Jerky Downhill Performance.
#1
Hello.

Introducing a grade seems to have highlighted a slight problem. Slow speed operation seems to create a jerky motion when going down hill. I've noticed that at speed step 1 the jerky motion does not seem to be too bad. However when increased to speed step 2, 3, 4 and 5 it only seems to exaggerate the problem ever so slightly. Especially at 2. I have a theory and I suspect it has something to do with slop or play in the truck gears which causes a load-no-load on the motor. A curious annoyance to say the least and something I honestly did not anticipate. At speed step 1 the loco goes slow enough to help hide the play by allowing gears to catch up or stay meshed together but there is still some excess movement there.

Going uphill it climbs like a champ. No problem there.

Please bear in mind that this is meant to be a slow speed down grade to a transfer slip, so putting the pedal into the fire wall is not an option in terms of operation. All that said, I haven't completely laid the full run of approx. 54 inches of track on the grade so perhaps as the loco gets a bit further into the slope the slop will "disappear". Or perhaps when I start pushing a ten car train on to the float the added weight pulling the loco down the grade will help disguise the problem.

As a refresher the loco is P2K GP 7 with a Tsunami decoder. It also does this with my P2K SW900 as well so its not just the one engine.

Thoughts? Any ideas for improvement other then getting some custom made precision gears for the locomotive's trucks? Has anyone else encountered this before?
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#2
Which DCC system are you using..?? Most Digitrax decoders have BEMF detection to prevent this sort of thing fromm ocurring. My locos go downhill with just a slight acceleration, but no "bucking", just as you would expect in the RL....
Gus (LC&P).
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#3
Well I'm not a DCC guy but your idea of slack in the gears does sound plausible. Do you have any other P2K units giving you that same problem, A few of mine suffered cracked gears on the axles, something like that could be the problem also. I was able to fix them with a drop of CA. you may want to check them just in case.
 My other car is a locomotive, ARHS restoration crew  
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#4
Teeters, if they are older P2K locomotives definitely check the axle gears, they are probably bad. If you can turn any wheel with your finger, the gear is bad. You can use Athearn gears to replace them and I think they were less than $2 for 6 of them. I can get the part number for you if you need it.
If the gears are OK, read the decoder manual. I'll bet there is a "coast" adjustment of some kind, some kind of a no load power adjustment. I only have had the chance to play with one Tsunami, and you can make them play a tune if the one I adjusted is a sample.
Charlie
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#5
Its not the cracked gears. I went through that drama a few years ago with all my P2K GPs and replaced all the wheelsets with the Atlas ones. So no its not cracked gears. Misngth

However it sounds like I might be able to adjust the Tsunami to compensate for the this behavior. There is BEMF adjusting on the Tsunami. I'll play with it and see what happens. If you guys have or think of anything else feel free to add more to this.

Thanks! Thumbsup
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#6
Downhill jerking is often caused by worm to gear mechanisms where the worm gear can slide back and forth. It doesn't affect uphill running because the load pushes the worm one way. Going downhill the worm gets pulled one way by the wheels and then the motor catches up and it slides the other way.
The solution to the slop is usually a washer or two someplace along the shaft the worm is on. On the old steamer motors where the one shaft did all, it often took a washer at the far end or one at the end of the motor where the worm is, but inside the motor (tricky, not recommended for tyros) or at the bottom end beyond the worm if there was anything for it to push against. If there's a gearbox you might be able to tighten that up.
David
Moderato ma non troppo
Perth & Exeter Railway Company
Esquesing & Chinguacousy Radial Railway
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#7
P2K locos do have a lot of play in the drive line. Some thrust washers on either side of each worm will help a lot. As it rolls down hill, it gets ahead of the motor and binds u, then the motor turns and unboinds it, then it runs ahead agein - over and over. I used ot have some Woodland Scenics inclines to test grades before I built anything, and my P2K locos, particuarly the Geeps. had a heck of a time going downill.

--Randy
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#8
It may be indeed the set up of your decoder that could help solving the problem if you have done the gears already. A P2K has slack, but no more than an Athearn BB drive, probably less than that. It may have to do a bit with torque control etc, as you say uphill no problem, because the engine is straining continuously. you'll need to play with the settings that are in the CV's in the 200's somewhere (forgot which numbers at present). If you have decoder pro software (JMRI) then they'll be under the Advanced tab. Anyway, they have to do with the several bands of the speed control, PID basically. There is a basic set up procedure in the manual , oh heck, I've just found the thread where I helped someone else set up a Tsunami, here's the quote:

torikoos Wrote:A Tsunami has CV's for (technical bit) Proportional, Integrator, and Derivative (PID) controls. Additionally, you can change the sample rate (the amount of times per second the decoder checks the motor responses) between 31 and 2 milliseconds. These factors had a big impact on how my loco runs. It does take time, but check it out if it's possible with your decoder. It is worth it.

good luck , Koos.

PS: The link to the full Digitrax decoder manual is here: <!-- m --><a class="postlink" href="http://www.digitrax.com/ftp/Decoder_Manual.pdf">http://www.digitrax.com/ftp/Decoder_Manual.pdf</a><!-- m -->
It is more comprehensive than the one shipped with your decoder I think, it describes a lot of functions, and not all will be applicable to yours perhaps. Check the manual of your decoder to find out which CV's it supports, and then check this decoder manual from this link, to find how to set it up. Hope that helps.

