Transcript CribPAD - the Department of Electrical and Systems Engineering

Crib Pediatric Dynamometer
Group 12
Authors
Brian Dodd EE ’07
Stephen Lin EE ’07
John Shon EE ‘07
Advisors
Dr. Jay Zemel
Dr. Babette Zemel
Demo Times
Thursday, April 19, 2007
10:00-12:00PM, 1:00-1:30PM
University of Pennsylvania
Dept. of Electrical and
Systems Engineering
OOOO
The Master PIC feeds data from the
sensors to the computer by serial
communication via USB. The GUI is
built in Matlab, which will process the
data from its raw format into a force
reading using the calibration curve and
the following formulas:
OOOO
Slave PIC
Slave PIC
Slave PIC
Signal
Conditioning
Signal
Conditioning
Signal
Conditioning
Sensors
Sensors
Sensors
Once data has been acquired from each of
the 72 sensors, the master PIC sends the
time stamped data through an Rx-Tx line to
a USB serial port for recording and
processing on the computer. The entire
board of sensors is sampled and the
corresponding data sent to the computer
every 5 milliseconds.
The data is then graphed in a threedimensional bar graph while being
written to a file on disk simultaneously.
Signal Conditioning Circuit
Sensor Array
The signal conditioning circuit serves two primary purposes: multiplex the
sensor array and prepare the signal for the slave microcontroller input.
The multiplexing is performed by an 8-to-1 analog chip controlled by the
slave microcontroller. The signal is then amplified to maximize the use of
the available quantization levels of the A/D converter in the microcontroller.
The last step involves offsetting the signal by 2.5V so that it is in the middle
of the voltage range of the microcontroller. The layout of the signal
conditioning circuit is given below.
An array of Measurement Specialty’s LDT2-028K piezo film sensors is
used to detect activity. When the sensor is subjected to a change in force it
generates an external current. An example waveform from the output of
the sensor when it is tapped is given below. The output from the sensors
had to be calibrated against actual force data as a layer of padding is
placed on top of the array. A Pasco Scientific CI-6618 Force Sensor was
used to log force data. The maximum force from the force sensor was
compared against a value obtained by integrating over the range of activity
from the output of the piezo film sensor. A calibration curve was created
using this data and is found below.
Calibration Curve
Sensor Output vs. Time
Buffer
Amplifier
Offset
A/D
35
3
2
Force Applied
1
0
-1
-2
0.5
0.7
0.9
Force Removed
1.1
Max Force from Force
Sensor (N)
All data is serially communicated to a computer from
the master microcontroller. At this point, the data
can be processed on a computer and analyzed
through a graphical user interface allowing the
researcher to visually and analytically examine the
data.
Master
PIC
Sensor Output (V)
An array of sensors that cover the area of a crib
mattress continuously measures changes in the
force applied by the infant.
A master-slave
microcontroller configuration is used to sample each
of the sensors. Each slave microprocessor samples
a column of sensors and the master microprocessor
collects data from each of the slaves. This masterslave configuration allows for easy expansion of the
system to meet different sizing needs.
Graphical User
Interface
GUI
Computer
Data collection from each of the sensors is
controlled and exported through a masterslave PIC scheme. Sampling of each of the
8 sensors in a column is controlled by a
slave PIC16F88 microcontroller, and the 8
data bytes are saved in the PIC. When
each of the slave microcontrollers have
acquired their samples, the master
PIC16F690 microcontroller coordinates the
orderly transfer of data from each of the
slave PICs to the master PIC.
MUX
Many researchers are currently studying the activity
of infants in search of a relationship between energy
expended at an early age and the increasing
obesity epidemic. Collecting meaningful data in this
area has many obstacles. Current data acquisition
techniques are very complicated, costly, limited by
age, and unnatural for the subject. The idea of
monitoring an infant’s activity in a crib with the use
of piezo sensors bypasses some of the obstacles
found in current methods.
Master-Slave PIC
System
Sensors
Abstract
2
R = 0.8777
30
25
20
15
10
5
0
0
Time (s)
0.01
0.02
0.03
0.04
0.05
0.06
Integrated Max Force from PVDF (V*s)
0.07