Position encoding in battery-powered electronics

October 09, 2014 //By Jonas Kupp and David Lin
Position encoding in battery-powered electronics
Jonas Kupp and David Lin consider the benefits of position encoding in battery-powered electronics.

The trend towards energy-efficient ultra-low power design is evident in all sensor technology areas. Especially portable devices and sensors with wireless networking and fail-safe protection require low-power measurement of position data. Furthermore, in many applications a change in position needs to be detected even when external power is not available. The energy required for the measurement can be generated from an energy harvesting solution [1] or provided by a battery. Magnetic position measurements using Hall sensors can be integrated into a 1-chip encoder including the complete signal conditioning circuitry.

Integrated Hall sensors are space-saving and cost-effective, however they do need a relatively large amount of power during operation. The solution here is temporary activation of the Hall sensors. Fast position measurements, like those required for motor control, require fast evaluation and pulsing of the Hall sensors, while a metering application needs only a slow sampling rate. Therefore, special solutions are necessary for energy-saving operation.

How to get to microamperes?

The signal voltage generated by a Hall sensor is proportional to the magnetic flux density and to the current in the Hall element. When implemented using CMOS technology the sensor characteristics are fixed by the process. Thus, current consumption can only be reduced by reducing the measuring cycle time for the Hall elements, reducing the supply voltage, and by using ultra-low power circuit design techniques (ULP).

The measuring frequency is set only as high as required by the position measurement. ULP circuit design activates the individual function blocks only when they are actually needed. A programmable power-down and wake-up circuit ensures no unnecessary activation and thus average current consumption is minimized. Reduction of supply voltage to 3.3 V or 1.8 V for I/O ports provides further reduction of current consumption and simplifies the selection of batteries.

To reduce interference from external magnetic fields, a Hall sensor pair is used in integrated Hall encoder ICs for differential acquisition of the magnetic field

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