Servo drive unit.

[grovenor-2685]

Here are the details for a Servo drive using the Exactoscale Tortoise adapter. Thanks go to Dave of MERG for the idea of using heat shrink sleeving to hold the wire in place, which in turn set my thoughts into motion.

First up is a video of my first test bed version.


The pictures are pretty well self explanatory so I'll add minimum text, anything unclear please ask.

Part 1. Parts needed.

servo-parts-1.jpg

Missing are a pair of 2.5 mm nuts and bolts to hold the Servo.
The brackets and wire guide can be made from any angle material or scrap brass to hand, the carpet edging was what I had available

servo-parts-2.jpg

servo-parts-3.jpg

What I have called the "drive pin" the manufacturer calls a "closed loop adapter". I have one left in my packet, not sure how many I started with but at least three. Could be made by filing and drilling the end of a 2 mm or 2.5 mm bolt. I had these in stock.

servo-parts-5.gif

Part 2 follows.

[grovenor-2685]

Part 2 - Assembly.

servo-assy-5.jpg

servo-assy-8.jpg

servo-assy-9.jpg

servo-complete-12.jpg

Here it is evident that I did not do a very good job marking out the wire guide holes giving a poor wire alignment, although it did work. Anyway an advantage of bolt together construction is that its easy to take it to bits for rework. So I took the guide plate off, marked it out with more care and re-drilled.

servo-complete-14.jpg

Part 3 follows

[grovenor-2685]

Part 3 - Completion

After redrilling the wire was much better aligned.

servo-complete-13.jpg

Here's the rear view---

servo-complete-11.jpg

And the complete unit---

servo-complete-10.jpg

Or as it will look under the baseboard--

servo-complete-10a.jpg

Just a note on the throw as you need to fit the wire guide plate to suit (or you could drill several mounting holes in the plate to provide adjustment).
My points need a throw of 1.5 mm at the blades and allowing for some slack and given the ability to adjust the servo accurately I tried for a bit over twice that. The distance from the servo arm to the throw bar drive pin is 58 mm on my version. It will vary a bit depending on the details of the servo mounting brackets that you make. I set the wire guide(or pivot) 9 mm from the drive pin and hence 49 mm from the servo arm. The throw available at the arm is 20 mm so on my unit the throw at the drive pin is 20*9/49 = 3.67 which should be ample for my P4 points. If you need more just rework the position of the wire guide plate. For example if it is set 12mm back from the drive pin the throw at the pin becomes 20*12/46 = 5.2 mm. If in doubt allow a bit extra as the limits are set electronically in the servo driver, mechanically you just need to be sure there is enough.
Regards
Keith

[David Knight]

Just as an aside Keith, this might be a good time to mention the possible use of servos for signals as well, bounce included! Unfortunately I have no link to the program that Mike Walton uses on his signals but I think it may have come from MERG.

Cheers,

David

[MarkS]

Dave, this is the web site you are thinking of - http://www.copnor.enta.net/FS/TheBouncer.htm

[grovenor-2685]

Just as an aside Keith, this might be a good time to mention the possible use of servos for signals as well, bounce included!

Not really Dave, that topic is already open here http://www.scalefour.org/forum/viewtopic.php?f=11&t=149 and people looking for info on Signals will look for it in the Signal topics rather than here.
Regards
Keith

[Waveydavey]

Keith,

How does this method compare costwise with using Tortoise motors and how are you achieving polarity switching of the point V?

Cheers

David

[grovenor-2685]

Like most things it depends on how much effort you put into sourcing things. if you buy commercial servo drivers then it will likely be more than Tortoises, using the MERG servo driver kit you are looking at £4.50 for each 4 servos, servos themselves somewhere between £2.50 and £5. For the frog switching it depends on how you are doing the layout, if convenient a spare pole on the panel switch can be used and I have some done that way, but otherwise the simplest is to use a relay operated in parallel with the Servo, relay maybe £1 to £2.
So by the time you have a few nuts and bolts, connectors etc say £8 each worst case, easy enough to beat that price.
The servo is also easily adapted to pretty much any existing mechanical linkage you may have using RC Model Aircraft fittings.
Regards
Keith