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Point motors, re-invented


jeff_p

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Well, I guess I ought to have known that I would end up doing this, so, yes, wheel re-invention was the order of the day.

 

Putting that to one side I think I have come up with something a little different, perhaps more flexible while being simpler to print and deploy (though that last point really will have to wait until I have actually installed one and got it working).

 

I have chosen to divide the whole "model railway point motor" thing into two distinct parts:  The component which interfaces to the point itself, and the component which provides the driving force to actually move the point.  So, following the picture painting a thousand words ethos:

 

The point interface:

20201007_145022.jpg.f93ac7fc7e9a8bd14651a546dbe3763c.jpg

 

and the Point Actuator:

20201007_145030.jpg.5f0284c8929a290b02a0c1dcf25a3e05.jpg

 

I'll quickly point out that I realise that the the interface unit only catches the "bottom" switches, I have to extend the 3D model and raise the post up another 4mm.

 

How would they work (I can't say "how do they work", yet :rolleyes:)?  Hopefully it's obvious they screw up under the base board, where a suitable metal pin inserted into the post of the interface unit would engage with the point itself.  The interface unit can be aligned and positioned such that it is centred lined up with the tie bar of the point.  This interface unit (in deed both units) have a maximum 10mm swing which should be way more than required for normal usage.

 

The actuator unit clips onto either side of the interface unit and (with the servo motor in it's "zero" position, as per above photo) moved until the point is accordingly set.  The 3D printed "springs" allow for both some freedom of alignment and also a little extension or compression hopefully making the whole arrangement easier to install and set up.

 

So how do you adjust the "throw"?  Two ways:  Firstly you could simply drive the servo through a limited arc rather than a full 180 degrees and 10mm of movement.  Or, secondly, change the offset of the hub in the actuator wheel for one where the centre of rotation is closer to the centre of the wheel.  At the moment I am thinking that a combination of the two is the best approach.

 

What I like about this design are the following:

 

  • The servo is not actually attached to the component interfacing with the point, in the event that something needs doing to he servo there would be no need to fiddle with the point and risk upsetting something that can be tricky get right (not to mention a whole bunch of wires)
  • There is no direct mechanical linkage between the servo and the point roding.  To remove the servo simply unscrew it and drop it off its actuator rod.  SImples.

 

Of course this is all, currently, hypothetical.  I think I shall have to make up a small "plank" test bed to see just how this is actually going to work. The best laid plans and all that stuff.

 

The switches on the actuator will perform the normally anticipated "frog polarity" thing.  "But wait", you say, "that only needs the one switch".  Yes is the reply, however, the software I have been writing can accept confirmation signals (in the electrical sense) that the point movement has completed, and this is the purpose of the other two switches:  they will pull down Arduino pins to earth allowing the software to know  when a point is set (and which way) and also when it is "in motion" between states.

 

Anyway, some more images:

 

20201007_150125.jpg.bfd4e6a7487a69a14f30fc385923f824.jpg

 

20201007_145047.jpg.8fa28ac079798ddd2d40944fab4951c2.jpg

 

20201007_145013.jpg.fac89237d88d1676aa08f4a6746a1304.jpg

 

For info:  CAD done using FreeCAD 0.18.4 exporting to STL and into Ultimaker Cura 4.4.1 under Linux Mint 20 Cinnamon.

 

Been trying to get these all "lined up" for a while now.  Some of my issues were software related, but there's been a fair amount of mental re-alignment required as well :D

 

Jeff.

Edited by jeff_p

  • Like 8
  • Interesting/Thought-provoking 2
  • Craftsmanship/clever 4

6 Comments


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  • RMweb Gold

What would be your estimated cost per complete point, using only one micro switch ( for us non-computer controlled layout builders)?

 

As an option, could you adapt the servo base to also incorporate the micro switches as well? I know this complicates the one unit, but if the servo itself is removable it should still fall in your remit of 'simple to fix'. The advantage is the under-point unit can be dispensed with and replaced by a simple rod with a pin.

Edited by Stubby47
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Clever, and the option for multiple m/switches is useful, when I fitted a 3-way point,

I needed the extra m/switches for interlocking (saves bending switch blades!)

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21 hours ago, Stubby47 said:

What would be your estimated cost per complete point, using only one micro switch ( for us non-computer controlled layout builders)?

 

As an option, could you adapt the servo base to also incorporate the micro switches as well? I know this complicates the one unit, but if the servo itself is removable it should still fall in your remit of 'simple to fix'. The advantage is the under-point unit can be dispensed with and replaced by a simple rod with a pin.

 

Estimated cost per switch is a little tricky.  The servos are really cheap at about £1.15 each off ebay, and a really cheap arduino to run it is about £3.50 but can control a number of servos. 3D printing is cheap but the printer isn't, unless you've a friend with one. How you make it all hang together and operate is really the question, to which there are many approaches, mine being just one (and technically far from the simplest).

 

Could the switches be incorporated into the servo unit?  Yes. The mount  for the servo has been deliberately centrally located meaning the switches could  be positioned either side as part of the same component. Then,  yes,  it could be used to drive a simple rod and pin to the point. 

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  • RMweb Gold

Thanks Jeff.

I've experimented with the MERG 3D servo bases,, the horn has a pin which fits into a staple/goalpost on a dowel rod.

As I've left the Peco point springs in place, the pin/ staple connection allows the point to be changed as the pin pushes the side of the staple far enough over.

The dowel itself is just supported by a drilled out cheap plastic corner bracket, and has another pin through the board into the tiebar. The MERG Esypoint board is run from half a DPDT switch , the other half changing the frog.

 

I like your method of replacing the horn with a rotary movement , and the built-in replacement Omega loops.

 

Hope the trials go well.

Stu

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Hi Stu,

 

I have looked at the MERG servo mounts, and thought they were a bit fiddly, especially the "pin" moving between the goal posts.  The wheel I've modelled screws in-place of the arm normally found on a servo so it feels more rigid and secure.  It's also put the loads closer to the motor so the twisting forces are reduced.  To be honest I couldn't quite see how I was going to fit the pin to the arm reliably.

 

On the subject of the built-in point springs, I would remove them.  They're essential for the electro-magnet point motors, otherwise the blades would not remain in one position or the other.  For a more driven solution like servo motors or stall motor based systems (e.g. tortoise point motors) then the spring is working against you IMHO.  I'd also electrically tie the point blades to their accompanying rail and let the mirco-switch power up the frog.  Of course this last statement is probably only really pertinent to electro frog points.

 

Jeff

  • Thanks 1
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