Electronic Design

With ZigBee Mesh Network, Remote Caregivers Keep Tabs On Patients

This year, almost 3 million Americans will turn 60. While rapid advances in health care are keeping people healthier longer, designers are increasingly looking for ways to enable individuals with medical problems to care for themselves in their own home. Wireless developer Cambridge Consultants is exploring how wireless technology can be used to implement home monitoring networks for the elderly and vulnerable.

The company demonstrated a mobile telemetry system based on a semi-static ZigBee telemetry network at the Freescale Technology Forum last July in Orlando, Fla. During the demonstration, the "patient" wore a wireless node that monitored physiological signs while providing zonal location sensing. Additional nodes provided 250-ms updates on the patient's breathing monitor.

A ZigBee network can be particularly useful for home monitoring because the technology is optimized for low current consumption. According to Cambridge, today's ZigBee chip sets have a transmitter current range of 17 to 30 mA and deep-sleep current consumption of 3 µA. Depending on the battery technology, infrequent-usage devices such as meters can achieve up to 10 years of battery life.

Additionally, ZigBee network control is resilient to link interruptions. It supports self-configuration. And, it achieves self-repair of the mesh configuration. The experimental practical location system demonstrated at the Freescale Forum is based on an ad-hoc distribution of nodes, each of which provides a wireless link to its nearest neighbors.

A gateway node supports the interface between the ZigBee network and the computer controller (Fig. 1). Static nodes are placed at various locations around the home on a 20- to 30-m grid (Fig. 2). Mobile nodes, which are attached to a mobile asset like the patient, act as reduced-function devices. Such devices generally are battery powered and are limited to a star topology. They cannot become network coordinators.

The wireless personal-area network (WPAN) physical layer is based on the IEEE 802.15.4 standard. In the international 2.4-GHz band, a WPAN based on ZigBee technology has a typical indoor range of 20 to 30 m and a raw data rate of 250 kbits/s. Zonal location capability is based on a proprietary algorithm that runs on standard ZigBee hardware and uses the standard ZigBee protocol stack.

A home monitoring network based on ZigBee technology could be manufactured using single-chip radio-plus-microcontrollers. Cambridge Consultants estimates resale prices starting at a few hundred dollars. Once the network is in place, raw data can be transmitted to a central location, using a broadband line, or analyzed and stored locally for review on demand.

A basic accuracy of 5 m rms can be expected from the system, due to noise and measurement resolution. The capacity of the system depends on the update rate.

Cambridge Consultants

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