Position encoding in battery-powered electronics

& -October 07, 2014

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 components. The magnetic field is generated by a magnet that rotates above the chip. By using three-phase sampling, only 3 Hall sensors are required instead of the usual 4, reducing current consumption by approximately 25%.

Always On

To allow permanent battery-powered operation, an integrated 1-chip ULP design must be able to completely turn itself on and off. Figure 1 shows such ULP architecture based on the iC-TW11 from iC-Haus. This device has been developed specifically for battery-powered applications, which require highly integrated energy saving and precise position measurement. It is connected to a central microcontroller—preferably also a ULP design—via an SPI interface. Position measurement and sampling of the Hall sensors only occurs when it is actually required.

 

There are no unnecessary measuring cycles which would waste energy from the battery. All circuit elements not needed are turned off after measurement and conversion is finished. The sampling of the Hall sensors, the downstream amplifier circuit with control and automatic calibration, as well as the interpolation for the angle measurement is designed to be fast and energy-saving at the same time. That way an average current of less than 3 µA at a sampling rate of 10 Hz and a resolution of 10 bits is achieved.

In the automatically activated standby mode between position measurements, current consumption of the complete 1-chip Hall encoder is only 100 nA maximum. The supply current as a function of the selected sampling frequency is shown in figure 2. The interfaces to the outside world operate at 3.3 V or 1.8 V. Therefore, no level shifter is needed for interfacing to a ULP microcontroller which uses a lower supply voltage. 

For position measurement, short measuring times are desired, which means a low latency time between the beginning of the measurement and when the result is available. In order for the ULP microcontroller to be able to switch to standby mode after a position measurement, an interrupt output to wake up the microcontroller is available. If multiple position values are required, like e.g. in a mobile multi-axis robot, cascading multiple iC-TW11s via an SPI chain and interrupt lines is possible by design. Since the maximum accuracy is not always required, the integrated filter function can be switched off to save even more energy.

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