As part of my RTC program, I've been trying to understand how the odometer in the engine works.

Part of the answer must somehow include the gear ratio between the motor and the wheel. I took a diesel, marked the tach cylinder and the wheel position. Then I twirled the tach wheel until I got an integral number of tach revolutions and wheel revolutions. I got 42 tach revolutions for 4 wheel revolutions which gives a gear ratio of 10.5 to 1.

My question is for the next step, trying to understand the number of tach pulses to distance traveled. The tach strip in this engine has 24 black bars. Then I need to know the circumference of the wheel.

The diesel has the typical wheel that uses 22mm traction tires. But when I measure the solid wheel without the tire, I get about 22.6 mm. It's a little tough to measure the wheel with the tire since the measurement depends on how well the tire is installed. I measured a few times and I could say that it might average around 22.6 mm.

Does anyone have any information as to whether or not I am using the right values for the wheel or the gear ratio?

 

 

Original Post

Mark I'm amazed at some of the stuff you've already come up with!  I'm pretty sure the byte 0x08 by itself tells a lot about the effective gear ratio, taking into account the driving wheel diameter.  If you take that value as an 8-bit integer and divide by 22.7, it will yield the approximate motor revolutions per inch.

I've counted the gear ratios on a lot of locos, and at one point several years ago, Dave Hikel shared a table with some common gear ratios.  Every MTH diesel I've ever measured is geared at 10.5:1.  I thought I measured the wheels at 0.875"; that loco may have had a lot of mileage with its tires worn smooth.  I no longer have physical access to all of the locos I measured.  In trying to relate the hex value to distance traveled, I had to guess at driving wheel diameters, so my suggested divisor of  22.7 is approximate.  It's known that slight speed variation exists among DCS locos; traction tire thickness could be one of the variables responsible for this variation.  I also know that MTH specifies the same tire for wheels having slightly different diameters.  In some cases, getting the tire on is a stretch!

The other variable is the flywheel diameter, and especially, the number of stripes.  Perhaps the flywheel diameter doesn't matter much, since the reader sees the stripes in linear fashion.  I'm pretty sure Gunrunner John said 28 stripes is standard.  There is probably a minimum stripe width required for reliable operation.  If the flywheel is too small to accommodate a tape of 28 stripes (at their minimum width), it's possible that MTH fudges the odometer variable so that the sensor's returned value at any given RPM matches that of other "standard" locos.  Maybe that's the purpose of byte 0x09h? 

I know it's possible to "trick" a loco by using a tape with more stripes.  Some of my PS2 locos are jerky below an indicated 4 mph.  Folks here on the Forum have claimed they could force a balky loco to run slower-- at a measured 1 or 2 scale mph-- by using a custom tape with more stripes.  (Of course at higher speeds, the actual speed would be very different from the indicated speed, and also from that of "factory" unmodified locos.)

I hope you're able to find out more on this issue, and I look forward to reading about it on the Forum!

Creep, coast, and pull.  We're not talking about cold fusion here.

I'd be very surprised if the MTH steamers are geared that low, I believe they're more like 18:1  I've recently worked on several Williams steamers where I actually was computing the gear ratio, a brass K-4 was 28:1, and the Williams brass Niagara was 44:1. 

Actually Ted, I have stated several times that 24 stripes is "standard" for MTH PS/2 or PS/3 locomotives.  As far as adjusting speed, dropping a stripe or adding a stripe from a 24 stripe flywheel will change the speed around 4%.

GRJ I'm sorry I misquoted you, no offense, I was working from memory. 

All diesels are 10.5:1.  Most early Premier steam is 16:1.  After 2001, MTH's engineers started looking beyond the Fisher-Price aisle... The 20-3047 Niagara and later 80" drivered Northerns were ~18:1.  Medium-drivered locos with an idler gearbox, like Consolidations and Mikados are ~20:1.  These are solid runners.  Most RailKings until recently were 14:1 (RS-545 larger motor) or 17:1 (RS-385 standard motor).  All of these ratios are "tall" (faster) compared to traditional norms for adult-oriented scale models.

