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HomeAircraftHelicopterRadio - Servo - Gyro - Gov - Batt › Wouldn't linear travel servos be better?
03-10-2008 03:39 AM  10 years agoPost 1
helicopter34

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New Jersey, exit 82

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I was just thinking today, the ball linkage on servo horns travel in a circular path. So the amount of linear displacement in your linkage per degree change in the servo horn depends on where in the servo horns range you are. If the servo arm is perpendicular to the linkage you have the greatest sensativity (greatest derivative of linear displacement of the linkage with respect to servo arm angle) and your sensativity decreases are the servo arm pulls towards the end of its range.

Wouldn't it be better to have a linear travel type servo design, say more like a rack and pinion (the pinion would pull the rack which would be connected to your linkage) so that you have constant sensativity throughout the range of displacement. You could always set your expo electronically.

I don't know if I explained that well enough. Let me know what you guys think.

Make it idiot-proof, and someone will make a better idiot

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03-10-2008 03:45 AM  10 years agoPost 2
wings19

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Tucson, AZ

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I know exactly what you mean... I'm sure servo manufacturers are aware of that also. IMHO it would be in designing a servo like this that would be problem. One that is light and small enough to fit and then there is the problem of mounting it...

Nick
If the wings are traveling faster than the fuselage, it's probably a helicopter

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03-10-2008 04:01 AM  10 years agoPost 3
GyroFreak

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Orlando Florida ...28N 81W

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I have seen linear travel servo link kits(?) at MA display area, so they are available for standard servos. I'll check around and repost this when I find a link.

EDIT: Not what I was talking about, but a linear servo is here.
http://www.emsjomar.com/SearchResult.aspx?CategoryID=18

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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03-10-2008 04:17 AM  10 years agoPost 4
GyroFreak

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Orlando Florida ...28N 81W

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Here are the linear link kits for Futaba servos by M.A. Don't know how they work, but they are available.
http://www.miniatureaircraftusa.com...asp?prod=0883-1

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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03-10-2008 04:45 AM  10 years agoPost 5
JKos

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Redondo Beach, CA

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> for Futaba servos

It says Futaba and JR servos.

- John

RR rules!

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03-10-2008 04:49 AM  10 years agoPost 6
gian

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AZ

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Check out the Yamaha Rmax

http://www.runryder.com/t239463p1

It makes use of linear servos.

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03-10-2008 02:29 PM  10 years agoPost 7
fdztec

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Windsor, UK

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Back in the late 70's and early 80's Skyleader (and others?) used to make linear servos, but they never caught on...

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03-10-2008 02:37 PM  10 years agoPost 8
Dr Lodge

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Guildford, Surrey - UK

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Surely it would be easy for transmitter manufacturers to build some programming into the Tx design - you tell it at which point the servo arm is perpendicular to the linkage, and the length of the servo arm, then it adds the necessary correction?

Vibe 90, Vibe 50, Vigor CS x2,
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03-10-2008 02:41 PM  10 years agoPost 9
Drunk Monk

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Preston, UK

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I don't really think it makes much difference since even if the servo moved linearly the parts they connect to aren't and are usually on bearings and rotate.. I can't see it catching on or even see a need since you would need to redesign a heli to make best use of them.


Stephen

I only open my mouth to change feet.....

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03-10-2008 03:56 PM  10 years agoPost 10
JKos

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Redondo Beach, CA

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> Surely it would be easy for transmitter manufacturers to build some
> programming into the Tx design - you tell it at which point the
> servo arm is perpendicular to the linkage, and the length of the
> servo arm, then it adds the necessary correction?

It already exists. The Airtronics Stylus has had it for a long time. Then Futaba got it on 14MZ, then 12Z, and now 12FG. JR put it on 7202 and 9303. Spektrum on the DX7. It is adjustable on the Stylus and Futaba radios; an on-off feature on the JR and Spektrum.

- John

RR rules!

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03-10-2008 08:38 PM  10 years agoPost 11
wonderboy

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Southeast, MI

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I don't think you want a linear servo for most applications we use servos for. Throttle and cyclic movements are best suited to rotary motion.

You're right, the servo arm travels in an arc, so the movement of the linkage rod is not linear. However, consider what you are attached to on the other end: usually some sort of arm that also travels in an arc.

Whatever nonlinearity the servo arm induces is taken out on the other end. (I guess this is true of most cyclic situations, but maybe not a mechanical collective pitch mechanism.)

Assuming equal arm lengths, 10 degrees of rotation of the servo gives 10 degrees of rotation of the driven component. If you make sure that the arms are parallel at neutral, then this direct relationship holds over the whole travel.

If you don't have equal arm lengths, then you simply multiply the angular motion (or divide it), but the key is the relationship is constant over the travel if you have the arms parallel at neutral.

Driving a throttle or swash (cyclic) with a linear servo would create a confusing non-linear movement.

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03-10-2008 08:51 PM  10 years agoPost 12
Greg McFadden

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Richland, WA

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actually, the "non-linearity" is not taken out because the linkage motions to the swash are decidedly non-planar (ala 3d motion) unless the pivots are directly in line with the pivot of the swash. There was a good thread with a lot of math on this topic probably 3-4 years ago... unfortunatly I have lost the link and the maple simulation files related to it.

that being said, consistency is what we all need, not necessarily linearity. your brain is very very good at pushing the stick left to get a certain roll rate,for example, so long as there are no significant changes to that based on other factors... which is why we have swash levelers to help with mixing out interaction if necessary.

The silence often, of pure innocence persuades, when speaking fails

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03-29-2008 10:37 PM  10 years agoPost 13
billrad

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Rineyville, KY

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I have used the linear servo conversion kits in a few cases, for airplanes. Unless there is a redesign, I wouldn't ever use one in a heli.

"EMS Linear Servo Conversion Kit for Futaba S-148 and S-3151 Servos
The Electronic Model Systems Linear Servo Conversion Kit with Ball Bearing for the Futaba S-148 and S-3151 Servos. Awarded U.S. Patent 7,243,562"

The internal setup is more like a curtain rod and string, instead of a completely geared setup, like you might expect.

It does work, and I use it for the elevators on lightly loaded v-tail combat planes. The linear travel geometry keeps the y-shaped pushrod from moving sideways, but nothing more.

billrad

I may not be able to turn a kit into a plane, but I can turn a plane into a kit...

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03-30-2008 05:11 AM  10 years agoPost 14
helicopter34

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New Jersey, exit 82

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Tail servo
I originally thought of this when I was aligning my tail servo. You want to set up your tail servo with your gyro in rate mode so that in hover your tail servo arm is perpendicular to your linkage so that you have the same nonlinearity (sensitivity to a change in servo angle) when you add tail pitch versus substracting pitch. Of course this can be achieved with proper adjustment of your tail linkage length, but I was thinking, I could avoid this if I had a linear servo.

Make it idiot-proof, and someone will make a better idiot

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