In my controlling interest #1 post I mentioned the change of turntable control system to the Model Railway Electronic Group (MERG) turntable control unit. Since I first built Fisherton Sarum the turntable was powered via a Frixinghall motor and gearbox that did not have any form of automatic indexing. This meant that the rotational speed of the table was not easy to control and track alignment had to be done by eye (sometimes hitting the mark easier than at othertimes) via a toggle switch.
Friend and regular operator of Fisherton Sarum, fellow High Wycombe and District MRS member and electronics wizard Mark Riddoch has very kindly built for me a MERG turntable control kit which drives a stepper motor and gearbox.
This clever bit of electronics can give up to 63 different positions (I only need 8 i.e. each end of 4 roads, on Fisherton Sarum) to an amazing precision of 0.03 degrees.
The turntable is simply operated from the panel via a rotary switch (a binary hex coded switch, for those that understand such things) to select the desired road and then pressing the start button. The controller then automatically works out the shortest distance to the desired alignment. When turning clockwise the deck is stopped automatically at the correct place or if turning anti-clockwise it over runs slightly before stepping back clockwise to the correct position ensuring any slack in the gearbox is taken up. The controller has a learn mode to allow the desired positions to be accurately set up which it then remembers.
The typical bank holiday weekend weather, just passed, encouraged working inside rather than other pursuits so the installation was completed. Installation has required a few slight changes to the control panel: firstly to incorporate the rotary switch, mounting inside the control panel the controller PCB card and changes to the wiring harness to get the required five wires to the stepper motor and gearbox that is being mounted directly under the turntable itself, replacing the previously installed traditional motor, gearbox and worm and wheel drive.
It is imperative that the link between the gearbox outlet and the table deck is positive and has no possibility of slip. The Peco deck is designed to be an interference fit on to a Meccano shaft so to ensure no unwanted movement is possible I have drilled through the boss and the shaft at 90 degrees and pinned it with a short section of Brass wire.
This shaft matching the gearbox output shaft has had a flat filed onto it and a brass collar joins to the two with grub screws ensuring a positive connection.
Programming the turntable in learn mode has not gone 100% to plan and I did not yesterday manage to get it working correctly, so need to some additional advice. Once I have mastered the programming I will upload a video of the turntable under its new control in action, so watch this space.
I am once again indebted to Mark, and offer my my sincere thanks for his time and assistance with this addition to the control of Fisherton Sarum which should, when fully up and running, improve the operation and visual effect no end.
Although Fisherton Sarum is not currently set up, I am using the time between shows to make a few changes and additions to the operating system. These modifications include: the addition of a third controller for main line operation (see below), the installation of a Model Electronic Railway Group (MERG) turntable control unit with stepper motor, moving of the signal control to the fiddle yard panels and changing all the point motors to slow acting Cobalt types. The other modifications will form the subject of future posts so to coin a phrase watch or follow this space.
Our default operation is one controller for the shed area and one for the main lines. This does mean that only one of the fiddle operators actually drives the main line trains and does so in both directions. The suggestion was to add a third controller so that two can be used on the main lines, i.e. one up and one down, allowing each fiddle yard operator to drive the trains towards their end. This will also increase the likelihood of trains passing on the main too (already possible but meaning the shed movements having to stop). The plan also includes the addition of remote signal controls to the fiddle yard indicator panels to ease operation too.
Time has been spent this weekend modifying the control panel wiring for the new controller, along with the installation of the new Turntable controller (but more of this soon). The final part of the controller installation will be the wiring on the west end baseboard to a socket for the new controller (the wiring harness is already in place with suitable spare cores). Fisherton Sarum’s first outing with the new operation will be Tring and District MRC show 13th October 2012.
As part of the layout control changes currently being made to Fisherton Sarum (more of which later) involves the turntable I thought it was about time I posted about the turntable itself. The turntable at Salisbury was originally a 65’ foot heavy over girder type. In 1958 this was replaced by a larger diameter under girder type. An unusual feature of Salisbury shed was the the access to the stores building unloading platform was via a shunt across the turntable and I have replicated this feature on Fisherton Sarum and it often raises a comment at exhibitions.
On Fisherton Sarum I wanted to represent the original style of over girder table installed at Salisbury. I used a Peco kit for the well and the deck, modifying the deck by scratch building the heavy over girder sides and a the addition of the turner’s platform.
Thick plasticard and Plastistrut sections was used for the girder sides along with an overlay embossed with a punch to represent the rivets along the top of the sides. A small outbuilding was provided to act as a mess room for the turners employed at the shed, on Fisherton Sarum this is a modified Wills kit.
Ironically the Peco kit unmodified matches the later style at Salisbury.
Since I first built Fisherton Sarum the turntable was powered via a Frixinghall motor and gearbox that did not have any form of automatic indexing. This meant that the rotational speed of the table was not easy to control and track alignment had to be done by eye (sometimes easier than others) via a switch. This is in the process of being changed to stepper motor and gearbox controlled via a Model Electronic Railway Group (MERG) turntable controller kit, kindly built by Mark Riddoch, more of this in a future post.
The model railway world and mainly Southern Railway meanderings of Graham 'Muz' Muspratt