I know for certain that some MTH steam locos with the tiny RS-365, such as RailKing train set locos and USRA 0-6-0 switcher, are geared lower-- 26:1, 28:1, and in some cases more.  Dave Hikel's table confirms this (and he would know!)

Based on my personal experience with two different RK Mohawks and analysis of data in the sound files, it looks like MTH has begun using the 26:1 ratio in more of the RK steam locos with the RS-385.  This is a good thing!  I remember our discussion about the 1980s Williams brass.  IMO 30:1 is about ideal, so the new ratios are a step in the right direction.  Unfortunately, the great bulk of die-cast locos already made are geared too tall and require speed control for smooth operations at single-digit speeds.

Creep, coast, and pull.  We're not talking about cold fusion here.

I suspect that motor torque has as much to do with performance as gear ratio.  My MTH 19th-century 2-8-0 has a gear ratio of 28:1, and slow speed performance out of the box was rather poor.  It got better after a great deal of breaking in, but this is the best it can do (under conventional control; I don't have DCS).  Not bad, but if you ask me, not that great.  And yes, its top end is way too fast.

 

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Walking out the door but can't stand not sticking in my .02cents.  I have two Railking steam locos a ten wheeler and a 2-8-0  H-10  both are around 28:1. They are among the best slow runners I have even without PWM motor drivers.  At 28;1 they still have plenty of top end speed and both run very quiet. Also looks like it would not be too difficult to ditch the 365 Mabuchi for a 385.           j

I opened up my 30-1660-1  P&LE (NYC) 0-6-0 USRA Steam Switcher  #9060 PS3 and twirled the tach wheel. I counted 18 revolutions for one rev of the driving wheel. The blind drivers and non-traction tire wheels are 27mm. Again the traction tire increased that a little bit to 27.2 mm. Tach tape has 24 stripes.

My goal along these lines is to figure out how to decode the data returned by the engine that represents the trip odometer (DTO) and odometer (DOD).

In the next few posts, I will describe what I have discovered so far.

Step 1: a fixed distance

I used RR-Track when I designed my layout. RR-Track has the ability to add up the length of a selected route. I picked my longest circular route and RR-Track told me that it was 1.5884 Smiles. Ellison fans will know that this was his term for a scale mile. In O gauge, it's 110 feet or 1320 inches.

Starting with the diesel's wheel diameter at 22.6 mm, 24 stripes on the tach strip and a 10.5 gear ratio, math and geometry tell us that we have 472.225 revolutions of the driving wheel to cover 1 Smile. That also works out to 119000.8 tach stripes per Smile.

My plan was to use that 1.5884 Smile distance as a standard length to try to understand the DTO and DOD commands.

Step 2: Collect the data

For several years, I've been studying the commands and responses between the remote and the engine. I learned a long time ago how to build a radio snooper to decode the commands and responses. Both the DTO and DOD commands elicit two responses from the engine, the first is a 16 bit number, the second is a 32 bit number. These numbers are read out from the RAM memory in the engine. The first number is always the same for any particular engine and is the same for both the DTO and DOD responses from that engine. The second number increases as the engine runs and does not change when the engine is stopped. The values returned for the DTO and DOD commands are very different but both increase as the engine runs.

My working assumption is that the 16 bit number somehow incorporates the gear ratio and wheel diameter. For the diesel that I'm working with, the value is 21637. In a lot of my documentation, I call this the Scale Factor.

Then the 32 bit number represents the trip odometer (for DTO) and the odometer (for DOD). The remote takes these two values and displays the Smiles on the display.

More to follow.

Ted S posted:

GRJ I'm sorry I misquoted you, no offense, I was working from memory. 

All diesels are 10.5:1.  Most early Premier steam is 16:1.  After 2001, MTH's engineers started looking beyond the Fisher-Price aisle... The 20-3047 Niagara and later 80" drivered Northerns were ~18:1.  Medium-drivered locos with an idler gearbox, like Consolidations and Mikados are ~20:1.  These are solid runners.  Most RailKings until recently were 14:1 (RS-545 larger motor) or 17:1 (RS-385 standard motor).  All of these ratios are "tall" (faster) compared to traditional norms for adult-oriented scale models.

