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OpenTx - what is the fuss really about?

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Biggles' Elder Brother - Moderator10/10/2016 22:36:20
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Those of us using OpenTx are very fond of shooting a line about how great it is and how much power it gives you and how it’s not really that difficult to use provided you accept that it is a different approach and you have to learn it from the ground up.

But, the fact is we actually give very few hard examples of the system being used in anger. Now that strikes me as a pity for two reasons:

  1. I genuinely think that if more non OpenTx users saw what could be achieved and how relatively easy it is to achieve it then some of the fear folks have of OpenTx being too difficult for them use might be eased.
  2. Some examples might help other existing users. I’d certainly welcome a chance to see other people’s set ups and programming projects were perhaps they have done something a bit different. I’m sure there is the potential for all of us to learn lots from each other.

So, with all that in mind, I thought I’d set an example and start the ball rolling in the hope that it would encourage others. I’m going to describe how I have gone about a project I’m working on using OpenTx on my Taranis with the help of some of FrSky’s incredibly low priced hardware on a complex multi-channel wing. I’m not claiming that my approach is the best, it's certainly isn’t the only way. But it’s my way! My hope is that as well as other OpenTx users looking in we might get some non-users who are just interested in what all the fuss is about!

So here goes….

BEB

Edited By Biggles' Elder Brother - Moderator on 10/10/2016 23:32:46

Biggles' Elder Brother - Moderator10/10/2016 22:51:59
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The Project.

I am assembling a Hangar 9 Blue Nose P51D Mustang. Nice ARTF, not cheap but a good quality kit. The wing for the Mustang has 6 servos in it – three in each side actuating: ailerons, flaps and retracts respectively.

wing overview.jpg

So the absolute minimum number of channels to drive this wing is three, one for each of those control elements. But then each of them, the ailerons, the flaps and the U/C, would have to be fed via Y-lead – so, for example, for the ailerons on one channel , that channel is split, via a Y-lead, and one leg of that Y-lead goes to the left aileron and one to the right aileron. Same with flaps and retracts.

Now that will work. It has the great attraction of being simple and it keeps the overall channel count down – adding in throttle, elevator and rudder and we get the whole kit and caboodle for just 6 channels. But it also has some limitations, mainly any change we make to, say ailerons, effects both ailerons. We have no independent control over each element. Now we can get round this by making any individual changes to the actual physical mechanisms – basically changing the control rod lengths and which hole we are in on the servo arms and control horns. But for this model, while that could be done, it would be very difficult. The control runs for both the flaps and the retracts are internal and difficult/fiddly to access. You would very soon run out of patience trying to optimise and match control throws and end points via lots of small adjustments.

The situation for the flaps is shown below,….

flap push rod.jpg

And this photograph shows how the retract servos are buried deep in the mechanism, making adjustment difficult,…

retract push rod.jpg

The solution to this is to put the controls on separate channels. So that’s two aileron channels, left and right; two flap channels, left and right; and two retract channels, left and right. This would give us the great benefit of being able to get each control basically right via mechanical adjustment, but to then fine tune it and get it spot on by adjusting sub-trims and end-point, for each control element independently, on the transmitter.

That is great obviously, but it has one major disadvantage – with 6 channels going into the wing we would have to have 6 wires going into the wing and make 6 connections every time we assemble the model. Not fun and there is considerable potential to foul this up! We could pass the wires in via a single multiple pin connector, like an Ashlock. We could have just a single power and ground wire then 6 signal wires – so we would need a least 8-way on the connector – something like this perhaps?

But there is challenge here to – we would be giving ourselves quite a soldering job to distribute that power and ground lead around 6 servos on the wing end and 6 connections on the RX end in the fuselage. A failure of one of those connections could have very serious consequences in flight. So we’re not going to do it.

So, what are we going to do? We’re going to use SBUS – that way we can carry all 16 channels into the wing with just one servo lead and one connector. Neat, but how? That’s what will come next – planning this installation,….

BEB

trebor10/10/2016 22:53:12
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I'm watching

Shaunie10/10/2016 22:54:14
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I'm waiting with bated breath, don't hang about BEB!

Shaunie.

Biggles' Elder Brother - Moderator10/10/2016 22:58:03
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Planning the Installation.

