Servo Controlled Points

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Re: Servo Controlled Points

Post by GAP » Sat Mar 13, 2021 10:55 am

Greg
OK might not go the micro servo route then.

Philip.
I am very familiar with omega (ohm) as a measure of resistance but had never heard of an omega loop
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Re: Servo Controlled Points

Post by gregh » Sun Mar 14, 2021 12:17 am

I'm just a simple electrical engineer and don't have a great grasp of mechanics. Can someone explain how using a spring between the servo and the throwbar will reduce the force on the points blades when they are pushed against the fixed rail?

Maybe the idea of a spring is relevant if the driving force is not a servo, but say a a solenoid, that can be pushed 'back' by the spring when the moving rail hits the fixed rail, thus removing/reducing the force?


Now what if a SERVO is driving a spring between the servo and throwbar? As the servo starts to move it will compress the spring until the throwbar moves. The spring then expands a little. Once the throwbar is right across to the fixed rail, and the servo keeps moving, the spring compresses. When the servo gets to the end of its 'commanded' position, the spring is still compressed.
So isn't the force pushing on the throwbar through the spring the same as the case with no spring?

Maybe one idea is to allow the whole turnout to move a little and not be fixed down.
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Re: Servo Controlled Points

Post by Jimmyb » Sun Mar 14, 2021 10:01 am

Greg, I don't understand the full mechanics of the loop, but my understanding is:

1) it takes the shock out of solenoid activated points
2) where the solenoid throw is greater than the point throw, it prevents shunting the track
3) having a "slight" spring tension on the point rail helps hold the rail in place

These are the only ones I can remember, as I was introduced to these over 40 years ago, it maybe with modern technology they are no longer needed.

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Re: Servo Controlled Points

Post by GTB » Sun Mar 14, 2021 11:48 am

gregh wrote: Sun Mar 14, 2021 12:17 am I'm just a simple electrical engineer and don't have a great grasp of mechanics. Can someone explain how using a spring between the servo and the throwbar will reduce the force on the points blades when they are pushed against the fixed rail?
With a spring in the throwbar, the force on the point blade will be set by the strength of the spring and the amount of spring deflection. Which will depend on the difference between how far the point blades move and how far the throw bar moves. If the throwbar moves so far that the spring is closed up, then the force will be whatever the servo can apply.

The omega loop is just a simple spring that can be easily fabricated by the average modeller from a piece of wire, using a pair of pliers. It doesn't have to be omega shaped, a zig zag will do the same job. Whatever is used is there to provide some longitudinal flexibility in the throw rod.

An omega loop is also there to take up any minor mismatch between the point blade throw and the throw of whatever mechanism is used to move the point blades, whether it be a manual lever, a solenoid etc. They are easily adjusted with pliers during the final setup so the point blade is held against the stock rail at each end of the travel with just enough force to ensure it doesn't open when a wheel hits it from the facing direction.

Modern devices used in the small scales like Tortoise motors, etc. don't need them, as the sideways flexibility of the drive rod does the same thing. Omega loops aren't needed with Peco points either, as the built in over centre spring does the same job of holding the blades against the stock rail.

Omega loops were once commonly used in HO and other small scales in the days of H&M solenoids, etc. Not sure how you'd go about using an omega loop in the garden scales, as the point rods need to be fairly heavy and wouldn't provide enough flexibility to be much use, I would have thought.

My garden track turnouts use 1/16" brass rod for the rodding and I made the rodding and tie bars adjustable, in much the same way as a real VR ng turnout. The point levers I use lock when thrown, so I don't need any springing to hold the point blades closed.

The only r/c servos I use are in steam locos and I make those linkages adjustable. That could be done for turnouts as well and I've seen adjustable linkages available in my local r/c shop.

I hope that all makes sense.

Regards,
Graeme

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Re: Servo Controlled Points

Post by GAP » Sun Mar 14, 2021 8:38 pm

The linkage I am using has a thread on end onto which screws a clevis pin, so there is some adjustment there; plus I am using a trimpot on the servo switch so that I can finely adjust the point blades at one end of the travel.
The other end of the travel is set by the potentiometer on the servo tester that is used to set the "start position" of the servo arm ie point blades hard up against stock rail.
The trimpot replaces the fixed resistor, added to the potentiometer wiper in the original design, which set the amount of servo movement when the control switch was thrown.
So effectively I have 2 methods of adjusting the point blades if needed and the servo provides the pressure to hold the blades.

