NFC Based Low Power Wearable Pedometer

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Mr. Kamaldeep Bansal, Staff Engineer, AMG Central Lab and Technical Marketing, STMicroelectronics, India

Introduction

In this technology driven world, all of us remain busy in handling our multiple priorities and rushing from here and there every day. Thus balancing fitness with work and time has become very difficult. Today everyone is willing to track his everyday activity which reflects level of effort throughout the day. Pedometer is one of application which can help in solving this problem.

Pedometer is a device to count the total steps taken by any person which can be further used to calculate total distance travelled, calories consumed and speed. It is a consumer application which is emerging quickly in the world of fitness.

STMicroelectronics offers a reference design to develop a NFC based wrist band shaped pedometer with an android application. This solution has been developed using 3-axes accelerometer, ultra-low power high performance microcontroller and NFC communication which helped in making low cost and portable solution.

Wearable Pedometer

Figure 1: Wearable Pedometer

NFC (Near Field Communication) is a wireless technology based on RFID (Radio Frequency Identification) at 13.56 MHz that establishes a communication between the devices by bringing them in close proximity.

Today most of the mobile phones have NFC interface. This can be used to communicate with pedometer helping to make it sleek, lightweight, portable and cost effective since the display of Smart-phone can be used for the information exchange with user. Ease to use and comfort to wear make it helpful for even old age persons to use it without any assistance.

This is an easy and time saving right solution to incorporate a fitness routine in your life System Key Components:

Following are key components of wearable pedometer system:

  • MCU(Microcontroller) – STM32L051K6U6D
  • Dynamic NFC/RFID tag IC – M24SR64-Y
  • Accelerometer – LIS3DH
  • Battery charging with voltage regulator – STNS01
  • Battery – 3.7V/35mAh Li-Ion
  • Android app

System architecture details:

Figure 2_System Architecture

System Architecture

Wearable Pedometer is implemented using STM32L series microcontroller which reduces the power requirement for concerned application & provides adequate processing capabilities ideal for this application. It also has various on chip peripherals like SPI, I2Cand ADC which helped in designing low cost and low power solution.

Dynamic NFC/RFID tag IC M24SR64-Y has the capability to operate from an I2Cinterface or by a 13.56 MHz RFID reader or an NFC phone. It helps in establishing low cost RF Communication between pedometer and phone. It embeds an EEPROM memory which is used to save the pedometer results and other required information. The I2C interface uses a two-wire serial interface, consisting of a bidirectional data line and a clock line. It behaves as a slave in the I2C protocol.

Output of ultra-low power high performance 3-axes linear accelerometer LIS3DH has been used as a part of algorithm to calculate different pedometer results. It has digital I2C/SPI serial interface standard output. MCU Communicates with accelerometer (LIS3DH) using on chip SPI peripheral.

Small size 3.7V/35mAH Li-Ion battery has been used to operate the whole application for few days. Micro USB connector has been provided to connect with external power source to charge 3.7V Li-Ion battery.STNS01 is a linear charger for single-cell Li-Ion batteries which has been used to charge the battery. It uses a CC/CV algorithm to charge the battery and is having several battery protection functions. Fast-charge current can be programmed using an external resistor. Inbuilt 3.1V voltage regulator helped in giving regulated power supply to Microcontroller, accelerometer & Dynamic NFC/RFID tag IC. Below are the top and bottom images of wearable pedometer assembled PCB.

Figure 3: PCB Top Side

PCB Top Side

Figure 4: PCB Bottom Side

PCB Bottom Side

OperationalFlow Chart:

Figure_5 Operational Flow Chart

Operational Flow Chart

Details of Operational Flowchart:

  • While powering on, microcontroller configures and initializes all system peripherals.
  • After this, system goes to stop mode to reduce power consumption.
  • Whenever any NFC compatible phone is brought in range of device, NFC RF communication begins.
  • After establishing RF connection, Android App running on the Smart-phone writes data into memory of Dynamic NFC/RFID tag IC. This data has commands and other information required for pedometer to operate.
  • Along with this, tag IC also generates an external interrupt to wake up Microcontroller from stop mode.
  • After waking up, controller reads NFC tag IC to check the received command.
  • Pedometer application starts with Start command. Otherwise system goes back to stop mode in case of Stop or unknown command.
  • Pedometer application runs continuously with any motion detected by accelerometer. If no motion has been detected for some time it goes to low power mode to save power and wakes up again with some motion detection.
  • System automatically saves the results into memory of tag IC in case of inactivity for long duration and goes to stop mode to save power
  • Whenever system gets a stop command, microcontroller saves the result in NFC memory using I2C interface and again goes to stop mode.

