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Old 11-17-2019, 09:50 PM
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Futaba endpoint numbers

I'm curious how the Futaba endpoint numbers relate to the servo and servo resolution. As I understand there's a pulse width that tells the servo whether to move, how far, and in what direction. I've read that a pulse of 1500us is centered, and then 1000us or 2000us is full deflection in either direction. Then I guess there's a certain number of steps the servo can respond to in either direction from neutral to full deflection.

So, in relation to that, what do the end point numbers on Futaba mean? The default travel setting is 100, but you can increase that to 140. Why is the default 100? Why 140 as the max? If the max is 140, why is the limit 155? What do these numbers represent?

I have a throttle application where the shortest hole on the servo arm leads to both end points at 140, but I'm just a tiny bit short of opening the butterfly all the way. The next hole puts me at about 100 or so each way. Do I actually get more steps and resolution at 140?
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Old 11-18-2019, 09:12 AM
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stick travel vs servo travel
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Old 11-18-2019, 10:17 AM
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Do you get more resolution at 140? Yes. Will you notice it in practice? Not likely.

100/100 is the default because if you use sub trim it slides that operating window around, lets say for example 10 points of sub trim your throw is really 90-110 but you don't notice it bottoming out because you can really go to 140.

Same thing with mixes, mix in 10 percent of something to another and it can actually drive the surface to 110. That's where travel limit comes into play, you can use that to not over drive a surface past its mechanical limit with a mix.
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Old 11-18-2019, 03:56 PM
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Originally Posted by dldeuce View Post

I have a throttle application where the shortest hole on the servo arm leads to both end points at 140, but I'm just a tiny bit short of opening the butterfly all the way. The next hole puts me at about 100 or so each way. Do I actually get more steps and resolution at 140?
If this is a gasser, I would not worry too much about making sure that the throttle plate is absolutely wide open.

Much more important to see that you can close it all the way. Gas engines develop most of their power early in the process of opening the carb plate. They'll be way ahead of 50% power with the throttle travel set at 50% of the total.

So I wouldn't worry about not getting it wide open, I'd be willing to bet that you wouldn't notice any difference between 90% open and 100% open.

JMHO.
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Old 11-18-2019, 04:56 PM
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Originally Posted by Zeeb View Post
If this is a gasser, I would not worry too much about making sure that the throttle plate is absolutely wide open.

Much more important to see that you can close it all the way. Gas engines develop most of their power early in the process of opening the carb plate. They'll be way ahead of 50% power with the throttle travel set at 50% of the total.

So I wouldn't worry about not getting it wide open, I'd be willing to bet that you wouldn't notice any difference between 90% open and 100% open.

JMHO.
Plus if it is set for complete WOT, carb damage is likely to occur. Between the vibrations and the power of the servo the throttle rod bore can get damaged. I always set the end point slightly before absolute WOT to prevent carb damage.
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Old 11-18-2019, 07:52 PM
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Originally Posted by Zeeb View Post
Gas engines develop most of their power early in the process of opening the carb plate. They'll be way ahead of 50% power with the throttle travel set at 50% of the total.
JMHO.
I'm not worried about it opening all the way. Actually, I'm playing around with getting my stick mid-point set at about 30% throttle opening. I see that ends up with your servo arm not having much of an angle off a straight line with the pushrod. With it at 140/140, idle trim and subtrim seem to become a self-defeating factor.



I think it's more achievable when I'm around 100/100, but then I'm wondering if the loss of resolution might be self-defeating.
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Old 11-18-2019, 08:01 PM
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Are you using end points to try to get a certain throttle curve? If so, set at 100% and use the AFR menu to set a curve to get smooth throttle response over the entire range.
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Old 11-18-2019, 08:07 PM
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Originally Posted by BarracudaHockey View Post
100/100 is the default because if you use sub trim it slides that operating window around, lets say for example 10 points of sub trim your throw is really 90-110 but you don't notice it bottoming out because you can really go to 140.
I noticed that when I had the end points at 140/140 and started adding either idle trim or subtrim. The end point could be set up to 140, but as I increased it from like 100, the servo stopped moving at like 108-116. When I got rid of both idle trim and subtrim, it would move all the way to 140. That's one of the things that got me wanting to ask questions.

So, like I said I understand the pulse width goes +- 500us from 1500us. Is that range wider with the end point at 140? I definitely get a wider arc out of the servo. It just makes me curious to understand exactly what's happening.

It's just a learning exercise. I realize there may not be a lot of room for real improvement here, and I'll probably end up leaving it just the way I started. Just trying to understand the tradeoffs.
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Old 11-18-2019, 08:12 PM
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Are you using end points to try to get a certain throttle curve? If so, set at 100% and use the AFR menu to set a curve to get smooth throttle response over the entire range.

My question is what is the technical meaning of 100 or 140 and what is the technical difference in resolution?
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Old 11-18-2019, 08:17 PM
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Plus if it is set for complete WOT, carb damage is likely to occur. Between the vibrations and the power of the servo the throttle rod bore can get damaged. I always set the end point slightly before absolute WOT to prevent carb damage.

Hadn't thought about that.
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Old 11-18-2019, 08:24 PM
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Originally Posted by dldeuce View Post
My question is what is the technical meaning of 100 or 140 and what is the technical difference in resolution?
Got it. Saw people talking about how far the throttle was open, etc. The other comments on the technical aspects have been right on. Higher resolution, but probably not going to be noticed by the average human being.
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