Monday, 31 August 2009

RF Energy Harvesting for Wireless Sensor Networks in the Outdoor Environment

RF energy transfer mechanism between a rotatable transmitter, attached to the wall of a property, and the receivers

Block diagram of the proposed RF energy harvesting system

Antenna measurement inside anechoic chamber

Field trial at Jealott’s Hill

Structure of an enhanced gain circular microstrip patch antenna with stacked parasitic ring

Simulated radiation pattern (realised gain) of the antenna placed on different soil surfaces at 867 MHz

Wireless Sensor Networks in Agriculture

In recent years, there has been a growing interest in the deployment of wireless sensor networks (WSN) within many sectors. These network systems, consisting of spatially distributed sensor nodes are used extensively in various applications such as structural monitoring, habitat monitoring, inventory tracking, and healthcare system. One emerging WSN application is in agriculture sector, where the sensor nodes are deployed in outdoor fields to monitor soil conditions, such as moisture, mineral content, and temperature. Data collected from these sensors could be used to manage irrigation and fertilisation, to predict crop yield, as well as to improve crop quality.

Motivation and Drivers

Energy supply has been a limiting factor to the lifetime of agricultural wireless sensors. These sensors are typically powered by onboard batteries which have fixed energy rating and limited lifespan. Hence, they need to be replaced in due time. Moreover, the cost is prohibitive when replacing the exhausted batteries since the sensor devices need to be unearthed. Apart from that, disposal of used batteries poses another major issue. Batteries containing heavy metals such as mercury, lead, or cadmium could be hazardous to human and environment if they are improperly disposed in the landfills.

A possible long-term solution to overcoming these problems is by using energy harvesting in which ambient energy can be extracted and converted into usable electrical energy to power the sensors. Several energy harvesting methods using different energy sources, such as light radiation, temperature difference, electromagnetic field, human power, and vibrations, have been reported in the literature. Selection of an energy harvesting scheme mainly depends on the operating environment and power requirement of the sensors.

Why RF Energy Harvesting?

The main application envisaged of this research is a wireless soil sensor network used for in-field pest detection and monitoring. This network comprises of several sensor nodes which are distributed across an outdoor field surrounding a property. The soil sensor nodes are non-moving and could be located in an open area, in the shades of trees, or even covered by dried leaves or mud.

Based on the given environmental conditions, Radio Frequency (RF) energy harvesting, which relates the concept of wireless energy transmission, is preferred. RF energy harvesting can not only be used to replenish the power required to operate the soil sensors, but it can also provide a more controllable and predictable power supply compared to other possible energy harvesting methods.

Through this approach, RF energy radiated from a controlled transmitter is captured by a receiving antenna attached to the sensor node and converted into usable DC voltage via a combination of rectifier and voltage regulator circuit. This DC output is then stored in an energy storage system before being used to power the sensor.

Research Objectives

The main objective of this project is to investigate and demonstrate the feasibility of using radio frequency (RF) energy harvesting in powering a wireless soil sensor network deployed in an outdoor field. In addition, a proof-of-concept RF energy harvesting device will be designed, built, optimised, and tested in the field.

Research Undertaken (so far)

(I) Investigation Phase

A series of lab and field trials has been conducted using a crude RF energy harvesting model built from commercially-available components – a transmitter and a pair of transmitting and receiving antennas. Performance of both antennas, in term of their gain and return loss, was evaluated inside an anechoic chamber. A field trial was then conducted in an outdoor field at Jealott’s Hill, Berkshire in order to gain a basic insight on the amount of RF energy which could be harvested using the proposed scheme. Trial results showed that this crude model is capable of powering the soil sensor nodes up to a vicinity of 3 metres. Moreover, it is found that the operating range of the proposed system could be further enhanced by:

(i) Using a transmitter with higher output power
(ii) Designing a receiving antenna with higher gain
(iii) Improving the efficiency of the power conversion circuit

(II) Design Phase - Receiving Antenna

Our design is currently focused on receiving antenna component. Receiving antenna is an important element of a RF energy harvesting system. It is responsible for capturing the radiated RF energy in the ambient, thereby affecting the amount of harvestable energy of the system. Microstrip patch antenna (MPA) has been chosen as the receiving antenna of the proposed system due to its low profile, low cost, low weight and ease of fabrication.

Up to date, the suitability of using a printed circuit board (PCB) receiving MPA for the intended RF energy harvesting application was investigated. Performance of a conventional circular microstrip patch antenna using five different PCB materials ranged from low cost fibreglass laminate (FR4) to advanced PTFE based laminates (RT5880, RO3003, RO3006, and RO3006) were evaluated in terms of return loss, radiation efficiency, and gain. Simulations were carried out using CST Microwave Studio to examine the antenna’s performance both in free air and in the presence of different soil conditions. It was found that an enhanced gain circular patch antenna stacked with a ring shaped parasitic radiator using RO3003 substrate could meet the minimum receiving antenna gain requirement (3dB) of the intended application, but with the need of a slight tilt to ensure correct directivity and also with the expenses of higher material and fabrication cost. We are currently investigating a cheaper alternative – an air-substrate-based, folded, MPA with tilted
beam capability. Findings on this antenna will be reported in the future.

Next Steps

1) Design of an air substrate based folded MPA.
2) Design of an energy harvesting circuit with energy storage system.
3) Validation of the complete RF energy harvesting system in field.

Research Partners and Principal Contacts

Mr Zhi Wei Sim, Syngenta Sensors University Innovation Centre, University of Manchester, UK

Dr Roger Shuttleworth, School of Electrical & Electronic Engineering, University of Manchester, UK

Would you like to become a research partner? Are there other aspects of this research that we should be bringing in?