Switch Point Detection continued

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RET
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Re: Switch point position detection

Post by RET » Sat Oct 21, 2017 10:08 am

Hi Chuck,

What you are describing is quite possible. Think of a tape deck reading head where the gap is a thousandth of an inch or less. What you want to do is create a small pole gap for the field to go through.

In practice, take a small button rare earth magnet (the kind you can buy almost anywhere). Epoxy a piece of mild steel to each face. Arrange these pole pieces so that they converge a short distance from the button magnet and almost touch. The field will be concentrated in the small gap between. Shape the pieces at the gap to get the field you want. Make sure the pole pieces only approach each other at the gap. Once you have got it right, encapsulate the whole thing in epoxy so nothing can change.

I haven't had any occasion to do this, but the concept is simple (most good ideas are), and should be easy to execute.

Hope this helps.

Richard Trounce.

RET
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Re: Switch point position detection

Post by RET » Sat Oct 21, 2017 8:50 pm

Hi again,

In practice, for your application, a gap of between 1/32" and 1/16" between the poles is probably the closest that is practical. You can also put a non magnetic material in between the pole pieces to maintain the desired gap.

Richard Trounce.

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pat1027
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Re: Switch point position detection

Post by pat1027 » Sun Oct 22, 2017 11:03 am

Use the switch point and rail themselves as the switch. If you have a working signal system you are making reliable insulated joints. The question would be if a weathered point and rail will make a reliable electrical connection. I don't have any aluminum rail at home. I checked the aluminum rungs on a ladder that resides outdoors and had zero or near zero resistant's with a light touch. From what I steel steel angle is pretty common for switch points. Perhaps a switch to stainless.

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ChuckHackett-844
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Re: Switch point position detection

Post by ChuckHackett-844 » Sun Oct 22, 2017 1:36 pm

pat1027 wrote:Use the switch point and rail themselves as the switch.
A good idea in concept but that would be less reliable than the current micro-switches inside the motors because, over time and after sitting unused for a time (i.e.: no traffic to cause slight rubbing to clean the contacts) they would not make reliable contact.

Case in point: Newly laid track with steel joint bars works great for months but then, one by one, the joints become unreliable - and these are joints between steel and aluminum that are:
  • a) bolted together, and
    b) do have a slight movement caused by thermal expansion/contraction.
The joints MUST be bonded (we use short green "ground bond" wires with terminals on each available at Lowes, etc. in the electrical department).

Our turnouts are bonded and insulated such that there are no "dead" spots (so all rail is live). I had a case where someone did not correctly insulate the bridal bar. This caused an intermittent false "occupied track" (i.e.: Red/Stop) indication (fail-safe but still annoying) when the point would sometimes make good contact with the running rail.

pat1027 wrote:If you have a working signal system you are making reliable insulated joints.
Ah, if only that were true. My system deals with a wide variation in track conductivity - both 'shunt' mode (conduction between rails caused by water, salts in the ballast, tie material, etc.) and 'longitudinal' mode (loss of conductivity due to imperfect joints) but there is a limit.

When one talks about the "reliability' of a signal system it involves the Entire system.

If any piece of the system fails - controls (in my case firmware in the controllers), wires, micro-switches, rail joints, signal lamps, yada, yada, even if not under your direct control (such as our turnout motors) - the engineer (the "customer" of the system) perceives a system that is "broken", i.e.: unreliable fair or not ...

99% of the 'reliability' issues I have on the railroad are due to:
  • * Incorrectly installed bond wires (fail over time because someone didn't tighten it or did not use anti-corrosion compound)
    * Broken bond wires (cut by a derailment, damaged by MOW tamping track, etc.)
    * Turnout motor not feeding the correct indication to the signal system
... or at least that is my experience over the last seven years ...

Chuck
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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ChuckHackett-844
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Re: Switch point position detection

Post by ChuckHackett-844 » Sun Oct 22, 2017 1:47 pm

RET wrote:.... In practice, take a small button rare earth magnet (the kind you can buy almost anywhere). Epoxy a piece of mild steel to each face. Arrange these pole pieces so that they converge a short distance from the button magnet and almost touch. The field will be concentrated in the small gap between. Shape the pieces at the gap to get the field you want. ....
The reed-switch I have in front of me is about 4 mm in diameter so I assume I have to have a gap of 4 mm + ... or do you think the goal would be accomplished using a gap less than 4 mm and just have the reed-switch approach the gap ... not have to pass into the gap?

I have also noticed that the reed-switch orientation to the magnet (held in my hand, no pole pieces) makes a difference ... more experimentation required ... so many projects, so little time ... sigh ...

Chuck
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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Atkinson_Railroad
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Re: Switch point position detection

Post by Atkinson_Railroad » Sun Oct 22, 2017 7:41 pm

Chuck,

Getting back on track to your original post, I’m curious about what is happening with the root cause failure of the micro switches.

