Large Scale Central

Servo Turnouts

Electronics
#1

I am going to use servos in my N-Scale layout, just bought 10 SG90 servos.

The idea struck me that the size of this servo just might fit the LBG switch machine and it dose.

Also if you eliminate the switch machine mechanism but retain the shaft with the magnet inside you can press fit the shaft pivot pin into the servo hole for the bell crank mounting screw, you would have to cut off the servo mounting tab opposite side of the wire entry into the servo body.

I have not tried to operate it yet, just a though to get the turnouts to look more prototypical in operation.

Looked at enlarging the two holes for where you put the wires and elongate it to pass the servo cable and connector, then seal it with RTV.

#2

Dennis, Dave Bodner has used servos as switch machines on his HO module. It does seam to be a good way of throwing switches, because his switches donā€™t slam the points over, but they move nice and slow.

Dave also has an article on how to control servo switch machines.

http://trainelectronics.com/Servo-simple-controller/index.htm

It would be interesting to see what you come up with.

#3

I use servos on my Z scale stuff, but you have issues sensing when to stop, you really want a controller that not only stores the limits you want, but ALSO senses the current increase and realize it has hit the limit.

Without these features, you risk damage. On Z scale, you destroy turnouts, the throwbar and break the points.

In G scale, you can probably tolerate a lot more ā€œovertravelā€ but the servo ā€œeatsā€ current if you are driving it beyond where it is already physically stopped.

Turnouts are a special case, since you want it to stop with just enough pressure to close the points but not so much that you are drawing a lot of current or possibly damaging the turnout.

Sounds easy, but becomes more complex quickly when you consider everything.

Greg

#4

Controlling servos for model railroading has come a long way.

There are many Arduino products and servo boards up to 16 servos that can be controlled on one board.

The software on some has the ability to program the limits on each servo, down to one step, and retain those settings, even be able to go back and adjust them if needed.

Got to get the other parts in and see if this will work.

#5

It would seem to me with an Arduino that you could also cut off power to the servo once you have completed the move.

#6

Michael Moradzadeh said:

It would seem to me with an Arduino that you could also cut off power to the servo once you have completed the move.

Thatā€™s what I do in my locomotives (remote coupler servos). Just put a power MOSFET in the ground line and tie the gate to a pin on the Arduino. The reason I do it on the locos is that the servos chatter when the traction motors are generating noise. Probably not a concern with turnouts. Of course it also saves power.

#7

Just watched a video yesterday and answered one of my questions about servo limits. A servo uses a potentiometer feedback to tell the position. Well the value of the Pots are all not equal and you really have to test each servo to find the limits. that is if you are using the full 180 degree range. Also will help you find the true 90 degree center and add that to your sketch for each servo.

A simple sketch in putting in the step value will let you know the limits.

It is a 23 minute video with a lot of information, at about 12 minutes he explains the servo testing.

#8

A couple of years ago at York, I think it was Cliff Jennings had a prototype of a servo-based switch machine at the Drag n Brag.

#9

I have two turnouts I am considering using a servo on. this would eliminate a constant duck under situation, especially when operating alone. looking forward to some recommendations.

Al P.

#10

Iā€™m using a home made magnetic clutch on my waterproof outdoor servos. It lets me have a bit of ā€˜slopā€™ in the final positions.

They make them for the little servos:

https://robosavvy.com/store/dagu-magnetic-servo-clutches-for-miniature-servos.html

perhaps they would help

#11

Martin Sant said:

Iā€™m using a home made magnetic clutch on my waterproof outdoor servos. It lets me have a bit of ā€˜slopā€™ in the final positions.

They make them for the little servos:

https://robosavvy.com/store/dagu-magnetic-servo-clutches-for-miniature-servos.html

perhaps they would help

Ive been pondering how to make a ā€œworks like the prototypeā€ switch machine for outdoors. Iā€™d like the ability to throw it remotely, electronically, but then also allow local operators to throw it via a ground throw (not pushbuttons, this would be trivial, and has been solved already). Ive looked at the idea of a magnetic clutch or two to do this.

#12

Yes, mine has a handle for manual operation.

Iā€™ve since changed the design slightly and use a bell crank so I can rotate the servo 90 degrees. That lets the handle move right to left instead of in and out toward the track.

#13

Neat. Iā€™d like to find a small electromechanical clutch so I can turn on and off local control. I wonder if the Dagu ones could be modified slightly. Hrmā€¦

1 Like
#14

I see one problem, is maintaining pressure to keep the switch points hard against the stock rails.

I donā€™t see how this is done without the servo motor using current to keep up this pressure. I guess if there was a worm drive somewhere it could work.

Can someone explain how a servo can maintain pressure without consuming current?

Greg

#15

I was pondering either using a z spring like I do with the barrel bolts, and just over throw a bit. Possibly a magnetic or spring servo saver would work too. Magnetic ones may do the same thing. Not sure, I havenā€™t got beyond the thought stage myself.

1 Like
#16

Are you guys using (or proposing to use) servos outdoors? Iā€™m using Tortoises for my indoor turnouts, but starting to consider my outdoor options. I like the idea of servos, but not sure about weatherproofing them.

#17

Bob McCown said:

A couple of years ago at York, I think it was Cliff Jennings had a prototype of a servo-based switch machine at the Drag n Brag.

Yep, and thanks Bob!

It was a heavy-duty servo (not the 9g), in a sealed container. I was trying to look that thread up, but Bob, it seems none of my buttons are working. Links yes, buttons nyet.

Those 9g servos are impressive though, for what they do in their size. The Train Li switch machines use them (I have a bunch).

https://www.trainli.com/switch-drives-stands-86/tl10-20004-p-312

The drive design isnā€™t very robust though; and the units are not sealed. Thatā€™s why I went on my own design journey. But after that York show 2 years ago, Iā€™ve not done much else on the topic. But best wishes Dennis with your endeavor, and thanks for the nice product links.

Cliff

#18

These are what Iā€™m experimenting with.

https://hobbyking.com/en_us/hobbykingtm-waterproof-analog-servo-3-2kg-0-17sec-40g.html

#19

Martin Sant said:

These are what Iā€™m experimenting with.

https://hobbyking.com/en_us/hobbykingtm-waterproof-analog-servo-3-2kg-0-17sec-40g.html

I like this unit, got to buy a few.

#20

Well I built one this week using an old LGB switch machine that seen better days, the coil assembly was rusted and had all the other parts.

It went together as I planned it and set the servo to center position. Still did not get my servo boards in so took it to the club yesterday and showed it a member that is developing a sketch using I2c interface for the servos and OLED display.

He reported back it ran the first time powering it up.

I am using just the normal SG90 servos I bought for $1.49 each with free shipping. USA dealer.

Need to buy a few of the water proof ones.

When I get it back next week will take photos and show the modification needed, was fairly simple to do.