I have now installed my solar panel, battery, charge controller and 12V to 5V buck converter (all working as advertised) so I now have the start of my 5V bus.
I was planning to tap off the battery so I could have a 12V bus but decided against as it would be in parallel with the 5V Buck converter input and may affect it I want smooth clean DC for the servos.
If I want a 12V bus in the future I can easily add another solar battery array.
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EDIT;
With all the talk about pressure on the blades I got to thinking about what current the servo would be drawing when it was pressing against the blades.
I connected my trusty ammeter and found the when the servo was stalled ie was making a "buzzing" sound it was drawing about 90mA but when I adjusted the potentiometer on the tester and the trimpot (which set the blade travel limits)till the buzzing stopped it drew only 4.7mA.
When the blades were moving the servo drew somewhere 100 and 200mA while it was moving.
So my fears about drawing to much current were allayed.
I was not able to move the blades with my finger by applying a fair amount of pressure so I do not think wheels will move them.
I have lost the ability at the moment to move the blades by hand but I can live with that.
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Re: Servo Controlled Points

Post by gregh » Mon Mar 15, 2021 4:34 am

Looks like you have a nice, cheap, workable system. Well done.
Good work measuring the servo current too-the perfect way to determine if the servo is trying to move too far
I don't think you need to worry about running the 12V around, interfering with the 5V servo supply.

You can always just run the extra wire now and connect to battery, and if you do get interference problems, go with your separate battery idea.
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Re: Servo Controlled Points

Post by GAP » Mon Mar 15, 2021 5:51 am

gregh wrote: Mon Mar 15, 2021 4:34 am Looks like you have a nice, cheap, workable system. Well done.
Good work measuring the servo current too-the perfect way to determine if the servo is trying to move too far
I don't think you need to worry about running the 12V around, interfering with the 5V servo supply.

You can always just run the extra wire now and connect to battery, and if you do get interference problems, go with your separate battery idea.
I have run 4 core cable 2 for the 5V and 2 for expansion, my big concern is running equipment off the battery terminals that the Buck converter is connected to and possibly loading the battery to much.
I could probably get away with running everything off the 5V but only have 3A to play with
The battery is 5AHr so perhaps loading would not be as issue but the cable I have used is only rated at 4.5A so again another limitation.
I would probably only run LEDs for lighting off the 12V so maybe I could get away with it, might have a cuppa and think about it, so many things so little time......
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Re: Servo Controlled Points

Post by Phil.P » Mon Mar 15, 2021 6:46 pm

The first thing your 12V bus should 'see' should be a resettable trip/fuse..
No problem of pulling too much current then.
:D

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Re: Servo Controlled Points

Post by GAP » Mon Mar 15, 2021 8:39 pm

Phil.P wrote: Mon Mar 15, 2021 6:46 pm The first thing your 12V bus should 'see' should be a resettable trip/fuse..
No problem of pulling too much current then.
:D
Already have a 3A fuse to protect the Buck converter so max current is limited to that.
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Re: Servo Controlled Points

Post by gregh » Mon Mar 15, 2021 9:49 pm

GTB wrote: Sun Mar 14, 2021 11:48 am
gregh wrote: Sun Mar 14, 2021 12:17 am I'm just a simple electrical engineer and don't have a great grasp of mechanics. Can someone explain how using a spring between the servo and the throwbar will reduce the force on the points blades when they are pushed against the fixed rail?
With a spring in the throwbar, the force on the point blade will be set by the strength of the spring and the amount of spring deflection. Which will depend on the difference between how far the point blades move and how far the throw bar moves. If the throwbar moves so far that the spring is closed up, then the force will be whatever the servo can apply.
Thanks for the long story, Graeme.
But surely it doesn't matter if the spring is 'closed up', which I'm assuming you mean is squashed as far as it will go.
If the spring is in any compression, won't the force still be as you state above?

Sorry to GAP for hijacking his thread. I promise to stop now and live with my ignorance.
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Re: Servo Controlled Points

Post by Phil.P » Tue Mar 16, 2021 9:26 am

No.
Think of squeezing a spring (even from a biro)...
If you squeeze it a little, the holding-force is less than if you squeeze it a lot.

You have had to do a greater amount of work, to get to where you are, if you have squeezed it more.

