Servo jitter and crosstalk

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SimonWood
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Servo jitter and crosstalk

Post by SimonWood » Sat Dec 07, 2019 1:23 pm

I've fitted RC to a couple of batter locos, but I'm basically new to it and haven't used servos before. I've been working on the RC for my Regner Konrad. and have hooked up the servos (Tower MG90S) to the receiver, a Deltang Rx 102. Everything works, but the servos jitter - it's hard to see, but it's audible. There is also some crosstalk (when one servo operates, the others move very slightly). None of this is actually preventing the system from working, but I'm wondering it means (a) faulty servos (b) faulty receiver or (c) it's just normal behaviour. If (a) or (b) I'm not sure whether it matters other than it might shorten batter life.

I'm not sure the following video is very illuminating really, the crosstalk is hard to see, but you can just see the tiny movement when an out of shot servo is operated (twice each way) and you can maybe here the jitter.


Today channel 1 on the receiver also seems to have stopped working which makes it seem that (b) is the most likely option, but I don't have another Rx102 on hand at the moment to swap in to test that theory...

Anyone experienced anything like this?

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bambuko
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Re: Servo jitter and crosstalk

Post by bambuko » Sat Dec 07, 2019 1:37 pm

Have you done anything to the RX antenna?
Does it happen with servos and receiver just connected on the bench (i.e. outside of the loco).
It's not likely to be faulty servo.
It is not a normal behaviour.
It matters - if it is that bad with transmitter only few feet away from the receiver, imagine what it will be like with loco some distance away and with obstruction to the signal...
I would take it all out of the loco and test with only one servo connected (are the symptoms the same?).
Then connect another servo (one only again) and only then connect couple of them.
That will confirm whether it is one dodgy servo, whether it is interference from battery (any bad/arcing electrical connections?).
Have you got another receiver to test it with?
Servo testers are very cheap and will allow you to test things without a radio.
Do it properly :mrgreen:
and p.s.
your video is fine and clear demonstration of the problem

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Re: Servo jitter and crosstalk

Post by metalmuncher » Sat Dec 07, 2019 1:49 pm

I had this when fitting R/C with separate regulator and reverser servos to a Ragleth. If I remember correctly, when the throttle servo was under load it would affect the reversing servo. I fixed it by putting the wiring from one of the servos through a ferrite ring a couple of times, something like this:
Image
The ferrite I used was salvaged from an old power supply. You can also get clip-on ones.

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Re: Servo jitter and crosstalk

Post by SimonWood » Sat Dec 07, 2019 4:20 pm

Thanks for the replies!
bambuko wrote:
Sat Dec 07, 2019 1:37 pm
Have you done anything to the RX antenna?
No - or at least, not intentionally.
Does it happen with servos and receiver just connected on the bench (i.e. outside of the loco).
It's not likely to be faulty servo.
It is not a normal behaviour.
It still happens on the bench. I've tested this by removing the receiver from the loco and connecting it to another couple of servos from the same batch (just to save unmounting the ones on the loco). Same thing. But - I also tried hooking up an SG90 (plastic gears) and that doesn't seem to jitter or be affected by the crosstalk...
It matters - if it is that bad with transmitter only few feet away from the receiver, imagine what it will be like with loco some distance away and with obstruction to the signal...
I have tested with the transmitter a few feet away, and with it several metres away, and seen little difference in the effect.
I would take it all out of the loco and test with only one servo connected (are the symptoms the same?).
Then connect another servo (one only again) and only then connect couple of them.
That will confirm whether it is one dodgy servo, whether it is interference from battery (any bad/arcing electrical connections?).
With one servo connected there are no symptoms. With two connected, there are. (No arcing or bad connections as far as I can see).
Have you got another receiver to test it with?
No, but I can see I'm probably going to need to get another one, since channel 1 on this one hasn't recovered!
Servo testers are very cheap and will allow you to test things without a radio.
I have now tested with a servo tester (why didn't I think of that?) and the jitter isn't there; can't see a way of testing for the crosstalk, but the rx is starting to look very much like the culprit.
metalmuncher wrote:
Sat Dec 07, 2019 1:49 pm
I had this when fitting R/C with separate regulator and reverser servos to a Ragleth. If I remember correctly, when the throttle servo was under load it would affect the reversing servo. I fixed it by putting the wiring from one of the servos through a ferrite ring a couple of times, something like this:
Image
The ferrite I used was salvaged from an old power supply. You can also get clip-on ones.
Thanks I'll have a look for something suitable.

