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EETE OCT 2015

Test & Measurement IoT wireless sensors and the problem of short battery life WBy Carlo Canziani ireless sensors provide great insight in applications like monitoring environmental conditions or industrial plants and machinery. Because they are simple to install, they can be deployed in a multitude of situations. In coming years, we will see an explosion of new uses for wireless sensors as the “Internet of Things,” or “IoT,” is widely deployed. But one of the factors that most limits the use of wireless sensors is their limited ability to do the job for a reasonable amount of time. When a wireless sensor’s operation is fully dependent on a battery, and the battery is depleted, it becomes just a piece of junk. If you are designing battery-operated wireless sensors, you face numerous challenges in ensuring your devices operate for a reasonable amount of time. The typical approach is to use energy for just the required activity, then put the device in lowpower use mode. The operation of a wireless sensor can be segmented in a series of activities, each one requiring a certain level of power for a certain amount of time. The most common activities: • Waking up, taking a measurement and processing data into a message • Powering up the RF power amplifier, transmitting the message, and powering the RF PA down again • In bidirectional sensors (transmit and receive): waking up, powering up the receiver, receiving, processing data, acting on a message, and powering back down It is easy to see that multiple actions play a role in discharging the battery. The simplest way to increase the battery life is to use a bigger battery, a battery with higher capacity. Nevertheless, your customers are likely to expect their sensors to be small and to offer high performance (so they can send lots of data and have local intelligence/data crunching capability). Clearly, your Fig. 1: Current levels during the three main states of a wireless sensor customer expectations are diametrically opposed to the easiest way to solve the issue of short battery life. How do engineers estimate battery life? As a design engineer, you need to start making compromises and find the balance between battery size and the wireless sensor’s functionality to get the best performance from a small battery with a sufficiently long time interval between battery replacements. The optimization process starts by understanding the energy requirements. Gathering data about energy usage is the first step to characterizing device performance. A battery has a defined amount of energy, specified in Watt hours (Wh) and capacity, specified in amp hours (Ah). If you know how much power is required to operate your device, you can calculate the battery life. Battery life (hours) = Battery capacity (Wh) / Average power drain (W) The battery’s energy is also the product of its voltage rating (V) and capacity (Ah). The voltage rating is a midpoint value on the battery’s discharge curve empirically determined to correctly relate the battery’s energy and capacity. Based on this, battery life can also be determined by the formula: Battery life (hours) = Battery capacity (Ah) / Average current drain (A) Carlo Canziani is Business Development Manager EMEA at Keysight Technologies - www.keysight.com CM7V-T1A Thin ceramic package kHz Crystal Consumer & AEC-Q200 automotive compliant, Size: 3.2 x 1.5 x 0.65 mm, Temperature range: -40°C to + 85°C and -55°C to + 125°C A u t h o r i s e d distributor MICRO CRYSTAL SWITZERLAND Our authorised distributor WDI AG is pleased to assist you.  +49 4103 1800-0  microcrystal@wdi.ag  www.wdi.ag/microcrystal 32 Electronic Engineering Times Europe October 2015 www.electronics-eetimes.com


EETE OCT 2015
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