Gimbal centering

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MikeB
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Gimbal centering

Post by MikeB » Mon Jan 26, 2015 12:02 pm

There has been quite a bit of discussion regarding the quality of gimbals in various transmitters, particularly about the accuracy of the centering.
I've just had a bit of a look at various gimbals and the values of the A2D conversion.
The pots on the gimbals vary somewhat for the range of output voltage they give due to some being 60 deg pots and others 80 deg pots etc.
I note the following:
On the firmware for AVR processors, the A2D is 10-bit, but by using over-sampling, this is increased to 11-bit.
On thr firmware for ARM processors, the A2D is 12-bit, but this is reduced to 11-bit for compatibility. This means that where the pots are 80 degree, some possible resolution is being lost. It is very easy to change to using the full 12-bit, the only downside is you would need to re-calibrate the sticks/pots.

Regarding the centering, on the whole the mechanical centering is actually quite good, where I think there is a problem is the actual pot body is not fixed firmly enough and so moves slightly when you move the stick. On some gimbals I can move the pot body very slightly. This shows up as poor centering where, after moving the stick one way and returning it to the centre, the stick output remains slightly off in that direction, then moving the stick the other way and returning it you get the stick output slightly off centre in the other direction.

One solution is to create a deadband at the centre of the stick so this "slop" is hidden. The main problems with this are defining where the "real" centre is to position the deadband correctly and it you lose a small amount of stick movement at the centre.

I'm wondering whether we could use a "clever" algorithm where the "centre" position moves slightly depending on which direction you move the stick, and how far you move it.
Clearly another solution could be bonding the pot body mechanically, although this wouldn't fix the problem if there is "slop" inside the pot.

Example, on one stick I've looked at, moving it fully right, then returning it to the centre, the channel output shows +0.5%. Moving the stick fully left and returning it to the centre the output shows -0.7%. From this -0.7%, if I move the stick slightly right (output say 0.0%) then release it, I see -0.3%, moving it a bit further (to say +0.5%) and releasing it I get 0.0%. This looks exactly like the pot body moving.

Mike.
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Daedalus66
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Re: Gimbal centering

Post by Daedalus66 » Mon Jan 26, 2015 7:43 pm

The 9XR stick assemblies vary quite a bit. I had one that showed 5% centering error on one axis. Most are about 1% between the extremes of centering variability. Right now, my original 9XR Pro gives about 0.3% centering variability (after replacing the stick assemblies). My second 9XR Pro is not nearly so good.

Regarding "clever' algorithms, what really matters is to have a repeatable center. The big problem is with models that can't be trimmed correctly because there is a range of centering positions. Losing a little bit of stick movement is less of an issue (for sport flying anyway) than not being able to rely on consistent centering.

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Re: Gimbal centering

Post by bob195558 » Mon Jan 26, 2015 8:31 pm

HI Mike,

Wild idea here :idea: :
Would it be possible to program a content Center value when the pot is near to its center or with in the common center error value that is being detected ?

Bob B.
RADIOS: ErSky9x on Taranis, ErSky9x on Sky9x, ErSky9x on 9Xtreme,
Er9x-FrSky on 9x-Radio with Smartieparts upgrade Programmer Board and 9x-TelemetrEZ Board.
Some help info here: (http://openrcforums.com/forum/viewtopic.php?f=95&t=7182).
Bob B.

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Kilrah
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Re: Gimbal centering

Post by Kilrah » Mon Jan 26, 2015 9:52 pm

bob195558 wrote:Would it be possible to program a content Center value when the pot is near to its center or with in the common center error value that is being detected ?
MikeB wrote:One solution is to create a deadband at the centre of the stick so this "slop" is hidden.
What he said but IMO it would be even worse than the original issue. You'd be knowingly causing a "break" in the travel everytime even when there isn't really one. Anything other than fixing the source of the problem at the mechanical level is kind of moot. I prefer a 1% centering imprecision than a 2% dead band that tries to hide it and screws up control response in the process.
MikeB wrote: On thr firmware for ARM processors, the A2D is 12-bit, but this is reduced to 11-bit for compatibility. This means that where the pots are 80 degree, some possible resolution is being lost. It is very easy to change to using the full 12-bit, the only downside is you would need to re-calibrate the sticks/pots.
Don't know about sky9x/9XR Pro but on the Taranis it's pointless, there's already 1LSB of noise at 11bit, so all you'd gain is more noise.

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Re: Gimbal centering

Post by bob195558 » Mon Jan 26, 2015 11:03 pm

bob195558 wrote:HI Mike,
Wild idea here :idea: :
Would it be possible to program a content Center value when the pot is near to its center
or with in the common center error value that is being detected ?
Bob B.
I want to clarify a little better what I was asking, if it were feasible, to program the firmware
it self, to make the pot to have a 0 center point when stick movement is near to its center value.
I know we can program in a dead center area, I have done this for my Quad's rudder control.

And what Kilrah is saying, it can cause a problem because it is causing a "break" in the travel
every time the pot moves through its stick center position.

I have never checked with the multimeter for the repeatability of the
control sticks returning to its 0 center position.
I do not know how much of a problem this can be also.
It may depend on how sensitive the controls are on each particular model
and/or also the repeatability of each particular Gimbals set to center.

