I have been playing with some of the CVs on my Bachmann 158 equipped with a Lenz Gold decoder. From what I have read, I thought that the minimum starting voltage (CV2)should be adjusted until the unit just moves at the first turn of the control knob (I use a Lenz Compact).
With the default settings (CV2=0) the unit starts to move very slowly after about 15 degrees turn of the knob. I assumed that increasing the value in CV2 would mean that the unit would start at progressively smaller turns of the knob, and I would have the correct value when it sets off at a crawl as soon as the knob is turned. This is not what happened though.
I found that as I increased CV2 the unit still started at the same position of the knob but it started at a faster speed. In other words it reduced the unit's ability to crawl slowly.
I then experimented with a Heljan Hymek fitted with a Lenz 1035 decoder and found exactly the same behaviour.
Have I misunderstood what this setting is supposed to achieve?
I've always found that anything [1] fitted with a Gold decoder has not required *any* messing with default settings, such is the quality of control. Having said that it doesn't really bother me quite *where* on the control knob that movement commences, providing that the initial movement and acceleration are smooth.
DCC is not yet quite 'plug & play' but the Lenz Gold and Silver decoders have certainly brought this a good deal closer.
[1] - I've witnessed a 'juddering' with two locos (both USA outline HO scale model) at low speed & under acceleration when fitted with a Lenz Gold - a Bachmann Spectrum 3-truck 'Shay' and a Stewart Hobbies Baldwin VO1000 switcher and have put this down to them both having a peculiar dislike to feedback control. I'm only using a Lenz Compact and consequently my programming options are somewhat limited, but with any higher spec command centres it ought to be possible to eliminate this problem. In the end I eliminated it in the Baldwin switcher by fitting a non-feedback LE1014E decoder, and significantly reduced it in the Shay by using an LE1035E, although I might have been tempted to use an N-scale LE0511 in the latter if they'd still been available.
Is your compact set for 14, 28 or 128 speed steps?
CV2 sets the proportion of full scale voltage applied to the motor when the decoder receives the command for speed step 1. It sounds to me like the compacts knob doesn't send speed step 1 until you've turned it 15 degrees. That's why increasing CV2 only increase the speed at which the loco starts.
How far do you have to turn it to get the next speed increase? Test that with acceleration disabled.
By default the Compact always starts in 28 step mode and it has to be changed for each loco to 128 steps after powering up. All my tests were done in 128 step mode.
Once the unit has started, further rotation of the knob results in a speed increase in all cases.
Phil: There is also a 'kick start' pulse CV (can't recall which number that is applied for initial moovement to overcome stiction - it may this CV that needs adjusting, rather than CV2 - CV2 would continue to apply after yhe movement has started
I agree with you John but when one has so many things to play with as there are in DCC I could not resist having a go! For example, I always try to set the top speed in CV5 to approximately match the top speed of the prototype by timing how long the loco takes to complete one lap of my main circuit.
I was just a little surprised that CV2 didn't seem to do 'exactly what it says on the tin'. Perhaps the explanation that the Compact's knob doesn't send speed step one until ten - fifteen degrees rotation is the correct one?
On a related issue, does anyone know the answer to this? If I am using 128 step control and lower the maximum voltage using CV5 in order to limit the top speed, do I still get 128 steps but of smaller increment or are the steps the same size and I just get less of them?
Any sensible decoder will still give you 128 steps, of smaller increments.
A related question is what effect do CV2 and CV5 have when the speed table is being used. I believe the answer varies between decoders but is not always clearly documented.
If you are using lights on your loco, the combined load of lights + motor determines the track or decoder voltage at which the engine starts to move. Incandescent bulbs use a lot of power. It's not uncommon for a loco with lights to sit until the track voltage reaches 6 volts on DC, and the equivalent on DCC. Add on-board sound, and the engine may not move until you get to 9 volts (effective).
The controller sends a signal that determines the on-time in the on/off cycle in the decoder. The longer the current is on per cycle, the higher the effective (roughly, the average) voltage, and the faster the motor spins. Each cycle takes exactly the same amount of time. Changing CV2 (CV5) on the locomotive changes the way the decoder interprets the signal from the throttle. It does not change the signal from the throttle.
CV2's and CV5's defaults are "no change", ie, the decoder just chops up the current into 128 steps for an effective voltage range of zero to 12. Ie, step 1 is "full off" and step 128 is "full on." Changing CV2 raises the starting voltage, ie, step one will have a longer on-time than the default setting. Changing CV5 reduces the on-time for step 128 (in your case), thus lowering the effective voltage.
128 steps means 128 different on/off cycles, but if they are spread over a smaller min/max range the differences between the cycles will be less.
If a loco starts at a DC track voltage of 3V, say, then it will always start at a track voltage of 3V regardless of any other load on the track.
I think what you are trying to say is that the extra load causes a higher current to flow which causes (a) voltage drop due to resistance along the track between the controller and the loco and (b) voltage droop in the controller due to overload. This means that the track voltage at the loco is lower than it would otherwise be. The loco will, nonetheless, start at the same track voltage measured at the loco.
In DCC the track voltage is constant and the boosters are usually capable of supplying a much higher load. If the loading from lights or sound has any effect on a DCC loco then you have either poor power distribution or a very marginal DCC system.
What I should have said is that when I disconnected the lights on a loco, it started at a lower _power_ setting. You're right about the starting voltage etc. I obviously wasn't thinking too clearly. Sorry 'bout that.
The following is based upon the NMRA RP 9.2.2, I haven't yet actually played with a DCC system although I've been thinking about it for several years :-) Also I can't say anything about particular decoders.
The CVs in question can be assigned values from the range 0 to 255 and represents a fraction of the maximum voltage available as 255ths, i.e.
