When is 120 SMPH really 120 SMPH?

They were travelling in different dimensions.

 

Pete

I know they are the same gauge, but are Bantam and Imperial the same scale?

The tach readers that maintain the smph of engines are very close but not perfect.

 

I have seen very little variation at the 25 - 50 smph range where I usually run trains.

 

It seems to me that the 120 smph range is much faster than most of us run our trains and that any slight variations would be accentuated at those speeds.

 

Jim

 

 

 

PJB

 

The number displayed on the DCS remote it the speed you are sending to the locomotive as the commanded speed.  The locomotive receives the command and compares the commanded speed to the speed it is measuring on its motor tachometer.  If the measured speed is lower than the commanded speed the motor control circuitry will increase the duration of the 12 volt DC pulse sent to the motor.  This is called pulse width modulation.  Once the motor control goes to 100% duty cycle, 12 volts all the time, it can't make the locomotive go any faster.  Some locomotives will hit full pulse width well before hitting 120 smph.

 

You might have your boys compare speeds between the two locomotives at much slower speeds, I'll bet they match.  Then ask them if they can determine the speed at which the one locomotive maxes out.  Running the locomotives together at say 30, 60 and 90 smph could help narrow down the precise point where one locomotive hits full pulse width.

 

Wikipedia has a nice brief article on duty cycle,

 

https://en.wikipedia.org/wiki/Duty_cycle

 

as well as a lengthy one PID controllers which are used to determine the amount of PWM change to make in response a difference between the commanded speed you send and the speed measured by the tachometer.

 

https://en.wikipedia.org/wiki/PID_controller

 

There is a heck of a lot of technology packed into each of our modern locomotives.  They can be a great educational toy if you and your boys are interested in electronics and their applications in everyday products.

 

 

On my 0-27 layout; it would be the actual point at which an engine goes airborn off a curve and headed toward the cement basement floor. 

 

Curt

What Ted said!

-Greg

Obviously the modern diesel had the computerized sanders going, and the 1:48 steam engineer was cursing his fireman for filling the sand-dome short. Wheel-slip encoders wouldn't have existed in the poor steam engineers time, so the electronic stool-pigeon caught him out. 

Can the fast one be adjusted down with new flywheel optical tape?

Can the fast one be adjusted down with new flywheel optical tape?

 

It could, but then it wouldn't match any other stock locomotive at any speed. 

 

 

 

 

 

If they don't run the right speed at slower settings, say 45-55 SMPH, then I'd consider tweaking the tach tape of the one that's off speed.

 

Removing a stripe will bump it about 4 SMPH, adding a stripe will reduce it about 4 SMPH, that's for standard tach tapes with 24 stripes. 

 

I have a set of custom tapes that I picked up here at one point, I believe they were generated by Ted Hikel.  They're primarily intended for upgrading non-MTH locomotives, but they would obviously work in this situation if you actually identify a speed problem over all the speed ranges.

 

 

I have to go with Ted on this one.

I'd try them again at less SMPH and see if they are still separated by that much difference.

Nothing I own will ever see this speed as the engines are not rated that fast

Depending on how old they are, turn it into a math problem, measure how fast the engines are running a measured distance, and scale it up using the scale of the engines, that will tell you how fast the engines are really going. 

Maybe the slow engine as to speed up?

 

Clem k

Frankly, how do you know either of them was actually going a scale 120 mph?   One could have been 5% high, one 5% low.  I imagine both were off and basically when they display 120 mph they really mean, "I going as fast as I can, boss."

I have a set of custom tapes that I picked up here at one point, I believe they were generated by Ted Hikel.  They're primarily intended for upgrading non-MTH locomotives

 

Actually, my brother Dave did that.

 

but they would obviously work in this situation if you actually identify a speed problem over all the speed ranges.

 

I highly doubt that PJB has a speed control problem with a consistent error that could be corrected by a tach tape.

 

I believe the Bantam J and the diesels have the same can motors but the J has a different gear ratio.  The J just sends 12 volts to the motor and runs out of motor RPM first.

 

 

Could well be Ted, if they're geared differently, one would presume they also have different counts on the tach tape?  They still should go the same speed when running at a slower speed, say 50 SMPH.

 

 

Thank you everyone who contributed substantive answers.

 

Ted - thanks very much!  Honestly, it's not a big deal to us, just a point of curiosity.  But my 10-year old sons are very mechanically inclined and math and science whizzes.  So I REALLY appreciate the explanation and the links, which I'm sure we will all find very helpful and interesting.  Should be right up their alley.  Really, the more science and math, the more interested they will be in our layout building project.   

 

Lee - huh?? No need to get out the Flux Capacitor on this one.  It was just a point of curiosity. 

 

Peter

 

 

Peter

 

They still should go the same speed when running at a slower speed, say 50 SMPH.

 

John

 

I agree.  If they test at lower speeds and work up they should be able to determine when the J runs out of revs.

 

my 10-year old sons are very mechanically inclined and math and science whizzes.  So I REALLY appreciate the explanation and the links, which I'm sure we will all find very helpful and interesting.  Should be right up their alley.  Really, the more science and math, the more interested they will be in our layout building project.  

