Sensor Networking for Condition Monitoring

Fraunhofer IMS has many years of experience in the field of energy-efficient wireless real-time communication with embedded systems, especially in sensor networking for condition monitoring. This includes the use of available hardware components and standardized protocols as well as the development of individual interfaces and proprietary protocol stacks.

One example of such an application-specific system is sensor networking for condition monitoring of overhead power lines. Here, sensor nodes mounted along the line are used to record the current load and the general condition of the line. The measured data is aggregated and visualized at a central location. In the event of a fault messages can be displayed there and measures initiated.

For data transmission from the sensor node to the control center the use of mobile radio technologies from the field of narrowband IoT is the obvious choice. However, in order to be able to connect sensors at remote locations without cellular coverage, it makes sense to first transmit the data via a separate radio network until they arrive at a location with sufficient coverage. Such a radio network should meet at least the following requirements:

• Operation in a license-free ISM band at the allowed transmission power prescribed there
• Transmission ranges at least in the double-digit kilometer range despite low transmission power and low energy supply of each individual sensor node
• High redundancy and reliability

These requirements can be met by using commercial LPWAN technologies such as Sigfox or LoRaWAN.

To ensure long availability of hardware components and avoid dependence on individual manufacturers, standard transceivers from the field of short-range devices should be used. To achieve a high overall range a special radio protocol is required which reliably passes information from one sensor node to the next until it arrives at its destination.

Fraunhofer IMS has developed such a protocol for sensor networking for condition monitoring. The basic structure of this protocol is based on a ring topology.

As shown in the sketch, such a logical ring consists of several sensor nodes (SK) and two gateways (GW A and GW B). The sensor data is passed along the ring from node to node until it arrives at the primary gateway (GW A).

The gateway takes the data from the ring and forwards it to the destination. In the event that the primary gateway can no longer reach the destination or fails completely, the secondary gateway (GW B) can take over the tasks of the primary gateway. This corresponds to simple redundancy.

Since in practice neither power supply lines nor pipelines have a ring-shaped structure, but rather a line-shaped structure, the developed protocol has the following differences compared to the pure ring topology:

• The left half of the ring is omitted. The two gateways are thus located at the beginning and end of a line.
• In order to maintain the required redundancy, each data packet is routed twice through a sensor node - first in the direction of GW B, then back in the direction of GW A.