868/915 Mhz as a viable frequency for submarines.

I have just bought a frsky R9 tx module and a jumper R900 receiver to give 868mhz a try. im using it on a rover to drive around the streets so signal penetration is the highest priority. my plan was to use 458 openlrs but its getting hard to come by and expensive.

I got the R9 module for £16 off ebay and the receiver was £16 new can do 16 channels with telemetry.

geofrancis, please keep us posted on your progress.

I have ran into some issues with unstable servo output but im not sure if its the tx module or the receiver or a combination of the pair, so i have ordered a frsky r9 receiver to try and figure out where the problem is.

geofrancis wrote:I have ran into some issues with unstable servo output but im not sure if its the tx module or the receiver or a combination of the pair, so i have ordered a frsky r9 receiver to try and figure out where the problem is.

you can build UHF TX modules using the r9 protocol, just look for the diy 5 in 1 modules. basically take a 4 in 1 multiprotocol module or even an arduino board and add a RFM95 (100mw) or RFM95PW (1000mw) to give it 868mhz capabilities.

https://github.com/pascallanger/DIY-Multiprotocol-TX-Module
https://www.rcgroups.com/forums/showthread.php?2165676-DIY-Multiprotocol-TX-Module

Geofrancis,

i ran into "jitters" when using the 433mhz equipment.  wrapping the servo wire through a ferrite ring got rid of it.

Geofrancis,

if you decide to stay with the r9 version of the 915mhz equipment, FrSky already has a couple transmitters that have two rf decks (their own 2.4ghz access protocol) and 915mhz (their r9 version of access)

https://www.frsky-rc.com/product/tandem-x20/
https://www.frsky-rc.com/product/tandem-x18/

they make "S" versions of both models, with better gimbles, etc.

then you would only need to add a "simple" 4-in-1 module to cover all the other 2.4ghz protocols.

I have a custom frsky taranis x9d with an internal 4 in 1 module that I added years ago along with a switch to select between it and the rear bay,  so converting my already installed 4 in 1 module to a 5 in 1 with an extra module isnt a big deal for me. My r9 external module wont be running frsky for long as its getting flashed with some experimental firmware for mavlink telemetry for use with boats and rovers as a backup to the 4G network connection. I just need to make sure the hardware works first. MLRS

you can buy the jumper T18 with an internal 5 in 1 module pre installed for around £200.

The unstable output I was getting was unrelated to servos, as I have my receiver connected to a pixhawk flight controller via SBUS and I could see the inputs fluctuating on the configuration software.

maxx78 wrote:

i use the RX with a flightcontroller for drones (Matek F411) so i set the failsafes in the FC. The FC also works to keep the Sub level (don't know the english term for  "Lageregler")
The nice benefit is that i get telemetry so i know battery voltage and used capacity.
Telemetry does not seem to work well while submerged but i get updates when surfaced.

Side note: the FC has a pressure sensor normally used for altitude on planes/drones. This actually can be used to see if the sub is watertight. The dive system creates an overpressure inside the WTC which reads as altitude in the FC. So no or small change in altitude = no leak.

so, i have had the flight controller (Matek f411-wte) long enough that i have it set up with INAV.  by set up i mean it controls two motors, a set of diving planes, a rudder and a servo output for the ballast tank controller, including the stabilized pitch control for the rear diving planes.

what i have not been able to do is configure anything remotely close to what i would call a proper failsafe.

maxx78, can you tell me what you did for your failsafe?   in my world a failsafe needs to set everything but ballast tank to neutral, which gets set to "empty" (what ever servo position that is for a specific sub)

in INAV, there is nowhere i can find where i can do that...  failsafe is a single page where i tell INAV which of 3 modes to use:  nothing, land, or RTH (return to home)  none of which work in any way i can imagine for a submarine.

Since its not an output that your actively controlling with the flight controller and you're going to be using the ballast in manual pass through its just going to obey the failsafe output values of your receiver. So you should be setting failsafe positions on your receiver to set the outputs of the ballast tank.

Hi Tsenecal,

failsafe took me a while to figure out. The drone related stuff does not make sense for a sub. it set it to "Do nothing"

There is a section in INAV where you can "program". What i have done there is something like:

If Failsafe = true then RC channel x is 2000.

2000 being the impulse length send on that channel.

i will try to get some screenshots this evening, please feel free to remind me if i forget  

I am also want to set it up in the future with a delay for the ballast tank. so if you loose connection first the dive planes set to "surface" and after 5-10 sec the ballast tank is emptied.
That way ballast only is emptied for longer failsafes and a short dip below receiving range does not effect it.


