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A pointless fiddleyard principle

Old wine in new bags

In real life trains run from one station to another often passing several other places. For most people it is not feasible to model an entire railway system or even a small part with only 2 stations. Lots of people have to do with only 1 tiny layout, and have to simulate the rest of the world with a fiddleyard. There all kinds of fiddleyards and in his book “ Layout Design “ Ian Rice sorts them into fans, sector plates, traversers, turntables and cassettes.

Having built a cassette fiddleyard quite some time ago, the limitations of that type of fiddleyard became quite a nuisance when fully switched over to FS160. Stock derailments when shifting the cassettes over became order of the day, limitations on train length etc made a nuisance to work with. A decent fiddleyard was to be built. And in my case it is called Montzen for good reasons. Montzen is the shunting station on the Belgium border near Aachen, where most east and north bound traffic passes. Requirements for Fremo use: useful length of tracks minimum 1m20, compatible with NEM with turned down flanges, good visual appearance thus ballasted code 40 track, prepared for DCC control.

This resulted in 2 baseboards of 1m25 ample satisfying the minimum length requirement. The Fremo requirements of a minimum radius of 1 m were followed in order not to run into problems with the couplings. This means that points of 1:7.5 form the maximum angle, but for visual appearance 1:9 looks much better. Having built one baseboard with a double track fan using triplepoint, double slip and several standard points the outlook to built another set of points was a bit daunting. Thus other solutions came under review. The most easy option is a sectorplate. A sectorplate normally has several moving tracks on a carrier board connecting to one or more fixed parts on the baseboard This means a (deep) cutout in the baseboard is necessary to accommodate the carrier board. This was not compatible with the visual appearance in my opinion. This lead into some rethinking on the principles: Can we do without carrier, using the baseboard as gliding surface? Can we combine a traverser with a fan?

The design idea

Playing around led to the idea of using a piece of flexible track one end fixed and the other end curving over to the left and the right just as in a stubpoint. The basic idea is to connect several fixed tracks with a single track traverser. The German name adopted now for this principle is "Schleppweiche".

Several small technical problems need to be solved: some way to accommodate the length difference of the curvature in the left hand and right hand rail, and for fiNe-scale wheels the connecting ends need to be lined up with an accuracy within 0.2 mm in 2 dimensions. What really came out you see in the accompanying photos and drawings. Of course some compromises were necessary. On top of the baseboard we see one piece of code 80 flexrail. This was selected because the American code 40 flexrail is hardly flexible whereas Minitrix has some flexrail that sit loose in their plastic trackbed, they even will fall out when kept vertical. The other compromise is in order to align the track ends with the required precision, below baseboard mechanics are necessary. This required a 6 mm wide curved slit in the baseboard at the moving end. The height difference between code 40 and code 80 was spread out over a 15 cm long raised section of the fixed connections.

The next two photo's show the traverser with above and below baseboard construction.

birds eye view below baseboard

Construction details

lengthwise cross section click for more details

Figure 1: Sketch of the cross section along the centerline.

principle of accomodation of length differences click for more

Figure 2: The upper sketch shows how to accomodate for the length differences, the lower one gives a cross section of how the flatbottom track slides without side play.

Figure 1 shows the centerline cross section, figure 2 shows the principle of how to accomodate for the tracklength differences due to curving the flextrack. In real life you will find rail connections which are able to take the expansion of several kilometers of welded track by sliding railends. The same principle is used here by a mechanism with sliding rails. This consists of about 25 mm track soldered to a pcb base. At one end the rails are halved over a length of 4 mm. The flexrail has the complimentary part taken out and thus these sections can slide along each other without a gap in the track. This one is symmetric, you will get away with using this principle in only 1 of the rails and fixing the other at both ends. However there will be some strain in the fixed rail possibly resulting into failure of the solderbond over time. The lower drawing clarifies how each (flat bottom) rail glides between 2 sections of brass angle keeping the alignment in vertical and horizontal plane thus making optimal use of the flatbottom section. This can be implemented at either end of the traverser but using the moving end simplifies the wiring.

Because of the precision necessary in the track alignment we cannot tolerate slop. Thus the moving part needs to be pulled gently against the baseboard and yet be moveable without too much force. The top drawing shows how this is obtained by using a kind of wagon that sits below the baseboard. This is bolted onto a M6 threaded rod hinged to a central pin some 25 cm away. The length of the rod is based on the maximum allowed radius. The wagon can thus rotate along a fixed curve taking out one degree of freedom in horizontal plane. The vertical plane is taken care of by a set of wheels on the wagon. The wheels are 2 ball bearings of 20 mm diameter. These are seated on a piece of 10 mm hexagonal brass with an excentered M4 at the fixing end. The excenter is used to take care of the vertical degree of freedom, thus the pcb on top is pulled against the baseboard with the wheels rolling at the underside of the board. Don’t think that I have been beavering away for weeks in the workshop, the whole shebang was made on a sunday afternoon using parts from a plotter mechanism out of the junkbox. So learn this lesson to keep an open eye for useable parts where you don’t expect them. The above makes up for the basic mechanism. The rest of this article illustrates how you can make simple life more difficult.