I know for certain that some MTH steam locos with the tiny RS-365, such as RailKing train set locos and USRA 0-6-0 switcher, are geared lower-- 26:1, 28:1, and in some cases more.  Dave Hikel's table confirms this (and he would know!)

Based on my personal experience with two different RK Mohawks and analysis of data in the sound files, it looks like MTH has begin using the 26:1 ratio in more of the RK steam locos with the RS-385.  This is a good thing!  I remember our discussion about the 1980s Williams brass.  IMO 30:1 is about ideal, so the new ratios are a step in the right direction.  Unfortunately, the great bulk of die-cast locos already made require speed control for smooth operations at single-digit speeds.

Hello Ted S

I have an Railking Imperial Challenger #30-1724-1 which has PS-3 and RS-385 motors and driver dia. is 32-33 MM.  What is the gear ratio for this engine ?  

God will save you from hidden dangers and from deadly diseases, Psalm 91:3

Tiffany 

Step 3 : expectations

Scaling factor 16 bit value - I would expect this to be a number that multiplies or divides the number to be scaled.

Like:   actual value = initial value * scaling factor

or:      actual value = initial value / scaling factor

In his reply above, Ted S, thinks this is not a 16 bit value but rather two 8 bit values. He might be right.

DTO and DOD 32 bit value - This has to be derived from the tach strip. In its simplest form, I would expect it to be a count of tach pulses. I would expect the DTO value to be a lower value with a lot of decimal places. A representation that could measure short distances accurately. I would expect the DOD value to be a higher number with few decimal places. A representation that could measure long distances. Think about the number of digits on the odometer and trip odometer in your car.

So if the DTO and DOD values were just a count of tach pulses, we could expect that the scaling factor embodies the gear ratio and wheel diameter to convert the raw values into linear distance.

Step 4: results

Now, running my engine over the measured 1.5884 Smile loop, I got the scale factor, DTO and DOD values from the engine. I used the remote to get these same values displayed as Smiles.

It took some experimenting and hints from Eric Linz, but I came up with these two equations:

DTO in Smiles =  raw_value / (11 * scaling_factor)

DOD in Smiles = (46.5 * raw_value) / scaling_factor

The constant factors in the equation were not exactly pulled out of thin air but I can't explain their values. I got those values by working backwards using the remote to tell me what DTO/DOD value I should see.

Step 5: tests

Over my fixed loop, the DTO and DOD values using the equations agree to within about 0.25% with the actual length of the loop. I tested this with one diesel engine and one steam engine that had different gear ratios and different wheel diameters. Both equations agree with the remote down to a few decimal places (but don't produce the exact same results as the remote).

I don't yet understand how to take a gear ratio and wheel diameter to produce the scaling factor value.

That's all I have been able to figure out about DTO and DOD.

raw_value is clearly based on the same tach stripe count for DTO and DOD.  Perhaps another way to look at it is to observe that 46.5 x 11 = 511.5.  That is suspiciously close to "exactly" 512.000 - in other words is a 9-bit binary shift which is arguably what you'd expect a presumably fixed-point microcontroller processor to do to represent two levels of resolution or scaling of the same underlying value.

 

@Tiffany your loco has a value of "96" in the sound file, byte 0x08.  Based on the info you provided (and my past experience), I would guess the gear ratio is 17:1.  You can verify this by taking the shell off and turning the flywheel by hand.  Make a tiny mark on the flywheel and driving wheel rim so you don't lose count.  Turning the flywheel by hand won't hurt the loco, and it's a good way to see if something is binding in the drive train.

Speaking of which, @49Lionel what kind of motor does your 19th Century loco have?  I can't help but feel that maybe something is binding on yours, because as John said, the 28:1 locos are usually very smooth runners.  The little RS-365 revs to 12000 RPM or more.  If the worm gear isn't pressed onto the motor shaft (and unfortunately many of them are  , you could probably swap the motor for another one with lower RPM and more torque.

