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Page Revision: 2010/10/07 11:43
System Overview
Requirements
The aim of the system is to sample in a synchronized fashion multiple accelerometers placed at different locations in a structure and forward the data to a central station for later processing. The most relevant application requirements were identified as follows:
- - XYZ accelerometer (triaxial)
- - Max. measurement range: ± 1 g
- - Minimum sensitivity: 1 V/g
- - Typical resolution: 1 mg
- - Max. resolution: 50 µg
- - Frequency response, 3 dB: 0 - 100 Hz
- - Max. sampling rate: 100 Hz
- - Max. sampling drift between sensors : 10 ms
- - ADC resolution: 24 bits
- - 0% sample lost during sampling process
System Architecture
The system architecture was designed in order to satisfy the identified application requirements and is illustrated in Fig. 1, considering a prototype system composed by four Sensing Nodes. Each Sensing Node is composed by a TelosB WSN platform with a signal acquisition board (SAB) attached to a MEMS acceleration sensor.
All four Sensing Nodes communicate with a Coordinator Node (also a TelosB WSN platform) via a standard communication protocol (IEEE 802.15.4). The Coordinator Node supervises the network and nodes activities (e.g. node configuration, start/stop sampling) and guarantees a tight synchronization between all nodes; it also forwards the configuration parameters and dispatches the acquired data to the Command&Configuration Application (C&C App).
The C&C App provides the system user with a human-machine interface (HMI) to configure the system and also an application programming interface (API) to integrate the WSN system with the data processing/analysis applications
cross link for the data processing/analysis applications. The latter enable to infer about the reaction of the monitored structure to vibrations (natural or external excitations).
WSN Architecture
The proposed SHM system aims at sampling several accelerometers placed at different locations in a structure, in a synchronized fashion. Sampled data is to be stored in each Sensing Node until it is retrieved by a central node for processing. To enable the analysis of the results, namely the modal shape analysis, it is crucial to guarantee the temporal correctness of the system.
Guaranteeing Synchronization
The IEEE 802.15.4 protocol
link to IEEE 802.15.4 provides a standard-based solution for synchronization (beacon-enabled operation mode) that fits the application requirements. Thus, it has been selected for the WSN communication infrastructure. The Coordinator Node (officially named PAN – Personal Area Network – Coordinator) schedules channel access and data transmissions in a messaging structure – the Superframe. This node is also responsible for periodically transmitting a beacon frame announcing the start of the Superframe. Upon beacon reception, each Sensing Node triggers an external GPIO (General Purpose Input/Output) pin on its SAB
cross link to SAB in order to synchronize it.
Communication Architecture
From the five WSN nodes, four act as Sensing Nodes and control the corresponding SABs, while one node acts as the Coordinator Node, assuming network management (including network configuration and synchronization), data collection and interfacing with the C&C App.
Coordinator Node
The Coordinator supports two types of commands:
- Board Commands – used to configure the SABs; these commands are transmitted to the corresponding node, and then directly forwarded to the SAB, using regular IEEE 802.15.4 data frames;
- Network Commands – used to manage the monitoring application.
The Network Commands are divided in two different types: - Node Management commands: commands sent to the Sensing Nodes using regular IEEE 802.15.4 data frames during the application Ready state. These include setting the behaviour of the node (active/passive), remote reset, channel selection, and requesting onboard sensor reading (temperature and humidity
- Application Management commands: commands sent within the payload of the IEEE 802.15.4 beacon frames so that all nodes receive and process the command at the same time, thus guaranteeing synchronization (there is no contention in beacon transmission).
Sensing Nodes
The Sensing Nodes control and synchronize the acquisition of the SABs, and carry out the acquisition of the embedded sensors measurements (temperature, humidity, voltage, luminosity).
The implementation the Sensing Node’s architecture was developed in nesC, over TinyOS. This concerned not only all the application and the open-ZB stack, but also the communications with the SAB. The last is handled using the UART serial interface of the TelosB
link TelosB. Two additional general purpose input/output (GPIO) pins of the TelosB
link TelosB are used to enable the synchronization of the SAB and to control the communication flow.
Signal Acquisition Sub-system
In order to satisfy the underlying application requirements
cross link for requirements, and consequently comply with the MEMS sensor characteristics, a custom-designed signal acquisition board (SAB) had to be conceived for supporting: a) a high resolution 24-bit ADC; b) enough memory for storing data samples.
Command & Configuration Application¶
In order to provide the necessary HMI and API for the data analysis applications a Command & Configuration Application (C&C App) was developed.
The available controls of the C&C App enable full control over the acquisition configuration parameters (i.e. axis selection, sampling rate, sampling period, sampling duty cycle, etc.) and also provides a quick evaluation of the presence of the system nodes. Several additional features are also built-in to assist the user with relevant information on the network and acquisition parameters configuration.
One additional goal of the C&C App was to provide a convenient interface between the WSN and the data processing/analysis application. The implemented mechanism allows a transparent interface with the system, in a very similar with the previously used, which are typically serial data interfaces.