# Adding a sensor to mavlink stream **URL:** \ **Category:** Companion (Retired)\ **Tags:** pymavlink\ **Created:** [August 17, 2020, 4:23am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985 "2020-08-17T04:23:15Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 17, 2020, 4:23am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/1 "2020-08-17T04:23:15Z")
Hello, I’m currently trying to send an ultrasonic thickness gauge sensor reading as a Mavlink message through the companion computer up to the ground station (running QGC). The sensor outputs serial data which I’m parsing with Python into a float value with optional metadata, but I’m struggling to find where the companion computer sends its Mavlink messages from. I’m imagining there’s some kind of event loop or similar that checks the existing sensors and reports their status to the ground station, but I haven’t been able to find that, and am thinking perhaps the companion computer just relays mavlink messages from the pixhawk without actually processing them or the sensor readings. Is it possible for me to have a script sending mavlink messages without interfering with the existing setup, or is there an event loop I can add a sensor-reading thread to? Also, I’m hoping that I’ll be able to just send a float message with the name of my sensor, and that QGC will pick that up and display it automatically. If that’s not the case, is there some convenient way to register an extra sensor in QGC? ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 18, 2020, 3:47am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/2 "2020-08-18T03:47:42Z")
Looks like the companion computer just runs `mavproxy.py` with some input parameters. Likely no no parsing of mavlink messages, just straight handover from pixhawk to ground station. Hopefully looking into how the ping sonar is added will yield some useful insights, in I dive. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 18, 2020, 3:55am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/3 "2020-08-18T03:55:39Z")
Well that was quick. Ping360 imaging sonar sets up its own port instead of transmitting via mavlink, which I suppose makes sense since it’s more like a camera than a single-value sensor. Not sure where to look now, time for some more googling and forum searching. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 18, 2020, 4:22am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/4 "2020-08-18T04:22:28Z")
Looking at the ardusub [pymavlink](http://www.ardusub.com/developers/pymavlink.html) reference page there aren’t any instructions for sending a message directly from the companion computer to the top computer, but maybe I can send a sensor measurement to the autopilot (Pixhawk), which I believe should get automatically related back to the top computer as part of the normal logging. Would be super useful as then I’m able to keep my sensor read+transmit script separate from the normal companion computer code. ---
**Author:** ![jwalser](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/jwalser/32/2234_2.png) [@jwalser](https://discuss.bluerobotics.com/u/jwalser)\ **Post date:** [August 20, 2020, 9:13pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/5 "2020-08-20T21:13:36Z")
> Also, I’m hoping that I’ll be able to just send a float message with the name of my sensor, and that QGC will pick that up and display it automatically. If that’s not the case, is there some convenient way to register an extra sensor in QGC? Unfortunately, no. You will need to recompile QGC to add a sensor that isn’t already supported. If you want to go this route I recommend looking at the NAMED\_VALUE\_FLOAT message. [https://github.com/mavlink/qgroundcontrol/blob/master/src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.cc#L217](https://github.com/mavlink/qgroundcontrol/blob/master/src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.cc#L217) Is it a hard requirement for the measurement to be displayed in QGC? It will be easier to print your value out in a terminal in another window. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 20, 2020, 10:48pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/6 "2020-08-20T22:48:57Z")