@SanDiegoMark Not sure if this helps, but do you know the "sampling period" for the tach sensor?  The scaling factor supports the speed as well as the odometer function.  In order to know the loco's current speed, Protosounds has to poll the sensor every second, every half-second, etc.  If 0x08 by itself is the scale factor, then multiply the static value by 1408 to get stripes per scale mile.  Multiplying 0x08 by about 0.4 gives the number of stripes per second, per smph  (in other words, the expected number of stripes per second at 1 smph.)  Higher values indicate a slower, smoother-running loco.  How does the sensor actually work?

Creep, coast, and pull.  We're not talking about cold fusion here.

The sensor clearly has to be using something like the uP counter input function, you could never individually capture all the pulses otherwise.  If the motor is spinning at 8,000 RPM (some do, I checked), and you have 24 stripes, you'd have 3200 stripes/second to sense, or one about every 300 microseconds.  You might be able to service interrupts at that pace, but it would be consuming a significant part of the bandwidth.  What I do in the Chuff-Generator is use an input pin counter trigger to count the stripes and then just react when I get to the correct count.

Ted S posted:

 

Speaking of which, @49Lionel what kind of motor does your 19th Century loco have?  I can't help but feel that maybe something is binding on yours, because as John said, the 28:1 locos are usually very smooth runners.  The little RS-365 revs to 12000 RPM or more.  If the worm gear isn't pressed onto the motor shaft (and unfortunately many of them are  , you could probably swap the motor for another one with lower RPM and more torque.

I don't know what motor it is, but here's the best picture I have:

DSCN4273

When I had the rods off I tested each component and didn't detect any binds.  I'd definitely be interested to see how a different motor would perform, but that's not something I'd try doing myself.

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@49Lionel the good news is, your motor is easily replaceable.  (The flywheel is removable, and there's a U-joint between the motor and the worm shaft.)  The bad news is that from your photo I can't tell what kind of motor it is.  It LOOKS like an RS-385, but the answer lies on the white label hidden by the electrical tape.

Creep, coast, and pull.  We're not talking about cold fusion here.

There is also a ton of the 3xx Mabuchi motors, so you want more than just the RS-385, as there are endless varieties of those.  You will want one that is a 12V or 15V model, and also has a top RPM to match your gear ratio.  When I went looking for motors, I was amazed at the difference in specification for the same 3xx model number, there are a bunch of letters/numbers after the motor type that denote various options.

gunrunnerjohn posted:

The sensor clearly has to be using something like the uP counter input function, you could never individually capture all the pulses otherwise.  If the motor is spinning at 8,000 RPM (some do, I checked), and you have 24 stripes, you'd have 3200 stripes/second to sense, or one about every 300 microseconds.  You might be able to service interrupts at that pace, but it would be consuming a significant part of the bandwidth. 

I figure that for a typical diesel, there would be 119,000 pulses per Smile. At 120 Smph, that would be 238,000 pulses per minute or 3966 pulses per second - one every 252 microseconds or so.  On a lower geared steam engine the pulse rate would be even higher.

So, as you noted, its likely this is a hardware counter.  But the DOD and DTO raw values are read out by reading 4 bytes of RAM each. If these are hardware counters, they could be memory mapped. I don't know what is available in whatever uP is used.

 

Ted S posted:

I'm pretty sure the byte 0x08 by itself tells a lot about the effective gear ratio, taking into account the driving wheel diameter.

I have 16 engines and I've looked at the bytes at RAM 0x08 and 0x09 to try to see some pattern.

All diesels have values like 0x54 0xAB, 0x54 0x85 and 0x55 0x45. These all have about 22mm wheels.

The 4 steam engines have these values

0-8-0     0x5D 0x64    27.55mm wheel

2-8-0     0x62 0x55     32.74mm wheel

Berk      0x50 0x23     36.27mm wheel

0-6-0     0x7B 0x90     27.11mm wheel

Maybe there are some hints in these numbers.

Mark, I'm not sure that 0x09 has anything to do with the speed or odometer function.  The byte at 0x08 does a pretty good job by itself of describing the effective gear ratio and by extension, motor revolutions per inch, stripe count per inch, stripe count per second at a given speed, etc.