 

Peter

 

That is great to hear.  If you look at old Lionel catalogs they sold toy trains in part as educational toys.  Wiring up old style accessories like crossing gates and signals can give an introduction to building basic electrical circuits.  With modern locomotives there is a ton of technology on board.  We are just now beginning to open the door to digital control and modern user interfaces in model railroading. 

 

Something I never thought of trying - but I no longer have DCS to check it out.

 

What is the maximum smph the handheld will show (although (as Ted said) you will reach full pulse width well before that)???

 

I imagine it's a case like the good old days when speedometers looked something like this even though the car never could do it!

 

 

Jim

The DCS remote tops out at 120 SMPH.

 

Thanks, John.

 

We probably should add a disclaimer. Something like - Don't try this setting at home, particularly with sharp curves and with an engine you value!

 

Jim

I guess after nearly 15 years the speed sensor starts to go......in this video, the GP-30 is going about 27smph and the Decapod 16.......in order to keep them a safe distance  apart.

 

 

Peter

Actually, the more likely candidate may be the tach tape, it could be an easy fix.

 

Could be that the drag of the steamer's running gear made it physically impossible to go as fast as the diesel at the voltage applied.  Try applying more voltage until the motors or boards burn out.

The J is geared lower; its motor hits "redline" at probably less than 90 scale MPH.  The diesels with two vertical can motors (which have always been geared ridiculously high IMO) can undoubtedly exceed 110 scale mph.  Neither loco might be able to reach the 120 indicated.

 

I don't believe the current version of DCS has a protocol for the loco to report its actual speed back to the controller (although earlier versions might have done this.)  At those high RPMs, it also might not be able to calculate the actual speed.

 

Ted Hikel, what's that layout diagram at the bottom of your second post?  Does MTH have a Layout Control System for DCS??  Thanks.

Seems to me that 120mph in the 1:1 world would be 176 feet per second.

 

In the 1:48 world that would be 3.6666666666666667 feet per second.

 

Or, if your choo-choo traverses an 11-foot section of track in 3 seconds,

a) You're running at 120 smph

b) You're in trouble if the next curve is less than O-480.

c) You might want to wrest the throttle from toddle---r.

d) The engineer/fireman in your gnu 3rd Rail ToT are singing "Nearer, my God, To Thee".

e) You probably should consider taking up R/C airplanes/helicopters.....gravity rules with a familiar 'fist' and is part of that hobby....and your LHS's income selling copious amounts of spare parts.

 

Of course, if you're running your Acela, fella, .......carry on, as you were.

 

KD

Don't try this setting at home

 

Jim

 

That is the best advice on this thread and right up there with

 

The engineer/fireman in your gnu 3rd Rail ToT are singing "Nearer, my God, To Thee".

 

for a laugh. 

 

I guess after nearly 15 years the speed sensor starts to go......in this video, the GP-30 is going about 27smph and the Decapod 16.......in order to keep them a safe distance  apart.

 

Peter

 

I agree with John on the likely diagnosis for your Dec.  Sensors usually work or they don't so their isn't a gradual failure.  But if a few stripes on a tach are dirty and the sensor can't distinguish the black from the white it will miscount and run faster than the commanded speed.  One way to confirm this is to check the chuff rate setting and watch the drivers at slow speed.  If the chuff rate is set for four and you see between 2 and 3 chuffs per revolution that would fit with sending a speed command of 16 smph and getting about 27.

 

Ted Hikel, what's that layout diagram at the bottom of your second post?  Does MTH have a Layout Control System for DCS?? 

 

Ted

 

That is the "Green Line" on the Northwest Trunk Lines.  It represents about 1/4th of the layout.  MTH does not have a Layout Control System for DCS.  But Hikel Layouts & Trains does.  If you are interested in PC and smart device control for your layout contact Dave. 

 

http://www.hikeltrains.com/contact

 

 

 

 

 

 

Actually Ted, I think you meant to say that if the sensor is missing stripes that it would run faster than the commanded speed.  Less counts per rev would have the electronics adding power to get the counts up to what it expects for the commanded speed.

John

 

Got it.  Thanks!

I'll do some chuff testing later this week,...Thanks!

 

Peter

Have you ever cleaned the opto itself Peter? The ones I've dealt with in other applications, did not tend to need it often. But on occasion a really dirty one has the same effect.

       
       


Nope. Never even opened them up. Not that important, as we were just curious as to why this was happening  But I'll definitely keep this in-mind if this sort of thing gets worse and I'm looking for solutions.

For what it's worth, the steamer has approx 50 scale miles on it and the CSX diesel was a gift from Santa this last Christmas and has maybe 15 scale miles on it. If these things need to be opened for fixes already then I am buying from the wrong manufacturer. Lol.

This whole thread has made a mountain out of a molehill.  The test of a loco is not how fast it will go, but how well it runs at very slow speeds, and how it pulls heavy trains.  99 44/100% of us want to run trains prototypically.  To fault any manufacturer because of a loco's top speed is the height of ridiculousness

Just a thought no one has mentioned that the slower engine may have at one time had MAX speed set. It would read the call for 120 but not go past MAX speed setting. Pull the stops then try ball to the wall.

       
       


Your post gave me a good laugh!  While I too was surprised by the number of responses, from your post it sounds like you never actually even read my initial post - it was to understand why two engines that are supposedly running at 120 smph are in fact traveling at different rates of velocity.