Servo tester and digital servo query

[Graham Bowers]

I'm setting up a new model, Sebart Mythos 50, and am using Savox digital servos - a first for me. 1251 MG's.

 

I installed the first servo in the first wing and connected the pushrod to the aileron. Then  attached my servo tester to set the neutral point and then manually test the deflections.

 

The servo jittered like mad when the tester is in manual mode. I tried the second savox servo loose on the bench, and it's the same. It wasn't too happy at the neutral point either -a bit of buzzing.

 

I have a couple of Hitec digital servos for the elevators and I tried one - smooth as butter. HS5245MG.

 

Next, I attached the loose Savox to one of the receiver aileron channels on a model I fly regularly. Nice and smooth.

 

Now I'm wondering why the Savox servos jitter when on the tester. I use the tester regularly and I've never had that behaviour with analogue servos, or the Hitec digital. Apart from innate curiosity, I do need a tester to set the Mythos up.

 

Anybody else had this type of thing please?

 

Edited to add, the 10MB video I attached wouldn't work, so I deleted it.

 

Edited June 12, 2023 by Graham Bowers

[Shaun Walsh]

How are you powering the servo tester?

I had this happen when running the tester from a run down 4 cell NiMH pack. Recharged the battery and everything worked fine.

[Graham Bowers]

19 minutes ago, Shaun Walsh said:

How are you powering the servo tester?

I had this happen when running the tester from a run down 4 cell NiMH pack. Recharged the battery and everything worked fine.

A 4.8V NiMH Eneloop pack, fully charged**. For fun I just tried powering it with a 6V fully charged** Overlander low self discharge pack and no better unfortunately. I was all hopeful for a moment as I hadn't tested the packs recently .................... ;-)

 

** Tested with a little tester that applies a load to the pack.

[RottenRow]

What kind of servo tester are you using?

It sounds like something within it is limiting the current that the servo is drawing.

Brian.

[Martin McIntosh]

Definitely a flat battery pack. Servo jittering is always the first sign.

[Graham Bowers]

Thanks @Shaun Walsh, @RottenRow and @Martin McIntosh. It turns out these servos are current hogs and have exposed two weaknesses. One in the switch I was using between the servo tester and battery, and the servo tester it's self.

 

I charged the 4 Eneloop pack and it took about 500 mAhr so it was known good.

 

I put one servo on the tester, supplied by the freshly charged battery and through a switch (the same switch I was using before, but didn't mention). No better.

 

Next test was to eliminate the tester.

I connected two of these servos to a loose telemetry receiver and powered that with the battery and switch. I exercised the servos using my transmitter on the aileron channels. Very jittery and the receiver voltage displayed on the Tx via telemetry was dropping to 3V as I waggled the stick enthusiastically - I saw 2.9V lowest and the Rx was dropping out (red led on Rx and servos stopped / telemetry warnings)

 

Next test was to eliminate the switch.

Same battery plugged directly into the receiver, same 2 servos. Everything worked well, servos lovely and smooth. The Rx battery voltage via telemetry stayed above 5V, I didn't note the exact voltage however the difference was night and day.

 

Next test was 2 servos on the tester with the same battery, no switch. As I manually cycled the servos using the tester they were a bit jittery, not buttery smooth as they were when being fed by the Rx. I applied a bit of resistance to one servo with my fingers and both the servo I was touching and the other one (not being touched) jittered in unison - suggesting voltage sag.

 

I've binned the switch and will replace the tester. It's one of these, by the way https://www.nexusmodels.co.uk/gt-power-servo-tester-gtp0028.html

 

Every day, as they say, is a school day. 

 

Any recommendations for a servo tester please??

 

Edited June 12, 2023 by Graham Bowers

[Chris Walby]

I had a HK UBEC and three small digital servos, just bench testing an idea before putting it to the 38inch model and stirred all the sticks. And the RX browned out!

 

Measured the voltage on the RX and everything worked until all 3 servos were asked to travel quickly at once. Changed the UBEC to another make and all was ok

[Graham Bowers]

5 minutes ago, Chris Walby said:

I had a HK UBEC and three small digital servos, just bench testing an idea before putting it to the 38inch model and stirred all the sticks. And the RX browned out!