Bob B.
RADIOS: ErSky9x on Taranis, ErSky9x on Sky9x, ErSky9x on 9Xtreme,
Er9x-FrSky on 9x-Radio with Smartieparts upgrade Programmer Board and 9x-TelemetrEZ Board.
Some help info here: (http://openrcforums.com/forum/viewtopic.php?f=95&t=7182).
Bob B.


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Re: Gimbal centering

Post by vinnieg » Tue Apr 21, 2015 4:24 pm

any more thought on this ? I have the centering issue on my left stick mode 2, but the replacement gimbal has been out of stock for over a month at hobbyking. I modified one model with mixes, but it's a pain to port over to all models as the parameters depend on expo, limits, trim, etc. It would really be nice to have a deadband feature. deviationtx has such a feature in it's curves menu.

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Re: Gimbal centering

Post by Daedalus66 » Wed Apr 22, 2015 2:01 pm

I assume the out of stock is temporary. They seem to be reorganizing the 9XR Pro line, so this should resolve itself eventually.

Meanwhile, various people have improved centering by doing things like securing the pot housing with hot melt glue. Take a good look to see if there is slop that can be removed.

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MikeB
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Re: Gimbal centering

Post by MikeB » Wed Apr 22, 2015 3:45 pm

Clearly, checking the mechanics is very worth doing, ANY movement of the pot housing WILL cause centering problems.
I have, however, added options in the hardware menu to allow a deadband to be set for each stick. We use an 11-bit value, therefore up to +/- 1024 for the stick position. At present I allow a deadband value of +/- 10, so around 1%.
The logic is:
If the stick value is is within the deadband value of the calibrated centre position, then use the centre position as the stick position.
If the stick value is is outside the deadband value of the calibrated centre position, then reduce the the stick value towards the centre position by the deadband value.

I've tested this on a 9XR-PRO, and have a solid 0.0 centre position on all three centering sticks. With a test servo, I can't detect any difference in the servo control with the deadband enabled and without.
Interestingly, I'm not convinced the servo actually centres exactly the same from the two directions, even with the stick giving a guaranteed ZERO position (OK, it's a fairly cheap Corona CS-929MG micro servo).

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Re: Gimbal centering

Post by elaerico » Wed Jun 28, 2017 7:12 pm

Hi Mike. Im having that issues with the center of the gimbal, but can't find that option in my radio. Where is it?
Thanks.

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Re: Gimbal centering

Post by jhsa » Wed Jun 28, 2017 7:18 pm

Turn your radio on while holding the left horizontal trim to the left for a second or two, and then release it. This enables the hardware menu. Now to enter the hardware menu, go to "Radio Setup / Hardware". There you find the stick deadband settings..

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Re: Gimbal centering

Post by elaerico » Wed Jun 28, 2017 7:24 pm

It's not in my radio(the hardware menu yes, but not the deadband one) . Version 1.784 Mike er9x-pr822.

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Re: Gimbal centering

Post by jhsa » Wed Jun 28, 2017 10:35 pm

You never said what radio you have.. Er9x and Ersky9x support different platforms.. If you don't say which one you're using, then you might get a wrong answer :)

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Re: Gimbal centering

Post by elaerico » Wed Jun 28, 2017 11:18 pm

Oh sorry, it's a 9x with atmega128.

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MikeB
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Re: Gimbal centering

Post by MikeB » Thu Jun 29, 2017 12:25 pm

I haven't put the deadband code into er9x, mainly because the original '64 processor doesn't have enought flash, and even the '128 will need to use some RAM for it to be added, and this is rather full as well.

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Re: Gimbal centering

Post by bob195558 » Thu Jun 29, 2017 12:55 pm

May be upgrading to the Ar9x board: (http://www.ar9x.net/).

Bob B.
RADIOS: ErSky9x on Taranis, ErSky9x on Sky9x, ErSky9x on 9Xtreme,
Er9x-FrSky on 9x-Radio with Smartieparts upgrade Programmer Board and 9x-TelemetrEZ Board.
Some help info here: (http://openrcforums.com/forum/viewtopic.php?f=95&t=7182).
Bob B.

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Re: Gimbal centering

Post by jhsa » Thu Jun 29, 2017 1:18 pm

MikeB wrote:
Thu Jun 29, 2017 12:25 pm
I haven't put the deadband code into er9x, mainly because the original '64 processor doesn't have enought flash, and even the '128 will need to use some RAM for it to be added, and this is rather full as well.

Mike.
Would it be ok on the 2561 processor? it has the double of the RAM..
Thanks
João
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Re: Gimbal centering

Post by elaerico » Thu Jun 29, 2017 2:48 pm

By the way, how can I use the taranis m9 gimbal in my 9x? It says it has 3.3v and I presume the 9x is a 5v board?

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Re: Gimbal centering

Post by bob195558 » Thu Jun 29, 2017 5:25 pm

Some info, start reading here and continue down reading: (viewtopic.php?f=122&t=9683#p123428).
For the old 9x you will need to change the 5 volts to the Gimbals to be 3.3 volts.
Ar9x upgrade board will do this for you.

Bob B.
RADIOS: ErSky9x on Taranis, ErSky9x on Sky9x, ErSky9x on 9Xtreme,
Er9x-FrSky on 9x-Radio with Smartieparts upgrade Programmer Board and 9x-TelemetrEZ Board.
Some help info here: (http://openrcforums.com/forum/viewtopic.php?f=95&t=7182).
Bob B.

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