FITtape
Digital Fitness Tape Measure with Data Syncing
Alex Chuang
Table of Contents
Introduction .................................................................................................................................................. 3
Product Overview ......................................................................................................................................... 3
Dimension Specifications .......................................................................................................................... 3
Features .................................................................................................................................................... 4
Hardware Systems ........................................................................................................................................ 4
Microcontroller ......................................................................................................................................... 4
Microcontroller Resources Used............................................................................................................... 5
Electromechanical Systems....................................................................................................................... 5
Magnetic Tape Latch ................................................................................................................................. 5
Measurement Encoder ............................................................................................................................. 6
Display ....................................................................................................................................................... 6
Human Machine Interface ........................................................................................................................ 6
USB Interface ............................................................................................................................................ 6
Bluetooth Interface ................................................................................................................................... 6
Power Supply ............................................................................................................................................ 6
Charging Unit ............................................................................................................................................ 6
Software System ........................................................................................................................................... 7
Software Development ............................................................................................................................. 7
Kernel ........................................................................................................................................................ 7
User Interface ........................................................................................................................................... 7
Encoder Algorithms................................................................................................................................... 8
Modules .................................................................................................................................................... 9
Sustainability ................................................................................................................................................. 9
Standards ...................................................................................................................................................... 9
Bluetooth Low Energy v4.0 ....................................................................................................................... 9
USB v2.0 .................................................................................................................................................... 9
ROHS Compliance ..................................................................................................................................... 9
Development Plan......................................................................................................................................... 9
Development Tasks ................................................................................................................................... 9
Development Platform ........................................................................................................................... 10
Demonstration Prototype ....................................................................................................................... 10
Specifications .............................................................................................................................................. 10
Included components ............................................................................................................................. 10
Sizing ....................................................................................................................................................... 10
Measurement Capability......................................................................................................................... 10
Wireless Sync .......................................................................................................................................... 11
Battery Specification ............................................................................................................................... 11
Power dissipation.................................................................................................................................... 11
Preliminary Parts List .................................................................................................................................. 11
2
Introduction
The FITtape is part of a fitness tracking system that allows the user to easily take body measurements by
digitally measuring and tracking the measurements of the body parts the user wants to track. This
system includes a handheld measurement unit and an application based system used for syncing data.
The user will be able to select the desired body measurement from the handheld unit’s GUI and then
record the measurement using the automatically controlled tape measure. The FITtape features
Bluetooth capability which can be implemented by interested developers to use the tracking
information for fitness applications.
Product Overview
The FITtape is part of a system that consists of a hardware and software approach to fitness metrics. The
FITtape tape product is the hardware interface for users to take measurements and temporarily store
data for performance metrics.
Dimension Specifications
The FITtape is a handheld measurement unit and charging adapter. The FITtape has a maximum
dimension of 3.5”x2.5”x1.25”. The user interface consists of a square LCD screen with a viewing area of
approximately 1.5” diagonal and a capacitive touch strip which also serves as a button. There will also be
a micro USB port on the device for charging and wired data syncing. The tape is 60” long since the
intended use is not for height measurements. The tape will feature a strong magnet at the end that
latches back to the measurement unit body to create a loop for easy one handed measurements. The
charging unit is an OEM USB wall charger, the dimensions will not exceed 2” x 2” x 3”.
Figure 1: FITtape product drawing #1, no capacitive touch strip visual test
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Features
Prioritized by development importance
1. Electronically controlled measurement control system: The most basic operation of the
FITtape is the electromechanical system that simplifies the measurement process into a
one click process.
2. Bluetooth/USB device data syncing: The key feature that sets the FITtape apart is device
connectivity. By having data syncing capability, the end user will have access to long term
data tracking and performance metrics.
3. Intuitive user interface on measurement unit: In order to make FITtape competitive, it
has to provide a streamlined and enjoyable user interface.
4. Rechargeable battery system: A rechargeable system provides convenience for the user
but in the event of development taking too long, an alkaline battery system will be used.
