Wireless communications are used in many industrial maintenance scenarios and are practically the only choice for transmitting data from rotating sensors. One such example in MANTIS is the shaft-mounted torque sensor in the press machine by FAGOR ARRASATE, shown below. The sensor sends the data to the receiving antenna mounted vertically from the machine’s ceiling.
The wireless signal must travel from transmitting antenna to the receiving one without overly high attenuation. Furthermore, the signal can travel via different paths, such that out-of-phase components attenuate each other. This so-called multipath effect is particularly strong in industrial environments that contain many large metallic surfaces. Correct placement of the antennas is therefore crucial. A good placement can often be found experimentally through trial and error. However, in certain cases where repeatedly re-locating a receiver or transmitter is not practical, a numerical simulation of radio wave propagation can be used instead.
As an illustration of concept, we present here a simulation of antenna placement for a simplified model of a part of press machine, shown below. The orange downward-pointing arrow shows the placement of the sensor and the orientation of its transmitting antenna. The white-and-gray shaded rectangle above the sensor is the receptor plane with the result of the simulation, as will be explained below. Note that the model is enclosed in a rectangular box from all six sides, but the front and top sides are not shown here in order to be able to see inside.
The simulation algorithm is based on the ray tracing method-of-images enhanced by double refraction modeling. It is computationally complex but highly parallel, and has thus been adopted to run on GPUs. In our case the runtime of a single simulation is approximately one minute on a high-end gaming GPU card. The simulator itself was developed by the Jožef Stefan Institute as part of the national research project ART (Advanced Ray-Tracing Techniques in Radio Environment Characterization and Radio Localization), co-funded by the Slovenian Research Agency and XLAB.
In order to use it in MANTIS, XLAB has developed a Blender plug-in that exports the model into the proprietary simulator format, and a similar import plug-in to import back the simulation results. We then ran a series of experiments simulating the rotation of the shaft, thereby changing both the position and the orientation of the transmitter antenna. The signal wavelength was 0.1225 m, corresponding to Bluetooth/WiFi frequency range. The video below shows the result. The color scale is from 0 dB loss (black) to 100 dB (white). However, values over 90 dB are replaced by red to highlight the areas that will most probably not have acceptable reception with common BLE or WiFi antenna setups.
Clearly visible are the vertical belts resulting from the obstruction by and reflections from the shafts and the diagonal patterns of reflections from the slanted parts in the model. Most importantly, within the belts of good reception we can see strong multipath interference patterns. Some of the individual, isolated red and white points are artifacts of the simulation where incidentally no ray has reached that exact area. These artifacts could be reduced by increasing the number of cast rays, which would also slow down the simulation considerably. Finally, it has to be noted that this experiment was only intended as an illustration of concept and no validation or comparison to actual signal measurement in the field was performed yet.