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Optimise wireless medical devices' battery life

Posted: 05 Nov 2013 ?? ?Print Version ?Bookmark and Share

Keywords:battery powered medical devices? BPMD? scope? DMM? power consumption?

The typical method to analyse the energy requirements of your battery powered medical devices (BPMD) is for the design engineer to assemble multiple instruments and miscellaneous external circuitry. Typically a scope, DMM, or digitizer is used for making measurements (usually two channels, one to measure voltage and the other current), a power supply and/or battery to power the medical device, and some shunt resistors for current measurements. A method to control the medical device (to test its different states of operation) as well as control the instrumentation to collect and analyse the desired data (current, voltage, power) will need to be developed. This can be manual, or semi-automated, by connecting the instruments to a computer and writing software to programmatically control the test. The result is typically multiple files with voltage and current waveforms for the different operating states of the device. It is left up to the designer to manipulate the data to determine power consumption for each of the operating states of the BPMD.

Figure 1: Traditional setup for measuring sleep current.

The information available from this approach is limited. Because of the finite dynamic range of scopes, digitizers, and most DMMs (8-21 bits), different shunt resistors are required to measure the peak values (100mA to Amps) and the sleep current values (low microamps). What is missed is the critical understanding of the transient behaviour from sleep mode to peak demand of your design. Additionally, many of the larger energy demands of the BPMD are dynamic and more challenging to characterise. In summary, this simple power consumption information doesn't provide much insight into how you would change your BPMD design to improve reliability and optimise battery life.

An integrated solution
The value of an integrated solution goes beyond saving the time and cost of integrating your own system. With a solution focused on the test challenges mentioned above, you are removed from low value-added tasks like gathering and integrating the instrumentation and writing test programs. Instead, you can spend time analysing more insightful results, giving you confidence to make design changes that lead to a more reliable and energy efficient BPMD.

The new approach uses a different instrument for measuring and analysing your BPMD's power consumption. A source measurement unit (SMU) is a standard instrument, available today, providing an ideal foundation for measuring power consumption. An SMU allows one to source a voltage/current and measure a current/voltage. Since the SMU knows what voltage it applies and the current that it measures, it is capable of making voltage, current, and power measurements without additional equipment. Assuming the SMU's power rating is sufficient, it can also replace the power supply needed in the traditional method. Additionally, an SMU makes current measurements without the need for external shunt resistors.

Combining this measurement capability with a data-logger function allows the capture of voltage, current, and power over time. Not only can multiple operating states of the BPMD be obtained during the datalog record, the BPMD's use of battery capacity (Ah) can also be calculated. Using an SMU based architecture significantly simplifies your setup to measure and analyse the power consumption of your BPMD. For our example, the Wi-Fi blood pressure monitor system (blip) was characterized.

Figure 2: Blip wireless blood pressure cycle: 1) Initial sleep mode, 2) Pressing 'User 1' button to measure blood pressure (BP), 3) Inflating BP cuff, 4) Measuring BP, 5) Passing information for Wi-Fi___33 communication, 6) Wi-Fi transmitting BP measurement information to internet, 7) Display of BP measurement, 8) Return to sleep mode. Markers are set to capture information within the complete measurement cycle. Peak current (during pump operation) was 619mA. Drain on battery capacity for one cycle is 2.37mAh.

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