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Let's Talk...INERTIA???
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 Posted: Tue Aug 15th, 2017 01:17 pm
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bobquincy
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David,
thank you for the information!  Are the 32 steps linear?  Now I am thinking of using a rotary switch with resistors to provide a positive selection.

boB



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 Posted: Wed Aug 16th, 2017 07:42 am
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DavidT
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Not quite. I believe these are the timings of the 32 steps:
0.028
0.117
0.352
0.704
1.408
2.347
3.520
4.693
5.280
7.040
8.213
9.387
10.560
11.733
14.080
15.488
16.896
18.304
21.120
22.528
23.936
25.813
28.160
30.507
32.853
35.200
38.720
42.240
44.587
49.280
51.627
56.320

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 Posted: Wed Aug 16th, 2017 02:44 pm
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bobquincy
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Thanks again David, this is very useful information!

boB



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 Posted: Thu Aug 17th, 2017 11:38 pm
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Bob D
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Bob and David, thanks for keeping this going!

I haven't been doing much on the train hobby lately, my daughter & family moved in with me back in April and just last week moved out and into their new house. Now I plan on getting my kitchen remodeled so I may have less (or hopefully more) time for trains in the next months.

Last edited on Thu Aug 17th, 2017 11:38 pm by Bob D



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 Posted: Wed Sep 13th, 2017 11:15 am
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Richard A-J
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Yes Bob, let's talk inertia. Great to see this thread on the subject, I look forward to studying it, but first I would like say something about the model railway world's relationship with inertia. It has always puzzled me that inertia is seen as an add-on feature. Something you can have or not have. This way of looking at inertia for a model I find very puzzling. To me it's the like asking yourself 'Shall I do it right, or not right?'. The way I see it there are models and there are toys:-

Toys are simple representations of the real thing
Models are accurate.

If you are going to make your model move, it means its movement must, by definition, model that of the real thing otherwise it is not a true model. It does not mean that it should only sometimes move like the real thing, just as it should not only sometimes look like the real thing. It also does not mean that the operator should be given a choice in the matter. Whether a model models the real thing or not should be down to the model maker, not the model operator. An operator of the real thing cannot extinguish inertia when they feel like it, and even if they could, would they be able to survive the G forces? The performance specifications of the real things are fixed, the locomotive driver / engineer cannot alter those specifications whilst driving (well not drastically anyway), they cannot make it accelerate or stop faster than its specifications just because they are late in the timetable or want to get some shunting done quickly.  If a certain locomotive has an acceleration rate of 0-60mph in 1 minute, the model should not be capable of scaled 0-60 in 1 second.

In the real world there is no such thing as zero inertia, it is not switch offable. The whole design and maintenance of track, wheels and the need for signalling, catch points, the speed operators drive and all safety features are all massively influenced by that one fact.

I believe that all models (to earn the title) should have inertia, and it should not be switch offable, so therefore the main means of giving a model inertia I suggest should be mechanical. The trouble with electronic inertia is that it can be switched off, either intentionally or unintentionally, consequently I think it should only be used as an aid to mechanical inertia so therefore I applaud Bernd's attempt at doing this … well done Bernd, excellent work. Of course mechanical inertia could also go wrong but with the biggest of flywheels possible even if it did go wrong it would hopefull not show in a sudden stop (one of the worst illusion spoilers). A flywheel should always be bigger than the motor, and preferably made out of tungsten (if anyone knows how the do that, where to buy it without costing a fortune, please enlighten me).

I think inertia should be easily adjustable by the operator whilst driving … BUT … there should be no zero inertia setting. The lowest setting should aim to replicate the inertia of a 'light' engine i.e. a locomotive with no train. The operator then should have the ability to adjust the inertia whilst driving in order to simulate that of pulling and stopping a train. Of course, if that could be done mechanically aswell then all the better.

Rich

Last edited on Wed Sep 13th, 2017 11:18 am by Richard A-J

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 Posted: Wed Sep 13th, 2017 11:31 am
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Tony Walsham
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In large scale I can never understand the attraction of any inertia options at all.
If you want to simulate the speed up and slow down of a heavy train, just turn the speed control knob more slowly.



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 Posted: Thu Sep 14th, 2017 01:11 am
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bobquincy
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Since we are trying to simulate accelerating many tons with a tiny flywheel our results are not likely to be realistic. Electronic inertia has a different problem where the control does not know actual motor speed. One solution is to look at back electromotive force (BEMF) and many DCC controls do just that. A potentially better method (especially at low speed) is to use a speed encoder to provide feedback. With either of these methods the speed control knows the actual motor speed instead of just providing a certain PWM and hoping it looks realistic.