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Re: Servo Controlled Points

Post by big-ted » Tue Mar 16, 2021 6:19 pm

I think the other thing being missed here is that a servo tries it's hardest to reach a position told to it by the receiver. One wants the point blade to be held against the rail with some amount of force, which means that, without any 'give' in the system, the point blade is actually preventing the servo from reaching the position it wants to get to. In this state, the servo motor will continue to run and will eventually burn out. Having a spring in the system allows the servo to obtain it's desired position whilst applying a small amount of holding tension to the point blade.


Further up someone mentioned the idea of using a micro servo to control a switch to in turn control LED signals. Maybe I miss-understood, but this is a pretty round-about way to achieve this. Better to use a board to convert the PWM signal from the receiver (or, in this case, servo tester) to an LED-drive signal (which could be either DC or PWM). Plenty of options on eBay by searching "RC receiver LED switch" or it's possible to convert a servo by removing all the moving parts:

https://www.instructables.com/Convert-a ... -off-LEDs/

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Re: Servo Controlled Points

Post by GAP » Tue Mar 16, 2021 8:44 pm

big-ted wrote: Tue Mar 16, 2021 6:19 pm I think the other thing being missed here is that a servo tries it's hardest to reach a position told to it by the receiver. One wants the point blade to be held against the rail with some amount of force, which means that, without any 'give' in the system, the point blade is actually preventing the servo from reaching the position it wants to get to. In this state, the servo motor will continue to run and will eventually burn out. Having a spring in the system allows the servo to obtain it's desired position whilst applying a small amount of holding tension to the point blade.

Further up someone mentioned the idea of using a micro servo to control a switch to in turn control LED signals. Maybe I miss-understood, but this is a pretty round-about way to achieve this. Better to use a board to convert the PWM signal from the receiver (or, in this case, servo tester) to an LED-drive signal (which could be either DC or PWM). Plenty of options on eBay by searching "RC receiver LED switch" or it's possible to convert a servo by removing all the moving parts:

https://www.instructables.com/Convert-a ... -off-LEDs/
Reply to point 1
The point blade in the method I am using is set at its max travel one way (ie the point blade is against the stock rail) by the servo tester's pot and the other max travel distance, by the value of the resistor that is switched in.
In my case I am using a trimpot to set that distance which happens to be when the point blade is against the stock rail on the other side.
Therefore the servo is at max travel both ways, I tested this by measuring the current draw and when the blades are in position the quiesant current is all that is being drawn, my reference quiescent current was achieved by using the tester's the neutral position function to set the servo then measuring the current.

Reply to point 2
That would be me.
The reasons for considering that method were;
1. the tester has 3 outputs that can be used
2. I have the micro servos
3. I have the micro switches
I also considered using a micro servo to operate a semaphore signal.

I have discounted using LEDs because they are not visible out in the sun.
I had another suggestion of driving the semaphore of the throw bar and that would remove the need for extra servos

I am trying to keep the amount of electronics out in the yard to the barest minimum, to reduce the number of more points of failure to consider if something stops working.
I would really like to run my signals, points and lighting control off a Raspberry Pi but the logistics of cabling and the environment put a stop to that outside, so I might take that project indoors to my HO layout.
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Re: Servo Controlled Points

Post by big-ted » Tue Mar 16, 2021 10:58 pm

GAP wrote: Tue Mar 16, 2021 8:44 pm
big-ted wrote: Tue Mar 16, 2021 6:19 pm I think the other thing being missed here is that a servo tries it's hardest to reach a position told to it by the receiver. One wants the point blade to be held against the rail with some amount of force, which means that, without any 'give' in the system, the point blade is actually preventing the servo from reaching the position it wants to get to. In this state, the servo motor will continue to run and will eventually burn out. Having a spring in the system allows the servo to obtain it's desired position whilst applying a small amount of holding tension to the point blade.

Further up someone mentioned the idea of using a micro servo to control a switch to in turn control LED signals. Maybe I miss-understood, but this is a pretty round-about way to achieve this. Better to use a board to convert the PWM signal from the receiver (or, in this case, servo tester) to an LED-drive signal (which could be either DC or PWM). Plenty of options on eBay by searching "RC receiver LED switch" or it's possible to convert a servo by removing all the moving parts:

https://www.instructables.com/Convert-a ... -off-LEDs/
Reply to point 1
The point blade in the method I am using is set at its max travel one way (ie the point blade is against the stock rail) by the servo tester's pot and the other max travel distance, by the value of the resistor that is switched in.
In my case I am using a trimpot to set that distance which happens to be when the point blade is against the stock rail on the other side.
Therefore the servo is at max travel both ways, I tested this by measuring the current draw and when the blades are in position the quiesant current is all that is being drawn, my reference quiescent current was achieved by using the tester's the neutral position function to set the servo then measuring the current.