Hardware Systems
The hardware of the FITtape systems is all contained within the handheld measurement unit. This
includes a microcontroller, Bluetooth wireless transceiver, electromechanical components for
automated measurement control, and user interface components such as a screen and buttons.
Figure 2: Block Diagram of the FITtape electronics
Microcontroller
The FITtape is designed to be controlled by a Freescale Kinetis KL25Z128VFM4 MCU. The KL25Z128VFM4
is 48MHz, 32 pin chip with 128 kB flash and 16kB SRAM. It comes with 23 GPIO pins, 12 of which have
interrupts. The RTC of the KL24Z128VFM4 will be used for logging the measurements according to
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calendar and time of day. The chip also features capacitive touch capability which will be implemented
in the form of a capacitive touch strip. The unit cost for 10k units is $1.74.
The prototyping platform will be the Freescale Freedom platform model FRDM-KL25Z. The KL25Z
features a KL25Z128VLK4 sharing the same clock speed and features. The memory is 128kB flash and
16kB SRAM. The chip also has nearly 3 times the GPIO pins at 66 pins, 23 which have interrupts. The
prototyping platform has a total of 84 pins, which needs to be taken into account during the prototyping
stage to prevent using pins that are not available on the final product.
Microcontroller Resources Used
The components of the FITtape will use approximately 16 data lines:
Component
Data Lines Used
Quadrature Rotary Encoder
2
Solenoid power FET
1
Capacitive Touch Slider
2
Switch
1
LCD Screen
5
Bluetooth Module
5
Data Type
GPIO
GPIO
GPIO
GPIO
SPI
SPI
Electromechanical Systems
The measuring unit contains an electromechanical system for controlling the measuring tape that
consists of a solenoid, ratchet and pawl gears, a constant force spring, and the measuring tape. The
microcontroller sends a voltage out to the gate of a MOSFET which is driving the solenoid to unlock the
pawl gear. This allows the measuring tape to retract. The pawl is held in place using a light spring and
the solenoid is a push solenoid which levers the pawl behind the pivot point. The tape is loaded with a
light constant force spring and is 60 inches long.
Magnetic Tape Latch
The tape measure has a magnetic latch at the end that locks back to the opening of the tape to create a
loop for easy one handed usage. The tape latch will most likely use small rare earth magnets in order to
have a usable holding force.
Figure 3: Magnetic
latching mechanism,
rare earth magnets
latch into body.
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Measurement Encoder
A rotary encoder will be mechanically coupled to the tape near the opening of the tape. This encoder is
a 64 pulse per revolution optical encoder with a minimum lifecycle of 1 million full revolutions. This
encoder was chosen due to its higher operational life compared to similar products. The quadrature
behavior of the encoder means that it takes two data lines to send the pulses to differentiate direction.
Display
The display is a 1.5” LCD display with a minimum resolution of 64x64 pixels. It will communicate via SPI
and will take 5 pins. The display will allow the user to select from 8 to 10 different icons representing
body parts along with an icon representing confirmation of measurement.
Human Machine Interface
The user interface is made up of a capacitive click-pad. This is a combination of a touchpad and switch
which is common to many devices such as MP3 players and laptops. The Kinetis KL25 series supports TSI
which is Freescale’s capacitive touch interface. The touchpad will be in the shape of a horizontal strip
and will behave as a slider. Once the user has initiated a measurement, no inputs will be read until the
measurement is done. The user will then have the option to keep or discard the measurement.
USB Interface
The KL25Z64VFM4 features a USB controller with on-chip full-speed and low-speed transceivers. The
USB module will be able to communicate with an application as a human machine interface in order to
isolate the data from accidental manipulation. The KL25Z is configured for full-speed USB and the
FITtape will also be configured to full-speed USB because the 5V USB line will charge the lithium polymer
battery.
Bluetooth Interface
The Bluetooth transceiver on the FITtape unit will use Bluetooth Low Energy, v4.0 standard. The
transceiver chip has an operating range from 1.8V to 3.6V and has a current draw of 16mA receiving or
transmitting. The desired chip is a surface mount chip with a 6mm x 6mm footprint. The chip uses UART
or SPI communication, I have chosen SPI in order to lower the number of pins used. The unit cost is
between $1.50 and $5.80 depending on volume.
