Arduino Controlled Loco Traverser

[BG John]

Not really sure where to put this topic, but as there's an Arduino involved, I thought Computer Control was probably best.

 

I need to convert, or rebuild, this hand operated traverser to motor driven.

 

 

I've just dismantled a DVD drive, and the disk drawer mechanism seems ideal for the purpose. It's the right size to fit under the baseboard, just a little narrower than the length of the traverser, and it looks as though I can just cut a hole in the baseboard and mount the existing traverser deck on top of it. The travel is more than double what I need, but I don't see that's a problem. It has a belt drive to gears that drive a rack moulded into the drawer.

 

 

This is the motor. It has two wires, so is presumably a simple DC motor. There seems to be a limit switch that I think reverses the direction. I think it runs too fast to safely move a 4mm scale loco, and would need new stop switches built in.

 

 

I'm wondering about replacing it with a stepper motor. I considered a continuous servo, but they're several times the price of a 28BYJ-48 stepper motor and controller, that seems to be the usual one used with Arduinos. The shaft is a lot bigger, so the old pulley wheel won't fit, but I'm sure I can get round that. I also wonder if the belt is too flimsy.

 

 

Does this seem like a workable idea? It's simpler than other options I've considered, where I'd have to make a new carriage for the traverser, as this one is ready made.

 

I also thought the drive for the laser would be a possibility, as it's a nice solid unit driven by a linear stepper motor, but it doesn't have enough travel. It might be enough for a standard gauge traverser, but not for this broad gauge one. I'll find a use for it later!

[Robin2]

Does the existing traverser work well physically - would it be sufficient to motorize it?

 

I don't understand "I considered a continuous servo, but they're several times the price of a 28BYJ-48 stepper motor and controller,". I only spend about £4 on servos and almost all of them can be converted to continuous rotation.

 

...R

[BG John]

The existing traverser doesn't work very well. It slides on a strip of Tufnol that isn't that good for hand operation. I was going to rebuild it for the originally planned hand operation anyway.

 

There are 28BYJ-48 stepper motors with controller on eBay from £1.25, from China. It's not worth converting a servo at that price. They also have screw fixing holes built in, that mean I just need to drill two holes in the plastic to attach one.

 

I've got the drawer off now, and can see the gears clearly. If I can find the right size gear to fit on the stepper motor, I think it should be straightforward to fit it with an all gear drive, so I won't need the belt.

 

What are the practical differences between a stepper motor and a continuous servo? It looks as though I may have a couple of small turntables to build too, and a direct drive with a 28BYJ-48 seems easy.

[BG John]

Good point. I'll try that later.

[Gordon H]

The existing traverser doesn't work very well. It slides on a strip of Tufnol that isn't that good for hand operation. I was going to rebuild it for the originally planned hand operation anyway.

 

There are 28BYJ-48 stepper motors with controller on eBay from £1.25, from China. It's not worth converting a servo at that price. They also have screw fixing holes built in, that mean I just need to drill two holes in the plastic to attach one.

 

I've got the drawer off now, and can see the gears clearly. If I can find the right size gear to fit on the stepper motor, I think it should be straightforward to fit it with an all gear drive, so I won't need the belt.

 

What are the practical differences between a stepper motor and a continuous servo? It looks as though I may have a couple of small turntables to build too, and a direct drive with a 28BYJ-48 seems easy.

 

A stepper motor will move the exact distance you want it to, simply by the number of steps you give it from the controller - assuming no slippage or missing steps due to mechanical problems. Their limiting factors are the step angular resolution and torque they can provide.

A simple continuously rotating servo in this context is little more than a geared down motor. Unless you can contrive a way to count the number of turns it makes, and what its angle is within each turn, you can't really use it as a position servo in the conventional sense. All it can really do is hit endstops.

Also bear in mind that the more gears etc. you add to the overall mechanism, the greater the backlash through it all, and consequent inaccuracies at the critical interface, i.e. where the rails are meant to line up consistently and repeatably.

