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Managing battery life of IoT wireless sensors

Posted: 12 Aug 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Wireless sensors? battery? RF power amplifier? DMM? oscilloscope?

The seamless current ranging is combined with two digitizers to measure voltage and current with simultaneous sampling at 200 kSa/s (5-?s time resolution). Digitized measurements can be captured over 2 seconds and displayed with full time resolution and proportionally longer time with lower resolution.

However, for long-term measurements, the internal data logger in the Keysight N6705B modular DC power analyser integrates the 200-kSa/s measurements over a user-specified integration period (20?s to 60 seconds) without losing any samples between the integration periods.

As the data logger is gap-free, all the samples fall in one integration period or in the next oneno samples are lost. With the data logger, engineers can now measure the current and energy drain performance of a wireless sensor for up to 1000 hours of operation.

Figure 3: Data logger: all the samples are integrated in consecutive sample periods. No samples are lost. For every sample period, min and max values are also available.

Figure 4: Recorded current drain over 200 seconds of operation provides new insight into a device's dynamic current drain.

Measuring the sleep current is just a matter of placing the markers and directly reading out the values provided. The measurement in Figure 4 is made with a single acquisition over a long period of time; we get the complete picture of the current drain as well as an accurate measurement of the sleep current at 599 nA.

With pan and zoom capability, it's possible to look at the current level and time spent at every power level. Details that traditional measurement tools do not see can now be identified and measured.

A clear example is the trailing pulses identified by "???" in Figure 4. The software revealed this surprise: the device drain pulsed energy at ~90?A peaks for 500 ms for an average current of 3.3?A.

When we add this current drain to the 599 nA sleep current, it moves to 730 nA, 22% higher current than we expected. This type of surprise can be one of the reasons for underestimating energy requirements and delivering a shorter battery life than anticipated.

In wireless sensor power optimisation, engineers get great value by understanding the details. Knowing how much energy it takes to send out a single packet of information is very important when balancing user experience against battery drain and answering questions such as "should I send information once every second, every 5 seconds or every 10 seconds?" Engineers can accurately estimate the battery drain impact of any firmware change and validate it in a reasonable time with real measurements.

Joule measurements made easy

Joules are useful in battery life estimation, as every activity has a defined amount of energy. We can also compare device performance using Joules/transmitted bits. But engineers rarely use Joules because they need to be calculated from voltage, current and time.

Figure 5: Using Keysight 14585A software, you can measure energy directly in Joules.

With the Keysight 14585A control and analysis software, energy in Joules can be measured directly. For example, you might measure the energy consumed by transmitting a packet (see Fig.5) captured with a triggered measurement.

This is one benefit of having two digitizers for voltage and current with simultaneous sampling that enable point-by-point power measurements. Joules can be easily read out as a value between the markers, and designers can go a step further by defining Joules/transmitted bit.

Summary
Engineers who design IoT battery-powered devices use advanced power management techniques to conserve battery life. Traditional measurement techniques are complex, time consuming and don't deliver the measurement accuracy required to optimise and validate battery drain, and often this causes engineers to underestimate the power required to operate the device.

About the author
Carlo Canziani is Business Development Manager EMEA at Keysight Technologies.


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