Projects:2014S1-14 Wearable RFID Antennas

Project Information

Smart Clothing For Patient Care

Wearable RFID systems have a large potential in medical and sensoring applications. In order to minimise the impact of the human body on the system (and vice-versa), special care has top be taken in the antenna design. I this project, a new compact wearable textile RFID antenna is designed, manufactured and tested. Technologies such as computerised embroidery of conductive threats will be used as building blocks of a truly wearable antenna

Project Introduction

When providing care for patients, it is vital that care givers know what state their patient is in at all times. As the demand for medical care increasingly strains the ability for care practitioners to maintain this awareness, technology begins to play a more important role in maximising carer awareness across a larger number of patients.

By incorporating sensing and communication technology into patient garments, smart clothing can be created. To maximise the service life of these garments, RFID communication frameworks present an attractive option by removing the need for a power source on the wearer and using rectified RF energy. Such RFID systems will target passive wwearer tags to create Personal Area Networks.

In order to make such a system possible, RFID antennas must be developed that are compatible with both RF and wearability requirements. The goal is to allow the system to be both functional and comfortable. With this in mind, the project team has set about designing flexible antenna systems to meet such a systems requirements. The antennas designed are all planar in topology and target efficient operation in the UHF RFID band. This band varies significantly from country to country which required consideration in the selection of antenna topology and the targeting of design bandwidth.

The planar antennas are fabricated using entirely flexible materials. Conductive layers are created from nylon fabric that is plated with thin layers of conductive material. Substrates for the antenna are provided by varying thickness layers of closed cell foam that provides a constant dielectric body beneath the antenna and feed structures. This allows antenna and feed impedances to be managed, a vital consideration when achieving energy efficiency.

Placing antennas on the human body presents an interesting challenge to the design team. The high water content and tissue structure of the body results in what well known as an absorbent and highly lossy dielectric substrate in close proximity to the antenna. If not managed correctly this can result in poor matching and antenna performance. In designing antennas that manage this fact, antenna and the body must both be incorporated into the design. This ensures that the antennas region of highest performance ends up in the target RF band once it has undergone the inevitable distortion resulting from human body proximity. The requirement for flexibility presents an additional complicating factor. Whilst the use of flexible materials ensures comfort to the wearer, flexing of an antenna is liable to alter its operation to varying degrees. This adds an additional consideration to the design problem.

With the above in mind, a number of slot antennas have been designed. In order to maximise the RF compatibility with global UHF RFID bands, many of the antennas designed are based around variations of Ultra Wideband topologies. One series of antennas has been used the Bowtie antenna as its fundamental base. Using electromagnetic equivalence principles, new antennas have been produced in a slot style. Various modifications have then been made to control their performance in the challenging body centric environment.

The antennas were fabricated and tested under a variety of conditions. Fairfield radiation was measured as this fundamentally impacts the read range of any RFID system utilising the antenna. The antenna matching and bandwidth was evaluated with the antenna under a variety of environments designed to simulate the body worn target application.

Fundamentals

This project will present the design and testing of dipole based antenna topologies. It has been established that in their common form dipole antennas do not work well when placed on the human body, in much the same way as they are negatively impacted by their placement on metallic surfaces. In these applications it is expected that a dipole antenna will experience detuning losses. In order to mitigate this effect, the basis for the behaviour is established.

When considering a simple half wave dipole the varying current within the dipole elements gives rise to a varying electromagnetic field around and between the elements. The components of the E-Field give rise to the radiation. Resultantly this antenna stores its energy in the near electric field which is readily impacted by close proximity loss dielectrics which has already been established as occurring in previous studies. The dipole antenna, whilst fundamentally omnidirectional will consequently suffer from the established issues arising from body proximity. This effect arises from the from the body’s inherently low wave impedance that serves to reduce the close proximity electric field. This effect can be mitigated through body-antenna separations of λ/4 however at UHF frequencies this would make the antenna excessively large. Whilst the near electric field is reduced, the opposite occurs to the magnetic field, which sees an increase in close proximity when the direction of magnetic current is normal to the body[12]. It has therefore been shown that magnetic dipoles have an advantage in this situation and exploit this effect.

Designs

There are five antenna designs investigated in this project that all broadly fit into the classification of slot antennas. The designs evolve from a Dipole Slot and build to Bowtie Slot, Flared Bowtie Slot, Folded Bowtie Slot and Radial Folded Bowtie Slot are designed. A parameter set is created for each design that promotes the development of a geometricly constrained modelling for each design. This model forms the basis for simulation within CST Microwave Studio’s Time Domain Solver. In order to generate simulation models that are representative of the practical design, the materials used to manufacture the antennas are investigated.

Outcomes

This project set out to develop wearable antennas optimised for RFID PAN Applications. In order to optimise the design for the application, a firm understanding of the PAN requirements is obtained. This allows a set of design criteria to be formulated and analysed. By understanding the impacts of the body on an antenna, topologies can be developed to specifically exploit its EM behaviour. In this project five planar antennas were designed that are fundamentally based on the magnetic dipole. This topology exploits the interaction of human tissue and the antennas near magnetic field to promote bandwidth, matching and efficiency when close to the body.

It is proven to be vital the simulation models account for the real world properties of the materials the antenna will be made out from. To this end, investigation into the material properties of metallised fabric, its potential manufacturing techniques and their impact on performance is discussed. A combined CGS and wireframe modelling technique has been shown to be an effective way of generating complex, parametrically constrained models to support simulation. By observing the geometric constraints of these models, manual and machine driven optimisation of these structures is simplified. This modelling technique was successfully utilised for all designs. With established parametric models that are based on accurately modelled materials the antennas can be simulated in an accurate manner. Placing these accurate models in real world body worn environments allows the antennas to be optimised for the target application. This helps ensure that when placed on the body and resonant modes shift due to nearfield impedances change, the design is able to meet the design requirements.

For each antenna designed, a prototype was manufactured to enable real world testing. This required establishing known manufacturing techniques or developing new ones. With all antenna designs using narrow CPW slot feeding, the manufacturing process was deemed to be more critical than other fabric antenna designs discuss in the current literature. As such, a manufacturing technique was developed specifically for this project that resulted in antennas with sensitive and tight tolerance geometry being translated effectively from design to hardware. Each antenna was validated for match and farfield radiation. Each design agreed largely with its predictions from the simulation models although a general loss factor appear to have been unaccounted for. This is thought to reside in the materials, likely to PVA glue or nylon fabric core. Some testing was performed using a human phantom which allowed validation of the numerical phantom results obtained from simulation. It also demonstrated levels of effectiveness for each antenna when bent across the shoulder of a person-like form.

Of the antennas designed, each displayed varying levels of application suitability. The Folded bowtie antenna displayed a wide bandwidth exceeding the design requirement. It was well matched across this band and maintained both bandwidth and match in a variety of body worn applications. Its simulated efficiency was sufficient however the unaccounted for loss mentioned above suggests that the efficiency should also be evaluated practically. Due to limitations of the test facilities this could not be performed but may form the basis for future work.

Group member

1.Joshua Brittain

2.Hassan Amirparast

Supervisors

1. Dr Thomas Kaufmann

2.Prof Christophe Fumeaux

3.Dr Damith Ranasinghe