Thanks for the reply! Yeah, I’d settled on named\_value\_float yesterday and put it in my script. Haven’t had a chance to test yet as I’m working from home at the moment. Alright, if I’m recompiling do I just have to update `_handleNamedValueFloat` to include my custom name, and add it as a ‘Fact’ in `APMSubmarineFactGroup` (including in the header file), or is there specific UI stuff that would need changing too? Mostly important for it to end up in the telemetry log so it’s with the rest of the data, but given we want to see the reading while we’re taking it (it’s a thickness gauge, so we need to see when it comes into contact with what we’re measuring and starts sending valid readings), it’d be preferable to have as part of the normal sensor value overlay as opposed to having a separate window just for that. If we were to decide to have a terminal output instead, can I read the same port as QGC, or would it be necessary to get the companion computer to send every message to the QGC port plus a separate port for the extra sensor parsing script? Lastly, I’m assuming I need to put in a meaningful timestamp for it to register at the correct time in the dive. My current idea is to `recv_match` on an arbitrary message just before it and then add the time delta from then to when submitting, but before I do that it’d be good to know if there’s an easier recommended way. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 21, 2020, 5:28am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/7 "2020-08-21T05:28:19Z")
Just saw in the “Run pyMavlink on the companion computer” example on the pymavlink page that they use `int(time.time())`, commented as `# Unix time` for the time specified in their ping. I’m confused about that since the mavlink [common](https://mavlink.io/en/messages/common.html) message set requires microseconds, but `time.time()` is in seconds. If the ‘time since system start’ that’s supposed to be used in these messages is indeed for the companion computer, I’m assuming I should use `int(time.time() * 1e9)` in `wait_conn`, and `int(time.time()) * 1e6` in my `autopilot.mav.named_value_float_send` (which specifies ms instead of us). Please correct me if I’m wrong, or otherwise fix the incorrect documentation (or let me know where I can raise an issue for it) 🙂 ---
**Author:** ![patrickelectric](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/patrickelectric/32/2162_2.png) [@patrickelectric](https://discuss.bluerobotics.com/u/patrickelectric)\ **Post date:** [August 24, 2020, 12:48pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/8 "2020-08-24T12:48:57Z")
Hi @EliotBR, Thank you for pointing this! We indeed used the wrong unit, we should be using microseconds over seconds for ping message. Be free to send pull requests if you catch any problem in our documentation. I have already created one to fix this issue here: [pymavlink: Fix ping message to use unix time in microseconds by patrickelectric · Pull Request #155 · bluerobotics/ardusub-gitbook · GitHub](https://github.com/bluerobotics/ardusub-gitbook/pull/155), it should be merged soon. Just a small correction, the scale for (nano)seconds is 1e9, (micro)seconds is 1e6 and (milli)seconds is 1e3. For the QGC related question, maybe @williangalvani can help you. ---
**Author:** ![williangalvani](https://avatars.discourse-cdn.com/v4/letter/w/6f9a4e/32.png) [@williangalvani](https://discuss.bluerobotics.com/u/williangalvani)\ **Post date:** [August 24, 2020, 1:29pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/9 "2020-08-24T13:29:27Z")
Hi @EliotBR, Check this [commit in QGC](https://github.com/mavlink/qgroundcontrol/pull/8570/commits/1b88e1427f3d86c5b2497ba61929cda57c0642b5). It adds support for the named\_value\_float “RollPitch”, which tells QGC if the user is currently controlling rotation or translation. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [August 31, 2020, 5:42am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/10 "2020-08-31T05:42:43Z")
> [@patrickelectric](#): > > I have already created one to fix this issue here: [pymavlink: Fix ping message to use unix time in microseconds by patrickelectric · Pull Request #155 · bluerobotics/ardusub-gitbook · GitHub](https://github.com/bluerobotics/ardusub-gitbook/pull/155), it should be merged soon. Thanks for this. I’ve seen that it’s merged now - should that auto-update the website display of the ardusub-gitbook, or does that require some form of release/rebuild to occur? The docs page (html file) I was referring to seemingly hasn’t been updated. > [@patrickelectric](#): > > Just a small correction, the scale for (nano)seconds is 1e9, (micro)seconds is 1e6 and (milli)seconds is 1e3. Haha, I must have been tired, thanks. > [@williangalvani](#): > > Check this [commit in QGC](https://github.com/mavlink/qgroundcontrol/pull/8570/commits/1b88e1427f3d86c5b2497ba61929cda57c0642b5). It adds support for the named\_value\_float “RollPitch”, which tells QGC if the user is currently controlling rotation or translation. Great, thanks @williangalvani - looks like I had everything except the json file update. Is there a reason `rollPitchToggle` wasn’t added as a `Q_PROPERTY` or getter method near the top of [ArduSubFirmwarePlugin.h](https://github.com/mavlink/qgroundcontrol/blob/master/src/FirmwarePlugin/APM/ArduSubFirmwarePlugin.h)? All the other properties are there, and it was added in all the other places where the other properties are used/specified. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 3, 2020, 3:41am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/11 "2020-09-03T03:41:00Z")
