Designing a Low-cost, Low-power Mote for Wireless Sensor Networks

Chapter 1: Introduction

The advances in the hardware and wireless technologies have permitted inexpensive low power communication devices that can be deployed throughout a physical space, providing dense sensing close to physical phenomena, processing and communicating this information, and coordinating actions with other nodes. Such a deployment is termed as a Wireless Sensor Network (WSN). To realize such a network, there are a new set of challenges. The individual devices in a WSN are inherently resource constrained and these have limited processing speed, storage capacity, battery backup, and communication bandwidth. These devices have substantial processing capability in the aggregate, but not individually. These individual devices are named as motesin WSN.

1.1 Mote

The term mote (a tiny particle or SmartDust) – are in fact embedded devices typically used in a WSN and are essentially tiny in size and hence the name. These tiny nodes have processing and computational capability and generally consist of an RF transceiver, memory, on board sensors/actuators and a power source. Some of the requirements of a mote for their use in applications are as under:

• The ability to have some amount of on-board processing.
• They should be able to communicate over the air and also have some basic networking capability.
• The motes should be alive for extended periods (up to a few months to a year) of deployment. Therefore they should be able to operate at low power.
• They should have sensors/actuators embedded in them so as to interact with the environment in which they are deployed and communicate the sensed parameter to a central location.

To be able to fulfill the basic requirements as mentioned above, a mote should have the following components: 

• Microcontroller
• Radio
• An embedded sensor or the ability to connect to sensors/sensor boards.
• On board memory: RAM, Flash.
• Power source

The usage of motes was originally intended for military applications but soon spread to other civilian applications like habitat monitoring [23, 27], environment monitoring [28, 26], volcano monitoring [30], wildlife tracking [20], industrial monitoring [22], structural monitoring [21] etc. These experimental deployments have enabled researchers and scientists across the globe to study effects / interactions in nature which was earlier not possible with wired technologies due to hurdles like hostile environment, physical obstacles in reaching a place etc.

1.2 Motivation and Problem Statement

There are a number of commercial off-the shelf (COTS) motes available in the market today but none of them are readily and easily available in India. There has been a similar development at IIT Delhi [ ], wherein they developed Rete, a mote which operates at 433 MHz. However, these motes have their sales division in USA and are not readily available in India. Moreover, the COTS motes have to be imported and not all educational institutions in the country have administrative procedures in place that enable them to procure these devices easily as and when required. It is therefore important that such devices are readily available to the educational and research communities in India.

Each of these motes costs a few thousands of rupees and thus are still costly enough to deter widespread use in educational and research institutes. Hence, the cost of these devices should be low enough to encourage their widespread use. With an increase in awareness about these devices, it is realistic to assume that new applications are developed which cater to the problems specific to India.

Problem statement

Design and if possible also implement a low cost, low power hardware platform for use in wireless sensor networks. Therefore, the main areas this thesis addresses are the following:

• Design of a low cost, low power hardware platform for use in sensor networks.
• Build a simple working prototype that can be improved upon. The prototype though simple should be able to deliver all the functionalities possible with commercially available state of the art motes.
• The motes developed should be easily available for research and educational institutions.

We have designed and implemented “ i-Mote “ which integrates an MSP430 microcontroller, a CC2420 radio chip which operates in the Industrial, Scientific and Medical (ISM) band (2.4 GHz). An 8 Mb flash for data logging and a temperature sensor have also been embedded into the platform. We have integrated with an i-Mote open source embedded operating system TinyOS so that it can be readily programmed for custom applications. The development cost of i-Mote is Rs 1625 and is less than one fourth the cost of the most popular commercially available mote, Tmote, which costs Rs 7740. We have designed the building blocks of the mote:  microcontroller and radio circuitry for their correct operation. We have also suggested out range measurements and have indicated ranges nearly twice that of Tmote. The i-Mote is powered by two AA batteries or 3.0 volts power source and consumes about 20 mA during transmission.

1.3  Thesis Organization

The rest of the report is organized as follows.

Chapter 2 discusses the essentials of wsn and literature survey and Background: evolution of various motes and comparison of commercially available motes.

Chapter 3 discusses the hardware platform ICs.

Chapter 4 discusses the design considerations and implementation of i-Mote. In this chapter we discuss the hardware choices available and the reasons for choosing the hardware used in. We also discuss the architecture of i-Mote and design issues while translating the paper design to a working model in the section on PCB design.

Chapter 5 discusses the testing methodology of i-Mote and forecasted results that may be obtained during the range and power consumption tests.

We finally summarize our work and discuss the future scope in Chapter 6.

The appendices at the end of the report give detailed schematics of the mote, bill of material required, procedure for integration of mote with TinyOS and the procedure to connect an external antenna to the mote to achieve higher ranges.