Power Supply
The FITtape will use a 3.7V rechargeable lithium polymer battery along with a boost converter to reach
the requisite voltage to drive the solenoid and encoder. In order to have stable operation, the system
will also have a voltage regulator to keep the power output at 3.6V during usage. In order to safely
charge the device on a 5V USB power line, it is necessary to have a charge protection circuit.
Charging Unit
The charging unit will be an OEM USB wall charger with an input of 110V to 240V and output of 5V, 2A
providing a 10W total power output. Because this OEM device is UL certified, there are no needs to
worry about power concerns for safety.
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Software System
The software includes the development of the firmware for the measurement device and a standard
data format for the measurements.
Software Development
The FITtape measurement module will be programmed in the Freescale Codewarrior environment using
ANSI C as the programming language.
Kernel
The FITtape will run on a real time kernel, checking between the machine interfaces and the human
interface. Upon user input to start measurement, the measurement operation will have top priority
without the ability to be interrupted except for low battery shutdown to save the unit from losing data.
User Interface
The screen on the FITtape measurement unit is a monochrome negative LCD with RGB backlight. The
capacitive strip selects a measurement type and clicking the touch strip will initiate the measurement
process. The device can change measurements between centimeters and inches via the settings
selection. The graphics of the user interface will be converted to data by using MATLAB to take the
predesigned icons and turning them into data arrays. This data will be loaded onto the SRAM including
alphanumeric font data.
Figure 4: Measurement Device user interface
screenshot simulation version 1
Figure 4: Icons representing body mesurements
Figure 5: Screen
showing icon in input
mode
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Figure 6: Screen
showing measurement
mode
Figure 6: User interface flow diagram
The user interface goes through two states, measurement and input. The input states can transfer
between each other by scrolling on the touch strip. In order to transition to measurement, the user has
to click to select a measurement. Once the measurement is complete, confirm will return to input
screen. Setting has a similar input fashion where select will move into the select menu and after settings
are changed, confirm will return to the input menu.
Encoder Algorithms
The encoder wheel is designed to be .64” in diameter. In order to get the increments, the circumference
is calculated to be .64” * π. This length is divided into 64 segments. This allows for a resolution of .01” as
intended. The measuring unit will measure natively in inches and convert to centimeters using this
conversion formula:
𝐿𝑐𝑚 = 𝐿𝑖𝑛 ∗ 2.54
The unit will store the inch measurements and only convert to centimeters as needed.
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Modules
The FITtape will utilize several self-written modules along with a few standard modules. The standard
modules includes the Freescale TSS capacitive touch sense software that is included with the Freescale
Kinetis KL25 microprocessors. The Bluetooth transceiver and LCD screen with both also have their own
separate modules. The electromechanical systems will also run in their own module to run as an
interrupt because the measurement a critical process.
Sustainability
Since the FITtape does not use replaceable batteries, the consumer will have to properly dispose of the
lithium polymer battery along with recycle the unit much like a modern cellular phone. The parts on the
measurement unit use enough electronics components that sending the unit in for recycling will be
beneficial to the environment and be more cost effective than destroying it.
Standards
The FITtape has to adhere to several standards for consumer usage. These include Bluetooth v4.0, USB
2.0, and RoHS.
Bluetooth Low Energy v4.0
In order for the FITtape to meet compliance for Bluetooth communication, we have to follow Bluetooth
Core Specification v4.0.
USB v2.0
The FITtape makes use of the USB controller on chip to communicate with host devices. This
communication has to follow the USB 2.0 specification. The device will behave as an HMI in order to
preserve the data on the device and prevent the user from overwriting any important data on the
FITtape.
ROHS Compliance
All ordered components have been RoHS compliant.
Development Plan
Development Tasks
The development cycle of the FITtape is very short with only enough time to build a couple prototypes.