Just noting here that the `pymavlink` complained when I used `int(time.time() * 1e3)` within my call to `named_value_float_send` claiming the number was too large. The error message specified that the variable the time is assigned to within `pymavlink` is `time_boot_ms`, so presumably it should be time since boot rather than since the epoch. Currently changed to `int((time.time() - boot_time) * 1e3)`, with `boot_time` set as `time.time()` at the start of my script. Will confirm if this works once I’ve managed to get my qgc build working. Apparently Qt needs ~50GB of space, so I’ll need to make the build on a computer with some more memory… ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 10, 2020, 3:20am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/12 "2020-09-10T03:20:18Z")
Have built custom QGC on my laptop for now, and a spot appears correctly for the sensor I’m trying to connect, which is great (thanks @patrickelectric and @williangalvani for the help!). Unfortunately no reading is displaying yet (I’ve confirmed that the reading is being processed correctly by my script and can display it printing at regular intervals in a `screen` session), so I’ll have to try to see if the mavlink message is sending correctly. The current process is 1. run script on startup 2. set time as `boot_time` 3. connect to mavlink port 9000 (to the autopilot) 4. wait\_conn (ping and wait for response) with microseconds since `boot_time` 5. parse next reading 6. `named_value_float_send` with reading value and milliseconds since `boot_time` I’m hoping that’s a valid process, but I’m unsure if the boot time should be determined in some other way, and I’m not entirely certain if the reading should be going via the autopilot or directly to the top computer instead. ---
**Author:** ![williangalvani](https://avatars.discourse-cdn.com/v4/letter/w/6f9a4e/32.png) [@williangalvani](https://discuss.bluerobotics.com/u/williangalvani)\ **Post date:** [September 10, 2020, 12:26pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/13 "2020-09-10T12:26:21Z")
I don’t think mavproxy is forwarding it to the topside computer, experiment sending that straight to the topside computer (192.168.2.1:14550). ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 11, 2020, 6:17am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/14 "2020-09-11T06:17:51Z")
Hi @williangalvani, thanks for the advice - I’ll give it a try on Monday when I’m back with the hardware. I’m assuming that’s using `mavutil.mavlink_connection(udpin:192.168.2.1:14550)`, since I want that address and port to accept messages, but if I’ve misunderstood that directionality I’ll try with `udpout` (ie if `udpin` doesn’t work) 🙂 ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 15, 2020, 7:04am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/15 "2020-09-15T07:04:18Z")
Couldn’t make a connection (raised an exception) when I tried using `udpin:192.168.2.1:14550`. `udpout:192.168.2.1:14550` connected but still didn’t come up with any value on QGC. Having looked over the pymavlink examples page again, udpout seems like it’s the correct way to send data from the calling computer (the companion in this case) to the receiving computer (the top computer here), given > **udpout** : Outputs data to a specified address:port (client). and the ‘Run pyMavlink on the companion computer’ example shows that it should be followed by `wait_conn`, so reasonably sure that that part of the process is set up as it’s supposed to be now. `wait_conn` completes successfully, but the measurement values (via `named_value_float_send`) still don’t seem to be getting through to QGC unfortunately. Not sure where else issues could be coming in - might have to try running pymavlink on the top computer and seeing if the messages are getting that far. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 24, 2020, 8:21am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/16 "2020-09-24T08:21:32Z")
