Marvelmind Precise Indoor Positioning System relies on two key technologies:
In this article we touch the point 1 and answer the questions how to choose between different ultrasound frequencies of Marvelmind beacons and why.
We don’t discuss the radio connectivity in detail here. Remember, though, that your beacons and modems must support the same radio band, for example, 868/915MHz band. You can’t combine Beacons HW v4.9 on 433MHz band and Beacons HW v4.9 on 915MHz band or Super-Beacons on 915MHz band.
The oscillograms below are the real examples from a steel making plant:
What to pay attention to:
In order 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 ultrasound frequencies, because some of the architectures that we have 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 sets – for robots, drones, forklifts, people, etc. You purchase them and they already the most recommended configurations of beacons with the right frequencies.
But why? What are the underlying principles behind those recommendations and beacons’ selection? Let’s discuss 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 to choose the resonating transducers in the first place? Why not to use wideband “ultrasound speakers” or something?
Answer: Yes, it is possible to use wideband ultrasound speaker to transmit ultrasound and that would bring a lot of simplification to the system. However, such speakers typically produce significantly weaker ultrasound signal, thus limiting the range the system can operate. For example, Marvelmind Indoor Positioning System can comforable work with the beacons placed up to 30 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 do. A
Question: But why not to just provide a lot of power to the ultrasound speakers and to produce 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 level of ultrasound signal as compared with the resonating transducers. Which means that the beacons with wideband speakers must always be powered from the external source that is often would become a limiting factor for such beacons.
Despite that, yes, it is possible to build the system with low-efficiency wideband ultrasound speakers. In many cases it could be a very good option and an alternative providing a new layer of flexibility of building the precise indoor positioning systems.
Question: So, how does different frequencies of beacons help?
Answer: If we can’t use modulation, we can separate different ultrasound signals either in time or in frequency.
Time division multiple access (TDMA) is easier. That is the basis for NIA. No ultrasound signals are transmitted at the same time within 50m or so of open space. The system is easy, 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 1 mobile beacon will be 16Hz; for 2 mobile beacons = 16Hz/2=8Hz, for 10 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 quite, but still rather observable “tick” – very short ultrasound pulse, that people hear as a “tick”. People can’t hear ultrasound, but the ultrasound pulse have weak elements of acoustic frequencies that can be heard that can simply disturb people, particularly, in quite 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 in order to distinguish between the simultaneously transmitted the ultrasound signals, they must be on different frequencies then.
It is difficult to filter the ultrasound signals that are close in frequency and short pulses have pretty wide transmitting spectrum as well. Thus, there is a practical limitation on how many ultrasound frequencies the system can have. But, in general, the more frequencies, the easier 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 8 ultrasound frequencies:
We are discussing the practical differences between different ultrasound frequencies below and give recommendations which frequencies to choose for your beacons, when ordering.
Remember, you can’t set the ultrasound frequency for transmission arbitrary. The ultrasound frequency of the beacon is defined by the ultrasound transducers of the beacons. Marvelmind Robotics produces special customized ultrasound transducers for each of the 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 and 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, summing up:
In 95% of real use cases, it is recommended to choose different ultrasound frequencies for your beacons, when you are ordering, because it gives you more freedom with your indoor positioning system system.
There are several cases when you must choose the same ultrasound frequency, or when it is preferred or recommended to choose the same ultrasound frequency for your beacons, and 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 on the most optimal selection of the ultrasound frequency for the beacon:
That is simple: the lower the frequency – the lower the attenuation. That means that for the highest range it is generally recommended to have lower frequency. Though, it shall be remembered that other factors, mostly, the external acoustic noise may significantly impact on the resulting signal to noise ratio.
Therefore, it it general 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 the 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 absolutely now 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 whether there are beacons or not in their vicinity.
Though the beacons operate in ultrasound, which is not heard by humans, and 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 to keep ultrasound transmitting beacons at least 2-3 meters away from people in order to minimize even a slightest probability of a potential impact of a long-term exposure to ultrasound.
Since the ultrasound signal is easily blocked, the ultrasound signal from beacons behind a wall, or a door, or a even a sheet of paper, are most likely completely blocked. This is why we pay so much attention to the importance of a direct line of sight for precise tracking and that is why the 2-3m recommendation is valid only to ultrasound transmitting beacons with direct line of sight to people’s ears.