By Frazer Barnes
My project is looking at the practical implementation of a new low power communications technology called LoRaWAN. This implementation is a wearable gas sensor that monitors the users exposure to respiratory irritants and transmits the data back to a server over LoRaWAN or to a mobile device using Bluetooth 4.0. The focus is on low power design and sensitivity.
Wearables and IoT devices are being deployed in huge numbers and the expectations of such devices are growing. These systems often must be small, low power, have wireless communications and a wide range of sensors. This creates a huge problem as every milliamp of power consumed becomes significant. Furthermore, the space requirements of wearables makes interference with sensors a problem.
• RQ1: Effects of low voltage (<1.2v) on LoRa Transceiver IC operation and stability.
• RQ2: How to minimize the digital interference created in high sensitivity potentiates in wearable/compact devices.
Background & State-of-the-Art
ST Micro have developed an excellent software stack for LoRaWAN which is integrated into their hardware abstraction libraries for the STM32 range of Microcontroller. As a result the device has been designed around the ST Micro ecosystem and provides an excellent software stack for the project.
For the gas sensing the project uses a sensor produced using screen printing technology. Screen printed gas sensors are relatively new and a result in advances in material science. This makes it possible to integrate a sensor with parts per billion accuracy into a very slim form factor. However the overall accuracy will be defined by electrical noise created by the electronics and environment.
Software Design & Initial Findings
LoRa WAN Stack
Most LoRaWAN modules operate at higher voltages such as 3.3, as a result, few designs on the market currently operate at the minimum voltage of the transceiver(1.2v). As a result the design has a very low power draw during operation, however, is very sensitive to sudden increases in current draw. I have adapted the software to ensure high power components like transmitters can only be enabled one at a time to prevent voltage drops. By ensuring the software can avoid sudden draws in current it is possible to operate the device at these low voltages.
Project Plan & Milestones
•Design/Manufacture of initial test PCB
•Population of test PCB
•Setup of base firmware including freeRTOS and WAN/BLE stack for communications.
•Measurement of voltage stability and current draw of the device.
•Write library for GPS communications
•Design of process to configure and read from potentiostat (connected to respiratory irritant gas sensor).
•Measure potentiostat output and determine noise on signal lines before and after filter circuits.
•PCB revision to optimize design based on previous tests.
•LoRaWAN Technology – STMicroelectronics . 2017. LoRaWAN Technology – STMicroelectronics . [ONLINE] Available at: http://www.st.com/en/wirelessconnectivity/lorawan-technology.html?querycriteria=productId=SC2150. [Accessed 24 January 2017].
•Ghenadii Korotcenkov, 2013. Handbook of Gas Sensor Materials: Properties, Advantages and Shortcomings for Applications Volume 1: Conventional Approaches (Integrated Analytical Systems). 2013 Edition. Springer.
•EE. 2015. LoRa Device Developer Guide. [ONLINE] Available at: https://partner.orange.com/wp-content/uploads/2016/04/LoRa-DeviceDeveloper-Guide-Orange.pdf. [Accessed 30 December 2016].