1 means 1/255th and 255 means 255/255ths (full power). A value of 0 or
1 for CV5 means ignore the CV.
CV5 is Vhigh, the fraction of available power to be applied to the motor at the highest speed step. By default you would expect this to be 255 for full power. The accepted wisdom is you reduce this to set a realistic top speed for your locomotive.
CV2 is Vstart, the fraction of available power to be applied to the motor at the lowest speed step (speed step one). By default you would expect this to be 1 for minimum power. A better name would have been 'Vnot stop' as the idea is that this should be the lowest value at which the locomotive keeps going. The accepted wisdom is to increase this (from 0) until the locomotive doesn't stall when the throttle is turned down to speed step one after turning it up far enough to get the locomotive running.
After adjusting CV2 some locomotives will always start at speed step one but many, particularly the older designs, won't. This is often referred to as stiction and most people will recognise the scenario on DC where you turn up the throttle and the locomotive suddenly flies off at a rate of knots (assuming a plain DC throttle without pulsing and/or feedback). Enter CV65.
CV65 is 'Kick Start', the fraction of available power applied, briefly, to the motor when the speed step is changed from zero to one. The accepted wisdom is to increase this (from 0) until the locomotive just starts moving when the throttle is turned from speed step zero to one and if you've set CV2 it will keep moving at the slowest speed it is capable of without stalling.
Think of the decoder having in it a straight line graph of applied power against speed step. Ignore speed step zero for now, at this setting the amount of available power applied to the motor is always
0/255ths, i.e. nothing.
By default at speed step one you get 1/255th of the available power but it might that your locomotive stalls at anything less than
19/255ths, roughly speed step two in 14 step mode or three in 28 step mode. This means you've effectively 'lost' one or two usable speed steps respectively.
By increasing the value in CV2 you raise the start point of the graph so your locomotive will keep running down to speed step one regaining a bit more control. This is further improved if you've set CV65.
Conversely at the maximum speed step (14, 28 or 128) the locomotive may be moving far faster than you ever want and again you're effectively loosing one or more usable speed steps.
By decreasing the value of CV5 you lower the end point of the graph and again gain a bit more control, the speed steps always split up the part of the graph between the values of CV2 and CV5.
There is also CV6, Vmid. This is the same thing but for the 'middle' speed step. Using this you can put a kink in the graph so it's shallower at the lower speed steps to give finer low speed control for shunting and such like.
CV67 to CV94 allow for a speed table which effectively means you can put in a graph with a shape other than a simple straight line. I wouldn't worry about this when starting off in DCC.
When using a feedback (EMF) decoder CV5 could be adjusted as described but I suspect CV2 and CV65 are best left alone. The decoder will dynamically adjust the amount available power applied to the motor. As I know nothing about actual decoders it might be that they actually do something useful with CV2 and CV65 but you'll have to ask someone else :-)
Somebody mentioned locomotives bucking or shuddering at low speed with a feedback decoder. This suggests the feedback gain was too high for the locomotive, i.e. the decoder thought the locomotive was running too slow but increased the power so much it then thought it was going to0 fast but then reduced the power so much it thought it was going too slow ... and so on. Find out if the decoder has a CV which allows you to reduce the feedback gain. As a guideline increase the feedback gain until the locomotive bucks or shudders then reduce it until you just obtain smooth running, this should give you the maximum amount of useful feedback.
By the way I could be wrong about the frations being 255ths but the basic principle holds good.
I forgot to mention but it's obviously easier to do this sort of tuning where you can directly control the speed step value, e.g. a push button throttle or computer control as opposed to a rotary throttle.
Ideally you'd want to have 'programming on the main' so you don't have to keep moving the locomotive on and off a programming track, unless you create a special test track that can quickly be switched to and from a programming role.
Better still would be a piece of software that does this for you. I don't know if Decoder Pro does but it should. The scenario would be something like: Start Set Up; Press space bar when locomotive starts to move; Press space bar when locomotive reaches desired maximum speed; Press space bar when locomotive stalls. The program just ramps the speed steps and CVs up and down then notes, and programs into the decoder, the appropriate value each time the space bar is set.
Lst comment is don't forget 'right' values of these CVs would be different when running light and when hauling a long freight or passenger train.
DecoderPro lets you run a loco using a throttle, so you can use the same peice of track to program and test. Another solutions is to use a rolling road.
The other week I suggested seeing if you could reduce the feedback gain. An alternative has occured to me.
Most modern decoders allow the user to change the PWM frequency.
For the benefit of anyone who doesn't know PWM simply means that the power is simply on for part of the time and off for part of the time. The amount of power is the average value. So for one quarter power the power is off for three times as long as it is on. For half power it is off for the same amount of time as it is on. And for three quarters power it is on for three times as long as it is off.
Most feedback controllers measure the EMF generated by the motor immediately after the power is turned off.
If you increase the PWM frequency the amount of time the power is on for is reduced, but the ratio of the amount of time it is on and the amount of time it is off stays the same.
This has two consequences:
The motor has less time to accelerate before the EMF is measured.
The EMF is measured much more often.
Both of these mean the decoder is likely to decide the motor has reached the desired speed and reduce the amount of extra power it applies sooner and hence it is less likely to overcompensate then overeact by reducing the extra power to too low a value.
If you were to increase the frequency a bit at a time you should start to notice the juddering reduce and eventually dissapear. You could opt to just whack the frequency up to a really high value but it's just possible you could find this diminishes the effectiveness of the feedback.
As a closing comment feedback isn't really a good idea when the motor has a decent flywheel. The change in motor speed in response to the extra power applied is too slow and the feedback is almost certain to overcompensate (well at least the sort of simplistic feedback used by most decoders). This could be mitigated by opting for a very slow PWM frequency but you'd be best advised to save your money and use non feedback decoders.
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