This is going to be a complex installation. Some folks are happy to do this sort of thing ‘on the fly’ (er, not literally, you know what I mean!) I prefer to work out all the main points up front so I know how this is all going to go together. I used to do this all on paper, in a notebook, but I have recently taken to making a soft copy version so I can keep it handy and refer to it as I need it. One of the beauties of this is that if you don’t fly the model for a few months when you come back to it you are not struggling to remember “what on earth is that connection there for and which switch on the trannie operates the retracts then?”

The picture below shows the Rx we will use, it’s a FrSky X8R – probably the most popular Rx used with Taranis. Its name is a bit misleading actually as it implies that it is a 8 channel Rx. It isn’t, it’s a 16 channel Rx but it only has 8 sets of outlet pins. But it also has an SBUS outlet that carries all 16 channels – so you can get at them. This receiver costs £28 – yes £28 for a 16 channel receiver with SBUS capability, incredible! I have about 20 of these, built up over the last two years in various models and they all work just fine.

rx.jpg

This second picture shows the end detail of the 8 servo connectors and the location of the single SBUS connector.

rx pin out 2.jpg

So here is a proposed schematic wiring diagram for the model. You can see the X8R as the black box in the lower centre. We plan to take six channels out of the standard 8 servo outputs, namely: rudder, elevator, throttle, mode, gun sound and aileron M. These are shown coming out of the left side of the X8R box. At this stage don’t worry about what all these are, I will explain them as we go along.

wiring schematic.jpg

 

The SBUS connection is shown coming out the top of x8R box (simply for convenience) and heading towards the wing which is represented by the orange dotted box – so this shows the single servo lead connection to the wing that we want.

This single lead is then connected to Y-lead which splits the SBUS single so it can enter both the right and the left wing. Note that both these righ and left versions of the SBUS signal have all 16 channels on board at this stage. We will discuss what then happens to these in the next post…..

BEB

Edited By Biggles' Elder Brother - Moderator on 10/10/2016 23:00:43

Edited By Biggles' Elder Brother - Moderator on 10/10/2016 23:06:53

Biggles' Elder Brother - Moderator10/10/2016 23:21:52
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Inside the wing.

Here we are going to discuss how will set things up inside the wing, but before we can do that we need to assign all the channels we will use. I do this with a Tx planning template – this shown in the picture below, again you might want to click on it so as to see it clearer:

channel map.jpg

As you can see this details exactly which of the 16 channels will carry the left aileron signal, the right aileron, the left flap, the right flap, the left retract and the right retract. Look at this alongside the wiring schematic in the previous post it becomes obvious that the right wing leg of the SBUS Y-lead will be delivering channels 9, 10 and 11. While the left wing leg of the SBUS Y-lead will be delivering channels 12,13 and 14. All very good, but how? How, especially as we will be using standard servos – not specialist high cost SBUS servos?

The answer is that we will use two of these, one connected to each leg,…

decoder 2.jpg

These are SBUS to PWM decoders, the SBUS signal enters on the single lead side and we get 4 conventional PWM channels out. So this will give us four channels in each wing. But how do we control which 4 of the 16 SBUS channels appear on the outlets? Easy, we use one of these gadgets, a SBUS channel changer,….

changer with numbers 2.jpg

The upper number, with the red arrow pointing to it, tells us what channel a particular PWM outlet on the decoder connected to the changer is currently set to. The lower number, indicated with the blue arrow, tells what channel number it could be set to by the changer. We can change which channel it would be set to by toggling the wheel on the left hand side of the device. We can select which of the four outlets we are operating on via a push button on the decoder shown in the picture below,…

decoder button 2.jpg

Each push of the button takes to the next outlet of the four.

So, we connect a decoder and a battery to the SBUS channel changer as shown in the picture below,…

decoder, changer and bat.jpg

By combining the use of the two controls, the channel selection button on the decoder and the set option on the channel changer, we can set the four outputs on the decoder in the right wing to channels 9, 10, 11 & 12; and the fours outputs on the decoder in left wing as channels 13, 14, 15 & 16. We can do this even though we will not actually use channels 12 and 16 at this point. For expansion smile

The picture below shows the whole wiring arrangement for the two wing halves all connected up,…

wiring dummy run.jpg

Now you might be thinking –“this is all very well, but I bet all this specialist kit, the decoders and the channel allocator cost a bomb!” Not so!!