[Robin2]

A friend of mine has had trouble getting a 28BYJ motor to line up a turntable accurately because of backlash in the gears. His turntable (N-gauge wagon turntable) is now working very well with a small servo.

 

However I have never tried one of those stepper motors myself.

 

From looking at your pictures, and assuming you still plan just a two-road traverser, it seems to me you could solder a small strip to the outside of the outside rail on each side so that traverser would come to a stop at the perfect position when the traverser rail touches the strip. Contact with the strip could also be used to tell the Arduino that it has arrived.

 

...R

Edited May 24, 2017 by Robin2

[BG John]

Before going any further, have you tried moving the tray with a weight on it, at least equal to that of a loco? There's little use it modifying the device if it locks up due to the excessive weight on it.

It seems happy enough with an O gauge Dapol Terrier, or a couple of OO gauge locos, so weight shouldn't be a problem with one 4mm loco.

[meil]

A friend of mine has had trouble getting a 28BYJ motor to line up a turntable accurately because of backlash in the gears. His turntable (N-gauge wagon turntable) is now working very well with a small servo.

 

However I have never tried one of those stepper motors myself.

 

From looking at your pictures, and assuming you still plan just a two-road traverser, it seems to me you could solder a small strip to the outside of the outside rail on each side so that traverser would come to a stop at the perfect position when the traverser rail touches the strip. Contact with the strip could also be used to tell the Arduino that it has arrived.

 

...R

The trick is to always finish the movement in the same direction.

 

So in one direction you need to over drive and then come back.

[BG John]

Also bear in mind that the more gears etc. you add to the overall mechanism, the greater the backlash through it all, and consequent inaccuracies at the critical interface, i.e. where the rails are meant to line up consistently and repeatably.

Looking more closely, it could involve a fair bit of work adding gears. The ones I've got don't match those in the DVD drive, so I'd have to find some different ones, or add one to the same spindle on an existing gear (if you see what I mean!). Fitting a gear on the stepper motor will be a bit challenging, as there don't seem to be many, if any, gears available to fit the shaft. I appreciate the point about having too many gears.

 

A friend of mine has had trouble getting a 28BYJ motor to line up a turntable accurately because of backlash in the gears. His turntable (N-gauge wagon turntable) is now working very well with a small servo.

 

However I have never tried one of those stepper motors myself.

I wonder if the 28BYJ comes from a number of manufacturers, and if some are of poorer quality. It seems to be the standard stepper motor for general use with Arduinos though, so must be pretty good. Looking at videos on YouTube, some turntables have been set up to automatically calibrate themselves when they start up, which eliminates any gradual creep over a long period. Presumably it's inconsistent backlash that's the problem, rather than it happening at all.

 

A current plan for a different layout uses two turntables, one for a tank loco and the other for a wagon. The big one will be O and O-16.5, and the smaller just O, so probably doesn't need to be quite so accurate as N gauge.

 

I haven't actually tried one of these motors yet, although I've got one!

 

From looking at your pictures, and assuming you still plan just a two-road traverser, it seems to me you could solder a small strip to the outside of the outside rail on each side so that traverser would come to a stop at the perfect position when the traverser rail touches the strip. Contact with the strip could also be used to tell the Arduino that it has arrived.

 

...R

It's two road, as it replaces the loco release crossover on a run round loop to save space. The contact strips could be a way to make accurate alignment. The current version uses the microswitches for switching the power, for alignment.

 

Due to the potential problems with gears, I think I'll try the existing motor first. I'll have to see how slowly it will run, as I think the speed it opens the disk drawer is much too high. This job doesn't have priority at the moment though. I'm just discussing it now so I can get any parts I'll need ordered ready for when they're required.

Edited May 25, 2017 by BG John

[Robin2]

The trick is to always finish the movement in the same direction.

 

I mentioned that and he had already tried it without success. (He is a retired signal engineer so he knows stuff)

 

From Reply #10

 

It seems to be the standard stepper motor for general use with Arduinos though, so must be pretty good.

