Using Python to Store Data From Many BLE Devices

We take a look at how to collect and store data from several BLE devices, a necessary issue to deal with considering the built-in limitations of BLE networking.

Erich Styger

Jun. 06, 17 · Tutorial

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BLE (Bluetooth Low Energy) sensor devices like the Hexiwear are great, but they cannot store a large amount of data. For a research project, I have to collect data from many BLE devices for later processing. What I’m using is a Python script running on a Raspberry Pi, which collects the data and stores it on a file:

Raspberry Pi with Python controlling a set of Hexiwear BLE Devices

Raspberry Pi with Python controlling a set of Hexiwear BLE devices

Getting Data From a Single Device

The following script gets sensor data from a single device:

# Using Hexiwear with Python
# Script to get the device data and append it to a file
# Usage
# python GetData.py <device>
# e.g. python GetData.py "00:29:40:08:00:01"
import pexpect
import time
import sys
import os

# ---------------------------------------------------------------------
# function to transform hex string like "0a cd" into signed integer
# ---------------------------------------------------------------------
def hexStrToInt(hexstr):
    val = int(hexstr[0:2],16) + (int(hexstr[3:5],16)<<8)
    if ((val&0x8000)==0x8000): # treat signed 16bits
        val = -((val^0xffff)+1)
    return val
# ---------------------------------------------------------------------

DEVICE = "00:29:40:08:00:01"   # device #24

if len(sys.argv) == 2:
  DEVICE = str(sys.argv[1])

# Run gatttool interactively.
child = pexpect.spawn("gatttool -I")

# Connect to the device.
print("Connecting to:"),
print(DEVICE)

NOF_REMAINING_RETRY = 3

while True:
  try:
    child.sendline("connect {0}".format(DEVICE))
    child.expect("Connection successful", timeout=5)
  except pexpect.TIMEOUT:
    NOF_REMAINING_RETRY = NOF_REMAINING_RETRY-1
    if (NOF_REMAINING_RETRY>0):
      print "timeout, retry..."
      continue
    else:
      print "timeout, giving up."
      break
  else:
    print("Connected!")
    break

if NOF_REMAINING_RETRY>0:
  unixTime = int(time.time())
  unixTime += 60*60 # GMT+1
  unixTime += 60*60 # added daylight saving time of one hour

  # open file
  file = open("data.csv", "a")
  if (os.path.getsize("data.csv")==0):
    file.write("Device\ttime\tAppMode\tBattery\tAmbient\tTemperature\tHumidity\tPressure\tHeartRate\tSteps\tCalorie\tAccX\tAccY\tAccZ\tGyroX\tGyroY\tGyroZ\tMagX\tMagY\tMagZ\n")

  file.write(DEVICE)
  file.write("\t")
  file.write(str(unixTime)) # Unix timestamp in seconds 
  file.write("\t")

  # App mode
  child.sendline("char-read-hnd 0x6d")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("AppMode:  "),
  print(child.before),
  print(str(int(child.before[0:2],16)))

  file.write(str(int(child.before[0:2],16)))
  file.write("\t")

  # Battery
  child.sendline("char-read-hnd 0x28")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Battery:  "),
  print(child.before),
  print(str(int(child.before[0:2],16)))

  file.write(str(int(child.before[0:2],16)))
  file.write("\t")

  # Ambient Light (0x2011)
  child.sendline("char-read-hnd 0x3f")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Ambient:  "),
  print(child.before),
  print(str(int(child.before[0:2],16)))

  file.write(str(int(child.before[0:2],16)))
  file.write("\t")

  # Temperature (0x2012)
  child.sendline("char-read-hnd 0x43")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Temperature:  "),
  print(child.before),
  print(float(hexStrToInt(child.before[0:5]))/100)

  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")

  # Humidity (0x2013)
  child.sendline("char-read-hnd 0x47")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Humidity:  "),
  print(child.before),
  print(float(hexStrToInt(child.before[0:5]))/100)

  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")

  # Pressure (0x2014)
  child.sendline("char-read-hnd 0x4b")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Pressure:  "),
  print(child.before),
  print(float(hexStrToInt(child.before[0:5]))/100)

