Gary Liebisch posted:... I assume distance is internally measured by counting total revolutions of the flywheel motors.
Which of course explains why it's a metaphorical fool's errand to use the record function to precisely position the engine after multiple loops over time since errors will accumulate. This is the same headache the DCS trolley/subway guys have trying to automatically stop right at the station platform after multiple loops over time.
The sky's the limit if you are willing/able to add sensors and write software for a computer-based system.
But if computer-based control is not in your comfort zone there have been sporadic discussions of non-software alternatives for DCS users. Specifically, the problem is how to automatically stop a DCS (command control) engine in response to some position based event...in this case being at the exact unload point for dumping coal, logs, whatever. The most common example I've seen is to stop an engine for block control or when approaching a drawbridge in the "up" position. There are 2 solutions that I'm aware of. 1) reduce the track voltage to a voltage that is enough to keep the engine alive (i.e., DCS electronics does not shut down) but not enough to spin the motor. This is a tricky affair with your-mileage-may-vary results. Or, 2) insert a relay in the engine to cut power to the motor(s). The relay is controlled by some external trigger like a light bulb in the track bed or whatever. The track voltage stays at full command voltage but the engine stops, the engine sounds will go to the idle engine sound even if the electronics is trying to drive motor. That is, in DCS the engine sounds are based on the flywheel activity rather than the commanded engine speed.
So, addressing the OP's specific application, the idea would be for the recorded program to activate an AIU output to enable the stopping trigger (e.g., the lightbulb in the track bed). If going slow enough, removing motor power to a DCS engine will reliably stop the engine within an inch or so every time. That is, the record-play would command the engine to go, say, 1 foot past the desired stopping point and the trigger-relay mechanism would stop it right on the dime so to speak. There would be no accumulated error after multiple loops. To be clear, this requires a modicum of DIY'er persistence though the actual cost would be modest compared to a computer-based system.
