Multiple Synchronized Inertial Measurement Unit Sensor Boards Platform for Activity Monitoring

The synchronization between wireless sensor nodes is a big issue in all those low-cost systems in which each node performs measurements using its own sensors and stores the measured values locally. In the absence of synchronization, it is impossible to relate all the measurements coming from the different sub-systems on a single time scale for the extraction of complex features. The importance of this issue is even bigger when the sensors' sampling rate is high as for the Inertial Measurement Unit platforms. The paper addresses the issue of managing a large multi-sensor platform for off-line activities with a reduced sensor node complexity and guaranteeing the perfect synchronization for each acquired sample. The new simple and light algorithm proposed overcomes the limits of the commonly used protocols and it is suitable for this kind of applications. The entire system consists of an Android smartphone, a master unit and several slave units. Each board has a 9 Degrees of Freedom (DoF) Inertial Measurement Unit, a micro-SD for data storage and an ARM® Cortex ™-M4 CPU SoC supporting the protocols for both Bluetooth Low Energy (BLE) 5 and 2.4 GHz custom communications.