Servos and DCC

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Dr Paul Willis
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Servos and DCC

Postby Dr Paul Willis » Sat Aug 22, 2020 1:24 am

Starting a new layout and wanting to power the points by servos hooked up to an Arduino. The points (C&L kits) require wiring for a live frog and the layout is intended for DCC. Anyone know of a wiring diagram for how to set up servo point control on a DCC layout?

nigelcliffe
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Re: Servos and DCC

Postby nigelcliffe » Sat Aug 22, 2020 7:32 am

Either switch the frog using a microswitch which is operated by the mechanical movement of the servo, or fit a relay controlled by the Arduino on a second output. Or, if the wiring isn't excessive, the frog could be switched from a control panel using another pole on the switch which controls the turnout movement.

A wiring diagram requires more information on intentions and setup. Just saying "servo and Arduino" isn't enough detail.


There isn't any difference between DC and DCC wiring of turnouts.

Various people have reported noise issues with Servos and model railways - electrical noise on the rails causes the servo to "twitch". Several designs have been proposed to deal with it - pull-up or pull-down resistors on the servo signal output, or opto-isolator between servo signal generator and the servo motor itself. Whether this problem is seen seems somewhat random. Not having long control wires from the servo signal generator and the servo motor will help.


- Nigel

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John Donnelly
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Re: Servos and DCC

Postby John Donnelly » Sat Aug 22, 2020 9:16 am

On my, under construction, layout, I'm also using servos albeit controlled via MERG Servo4 boards. In an effort to avoid having even more wires going back to the panel where the switches are, I've decided to use Gaugemaster Frog Juicers to switch the polarity, mounted under the baseboards next to the frog to minimise the wiring...

John

DougN
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Re: Servos and DCC

Postby DougN » Sat Aug 22, 2020 9:33 am

I'm with John. The gaugemaster frog juicers are relatively cheap. I am using these specifically for a single single slip as I have heard these are more difficult to wire due to switching not required for certain moves. I have gone back to using analogue control for switching but DCC for train moves. I must do more on my P4 layout as it has been on the back burner for about 12 months or more!
Doug
Still not doing enough modelling

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grovenor-2685
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Re: Servos and DCC

Postby grovenor-2685 » Sat Aug 22, 2020 10:46 am

I use servos with MERG Servo4 units and have for years now. Used as intended I have not had twitching problems. From what I have seen those problems are more likely on DC layouts than DCC.I have used microswitches for frog switching in a few cases but prefer to use relays. The chear chinese relay modules from ebay can be operated in parallel with the Servo4 and have very robust 10 Amp rated contacts
Are you specially wedded to using an Arduino?
Regards
Keith
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Dr Paul Willis
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Re: Servos and DCC

Postby Dr Paul Willis » Sun Aug 23, 2020 5:37 am

Thanks for all these comments guys, most constructive!

No, I'm not wedded to an Arduino, and the suggestions around microswitches or frog juicers sound like a way forward.

I'll keep you posted as to how I go and welcome any other input to this project.

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grovenor-2685
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Re: Servos and DCC

Postby grovenor-2685 » Sun Aug 23, 2020 9:19 am

From what I've seen on MERG using Arduinos seems to very quickly become a hobby in itself. I'm pretty sure there are Arduino sketches out there for operating Servos so if you can resist the Arduino take over might bbe worth a try. On the other hand MERG have servo driver kits optimised for operation of points in a variety of sizes from single point drivers (Ezypoint), 4 point drivers (Servo4), 8 point drivers (via CBUS, not standalone) and 12 point drivers.
Regards
Keith
Grovenor Sidings

davebradwell
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Re: Servos and DCC

Postby davebradwell » Sun Aug 23, 2020 3:10 pm

There just seems to be something wrong or even unethical about a frog-juicer causing a short circuit in order to avoid one so for that reason alone I prefer relays or microswitches. You're putting a spike on the power supply too, for what it's worth. I'll accept that they work very well and a friend has one on his turntable deck feed.

A servo is a sophisticated drive that can be stopped accurately anywhere in its travel so it would normally seem inappropriate to use one just to bang a tiebar back and forth. The reason for their popularity is, I suppose, low price - something that would otherwise be more appropriate costs more or hasn't been developed to be as convenient. The big question for anyone choosing this path is can you deal with it if it goes wrong?

Single slip wiring is straightforward - tiebar at one end switches crossing at other.

This all ends up at a control panel and I suggest you will find that anything not having the concept of a visual "normal" as in a lever frame or modern equivlent will likely lead to repeated human errors. This an area that is rarely done well on models except by copying the prototype very closely. I have a geographical panel in place of hand levers in my NCB yard and operators are always going the wrong way. Imitation of lever frame with toggle switches on BR side works much better as there is a "normal" (back) position when all is safe.

