Servo Controlled Points

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GAP
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Re: Servo Controlled Points

Post by GAP » Tue Mar 09, 2021 8:03 pm

gregh wrote: Tue Mar 09, 2021 8:57 am I've had a few more thoughts from my post above.
If you set the pot on the board to mid position, it can be the neutral position where the servo sits unless it is moving the points.
Then the additional resistor and switch connected to 0V shown by Bodnar, would become the position where the points move to in one direction. Then add another resistor and switch connected from the wiper but to +5V. When that switch is closed, it will move the points in the opposite direction.

I haven't tried it but it should be simple for you to try. I'd try 4.7k resistors to start.

In practice the resistors would probably need to be pre-set pots so you can adjust how far the point blades are moved.
I already tried that setup but the servo didn't move when switched to the 5V. The pot is used to set one extreme of the servo and when the resistor is switched in moves it to the other extreme depending on the resistor value.

My main issue with manual movement is that 5 out of the 8 sets of points I have do not have over centre springs, I could make them using a safety pin the same as I saw on the S&T.
The main reason I am doing this is because my manual LGB points do not move when I throw the lever, so I have to tap the blades to get them to move into place. I have tried all sorts of lubricants from light oil thru to graphite to free them up but had no luck.

I have even toyed with using throws like Rik did but they make and break a microswitch instead of having a main panel, I would just have to go to the point to throw it the same as I do now and that is still an option but if I did that I may as well just use straight rods.
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Re: Servo Controlled Points

Post by gregh » Wed Mar 10, 2021 5:02 am

GAP wrote: Tue Mar 09, 2021 8:03 pm I already tried that setup but the servo didn't move when switched to the 5V. The pot is used to set one extreme of the servo and when the resistor is switched in moves it to the other extreme depending on the resistor value.
Rats! Thought that would work. I still don't understand why not.
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Re: Servo Controlled Points

Post by GAP » Wed Mar 10, 2021 6:23 am

I did find another funny; the small servos (HTX 900) I have move in the opposite direction to the large ones (HK15288A) I have, when connected to the tester.
No idea what is happening there.
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Re: Servo Controlled Points

Post by GAP » Fri Mar 12, 2021 6:39 am

This is a photo of my test rig with a mock up of how I plan to fit the servos, I have had good a suggestion about reinforcing the throw bar with a piece of brass or styrene to add strength.

I have gone with waterproof servos so I can mount them through the roadbed.

The micro servo on the right is for a signal, as it moves at the same time as the points servo I can add a semaphore to show which way the blades are set, I could even use it to open/close a micro switch to turn on and off a signal light. It would most likely go under the roadbed in a weatherproof enclosure.
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Re: Servo Controlled Points

Post by philipy » Fri Mar 12, 2021 7:24 am

GAP wrote: Fri Mar 12, 2021 6:39 am ...as it moves at the same time as the points servo I can add a semaphore to show which way the blades are set, I could even use it to open/close a micro switch to turn on and off a signal light. It would most likely go under the roadbed in a weatherproof enclosure.
That is essentially what I did with my bracket signal indicating which way my three way junction is set.
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Re: Servo Controlled Points

Post by GAP » Fri Mar 12, 2021 10:13 am

I have had a wild idea of making dummy rods using stiff fencing wire driven by the servo going back to the signal box.
It would be a bit of mucking around but could look believable.
I was thinking of using pieces of 4mm micro irrigation risers as guides.
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Re: Servo Controlled Points

Post by Jimmyb » Fri Mar 12, 2021 10:56 am

watching this with interest, though one point (no pun intended) when I had solenoid operated points in smaller gauges we use to use omega loops to prevent damage, as it would take up any over/under throw, have you considered this. If this has already been discussed and I have missed please give me a virtual :shock: slap

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Re: Servo Controlled Points

Post by Phil.P » Fri Mar 12, 2021 1:25 pm

The problem with omega loops, is in the garden, they rust quite quickly..