Pololu sells inexpensive ($8.95) encoders (optical or magnetic) that are made to fit some of their motors but could be adapted to many others by using NWSL adapters/joints/shafts. We would need something to read the encoder output and control motor power accordingly, probably a microcontroller and a motor driver board. Hey, who said realistic inertia was easy? With a microcontroller reading 12 pulses per revolution of the motor we can precisely control acceleration to simulate any amount of inertia required.

Total cost would be less than $20 for a do-it-yourself kit (programming not included).

Reading BEMF is not difficult either, we just shut off power to the motor for a brief time and read the voltage into an A/D converter. The microcontroller I use converts the voltage in about 25 uS and takes another 25 uS to send a correction signal to the motor. The downside of BEMF may be poor response at low motor speeds where the voltage is too low or erratic to provide a good signal. Some testing will determine which method works best.

I like the eddy current idea (and may try it) but the electronic methods look easier and even the eddy current plan would probably work better with an encoder and feedback.

Or we can turn the knob slowly, which is what I do for now. :)



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 Posted: Thu Sep 14th, 2017 11:01 am
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Helmut
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As I'm using DT's transmitters, the 'inertia control' on them does it for me satisfactorily. BEMF as such has nothing to do with inertia simulation, it only helps to follow the set speed value.



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 Posted: Thu Sep 14th, 2017 07:27 pm
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Richard A-J
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Tony Walsham quote "If you want to simulate the speed up and slow down of a heavy train, just turn the speed control knob more slowly."
Bobquincy quote "Or we can turn the knob slowly, which is what I do for now. :)"

What I am pointing out is that I think prototypical movement should really be the job of the model, not the operator. A driver of the real thing does not have to slowly push the throttle lest the locomotive should shoot off like a rocket … or likewise be carefull with the brake in case it stops dead. (Sorry Bob, I'm not knowledgeable enough about electronics to understand your idea).

There seems to be a thinking that inertia is just for heavy trains. Blimey, locomotives are heavy beasts also and they have some considerable inertia all on their own. I still think electronic inertia should be kept to adding the final touches to mechanical inertia. I reckon the key to inertia is the fact that with the real things, starting and stopping is difficult. With our models it is ease peasy lemon squeezy. We need to make it difficult and I think the way to do that is by using weaker motors, motors that are too weak to shoot off your locomotive like a Formula 1 racing car.

So, how do you then get your locomotive moving if the motor is too weak? Answer - you use a higher gear ratio, in fact you use a CVT (Continuously Variable Transmission). A weaker motor could get a model moving if it is starting off in a gear ratio of, say 400:1. If it also has a massive flywheel, once it has got going it will have the energy from the flywheel to keep it going and give it extra power. As the locomotive increases in speed the CVT moves down in the gears.

Of course, making a CVT is going to be difficult, but I think it needs to be tried. A one gear ratio to fit all speeds is not really a good system in my opinion.

I think such a system might also make wheel slip starting much more realistic, in fact, I get the feeling it might mean a locomotive could pull heavier loads, because it is not trying to start in top gear, but I don't know. The motors being smaller and the help of a flywheel might also be an advantage for battery life, not sure about that either, does anyone have any ideas on that?

Rich

Last edited on Thu Sep 14th, 2017 07:38 pm by Richard A-J

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 Posted: Fri Sep 15th, 2017 12:58 am
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bobquincy
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As an electrical engineer (retired) I will leave the flywheels and CVT to the mechanical engineers, electronics is more my thing.  A weaker motor is easy to do, on a very basic level the motor power is from the magnetism which is governed by amps x turns of wire.  The turns of wire being set, less amps = less power and we can easily control the amps.
The problem is how do we control the amps properly in order to provide a smooth, consistent acceleration?  A lookup table (as used by Deltang) is one way (also the simplest way) but cannot account for stiction; motor nonlinearity; or any other of the complex parameters of a moving device.

A speed control system without feedback does not work well: think of cruise control in a car where the system could only set a certain throttle opening (from a lookup table) and assumes that will maintain a certain speed.  It doesn't work, feedback (in the form of a signal reporting the actual speed) is necessary for the system to maintain a given speed.  Our trains require a certain current to start but once friction is overcome the current should be quickly reduced to provide a smooth (and slow) acceleration.  Feedback on motor speed can provide the correction.
Disney's Disneyland Mark VII monorail model comes with a remote control, on/off, zero inertia!  I added a microcontroller programmed to ramp up a PWM output to the motor from zero to full power over 5 seconds, it works nicely to provide a soft start/stop but I can see the nonlinearities and quirks in the system.  It is still a lookup table type with 64 steps of PWM and no feedback.
I plan to add a speed sensor to the motor and program the microcontroller to maintain a constant acceleration, it should work a lot better.  You may think this is a lot of electronics and programming just for inertia, you would be correct!  But it is probably less than the DCC manufacturers did to add BEMF to their decoders, apparently they considered it useful and worthwhile.



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