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Hitec QPCM a disappointment?

Old 08-29-2005, 04:06 AM
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Default Hitec QPCM a disappointment?

Why is it that in PPM mode (with a range of receivers) the Optic 6 delivers smooth response to movements of the control sticks (usnig a range of Hitec servos) but when using the QPCM receiver the servos move in decidedly noticeable steps?

I'm assuming that the resolution of the QPCM system (512 steps?) is lower than when operating in PPM mode.

Sensitivity is great, selectivity is excellent but resolution is decidedly third-world -- to the extent that I'm hesitant to use the QPCM receiver in my F3A/3D I'm building -- compared to all of my PPM receivers (Hitec and others) it makes the control surfaces appear to respond in a "rough" fashion to stick movements.

I'm using a hi-capacity, hi-current battery (5-cell 2250 mAH pack made from A-sized cells) and the effect is the same using any servo from an HS5925 to a good old HS425BB.

I seem to recall that there was another post from someone who'd noticed something similar with these receivers or the QPCM system -- was that a faulty unit or just an intrinsic limitation?

This stepping wasn't noticed in bench-testing, only once the receiver was installed in a ship with large surfaces and long throws. It's not servo-binding because putting a PPM (even DSP) receiver in the same plane fixes the problem.

Ideas?
Old 08-30-2005, 12:51 PM
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Default RE: Hitec QPCM a disappointment?

XJ

Hitec QPCM is 512K, but the processor speed on the front side is enhanced compared to others to minimize shortcomings of 512K limitations.

Yes, we have heard of this problem previously. I have no direct experience with same. But have seen many modelers use the QPCM gear without issue, small models all the way to GS.

Is it possible you linkage setup or linkage ratio contributes to the notchy realization?
Old 08-31-2005, 03:54 AM
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Default RE: Hitec QPCM a disappointment?

ORIGINAL: mglavin
Is it possible you linkage setup or linkage ratio contributes to the notchy realization?
This model has large surfaces and throws (3D stuff) so the effect of any granularity in the resolution will be amplified by that. It's quite possible that I'm just seeing the results of the 9-bit position encoding (something that no amount of processor speed will compensate for at the receiver end).

As I said, the effect is not at all visible in PPM mode (I suspect the transmitter uses a 10-bit ADC which isn't downsized to 9 bits in PPM mode).

Likewise, a Futaba 9C in PCM mode shows no visible granularity when used in the same aircraft.

I'll try flying this plane with the QPCM receiver but I'll probably have to go back to PPM or use a Futaba set because this is a plane that is 3D capable but also has a very high top speed when flown for F3A stuff. To get a smooth response (ie: not overcontrol it) when flying at speed, the low-rates have to be dropped to about 25% of full rate. Doing this with the current level of granularity would make it *very* difficult to maintain straight/level flight as the difference between one step and the next would result in several degrees of difference in the aircraft's pitch.

Was there a reason that Hitec opted for 9-bit resolution when everyone else is using 10?
Old 09-06-2005, 10:29 PM
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Default RE: Hitec QPCM a disappointment?

The QPCM receiver does not seem to be very compatible with some Hitec Servos. I noticed servo oscillations with the QPCM and HS-605BB. The servos were solid with a PPM receiver. I sent the QPCM receiver and servos back to Hitec and they replicated the problem, but had no solution other than using a PPM receiver. So I am using a Superslim 8 channel now. I suspect the QPCM would work pretty good with other servos. What kind do you have?
Old 09-07-2005, 03:23 AM
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Default RE: Hitec QPCM a disappointment?

The effect was most noticeable with the HS475 heavy-duty servos.

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