Are the switches unable to perform consistently at the functioning operating current?

Are the switches within the turnout motors not being actuated at the proper moment in relation to the position of the turnout points?

In thinking about a switching device being fastened directly to the rail points and requiring tight tolerances…
What about vibration and likely positioning of the mounted item being jostled ajar with varies weights passing over the rail?
(A permanent magnet for example being knocked off position, etc.)

While there may be a negligible amount of “hysteresis” with the linking of a detector rod arrangement as in full-size practice,
the switch method whatever it may be, would be off to the side/below the tie line more “fixed” and protected from rail vibration.

Also, further conversation on why separate (suitable) limit switches actuated by each rail point would not work,
will be helpful for clarification too.

Following the discussion,

John

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ChuckHackett-844
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Re: Switch point position detection

Post by ChuckHackett-844 » Sun Oct 22, 2017 9:59 pm

Atkinson_Railroad wrote:Getting back on track to your original post, I’m curious about what is happening with the root cause failure of the micro switches.
The micro-switches are fairly well protected from water (under a good cover) so I don't think that is the issue.

The internal micro-switch that is 'failing' actually provides two functions, one set of contacts drives the motor (this does not seem to fail). The other set (same micro-switch, just the other set of contacts) feeds the signal system and is the part that is failing.

I think the issue comes down to:

To my knowledge, micro-switches come in three major types determined by the construction of their contacts (ignoring how they are actually activated - lever, button, etc.):
  • * heavy current - these contacts are designed to carry large currents
    * low current - these are designed to carry sub-ampere levels of current (ma)
    * millivolt switching - these are designed for the very small currents required by solid state logic inputs
I believe the turnout motors we use are using the "low current" type. The small dc motor draws something like 100ma to start and ~20 ma when running.

It is my understanding that these low current contacts partially rely on a small amount of arcing to create a surface that makes reliable contact.

When used to switch millivolts - as on the side that feeds the signal system, there is no arcing because the current is very low and thus the contacts do not self-clean.

In a micro-switch designed for very low currents the contacts are arranged such that there is some degree of physical wiping (rubbing) taking place to do the contact cleaning. In some cases the contacts are sharp edges (edges of triangles) arranged at 90 degrees so that there is a small contact area and thus higher force per sq inch.

Knowing this, one solution would be to attach a light bulb to each lightly loaded contact (to drive a switch dwarf which i believe was the original purpose) which would increase the current but we currently run 19 controllers and 60 signals off of one 16v power supply feeding about 2 amps to the system (i'm guessing at about 4,000 feet in length). At the far end of the railroad the voltage is down to about 10 volts. Adding all this intentional load (one lamp would always be on at every turnout motor) would cause a much higher voltage drop at the far end of the railroad.

I won't go into detail but it is impractical at the moment to feed power from multiple places ... and no, solar does not work in the woods where we are and we can't put solar up on a pole above the trees .. besides the cost, the last thing we need on a signal system in Florida is a lightning rod! :-)

I'll address your mounting and "suitable microswitch" questions tomorrow ...

It's late - I hope I explained all that correctly :-)

Chuck
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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Atkinson_Railroad
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Re: Switch point position detection

Post by Atkinson_Railroad » Mon Oct 23, 2017 6:15 am

Yes, your explanation makes sense to me Chuck. I somewhat suspected the “low current” passing across the
contacts was a portion of the problem, and your description of that issue is cleared up for me.

I’ve been looking through an old Magnecraft Electric Company Designers’ Handbook & Catalog of
reed and mercury wetted contact relays. Within the booklet it covers the many issues related to mechanical
contacts such as; contact resistance, sensitivity, contact bounce, contact capacitance, contact protection, etc.
While mercury wetted contact reed relays solve some of the above issues, they also open up another set of problems.
(If it’s not one thang, it’s another ; )

To be continued… on all of the above.

Would like to also re-stir the pot a tad related to the subject at hand, and do it in a manner
that is said in a friendly tone for other’s that may be peeking in on this discussion.

We’re sparring on a subject outside the parameters of the declared Subject Matter for this portion of the board
as noted at the beginning of [Subject matter -------PLEASE READ--------] clearly stated at the thread below.

https://www.chaski.org/homemachinist/vi ... =8&t=82438

As a student of observation and being one of self awareness, I’m just highlighting the obvious trend that seems to be
taking place in this 21st century. Recreational outdoor railroading activity is a vast subject covering multiple disciplines.
I’m continuing to see where this forum may, at some point have to restructure its marketing platform.
Perhaps at some point merely calling the entire forum board Home Hobbyist with the various sub subject categories
falling into their respective place.

