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Adding R/C to a K-4

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#23 ·
Carl,

I’m very familiar with the Hitec servos you mention…. The maximum servo angle or travel arc the HS-81 and most other servos can work with is 120 degrees and or sixty degrees either side of center, there are exceptions… Transmitters are not capable of overdriving a servo much past the aforementioned 120 degree total servo angle.

You mentioned “mechanical advantage” in error I believe as this is a value realized or not specifically of the linkage and or gear ratio in-play.

Think of your gears pitch radius as levers, essentially that’s all they are and they abide by the same math as lever arms. There is NO benefit that I can factor gears verse levers, i.e., travel volume arcs its all the same… No matter how you get there your limited by the servo angle factored and or realized.

Michael
 
#24 ·
Posted By steveciambrone on 01 Apr 2012 04:38 PM
A lever that is 1/2" long connected via a pushrod to a servo arm that is 1" long will approximittely double your travel.

Steve

Steve,

“Approximately” validates your assertion IMO, in a perfect world it works as noted. Trains are not perfect and the specific linkage geometry in play skews the net results. Linkage binding more often than not plays a factor in reducing the calculated values and there is miss-matched linkage arms, vertical height, horizontal and depth offsets of the servo axis and throttles needle, as well as the distance between the servo axis and throttle needle axis. Binding can be mitigated with some attention to linkage arm lengths, pushrod length (as you note) and the servos neutral angle.

Michael
 
#25 ·
Posted By Greg Elmassian on 01 Apr 2012 10:37 PM
since RJ has a D6i, maybe the thing to buy is a "stretcher"...

Has anyone used one of these?

http://servocity.com/html/180o_servo_stretcher.html

Greg
Greg,

I haven't played with the one you note, but have tested many similar products in the past. They seem to work quite nicely. Some of these ancilalry gadgets offer lots more features than others, the one you link to is a simple offerring.

Michael
 
Discussion starter · #26 ·
Thanks for all the replys. For one Greg left out that the transmitter is a DX6i not a D6i. I have tried to adjust the trow of the Servo as it is suppose to have a max travel of 180 but I barely get 90 degrees. I have tried to adjust but can not get it to travel any further. The servos are hyteck HS-82MG. If I can gain a little more throw and yes redoing the linkage I think I'll get enough travel on the throttle to run. For now the best movement I get is from about 12 o'clock to 3 o'clock. Any other pointers will be appreciated. Later RJD
 
#27 ·
With a linkage, of course, the most rotation you can get on the throttle approaches 180 degrees, in reality it will be less than that; perhaps 150 degrees. It does not matter what the rotation of the servo is. if you need more rotation on the throttle or more you will need the chain and sprocket system.

Of course none of my engines seem to need more than about 90 degrees of rotation, which is easy enough. I can easily move the link from one set of holes in the servo horn (or the throttle horn) to the next to tweek the rotation as needed. It may also not be necessary for your engine to have the throttle completely close or completely open. you can stop the engine by shifting the Johnson bar to neutral once it has slowed down a bit.
 
#28 ·
RJD,
With the Dx6i transmitter, servo throw adjustments are done at both ends of the control stick. Full up AND full down position, or full left AND full right positions. You may already know this, but I'm just making sure. You should be able to get over 180 deg throw if adjusted properly.
 
#29 ·
In your example the K28 36/16 nets a linkage ratio of 2.25:1. Which equates to a 1" servo arm and a .444" needle arm 1/.444 = 2.25:1.
Michael,

While I can't fault your math, I'd love to know how that 1" servo arm turns the throttle through 270 degrees!
 
#30 ·
Posted By Phippsburg Eric on 02 Apr 2012 06:24 AM
With a linkage, of course, the most rotation you can get on the throttle approaches 180 degrees, in reality it will be less than that; perhaps 150 degrees. It does not matter what the rotation of the servo is. if you need more rotation on the throttle or more you will need the chain and sprocket system.

Of course none of my engines seem to need more than about 90 degrees of rotation, which is easy enough. I can easily move the link from one set of holes in the servo horn (or the throttle horn) to the next to tweek the rotation as needed. It may also not be necessary for your engine to have the throttle completely close or completely open. you can stop the engine by shifting the Johnson bar to neutral once it has slowed down a bit.



