Controlling crossover with a servo

[bordercollie]

Hi

 

I have finally decided that I will use MERG Cbus. I have also decided to use servos.

My questions are:

       a) Can a crossover be controlled by just one servo?

       b) If so, what be a good, and preferably simple, way to connect the servo to the two switches?

       c) Is it a good idea anyway, considering the relative cheapness of servos?

Regards

[Ray H]

It wouldn't be the easiest thing to do even if you were using something other than servos because you need to get the blade travel distance more or less exactly the same and the linked mechanical drive would be a challenge.

 

A crossover normally works off a single lever/switch so you shouldn't need to worry about using two switches.

 

You can work two servos from one switch. One way is to use a double pole double throw switch with one pole connected (indirectly) to one servo and the second pole to the other servo but that can be wasteful on wiring. You may also be able to connect the two servos electrically on the input side of your chosen servo controller.

 

You can probably find all the help you need on the MERG website/forum.

 

Hope that helps.

[Gordon A]

You can by servo leads that are Y shaped so you can connect two servos to the same control. As Ray H above has mentioned you may need some form of adjustment for the blade travel as per the prototype. 

[2E Sub Shed]

 Best use two servos and separate control boards as you can adjust them individually for travel, i.e the throw of the point.

 

If the control boards work by applying a ground to switch direction, i.e. move the servos, then a couple of diodes will combine this to a single switching point, e.g. Single Pole switch, however the servo travel of direction needs to be in sync, so they will each need to be mounted "the right way round" to operate together to switch the crossover.

 

Assume if you are using CBUS, you can get over the both moving the same way by using logic in CBUS.

 

Suggest you ask the question on the MERG Forum as suggested above.

[jcredfer]

Simple answer, yes - to all three questions.

4 hours ago, bordercollie said:

Hi

 

I have finally decided that I will use MERG Cbus. I have also decided to use servos.

My questions are:

       a) Can a crossover be controlled by just one servo?

       b) If so, what be a good, and preferably simple, way to connect the servo to the two switches?

       c) Is it a good idea anyway, considering the relative cheapness of servos?

Regards

 

a)  Place the servo to one side of the rails [over / under the baseboard], halfway between the two sets of Points.  Use the shortest servo arm out of the pack and remove one of the two arms.  Use a pivot pushrod connector [cheap and readily available] in the nearest servo-arm hole, to the axle and run a single pushrod, both ways to end level with the points Stretcher Bar.  [horizontal brown below]

b)  Set the servo throw from 90deg to 270 deg.  Take another 2 of the servo arms, with the arms at a rt angle [black lined yellow, below] and connect the long pushrod to the longer arm on the servo arm.  Take another short pushrod from the inner hole on the shorter servo arms, to the ends of the Bar on the points.  This will reduce the throw on the pushrod to the points and can be adjusted by moving the long pushrod to different holes on the longer pushrod arm.  Throw on the points is also cushioned with "Z", or Omega bends in the pushrods to the points, or coil springs either side of the connection to the short arm. [Buffer springs / 3 link connector springs - fitted similar to the old fashioned rotary governor on steam / diesel engines - I can take a picture if you are interested]  The action of the cushioning will also press the Switch Rails to the Running rails.

 

 

It is also easy to change the Frog polarity, with those short, horizontal, arms pointing to a micro-switch, like a Peco PL - 15, on each end.

 

c)  Servos are around the same price, overall as most other means to change the points, and a pair like this for a crossover / other paired points, is half the price.  They are also very flexible in placement, with the pushrods and some servo arm bell cranks [spare arms come in the pack].

 

I had some pictures and a couple of videos, but they went with a computer glitch, so I have just put the above simple schematic.  If you are interested, I can take some more pictures and a video of the set-up.

 

Julian

That said, the, above, [Gordon A] recommendation for a "Y"  servo wire lead, is a bit easier and the price of an additional servo only.  The servo decoders will handle the power for both very easily.

 

Edited March 6, 2022 by jcredfer

[Michael Hodgson]

It is not unknown on the prototype for one point motor to work more than one point, but they have to get the mechanical linkage right.

So yes, you can do the same, but you'll also have to get the mechanics right, as per suggestions above.  

 

Personally I wouldn't bother - both the MERG module and servos are fairly cheap, and I think it's easier just to fit one servo per set of point blades.  I think you would probably get a similar reply on the MERG forum, servo mounting brackets also available through MERG.

[jcredfer]

2 hours ago, Michael Hodgson said:

It is not unknown on the prototype for one point motor to work more than one point, but they have to get the mechanical linkage right.

So yes, you can do the same, but you'll also have to get the mechanics right, as per suggestions above.  

 

Personally I wouldn't bother - both the MERG module and servos are fairly cheap, and I think it's easier just to fit one servo per set of point blades.  I think you would probably get a similar reply on the MERG forum, servo mounting brackets also available through MERG.

 

I had gone for the Peco Servo kit several years ago, which works quite well and is pretty much connect up the wiring and off you go, as with a good number of the ready to go ones.  The MERG kits are much cheaper than I had imagined, so thank you very much for the prompt.  Your comment about MERG prompted me to go and do what I had thought about a number of times, join.  I have a couple of more tasks for servos, so MERG will definitely be the route for those.

 

You are so right, the mechanics definitely have to be right, as a good number of folks have discovered, much to their disappointment.  For Points, there is only a requirement for a movement of ~ 3mm, at the tie bar and a "little" pressure on the blades onto the Stock Rails.  What is also irritating is when a servo is left to apply a static rotational pressure, it buzzes constantly, hence setting the servo to go from 090deg to 270deg.  All the joints are then inline and consequently no rotation forces.  A reduction of [in this case] 12mm servo arm throw to 3mm throw is a simple 4:1 reduction, slightly less will give a little blade to stock rails pressure.  My layout pictures are, sadly, gone, but a mock-up might serve to illustrate.  Obviously there would be a similar linkage off to the right hand end of the push rod, for the other points.  I included both a wire "Z" bend spring, which is very simple but takes quite a lot of space, with the coil springs, which are much neater and very easy to adjust the pressure at the tie bar.  I won't be using the Z spring for points, but it is simple and very cheap.

  

 

Apologies for the less than tidy bits left on the screwed alloy collars, which are in need of a dressing with a file, to rid them of that unsightly bit left after the connector peg was cut away.

 

[bordercollie]

Hi All

Thanks for your replies. 
I think I will use 1 servo for each point. Mechanically linking 1 servo seems to be beyond me at present.

Best wishes

Graham