Human Posture Measurement System

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Human Posture Measurement System
Brandon Ngai Lawrence Wong Josephine
Wong ensc440-u-sense_at_sfu.ca
Team Personnel Contact Email
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Overview

• Overview of project
• Functional specifications
• System design
• System testing
• Current progress
• Future improvements

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The Objective

• To develop a portable human monitoring device
  that tracks and records the movement of different
  parts of the body
• Should combine motion-sensing, data-logging, and
  data-transmission capabilities

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The Motivation

• Motivated by the research needs of Dr. Steve
  Robinovich (SFU) and Dr. David Rempel (U of
  California)
• Limited number of tools available for human
  movement and injury prevention research

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Intended Applications

• Study of the loss of balance and falling in the
  elderly
• Prevention of hip-fractures and possible
  life-threatening injuries
• Examination of the causes of work-related
  injuries in construction workers
• Prevention of carpal tunnel syndrome

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Current Solutions

• Similar devices are commercially-available
• Data-loggers
• MicroStrain Virtual Corset
• Motion-capture system

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MicroStrain Virtual Corset

• Measures the angle of inclination of the trunk of
  the body
• Operates at 900 µA at 3.6V
• Commonly used in human movement research studies

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MicroStrain Virtual Corset

• Limitations include
• High cost
• 1,000US per unit
• Uses specialized batteries
• Limited measurement range
• Reduced resolution between 85 and 90

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Motion-Capture System

• Markers are mounted on the subject
• Video cameras are used to track the markers
• Requires a minimum of 3 cameras

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Motion-Capture System

• Limitations include
• High cost
• Difficult to transport
• Mostly limited to lab settings
• Complex software packages are required to extract
  the data from the video feed

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The microSense Solution

• Pager-sized sensor units that operate
  autonomously from other units
• Measuring the angle of inclination of a body
  segment in three-dimensional space (with respect
  to gravity)
• Stores acquired data in internal flash memory for
  up to 12 days
• Uploads data to a computer via USB

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The Device In Action

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Functional Specifications

• 0.5 degree resolution
• Measures 360 of rotation about the x-axis and
  y-axis
• Acquires data at 32 samples per second
• Stores up to 1 Gbit of data (12 days)
• Powered by 2 AA batteries (final design)

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Device Limitations

• Insensitive to rotation about the vertical axis
• Sensor measures changes in orientation with
  respect to gravity
• Unavoidable with the chosen sensor technology

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System Overview

• Uses two micro-electrical system (MEMS)
  accelerometers to measure the angles of
  inclination
• Controlled by a PIC18 microcontroller with
  built-in USB capabilities
• Raw data is stored in 1 Gbit flash memory chip
• Angle outputs are calculated by the computer
  terminal

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System Block Diagram
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Device Schematic
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Inclination Sensors

• 2 x Analog Devices ADXL203E MEMS accelerometers
• Mounted perpendicular to each other
• Most sensitive when the measurement axis is
  perpendicular to gravity

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Data Acquisition

• Continuous data acquisition at 32 samples per
  second
• Sensors provide analog outputs
• Digitalized using the 10-bit ADC on the PIC18
  microcontroller
• Non-linear relationship between sensor output and
  angle of inclination
• Calculated using arcsin function

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Flash Memory

• Toshiba TC58DVG20A1 1-Gbit NAND flash memory
• Holds 128Mb of data
• Durable and reliable
• Interfaces with the PIC18 microcontroller via 8
  address/data lines and 7 control lines

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Data Structure

• 2 bytes per sample

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Data Storage

• A 1-Gbit chip can store up to 12 days of data at
  32 samples per second
• 2-Gbit NAND flash memory chips are also available
• More difficult to acquire

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Data Transmission

• Data is transmitted to a computer via an USB
  cable
• The computer automatically recognizes the sensor
  device
• Data transmission begins at the users command
• Graphical user interface

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Data Conversion

• Angle measurements are calculated from the raw
  data by the computer

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Graphical User Interface

• Windows-based GUI
• Allows user to establish and terminate data
  communications with the sensor units
• Allows user to initiate data transfer

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Data Output

• System outputs a comma separated value (CSV) file
• Lists the angles of inclination with respect to
  the x-axis and y-axis at each sampling time
• Readable using Microsoft Excel

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Design Challenges

• Component identification and acquisition
• Long shipping delays
• Need for adapter boards for small packages
• Subdividing the system
• Difficulties in integrating the system modules

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Testing Protocol

• Real-time testing
• Used to verify sensor accuracy and
  sensor-to-microcontroller communications
• System testing
• Required to verify data storage and retrieval
• Ensures the proper integration of the system

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Real-Time Testing

• Device is connected to a computer via USB
• Enables sensor calibration
• Device outputs are compared to a 1-axis digital
  level for accuracy

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System Testing

• Need for extensive system testing to determine
  the accuracy and reliability of the device

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Testing Challenges

• Difficult to test each system module
  independently of other modules
• Heavily reliant on USB-to-PC interface during
  testing
• Difficult to verify timer operation
• Hard to pinpoint problems and debug the system

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Device Characteristics
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Device Characteristics
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Power Consumption

• Prototype is powered by 3 AA batteries
• Standby mode
• 13mA of current
• Data acquisition mode
• 45mA of current
• Data transmission mode (USB connected)
• 50mA of current
• Need to minimize power consumption

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Current Status

• Device can measure, record, and transmit data
• Can also operate in real-time mode (for system
  testing)
• Currently developing a time-stamping algorithm
  (to synchronize data from multiple sensors)
• Need to test the system for accuracy and
  reliability

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Future Improvements

• Final prototype will require 2 AA batteries
• Users will be able to mark key events in the data
  with the press of a button
• May implement wireless data transmission

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Questions?