Cabled based sensing systems for structures have high installation and maintenance costs
Wires vulnerable to ambient signal noise corruption.
Wired links are prone to breakage and environmental wear.
centralized approach with all system sensors sending measurement data to one data server. Such an approach adds latency to the system during real-time data processing and represents a single point of failure.
Use of a wireless communication system for inter-sensor communication. Low cost wireless technologies have significantly contributed towards this direction.
B luetooth . Operate in the unlicensed, 2.4 GHz radio spectrum. These radios use a spread spectrum, frequency hopping, full-duplex signal at up to 1600 hops/sec. The signal hops among 79 frequencies at 1 MHz intervals to give a high degree of interference immunity.
Sleep . Sensor module and R/F transceiver in sleep mode. SU controller and the low sensitivity accelerator ( Acc-Low ) intermittently powered. Right After Acc-Low startup time elapsed switches on the A/D converter and samples the Acc-Low output. If the data do not indicate an extreme seismic event Acc-Low and A/D are powered off.
If an event ( 5mg and above ) is detected unit enters Awake state. Sampling rate is increased accordingly and data are saved in SRAM. Event time is noted. Synchronizes with LSM and adjust s event time accordingly. Awake state is synchronized with TDMA slot.
It passes in Semi-Awake state all modules but transceiver are on. Active sampling of both accelerators output. Semi-Awake periodically alternates with Awake state were radio transceiver is also on , allowing transmission of sensor data to LSM along with active sampling
After event has passed and all collected data are sent SU enters sleep state.
CSM determines at which instances ambient vibration info is to be recorded to enable periodic monitoring. SUs usually in sleep states are waken up in fixed number of times per day and enter the Update State.
In the Update state state sensor module is off, SU ctlr and transceiver on and Ac-Low cycle powered. It synchronizes with LSM and sets a wake up timer for the next scheduled monitoring phase. After that enters Awake State alternating with Semi-Awake transmiting data
After a predefined time interval it goes back to sleep.
A special purpose customized TDMA protocol. For N sensor units per cluster we have a round of N+2 slots. N for data, 2 for control. A data slot assigned per SU. 1 st control slot for Synch-Ack and second for Global-Synch signal. One packet per data slot. Frequency hopping at every slot.
LSM acknowledges SU packets broadcasting a Synch-Ack signal containing ack bits for all packets received in the current and the previous round. Of course contains the local clock information for synchronization of SU clocks with the global clock.
SU notifies LSM before entering the sleep state at the end of a monitoring or update state. Uppon notification LSM is able to use the SU slot for startup packet.
Synch-acks are appended with the ambient monitoring schedules enabling schedule updating upon synchronization.