@ geofrancis:

you can't do failsafe settings in the ExpressLRS receivers when used in serial mode.
They send a info to the flightcontroller, that a failsafe occured and the FC has to handle that. Which makes sense for drones where you can have different failsafe settings depending on the configuration.

Geofrancis,

as Maxx78 stated, the elrs receivers are not really made like the receivers we know and love.  they communicate with the flight controller using a 64 byte block of data using a serial protocol.   one of the values in that block is a failsafe value.

this allows the Flight Controller to be notified that the receiver is no longer communicating with the transmitter.  to them, at that time, nothing else really matters.

for elrs, the Flight Controller is the master of all.  "Failsafe" is just another mode the Flight Controller knows.  INAV gives you three choices of what to do when in "Failsafe" mode:  do nothing, land, or return to home, with "home"  being the GPS coordinates of where the flight started.

...except if you are using what is called an elrs pwm receiver, like the Happy Model epw6.  They are designed to work with standard fixed wing aircraft, just like classic receivers, and do have a way for you to set the pwm ouput setting for all channels when failsafe happens.


Maxx78,

thank you very much for the heads up on the "program" feature for INAV.  that does all make sense, I knew that this is not a special situation that some one else had not come across.   my assumption was i would set the failsafe option to "land", and then be able to define what "land" meant, but i could not find anywhere where i could set my own meaning for "land".    i will look at the programming tab tonight.   in my digging around, i did find using the CLI, you can set a failsafe delay variable (failsafe_delay) to the appropriate time to wait before invoking the failsafe procedure:

https://github.com/iNavFlight/inav/blob/master/docs/Failsafe.md#failsafe-settings



one final note. there are a bunch of "adapters" that convert that 64 byte block to pwm outputs, so you can theoretically use the standard serial receivers with servos directly, but how they set up failsafe values for servos differs from device to device, and sometimes is less than optimal. the ones i bought for two of my receivers to test with have hardcoded values that can't be changed, and are less than optimal for a submarine... basically set channel three to "low" and everything else to "neutral".

So here is the screenshot:

ignore line #0 and 1




Logic condition 2 checks if the CRSF LQ (link quality) is lower than 50. Dont remember why i used that value, i think its the failsafe limit
Logic condition 3 overrides RC Channel 4 to a value of 1500. This puts the ESC in neutral.
Logic condition 4 overrides RC Channel 1 to a value of 1000. Which is for the balast tank.

i am somehow missing the dive planes....strange.

i am using INAV 5.0

latest update on all my 915mhz testing...


I HATE flight controllers enough to say that i will avoid them at all costs.  getting the one i bought for testing to a usable state has made me decide that it was better to spend $14 and two weeks building a CRSF servo driver to wire to a classic ELRS or TBS receiver than it was to spend any more time fighting with flight controllers.   I understand their place and would have loved to have one when i was learning to fly helicopters, but they do not belong on a submarine. :)

I have been doing a lot more testing on the ELRS front, both building the servo driver, as well as setting up a total of 5 submarines on the system...  I have discovered that the Happymodel transmitter module is the most robust, but the betafpv receiver seems the best receiver.

It has reached a point where I have moved the TBS transmitter module and receiver to a different submarine for initial testing, one that worked relatively well using the ELRS system, so i will be able to verify/validate the TBS with known ELRS hardware.

I bought a "radiomaster zorro" with an ELRS module as a christmas present, and it along with the "Fantastic Voyage Proteus" that i posted pictures of on TAMS facebook page have gone through bathtub tests together, and all that remains to do on the sub is paint it.  then wait for my local pond to return to a liquid state.

Sorry to hear that you had such a bad experience with the flight controllers, its my "standard" solution.
May be you can share why you think they are not suitable for Subs ? are they lacking some important fuctionality? Just to be sure thht i did not miss something important in my setups.

So you can mix and match the eLRS Tx & Rx ?
then i will get some betaFPV Rx

Edit:

i still have the idea to try the Elrs @ 433Mhz, that should be even better than 868MHz.
i just need some new antennas and time. The second one is the bigger issue

It is more of a "why do i want to use a $50-$100 sledge hammer to fix my problem, when a $14 ball peen hammer will do it". secondary was the amount of "futzing" i had to do to make it work... the level of effort to get s1 and s2 to output both the full range and center neutral the ESC were expecting took days to figure out, and some of the settings for that could only be done via the command line. why go through all the expense and headache to do what a receiver can/should do all by itself. i do agree that there is one thing a flight controller can do better than anything else made: stability. pitch and roll stability with a flight controller should be no extra effort at all.