Your sample data only confirms what I already knew.  All diesels are geared the same at 10.5:1 and typically have the same size wheels.  With my understanding, and never having had any of your locos apart, I predict the gear ratios of your steamers as follows:  0-8-0 is 14:1, 2-8-0 is 18:1, Berk is 16:1 (I'm guessing yours is Premier- I have one), 0-6-0 is 18:1.  The 0-6-0 has the highest 0x08 value and would probably be the smoothest runner of this group.

The formula for the approximate gear ratio is: Driving Wheel size in mm * the value of 0x08h as an 8-bit integer * 0.00552. 

I'm not sure why some diesels have 0x54 and others 0x55.  But the difference is only 1 in 84, or about 1.1 percent which is within tolerances for double-heading.   I also can't explain why the formula doesn't work out to the exact value in every case. 

Some theories are: the exaggerated curvature of a very small flywheel may impact the accuracy of the tach reader, leading to missed or double-counted stripes.  Also, there may be a fudge factor to account for expected skidding / slippage on curves.  On tight curves, the inside rail is significantly shorter than the outside, so one or both wheels have to skid, and this may translate to lost motion.

A long time ago MTH warned that operating scenarios recorded in the TIU wouldn't be suitable for repeated playback, without eventually resetting the loco to its exact starting position.  They predicted that position errors would accumulate over time, leading to fouled switches, running through the block, etc.  Lionel's LCS tries to get around this problem by using track sensors to establish the position of the loco, and to trigger events.

Creep, coast, and pull.  We're not talking about cold fusion here.

SanDiegoMark posted:
gunrunnerjohn posted:

The sensor clearly has to be using something like the uP counter input function, you could never individually capture all the pulses otherwise.  If the motor is spinning at 8,000 RPM (some do, I checked), and you have 24 stripes, you'd have 3200 stripes/second to sense, or one about every 300 microseconds.  You might be able to service interrupts at that pace, but it would be consuming a significant part of the bandwidth. 

I figure that for a typical diesel, there would be 119,000 pulses per Smile. At 120 Smph, that would be 238,000 pulses per minute or 3966 pulses per second - one every 252 microseconds or so.  On a lower geared steam engine the pulse rate would be even higher.

So, as you noted, its likely this is a hardware counter.  But the DOD and DTO raw values are read out by reading 4 bytes of RAM each. If these are hardware counters, they could be memory mapped. I don't know what is available in whatever uP is used.

In my little C-G, I just let the tach counts accumulate in a 16 bit timer.  In the mainline loop I compare that count to my calibration constant.  When it equals (or exceeds) my calibration constant, I trigger a chuff and zero the hardware timer for the next cycle.  Although my cycle time for the simple functions of the C-G probably never missed a count, if I did miss one, it wouldn't really make any difference in the greater scheme of things, so that wasn't a major issue.

Ted S posted:

Mark, I'm not sure that 0x09 has anything to do with the speed or odometer function.  The byte at 0x08 does a pretty good job by itself of describing the effective gear ratio and by extension, motor revolutions per inch, stripe count per inch, stripe count per second at a given speed, etc.

Your sample data only confirms what I already knew.  All diesels are geared the same at 10.5:1 and typically have the same size wheels. 

The formula for the approximate gear ratio is: Driving Wheel size in mm * the value of 0x08h as an 8-bit integer * 0.00552. 

I'm not sure why some diesels have 0x54 and others 0x55.  But the difference is only 1 in 84, or about 1.1 percent which is within tolerances for double-heading.   I also can't explain why the formula doesn't work out to the exact value in every case.

Ted, I'm going to spend some time considering your point about the byte at 0x08. I can modify RTC to use only that byte in the DTO and DOD calculations. I'll post my results.

The one strong point against it (at least to me) is that when the remote reads up the scale factor, it always reads up two bytes. Reading bytes out of RAM is fairly expensive in terms of bandwidth over the track. In every command that I've looked at, the remote never reads up any extra bytes. If it could work with just the byte at 0x08, it would not have read up the byte at 0x09.