Finally got back to working on this a bit more. Got pymavlink today and found that my sensor readings are coming through correctly as named value floats, but their ‘time since boot’ was much larger than the normal named value float messages (6.1x10^6 vs 6.5x10^5 ms). Will have to check next week if it’s a scaling issue, or an offset issue from my sensor reading script starting earlier than the normal telemetry, or the normal telemetry script restarting (or at least restarting its boot count) at some point while the sensor script continues. Also not sure if that’d be causing no displaying, so it’s still possible I’m checking the wrong value in my custom QGC build or something. At least now I know the messages are getting to the top, and can debug a bit more with pymavlink 🙂 ---
**Author:** ![williangalvani](https://avatars.discourse-cdn.com/v4/letter/w/6f9a4e/32.png) [@williangalvani](https://discuss.bluerobotics.com/u/williangalvani)\ **Post date:** [September 24, 2020, 1:11pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/17 "2020-09-24T13:11:09Z")
Hi @EliotBR, Thank you for reporting your progress! You could use mavlogdump to check where your messages don’t match the ones sent by the ROV. I’d take a look at the sysID and CompIP fields as QGC could be using those. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 25, 2020, 1:41am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/18 "2020-09-25T01:41:38Z")
No worries - forums work best when later viewers of posts can refer to useful work and solutions that have already been gone through, including the mistakes that were made along the way. I’m hoping this will be a useful reference for others looking to do something similar in future, and am planning to make a summary once it’s successfully working 🙂 Thanks! I wasn’t aware there were extra fields that don’t come through to pymavlink’s `recv_match`. I’ll make sure to check those if it’s still not working once I’ve sorted out the timing issue and double-checked the parameters I’ve put in QGC. ---
**Author:** ![EliotBR](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.bluerobotics.com/eliotbr/32/6937_2.png) [@EliotBR](https://discuss.bluerobotics.com/u/EliotBR)\ **Post date:** [September 28, 2020, 7:34am UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/19 "2020-09-28T07:34:34Z")
Timing difference is now just the small difference between when the two scripts start, which may be problematic for accurate timing later but should at least still allow the readings to display because they’re quite close together. That didn’t solve the issue unfortunately, but one less major thing to be concerned about for now. @williangalvani I’m not sure how I’d be able to use mavlogdump in this case. I got one of the .bin log files off the pixhawk, but when analysing it I couldn’t find any named\_value\_float messages at all (there were a bunch of supposedly invalid headers, and then a bunch of parameters with 3 or 4 letter names) - maybe I need to use verbose, although I think I tried that and it didn’t help. I also realised that the bin files won’t help me to tell the difference between pixhawk and companion mavlink messages anyway because my sensor script sends directly to the top computer, so my custom messages shouldn’t even appear in the log file. The tlogs have both my custom and the normal named\_value\_float messages, but they seem to just have the same information as a live pymavlink connection (no extra sysID or compIP fields, or anything else, just the usual name, time\_boot\_ms, and value). I can’t remember if I tried reading the tlogs with verbose, so will do that tomorrow, but it seems like that’s unlikely to be the missing piece given the other things I’ve tried. ---
**Author:** ![williangalvani](https://avatars.discourse-cdn.com/v4/letter/w/6f9a4e/32.png) [@williangalvani](https://discuss.bluerobotics.com/u/williangalvani)\ **Post date:** [September 28, 2020, 4:44pm UTC](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985/20 "2020-09-28T16:44:23Z")
Hi @EliotBR > [@EliotBR](#): > > I got one of the .bin log files off the pixhawk, That is actually a Dataflash Log. It doesn’t have mavlink communication, so it is not very relevant to your use case. This seems to work here: ```python from pymavlink import mavutil master = mavutil.mavlink_connection('udpout:localhost:14550', source_system=1) master.mav.statustext_send(mavutil.mavlink.MAV_SEVERITY_NOTICE, "Qgc will read this".encode()) ``` change localhost to the topside IP, if your qgc read this message, the communication is working and you can try the named\_value\_float message next. [Next page](https://discuss.bluerobotics.com/t/adding-a-sensor-to-mavlink-stream/7985.md?page=2)