Transcript Background Lecture - IEEE Real World Engineering Projects

Birth Weight Measurement
in Midwifery and Mobile
Health Care Delivery
Dr. James Andrew Smith *
Dept. of Electrical & Computer Engineering
Ryerson University
Toronto, Ontario, Canada
*Member, IEEE
Your mission:
Create a baby weighing system!
• Weigh a baby up to 5kg
• Over two month period
• Compare to CDC “Weight for
Age” Chart
• Recommend transfer of care
(to hospital) if baby becomes
underweight.
Figure: Midwife measuring a three-week
old baby in a home environment.
Importance of Birth Weight
• The percentage of disabilities increases
with lower birth weight.
• Decreased weight after birth can be an
indication of health problems, too.
Source: (Laura E. Berk and Stuart G. Shanker. Child Development.
Pearson Education Canada, Toronto, Canada, Second Canadian edition, 2006. )
Engineering
Development Process
1.
Identification of problem
–
–
2.
What is possible?
Keep it simple & effective!
Risk evaluation
–
5.
6.
7.
8.
Prototype
What actuators and sensors? At what cost?
Determine level of functional replacement
–
–
4.
What function is missing?
Talk to the clients/users!
Identification of affordable technology
–
3.
Start
Never underestimate what can go wrong!
Prototype device, test & start again (Steps 1 -5)
Test on larger population set
International certification
Manufacture & distribute device

Test

Manufacture
End
Rapid Prototyping
• “Express - Test - Cycle” approach to design
–
–
–
–
–
–
Identify a need & design objectives
Brainstorm for solutions
Express an idea in a physical device
Test the device
Discover problems that you weren’t aware of
Repeat until you’ve met the design objectives
• Rapid prototyping systems
– Combine modular, off-the-shelf components
– Great for quick mock-ups & functional testing
Breadboard system
Project Design Overview
2 kg
2 kg
Step 1:
Place baby
on scale.
Step 2:
Scale bends
slightly.
Measure
mechanically or
electrically?
Step 4:
Step 3:
Display
Amplify
measurement. measurement.
Display
mechanically or
electrically?
Hooke’s Law:
Measure bending force
• Relate Force and Displacement
• Force in a spring (F)
– Proportional to change in length (∆l)
– Spring constant: k
F  k  l
Electricity Background
• Voltage [Volts]:
– A “force” that tries to move electrons
– Provided by devices such as batteries
• Current [Amperes, Amps]:
– This is the “flow” of electrons
– Carried by wires
• Resistance [Ohms]:
– Resist the flow of electrons
– Intentionally provided by resistors.
– Unintentionally provided by almost all real-world
components
Ohm’s Law
• Relates the main
electrical measures
• I=V/R
– Battery has constant
voltage [V]
– Current [I] varies
with resistance [R]
– Larger resistance
means smaller
current
Potentiometers
• Change resistance has you turn a knob.
• Often found in stereo volume controls
Actual Potentiometer
Schematic symbol
Images: Wikipedia
Power and Energy
• Energy is measured in Joules
• Power, measured in Watts, is
Energy per unit time:
• Electrically, the power being
used in a circuit with fixed
voltage and current is:
P
E
t
P  IV
Example: if a 12V battery provides 1 Amp of current to power the stereo in
your car, it is providing 12W of power. If the stereo is on for 1 hour, the
battery provides 12W(60 minutes)(60 seconds/minute) = 43,200 Joules.
Batteries as an Energy Source
• Batteries are made of individual cells
• Series cells: more voltage
• Parallel cells: same voltage, longer life
Single Cell
Series Cells
Series & Parallel Cells
Resistances in Series & Parallel
• Resistances in
series add up.
Req  R1  R 2
• Resistances in
parallel:
Req 
Series Resistance
1
1
1