Reply to point 2
That would be me.
The reasons for considering that method were;
1. the tester has 3 outputs that can be used
2. I have the micro servos
3. I have the micro switches
I also considered using a micro servo to operate a semaphore signal.

I have discounted using LEDs because they are not visible out in the sun.
I had another suggestion of driving the semaphore of the throw bar and that would remove the need for extra servos

I am trying to keep the amount of electronics out in the yard to the barest minimum, to reduce the number of more points of failure to consider if something stops working.
I would really like to run my signals, points and lighting control off a Raspberry Pi but the logistics of cabling and the environment put a stop to that outside, so I might take that project indoors to my HO layout.
Apologies. Reading my post back I sound like I consider myself some kind of authority. That wasn't my intention!

Having bits on hand is, of course, the perfect reason to do things a certain way. And I think if I were you I'd prefer semaphore signals to LED lights also. I think a while ago someone on here had used an Arduino or similar to program servos to mimic the bounce and heavy motion of a semaphore signal. It looked really impressive, but as you say, more gubbins outside to go wrong, for sure.

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Re: Servo Controlled Points

Post by GAP » Wed Mar 17, 2021 4:16 am

No apology necessary, I get some of my good ideas from peoples comments.

Servos have just turned up so as soon as it stops raining I can get out into the backyard again, till then I will continue with adding the trimpots to the 12 testers I have.
Good way to pass a rainy day.
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Re: Servo Controlled Points

Post by gregh » Wed Mar 17, 2021 7:21 am

big-ted wrote: Tue Mar 16, 2021 10:58 pm I think a while ago someone on here had used an Arduino or similar to program servos to mimic the bounce and heavy motion of a semaphore signal. It looked really impressive, but as you say, more gubbins outside to go wrong, for sure.
That was me. Using a Picaxe control.
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Re: Servo Controlled Points

Post by GAP » Fri Mar 19, 2021 3:27 am

I have modified all 12 of my Servo Testers, so now I am ready to set them up on the layout.
I will be putting each one next its relevant point so that way I can adjust with a bit of ease.
They will be housed in plastic food containers, with a seal-able lid under the baseboard, we have had torrential rain this week and where I am planning on putting is still dry so they should be OK, the only water that gets under there is from the irrigation and it is only a fine mist anyway.

I added a On/Off switch for the 5V bus that I had in a box it is rated at 500V and 30A so I think it should handle a 5V 3A bus OK.

I did consider a "service" switch in line with the main switch that would isolate it while I made adjustments so that I do not have to walk back and forth to the control panel while adjusting, I may still retrofit one if the becomes necessary.

I also considered a 1ohm resistor in line with the servo +ve line so I could put a voltmeter across it to measure the current while adjusting but at this stage, I'll just listen for the servo buzzing to indicate over driving of the motor.

Edit; added enclosure to picture.

A picture of the way I modified the testers and mounted them in an enclosure, Power on the Right, Servo on the Left and switch input at the bottom, all cables will come in from the bottom through a piece of cut down irrigation fitting.
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Last edited by GAP on Mon Mar 22, 2021 1:50 am, edited 2 times in total.
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Re: Servo Controlled Points

Post by gregh » Sat Mar 20, 2021 4:01 am

A great solution. I'm impressed and will use this method when/if I add any more turnouts.
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Re: Servo Controlled Points

Post by GAP » Sat Mar 20, 2021 9:01 pm

I had a thought about how I could control them via radio control by replacing the toggle switch with one of these https://core-electronics.com.au/pololu- ... mbled.html and a 2.4GHz receiver at each point.
Would get expensive and defeats the purpose of doing it on the cheap though, plus I would need 12 radio channels.
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Re: Servo Controlled Points

Post by ge_rik » Mon Mar 22, 2021 9:49 am

Hi Graeme
That all looks very elegant.

Can I just clarify how it all works?

Am I right in thinking there will be a servo tester assembly close to each turnout, so you can tweak the amount of throw?
Will the switching for each turnout be done from a central control panel, using hard wired connections, or will you have a series of smaller control panels for doing the switching at various locations?

Rik
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