Mark

 

Mark I'm not arguing with you, I would love to help.  I just don't know what 0x09 means or how it's used.  0x08 by itself gets you pretty close to the scale factor.  Way too close to be a coincidence, but not exact, and that bothers me too.  Unless 0x09 is some kind of "fudge factor" (see theories in my post), I just don't know.

Creep, coast, and pull.  We're not talking about cold fusion here.

Ted S posted:

@Tiffany your loco has a value of "96" in the sound file, byte 0x08.  Based on the info you provided (and my past experience), I would guess the gear ratio is 17:1.  You can verify this by taking the shell off and turning the flywheel by hand.  Make a tiny mark on the flywheel and driving wheel rim so you don't lose count.  Turning the flywheel by hand won't hurt the loco, and it's a good way to see if something is binding in the drive train.

Speaking of which, @49Lionel what kind of motor does your 19th Century loco have?  I can't help but feel that maybe something is binding on yours, because as John said, the 28:1 locos are usually very smooth runners.  The little RS-365 revs to 12000 RPM or more.  If the worm gear isn't pressed onto the motor shaft (and unfortunately many of them are  , you could probably swap the motor for another one with lower RPM and more torque.

@SanDiegoMark Not sure if this helps, but do you know the "sampling period" for the tach sensor?  The scaling factor supports the speed as well as the odometer function.  In order to know the loco's current speed, Protosounds has to poll the sensor every second, every half-second, etc.  If 0x08 by itself is the scale factor, then multiply the static value by 1408 to get stripes per scale mile.  Multiplying 0x08 by about 0.4 gives the number of stripes per second, per smph  (in other words, the expected number of stripes per second at 1 smph.)  Higher values indicate a slower, smoother-running loco.  How does the sensor actually work?

Hello Ted S

Thank you for the information on my Imperial Challenger's 17:1 Ratio, I guess isn't too bad as it really is smooth running engine so I am happy about that.  This engine weight is 8.55 pounds by itself if that matters ?  It is heavy for my weak arms so I had to use both of my arms to pick it up and this engine is about 1/50 or 1/55 scale.   I am NOT the smartest cookie in this world but do get by okay LOL.   I don't understand what "value "96" in the sound file, byte 0x08 means ?   I read what other guys in this tread talking about but I don't get it ?   I am wanting to "learn" more about this wonderful die cast 12 coupled steamer.  I believe all of the lighting is LED's so it even better.

You will have power to trample on lions and poisonous snakes.  Psalm 91:13 ERV (Easy to Read Version)

 

Ted,

I appreciate all of your comments. Makes me think further.

I ran a bunch of comparisons today between my equations and the remote. I am using the scale factor as a 16 bit number. My equations are:

DTO = ((RawValue) / ((ScaleFactor) * (11)))

DOD = (((46.5) * (RawValue)) / (ScaleFactor))

To come up with these equations, I first guessed that the raw value had to be divided by the scale factor. Then there needed to be a constant to get the correct value. I used a comparison with the value generated by the remote, worked backwards and found the constants needed. It amazes me how well these constants work in the equations as shown in the comparisons below. I can't explain the significance of these constants.

I'm still looking at the scale factor and how it could be derived from the gear ratio and driving wheel diameter

DOD
11 Sep 2019

Engine RTC Remote
1 646.41 646.4 U28B
4 297.30 297.3 SW1200
4 298.16 298.2
5 162.29 162.3 2-8-0 H9
5 163.41 163.4
6 159.58 159.6 Berkshire
11 290.29 290.3 0-6-0
11 290.55 290.6
13 100.52 100.5 GP7
14 38.73 38.7 GP38-2
14 40.92 40.9

DTO
11 Sep 2019

Engine RTC Remote
1 0.25967 0.3 U28B
4 0.88183 0.9 SW1200
4 1.73594 1.7
5 1.11786 1.1 2-8-0 H9
6 0.40387 0.4 Berkshire
6 1.09274 1.1
11 0.25700 0.3 0-6-0
11 0.51748 0.5
13 1.55870 1.6 GP7
13 2.08773 2.1
14 2.19460 2.2 GP38-2

stan2004 posted:

Perhaps another way to look at it is to observe that 46.5 x 11 = 511.5.  That is suspiciously close to "exactly" 512.000 - in other words is a 9-bit binary shift which is arguably what you'd expect a presumably fixed-point microcontroller processor to do to represent two levels of resolution or scaling of the same underlying value.

