Hod Carrier

[MOC] Differential Drive - Variable Reduction Gearing

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Posted (edited)
43 minutes ago, zephyr1934 said:

I think what you mean is that with two motors you get the same net power output (ignoring the loss due to the differential gears) but twice the number of resolvable power steps along the way if you step motor A from 0 to 7 then motor B from 0 to 7 (16 steps), that is more than the 8 steps if both motors go from 0 to 7 together. If you take into account the power curves are probably not linear, then you might be able to get 8x8 different speeds (well 64-24 = 40 assuming 6&4 is the same as 4&6) but it would be hard to do so in a constantly increasing manner.

That’s precisely it.

The code as it stands has 41 evenly spaced power steps in total (stop plus 20 steps to full power in each direction). This is likely to need a bit of tuning to suit the characteristics of different motors in order to give a smooth transition through the acceleration/deceleration phase.

Rather than increasing the speed of each motor individually as you suggest, the custom controller treats both motors as a single system. The buttons only increase and decrease train speed “a” which is resolved into the individual motor inputs “b” and “c” by the program.

That said, you probably could drive the system manually through a remote by increasing and decreasing the motor speeds individually with the buttons, but I think you’d lose some flexibility. As you can see from the motor map graph, there are times when motor A is driving in reverse at 50% speed relative to motor B. This acts to increase the ratio of the reduction.

8 minutes ago, zephyr1934 said:

To get the highest speed possible you would want both motors running, so I would think start the high torque motor first, and then at some point bring in the high speed motor. Use the high torque (lower max speed) to fine tune the speed if necessary, otherwise, rely primarily on the high speed motor for the speed level. The max rotation of the axle coming out of the differential is then the average of the max rotation speed of the two motors taken individually (where the motor includes whatever gearing ahead of the differential input)

I think that sounds workable. 

Edited by Hod Carrier

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5 minutes ago, Hod Carrier said:

I think that sounds workable. 

Mind you, I suspect two L motors (or whatever motor that is at the halfway point) in direct drive would perform better than an M and an XL connected with a differential, but it is a fun concept to contemplate... and maybe having the high power at low speed is an advantage over two matched motors

6 minutes ago, Hod Carrier said:

The code as it stands has 41 evenly spaced power steps in total (stop plus 20 steps to full power in each direction). This is likely to need a bit of tuning to suit the characteristics of different motors in order to give a smooth transition through the acceleration/deceleration phase.

Ah yes, I was thinking manually. With the right code you could hit all of the stops progressively

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Just now, zephyr1934 said:

Mind you, I suspect two L motors (or whatever motor that is at the halfway point) in direct drive would perform better than an M and an XL connected with a differential, but it is a fun concept to contemplate... and maybe having the high power at low speed is an advantage over two matched motors

Having tried a pair of M-motors with the differential I’m unsure of their suitability for high speed application. I simply couldn’t get them to turn a 1:3 ratio even with both on full power. Even at lower speeds (assuming it could be made to move at all) I’d find that either one motor would stall or the inertia in the drivetrain below the differential would be so great as to cause the two motors to simply spin their power away through the differential.

I still haven’t modified the H10-44 to take L-motors, so I may try a few things first in it’s initial configuration to see whether I can improve it's performance.

There may be an advantage to mismatched motors as you suggest, with L and XL being the most likely combination. I can feel my credit card trembling with fear already.

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Posted (edited)

Just a little update, if I may be permitted.

51269870851_afdb71fb78_z.jpg

The Fairbanks Morse H10-44 switcher has had an upgrade and now uses Technic L-motors. It's a wee bit of a squeeze and routing the cables was a bit of a headache, but everything goes in nice and snug.

I'm sure that there is a better arrangement of parts, but I wanted to keep the motors away from the bogie pivots so that they could be lowered enough to permit them to be stacked. The problem otherwise is that the motors would have needed to be raised in order to give enough clearance for the bevel gears that drive each bogie.

51269870756_37ae1d1284_z.jpg

The arrangement of gears and shafts for the differential is the same as that used on the English Electric GT3 but it retains option to swap the final gear ratio between 1:1 and 1:3. This is done by sliding different gears along the prop shaft to engage with the differential which can be accessed through a hole left in the floor of the loco.

51269130272_d8328e4ba2_z.jpg

A little spin on the test track to show that the loco is now capable of turning the higher 1:3 gearing and the range of speeds that can be achieved.

In spite of all the effort put into the Powered Up custom controller to make use of the L-motor's rotation sensor to adjust it's speed, this run has been done without making use of this capability. Instead I have used the simple version of the controller that just feeds the motor power variables directly into the motors using the SetPower command. As you can see, it still manages a good range of power and control thanks to the greater power of the larger motors.

In other developments, I did try pairing an XL-motor with an L-motor on the GT3 chassis using the existing gearing. It was a bit of a lash-up and certainly wouldn't fit inside the bodywork, but it was functional. Taking @zephyr1934's idea of running up the XL-motor first as a low-speed high-torque starter and then adding the L-motor for high-speed running, the loco performed well but didn't quite have the same range of low-speed control. I'll test some more, but I suspect that some sort of hybrid control system where the second motor turns the differential in reverse to increase the reduction ratio in the gearing would replace this loss of truly low-speed capability. Whether the additional grunt of the XL-motor offers other advantages, such as increased hauling ability, is something someone with access to more rolling stock would have to test. Also still to be tested is gearing-up the L-motor ahead of the differential to see what difference this makes.

Edited by Hod Carrier

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I'm not sure how to make this sound less destructive, but I still think the benefit of this setup -- other than using a cool differential -- is... pretty nonexistent: The PU motors can afaik be controlled in 125 steps (only considering one direction). Your setup now doubles this resolution to 250 by providing the in between .5 steps, too. But in terms of power or speed, you gain nothing.

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I'm glad you're not trying to be destructive because we're not going to fall out over this. :classic:

I will just say again, however, that I am not trying to get more power or speed out of this idea. That was never the idea to begin with. It is just a way of trying to give LEGO trains a larger usable range of speeds by means of a compact variable reduction gearing.

I am also well aware that some PU motors allow for slow-speed control due to the in-built rotation sensors, but would remind you that this functionality is not currently fully supported by the 2 I/O "City Hub" that is favoured by train builders due to it's size compared with the more capable 4 I/O Technic Hub. It's also worth stating again that this idea is not specific only to PU but can be used with any Technic motor and is, therefore, of potential use to those builders still using PF motors.

I understand that you consider this idea to be useless, and that's fine. Not every idea is going to resonate with every builder and I accept that there is always likely to be a whole range of reactions. Some builders may be happy to embrace third party electronics and other components in order to achieve certain functionality, but not everyone is confident to follow that path, and I include myself in this. I simply wanted to explore another path to achieve these outcomes that falls more easily within my comfort zone.

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