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Judder on all servos with DFT module

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Engine Doctor09/12/2019 11:59:04
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2370 forum posts
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Can anyone shed any light on the following ? I recently obtained a FrSky DFT Futaba Tx module . I don't need or want any telemetry and am using a Vr8 11 Rx

Have set the switches on the back according to the instructions for use with this Rx , Binds instantly but all servos judder as if the frame rate is wrong . I have checked back on much earlier post and there were some dodgy modules in early batches. This one I was assured is from the later batch so in theory should be ok . Were there any fixes found ?

Peter Christy09/12/2019 12:47:00
1643 forum posts

Not come across this one personally, but the first thing I would advise is making sure the firmware is up to date.

There's no way of checking what is actually in the module, you just have to download the latest version and apply it. There are a few ways of doing this, and a quick google should find everything you need.

--

Pete

Engine Doctor09/12/2019 13:44:50
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Thanks Peter I'll pass that on

Denis Watkins09/12/2019 14:04:37
4006 forum posts
73 photos

Have only come across this with near flat Rx pack, or faulty flight switch

cymaz09/12/2019 18:03:05
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8883 forum posts
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Could the Rx be set to High Speed mode?? I know some of the Frsky Rx you can change it. I’ve had one that came out of the packet in high speed mode, my analog servos went bananas 

Edited By cymaz on 09/12/2019 18:04:38

Peter Christy09/12/2019 18:17:10
1643 forum posts

The Vr8-11 is one of the first generation receivers (non-telemetry) and doesn't have a high speed mode. BUT, it does have the ability to bind to D series transmitters, albeit without telemetry.

ED: It might be worth checking that the receiver was bound in the right mode. I don't think it will bind in the wrong one, but you never know!

V8 mode: Set the module to V8 mode (Sw 1 OFF, Sw 2 ON) and bind receiver by holding the F/S button when powering on. Once bound (its almost instantaneous), switch everything off and on again, and it should be bound in V8 mode.

D8 mode: Set module to D8 (both switches OFF). On the receiver, connect the Ch1 and Ch1 signal pins with a jumper and then power it on WITHOUT holding the F/S button. After binding, power everything down and remove the jumper. It should now be bound in D8 mode - but you still won't get telemetry.

Maybe one mode will work better than the other? Worth a try before going through the hassle of flashing it!

--

Pete

Mike Blandford09/12/2019 18:53:46
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You don't mention what transmitter you are using.
You say the servos judder. What do you mean by this? Do they move in response to the sticks, but don't stay still in any position?

If your Tx has a protocol setting, the DFT module requires PPM (not PCM).

Mike

Alan Hilton10/12/2019 09:23:53
112 forum posts

Hi Doc

Try binding with both module switches in the off position

Alan

Engine Doctor10/12/2019 11:09:17
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2370 forum posts
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Hi Mike The Tx is a FF9 Super . The judder might better be described as stepping? they go move at an inconsistent speed hard todescribe

Alan have tried binding in telemetry and non telemetry modes the Rx binds ok in either mode and the servo response is better/smoother in telemtry mode .

Peter . we will try binding using the jumper method . It currently binds in either mode using the F/S button but will try the binding plug and see if any improvment

Thanks for all the input . We intend to go to the flying site when its a bit drier and try range checking and also see if some distance between the Tx an model changes anything.

Will let you know the outcome

RottenRow10/12/2019 12:11:59
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17 forum posts

I have exactly the same symptoms with a later DFT module (the two-way one) and the VR8 receiver (more than one). In my case the transmitter is a Futaba 9ZAP and it's definitely set in PPM mode.

As Engine Doctor says, it is a kind of stepping rather than a judder, most noticeable if the sticks are moved slowly, and more noticeable by the sound made by the servos (mostly 148s in my case) than by actually looking at the control surface movement (though you can see it if you look carefully). It is also much more apparent on some models than on others, but I suspect that this is down to the accoustic properties of the different airframes and servo mountings. It's only really noticeable on one particular model, the others you have to listen specifically for it. You would not notice it on an ic powered model with the engine running.

I have just transferred the module to a different transmitter - an FF7 (the old type with mechanical trims) as a test, and the symptoms are still present. So it's not specific to my 9ZAP. Also putting the module into low power (range check) mode doesn't change anything.

I have flown the models like this for 100s of flights now and there appears to be no side effect whatsoever. Until I noticed this forum post appear I had thought nothing of it.

I have some of the original (non-telemetry) Futaba FrSky modules, and some receivers of varying vintages, so will set up some kind of test later and see if a pattern emerges. I will also try to connect the module to a non-computer transmitter to see if that's any different.

Incidentally, some time ago, there were a few occasions when on switching everything on I had no output from the receiver. This turned out to be one of the two DIP switches on the back of my tx module which had had poor contact integrity (several thousand ohms when closed) - this meant the module was firing up in the wrong mode. I soldered a bridge across the switch and have not had an issue since. I don't use telemetry mode.