Another reason i am making my own servo driver is to eliminate some of the drone centric tendencies, like to have channel three failsafe to 988 microsecond pulse... (something the matek servo drivers hard code) i would prefer to be able to press a button when the servos are where they need to be, and record those values to the driver's eeprom for failsafe. there is no need to put the system in wifi mode, or connect it to inav to do some of these things.

one other thing that David seems to have reinforced with his testing of one of his homegrown 915mhz systems... the TBS, R9, and ELRS 915mhz systems get their 40km plus drone flight range by using a LORA based protocol/hardware. it seems LORA is the way to get the penetration for both long range (ie LORA) flights, as well as the penetration needed to go through water.

And Maxx, yes all the ELRS hardware is interchangeable with 2 caveats:

1) they have to be using the same major version of the firmware. (ie, 2.0.1 will work with 2.1.0)
2) they obviously have to be using the same frequency (2.4 -> 2.4, 915 -> 915, 868 -> 868)

beyond that, it should work together without issue. (and it does... i am currently mixing betafpv and happymodel, i want to pickup a couple other brands like HERMES and JEHEMCU and checkout their quality control.

it is very much just the latest generation of the LRS tech. (i started all this with 433mhz OpenLRSng)

Thanks for your answers.

So it is "just" the setup hassle, which is a pain in the backside the first time. After that its just copy & paste.
But i was already used to it from Drones & planes, so the transition was'nt that hard ;-)
For me the tought is: why spend 50 € on a stabilzer, when i can get an FC for 25-50€.

And thanks for the info about mixing and matching receivers.

For those of you that don't hang out on Facebook

The latest in models using the 915mhz equipment.

In this specific case: Radiomaster Zorro transmitter with BetaFPV nano ExpressLRS transmitter module (915mhz) and a HappyModel EPW6 pwm 6 channel ExpressLRS (915mhz) receiver.

https://www.youtube.com/watch?v=zGnXbPrkhzc

Let's hope that the Radiomaster equipment continues to be available. I assume that this must now have the distinction of being the only Commercially available equipment which can give true underwater communication? (Supply of the openLRS gear dried up some time ago.)

David

David, it is a better situation than that...

Three different 915mhz systems are in play right now, and readily available: ExpressLRS, TBS Crossfire, and Frsky R9.

all can be used with either the "lite" style modules (also called nano) and the JR style modules.

Crossfire can use either the CRSF serial protocol to communicate with the transmitter, or standard PPM.

ExpressLRS only uses the CRSF serial protocol.

Frsky R9 uses either their own serial protocol (XJT format) or PPM.

the CRSF and XJT format are available in the OpenTX and EdgeTX transmitter firmware.

TBS is the only maker of the TBS Crossfire equipment.

there are several Chinese makers of the ExpressLRS equipment. I have used Happymodel, Betafpv, and HGLRC Hermes branded products.

Frsky is the only maker of the R9 products.


Of the three systems, ExpressLRS and TBS work equally well, the Frsky stuff works, but will have "telemetry lost" dropped packet warnings, while the other two won't... this is to how the power output escalates automatically in those protocols, but does not in the R9 systems.

so... yes, the OpenLRS systems seem to have been replaced by the ExpressLRS in most situations... but the ExpressLRS does not offer PPM or futaba style modules, so for those of us that have the older futaba equipment (a total of 6 transmitters with futaba style modules for me) it is time to upgrade.

I do like the Radiomaster transmitters, and specifically bought the Radiomaster Zorro based on several reviews that indicate it to be the best of the "game controller" style. (hall sensor gimbals, well thought out position of the secondary switches and sliders, and a larger LCD screen)

perhaps some individual topics that are reviews of all the different transmitters and modules i use might be worth while.

Hi Tim,

Good to see that we still have options!

I have created three new topic threads as you suggested.

David

I finally got around to adding a RFM95PW radio module to the 4 in 1 IRX4 multiprotocol module inside my Frsky X9e radio making it into a diy 5 in 1 with 500mw 868mhz frsky r9 support. I had to go this route as the actual frsky r9 module thats in the module bay is running another incompatible firmware.

https://www.aliexpress.com/item/1005003176028870.html
https://www.aliexpress.com/item/1005001690023300.html
https://github.com/pascallanger/DIY-Multiprotocol-TX-Module/issues/303#issuecomment-578279156

The main reason for it is so I can use these really cheap r9 compatible receivers, most 868 receivers are about £30-40, at £18 these are a bargain.I have one for my sub so im looking forward to testing it.

https://www.flyingtech.co.uk/electronics/jumper-r900-receiver-900mhz-long-range-rx

do you have a setup that works with those r900 receivers?

I was unable to get them to work with any of the real FrSky r9m modules i own.

It connected right away to my diy module on 868.

are you using opentx? did you have the latest flex firmware on the module?