 

Stan, you may be onto something here. The 46.5 number I use was derived by working backwards. With a little more 'tuning', it really could be 46.545454 and then 46.545454 * 11 = 512.

Wow, the conformance between RTC and the value from the remote is excellent.  So I think I understand now that the 2nd byte, 0x09, adds a level of precision which eliminates the fudge factor I was struggling with by using 0x08 alone.

One thing I don't quite understand though... All of the diesels have the same size wheels, the same gear ratio, same or similar motors, and in some cases, the same flywheel diameter (if that even matters.)  So why would the scale factor in the sound file differ at all among diesels?  Does MTH test a prototype, and then set the scale factor based on some observed value?  This would make the scale factors very close but somewhat random, perhaps to frustrate attempts at reverse engineering?

Of the locos in your list, the GP7 and GP38-2 would be the closest, in terms of what's under the hood, so perhaps those two merit close examination for differences.

Creep, coast, and pull.  We're not talking about cold fusion here.

A shot in the dark. Could it represent manufacturing variables?  Regardless of the level of precision there are some variables. The weight of each locomotive is different even if the drive ratio and the motor are the same. An F3's weight is different than a GP7 or Gp-38 and just like paint, motors with the same specs on paper can very from one production run to another.  The most common variable is the flux density of the magnets. As the flux increases from weak to strong on a typical motor there are two sweet spots one for rpm and one for torque. I bet on a well designed motor those points are much closer together on the curve.    Flux density is less variable today than back when I was rewinding slot car motors in the sixties. Back then you could find two "identical" new motors where without a pinion gear your fingers could barely spin one over the cog and one you could spin like a top. The one with stronger magnets would always have more torque, run cooler and pull taller gearing. Racing and dynamic braking would eventually weaken those strong magnets and for a buck or so most slot car tracks could re-magnetize your magnets. You do reach a theoretical max flux density where an iron core armature cannot turn at all before the commutator and windings fry. Fairly sure that does not occur with coreless motors. Back to mine and Teds hot button, as the gear ratio goes up variables in the motor have less effect.  And then there is this.  Back when Lionel made the 18000 PRR B6 I was not at all happy with the slow speed performance of mine so I dug through my box of slot car gears till I found a pinion and spur that would properly mate with the given fixed space between the motor shaft and the shaft the worm was on. I cannot remember what the factory ratio was and what I came up with it was only a small increase in ratio. If 1:2 was stock my gears only made it something like 1:2.5~2.6.   Anyway the final drive ratio went from about 14:1 to about 22:1. You would expect the slowest speed would have been slower.  NOT with the ZW which has a low voltage setting no lower than about 6v. Without a load the loco was faster at 6v than before I changed the gear ratio.  However when pulling a load it would go noticeably slower before stalling.  Had you been able to dial the ZW down to one or even three volts the loco's slowest speed before stalling would have been slower.  Shows the importance of the power supply in the complete picture.  What I did in response to the increase in speed without a load was to put several diode pairs on the pickup rollers. Which may have accomplished what I set out to do without the PITA that changing the gears was.

   I'm sure if you could test enough MTH locos you would see a pattern but simple manufacturing variables could account for the difference between the 0x08 vs the 0x09.          j

John, with the speed control and tach sensor, identical gear ratios and programming should yield identical results.  The is one variable that can change it slightly, that would be traction.  If one design happens to slip a bit more on curves, for instance, that might skew the results a bit.

SanDiegoMark posted:
stan2004 posted:

Perhaps another way to look at it is to observe that 46.5 x 11 = 511.5.  That is suspiciously close to "exactly" 512.000 - in other words is a 9-bit binary shift which is arguably what you'd expect a presumably fixed-point microcontroller processor to do to represent two levels of resolution or scaling of the same underlying value.

