N Gauge Low speed

[Robin2]

I started a couple of other threads about writing a PC program to control my trains through a Hornby Elite controller. I had grand ambitions to have a throttle and brake and make the train accelerate based on scale power and weight data.

 

However when I got the program sufficiently finished to try it out I discovered that it is quite impractical on my small (6 ft) layout. Before the train has stopped decelerating it will have hit something. It does not seem to be possible for it to go slow enough to allow any significant time - ie several seonds -  for deceleration.

 

So I have simplified my program to remove acceleration and braking and just directly control the speed. It is still much better than the silly knobs on the Elite which go round and round forever with no sense of when they are at full throttle or stop.

 

I don't think it will make any difference using the acceleration feature within the decoder.

 

I would like to hear other people's experience of slow speed control of N Gauge trains.

 

I should say that my 2 steam locomotives are about 15 year old Graham Farish designs.

[Nigelcliffe]

Its not quite clear to me what you are asking.

 

If it is "how to do reasonably close to scale inertia in 6ft?", then I think the answer is that it will only work with very short trains operating at low speeds; eg. around a loco yard, a dock (pre container period), or similar constrained space.

 

If it is "how slow can you get a Farish to run", then I've managed just over 0.5rpm at the wheels of a Crab for four minutes without stalling, running on a Digitrax Zephyr (around 13v at the track). When using an Elite the performance is similar, though I did not time it. That is stupidly slow, but it give a very good impression of mass when accelerating and decelerating from stand-still - there is no instant change from walking pace to stopped. It requires clean wheels and track, and good pickups (and tender pickups). I do use the acceleration parameters in decoders, not just CV3/4, but any others that the makers provide (eg. Zimo's "exponential acceleration" to stretch things at slower speeds). And I spend a fair bit of time tweaking motor control features, such as reference voltages, BEMF frequencies, K and I parameters, etc.. depending on what is offered by the decoder maker.

 

Dropping the track voltage should improve low-speed control. I don't think the Elite has any controls for the track voltage, but you can drop it by putting the track feed through a pair of diode strings; each diode pair will drop the track voltage by 0.6v. Taking it down possibly as low as 10v may give you the slow speed you desire.

 

- Nigel

[Robin2]

That is very interesting. Do you know what speed step (out of the 128 possible) you loco is operating at? Mine is working at 6 but is not really slow. At the same time my track may need more cleaning.

 

I must try lowering the voltage. I guess that would mean that you would need a higher speed step for the same effect.

 

 

[Nigelcliffe]

When it left me (it was a commission install), the loco would have been set to run at its slowest speed possible on step 1 of 128. I have no idea what the lowest knob setting on an Elite sends to the track.

 

To run well, a loco has to run well on DC first. Then choice of decoder makes a big difference. Finally time spent setting up CV's to run properly.

 

- Nigel

[Robin2]

My PC program is setting the speed steps so I can make it do what I want. I am just using the Elite to create the necessary DCC signals in the same way that you would use a Sprog. Unfortunately I did not know about Sprog when I bought the Elite. 

I guess if you set the minimum voltage in the decoder the speed steps don't compare with another identical decoder with a different minimum voltage.

 

My feeling is that it is easier to leave the decoder at its factory settings and make the adjustments within my PC program - its a lot less trouble than changing CVs. However I'm new to DCC and there may well be features in the decoder that I am not conscious of and which are not the same as changing the speed steps. 

 

 

I don't have a DC system any more. 

[Nigelcliffe]

My PC program is setting the speed steps so I can make it do what I want. I am just using the Elite to create the necessary DCC signals in the same way that you would use a Sprog. Unfortunately I did not know about Sprog when I bought the Elite. 

 

I think you might find it easier with a different controller with a more reliable and predictable computer interface.

If JMRI development is anything to go by, the Elite is unpredictable, tends to drop commands, fails to acknowledge them correctly, etc.

 

I guess if you set the minimum voltage in the decoder the speed steps don't compare with another identical decoder with a different minimum voltage.

 

That depends what you are trying to achieve. If you want all locos to do the same MPH at the same throttle position, then you will need to set the speed curve in the decoders. Its commonly called "Speed Matching", and is done a lot by US modellers as they tend to run trains with four or more locos on the front.

 

My feeling is that it is easier to leave the decoder at its factory settings and make the adjustments within my PC program - its a lot less trouble than changing CVs. However I'm new to DCC and there may well be features in the decoder that I am not conscious of and which are not the same as changing the speed steps. 

 

If you want really good control, the factory defaults do not necessarily work the best.

 

The decoder is controlling the motor speed, sensing the back-emf from the motor. The motor is highly variable (different makers, designs, sizes, quality control, mechanical loads), so the factory settings in a decoder are a best guess. If you want it to work exceptionally well, spend time adjusting a lot of motor control parameters (which is not CV2!); I wrote up my approach to setting one type of decoder some months back:

http://www.2mm.org.uk/articles/CT-decoders.htm

 

 

And I suggest any difficulties changing CV's are really "problems with the Elite" again; the Elite's keypad is a nightmare to change CV's, and the Elite's troublesome computer connection means software like JMRI/DecoderPro has to run at a snails pace to be sure the Elite has done all it was asked ( around eight times slower than some other systems in reading the same decoder ).

 

I don't have a DC system any more. 

 

I think that a DC controller is a must if you want to test locos thoroughly. Without one, you cannot tell if the problem is a chip setting or mechanical in the loco. I spend a fair bit of time with an analogue ammeter running locos under DC to find and remove tight-spots in mechanisms before doing anything DCC to locos.

 

 

- Nigel

[Robin2]

I think you might find it easier with a different controller with a more reliable and predictable computer interface.  

 

Can't afford one now

 

 

 

 

 I wrote up my approach to setting one type of decoder some months back:

http://www.2mm.org.u...CT-decoders.htm

 

 

 

This looks very interesting - thanks

 

 

 

[Frutigen]

Smooth slow running is important for me, too. I run swiss electric outline n gauge, including some locos that are about 30 years old. Choice of decoder makes a big difference and I find the Zimo Mx620 is by far the best for slow running, especially with older, rougher models. Lenz silver mini are good, too, but they lack the soft transition from stationery to walking pace and back again.

 

First thing I do is to set vmax (cv5) to around 20% of the model's own max, especially on older models, which tend to have higher top speeds. This gives a satisfying speed through the points and curves on my single track cross country route - anything faster looks silly as the train makes a sudden change of direction through the points. The added benefit is that the 128 speed steps are working on a much shorter speed scale, so each step is finer.

 

Next I work out at which speed step the loco begins to move, and set this as the vmin (cv2). This reduces the dead zone you can get with reluctant locos at the low speed end of the controller.

 

If necessary, you can refine the running of the motor with all sorts of strange cvs but it takes a while to work out what they all do, even with the manual.

 

For the record, I use a Lenz Compact controller with a Roco Multimaus as a slave.

 

Keith

[Etched Pixels]

The decoder also matters as does the mechanism. It's pretty hard to get a 15 year old Farish small steam loco to do very low speed due to the motor and gearing. For tender locos you can get close if you've got tender pickups to get good power pickup, a decent decoder and are willing to spend time tweaking the mechanism.

 

From the Bachmann era the diesels can usually be made to crawl at lower than scale walking pace given a good decoder like a CT.