An Energy-Efficient Current Measurement Method for Wireless Sensors

Battery life and available energy are critical parameters in wireless sensors. As the battery voltage is often not a good indicator of the state of charge, it is of interest to measure the charges that go in and out of the battery. This can be achieved by integrating the current over time. An accurate current measurement circuit may involve an ADC with a high sampling rate to cope with the current's fast transients and high crest appearing due to duty cycling and DC-DC converters. However, the low power constraints prevent the use of such sampling approaches. In this paper, we present an energy-efficient method for measuring charges and average currents flowing in and out of an energy storage device. The proposed approach uses a capacitor as an integrator in the analog domain. Averages of currents with high crest factors can be determined with good accuracy (RMSE <= 7.6%) and low power consumption, where the measurement costs about 0.8% of the total power. The approach is demonstrated on an autarkic wireless sensor for the grippers of a robotic manipulator.