Eric,

You suggested "it does not matter what the rotation of the servo is" if the servo angle is less than 120 degrees total you'll never see the 150 degrees you suggest is "reality" of which I concurr. 180 degreees maybe plausible in some circumstance. I rigged up some servos arms attached to a piece of plywood earlier with photo copied protractors fixed on the axis of each lever with a horizontal offset of 2". I noted 160 degrees output with 120 degrees input with a 5/8" servo arm and a 1/2" driven output arm before binding occured. I tried other arm lenghts too, the 5/8"-1/2" seemed to be the best combination.

Michael
 
#31 ·
Posted By Pete Thornton on 02 Apr 2012 09:06 AM
In your example the K28 36/16 nets a linkage ratio of 2.25:1. Which equates to a 1" servo arm and a .444" needle arm 1/.444 = 2.25:1.
Michael,

While I can't fault your math, I'd love to know how that 1" servo arm turns the throttle through 270 degrees!
Pete,

All right then I see the light, while the math supports the 2.25 ratio it does not take into consideration the binding that occurs once the driven arm rotates over center due to the dissimilar arm lengths. The chain simply goes on by unloading at half the diameter of the sprocket while the arm is still hard connected. To that end 270 degrees rotation of the throttle needle is available with 120 degrees servo angle with sprockets.

Michael
 
#32 ·
Posted By aceinspp on 02 Apr 2012 06:02 AM
Thanks for all the replys. For one Greg left out that the transmitter is a DX6i not a D6i. I have tried to adjust the trow of the Servo as it is suppose to have a max travel of 180 but I barely get 90 degrees. I have tried to adjust but can not get it to travel any further. The servos are hyteck HS-82MG. If I can gain a little more throw and yes redoing the linkage I think I'll get enough travel on the throttle to run. For now the best movement I get is from about 12 o'clock to 3 o'clock. Any other pointers will be appreciated. Later RJD

RJ,

The Hitec HS-82MG is capable of swinging a travel arc of 120 degrees or servo angle out of the box. Sounds like you are not driving the servo to its maximum travel arc. In my previous post I suggested how I did a real world test evealuation with servo arms or levers, 120 degrees input netted 160 degrees output with 5/8" and 1/2" arms.

Michael
 
#33 ·
The servo is mechanically capable of 120 degrees, but most transmitters do not put out a pulse that yields more than about 80 degrees...

The specs on that servo state both the max angle, and the angle per pulse...

Carl Weaver's post has the info... although what you really want is the spec of the length of the pulse in microseconds that your transmitter puts out, then you can see what angle you get.

If you read the specs on this page for the servo: http://servocity.com/htm...trong>


It states the pulse signals required for desired angles.... to get the full mechanical travel, you need a transmitter that goes from 600 microseconds to 2400 microseconds... as stated right on that page, your transmitter might not do this.

Michael, you did state that the servo "stretcher" I found was pretty basic. While I think it will work, could you provide me with some links or brand name and models of the more advanced ones you have tried? Very interesting stuff, and I'd like to learn some more.


Thanks, Greg
 
#34 ·
Posted By Greg Elmassian on 02 Apr 2012 11:22 AM
The servo is mechanically capable of 120 degrees, but most transmitters do not put out a pulse that yields more than about 80 degrees...

The specs on that servo state both the max angle, and the angle per pulse...

Greg
Greg,

Most "LOW END Transmitters" suffer from the limits of the pulse train code as you suggest, in fact most spec 100% travel volume which nets the 80 degrees servo angle as their benchmark. Typically transmitters operate with 1200ms bandwidth, some Transimitters can provide up to about 1300ms. This OEM specific anomally wreaked havoc with OEM compatibilty years ago when Hitec's programmble digital servos were designed to work with the aforementioned 1200ms pulse code bandwith limit. The net result caused the programmable digital amplifiers to "REBOOT", this was NOT desirable in flight......

Michael
 
#35 ·
What I am trying to say is that a linkage cannot reliably drive the throttle more than 180 degrees (probably 150 or possibly 160 degrees) because it cannot push the throttle around farther than that because the angles will not allow it to happen. Even if you could rotate it 180 degrees, from the farthest "open" point the throttle would have a random choice whether to close again or just open farther. you can have this problem in any case if the link between the servo and throttle arm line up with the servo arm at any point in its range of motion. Ideally you want the servo and throttle arms to be working more or less parallel to each other and perpendicular to the link between them (in the middle of the range of motion). If the servo tries to push the throttle too far it will stall or break something.