 

Stan, you may be onto something here. The 46.5 number I use was derived by working backwards. With a little more 'tuning', it really could be 46.545454 and then 46.545454 * 11 = 512.

What I'm suggesting is only one of the DTO or DOD values gives you "all" the info you can expect in terms of raw_value as the dependent variable.  In other words DTO and DOD represent simple linear equations  DTO=m1 * x + b1,  DOD=m2 * x + b2.  x is the same raw_value (tach stripe count).  Different slopes and different offsets but with a fixed-ratio (512) between m1 and m2.  So for the purpose of messing with the gear ratio and wheel diameter, use the higher resolution (larger slope "m") which is undoubtedly the trip odometer since its "b" gets reset on every new trip and hence does not require the range of the overall-cumulative odometer.  This gets you the best resolution for the task at hand.  My 2 cents.

 

stan2004 posted:
SanDiegoMark posted:
stan2004 posted:

Perhaps another way to look at it is to observe that 46.5 x 11 = 511.5.  That is suspiciously close to "exactly" 512.000 - in other words is a 9-bit binary shift which is arguably what you'd expect a presumably fixed-point microcontroller processor to do to represent two levels of resolution or scaling of the same underlying value.

 

Stan, you may be onto something here. The 46.5 number I use was derived by working backwards. With a little more 'tuning', it really could be 46.545454 and then 46.545454 * 11 = 512.

X is the same raw_value (tach stripe count).  Different slopes and different offsets but with a fixed-ratio (512) between m1 and m2. 

 

Stan,

I can easily test this part.

I took the DTO and DOD raw values, ran the engine for about 1.5 Smiles and took the raw values again:

 StartEndDifference
DTO0392296392296
DOD2711727883766

 

The DTO difference is (almost) exactly 512 times the DOD difference. Its off by just a little which could be the result of how the engine accumulates the tach counts.

So you have added another piece of the puzzle.

I did some calculations using the measured gear ratio, measured diameter of the driving wheel and the observed number of stripes on the tach strip.  I was able to determine the number of tach stripes per Smile. For a typical diesel, this was 119000 stripes per Smile (it is different for each gear ratio and wheel diameter).

I ran the engine for as close to 1 Smile on my layout as I could (1 Smile is 110 feet). I read up the raw DTO value and raw DOD value at the beginning and end of that run.

For DTO, the raw value delta over 1 Smile is 237012, for DOD, the raw value delta over 1 Smile is 462.

Looking at these numbers shows that 237012 is about 2 * 119000 and 462 is about 119000 / 256. I confirmed this general pattern with two other engines with different gear ratios and wheel diameters.

This tells me that for the DTO, each tach stripe generates two raw tach counts, possibly a count for the white to black edge and a count for the black to white edge.

The DOD raw value is just the DTO raw value divided by 512 (8 fewer significant bits, yes 8, not 9).

Ted S posted:

Your sample data only confirms what I already knew.  All diesels are geared the same at 10.5:1 and typically have the same size wheels.  With my understanding, and never having had any of your locos apart, I predict the gear ratios of your steamers as follows:  0-8-0 is 14:1, 2-8-0 is 18:1, Berk is 16:1 (I'm guessing yours is Premier- I have one), 0-6-0 is 18:1. 

You are exactly right!

I have developed an equation which I think will let you calculate the gear ratio from the Scale Factor Value, the measured wheel diameter and the number of stripes per rev of the tach wheel (usually 24). The measured wheel diameter is the real variable in this equation. Its hard to measure with the traction tire in place. But its the traction tire that really determines how far the engine goes in one rev of the driving wheel.

Here are the values that I get which need to be rounded:

0-6-0   18.41 (I actually twirled the tach wheel and counted 18:1 on this engine)

2-8-0   17.86 (you predicted 18:1)

Berkshire  15.98 (you predicted 16:1)

0-8-0   14.14 (you predicted 14:1)

The equation can also be solved for the Scaling Factor given the gear ratio, the wheel diameter, and the number of stripes per revolution of the tach wheel.

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