Power Budgeting

The ZigBee Alliance and the Bluetooth® SIG has worked hard to enable personal health devices into the market. These are low power, short range radio applications which have a wide portfolio of applications, but both have solutions which are directly or indirectly designed for medical applications. Both technologies have low power solutions: the Bluetooth technology industry has recently developed and released the Bluetooth Low Energy (BLE) product and ZigBee has been designed from inception to be low power. There are of course other radio technologies which are equally suitable for medical applications, such as WiFi; however, Bluetooth technology and ZigBee have been particularly identified as suitable choices. This is further illustrated in the fact that the Continua Health Alliance, an open industry coalition of healthcare and technology companies joining together in collaboration to improve the quality of personal healthcare, has identified Bluetooth Wireless Technology and ZigBee as two of their main transports for medically related data. Even with the low power consumption which BLE and ZigBee offers, engineers are striving to maximise efficiency in their applications and reduce the power consumption even further by intelligent implementation of their operation, such as an efficient duty cycle. Consider a potential application which would require long period, low power consumption, such as a pedometer which is to be worn by a patient at home to monitor perambulatory activity. This could be a requirement of medical professionals monitoring an obese patient or a patient recovering from (lower limb) surgery. The medical professional may wish to monitor that the patient is not doing too much or too little movement. In the traditional monitoring situation this would require a visit to the patient by a health visitor to obtain this information. The pedometer could have a BLE or ZigBee radio module employed which sends the recorded data to a hub in the house of the patient which in turn could transport the data, via internet or GPRS, to a monitoring station at a central location. Such a patient worn device could transmit the recorded steps at say, hourly intervals. A higher power consuming device may require that the pedometer be put on charge at the end of each day, which would require the patient to remember to place it on charge, however, with a low power consuming device this could operate for days or even weeks eliminating this requirement. Of course, the question could be asked, "well if we are eliminating the requirement for the patient to remember to place the device on charge, how do we know that the patient will remember to wear the device". The answer is simple - in an application such as this, if the device reports "no steps" to the hub, the hub could emit an audible reminder or if the external monitoring station receives "no steps" data they could call the patient to investigate. This is an excellent example of low power technology in use. In addition to this, there have been considerable advances in energy harvesting techniques and the device could be further designed to harvest the kinetic energy of the patient to "top-up" the power supply to the transmitter. Clearly designers are presented with some clever and advanced technology which they can employ in their products to provide added value and quality of service. However, incorporating a radio device into a product does present other requirements which must be taken into account at the design stage; that of certification and regulatory. Bluetooth wireless and ZigBee technology have been developed over many years with input from many thousands of engineers costing many millions of pounds, and the intellectual property is protected. Therefore, it is usually a requirement when using these technologies that the final product must be submitted to a qualification (Bluetooth) or certification (ZigBee) assessment to ensure that the technology is being used correctly. Such assessment has many benefits such as improving interoperability of devices as well as allowing the product to bear the appropriate logo of the technology. This type of assessment is a requirement of the owners of the intellectual property and is not a legal requirement per se, whereas to market or put the device into service in countries around the world there are specific legal requirements which must be satisfied. These requirements do vary from country to country but for a radio device it is typically required that the radio, EMC and safety requirements of the target country be met. For a professional test laboratory, the most common domains requested are Europe (EU), North America, Japan and Australia/New Zealand. There are various routes to demonstrating compliance but the most common is to test products at a recognised laboratory. The radio testing assesses the radio parameters of the transmitting device, such as maximum transmitted power, power spectral density and harmonics. The EMC ensures that the device does not affect or is not affected by other electronic devices. Safety testing can be a bit more convoluted in that the requirements can differ considerably depending on where the product is to be marketed. This is particularly so when putting the product into a domain such as North America where it may be a requirement to adorn the product with a nationally recognised safety mark. In summary, the developments in medical technology are presenting some interesting challenges but clearly the technologies are available to enhance these devices and maximise power efficiency. Using the new technologies presents additional challenges for compliance which must be considered at the design stage. However, there are communities and organisations which are available to assist in the progress of this fascinating industry. www.tracglobal.com