The decoders cost £10.79 each, and the channel changer costs £13.49. So the all up cost of all the kit used in this post was just £35.07 and bear in mind only the decoders will go in the model, the channel changer is a one-off cost as it can be reused again on subsequent models. This FrSky kit is not expensive for the power it gives us I am sure you will agree.

Next we can turn or attention to the business of programming the OpenTX in Taranis to successfully drive all of this.

BEB

Gary Manuel10/10/2016 23:56:50
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Hi BEB.

I understand the "logic" but one thing is puzzling me.

Can the sbus output from the receiver and the relatively thin servo Y-lead meet the power demands of all 6 servos, or is there a separate power supply to the decoders?

john stones 111/10/2016 00:01:43
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Cuppa in hand, great stuff BEB yes

Biggles' Elder Brother - Moderator11/10/2016 00:05:33
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Good question Gary - and one I puzzled about myself. Would I need extra heavy duty servo cable?

But as Chris Bott pointed out very wisely when I was chatting about the project with him - when you think about it we commonly feed the Rx, and all the servos, via a single servo lead from either the ESC or the UBEC - and we don't worry about that.

The actual current draw by these servos - provided none of them stall - will be quite low - and even then transient. I think the chance of exceeding the capacity of the single feed for any significant time would be very slim. But you'll see later that I go some lengths to protect against servo binding/stalling

BEB

Frank Skilbeck11/10/2016 05:58:31
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The only servo that could be an issue in the wing is the retract servo, if the retract binds then it would draw a high current. I think the standard servo plug is good for around 6 amps and as BEB is driving all the servos from the ESC BEC it would probably be that which would give up before the servo wire. Personally I would have a multipin plug (Ashlok) on the wings and separate signals and power etc direct from the Rx, in the Frsky case you could use their Redundancy bus for this and then protect each output from overload.

The only comment I would make on your set up is the chosen switches, I'd keep the retracts away from the rates switch so you don't accidentally get the wrong one.

BTW this isn't really a complicated set-up, now if you add some sequencing doors in etc etc.

Charles Smitheman11/10/2016 08:01:26
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Thanks for this, very interesting!

Attilio Rausse11/10/2016 09:31:23
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BEB excellent example but I would have thought more suited to an experienced frsky user, another way for potential frsky users would be 2 receivers, 2nd one in the wing with the same one wire connection.

Til

Biggles' Elder Brother - Moderator11/10/2016 09:36:37
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Thanks guys - always nice to know someone is actually reading it!

Frank - no BEC in this one. I agree the wiring schematic looks like I will use the BEC but actually the ESC I'm going to use doesn't have one! So it will be a separate regulated power supply.. Not that it makes that much difference unless I was to go to something like a Powerbox - which I'm not. I plan on using something that will give me upto about 10A into the wing - frankly if I need more than that I'm in big trouble!!

Regarding sequencing etc - ah, well, wait and see! We will be using our independent channels to do a few little fancy things (modest but interesting I think) with the U/C and flaps. So it will get a bit more complex yet!

But of course if you would like to contribute a thread on fully sequencied doors etc in OpenTx that would be great - and I for one would be very interested - I have only done relatively staright forward sequencing so far in OpenTx so I'm keen to look at it in more depth. Similarly if there are any OpenTx using glider guiders out there, you guys really do have very complex wing set ups sometimes - we'd love to hear from you in a thread - the more the better.

BEB

Biggles' Elder Brother - Moderator11/10/2016 09:39:11
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Posted by Frank Skilbeck on 11/10/2016 05:58:31:

The only comment I would make on your set up is the chosen switches, I'd keep the retracts away from the rates switch so you don't accidentally get the wrong one.

.

Oh forgot that bit - that is pretty much my standard set up on most models - haven't hit the wrong switch yet - but I suppoose there is a first time for everything! wink 2

BEB

Biggles' Elder Brother - Moderator11/10/2016 09:45:50
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Posted by Attilio Rausse on 11/10/2016 09:31:23:

BEB excellent example but I would have thought more suited to an experienced frsky user, another way for potential frsky users would be 2 receivers, 2nd one in the wing with the same one wire connection.