Optical feedback

Now for the actual line up I didn’t thrust my eyesight to be able to see 0.2 mm difference from 2 m distance thus I made use of some electronics. There exist simple optical sensors that can do this job with higher precision and more attention than I can. They cost less than a pound when new, but you might inspect the junkbox again because you can also save them from floppy disk drives. This sensor, an open opto coupler, with a led on one side and a light sensor facing it, detecting things (such as a floppy) in the light path. Normally this is an infra-red led so you cannot see any light at all. This sensor comes to good use by taking a strip with holes as alignment help. If light passes through, the wagon is in the right position. Actually I used a steel strip with slits cut by the narrowest jewellers saw, with some 40 teeth/cm, this leaves a cut of about 0.2 mm.

sketch of feedback and
drive unit

Figure 3

electronic scheme

Figure 4

Figure 3 shows the principle and layout of the optical feedback unit as we call this thing deftly. The accompanying electronics are shown in the left hand section of the fourth sketch. To narrow down the position of the detection further, use is made of an opamp comparator which switches a led on. Only when the voltage of the sensor passes that of a fixed threshold voltage, made with the normal diode, the led burns. You need this because the slit is narrower than the field of view of the sensor. The comparator now switches only when the slit passes the invisible light beam at its peak intensity instead of between the edges of the beam area. You can change the diode into a small print potmeter for adjustable control if you wish. The 5V supply bears a relation to the righthand section of figure 4, but apart from that you can use any other available voltage level if you adapt the resistors. Of course it is not impossible to cut the slits at the exact position of the tracks. However doing it the other way round makes life a lot easier. Thus first mark the positions of the slits at roughly the right place, make the cuts and test/finish the electronics, and mount the complete wagon and moving track on top of the baseboard. And then complete it by soldering the fixed rails at those spots where the led comes on. The fixed ends are soldered to a curved piece of pcb screwed permanently to the baseboard. However take care for a bit of expansion of the track, I had to file some track ends when temperatures ran into the 30’s. That is just all about it to get a simple hand moved traverser with feedback into working order.

Wiring the unit

This design dates from the analogue age. This requires extensive wiring in order to keep only 1 track live. Therefor the analogue version was extended with a set of electric wipers to distribute the power to the connected tracks. The main lead feeds the others, an extra rotary multi-position switch overrules this in order to carry out shunting manoeuvres with the traverser pointing to another track. Now in digital age this is all superfluous. All tracks and traverser can be connected parallel to the digital booster.

A more interesting option might form the mechanical drive. I used a model servomotor coupled to a microcontroller to drive it. A servomotor reacts to a signal pulses with pulslengths between 1-2 ms. The position of the axle being in fixed relation to the pulse length for a particular servo type. There are servos with different amounts of rotation, 90 or 180 degrees being the most common. I am using a 180 degree type. The pulses are generated with the microcontroller. The righthand section of figure 4 gives an idea of the basic principle and useable components. A programmable microcontroller can be used to steer the servomotor directly. This servo motor is coupled to the connection rod as shown in figure 3. The controller gently steers the servomotor to the rough position and then creeps further until the led comes on. Running stock on the flexrail during passage is possible and won’t run off due to the movement. Care is taken to arrive at the positions always from same side so that friction and/or any slack in the connecting rods won’t play havoc. In my case the commands for track selection are send by infra-red communication as this saved building a control interface and a lot of wiring. The SFH 506 (or SFH 5110) is a IR demodulator/receiver making a clean digital signal from the IR beam. The microcontroller further decodes the digital signal and interprets commands for selecting a track. A general replacement IR controller for domestic appliances is used. Of course this might interfere with your television set if you play trains thus some care in code selection is necessary in order to keep the peace at home. Costs of chip components is low, 10 euro or thereabouts would suffice, a cheap servomotor you find new for 8, but a cheap IR controller needs shopping or visit the junkbox again? Complete description of the ins and outs of programming such devices is not given here but you can explore the internet for related subjects and projects. Chip specifications can be found at the manufacterers sites like Atmel of Microchip together with useful application notes and Siemens in Germany for the SFH receivers. Although much more expensive using a ‘basic stamp’ (www.parallax.com) is an alternative, programmable in basic instead of assembly language but requiring more extensive IR decoding circuitry. A good working solution is using an extra chip between IR receiver and stamp in the form of a Philips SAA3049 IR serial to parallel decoder together with appropiate RC5 IR code. This will give you a fast entry with minimal development time.

This article is based on work from last century and can be used by everyone for personal applications. However all rights in any form for further publication in magazines that require any form of subscription are reserved and require written permission.

Last Updated: 13 June 2002
Author : Henk Oversloot
all photo's copyright : Henk Oversloot