Method of synchronization of a wireless sensor system and a sensor system implementing such a method

1) A synchronisation method for a wireless sensor system for monitoring civil structures comprising the steps of:

• providing and arranging a sensor system on a civil structure, said sensor system comprising:
• a plurality of sensors configured to communicate wirelessly and suitable for detecting quantities useful for monitoring civil structures, each of said sensors comprising a counting and timing device and a peripheral processing and control unit associated with said counting and timing device, said counting and timing device comprising an oscillator capable of generating a clock signal at a predetermined oscillation frequency fc, said peripheral processing and control unit being configured to operate on the counting and timing device to generate a clock signal having a frequency equal to a nominal sampling frequency fs based on a predetermined relationship between the predetermined oscillation frequency fc and the nominal sampling frequency fs, said oscillator having a nominal maximum tolerance δfmn, said sensors being in number equal to M, each of said sensors being configured to detect and sample a continuous time signal xm(t) wherein m ranges from 1 to M and indicates the m-th sensor;
• a central processing and control unit configured to communicate with said sensors wirelessly;
• calculating the limit maximum tolerance of one of said oscillators δfml on the basis of the nominal sampling frequency fs, a predetermined maximum tolerable synchronisation error τd and the number of samples to be acquired in a nominal sampling window Tc at said nominal sampling frequency fs;
• comparing said nominal maximum tolerance δfmn with the previously calculated limit maximum tolerance δfml;
• if δfmn≤δfml performing a first-synchronisation comprising the steps of:
• at an initial tuning instant equal to ta−Tdiff, by means of said peripheral processing and control unit, placing the sensors in a state of continuous reception for a continuous reception time window having a duration greater than 2Tdiff wherein ta indicates the monitoring start instant and Tdiff indicates the maximum time offset between said counting and timing devices;
• at the monitoring start instant ta sending wirelessly via said central processing and control unit a single multicast monitoring start message addressed to all the sensors;
• receiving via said sensors the multicast monitoring start message;
• initiating via the peripheral processing and control units of the sensors the sampling of the continuous time signal xm(t) at a sampling start time t0.