# Servo Throws over 90 degrees.



## Pete Thornton (Jan 2, 2008)

Anyone who has converted a live steam engine to radio control has bumped in to the question of how much the throttle servo has to turn the throttle shaft in order to make the engine go properly. In general, most engines will move on 10-20 degrees of shaft movement, but on an uphill line with 6 heavy coaches you may feel that 180 degrees will be required.

Some years ago I converted my Accucraft C-16 and managed to get about 135 degrees of movement (see below.) My Accu EBT #12 had only 90 degrees as I didn't try anything special, though I recall Kevin saying he wanted 180 to be able to give it a big burst of steam when needed. Most live steamers only seem to need about 90 degrees max to function, though you hear that more might be better. With Jason selling longer tapered fine control throttle shafts, you will definitely need to think about the shaft rotation.










My C-16 originally had a regular servo and a long servo arm connected by a wire to the throttle shaft crank, which was shorter than the servo arm. Simple geometry will show that a long arm moving a shorter one will make the short one move further. I think I got about 110 degrees, but it is tough to go further as you run in to the mechanical problems of such arms.
My C-19, on the other hand, had chain drive (1/8" nylon from Servocity.) In theory, this allows you to alter the ratio of turns between the servo and the throttle shaft and thus move the shaft as much as you want. A 12 tooth sprocket on the throttle being turned by an 18 tooth on the servo will give you a 1.5 times advantage (i.e. if the servo turns 90 deg, the throttle turns 135.) 
There are 2 problems - the torque needed is directly proportional to the ratio of the connection (i.e. in the example above, you need 50% more torque than a 1:1 connection,) - and there is a size limit to what can be fitted inside a (model) loco cab. (Followng pic is my EBT#12, but the C-19 was similar.)










I found a 180 degree servo, and replaced the servo in the C-19. As it turned out, my TX (Deltang TX20) was only sending a 75% move command so my 180 degree servo was only moving 135 degrees! However, it was more than adequate. I did research changing the TX to a 100% throttle throw, but it got too complicated and the engine was performing fine, so it was left in that state.

Recently I added a throttle servo to my K4 (another thread, when I get time) and pondered the same issue. I found a bunch of servos on aliexpress that promised 180, 270 or 360 degrees of rotation, for the robot guys I believe. However, when they arrived I could see no advantage - most just turned 90 degrees on my test receiver. One did almost 180 degrees - a Futaba S3003 which was supposed to do 270 (as ordered.) More research was needed.

It turns (!) out that the pulse sent to the servo (from the RX, under command of the TX) needs to be extended to make it turn more. The throw on the servos can sometimes be adjusted at the TX - the Spektrum Dx6i will allow you to request a 150% throw on any channels. At the moment I have not found a TX that can ask for 200%. Most of the robotic guys seem to be using microprocessors and generating servo pulses in software that are as long as they like.


Does anyone know of a TX that can make a servo turn 180 degrees?


_More to come as my musings and research continue. . ._


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## Pete Thornton (Jan 2, 2008)

Here's a bit of research I did on making servos go the distance.


" The standard servo . . . is a 485, which if you look at the specs is a 90 degree servo when supplied the standard pulse range of 1 to 2 msec. The specs also state the 485 can rotate 180 degrees with a pulse range of 0.6 to 2.4 msec."


From other research, it appears the RX sends a pulse of width dictated by the TX. So it's the TX that decides how wide the pulse and thus how far the servo turns.


I did find a 2.4Ghz TX/RX that can be adjusted to 150% throw for a mere $35 (+$12 shipping) but it isn't DSM2 compatible:
https://www.aliexpress.com/item/Fly...ad-48c6-a3fa-830e60ceb2d0&transAbTest=ae803_4


I already have 4 TX so I don't want another.


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## placitassteam (Jan 2, 2008)

Pete, I have a couple of inline range extenders that I bought because I thought I needed them. Turns I run such short trains that I can do without them. I'm not sure how well they work but if you would like one to play with I'll send it to you. PM me an address.