 

Measured the voltage on the RX and everything worked until all 3 servos were asked to travel quickly at once. Changed the UBEC to another make and all was ok

Good save.

[RottenRow]

I’m glad that you have found the cause of the problem Graham.

Some switches are absolutely rubbish, I often replace them with a larger slide switch and reuse the wiring from the original cheapo switch.

 

With your servo tester, it will be the supply tracks on the circuit board inside it that have insufficient current-carrying capacity. If you are good at soldering you could add a couple of bypass wires internally.

 

Alternatively you could make something up like the attached drawing, using a couple of spare servo extension leads (the shorter and thicker the better).

 

The bypass wires will carry most of the servo current.

 

Brian.

[Graham Bowers]

7 hours ago, RottenRow said:

I’m glad that you have found the cause of the problem Graham.

Some switches are absolutely rubbish, I often replace them with a larger slide switch and reuse the wiring from the original cheapo switch.

 

With your servo tester, it will be the supply tracks on the circuit board inside it that have insufficient current-carrying capacity. If you are good at soldering you could add a couple of bypass wires internally.

 

Alternatively you could make something up like the attached drawing, using a couple of spare servo extension leads (the shorter and thicker the better).

 

The bypass wires will carry most of the servo current.

Brian.

Thanks for that Brian.

I opened it up and see what you mean.

I think I favour the external bypass method, I make my own leads so have all of the bits.

Cheers

Graham

[Trevor]

Hmm. If the + and - connections run straight through the tester, couldn't you just Y lead the battery and servo into the output port and ignore the input completely? Depends how sophisticated the tester is perhaps, and it's early in the morning so I might have missed something! Try at your own risk!!

 

Trevor

[RottenRow]

Yes that would work too, and would be a bit easier to do.

 

Brian.

[Graham Bowers]

Having slept on it, I think I'll research more capable testers. 

[MattyB]

You’ll be lucky to find anything that isn’t either cheap and cheerful with similar issues to the one you already have, or a whole lot more expensive. There is a Toolkit RC one, but that is an awful lot to pay for something so (relatively) rarely used.
 

Personally I would just do the mod mentioned above, or use your TX with a receiver bound up to it - I have a servo testing model memory in my Taranis especially for the purpose of testing and burning in new servos, and it is more representative of the real world too. YMMV.

[PeterF]

I do not use a servo tester to set up a servo / control surface, always the transmitter and receiver. Then I know I am setting up to what the transmitter calls centre as I have found the centre point on the cheap and cheerfull servo testers is not always the same position as my transmitter.

[Graham Bowers]

Thanks @MattyB and @PeterF

 

Always more than one way to skin the proverbial..............

[Ron Gray]

On 13/06/2023 at 10:25, Graham Bowers said:

I think I'll research more capable testers. 

Those small one are, generally, rubbish! A few years ago I bought a GT Power one (4-Max) and it's been excellent, but tbh I only use it to centre the servos before fitting then the rest is done using the radio so is it really worth it?

[Peter Jenkins]

Never had or used a servo tester.  If I want to test a servo I use a spare receiver and my Tx.  One of the useful things is to minimize any sub trim to get the control surfaces to neutral.  Futaba 4 horn servo arms are a very useful way of doing so.  If you couple the servo arm adjustment with a turnbuckle connector you can set up the model with zero sub trim and that helps to avoid asymmetric control throws if you have dialled in a lot of sub trim.  Always make physical adjustments before resorting to sub trim.

[Engine Doctor]

When all set up make sure your power supply or battery in the model can supply enough current for all the servos and rx .

[John Lee]

Just to throw in the other side of the argument I always use a servo tester when constructing & setting up my models. I find it much more convenient to keep it on the bench with an old NiMh pack than use a radio.

 

The neutral is spot on to those of my Jeti & Spektrum radios & the throw almost exactly matches 100% on the radios.

 

Of course I complete the set up with the Tx & Rx but if the tester were to break today I'd get another tomorrow.

[Nigel R]

16 minutes ago, John Lee said:

I find it much more convenient to keep it on the bench with an old NiMh pack than use a radio.

 

I'd agree. Clearly, experiences differ, but it seems to get the neutral 'close enough' until the point where I start fitting the RX and setting up subtrims and so forth.