Integrating a network of wireless sensors with standard control systems

Thursday, 02 June, 2011

Wireless mesh networks are designed to provide high bandwidth communications channels between wireless sensing nodes over a specific coverage area. Wireless nodes cooperatively make data forwarding decisions based on their ‘knowledge’ of the network. In a mesh network each node may be communicating to other neighbouring nodes in the same area as long as they share the same network credentials. A wireless node normally needs only to transmit to its ‘parent’. If a node drops out due to hardware failure or other reason, its neighbour should be able to find another route to transfer the data to the next station.

Mesh networks use various communications protocols. One protocol is the Ad hoc On-Demand Vector (AODV) routing. When using AODV protocol the network is silent until a connection is needed. When a node requires a connection it broadcasts a request for connection to the network. Other nodes on the network forward the request recording, at the same time, the ID of the requesting node. Assuming communications routes were initially established throughout the network, the receiving node sends a response to the sender. Communications between the stations are generally established based on the least number of ‘hops’ through other nodes. When a communications link fails a routing error is sent back to the original transmitting node and the process repeats itself.

Nowadays, wireless sensors are a viable solution for a broad spectrum of projects.

The first and main advantage of using wireless sensors is substantial savings in installation costs of cables. Consider a simple warehouse environmental monitoring system where multiple temperature and relative humidity points are monitored. Using wireless nodes that include both types of sensors there is no need to install cables in roofs, ducts or ceilings.

Data from multiple sensors distributed throughout the warehouse can be transferred reliably over a wireless network to a central station. The wireless network is managed by a central node known as the network coordinator or gateway. The coordinator also enables a simplified interface to external systems such as PLC, SCADA stations or HMI panels.

The second advantage of using wireless sensing nodes is flexibility. In a wired world the decision to move a device from location A to location B almost always dictates new ducts, additional cabling and additional man-hours. The need to add a sensor such as energy metering at the last moment can easily be accommodated by multisensing, multifunction wireless nodes.

The third advantage of using wireless sensors is the relative low price of a wireless node. Wireless sensing nodes are available as either single sensing or multisensing nodes. The advantage of multisensing or multifunction nodes is translated into a lower cost of ownership per node. In addition to multi inputs of analog and digital signals, multisensing nodes may include digital and analog outputs to facilitate control of external devices such as fans, refrigerators, lights, valves, pumps and even motors.

Interfacing to standard control systems such as PLC, SCADA stations, HMI panels is straightforward. All that is needed is for the wireless network coordinator and gateway to use a standard communications protocol. One of the most popular ones throughout the world is Modbus. Other communications protocols may be implemented when the need arises.

Finally, when considering a network of wireless sensing nodes, it is highly recommended to conduct a wireless site survey. A wireless range tester tool provides the system integrator or installer with good indication of the wireless path, wireless signal strength and wireless signal quality between any sensing nodes and the central gateway station.

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