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## Nick Jr (Jan 2, 2008)

I don't know if this helps anyone. I use the smallest and less expensive servo's that will fit the situation. Most of them are not strong enough to fully shut the throttle, and if they do when the loco cools down may be locked closed until heated again. I set my throttle servo to go as slow as possible and then use the directional servo to slowly move from direction of travel to neutral which brings the loco to a stop. I'm happy with the results. LiG.


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## Pete Thornton (Jan 2, 2008)

Further musings.


Tony Walsham at RCS informs me the standard config for his TX range is 100% throw on the throttle and 150% on the other channels. It turns out his reversing channel, #3, is the same hardware as the throttle, so merely by swapping the knobs you get 100% reverser and 150% on the throttle. I have ordered one and am planning to try it.


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## placitassteam (Jan 2, 2008)

Pete, That is good to know as I use Tony's RC. If I ever need the extra travel I will try that. check my PM


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## ktucker8 (Jul 23, 2014)

I use a cheap FlySky FS 6 channel transmitter and Receiver with micro Futaba metal geared servos or Hitec's...These older 2.4ghz are getting harder to find but have the two silver knobs on them left and right top which make it easy to adjust the locomotives speed or direction I have found out. I have it currently set up on my Accucraft Live Steam Allegheny and plan to add it to my Big Boy and a BR 5MT. The Flysky you download a program off the internet to your PC or Laptop and using a HDMI adapter cable from the Transmitter to the PC can adjust your throws to however far you wish with the software.The FlySky is 2.4ghz. around $45 for both radio and receiver. I use the two knobs to control forward reverse and throttle. I also use the same Transmitter to run my electric locomotives with a servo moving my 15 amp Transformer throttle and directional switch PC Hobbies brand. So you can do a search for the 
*FlySKY FS 6CH 2.4G FS-CT6B or
*

eBay item number: 282533145612 or


https://www.ebay.com/itm/FlySKY-FS-...=282533145612483673ff540c4ede9fb6b0cca63b6ffa


Hope this helps. Ken


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## TonyWalsham (Jan 2, 2008)

Hi Peter.
My RCS Live Steam handpieces all use a standard Deltang produced DSM2 compatible Tx2 core module.
They do indeed come standard with Ch # 1 on 100% throw and the rest including the Ch # 3 reverser have 150% throws.
As you said the handpiece itself uses two identical pots with D shafts. So why not swap the knobs around. Simpleness!!!
The two knobs can go on either shaft. The large regulator knob can use Ch # 3 and thus go instantly from 100% throw to 150%. The reverser control is quite comfortable using the 100% throw on Ch # 1.
All I needed to do was make some minor layout changes to the hand-piece label and the hand-piece went from this;










To this;











All you need to do in the loco is move the regulator servo lead from Ch # 1 to Ch # 3.
The reverser servo goes from Ch # 3 to Ch # 1 on the Rx.



They are now available at the same price as before.


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## Pete Thornton (Jan 2, 2008)

ktucker8 said:


> I use a cheap FlySky FS 6 channel transmitter and Receiver with micro Futaba metal geared servos or Hitec's...These older 2.4ghz are getting harder to find but have the two silver knobs on them left and right top which make it easy to adjust the locomotives speed or direction I have found out. I have it currently set up on my Accucraft Live Steam Allegheny and plan to add it to my Big Boy and a BR 5MT. The Flysky you download a program off the internet to your PC or Laptop and using a HDMI adapter cable from the Transmitter to the PC can adjust your throws to however far you wish with the software.The FlySky is 2.4ghz. around $45 for both radio and receiver. I use the two knobs to control forward reverse and throttle. I also use the same Transmitter to run my electric locomotives with a servo moving my 15 amp Transformer throttle and directional switch PC Hobbies brand. So you can do a search for the
> *FlySKY FS 6CH 2.4G FS-CT6B or
> *
> 
> ...



