Idea for adding expo to servo

Yes, but you would only get one direction of travel on the control surface. Another easy way to get non-linear travel is to make the servo arm attachment longer than the control surface. The greater the difference, the more non-linearity you get. It may not be perfect expo but it is a pretty good representation. This setup will reduce the travel around neutral and increase as you go farther from center. Be careful not to overdrive the control surface!

While that may make some minor changes in linearity it is mostly just changing the how much the control surface will move. I have an idea on how to do this but I have to do a few calculations first, to make sure it works.

-- Chris W

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

Certainly would.

Along with precious little travel volume . . . .

Cheers, Fred McClellan the dash plumber at mindspring dot com

It can make HUGE changes in linearity. As the control horn approaches it travel limits it takes very little servo travel to move a lot. Like I said, it depends on how much difference there is between the servo and control horn lengths.

On my throttle setups I build in expo on the low end for very steady idle adjustments.

I set it up so both arms are at 90* to the servo case at full throttle.

It would provide LOTS of travel, but as Paul said, only in one direction.

-- Chris W

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

I just drew up a test case to check and I am still going to have to disagree. I set it up with a servo horn length of 2 and a control surface horn length of 1. When at neutral position 1 degree of servo rotation resulted in 2 degrees of control surface movement. When the control surface was at 45 degrees (maximum deflection) 1 degree of servo rotation resulted in 2.8 degrees of servo rotation. I don't consider that to be a significant amount change in linearity. On top of that, with the control surface 45 degrees from neutral the servo horn is only 20.6 degrees from neutral, that's not a very practical setup. It also gives your control surface a pretty good mechanical advantage over the servo and you are only using about 1/3 the force that servo can put out. On the up side the control surfaces will move really fast. But this all got started because of a way to do things without a computer radio. Without a computer radio you are going to want to have control horn lengths set up such that full servo travel gives full control surface movement.

-- Chris W

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

Chris W. wrote,

Yes, I was suffering a 'senior moment' and was semi-comatose (not enough coffee) when making the original post. Obviously as Mr. McIntosh said, the movement will be in only one direction. Bill (oc)

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The question was how to get expo without a computer radio. My method was the only way you are going to get anything like that. And, I consider a 30% increase in relative travel at 45 degrees deflection to be very significant! I never said there weren't drawbacks ;^)

Also, setting the control surface for full travel with full servo travel will usually result in reverse expo where you get more surface travel relatively around neutral and less at the ends of travel.

Hes right. If teh servo arm and control horn are not the same lenght, and parallel, you will get assymetric and non linear response.

MOST people I have seen in magazines trying to go for e.g. asymmetric ailerons (more up than down) get it the wrong way round tho.

Paul has got it right for expo.

For asymmetry, lean the horn towards the servo in the direction you want MORE travel. This seems counter inutitive. You of course get less

The world of mechanical engineering is full of useful little linkages that do this. Mostly these days they are not used much, as computers have taken over everything.

However you will find something useful if you study an engine camshaft...it may even be adaptable.

Using cables over offset pulleys is another one.

Actually the reason I brought up the subject of expo, was because I have a Graupner Tipsy (don't laugh) that does neat barrel rolls with the rudder. But the rudder is set up on maximum throws which makes it extremely twitchy around neutral. That's why expo on the rudder would be nice in this particular instance.

Bill (oc)

Wouldn't use of a dual horn (push pull) with the servo horn at 90 degrees do the job?

Jim - AMA 501383

Bill Sheppard wrote:

Now that is a fairly easy one to implement and would work great. Just wrap a cable around the pulley and hook each end of the cable to a control rod. Then drill holes in the pulley to mount it to a servo wheel off center. Experimenting with how far off center you need to get the desired effect. If the original poster wants to follow through with the idea I will draw it up to get a better idea of the geometry he needs for his application.

-- Chris W

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

That won't work. An eccentric wheel gives different travel from each side of the wheel. As the wheel rotates the side with the larger end of the wheel will be pulling farther than the other side will be releasing. This will cause increased stress as the travel increases. It is no different than using solid pushrods and having the pushrod pivots offset from the servo pivot point.

It will work. I'm not sure I explained it very well though. A picture would make it much more clear so take a look at

-- Chris W

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

OK, I can see how that would work in theory. The application would be fairly difficult. You would need a wheel like a sail winch wheel with a groove in the circumference to guide the pushrod. To attach the pushrod, you could use some kevlar thread, attach one end to the pushrod about an inch from the servo wheel, wrap the thread a couple times around the wheel and then attach it an inch away on the other side. You would have to keep tension on the thread to keep it from slipping.

All in all it seems a lit more complicated than just calibratign your thumbs.

I saw something like that once where the idea was to change the rotational motion of a servo into linear motion where the wheel was centered and there was a steel cable around the pulley hooked to the rod like you describe and the cable was also fastened to the wheel on the opposite side to prevent slipping.

But I agree a fairly complicated thing to make and that is why I have a computer radio.... you can get computer radios that do exponential and endpoint adjustments, pretty cheap these days too.

-- Chris Woodhouse

"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."

-- Benjamin Franklin, 1759 Historical Review of Pennsylvania

Use an eccentric servo horn as well :)

Seriously, lots of mechanisms exist. Sadly none are trivially as easy to implement as a bit of bent wire in a lever.