Tag's TWR strategy

Hi,

I have a question about TWR strategy.
I have a network with 30 anchors and 1 tag and I save the location data in a .csv log file to be able to make some statistics and retrieve the trajectory offline.
In some cases I observe that in the log files there are “holes” in the trajectory, meaning that for a certain period of about 15 seconds the calculated locations of the tag are always (0, 0, 0).
What could be the problem? The anchors are spaced about 20 meters from each other and in LOS as adviced in the documentation…
In section 4.9 Tag’s TWR strategy the document DWM1001 System Overview and Performance, version 2.0 says “(if it does not know its position it will use 0, 0, 0)”.What does it mean? Why the tag does not know its position? In some cases the tag receive the positions from just 2 anchors but in that cases in the log file i see the repetition of the last position known… Maybe there can be another reason?

Thanks for reply,
Simone

Hi Simone,

are you using PANS Release 2 firmware? In Release 2 if Location engine cannot calculate position it should return you NaN and not 0,0,0. The description in the documentation is probably obsolete and referring to Release 1.

How did you log the positions? If the Location Engine could not calculate the position you should not get position via the on-module shell, only distances.

Cheers,
TDK

Yes, I am using PANS Release 2 firmware both in the anchors and in the tag! I simply receive some superframes correctly formatted and I write the distances like an int32 in millimeters on the .csv file, but in that period the superframe is full of zeros, and after some seconds I newly have some useful data…
I downloaded the documentation about DWM1001 Module from the Decawave website https://www.decawave.com/product-documentation/ and I think that it’s not obsolete…
The accuracy of the positioning is ok and I should extend the network to a much large area adding much more anchors to the network, but for my indoor application I can’t afford such a long time (or maybe indefinite time) with correct superframe structure but without measured distances! Can you suggest me a solution to this problem?
I add to this post an extract of one of the .csv log files, where you can see the zeros from line 180 to line 328.
I also attach a map where you can see the position of the anchors marked with a red X.

Thanks a lot,
Simone

20190709_115338_DWM extract.csv (74.7 KB)

Hi Simone,

How di you get the data logged in the CSV file ? I can’t make sense of it so far.

Are you saying that there is an area within your network for which the tag always reports a 0;0;0 position ?

Your anchor placement looks ok, could you indicate the distance between two anchors for scale, and also is the lines represents wall or something as such that could potentially reduce line of sight ?

Thanks
Yves

Hi Yves,

Sorry for answering with so much delay.

I get the data from the tag trough SPI connection with a micro which send the data to a computer and then the data are saved in the CSV log file.

I attach the new map with the distance between two anchors.

The lines in the map don’t represent walls but only wall beams of the warehouse and does not influence the LOS between the anchors. All the anchors are attached at the ceiling at about 6 meters height, and the tag is mounted over a forklift at about 2,30 meters height.

When the tag is in that area I don’t get always 0;0;0. This happen only occasionally and for some seconds.

The only problem for the LOS between the tag and the anchors could be the presence of paper pallet inside the warehouse, but the in most cases the tag is in LOS with 3 or 4 anchors. Moreover, in other areas of the warehouse where there are high columns of paper pallet, this problem does not occur!

Hi Simone,

This looks fairly odd indeed. Do you observe the same directly with the LES command over UART interface ? At some stage the tag completely lose track of the anchors ?

Thanks
Yves

Hi Yves,

Yes, I obtain the same also over the UART interface. It seems like sometimes the tag receives all zeros from the anchors, and after some seconds it restart receiving correct data. I can’t explain this behavior because in different log files, also in the same area, there is not this problem…

Thanks,
Simone

Hi Simone,

would it be possible for you to create a log using the ‘les’ command only? Just to rule out issues generated outside the module, e.g. obtaining the data via the SPI. Also please mark where is the [0,0].

Does it mean that you don’t see always 0,0,0 within the area in the picture above? Where does it have problem then and how long the issue last? Do the tag need to move until the problem is fixed.

Thanks,
TDK

20190917_110754_DWM.xlsx (787.8 KB)
I can’t create a log file using the LES command because I’m not using shell commands! I get the row data through SPI connection and simply save them in the log file!
Here is a new complete log file in which the tag were moving around the whole warehouse. As you can see in the second sheet of the file, the trajectory presents some points where it receive (0, 0, 0) for some seconds, then it restart following the real trajectory of the forklift.

I mean that I don’t see (0, 0, 0) always in the same area! maybe the second time in that area I have correct measurements, but I have (0, 0, 0) in another area of the warehouse!
It looks like a bug because the timestamp, the message count, and the format of the messages are correct!

Thanks,
Simone