wearable computing - university of...
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Wearable Computing
Alexander Nelson
November 2, 2020
University of Arkansas - Department of Computer Science and Computer Engineering
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Review: Applications of Wearable Computing
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Fitness & Wellness Tracking
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Fitness & Wellness Tracking
One of the more well-known applications of wearable computing
Application Specific – Detect physiological parameters & report
Examples:
• Inertial Sensors
• Pedometers/Step Counters
• Sleep (w/ microphone)
• Accelerometers/Gyroscopes/Magnetometers (IMU)
• Evoked Potential (Electrodes)
• Heart-rate
• Sweat sensors
• Location (GPS/Wi-Fi/Cellular)
• Air Quality
• Respiratory rate
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Function Determines Form
Many Fitness & Wellness applications require specific locations
e.g. Heart-rate/Pulse monitor requires skin contact near pulse
location
Respiratory rate around chest or near mouth/nose
Some of these sensors can be moved off-body
Only works under certain circumstances
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Example: Wi-Breathe
Wi-Breathe – Uses phase
change of Wi-Fi signals to
detect respiratory rate
Requires person to be in
the home, & not occluded
by another individual
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Function Determines Form
The chosen application determines what sensors/data are needed
• Pulse – Voltage Amplifier + A/D Converter
• Step Counter – Mechanical Switch/Accelerometer
• Air Quality – Chemical concentrations and Particulate
Counters
Also determines how accurate the sensors and data need to be
• Step Counter – ±5% average sensor – No need to be exact
• Pulse – Depends on application
e.g. medical vs. activity and amateur vs. professional
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Measuring/Evaluating Accuracy
With real valued numbers, “accuracy” often refers to both
accuracy and precision
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1By Pekaje at English Wikipedia - Transferred from en.wikipedia to Commons.,
GFDL, https://commons.wikimedia.org/w/index.php?curid=1862863
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Accuracy/Precision/Resolution
Precision – Description of random errors/statistical variability
Accuracy – Description of systematic errors/statistical bias
These two parameters are independent of each other
Resolution – Smallest change in a quantity that can represent a
change in the measurement device
Properly evaluating these quantities in your system is important
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Example: Step Counting
How do you measure accuracy of a step counter?
• How do you define a step?
• MW – “a movement made by lifting your foot and putting it
down in a different place”
• Orendurff – “unweighted, moved to a new location, and then
re-weighted, in the load path of the body ”
• Binary classification accuracy of a single step?
• Steps counted in a given time frame? (e.g. 79 counted, 81
actual)
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Example: Step Counting
Figure 2: Raw Acceleration Signal2
2Mladenov et al 20099
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Example: Step Counting
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3Mladenov et al 2009 10
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Example: Step Counting
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4Mladenov et al 2009
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Example: Step Counting
The above example used the following approach:
1. Convert and filter 3-dimensional raw timeseries to
1-dimensional time series
2. Hill-climbing and thresholding to count steps
This method means that classifying steps only works in the context
of other steps (1-step motions would not be counted)
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Reporting Sources of Error
It is important to document your assumptions that could represent
sources of error
Example: Bassett Jr. et al 2016 show that undercounting steps is
a systemic problem with step counters
“... and at 26.8 m/min (1 mph) most devices will record only
50-75% of actual steps”
Overcounting can occur with extra mechanical motions
Example: Fitbit wrist-worn users would trigger steps while brushing
their teeth
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Reporting Error
In an estimator/predictor system (like counts of steps), the Mean
Squared Error (MSE) is often used as a descriptor
MSE (θ) = Eθ[(θ − θ)2]
MSE = 1n
∑ni=1(Yi − Yi )
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Error parameter that penalizes larger errors, regardless of sign
May also use Mean Absolute Error
MAE = 1n
∑ni=1 |yi − xi |
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Binary Classification
Binary Classification Errors
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Precision vs. Recall
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