R1 R 2
Parallel Resistance
Voltage Drops
• Batteries increase circuit voltage
• Resistors & other devices “drop” voltage
– Sum of “drops” equals battery voltage
• Imagine walking on a mountain.
– Battery raises you to the top
– Resistors, etc. drop you down.
How can we measure
forces applied to objects?
• We can’t!
– Not directly, at least.
• We measure secondary effects, instead
–
–
–
–
Acceleration
Bending
Compression
Structural failure
• These are related to force but are not the
force itself.
History of Measuring Strain
• Historically, we used “Extensometers”
– Invented by Charles Huston
– Measure the length before and after a
stress is applied to the material
– Levers amplify the strain so that you can
see it.
Extensometer
http://upload.wikimedia.org/wikipedia/en/6/6e/Extensometer.jpg
Modern Strain Gauges
• Better: Metal foil is glued onto the
material to be tested
– Moderate temperature sensitivity
• Best: semiconductor processes
– Uses photolithography & diffusion
– “Spray” a conductor onto an electrical
insulator
– More stable vs. temperature
Poisson’s Ratio: Demonstration
• What happens when you stretch an elastic
with your fingers?
Poisson’s Ratio: The Math
• When you pull on an object it becomes
narrower
• This is Poisson’s ratio:
How do Poisson’s Ratio and
Electrical Engineering go Together?
• Through flexible circuits and resistivity
• A thin resistor resists more than a thick
one
– Resistance is inversely proportional to
diameter & length
Less
More
ResistiveResistive
AWG
Diameter
(“Gauge” (m)
)
Resistance /
Meter
(Ohms)
18 (thick)
0.0010
0.02
24
0.0005
0.09
32 (thin)
0.0002
0.55
What is a Strain Gauge?
• A device that measures deformation (strain) in
a material cause by forces (stress)
• Kinds of Stresses:
– Tension: pull apart
– Compression: push together
– Shear: parallel to the plane
• Stress is the force applied over an area
– Compressive stress
– Tensile stress
More about Strain
• Strain results from the applied force
– It’s the ratio of change in length over original
length:
– Negative if compressive (push)
– Positive if tensile (pull)
• Longitudinal strain: same axis as stress
• Transverse strain: perpendicular to stress
– A rubber band’s thinning is “transverse” strain
– No force is associated with it
Modulus of Elasticity
• It relates Force and Deformation
– Deformation = change in shape
• Also known as “Young’s Modulus”
– Ratio of stress over strain:
• Poisson’s Ratio is a ratio of strain
– Logitudinal strain over Transverse strain
Back to Strain Gauges
• Metal foil, in a zig-zag pattern, glued to
the material that will be strained
• Both the foil and the material will deform
when stressed
A Load Cell: Strain Gauge + Extras
• Three fixed
resistors (R1, R2,
R3)+ strain gauge
(Rgauge)
• Has good
sensitivity
• Output voltage,
Vout, changes with
change in
resistance of strain
gauge, Rgauge.
R1
Vout
Rgauge
Vi
n
R2
R3
Concept: Sensitivity
• Low sensivity:
large change in
input results in
small change in
output
• High sensitivity:
small change in
input results in
large change in
output
Sensor Limitations:
Error vs. Precision
• Understand the limitations of
the sensors you use
• Error evaluates a
measurement’s closeness to
High Precision, Low Accuracy
a “true” value
– Opposite is “accuracy”
• Precision is degree to which
the measurement is
reproducible
Low Precision, High Accuracy
Error & Precision Measures
• Use “error”, the opposite of “accuracy”.
– Express as “full scale” (max reading) of
sensor
xt  xm
eFS 
100%.
x max  x min
• Use “standard deviation” for precision:
N

s
2
(x

x
)
 i m
i1
N 1
.
Final Project:
Goals
• Criteria:
–
–
–
–
Mass range of 0 – 5kg (see Note 1)
Operating temp. ~20 C
Accuracy of 0.1 kg (see Note 2)
Measurement time of under 5 seconds
• Keep in mind:
– Target group for the final design (midwives & babies in remote
locations)
– What socio-economic factors affect engineering projects?
– Where will the device be used?
– How will the target group use the device?
Note 1: low birth weight: 2.5 kg (http://www.cdc.gov/nchs/fastats/birthwt.htm)
Note 2: gains every 12 days should be 0.2 to 0.3 kg. (http://www.nlm.nih.gov/medlineplus/ency/article/007302.htm)
Final Project: Options
Spring
Baby
to be
weighed
Spring
Load Cell
Lever / Gear
Mechanical
Measurement
& Display
Potentiometer
Resistance
Measurement
& Display
Amplifier
Voltage
Measurement
& Display
Packaging for the Real World
•
KISS: “Keep it Simple, Stupid!”
– Simpler designs have less flaws
– Murphy’s Law: “If it can go wrong, it probably will.”
•
Intuitive usage
– Nobody reads the manuals
– Must be easy to recharge & operate!
•
Rugged design
– Can you drop it without breaking it?
•
Design for the local environmental conditions
– Ex.: dust, sand, snow, humidity, etc.