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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  ma  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  mv 
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.