The Sensor Node Design and Database

The sensor node design is a key component of the generic design conceptual framework. It aims at ensuring that the sensing, monitoring and tracking requirements are thoroughly examined and the specifications of a device sensor node that fully meet these requirements are dynamically elaborated, performing an automatic update when these requirements have changed.

5.3.2.1. The Sensor Node Composition

The sensor node is defined in this implementation in its generic meaning, referring to a universal fixed device that can be repeatedly used in a configuration as a homogeneous device in the WSN. It includes, basically, all devices (sensors, RFID tags and other data, support and control components) that be used for sensing, monitoring and tracking, grouping different capabilities inherent to the integrated use of different technologies such as WSN, RFID and other technologies supporting the development of smart devices.

In this work, the sensor node design consists of grouping a set of sensors, the number being limited in this case study to six sensors per sensor node with the option of incorporating a RFID tag or reader, depending on the sensing, monitoring and tracking requirements.

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Of great importance in the sensor node design, is the calculation of the device specifications (power consumption, storage and processing) elaborated should this device become fully active to perform local operations required in network processing and data storage. This calculation can be performed with different scenarios of sensor and sensor node configuration, depending on the setting of each sensor and device sensor node in the ad hoc WSN.

5.3.2.2. The Sensor Node Specifications and Resources Calculation Models

The rules for the calculation of the fixed device specifications are the result of the domain knowledge elicitation which focuses on various aspects of device deployment and utilisation in the WSN, ensuring that reliable procedures for fast activation and verification are elaborated, and re-tasking procedures are considered when reprogramming the sensor node devices. They are stored in the system model base as the resources calculation models. They support a decision tool for the sensing area for each device, ensuring that the sensing spacing distance is the sensors minimal distance (SSD) of each sensor composing the sensor node device, as shown in Equation 5.1.

SSD device = min (SSD sensor i) for i=1,...,6 [5.1]

There are a wide variety of calculation models for evaluating the power consumption of wireless sensor network applications [214]. These models are based on the use of different strategies to help predicting the WSN lifetime depending on the applications specifications for the use of the WSN, and optimize the required energy consumed using an effective energy plan.

The integrated functional unit aimed at the design of the generic sensor node is illustrated by the interface screenshot shown in Figure 5.4.

It is essential that the designed sensor node device meets the following quality requirements:

 High detection or identification rate

 Low false alarm rate,

 Low reporting latency, and

 Sufficient energy for continuous operating even when duty cycling is used.

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5.3.2.3. The Sensor Node Design Implementation

The generic sensor node design process is based on the use of data described in the device sensor design data model illustrated in Figure 5.4.

Figure 5.4: Device sensor node design interface

The content of this model can be context application specific in the sense that it can be updated depending on the general characteristics description and functional specifications of the needed sensors or RFID tags composing the desired device sensor node. This updating is supported by the system database management component which provides the flexibility to support both the definition and manipulation of data operations. Additional data functions need to be developed to support the data warehousing. An exhaustive list of sensor properties is published in technical documents.

5.3.2.4. The Mobile Sensor Node Device

Of great interest in the definition of the sensor nodes architecture is the composition and representation of people and objects interacting in the premises covered by the WSN. The solution suggested in this work is the attachment of RFID tags to people and objects. The architecture of these tags enables the incorporation of different types of sensors, and also a set of data pre-coded and modifiable depending on the WSN applications invoking these mobile

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devices. This data is structured using a language task description of the activity involving the tags correspondents. This language is required to implement the sensor node architecture:

composition plans are converted into assembly commands which when performed result in perceptual data reflecting the status of each component of the node.

The composed devices are wirelessly connected to the WSN, interacting with the RFID readers installed in each room of the building, as explained later, in the section about the heterogeneous devices allocation.

5.3.2.5. The Sensor Node Data Model

The data model shown in Figure 5.5 is specific to the description of the fixed sensor node device represented by "Detector.db".

Figure 5.5: Device sensor design data model

Mobile sensor node devices will be linked to a table of people or goods to highlight the concept of device attachment to these two entities which can be grouped in the same database using an entity code to dissociate the two different types.