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Initial situation

Until now, the storage silos of the Werkhof of the city of Rapperswil were equipped with laser level sensors to regularly transmit the levels of the salt reserves by means of a 2G mobile network. This meant that the purchasing department always had an overview of the available reserves and could reorder salt in good time if necessary, thus ensuring the management of the roads in winter.

With the discontinuation of the 2G network infrastructure by Swisscom, the existing solution became unusable. In cooperation with CRA AG from Jona, adnexo designed a successor solution.

Our goal was to implement a cost-efficient solution in the form of a retrofit. This means to continue using the existing infrastructure & sensor technology and to renew only the necessary parts.

Choice of radio technology

In order to replace the outdated 2G radio technology, the first issue was the choice of alternative. The next generations of mobile networks like 3G, 4G or 5G or LPWAN technologies like LoRaWAN are available. The classic mobile networks require a SIM card and therefore basically a classic mobile subscription, which usually costs several dozen francs. High costs for the transmission of a few sensor values. In contrast, Swisscom's LPN LoRaWAN network was designed for exactly this type of data transmission: Low bandwidth, low power consumption, but a lot cheaper.

Because of the existing mains voltage there were no requirements for the power consumption. the connection costs are of crucial nature. The low costs, about one tenth of a SIM card with subscription, therefore speak clearly for the new LoRaWan technology. After successful reception tests on site, the choice of LPN radio technology was made.

Implementation

We replaced the 2G modem with a LoRaWAN transmitter and integrated the sensor measurements into our ax-track platform. We migrated the data transmission from the obsolete 2G GSM to the modern LoRaWAN radio standard of the Swisscom LPN network. Originally, the data was managed in a fat-client desktop application, which was already somewhat outdated. By moving to our ax-track platform, the municipal utilities now have access to the data at any time, even from mobile devices. In addition, we have configured notifications to automatically trigger alerts should inventory run low and need to be reordered.

Sensor

The existing sensor uses a laser beam to measure the distance from the lid of the silo to the surface of the contents and relays this in the form of a 4-20mA signal. We were able to reuse the sensor itself by tapping the signal from the sensor with a LoRaWAN transmitter and then forwarding it to our ax-track IoT platform.

Calculation

For the level calculation, we had to consider a few things. Firstly, the silos do not have a simple geometric shape, i.e. they are not a simple cuboid or cylinder, but a combination of several shapes. More cylindrical in the upper part and then a cone in the lower part. In addition, the salt is not distributed evenly in the silo, but valleys or mountains are formed, depending on the filling level. These factors make the level calculation a bit more complicated. On the basis of the available construction plans and dimensions, we decided to use a multi-point linearization and performed this for each silo individually, thus calculating a good approximation for the fill levels. In addition, at the customer's request, we also calculated the mass and volume of the salt present.

Visualization on ax-track platform

The fill levels are visualized simply and clearly on the ax-track platform. Changes in the fill level can be shown over different time periods. With the linking of weather data, it would now also be possible to monitor consumption and better plan salt requirements for the future.

With the "Rules" function, rules can be defined independently, which trigger a reaction when a certain action occurs. Automated messages about the filling level, automated orders of salt or information about the currently purchased quantity are just a few application examples.