Koos
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#9
BR60103 Wrote:Downhill jerking is often caused by worm to gear mechanisms where the worm gear can slide back and forth. It doesn't affect uphill running because the load pushes the worm one way. Going downhill the worm gets pulled one way by the wheels and then the motor catches up and it slides the other way.
The solution to the slop is usually a washer or two someplace along the shaft the worm is on. On the old steamer motors where the one shaft did all, it often took a washer at the far end or one at the end of the motor where the worm is, but inside the motor (tricky, not recommended for tyros) or at the bottom end beyond the worm if there was anything for it to push against. If there's a gearbox you might be able to tighten that up.

Usually, this problem was where the motor armature would be forced to one side or the other. Very common in old steamers when the worm was mounted directly on the motor shaft. But, most diesels incorporate a slip joint in the universals, so I doubt this is the problem here. You could have the same problem with thrust in the individual worms on the trucks, but it seems less likely.
--
Kevin
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#10
Wow... I'm glad I posted up. Lots of good suggestions and ideas. Thanks guys. I'll try playing with the decoder settings, if things don't improve then I'll take off the shell and look under the hood, so to speak. It may be a combination of the two as well, but I'll try and keep everyone posted on my progress and solutions.

Thanks again! Thumbsup
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#11
Yeah, I noticed I made an error on my post, referring to a digitrax manual, should have been soundtraxx of course :-)

Here's a bit from the manual (the diesel user guide) that you might need. Notice they're talking about 'yerkiness' in places, this may be exaggerated if going down hill.
Tsunami’s Hyperdrive system features high-frequency PWM drive for silent motor control along with load compensation (or back-emf) to improve low speed operation and maintain constant speed regardless of changes to the motor load.
The PWM frequency is fixed at 22.000Khz and there are no adjustments.
Tsunami uses back EMF and a PI control loop to sense and control motor speed. There are four CVs associated with the load compensation feature:
CV 10, Back EMF Cutout
CV 209, Kp Coefficient
CV 210, Ki Coefficient
CV 212, BEMF Feedback Intensity
Control Loop Coefficients
Tsunami compensates for load changes by comparing the locomotive’s speed, as measured by the motor’s back-emf voltage, with the current throttle setting. When a difference is detected, the motor voltage is adjusted up or down so that locomotive speed stays constant. This is done using a P-I (proportional-integral) control loop that is tuned using two CVs:
CV 209, Kp, controls the Proportional gain of the control loop and compensates for dynamic changes in the motor load by applying a correction based on the difference between the throttle setting and the current motor speed. This CV may be set between 1 and 255. Higher values produce an increasing level of compensation. However, setting this value too high can lead to irregular and rough performance.
CV 210, Ki, controls the Integral gain of the control loop and compensates for static motor load by applying a correction based on the historical (or integral) difference between the throttle setting and the current motor speed. This CV may be set between 0 and 255. Higher values produce an increasing level of compensation and set the ‘snappiness’ of the control loop. However, setting this CV value too high can lead to oscillations in locomotive speed and we suggest you use the lowest possible setting that still provides optimal performance.
Note: Setting both Kp and Ki to zero will cause the motor to stop.

Strategy for CV Optimization
Finding the right combination of CV settings requires a certain amount of experimentation and patience to get the best performance. Be aware that these settings will also vary from one locomotive to another. Take notes of your CV settings so you can use them as starting points when configuring other locomotives.
Overdoing a particular setting can actually degrade performance so changes should be made in small increments (+/- 10 or so) until the desired performance is attained. The default settings should provide satisfactory performance when used with most HO and N scale motors.
We suggest the following procedure to help fine tune the CV settings:
1. Begin with each CV set to its default value: CV 10 = 0, CV 209 = 25, CV 210 = 20, and CV 212 = 255. In addition, set CV 2 (Vstart) to 0.
2. With the locomotive on level track, set the throttle to around speed step 20.
3. Adjust the value in CV 209 (Kp) with increasingly higher values just until you begin to notice some jerkiness in the locomotive speed. Reduce the setting in CV 209 to the value just before the jerkiness started.
4. Adjust the value in CV 210 (Ki) with increasingly higher values until you begin to notice some oscillation in the locomotive speed whenever the throttle setting is changed. Reduce the setting in CV 210 to the value just before the oscillation started.
Note: When using load compensation, it is unnecessary to set large values into CV 2 (Start Voltage). This is because the load compensation algorithm will automatically adjust the motor power up to reach desired throttle setting. Therefore CV 2 can usually be set to 0 when load compensation is enabled.
Setting the Load Compensation Curve
Tsunami provides two degrees of freedom for controlling the amount of load compensation applied to the motor over the throttle range using CV 10, Back-Emf Cutout and CV 212, Back-Emf Feedback Intensity.
CV 212 sets the amount of load compensation that is initially applied to the motor and may be set anywhere between 0 and 255. A value of 0 disables load compensation altogether while a value of 255 corresponds to full (100%) compensation.
CV 10, Back-Emf Cutout, determines the degree by which the load compensation is reduced as the throttle speed is increased.
When CV 10 is set to 0, the amount of load compensation set by CV 212 is constant over the entire speed range. Setting CV 10 between 1 and 127 will determine the speed step at which the load compensation is reduced to zero as illustrated in the graph on the next page.
Setting CV 10 between 128 and 255 selects a partial reduction of load
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#12
Slop in the worm gears on the trucks was almost a standard feature on blue box Athearn diesels. Thin brass shim washers between the worms and the bearings to allow just slightly perceptible play would solve the problem.
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