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Old Mar 12, 2013, 07:19 PM
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Hitec programmable servos, Spektrum, and the HPP-21

The hitec HPP-21 programmer is pretty neat. It allows one to program end points center for a servo. Programming the center of your servo helps when the splines don't provide the 90 degree servo arm orientation at center. You can do this with programmable radios with sub-trim, but doing so results in losing throw in the direction you subtrim.

Based on my measurements of my HPP-21, this is how it performs in test mode:

HPP21 pulse width range: 900, 1500, 2100 uS (left, center, right)
Accuracy: 0.2 uS
Resolution: 1.0 uS

This is pretty good in my view.

However, using my DX8, with 150% throws and no subtrim, the AR8000 provides this:

Pulse width range AR8000 with DX8 @ 150%: 919, 1504, 2090 uS (left, center, right)

Notice that the center pulse width is 15004 uS and not 1500 uS. This is probably due to temperature effects on the oscillator in the receiver as well as the accuracy of the receiver's oscillator. I've found that different receivers have different offsets, even when the transmitter has no trim or subtrim programmed.

This leads to problems when you take the time to program your radio. If you're using Spektrum (and probably other radio brands) you will find that the center pulse width is not 1500 uS.

solution to pulse width center deviation from 1500 uS


The solution to this is pretty simple.

If your servo is not reversed, add 3R sub-trim to the channel. If the servo is reversed, add 3L sub-trim to the channel. In either case, the pulse width range of the channel will be something like what I got:

Pulse width range AR8000 with DX8 @ 150% and 3 clicks of subtrim: 915, 1500, 2085 uS

Better yet, the center you programmed with the HPP21 will match the center you have on your AR8000, and you will have symmetry on your pulse width.

You won't need fancy equipment if you're not using Spektrum, or if your Spektrum receiver isn't working as I describe. All you have to do is program your center and end points with the HPP21, and then when you install the servo to your receiver, add sub-trim to re-center it.

All of this assumes that the holes on the control surface control horns are perpendicular to the hinge line, and that you've programmed the servo to have a 90 degree servo arm position at center with the HPP21, and that you've mechanically adjusted the length of the control arm so that the control surface is level at center.

This is based on a sample of one transmitter and one receiver. I'll be testing other combinations and spektrum transmitters, and will post addendums if any recommendations change based on this
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Last edited by 7oneWo1f; Mar 30, 2013 at 12:35 PM.
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Old Mar 12, 2013, 07:40 PM
can't buy vaporware
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Just checked with a DX18 and the same AR8000. This time I needed one click of subtrim if the servo channel was not reversed, and 7 clicks of subtrim if the servo channel was reversed. In both cases, I was able to get the 915/1500/2085 uS for left/center/right.
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Old Mar 12, 2013, 08:10 PM
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Just checked with a different AR8000, and its center was off by 1 uS from the other AR8000, using the same radio, subtrim set to 0 for both. Looks like it varies by radio and receiver.
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Old Mar 14, 2013, 10:07 AM
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This has been mentioned before and it's due to factory allowed tolerances.

The difference would be greater between radios IMO due to pots being used while the difference on the receiver would be smaller.

Using same radio, same power source and same servo all rigged up I was not able to see the difference at the end of the servo horn tips from 3-4 difference receivers.

If one is using a digital device to read the center value one would probably see minor difference.

The difference in radios is noticable when using a BUDDY box system. Giving comman over to buddy and the servo moves a little.
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Old Mar 30, 2013, 12:48 PM
can't buy vaporware
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Quote:
Originally Posted by freechip View Post
This has been mentioned before and it's due to factory allowed tolerances.

The difference would be greater between radios IMO due to pots being used while the difference on the receiver would be smaller.

Using same radio, same power source and same servo all rigged up I was not able to see the difference at the end of the servo horn tips from 3-4 difference receivers.

If one is using a digital device to read the center value one would probably see minor difference.

The difference in radios is noticable when using a BUDDY box system. Giving comman over to buddy and the servo moves a little.
Good point about the TX side. However, I used the same radio, same model #, just bound to a different receiver. But when I set up two radios using buddy box, I also noticed a change in subtrim...but that was over a year ago, and I didn't know what to make of it at the time.

Temperature also plays a role, since crystal oscillators change frequency with changes in temperature, unless one uses a temperature controled crystal oscillator, which would be cost prohibitive for most receivers.
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