sensor with spring steel backing sits under the heel of a child’s foot, unobtrusive and fixed to the bottom of a shoe insole. When the sensor is deformed as during a typical step, an external current proportional to the applied force is induced. 2 A low power, single supply instrumentation amplifier acts as the interface between the sensor and the microcontroller ADC. dF/dt System Block Diagram LabVIEW GUI Signal Conditioning Demo Schedule Sensor Operation Digital Subsystem The PIC16F88 microcontroller digitizes the analog signal. The dF/dt samples are integrated over time to extract force. In order to conserve memory space, the data processing algorithm looks for one “step” (one deflection and relaxation cycle) in the signal and stores the peak and average forces for that cycle. Results The calibration curve below shows the relationship between the device output signal and the magnitude of the applied force. The curve was obtained using a force plate, test subjects of different weights, and the pictured apparatus. Energy expenditure is displayed in Lab VIEW.