[NSRCA-discussion] SD-10

Tue Aug 25 07:14:20 AKDT 2009

Yes I understand that. I guess I oversimplified a bit. I was assuming that their tradeoff decision took into account the frame rate of the data. As you know, you can get more bits at a slower frame rate or have a higher frame rate with less bits - given a fixed bandwidth. As all servos are now pretty fast, it would seem that increasing the frame rate would work in your favor AND make the control system more responsive, especially in a Pattern setup. A "fast" servo that is being updated with a position signal at a "slow" rate is not a good thing.

I remember when PCM was becoming popular, a few guys took some measurements of the different systems (Futaba, JR, etc.) comparing frame rate, resolution, latency, etc.. This was really interesting stuff. Now that we have 2048 resolution and 2.4GHz systems in widespread use, I think it's time we did this again. :)

John Pavlick

--- On Tue, 8/25/09, Anthony Frackowiak <frackowiak at sbcglobal.net> wrote:

From: Anthony Frackowiak <frackowiak at sbcglobal.net>
Subject: Re: [NSRCA-discussion] SD-10
To: jpavlick at idseng.com, "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
Date: Tuesday, August 25, 2009, 10:12 AM

Some good points, but I must correct a misconception. There is system "latency" and then there is servo speed. These are two different things. Reduced latency does not make the servo faster. It only very slightly reduces the time between stick movement and when the servo begins to move. It doesn't make full travel servo speed any faster. This is a subtle point but an important one in this discussion. 

Also, when I want my control throws measured properly I never use the hanging weight devices. They are not accurate enough. I use a 7" protractor and pointers on the surfaces. This is much better and lets me detect even a single digit of travel adjust.

I still feel that improved resolution is better for pattern flying then a slight decrease in system latency. But that is just my opinion!

Tony

On Aug 25, 2009, at 6:40 AM, John Pavlick wrote:

Good point Jay. Remember, no matter WHAT people say (or would like to have us believe), we live in an analog world. Trying to digitize an "analog" motion is best accomplished with the largest number of bits that bandwidth (and memory) allows. But there's more to our control systems than meets the eye.

One thing to keep in mind is that Pattern setups are almost totally opposite from "3D" setups. Pattern strives for maximum control precision with the least amount of surface travel whereas "3D" does the opposite. What this means is that our linkages are configured to give the servo maximum mechanical advantage. This is like putting a transmission in low gear. You have lots of torque but not a lot of speed. It also means that the "resolution" of the control system (meaning servos and encoding process) does not play as large a part in the total positioning accuracy as it does with a "3D" setup. Not that high-resolution numbers are "bad" - they're certainly not, but in a Pattern setup they're not as important as you might think. If the servo has a 3:1 mechanical advantage over the control surface (as opposed to a "3D" setup which may be close to 1:1) then a 1 degree "error" at the servo only gives us .3 degrees at the control surface. Can you even measure
 that with one of the "standard" weighted-pendulum deflection gages? On the other hand a "slow" servo will become very noticeable in this case. When the servo must rotate to it's extremes while only moving the control surface a slight amount, it will take a noticeably long time if the servo is slow. When you look at the mechanics in detail it becomes apparent that a good Pattern control system could actually compromise on "resolution" and instead look to improve the "speed" + "response". So maybe the Airtronics Engineers hit the nail on the head. I'm sure if you told them to build the BEST control system possible they would throw in 2048 resolution if they could, but in this case they had to make a decision and it looks to me like they made the right one. As long as you're a Pattern pilot anyway. :)

John Pavlick

--- On Mon, 8/24/09, Jay Marshall <lightfoot at sc.rr.com> wrote:

From: Jay Marshall <lightfoot at sc.rr.com>
Subject: Re: [NSRCA-discussion] SD-10
To: "'General pattern discussion'" <nsrca-discussion at lists.nsrca.org>
Date: Monday, August 24, 2009, 7:58 PM

A liner, non-digital system such as PPM, has infinite resolution and
therefore the ultimate "smoothness". I agree, however, that you probably
can't "feel" a 1/2048 digital step. I suspect that the major improvements
have been in less latency.

Jay Marshall

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Atwood, Mark
Sent: Monday, August 24, 2009 4:19 PM
To: 'nsrca-discussion at lists.nsrca.org'
Subject: Re: [NSRCA-discussion] SD-10

I'll take a stab, and I'm blind copying Mike Greensheilds (don't feel
comfortable putting his email out on the list) who is the product manager
for ATX here in the states.  He may chime in with additional background. 

My understanding is that when Sanwa was evaluating speed vs resolution,
speed was deemed a much more critical factor.  1024 resolution is already to
the limits of our ability to "feel" any difference where as the
speed/latency was much more noticable.   Cost does come into the equation
such that for a $500 radio, they couldn't do both.   

1024 resolution provides better than 1/10deg at 90 deg of rotation.  That's
less than 1/1000" on a 1" servo wheel. 

The speed on the other hand is just now approaching our limits of "feel".
The faster systems clearly feel more connected and are noticable when flying
Snaps and other rapid input maneuvers and especially flying 3D with the
helis.   

So the choice was to invest in a fast response system with 1024 resolution.

Does anyone know the tolerances of our servo accuracy?  I'd be curious to
know how high a resolution they can accurately replicate.   

Mike?  Any input from the source?

-Mark
--------------------------
Sent from my BlackBerry Wireless Handheld

----- Original Message -----
From: nsrca-discussion-bounces at lists.nsrca.org
<nsrca-discussion-bounces at lists.nsrca.org>
To: 'General pattern discussion' <nsrca-discussion at lists.nsrca.org>
Sent: Mon Aug 24 15:30:03 2009
Subject: [NSRCA-discussion] SD-10

Hi Anthony, Jim, Mark , and Others who Responded,

Thanks for your valuable input. I've read that this radio's resolution is or
is among the very  fastest; so much so that even digital servos can't keep
up with it. I understand  that the system is 1024 , but can't help but
wonder why it isn't 2048. I'm sure I'll get past my  hangup, but would
appreciate everyone's insight's on that one.

Thanks,

Frank Imbriaco

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