If the angle between the throttle arm and the link get small you will not provide as much torque to turn the throttle. if you are looking for angles of 120 or more you may get into trouble pretty easily.


If you need more rotation on the throttle than the servo produces, you can lengthen the arm on the servo or shorten the arm on the throttle until you are in the 160 degree range (on the throttle arm), beyond that you will have to use sprockets and chain. use the big sprocket on the servo and a smaller one on the throttle. if you need more torque to drive that sprocket, get a more powerful servo...they come in all sizes and torques and operation angles. For my model sailboats I use a very hi torque servo (as a sail winch) with seven or so turns from full in to full out.
 
#36 ·
So, the Spectrum series is a "low end" system in this perspective? Interesting. To me, and I certainly am not an expert in this realm, 100% "travel volume" when it only controls about 50% of the servo capability seems weird.

But, I guess maybe from a historical point of view, maybe servos used to only have about 80 degrees of travel, so maybe a while ago that would be viewed as 100%... (man, enough maybe's in that sentence? )

"growing" the capability of a system to greater performance / features can sometimes be painful.. yes, I'm not happy when my phone reboots mid call, a reboot mid flight must have been pretty nasty!

Regards, Greg
 
#37 ·
Eric,

Seems you have an excellent understanding of servos and linkages… I’ve been playing with this stuff for many years with all things radio controlled. Most modelers fall short in understanding the basics and more specifically when it comes to calculating torque values with lever or arm changes and not to mention when we get into precision set-ups desired of precision aerobatic models. The most common problems are realized by using the transmitter to correct bad linkage geometry which creates offsets in equal stick movement as compared to equal movement or travel in either direction of the TX stick and the control surfaces.

To clarify for others, servo torque ratings are specified with 1” arms. A smaller arm will provide a higher torque value at the servo axis. The linkage ratio will dictate the torque at the control surface axis or in the case of steam engines throttle needle axis.

Michael
 
#38 ·
Greg,

Yeap, Spektrum equipment is NOT top of the line stuff….. Many believe Spektrum is a JR Radio sub-product, not! That said JR like other OEM’s offer low end equipment too! The 100% travel volume benchmark is typical, many but not all TX’s are able to be trimmed or programmed to deliver more than 100% travel volume and still work within the 1200ms pulse train code limit mentioned previously.

The 100%/80 degree servo angle or travel volume mitigates over driving servos and allows for some headroom with the typical travel volume offsets realized with bad linkage geometry and TX programming, hereto with the TX trims, sub-trims, programmable mixes, conditions and more.

I never had a problem with the aforementioned “reboot” unfortunately many lost expensive models when combining Hitec’s programmable digital servos with other OEM receivers and transmitters. I always take the time to set-up the model linkage correctly and never use the TX to accommodate poor set-ups. Had Hitec had the foresight to test and evaluate other OEM equipment this phenomena would have never seen the light of day. Of course if there was such a thing as “standards” in the RC industry this would have been a non issue too. After the fact Hitec reprogrammed their amplifiers to recognize control signals outside the 1200ms base of theirs and may others equipment offerings.

Michael
 
#39 ·
Greg,

Find below devices that offer additional features as comapred to the aforemetioned "stretcher". There used to be more devices, some were simply inline devices with “trimmers” others were more in line with what you linked to with single servo capabilities. To be honest I haven’t paid much attention to the RC Industry for several years, I used to live and breathe this stuff, accordingly I may step on my toes when I comment on things it seems I have forgot a lot more than I recall of late.

I did note the info Servo City is providing suggests Hitec’s Analog servos are now able to realize 180 degrees total servo angle with the proper pulse train out of the box, woohoo….


Ganging multiple servos is a more common need in todays giant scale aero modeling.

http://www.horizonhobby.com/product...em-JRPA900
http://www.horizonhobby.com/products/matchmaker-JRPA915
http://www.smart-fly.com/products/EqualizerII/equalizer.htm
http://www.futaba-rc.com/accessories/futm4155.html
http://www.troybuiltmodels.com/ns/manuals/wrcservomatch_.pdf

These allow all of the above and more:
http://shop.rc-electronic.com/e-vendo.php?shop=k_emcotec_e&SessionId=&a=catalog&t=6&c=6&p=6
http://www.powerbox-systems.com/e/powerbox_systeme/produktuebersicht/start.php
http://www.min.at/prinz/oe1rib/I2C-Servo/
http://www.pololu.com/blog/17/servo-control-interface-in-detail

Michael
 
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