I have wondered if link, possibly with the pins included, could be 3D printed, and include an omega loop in the print?

I guess the choice of filament material, would be key to this being possible?

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Re: Servo Controlled Points

Post by Jimmyb » Fri Mar 12, 2021 6:00 pm

Phil.P wrote: Fri Mar 12, 2021 1:25 pm The problem with omega loops, is in the garden, they rust quite quickly..

I have wondered if link, possibly with the pins included, could be 3D printed, and include an omega loop in the print?

I guess the choice of filament material, would be key to this being possible?

Phil.P
I always used brass ones :)

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Re: Servo Controlled Points

Post by philipy » Fri Mar 12, 2021 6:24 pm

Jimmyb wrote: Fri Mar 12, 2021 6:00 pm
Phil.P wrote: Fri Mar 12, 2021 1:25 pm The problem with omega loops, is in the garden, they rust quite quickly..

I have wondered if link, possibly with the pins included, could be 3D printed, and include an omega loop in the print?

I guess the choice of filament material, would be key to this being possible?

Phil.P
I always used brass ones :)
Me too.
I've also used brass ones in the garden on manual points, but I think the tension/pressure balance could be a problem with servos. On my fairly short working rodding runs there is sufficient flex in the brass rods to allow some push pull flexibilty.

I'm not sure that the standard ABS would be man enough for printed omega loops of a sensible size, and PLA would be too rigid. I guess that Nylon or PetG filament might work but I've no experience of them myself. These days there are other more exotic materials but no idea what.

I do have PLA 3D printed angle cranks in my rodding runs and they seem to be OK.
Philip

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Re: Servo Controlled Points

Post by gregh » Sat Mar 13, 2021 6:53 am

Just another idea for operating the signal. Instead of using a 2nd servo, I used a piece of fishing line from the throwbar to the signal. With the signal arm weighted to go to stop when the line is not pulled, and the the fishing line pulling it to clear.
Greg from downunder.
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Re: Servo Controlled Points

Post by GAP » Sat Mar 13, 2021 8:45 am

gregh wrote: Sat Mar 13, 2021 6:53 am Just another idea for operating the signal. Instead of using a 2nd servo, I used a piece of fishing line from the throwbar to the signal. With the signal arm weighted to go to stop when the line is not pulled, and the the fishing line pulling it to clear.
Used at "Black Wall" passing siding?
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Re: Servo Controlled Points

Post by GAP » Sat Mar 13, 2021 8:47 am

Jimmyb wrote: Fri Mar 12, 2021 10:56 am watching this with interest, though one point (no pun intended) when I had solenoid operated points in smaller gauges we use to use omega loops to prevent damage, as it would take up any over/under throw, have you considered this. If this has already been discussed and I have missed please give me a virtual :shock: slap
What are Omega Loops?
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Re: Servo Controlled Points

Post by gregh » Sat Mar 13, 2021 9:49 am

GAP wrote: Sat Mar 13, 2021 8:45 am
gregh wrote: Sat Mar 13, 2021 6:53 am Just another idea for operating the signal. Instead of using a 2nd servo, I used a piece of fishing line from the throwbar to the signal. With the signal arm weighted to go to stop when the line is not pulled, and the the fishing line pulling it to clear.
Used at "Black Wall" passing siding?
Yes. And I had them at Melaleuca, but converted to small servos, but I should have stayed with the fishing line as the servos failed with water.
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Re: Servo Controlled Points

Post by philipy » Sat Mar 13, 2021 10:01 am

GAP wrote: Sat Mar 13, 2021 8:47 am

What are Omega Loops?
Omega is a letter in the greek alphabet and is the traditional symbol for electrical Resistance, although commonly an upper case R is used now since that is available on the keyboard.

An omega loop uses the same basic shape formed in wire, and because of its shape it acts as a kind of spring since it is flexible enough to expand or contract by a small amount in a lateral direction.
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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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