PIMU: A wireless pressure-sensing IMU

Inertial measurement units (IMU) are included in uncountable consumer electronic products, e.g. smart phones. IMUs are state of the art for research in human motion analysis, context and activity recognition. Sensor setups to estimate the activity of a subject, e.g. standing, sitting, walking, climbing stairs etc., typically require a multitude of sensor nodes to provide reliable estimations. In our paper, we present a sensor system for enhanced context and activity recognition while requiring a minimal set of sensor nodes. Our system combines a pressure sensor featuring high spatial resolution (1248 sensitive points) with an IMU. The pressure sensor is typically placed inside a shoe and measures the force exerted on the feet. Our system eases the detection of many situations: differentiating between sitting and standing, walking and climbing stairs etc. It is adjustable to virtually any shoe and therefore could be included in projects requiring specialized footwear (e.g. running, weight lifting or biking shoes). Data can be stored on the sensor node or it can be transferred wirelessly to a remote aggregator device, i.e. a device responsible for data stream reception, sensor control and analysis. The aggregator can be implemented in a smart phone or a PC. Our system can in real-time detect situations of sitting, standing and walking automatically. The system's low-power implementation allows for run-times exceeding 14 hours. In a multi-system setup the systems can be synchronized with a node-to-node offset in the order of 10μs. The pressure can be sampled at a maximum of 120Hz, the inertial data is available at a rate of 512 Hz. Our small-footprint real-time classifier can distinguish between the situations standing, sitting, and walking with an accuracy above 99%.