Til

Hi Til,

thanks for this. Yes its tricky - I'm tring to hit a middle way - enough depth to be interesting for those who have used OpenTx/FrSky, while at the same time keeping it basically understandable for those that haven't, so that they can see what it is we find so interesting about this stuff! I may fail in both respects! But hey ho, that's life isn't it!!

Yes the two Rx solution would be another inetersting way to do this - basically putting a second X8R in the wing and using it to supply a second set of channels. And that is one of the great things I think about both FrSky kit and OpenTx - as well as being really cheap (while still reliable) its increadably flexible and there are always many different ways to the same end. Fancinating.

BEB

trebor11/10/2016 10:16:20
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Probably a totally novice question ( I am ! ) but why have you split the elevator and ailerons on different switches for your Rates.

Bob Cotsford11/10/2016 10:26:11
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Posted by Attilio Rausse on 11/10/2016 09:31:23:

BEB excellent example but I would have thought more suited to an experienced frsky user, another way for potential frsky users would be 2 receivers, 2nd one in the wing with the same one wire connection.

Til

For twin receiver setups I prefer to use an XPS X10+ channel expander which acts as an SBus decoder with up to 11 programmable channel decoding or 10 channel and receiver redundancy with a built in failsafe. It also has tabs to solder a power supply so there's no heavy current draw going through servo or sbus leads. On a couple of models with one-piece wings I've put an X10 board in the wing with just the sbus lead and a power lead to connect the wing.

On two piece wings I use two FrSky decoders, one in each wing half, each driving one aileron, one flap and one retract unit. Most electric retracts have a stall current under 2 amps so even a stalled leg won't overload the sbus lead (6 amp rated) before the stall protection kicks in.

If I did go the separate receiver in the wing route I'd go a step further and give it it's own battery and switch also in the wing. Bet I'd forget to switch it on sooner or later frown

Edited By Bob Cotsford on 11/10/2016 10:33:15

Andy4811/10/2016 10:46:10
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Posted by Biggles' Elder Brother - Moderator on 11/10/2016 09:39:11:
Posted by Frank Skilbeck on 11/10/2016 05:58:31:

The only comment I would make on your set up is the chosen switches, I'd keep the retracts away from the rates switch so you don't accidentally get the wrong one.

.

Oh forgot that bit - that is pretty much my standard set up on most models - haven't hit the wrong switch yet - but I suppoose there is a first time for everything! wink 2

BEB

Lets not forget that in OpenTX you can add a spoken message to each switch position so you have confirmation that you have put the correct switch in the correct position. Its easy too to create your own messages.

Great post BEB and love the untidier workbench. cheeky

Biggles' Elder Brother - Moderator11/10/2016 10:52:27
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Posted by trebor on 11/10/2016 10:16:20:

Probably a totally novice question ( I am ! ) but why have you split the elevator and ailerons on different switches for your Rates.

Hi trebor

simply for the flexibility. Sometimes, for example, with a taildragger that is having problems keeping its backside down on taxying its useful to have an extra high rate setting on elevators alone that you can switch in just for take off without effecting the ailerons. Similarly if the model is a bit sluglish on roll, but has a lively elevator because you have the CoG set fairly backward you might want to inject a bit more roll rate for some manoeuvres, but without making an already sensitive elevator even more so!

Actually uisng OpenTx you could have both set ups! You'd have a separate switch that decided whether you were switching both sets of rates or handling them separately - now there's an interesting OpenTx programming challenge for someone - any takers?

BEB

Biggles' Elder Brother - Moderator11/10/2016 10:54:22
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Posted by Andy48 on 11/10/2016 10:46:10:
Posted by Biggles' Elder Brother - Moderator on 11/10/2016 09:39:11:
Posted by Frank Skilbeck on 11/10/2016 05:58:31:

The only comment I would make on your set up is the chosen switches, I'd keep the retracts away from the rates switch so you don't accidentally get the wrong one.

.

Oh forgot that bit - that is pretty much my standard set up on most models - haven't hit the wrong switch yet - but I suppoose there is a first time for everything! wink 2

BEB

Lets not forget that in OpenTX you can add a spoken message to each switch position so you have confirmation that you have put the correct switch in the correct position. Its easy too to create your own messages.

Great post BEB and love the untidier workbench. cheeky

Yes indeed and I'll be showing that as we go along - all my switches speak to me!!

BEB

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