Building Blocks For Wireless Sensor Networks
The essential building blocks for wireless sensor networks can be segmented into hardware, middleware, software, communications protocol, and user interface.
This is a simplified representation of a wireless sensor network (WSN), but serves as the basic configuration. Please see additional articles on this site for more details of implementing WSN for personal use, industrial automation, building automation and various configurations (e.g, mesh network, star network, etc.).
In the hardware category are the nodes. Each wireless sensor network has tens, hundreds or possibly thousands of nodes. An individual node is made up of the following elements:
- sensor
- controller
- data collection / storage
- communications
- power system
The sensor is specific to whatever parameter is to be measured or monitored. This can be anything such as temperature, humidity, pH, water level, smoke, vibration, sunlight, radiation, sound, proximity, movement, magnetism, flow or pressure. A sensor in most cases is an analog device which either has an embedded analog to digital converter (ADC) circuitry or attaches to an external ADC.
The controller part of the node contains the intelligence for the system. This will determine how frequently to take a measurement, whether the measurement is valid, how many measurements to store and when to transmit the data from the node. Depending on the complexity of the node, there may also be two way communications from a central site to the node, rather than one way communications of the node sending out its readings. The controller is also responsible for monitoring the power reserves of the system and the overall health of the node.
Data collection and storage is the process of translating the output of the sensor into a digital format which can be communicated to the network. This may also include some level of encryption to help maintain security of the data. Data storage can be as simple as a loading a transmission buffer in the communications device to temporary accumulation of a number of measurements in a short term memory element, to a permanent data log of all measurements.
The communication system has an antenna, a transmitter and in some cases a receiver for two way communications. Communications in a wireless sensor network balance the requirements for power conservation, distance and security. The majority of communications use standard protocols such as ZigBee or HART, but can also use a proprietary standard for greater security, integrity or power usage.
The most common method to implement the communications portion of the wireless sensor node is to purchase one from a third party developer that has already tested, optimized and cost reduced this function to a module with a standardized interface. This greatly simplifies a developers task by reducing this portion to a selection matrix of systems requirements – such as protocol (e.g., ZigBee, IEEE 802.xx), communications frequency (e.g., 900MHz, 2.4 GHz), distance and special features for power management or network configuration.
One company which provides very easy to use communications modules to integrate into a sensor node is Digi International . One small, low cost module that operates in the 900 MHz band can communicate at up to 28 miles under unobstructed line-of-site (LOS) conditions. This is the Digi ” Xbee-PRO 900HP ” solution. For initial proof of concept and development, they also feature complete kits to help designers get a complete product working and to market or deployment as quickly as possible.
Critical to the sensor node is the power system. In many cases this is simple a battery or other energy storage cell. In others, it can also include a more complex energy harvesting system which allows the node to extract energy from its surrounding environment. Energy harvesting is most commonly achieved through converting light (e.g., solar) or electricity, but can also make use of audio energy, RF energy or vibration.
Related Article – Energy Harvesting In Wireless Sensor Networks
The firmware, middleware and software are responsible for all communications from the sensor node to the central site. A wireless sensor network may have a large number of intermediate communications elements, called routers and gateways, which “hop” the data from point to point until it eventually reaches the central site. Each gate communicates with a subset of nodes that are in its communication range, creating a sub-network within the entire network. Similarly, a router communicates with a number of gateways.
In addition to providing a communications pathway from the sensor node to the central site, the gateways and routers can also be responsible to providing security and reliability to the overall network. Security is accomplished by implementing additional levels of data encryption to protect the underlying sensor measurements, as well as requiring authorizations for anyone attempting to access the network itself. Reliability is achieved through creating a redundancy in the network where if one (or more) gateways or routers stops operating, it’s possible for another in the network to take over its function and continue to communicate with the sensor nodes.
Depending on the complexity and application of the wireless sensor network, either a portion or the entire communications infrastructure can be supported by an external provider. Etherios , a division of Digi International, has a cloud based model to allow connections of any enabled device anywhere in real time.
The user interface may be the most critical portion of the wireless sensor network. This is typically a software application which displays the data from the sensor node. It can be as simple as a spreadsheet of data, to a app running on mobile device, to a dashboard on a computer to massive control panel at an industrial complex. The complexity of the user interface is typically directly related to the importance of the element being monitored – algae growth in the South Pacific Ocean is different from radiation levels in a nuclear reactor.
The building blocks for wireless sensor networks can be summarized as hardware, middleware, software, communications protocol, and user interface – however the realization of the actual network contains many more elements within each of these categories.
To learn more about this fascinating topic we recommend these books available through Amazon:
Industry 4.0: The Industrial Internet of Things
Industrial Automation
Wireless Sensor Networks
Principles of Wireless Sensor Networks
Introduction to Wireless Sensor Networks
Protocols and Architectures for Wireless Sensor Networks
(Photo Credit – SxC)
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