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 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.
In normal operation, iC-TW11 reaches sampling rates of 4 kHz with activated filter and automatic amplifier calibration for maximum accuracy with 10-bit resolution. If the filter is deactivated, the latency period is reduced to 50 µs and sampling rates of up to 20 kHz are possible. With the same sampling rate, the current consumption is typically reduced by almost 90 % due to the filter deactivation. A measuring cycle can be started by the microcontroller or via a separate trigger input, i.e. through an external event. If a new position value is available, the microcontroller is switched from standby mode to active operation via an interrupt. The absolute position data is read via the 4-wire SPI interface with clock rates up to 16 MHz. A high clock rate also affects a reduction in current consumption because the active times of the iC-TW11 and the microcontroller are reduced. Optionally, the internal raw Hall sensor values (10 bits), the sine and cosine values (12 bits), and the amplification factor (18 steps) can be requested for test purposes or for the measurement of the magnetic airgap.
An energy harvesting solution, e.g. with wireless sensors, can be used instead of a battery if the peak currents are supplied from a capacitor. For digital control buttons, ultra-cap buffering to ride through power failures is possible.
Switching Supply Lines
In some applications, an energy-efficient position measurement is required even when external power is not available. For robots, which due to their inertia can move even after power failure, it is dangerous if the change in position is not recognized and the restart occurs with false data. Therefore, all continued movement in these actuators has to be safely detected by a multiturn encoder. They either have a mechanical gear or an electronic recording with power failure protection, e.g. through a battery. For consumption measurement devices, such as industrial gas or water meters, the acquisition has to be possible even without external supply voltage. An automatic switching between battery and external supply is then necessary.
Figure 3 shows the block diagram of a gas or water meter using the iC-PV to magnetically scan an impeller. This ULP 1-chip Hall encoder automatically switches between energy-saving battery operation and normal grid operation.
If the supply voltage VDD falls below a defined level, the iC-PV automatically switches to battery supply VBAT. After the power supply has been restored, the iC-PV provides the readout device with the counter value via a serial interface for the consumption calculation. The measuring results are checked via 8-bit CRC and errors are indicated via an active-low NERR output as well as an error bit within the serial data transmission.
Four Hall sensors detect position changes with an adjustable 1- to 3-bit resolution. The number of rotations is counted in a multiturn counter with a length of up to 40 bits. The iC-PV also has parallel outputs for octal resolution of 3 bits. The external EEPROM is programmed during calibration via a separate I²C interface and the iC-PV loads the CRC-secured configuration data at power-up.
The iC-PV’s ULP design uses similar methods to the previously described iC-TW11 to reduce current consumption during active operation and in standby. However, the iC-PV has its own independent cycle and timing control to cyclically activate a defined measuring cycle without the need of an external microcontroller. Depending on the set sampling rate, revolutions can be counted at speeds from 12,000 to 100,000 rpm. The average current consumption ranges from only 2 µA to 30 µA allowing battery-buffered operation over several years.
Summary
As shown in two examples, position measurements using ULP 1-chip Hall encoders can be performed efficiently with direct battery operation or even during power failure. In the first case, measuring cycles are controlled and processed externally via a microcontroller. In case of power failure, the 1-chip Hall encoder iC-PV is fed through a backup battery. Changes are detected and saved so that they are available for transmission when external power returns. The complete integration of Hall sensors with the analog and digital evaluation circuitry in a 1-chip encoder is advantageous because solutions with extremely low current consumption (less than 10 µA) can be achieved with ULP design techniques.
[1] Energy-Harvesting Multiturn Counter/Encoder
[2] Sine/Cosine-to-Digital Conversion: Methods and Design Challenges (EDN 02/2014)
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