Accelerometer role in Step count for pedometer application:

Figure_6 Step Count using Accelerometer

Step Count using Accelerometer

3-axes accelerometer has been used to measure acceleration of a person in X axis, Y axis & Z axis. Output of accelerometer has been sampled periodically in all the three axes. This acceleration data has been used as input to pedometer algorithm helping in calculating the step count, distance travelled, calories consumed and speed. AccelerometerLIS3DHis based on MEMS (Microelectromechanical systems) technology features ultra-low-power consumption and small size is very suitable for this application. This helped in making more accurate, low cost, compact and low power pedometer system.

Low Power Implementation

Microcontroller is used to communicate with Dynamic NFC/RFID tag IC to take the information from user using mobile phone or tablet. Depending upon the command it starts processing data coming from accelerometer to calculate different exercise measurers like step count, calories burn etc. It also helps to give power to accelerometer & Dynamic NFC/RFID tag IC to achieve lowest power consumption. Consumption of pedometer is mentioned below:

  • Low power mode(Stop Mode) – Approx 6uA
  • Run mode – Approx 350uA

M24SR is a family of dynamic NFC Forum Type 4 Tags in which memory organization and access complies with the associated NFC Forum specification.NFC Data Exchange Format (NDEF) specification defines a message encapsulation format to exchange information, e.g. between an NFC Forum Device and another NFC Forum Device or an NFC Forum Tag. Each NDEF message can consist of multiple NDEF Records as per application demand.

Android Application for Pedometer Device

An Android Application known as “ST NFC Based Pedometer” is available on Google Play Store. It is used to Start and Stop Pedometer Device explained in above paragraphs.

Figure 7: ST NFC Based Pedometer” on Google Play Store

 ST NFC Based Pedometer” on Google Play Store

ST Pedometer Android Application based on Android platform is a user friendly application and uses NFC (Near Field Communication) technology. It can be easily downloaded on a NFC Compatible mobile phone from the following link:

https://play.google.com/store/apps/details?id=st.pedometer.pedometerst&hl=en

Figure 8: ST Pedometer Android Application

Figure_8 ST Pedometer Android Application

Features of Application:

It is capable of starting, stopping the pedometerand reading values(Step Counts, Calories, Distance travelled) etc. in NDEF format from the Pedometer device.NDEF format offers the following features:

  • Demo mode in App to access application without registration as a User.
  • Start and Stop both implemented on single button to avoid confusion.
  • History implemented for individual registered user as well as demo users.
  • Interactive graphs to show user’s progress in Steps, Calories, Distance and Speed.

Graphical Representation of results

Figure 9: Graphical Representation of results

• User can delete a single entry in History as well as clear all History.
• Speed can be seen either in Km/hr or m/s by tapping on units in App.
• Registered users can edit their Profile in Edit Profile module on the top right corner.
• Once a user has logged in, each time he/she opens the application it is by default logged in until user Logout.
• User can close the application after starting Pedometer device. Whenapplication is opened again to stop pedometer device, application will start from the state left with while closing the application i.e. Stop button will be shown.

Application makes use of SQLite Database to save all the readings read from Dual EEPROM (which is embedded in Pedometer device) through NFC (Near Field Communication).

Figure_10 History of Readings

Figure_10 History of Readings

Conclusion:

Pedometer is a valuable device which provides a convenient way to monitor health related data anytime and anywhere. It just requires a wristband pedometer device and a smartphone. It helps us to not only to monitor but also to record, analyze our physical activity and to take necessary precautions on time.

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