I've volunteered to take the tractor test bed to the Tutbury gathering next month so I've been doing some more work on it ahead of that. All of the work so far is 'below stairs' so visitors may not see much visible difference from it's state last year at Chelford but in fact quite a lot has changed.
The first big change is that the curtain track has now been relaid to run dead straight instead of following the line of the Esso siding. This makes manual steering more predictable - previously it was a bit tricky to drive when the carriage was on the curve, so trying to steer the tractor along the straight road of the Esso siding turnout was not easy.
Second I've made the swing arm about 20mm longer and added a new gear into the train. This means that the magnet can reach 20mm further out on either side of the centre line. Several of the el cheapo moulded chinese gears have been replaced with machined ones from HPC. A couple of cheapo ones remain where I still need thin gears.
Thirdly there is a little sensor (called a slotted optical switch) part way along. The carriage has a rectangle of black plastikard that passes through the slot, breaking the light beam. This allows the Arduino to 'know' how far along the track the carriage is.
So why bother when it was working OK anyway? Well the other big item on the 'to do' list for the mockup was to deal with the gravity shunting that was used in the 1950s to gravitate oil tanks from the bromine works gate back onto the road outside the Esso depot. My original plan was to build a separate contraption with an electromagnet moving along another track. I did some experiments with small electromagnets and found that anything small enough to fit just wasn't strong enough to do the job. Plan B was to have a rare earth magnet mounted on a servo so that it could be either engaged or not with the tank to be moved.
After sketching several ideas which all looked like a lot of work I started wondering if I could avoid the job entirely and do the gravity shunt using the tractor drive. To cut a long story short I found that I could... but the tractor drive would need more 'reach' and the Arduino (the brains of the operation) would need to know exactly where the magnets were instead of just knowing the general direction in which they were pointing. The Arduino software naturally needed a whole lot of work to make it do new things.
The final piece of work has been to replace all of the curtain track mountings. They had been done fairly quickly (i.e. badly) by just screwing down into bits of strip wood glued to the cross members along with Blu Tack and bits of wood or card to pack things to vaguely the right height. This was OK to get things working but on the real layout they wont be accessible from above so I needed to design some new mountings that can be adjusted and removed from below. The new ones use Tufnol blocks drilled and tapped to take machine screws and an aluminium angle cross member that is removeable from below.
Finally while the thing was turned over I took a picture of the carriage from underneath from where you can see the two stepper motors and the bits that keep it on the curtain track.