In the meantime, back to the switch point detection problem : )

John

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Re: Switch point position detection

Post by Greg_Lewis » Mon Oct 23, 2017 10:51 am

If part of the issue is the conflict in current requirements of a two-pole microswitch, what about running two switches in tandem, one being for the higher current, one for the lower?
Greg Lewis, Prop.
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Re: Switch point position detection

Post by fly5150 » Mon Oct 23, 2017 11:18 am

Why not look at some of the low cost PNP prox sensors? it would require a junction box, a pair of relays and a power source. 2 of these, mounted in the tie below the throw bar with 2 bolts adjusted to just come into field when the bar is fully thrown each direction. They are waterproof and used everyday in industrial applications. They come in different sense distances, and can be adjusted to perfection. I would Not introduce a second throw-bar, the more parts that can easily get bent in a derailment.
https://www.ebay.com/itm/LJ12A3-4-Z-AX- ... DHFC6Gvd4g
-Dave

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Re: Switch point position detection

Post by rkcarguy » Mon Oct 23, 2017 11:43 am

I've weighed many options for my soon to be RR as I am currently building signals and a relay system to control them. I have crossed out anything with complex electronics as I don't want a voltage spike to fry it all. Magnetic devices can pick up metal debris and stop performing properly. I'm just going to use one of these:

https://www.amazon.com/Moroso-74120-But ... B000CON4MU

Mount it in a project box, mount the box so that the turnout crossbar pushes the button, seal the lid with sealant, and use the threaded portion to adjust it so when the switch is truly in position it closes the circuit. I have one of these I built on my boat so I can raise and lower the drive from the stern. It's used in saltwater and when I've been crabbing the pots drip saltwater all over and it see's about the worst environment possible, and it has been working great for over 5 years.
Another thing that has worked great on wiring for me in the saltwater environment(boat and boat trailer wiring) is grease, lots of it. Moisture tends to wick inside the wires and eat them under the jacket. I found by crimping, soldering(forward of the crimp only so the wire can flex), then applying grease and heat shrinking the whole thing, I get a pretty much bulletproof connection.

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Re: Switch point position detection

Post by ChuckHackett-844 » Mon Oct 23, 2017 1:06 pm

I am going to answer multiple questions here to try and keep the traffic down ...
Atkinson_Railroad wrote:In thinking about a switching device being fastened directly to the rail points and requiring tight tolerances…
What about vibration and likely positioning of the mounted item being jostled ajar with varies weights passing over the rail?
(A permanent magnet for example being knocked off position, etc.)

While there may be a negligible amount of “hysteresis” with the linking of a detector rod arrangement as in full-size practice,
the switch method whatever it may be, would be off to the side/below the tie line more “fixed” and protected from rail vibration.


I'm thinking that the devices (magnet and reed-switch) would hang below the rail such that they are out of harms way. Yes, being mounted as close as possible to the point (to increase position sensitivity) there would be vibration that would hinder getting very close tolerance.

In full-size, any mis-fit in the rod linkage would be small compared to the distance being measured but I think that any mis-fit in the linkage (poor construction, poor maint, etc.) would quickly swamp the ability to detect down to 1/32" in 7.5" gauge.
Atkinson_Railroad wrote:Also, further conversation on why separate (suitable) limit switches actuated by each rail point would not work,
will be helpful for clarification too.
I'm confused, this is the approach I think works best (i.e.: separate switches)
Greg_Lewis wrote:If part of the issue is the conflict in current requirements of a two-pole microswitch, what about running two switches in tandem, one being for the higher current, one for the lower?
There is no room on the motors we purchase and even if there was, due to the springs in the linkage, they would still only detect the position of the motor, not the position of the points - i.e.: in the case of a rock in the points.
fly5150 wrote:Why not look at some of the low cost PNP prox sensors? ... They come in different sense distances, and can be adjusted to perfection. ....
https://www.ebay.com/itm/LJ12A3-4-Z-AX- ... DHFC6Gvd4g
-Dave
I could not find any information on their Hysteresis (i.e.: distance required to go from off to on or vice versa). This is different from "sense distance" (how far away the switching will occur).
rkcarguy wrote:I've weighed many options for my soon to be RR as I am currently building signals and a relay system to control them. I have crossed out anything with complex electronics as I don't want a voltage spike to fry it all.
I think you will find it difficult to find relays that will work on long sections of track under both wet and dry conditions - doable, but not easy.

I have modified and tested my design for at least 5 years in Florida (lightning capitol of the world :-) ). I can now draw a 5,000 volt arc to a track terminal without damage. Direct strike - forget it, but anything would be fried then.

It's not easy to build a relay system that guarantees only one train permitted at a time with an arbitrary number of block entries (ABS signals do not).
rkcarguy wrote:I'm just going to use one of these:

https://www.amazon.com/Moroso-74120-But ... B000CON4MU
Wow, you are going to need a strong turnout motor to activate those! Also, I think you'll find that:
  • * they work fine for high currents but will eventually fail on low currents (which is what I need)
    * their hysteresis will prevent you from detecting down to 1/32"
Regards,

Chuck
Regards,

Chuck Hackett, UP Northern 844, Mich-Cal Shay #2
"By the work, One knows the workman"

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