Short answer suitable for 95% of cases: if you purchase Super-Beacons or Industrial Super-Beacons, choose as many different frequencies as available. It will give you the most flexibility with the system and the most freedom:
Marvelmind Precise Indoor Positioning System relies on two key technologies:
In this article, we mostly touch on the first topic – trilateration – and answer the questions of choosing between different ultrasound frequencies of Marvelmind beacons and why.
We don’t discuss radio connectivity in detail here. Remember, though, that your beacons and modems must support the same radio band, for example, the 868/915MHz band. You can’t combine Beacons HW v4.9 on the 433MHz band and Beacons HW v4.9 on the 915MHz band or Super-Beacons on the 915MHz band.
See more about radio connectivity.
The oscillograms below are real examples from a steel-making plant:
What to pay attention to:
The Oscilloscope is a great tool for deeper system debugging. It allows you to see what the system hears and identifies:
To address the needs of as many different applications as possible – from autonomous robots, drones, forklifts, and humans to sports and archaeology – we produce beacons with different frequencies of ultrasound because some of the architectures that we have to require different frequencies (IA and MF NIA) and because different ultrasound frequencies have slightly different characteristics.
Naturally, potential users may have questions:
To simplify the decision process, we have different sets – for robots, drones, forklifts, people, etc. You purchase them, and they are already the most recommended configurations of beacons with the correct frequencies.
But why? What are the underlying principles behind those recommendations and beacons’ selection? Let’s discuss this deeper.
Question: Wouldn’t it be easier or better to have only a single frequency for all beacons?
Answer: Yes, it would be if it was practically feasible. However, there are several factors against it. Thus, multiple frequencies are better in practice:
Question: OK. But why choose the resonating transducers in the first place? Why not use wideband “ultrasound speakers” or something?
Answer: Yes, it is possible to use a wideband ultrasound speaker to transmit ultrasound, which would simplify the system. However, such speakers typically produce significantly weaker ultrasound signals, thus limiting the range the system can operate. For example, Marvelmind Indoor Positioning System can comfortably work with the beacons up to 30 meters away. With Marvelmind Horns – up to 120m away.
Low efficiency, i.e., it would require significantly more energy to produce the same level of ultrasound signal as the resonating transducers. A
Question: But why not give the ultrasound speakers a lot of power and produce a strong ultrasound signal?
Answer: It is also possible. However, since the non-resonating wideband ultrasound speakers typically have low efficiency, it would require significantly more energy to produce the same ultrasound signal level than the resonating transducers. This means that the beacons with wideband speakers must always be powered from an external source, which would often become a limiting factor for such beacons.
Despite that, it is possible to build the system with low-efficiency wideband ultrasound speakers. In many cases, it could be a perfect option and an alternative providing a new layer of flexibility for building precise indoor positioning systems.
Question: So, how do different frequencies of beacons help?
Answer: If we can’t use modulation, we can separate different ultrasound signals in time or frequency.
Time division multiple access (TDMA) is more straightforward. That is the basis for NIA. No ultrasound signals are transmitted simultaneously within 50m or so of open space. The system is easy and robust, but with the growing number of mobile beacons that are emitting ultrasound, it becomes proportionally slower per mobile beacon, i.e., if the update rate per system is 16Hz, then the update for one mobile beacon will be 16Hz; for two mobile beacons = 16Hz/2=8Hz, for ten mobile beacons => 1.6Hz, etc. For many applications, it is too slow.
Also, in some cases, it is simply inconvenient to have emitting ultrasound mobile beacons, for example, on people, because the beacons produce, albeit quiet, still somewhat observable “tick” – a very short ultrasound pulse that people hear as a “tick”. People can’t hear ultrasound, but the ultrasound pulse has weak elements of acoustic frequencies that can be listened to that can disturb people, particularly in quiet areas. It is much better to have “ticking” beacons – ultrasound transmitting beacons 5-30m away, where the ticks are not even observed in practice.
But to distinguish between the simultaneously transmitted ultrasound signals, they must be on different frequencies.
It isn’t easy to filter the ultrasound signals that are close in frequency, and short pulses have a pretty broad transmitting spectrum. Thus, there is a practical limitation on how many ultrasound frequencies the system can have. But, in general, the more frequencies, the easier it is to build large maps consisting of dozens of submaps or to have a better performance of swarms of drones.
Marvelmind indoor positioning system supports eight ultrasound frequencies:
We discuss the practical differences between different ultrasound frequencies below and recommend which frequencies to choose for your beacons when ordering.
Remember, you can’t set the ultrasound frequency for transmission arbitrarily. The ultrasound transducers of the beacons define the ultrasound frequency of the beacon. Marvelmind Robotics produces specially customized ultrasound transducers for each ultrasound frequency we use in our beacons.
At the same time, Super-Beacons, as well as Industrial Super-Beacons and Mini-RXs and resulting products supporting IA, such as Helmets, Jackets, Badges, Caps, etc., can receive an ultrasound frequency or several of them or all of them at the same time, because the receiving microphones are wideband and digital filters of the beacons allow beacons to distinguish between different ultrasound frequencies.
So, to sum up:
In 95% of real-life use cases, choosing different ultrasound frequencies for your beacons when you are ordering is recommended because it gives you more freedom with your indoor positioning system.
There are several cases when you must choose the same ultrasound frequency or when selecting the same ultrasound frequency for your beacons is preferred or recommended. Then there are cases when it doesn’t really matter what frequency to choose, and some other factors may influence the final decision, for example, availability or price:
There are several factors affecting the most optimal selection of the ultrasound frequency for the beacon:
That is simple: the lower the frequency – the lower the attenuation. It is generally recommended to have a lower frequency for the highest range. Though, it shall be remembered that other factors, primarily the external acoustic noise, may significantly impact the resulting signal-to-noise ratio.
Therefore, it is generally very much recommended to run practical tests on the factory floor, for example:
For more information, see:
For example, the formulas return for 19kHz the attenuation of ~0.5dB/m and for 45kHz – ~1.5dB/m, which gives ~30dB difference between the two frequencies on 30m. That is a lot! – more than 30 times in magnitude and 1000 times in power.
Surprisingly, we see a big difference between the frequencies in practice, but not as big. However, as discussed, there are many affecting factors (sensitivity, transmission efficiency, directivity, etc.)
Practice shows that while people can hear quiet ticks produced by ultrasound transmitting beacons in quiet rooms, animals, even those that potentially can hear ultrasound, pay no attention to it. At least, their behavior doesn’t change at all.
We believe the short (0.5-1.5ms) ultrasound pulses coming with ~8Hz rate from the ultrasound transmitting beacons are heard by the animals as just loud ticks, but nothing more. Similar to what people can hear, but louder. However, the animals remain indifferent to whether there are beacons in their vicinity.
Though the beacons operate in ultrasound, which humans do not hear, the beacons emit very short pulses. Thus the average transmitting signal or the level of ultrasound noise is about 1/1000 of the peak level; we recommend keeping ultrasound transmitting beacons at least 2-3 meters away from people to minimize even the slightest probability of a potential impact of long-term exposure to ultrasound.
Since the ultrasound signal is easily blocked, the ultrasound signal from beacons behind a wall, a door, or even a sheet of paper, is most likely completely blocked. This is why we pay so much attention to the importance of a direct line of sight for precise tracking. The 2-3m recommendation is valid only for ultrasound transmitting beacons with a direct line of sight to people’s ears.