Monday, 7 April 2008

4mm Scale Digital Alex Jackson (AJ) coupling

As a break from 2mm scale modelling, I make some items to Scalefour standards. A while back, I started looking at DCC control of model trains and started to think of the control options a chip inside a model might offer.

One such is digitally controlled couplers; a function button on the controller can activate the uncoupler. This is not particularly new; European makers have offered such uncouplers for years on HO trainset models. The only real challenge is to make the system work with the uncouplers in common use in UK finescale model making.

My initial idea came from seeing the latest version of the 2mm scale Electra coupler. This is a small rotary acting coupling whose origins dates back to the 1970's. Its somewhat simpler to make than a 2mm scale Alex Jackson (AJ). There is a similarity in the coupling action of the AJ and Electra, and, as I have 8 times the volume inside a 4mm scale model, I decided to experimented digitally controlled uncouplers in 4mm scale as a route to 2mm scale operation.

The main shank of the coupler is a standard AJ. It rotates in two small holes arranged behind the buffer beam. A steel bar weight is fitted to the coupler shank, and ensures the correct coupler rest position. A home made electromagnet coil can attract the steel weight away from vertical, rotating the coupling head to the "uncouple" position.

The electromagnet is made from 45SWG enamelled wire (from the Scientific Wire Company), wound onto a ~1mm iron core. The core is a piece of garden wire with the green plastic coating removed. I have found that approximately 12m of wire works well (around 50 ohm resistance), though the exact amount will depend on the available space. There are various aspects of coil and magnetic field design which need experimentation in the future, such as turning one end of the core round to be near the opposite pole, or fitting a metal disc to one end of the coil.

Winding was done on my lathe in lowest back-gear, using a close-up TV camera to watch the wire was laying down tight against preceeding turns. Temporary end-cheeks from PTFE rod controls the length of the wound core. Care in winding makes a huge difference to final coil diameter - sloppy windings have lots of air spaces which make the coil bigger. My early coils were secured with occaisional dabs of super-glue, though I'll change to shellac for future ones. The PTFE end-cheeks are carefully pulled away once the coil is complete.

DCC control uses a function output from the chip. Both Zimo and CT Elektronik chips have support for uncouplers within their firmware, with CV values to set the time for the full power applied to the electromagnet (to initiate the pull) and then a lower power to hold the coupler open. Zimo have a function which can, optionally, back the loco up a few scale inches, uncouple, then drive away about a scale foot (though not yet working on the MX620, the manual is wrong on CV116, Zimo promise a fix during spring/summer 2008). I would expect that almost any maker's chip would work using their normal function outputs (lights, fans, etc).

Due to the rotary action, if the loco is not driven away and the coupler closed (magnet off), the train remains coupled. One function key can operate both uncouplers simultaneously. This simplifies the selection of function key on the DCC control panel. Selection of which function key is down to chip programming and individual choice; one or two handset makers suggest F3, but other makers use F3 for "half speed" mode. At the moment I am using F1.

With two coils (two couplings) in series on a single function wire, I measure about 90mA at full power from a Zimo chip, well within the 300mA limit for function output of the smallest Zimo (MX620). Its also just within the limit for the tiny CT DCX74/75 (100mA), though I've not yet tried these chips with the uncoupler; I would be tempted to slightly increase the resistance (small resistor in series) in the circuit with a CT to drop the current a little.

The more current through a given coil, the stronger the magnetic field. On a bench test, I found my coils were getting rather warm with 20 seconds at 150mA, but could not detect any heating at 100mA.

There is a video of its operation on the Google Video site and now duplicated on YouTube

The loco fitted with the digitally controlled AJ will work alongside conventional under-baseboard magnets. The loco acts as any other loco with a fixed coupling, and the wagon uncouples in the conventional manner.

It would be possible to put the uncoupler under rolling stock, though the cost of fitting a function decoder (a DCC chip which can respond to function commands but lacks motor control) under every wagon could get quite high, and having to address a wagon before uncoupling would be tedious on most DCC systems. I can conceive of ways of making it very easy to operate with minic diagrams, though the computer complexity and price of assembling such a system would be very high. There is a half-way house where some key items of stock could have an uncoupler fitted; brake vans, vehicles in trains which are regularly split, etc.

Having retro-fitted the coupler to an existing locomotive, I think it would be a lot easier to design the coupler components when building rather than afterwards. My next loco will be done that way round; design the coupling components before building. It should result in better shape of magnetic field and a smaller air gap for the moving part to move across, that means less current (or smaller coil) to operate. I'm still thinking about the 2mm scale version !

Comments welcome, pictures and drawings will follow when I have some time to add them.

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