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Body sensors that communicate with watch-sized display devices over wireless personal area networks (PANs) are beginning to morph from a relatively small niche of expensive monitoring systems for professional athletes into a much broader market.
Three components account for the body-PAN market's expansion. The largest is the fitness segment. It is typified by recreational runners who already listen to a radio while they run and would like to know at a glance, their pace, how far they have run, and how many calories they're burned. Weekend bicyclists also fall into this segment.
A health-care component is also emerging. While not subject to the stringent regulations of medical devices, these products are helpful in monitoring patients' activities and base line physiological data. Products exist today but this component's growth will ramp only when sensor data can be efficiently transferred to the health care provider using the mobile phone network.
The third component—medical applications—is further out largely due to regulatory requirements and the need for a high degree of robustness and accuracy.
Professional and semi-professional have been using body PANs to monitor pace, heart rate and distance for several years. Two Swedish companies-- Polar Electro OY and Suunto OY--still dominate the field.
But Nike's recent entry into this market with a product for recreational athletes—and with a goal to sell more shoes—marks a significant departure. An accelerometer embedded in the running shoe communicaties with a watch-sized display that can also download the day's fitness data into a PC and potentially onto Nike's web site.
Technology enablers
Complimenting the market drivers are several technology enablers that were not available even a few years ago. The most important are ultra-low-power wireless radios that can run for a year or more on a watch battery.
An important goal for the ULP wireless community is to standardize a ULP communications link. This is well underway at the Bluetooth SIG where its ULP Bluetooth spec will be ready sometime in mid-2009, according to Executive Director Mike Foley.
Much more energy efficient protocols have also made relatively recent appearances, and the sensors themselves are becoming more sophisticated and reliable thanks to the maturing of MEMS technology. The most commonly used MEMS device is the accelerometer.
Defining ULP
A clear definition of ULP is useful for comparing silicon products in any new application and a definition proposed by Nordic Semiconductor is a good place to start. Nordic's wireless PAN technology was being adapted to Nokia's short-lived WiBree specification before it was merged with the Bluetooth SIG's ULP Bluetooth initiative.
Nordic bases its ULP definition on available battery power: ULP means the body PAN can run for a year or more on a single watch battery. It specifically refers to the CR 2032 coin cell. Translated into numbers, this means: (1) a capacity of 180mAh to be consumed over a year or more (average current of approximately 20μ A); and, (2) a peak current of just 20 mA because stressing the battery's instantaneous current draw damages the battery and reduces the total charge available.
According to Nordic product manager Thomas Embla Bonnerud, the 12mA peak of Nordic's nRF24L01 2.4 GHz transceiver is well under the 20mA required to keep the battery healthy.
Techniques usually associated with low-power chip design such as deep sleep modes, clock gating, steep transitions between on and off, and using low- leakage transistors (and lower performance) in the IC process are all appropriate for ULP body PAN transceiver design. Design interest even extends to the crystal oscillator when designers are serious about ULP, Bonnerud says.
The nature of the application—small data packets and the ability to tolerate relatively long periods between transmissions, for example—puts additional tools at a ULP wireless company's disposal. Despite the small amount of data being transmitted (as little as a few hundred bits), for example, a high data rate—on the order of 2 Mbits/s—is desirable because it allows the transmitted to be on for only microseconds.
This implies a sensitive transceiver design, of course, but it also has important implications for the protocol.
The average current measured over a year depends heavily on the protocol used to transfer data. Figure 1 illustrates how battery lifetime relates to average current, time the transceiver is active, and time the transceiver is in sleep mode.
Click here for Figure 1.
Figure 1: Battery lifetime is a function of charge available, peak current and average current..
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