  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")

  # HeartRate (0x2021)
  child.sendline("char-read-hnd 0x52")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("HeartRate:  "),
  print(child.before),
  print(str(int(child.before[0:2],16)))

  file.write(str(int(child.before[0:2],16)))
  file.write("\t")

  # Steps (0x2022)
  child.sendline("char-read-hnd 0x56")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Steps:  "),
  print(child.before),
  print(hexStrToInt(child.before[0:5]))

  file.write(str(hexStrToInt(child.before[0:5])))
  file.write("\t")

  # Calorie (0x2023)
  child.sendline("char-read-hnd 0x5a")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Calorie:  "),
  print(child.before),
  print(hexStrToInt(child.before[0:5]))

  file.write(str(hexStrToInt(child.before[0:5])))
  file.write("\t")

  # Accelerometer
  child.sendline("char-read-hnd 0x30")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=5)
  print("Accel:  "),
  print(child.before),
  print(float(hexStrToInt(child.before[0:5]))/100),
  print(float(hexStrToInt(child.before[6:11]))/100),
  print(float(hexStrToInt(child.before[12:17]))/100)

  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[6:11]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[12:17]))/100))
  file.write("\t")

  # Gyro
  child.sendline("char-read-hnd 0x34")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=10)
  print("Gyro:   "),
  print(child.before),
  print(float(hexStrToInt(child.before[0:5]))/100),
  print(float(hexStrToInt(child.before[6:11]))/100),
  print(float(hexStrToInt(child.before[12:17]))/100)
  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[6:11]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[12:17]))/100))
  file.write("\t")

  # Magnetometer
  child.sendline("char-read-hnd 0x38")
  child.expect("Characteristic value/descriptor: ", timeout=5)
  child.expect("\r\n", timeout=10)
  print("Magneto:"),
  print(child.before),
  print(hexStrToInt(child.before[0:5])),
  print(hexStrToInt(child.before[6:11])),
  print(hexStrToInt(child.before[12:17]))

  file.write(str(float(hexStrToInt(child.before[0:5]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[6:11]))/100))
  file.write("\t")
  file.write(str(float(hexStrToInt(child.before[12:17]))/100))
  file.write("\t")

  file.write("\n")
  file.close()
  print("done!")

  sys.exit(0)
else:
  print("FAILED!")
  sys.exit(-1)

Calling it with the device ID, it gets the sensor node data, appends it to the file, and shows it on the console:

pi@raspberrypi:~ $ python GetData.py "00:2A:40:0B:00:4E"
Connecting to: 00:2A:40:0B:00:4E
Connected!
AppMode: 02 2
Battery: 64 100
Ambient: 0b 11
Accel: 05 00 02 00 9b ff 0.05 0.02 -1.01
...
done!

The data gets stored into a CSV text file.

Getting Data from Multiple Devices

That brings us to the following script file:

# Python script to get data from multiple devices
# Usage:
# python GetDataAll.py

import subprocess

# list of devices
devices = [
  "00:32:40:08:00:12",  # device 01
  "00:2F:17:03:00:35",  # device 20
  "00:28:22:0C:00:15",  # device 21
  "00:15:77:03:10:01",  # device 22
  "00:2A:40:0B:00:4E",  # device 23
  "00:29:40:08:00:01",  # device 24
   ]

for x in range(0,len(devices)):
  cmd = "python GetData.py " + devices[x]
  subprocess.call(cmd, shell=True)

print("finished all devices!")

The script has a list of BLE devices, which are used to poll the data.

Summary

With a Raspberry Pi and Python scripting, I can collect data from multiple BLE devices and store the information into a file for further processing. Because I don’t need to keep the BLE connection active, I can extend this to an almost unlimited number of devices, without running into the typical BLE connection limit of around eight devices.

I’m using a similar method to update all the devices (e.g. with the current time/date), plus I have implemented a remote shell in each BLE device: That way, I can send commands to each node, which then are executed on the BLE device.

The Python scripts used can be found on GitHub.

Happy saving!

Links

Data (computing) Python (language)

Published at DZone with permission of Erich Styger, DZone MVB. See the original article here.

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