New MOC: Switch Point Motor with PF Servo Motor

[Brickthus]

The new PF Servo motor from set 9398 lends itself very well to the control of switch points.

A point motor has these requirements:

1) The motor shall operate within the end stops of the switch point slider without putting too much stress on it.

2) The motor shall be capable of remote control.

3) The motor and mechanism shall be robust and reliable in out-of-reach places for at least a 2-day show.

4) The motor and mechanism shall not foul the loading gauge of rail vehicles.

For the new Mk3 mechanism, the Servo motor has built-in end stops, so it was natural to find a gear ratio that matched these with the end stops of the switch point slider. A gear ratio of 12:20 works well, with 12-tooth cog onto a long rack (2 available in set 9394 Jet Plane and others). The long rack has a hole at each end for overlapping the switch point slider. I included a tell-tale, which also serves to prevent Servo movement in the other direction and thereby takes the stress off the switch point slider. I enhanced the clamp that fixes the mechanism to the switch point for reliability.

Pictures of the Mk3 mechanism (when moderated):

Pic1 shows the mechanism in the straight position, tell-tale down, panel switch centred.

Pic2 shows the mechanism in the curved position, tell-tale up, panel switch to one side (configurable on the switch).

Pic3 shows the mounting of the Servo motor on the baseplate. Pegs into the beam and jumper plates into the other holes.

Pic4 shows this from above.

Pic5 shows the clamping of the switch point to the mechanism. This is key to reliability because the clamp has to resist the forces as the mechanism changes the points.

Folder when moderated

The rack-with-end-hole principle worked previously with the Mk2 mechanism that used the PF M-motor, 71427/43362 gearmotor or 47154 monkey motor, the earlier racks and an 8-tooth cog. The Servo motor has simplified the mechanism compared to the M-motor solution, which needed a clutch gear for the end stops and another gear reduction.

Testing of the Mk3 mechanism has gone well. It seems to be reliable and also has more torque than the Mk2 mechanism. This means it should overcome stuck points more easily. I had one that switch point that needed more torque than a single clutch gear could provide.

There are other train applications of the servo motor:

1) Level Crossing Barriers - could use proportional control for smooth raising, or switched is easy if you don't mind the sudden movement.

2) Decoupler on the loco - pushing a device out from either end to push the adjacent vehicle away. Controlled by Infra Red switching commands to the loco. Needs enough torque to break the magnetic contact but the Servo motor has enough. Allows shunting without "Hand of God" intervention.

3) Signals, either semaphore actuation by levers or flex, ground-based signals by rotation or colour light signals by linear movement of red/yellow/green filters in front of a PF light brick LED. All applications need 2 or 3 fixed positions so these are ideal to be driven from the PF pole reverser switch. A single Servo motor could operate 2 semaphore signals with either end stop for one green and one red and the centre position for both red.

The fact that the Servo Motor stays in its commanded position when controlled from the PF pole reverser switch without running the motor continuously is great for train applications because most applications use occasional movement and long periods at fixed positions.

I look forward to using the Servo motor for more train applications. The Servo motor should be available as a single item sometime in 2013.

How will you use the new Servo motor in train applications?

Mark

Edited August 7, 2012 by Mark Bellis

[Selander]

I've earlier built a PF based remote decoupling system using a medium motor installed in a MOC locomotive, (short video in my flickr account) so as soon as I heard about the servo motor I thought that new motor would be a much better choice. When it becomes available I'll look deeper into the subject.

So far I haven't considered any more application.....but it'll be interesting to see what you other trainheads come up with.

Edited August 5, 2012 by Selander

[Hrw-Amen]

When I read the review of the set using this new motor I also thought straight away of using it for powered switch tracks. I would like to think that TLG would come up with a set comprising of a switch using this as it is at least in my mind one of the most obvious uses for that type of motor. But then again I am in train mode so that is probably why!

[Doctor Whom]

How about using this trick to switch the track:

http://www.brothers-brick.com/2012/05/30/automated-points-switch/

And installing the servo on the train so that the train goes left or right depending on how the servo is positioned?

[Brickthus]

I've earlier built a PF based remote decoupling system using a medium motor installed in a MOC locomotive, (short video in my flickr account) so as soon as I heard about the servo motor I thought that new motor would be a much better choice. When it becomes available I'll look deeper into the subject.

So far I haven't considered any more application.....but it'll be interesting to see what you other trainheads come up with.

That looks good. I see how a 4M length of rack is needed to push the wagon away enough to break the coupling. That sets the gear ratio for the servo motor at 4M for 1/4 turn (assuming the loco decouples at the other end when the servo is commanded the other way). 4M is equivalent to 10 teeth and would suit a 40-tooth cog if one would fit! A 20:12 ratio and a 16:8 ratio with a 12-tooth cog on the rack would be sufficient, also allowing a right-angle turn in the gear train.

How about using this trick to switch the track:

http://www.brothers-brick.com/2012/05/30/automated-points-switch/

And installing the servo on the train so that the train goes left or right depending on how the servo is positioned?

This is a nice idea for cab control but it needs to change in its implementation. So far the height of the mechanism between the rails is 1 plate + 1 brick width. In order for a train motor to go over the mechanism, it should not be more than 2 plates (and no studs) above the sleepers, equal to the height of the rails. Conversely the red dot could not be lower than the height of the rails or it would foul the rails on the points. Therefore a magnet on the bogie might be better because no physical contact would be needed.

The mechanism also needs to allow a train entering from one of the two roads to be able to change the points even if the leading vehicle is light (e.g. reversing a train down a siding). This means pushing the switch rail must be easy so the latch of the mechanism needs to be not too strong.

I reloaded my pics at the top because the thumbnails didn't work properly the first time. I edited the links so hopefully they will be moderated soon.

Mark

[JopieK]

I think that servo is way too big for a switch. I use small servo's very effectively as point motors (without the need to modify the switches).

[deejay]

Video from the Switches.

For those who like small switches. But the Track must be modificated. The Plasticspring must be cut because the little Motor is not strong enough.

Edited August 13, 2012 by deejay

[JopieK]

my solution only switches the rail switch part, no modifications needed for normal switches (just for the cross switch).

[eric87]

Still waiting to see this new servo motor switch a track????