DaveB

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John Donnelly
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Re: Servos and DCC

Postby John Donnelly » Mon Sep 07, 2020 5:18 pm

davebradwell wrote:There just seems to be something wrong or even unethical about a frog-juicer causing a short circuit in order to avoid one so for that reason alone I prefer relays or microswitches. You're putting a spike on the power supply too, for what it's worth.


I'll be honest, despite the fact that I've decided to use them, I do have the same misgivings. So far, in testing, they have worked flawlessly and while I do appreciate it that a short circuit is being created, there is no sparking from the track or wheels and the juicer does it's thing before the DCC system picks up the short. I'll so how I get on and, if push comes to shove, installation of micro switches will be an option as I'm surface mounting the servos so they are easily accessible to mount the switches to.

John

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Allan Goodwillie
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Re: Servos and DCC

Postby Allan Goodwillie » Sat Sep 26, 2020 9:15 am

I will say first of all that I am not an expert in any way on the subject although I am using Merg servos and their simpler individual boards for controlling my points on the new layout, but I thought I might mention that on the Alloa layout when they went over to using servos for points and signals they had problems with twitching-annoying when signals were involved but even more so with points.

I was told that it was the locomotive motors going past too close to the electronic boards that was the cause, so sighting them away from the tracks is probably something to watch out for. I am trying to do this on my own layout and it seems to be OK so far. The reason I chose the simpler type of board was that I could show my son how to adjust them as all you need is a screwdriver, you do not need any computer skills, or a computer for that matter when at a show. We managed to get caught out at a show with John Stocks layout when the servo 4 boards lost their programming and he had not brought his computer along. ( at that time the small reprogramming gismo now available had not been introduced.)

Similarly when we went over to servos for the signals on Burntisland we were always having to reset the programming each time we were away - and that required having someone who knew what they were doing and redesigning the signal plugs so that they could not be put into the wrong sockets when setting up - a different set of problems. All part of the fun! :D :? :cry: :D :!:

Allan

CeeJay60
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Re: Servos and DCC

Postby CeeJay60 » Sun Sep 27, 2020 5:11 pm

Paul,
I'm installing servos on the layout I'm constructing. I've chosen to go with the MegaPoints Controllers system for servo control from multiple control panels (which have not yet been built). Remote set up of the MegaPoints servo controllers is just a plug in cable and four pushbuttons - I made my own, but the MegaPoints version is not expensive.

I've not suffered from it, but my understanding is that "Servo twitch" can occur when the servo control wire acts as an aerial and picks up induced electrical noise that affects the servo position signal, most likely from older noisy motors in locos. Keep the wires between the servo and the control board as short as practicable, and if you do have to cross below the rails, or cross your track power bus wires, try to make the crossing perpendicular.

I find that a little bit of "overdrive" on the servo end positions is a good thing, taken up by some flex in the mechanical connection to the point tie bar - an omega loop, some flex in the spring operating wire, a torsion bar, or whatever.

My engineering sensibilities are offended by the idea of frog juicers allowing a short circuit to occur so that it can be switched out, and the juicer must work faster than your DCC power district cut outs. So I'm another one who prefers to switch the crossing vee by other means.

I've tried micro switches, but I find it annoyingly difficult to set both servo end points and micro switch trigger points in the same adjustment, although that can be resolved by choosing an appropriate servo mount (at a cost, of course). I've got a lot of them to do, so I'm looking for a more cost effective solution.

Instead, I came up with an infra red position detector and relay switch solution - see the latest MERG Journal for my write up. I'm now installing numbers of these as my layout build progresses, local to the points. Magnetic position detection to switch a relay is equally feasible, and if you really wanted to avoid electro-mechanical relays, power MOSFETs or similar electronic wizardry could certainly do the job.

To my mind, an Arduino or Raspberry Pi computer solution for servo control is the proverbial sledgehammer to crack a nut - a simple PIC is at the heart of all the servo controllers I am aware of, and is all that is needed.

As I said, I am using the MegaPoints Controllers system, which has a DCC interface so that DCC commands can be used to switch the points, or if the DCC system is hooked up to a computer running JMRI or similar, DCC commands from the computer. I've just realised that I haven't tried point control from the WiThrottle app on my iPhone ...

Other layout control bus systems are available which can interface between DCC (or DC) systems and servo control systems - MERG CBUS comes to mind, of course, although I haven't tried it myself (yet?).

Hope this helps.
Cheers,
Colin

I promise I'll get some of it right some of the time!

Tony Wilkins
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Re: Servos and DCC

Postby Tony Wilkins » Tue Sep 29, 2020 12:41 pm

Frog juicers always seemed to me like a technical solution to a non problem. Even the very name makes me cringe. Oh well, this seems to be the age we live in. Throw technology at it.
On the EMAG test track, we used Peco lectric servo boards to control some of the turnouts and these came with a relay add on which could be used to switch the crossing polarity. Personally I prefer a proper point motor to switch the blades and use the internal switches to control crossing polarities and the like. Where additional contacts are needed external relays can be added. I may consider using servos to control some of the signals when I get that far as I like the capability of programmable bounce.
Regards
Tony.


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