Tuesday 8 April 2008

Pictures of DCC AJ Coupling

In an earlier blog entry, I described the DCC Alex Jackson Coupling which I have been developing.

I've now taken some pictures of the first version, and present them here with some annotations. Clicking the pictures should cause them to appear much larger.

The first photograph is labelled to show the main elements.

  • (A) is the Alex Jackson coupling, formed from 0.010 guitar string.
  • (B) is a nickel silver u-shaped bracket which forms the support for the pivoting wire (A).
  • (C) is a steel counterweight. 2mm diameter, with a 12BA thread at one end. The thread is cross-drilled 0.3mm below the brass nut. When installed, the nut is wound down to clamp the counterweight onto the AJ wire (A). I have a better design for the counterweight which will be much easier to install than this first version.
  • (D) is the electromagnet coil, squeezed in behind a gearwheel which drives the locomotive jackshaft.

Because the locomotive is upside-down, the counterweight (C) falls to one side under gravity. When the right way up, it hangs vertically downwards.

This second photograph shows the same elements from the other side. The wires from the coil can be seen running over the jackshaft axle before heading inside the locomotive. There is a small piece of PCB mounted below the motor where the coupling wires are connected to the DCC chip wires.

The third picture has a propelling pencil holding the counter-weight approximately vertically. This shows the (approximate) rest position of the system if the loco was the correct way up.

The final picture shows the counterweight pulled over by the electromagnet. The counterweight strikes the side of the coil, not the iron core; this avoids it "sticking" when the electromagnet is turned off. An alternative to avoid "stick" is to cover one of the moving parts with something non-magnetic, such as a tiny piece of brass, thin paper, or even blob of glue.

There is massive scope to improve the engineering; the pivot would benefit from an etched design, the counterweight can be made much easier to install, and the coil pole pieces could be designed to deliver a more effective magnetic field. Even the use of Blu-Tac to fix the coil in place should be avoided in later version !

The locomotive is a High Level kit of a small Armstrong 0-4-0 diesel shunter (original at Shildon museum). Prior to fitting the couplings, it was assembled according to the instructions, with P4 wheels, simple beam compensation on one axle. The next loco to have a similar coupling fitted will be built up with the coupling in mind; its a lot easier to design space for parts before assembly !

Photographs taken with a Canon Powershot A710, with addition of a x4 and x2 close-up lenses fitted to a Canon accessory lense holder. The camera is on tripod, and taken with timer release. Its a pretty cheap setup for closeup work.

Ski Pictures from Norway 2008

Photographs from Geilo, Norway, Feb/March 2008.

Nigel's Photos

Sarah's Photos

There were also photos from Chris and Janet, though they require site registrations/invitations, so consult your personal email if you were on the trip.

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.