Inductive Power Transfer for On-body Sensors. Defining a design space for safe, wirelessly powered on-body health sensors.

Abstract

Designers of on-body health sensing devices face a difficult choice. They must either minimise the power consumption of devices, which in reality means reducing the sensing capabilities, or build devices that require regular battery changes or recharging. Both options limit the effectiveness of devices. Here we investigate an alternative. This paper presents a method of designing safe, wireless, inductive power transfer into on-body sensor products. This approach can produce sensing devices that can be worn for longer durations without the need for human intervention, whilst also having greater sensing and data capture capabilities. The paper addresses significant challenges in achieving this aim, in particular: device safety, sufficient power transfer, and human factors regarding device geometry. We show how to develop a device that meets stringent international safety guidelines for electromagnetic energy on the body and describe a design space that allows designers to make trade-offs that balance power transfer with other constraints, e.g. size and bulk, that affect the wearability of devices. Finally we describe a rapid experimental method to investigate the optimal placement of on-body devices and the actual versus theoretical power transfer for on-body, inductively powered devices.

Garment level power distribution for wearables using inductive power transfer

“…Energy harvesting techniques such as solar, thermoelectric and piezoelectric [9] aim to scavenge power from the local environment, though the energy produced in wearable form factors often falls short of smart device requirements with power-intensive displays or applications where continuous wireless communication connections are required. Energy transfer techniques such as inductive power transfer have been proposed to assist with powering electronics located on and around the body [3,10,11]. However each of these approaches suffers from the limitation of not being able distribute power around the body and the energy, in the form of a magnetic field, must be applied where required.…”

“…Our work builds upon the work of Worgan et al [10]. Worgan et al propose using inductive power transfer from transmit coils within the ambient environment to receive coils at a specific location on the body.…”

“…Following from Maxwell's equations, a time-varying magnetic field in close proximity to a medium with a non-zero conductivity will induce an electric current in the medium [10], where these induced currents are often referred to as eddy currents. International guidelines exist to protect users exposed to time-varying electric, magnetic and electromagnetic fields.…”

“…For the power sharing system we comply with the ICNIRP 1998 basic restrictions for general public exposure, Table 4 in [13], the most stringent basic restrictions. An operating frequency of 99 kHz was selected for on-body power sharing for analogous reasons to Worgan et al [10]; maximising the time-varying magnetic field to produce higher induced receive voltages in the receive coil, whilst remaining below 100 kHz. Above 100 kHz the ICNIRP 1998 basic restrictions require a current density measurement and specific absorption rate (SAR) measurement, with SAR typically obtained from specialist electromagnetic modelling software such as finite difference time domain methods.…”

“…Above 100 kHz the ICNIRP 1998 basic restrictions require a current density measurement and specific absorption rate (SAR) measurement, with SAR typically obtained from specialist electromagnetic modelling software such as finite difference time domain methods. Below 100 kHz the ICNIRP 1998 basic restrictions require only a current density measurement, which can be practically inferred using a search coil and oscilloscope, as detailed in [10]. To facilitate reproducibility of our garment level power sharing approach, we opt to restrict our operating frequency to sub 100 kHz, where compliance can be practically inferred using common test and measurement equipment.…”