In developing the FITtape, it is important that the first prototype comes out as rapidly as possible in
order to test the functions and long term durability of the product. The development plan occurs mostly
in the winter months. The order of project scheduling is as follows:
Weeks
Task
WINTER 1-3
Completing a working prototype for the electromechanical system
WINTER 4-5
Building working prototype for the encoder and MCU ADC
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WINTER 6
Combine electromechanical control with encoder system for full measurement.
Implement click wheel functionality.
WINTER 7-10
Setting up screen functionality: develop fonts, icons, port to arrays, get LCD to
display
WINTER 11-12 Interface Bluetooth module with device. Begin wireless transmission development
SPRING 1-4
Develop app to take Bluetooth communication, debug data transmission system
SPRING 4
Hardware Design Review
SPRING 5
Software Design Review
SPRING 4,5
Test systems
SPRING 6-8
PCB design, simplification of systems
SPRING 8-11
Final testing, video shooting, project presentation design.
The delivery time critical parts are the initial parts for each system. The electromechanical, HMI, and
Bluetooth interfaces can be developed concurrently. The development tasks laid out in the schedule use
a logical progression of what components are most vital to the function of the FITtape.
Development Platform
The development platform is the Freescale Freedom KL25Z development board and the Freescale
Codewarrior environment for microcontrollers. This development platform supports all KL series chips
as long as the program doesn’t run over memory requirements. Since the device is a handheld, the
development requirements will be fairly light as the FITtape is a microcontroller based device.
Demonstration Prototype
The demonstration prototype will be the 2nd or 3rd iteration of the FITtape. This iteration will have a PCB
and a monocoque ABS plastic chassis. The measurement unit PCB will use SMD components where
possible to minimize footprint. The display will use an android device showing the tracking app along
with having a live prototype of the FITtape. Along with these devices, the display will include an
informational poster and 2 minute promotional video showing the function and benefits of the device.
Specifications
Included components



FITtape measurement tracker
Charging cable
Charging unit
Sizing
Length
Width
Height
Tape Length
Screen Size
3.5 inches (88.9 mm)
2.5 inches (63.5 mm)
1.5 inches (38.1 mm)
60 inches (1524 mm)
1.5 inches (63.5 mm)
Measurement Capability
FITtape will keep measurement data for each measurement up to 20 previous measurements. The
measurements are in inches with an accuracy down to .01 inches.
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Wireless Sync
FITtape has a close range wireless sync of a maximum range of 15 feet.
Battery Specification
Battery
Battery life
Battery Type
Charging Type
2200mAh
10 days (nominal use of once every three days)
Lithium Polymer
USB 5V or 5V wall charger
Power dissipation
KL25
Solenoid
LCD
Boost Converter
Active Power Dissipation
70mA worst case
133mA nominal 100% duty, 2.3A
worst case
3mA
35µA
Sleep Power Dissipation
10µA worst case
-
Average use
2 times a week
10 times a usage
-
15 minutes active
15 minutes active
The total power dissipation is ultimately up to the end user. Based on the recommended intervals for
taking measurements, the user should be taking measurement a maximum of once every three to four
days. From this type of usage, with the solenoid being cycled 10 times per usage the total worst case
would completely drain the battery if the solenoid was stuck on 100% of the usage time. For nominal
usage, there cycle time is no longer than 1.5 seconds and the current draw is drastically lower, allowing
for three uses before requiring a recharge. This gives a standard battery life of 9 to 12 days.
Preliminary Parts List
Component
Freescale KL25Z128VFM4
LCD module
Boost Converter
Bluetooth Module
MOSFET
Rotary Encoder
Resistors
Capacitors
Battery Management Module
Battery
Touchstrip components
Mechanical components
Rare Earth Magnets
USB components
Case Material
TOTAL
Lead Time
n/a
1 month
2 weeks
2 weeks
2 weeks
2 weeks
2 weeks
2 weeks
-
Cost
$1.74
$3.71
$1.37
$2.47
$0.44
$14.43
$2.00
$4.00
$2.00
$9.99
$1.00
$5.00
$2.50
$0.79
$5.00
$51.94
The cost of unit is mostly the rotary encoder and battery. If sourced properly, the total cost of the
FITtape can be driven down 20%
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