Design and test of a low-power acceleration sensor with Bluetooth Low Energy on ear tags for sow behaviour monitoring

Abstract

Short battery life limits the applicability of wearable sensors in animal behaviour monitoring. This research developed an ear-attached acceleration sensor of low power consumption for sow welfare studies. The sensor weighed only 10.0 g and was composed of a 3-axis accelerometer, a Bluetooth Low Energy module, a printed circuit board antenna, and a 3-V, 230-mAh CR2032 battery. It entered into a sleeping state when no activities of the monitored sow were detected, to conserve battery energy. In the sleeping state, the sensor still read data from the accelerometer at 1 Hz, but did not transfer data wirelessly to a receiver if a first-order difference that differentiated sow resting from its other activities was below a defined threshold. Laboratory and 8-sow field tests were conducted to assess the sensor performance in three operational modes (continuous, data grouping, and power saving). Results showed that there were three major behaviours of the sows (resting, moving, and eating) and the behaviour patterns differed from sow to sow. The ear-attached sensors could in principle be used to monitor and predict sow farrowing while not causing sow abnormal behaviours. In the continuous mode, data broadcasting consumed 97.6% energy and the battery life was only about 31 days. The average battery life could be extended to about 288 days in power saving mode when the first-order difference threshold was set at 0.1 g. Compared with reported similar acceleration sensors, this new sensor provided more than double the battery life while reducing the sensor mass by more than 70%.