That is not a safe conclusion. I think its main attraction is its price.

 

...R

[melmerby]

I used the guts out of a Traintronics TT300 DCC point motor which cost £22 but was less than successful as a point motor

 

Microswitch at each end of deck controls the limit of travel as it does with the normal TT300

Original motor (same as a DVD tray) geared down to drive a 6mm screwed rod which moves the traverser.

One DCC address, like a turnout. Just thrown or not thrown. Works a treat.

 

 

Just found the pictures I took:

 

This is the frame:

 

Deck

 

Underside showing PCB fromTT300

 

Apart from the extra orange gear which had to be adapted to fit the shaft the others are the original TT300 ones

 

Completed view

 

It's based on this at Birmingham Moor Street:

http://www.warwickshirerailways.com/gwr/moorstreet/gwrms1735.jpg

 

Keith

Edited May 30, 2017 by melmerby

[BG John]

Wow! £22 for a point motor that's no good. I'm glad I'm paying throwaway prices for the stuff I'm using! I should be able to motorise around 10 points for about that! Assuming they work, or course . I'd originally thought of something like that, but much smaller. It just seems easier to use something that's ready made and about the right size. I haven't done any experimenting with it yet, as I'm playing around with radio control at the moment, but I hope to have a go at it soon.

 

I've got the DVD drawer motor off the circuit board, so I'll try it rather than a stepper motor first. Maybe it will run slow enough with less volts. I assume yours runs at a constant speed. I was wondering whether it would be better to accelerate it gradually, especially if I try the belt drive. It won't travel far, so won't have time to accelerate much, but the Arduino can handle it.

[melmerby]

Wow! £22 for a point motor that's no good.

I tried most things once!

H&M point motors, Peco solenoids Hi and Lo resistance, Hornby, Cobalt and Tortoise. Plus the two TT300s which worked fine for a year or more, first one, then the other started slipping at the end of travel and not turning off.

The only ones I have never had a problem with are the Tortoise, so that's what I have now.

The one TT300 operates the traverser, the other will be probably something like level crossing gates.

 

Keith

Edited May 30, 2017 by melmerby

[BG John]

I'm hoping that cheap SG90 servos will operate my points reliably. Around £1.20 each from China, plus an Arduino Uno for under £4, and a 16 channel servo board for under £2 from China if there aren't enough suitable outputs on the Uno. The Arduino will do other stuff too, so it's not fair to charge the full cost of it to point operation! There will be other bits as well, but not many, and not much money. The first two setups are part built on my workbench at the moment, so I can't be sure I'm on the right track yet!

[melmerby]

I'm hoping that cheap SG90 servos will operate my points reliably. Around £1.20 each from China, plus an Arduino Uno for under £4, and a 16 channel servo board for under £2 from China if there aren't enough suitable outputs on the Uno. The Arduino will do other stuff too, so it's not fair to charge the full cost of it to point operation! There will be other bits as well, but not many, and not much money. The first two setups are part built on my workbench at the moment, so I can't be sure I'm on the right track yet!

But this Arduino type stuff and cheap servos weren't available some years ago so £20 or so for a one off DCC point motor wasn't a bad price. If you had lots of points Tortoises and Switch - 8s were cheaper

 

At the moment I have on the bench an Arduino Uno, 16 Channel servo board 16 x FS90 servos, A stepper motor driver and a stepper motor out of a scanner.

I went for branded clone boards to start just in case the super cheapos were crap (you wouldn't much comeback if they didn't work).

 

BTW what about some postage? - It must add a few pence.

What I bought was free delivery as they were UK stock.

 

Keith

[BG John]

I've been buying the cheapest Arduino bits I can find. Postage has been free on most of it, from UK and China. I haven't tried everything I have yet, so can't comment on quality and reliability, but what I have tried works. I'm working on almost totally Arduino based control for four small layouts at the moment, but keep having to stop while I wait for extra bits to arrive. Trying to make space on my workbench is holding things up too!

 

Time will tell if my cheapskate ways are a false economy .