The Flysky FS i6 2,4G 6 ch tx/rx is available on Aliexpress for $35-47.
https://www.aliexpress.com/item/Flysky-FS-i6-FS-I6-2-4G-6CH-AFHDS-RC-Transmitter-Controller-w-FS-iA6-FS/32702661508.html
















I have looked it it, and it does allow the rthrow end points to be adjusted (though I have not found anything that tells me how much they can be increased!):











However, it is not DSM2 compatible, and I have a lot of DSM2 equipment that all plays nicely together, (not to mention a bunch of weird stuff that doesn't, like a Code RC system in one UK loco,) so I don't want yet another strange system


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## Pete Thornton (Jan 2, 2008)

Maybe I do need one of these. Poking around Aliexpress, I found this RX:










11 * 25MM which makes it <1/2" x 1". Hmmm . . . 



(Btw - watch the shipping charges for inexpensive items on ALiexpress - and eBay - as the vendors are getting sneaky.)


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## Pete Thornton (Jan 2, 2008)

Winn lent me a Servocity "servo doubler" which seems to work. Here's a video of my first set of tests. I did some more which I will post later today.


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## Pete Thornton (Jan 2, 2008)

Second tests. Surprising results from the 180 degree servo. Clearly the RCS TX3 on 150% works as advertized, and the Servo-doubler also works.








Next is a Hitec HS81 that is on the way. It's rated for 162 degrees max.


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## placitassteam (Jan 2, 2008)

Very interesting. That would not work very well with a push rod but would be great with chain and sprocket.


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## llynrice (Jan 2, 2008)

I have solved the problem two different ways.

First, I always use chain and sprockets to drive the throttle. It's easy to use a ratio which increases the throttle shaft rotation beyond 90° while allowing the servo to turn the default 90°.

Second, I always buy digital programmable servos. I bought a programmer which will change how far the servo can rotate, change the direction of rotation and other lesser characteristics. When I have connected a servo to a reversing lever, I'll often reprogram it to reduce the throw so that the lever won't go too far at either end of its travel.


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## TonyWalsham (Jan 2, 2008)

Hello Llyn.

Seeing more torque is required step up the ratio of chain and sprocket, what size servo would you recommend for the extra load?

What sort of money do digital servos cost compared to say Hi-Tec HS82MG.
It might be as cheap or perhaps even cheaper, than the cost of a regular servo and an extender/reverser add on.

Follow up.
I have since done a bit of research with the Hi-Tec distributor here in Australia.
The Digital versions of the Hi-Tec HS65MG and HS85MG are just 10% more expensive than the non Digital.
I can supply the field programmers to Australian Live Steamers for a bit more than the cost of a Hi-Tec digital servo.


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## llynrice (Jan 2, 2008)

Hi Tony,

Depending on how stiffly the throttle shaft turns, the digital equivalent of the HS85 can be enough. I often play it safe and use something from the "Mini Series" such as the HS5245. I have one loco with such a stiff throttle that I used metal sprockets and chain and an even larger servo. But that was the exception.

All of the solutions discussed in this thread have merit. I've settled on using programmable digital servos for their flexibility. Also, I always use sprockets and chain for throttles because the ratio of angular travel between the servo and the throttle shaft remains constant over the full range of travel.

Llyn


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## TonyWalsham (Jan 2, 2008)

Thanks Llyn.


There is a fair bit of development work going on at the moment that will revolutionise the R/C Tx, Rx and servo relationship.
Sadly I cannot talk about it yet, but it certainly will be interesting.


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## Pete Thornton (Jan 2, 2008)

I just ran a field test of my new AML K4 with throttle servo. It doesn't need more than the 90-ish degrees. At rest, the servo is at midpoint ( as in this photo) and the zero position lets me slow it to a stop - hopefully downhill!
















(Even better, it runs nicely at this throttle setting, so if I turn off the TX it continues to run at a nice speed. See video


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## Pete Thornton (Jan 2, 2008)

Final set of tests, on a Hitec servo and a Spektrum TX/RX. Same results.


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## Kovacjr (Jan 2, 2008)

Nice to see that it actually works. I bought one a long time ago and it did nothing for the servo throw but make it buzz at its ends during testing. Was a HS81


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