Brian W.

Peter Christy10/12/2019 15:05:42
1643 forum posts

ED: Just noticed a typo in my instructions! It should read join Ch1 and Ch2 signal pins with a jumper - but you probably guessed that.

If you didn't get full instructions for the receiver, you can find them here:

**LINK**

--

Pete

Engine Doctor10/12/2019 15:26:52
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2370 forum posts
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Thanks for you input Brian .

hello Peter yes not a problem worked that one out .

Thanks

E.D. (Phil)

RottenRow10/12/2019 23:01:10
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17 forum posts

I have been playing about with a few combinations of transmitters and FrSky receivers. It does look like the issue is something to do with the frame rate as you thought Phil. I didn't notice any difference between several FrSky receivers of different ages and versions.

With the DFT Module in the tx, and set to V8 mode, I measured the frame period (ie the gap between each successive pulse to the servos) at around 18ms with upto 0.5ms variation. Looking at the signal to the servo (on an oscilloscope) I could see the frame rate varying when no tx sticks were being moved. I tried this with both the two-way DFT module and the older non-telemetry type and results were similar. This is a frame rate of around 55 per second, but with variation of plus or minus 2.

I then set the module to D8 mode, rebound the same rx and the frame period was still 18ms but was solid with no variation. The servo operation was definitely smoother but the stepping was still there to a degree.

For reference I installed a 35MHz module into the same transmitter (9ZAP, still set to PPM mode) and measured the frame rate out of a suitable receiver - it was 22ms and very stable. The same was found with a 9C tx (FF9), and a FF7 had a frame rate of around 21ms (there are less pulses per frame as there are fewer channels). This is about 45 frames per second. This is being generated by the tx and in 35MHz PPM is what is received by the rx and then decoded and sent to each servo. Servo operation was, as expected, very smooth.

The FrSky module takes in this same signal from the tx. So it is getting around 45 frames per second from the tx but is sending them out (to the rx) at a faster rate. I guess what is happening with the FrSky module is that, in trying to send 55 frames per second, it will sometimes send the same frame twice (which will halt the servo for a moment) then the next frame will be correct again and the servo will respond with a step.

I have also tried a number of different models of servo and the effect is more pronounced on some types than others. Futaba 148s and S3003s seemed worse, a HiTec HS-311 was definitely smoother. Perhaps it is slower to react to input changes.

Finally I tried a FrSky Taranis tx with the V8R-II receiver set to 'D8' mode. The servo operation was smoother and nearly to 35MHz standard. The frame rate was 55 per second again and dead stable.

So I suspect that the main reason for the servo 'stepping' is the difference in frame rates between the transmitter itself, and the rate generated by the FrSky module. The faster rate to the servo (compared to 35MHz) may also be having a slight effect.

If I am correct in saying this, then using the module in D8 mode and perhaps choosing different servos is probably all you can do. Once in the air I doubt if the difference will be noticeable.

Sorry for the long post, I hope you are able to follow it....

Brian W.

Mike Blandford11/12/2019 00:50:20
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564 forum posts
22 photos

In D8 mode, the module sends all 8 channels at a 9mS period, but every fourth 9mS period it doesn't send anything, instead this time is used by a telemetry receiver to send the telemetry data back to the Tx.

In PPM mode, the module has to measure each PPM pulse, and then it is sent in a digital format. The receiver (I believe) synchronises to the 9mS period and sends the servo pulses out every 18mS.

I just tried a DJT module (same hardware and firmware as the DFT) in a 9X with a SKY board upgrade, to a D8R-IIplus. The protocol to the DJT is PPM, and I can change the frame rate (erskyTx firmware on the Tx). I increased the frame period to 31mS, and I couldn't detect any significant stepping of a servo. Just possibly, the servos were slightly smoother when I reduced the frame period to 18mS.

This Tx creates the PPM frame by measuring the stick positions, then storing all the required pulse widths ready for transmission shortly before the start of the first PPM pulse.

I then swapped in a XJT module, configured for D8 mode, but using the PXX protocol. This sends the data to the module every 9mS in digital format. A servo was definitely smoother (I was using an old Hitec HS-303 with a 1 foot long balsa stick on it).

I would agree that any stepping being seen is due to phasing between the different frame rates, causing a significant variation in latency from stick movement to servo output pulse.. The more channels a Tx is sending, then the longer the frame period will be.

Mike

Engine Doctor15/12/2019 10:48:39
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2370 forum posts
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Re the jerky or stepping servo action with FrSky radio . Friend in club has just found out that by holding the Fail-safe button on the Rx down for about 6 seconds when turning on changes the frame rate from a higher rate for digital servos to a lower rate for analogue servos . He said its seemed to have cured the problem . Hope this helps anyone else who has the problem.

Thanks for the help and suggestions .

E.D.

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