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bambuko
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Re: Servo jitter and crosstalk

Post by bambuko » Sat Dec 07, 2019 5:25 pm

SimonWood wrote:
Sat Dec 07, 2019 4:20 pm
...I have now tested with a servo tester ... and the jitter isn't there; can't see a way of testing for the crosstalk, but the rx is starting to look very much like the culprit....
Yes, I think you have narrowed down the problem.
If it was servo problem it would also be a problem with servo tester.
Whether it is dodgy receiver or simply interference (more likely in my view) is another story.
Second receiver will help in confirming :thumbright:

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tom_tom_go
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Re: Servo jitter and crosstalk

Post by tom_tom_go » Sat Dec 07, 2019 7:04 pm

Send the Rx back if you suspect it of being faulty.

Have you tried different batteries in both the Tx and Rx?

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Re: Servo jitter and crosstalk

Post by ge_rik » Sun Dec 08, 2019 4:35 pm

Try metalmuncher's idea with the ferrite ring. Could help.


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Re: Servo jitter and crosstalk

Post by SimonWood » Sun Dec 08, 2019 6:38 pm

Thanks for the replies - today has shot by without a chance to have another play with this problem. I haven't got a spare Tx battery to hand, but I recharged the Rx batteries. Then my next steps will be (a) get another Rx102 as I need another one anyway, and then I can compare directly by swapping, and (b) get a ferrite ring and see if that improves things. Will keep you posted.

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Re: Servo jitter and crosstalk

Post by tom_tom_go » Sun Dec 08, 2019 6:47 pm

I have some second hand Rx102's, PM me if interested.

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Re: Servo jitter and crosstalk

Post by SimonWood » Wed Jan 15, 2020 7:06 pm

Brief update: With new (s/h) Rx102 from tom_tom_go there's no crosstalk. In fact there's no crosstalk anymore with the original receiver. This is puzzling, as I haven't changed anything other than leaving it in a cupboard for several weeks, but hey it works. Plus I've now got a receiver for my next r/c project. There's still a bit of jitter (though it seems less) but I have still to test a ferrite ring so the next step will be to see if that eliminates it.

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tom_tom_go
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Re: Servo jitter and crosstalk

Post by tom_tom_go » Wed Jan 15, 2020 7:08 pm

Did you try a different battery?

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Re: Servo jitter and crosstalk

Post by SimonWood » Wed Jan 15, 2020 7:35 pm

tom_tom_go wrote:
Wed Jan 15, 2020 7:08 pm
Did you try a different battery?
No, that's the odd thing. Same batteries in Rx and Tx. I'd borrowed the Rx batteries for something else so I recharged them. But I'd just given them a full charge before testing it last time...

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Re: Servo jitter and crosstalk

Post by tom_tom_go » Wed Jan 15, 2020 8:00 pm

So when the batteries are fully charged you get the jitter?

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Re: Servo jitter and crosstalk

Post by SimonWood » Thu Jan 16, 2020 11:18 am

tom_tom_go wrote:
Wed Jan 15, 2020 8:00 pm
So when the batteries are fully charged you get the jitter?
I do, but it's distinctly less jittery than it was before.

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Re: Servo jitter and crosstalk

Post by tom_tom_go » Thu Jan 16, 2020 11:23 am

I would replace the battery, are you using single cells or a pack? If you are using AA or AAA batteries then you might have a dodge battery in the mix.

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