KB926: CV55, CV56 & CV57 - Scaleable Speed Stabilization (Back EMF)

Some Digitrax decoders offer scalable speed stabilization (also called back EMF) control to help smooth out operation in the low end of the speed range.  This feature can be beneficial especially in N scale switching situations especially when the decoder can select how much of this effect to implement (scalable speed stabilization).  Speed stabilization is a decoder feature that can be used with any DCC system.  Most Digitrax decoders that support this feature are shipped with scalable speed stabilization turned off so, to use it you will need to set program them to use this feature.  Other Digitrax decoders are shipped with this feature enabled.  The tables below show ranges and examples for how you can set these CVs:

CV55 controls the STATIC compensation or how much the decoder considers the difference between the current motor and locomotive speed and the target speed set on the throttle when determining the next speed command to send to the motor. This is like the stiffness of a spring. The stiffer the spring, the more compensation you will get.  Higher values give a more intense reaction and lower values give less intense reactions.  CV55 will have no effect on decoder operation until you program CV57 as described below.

CV56 controls the dynamic compensation or how much the decoder considers the historical difference between the current speed and the target speed when determining the next speed command to send to the motor. This setting is like a damper or shock absorber on the spring that helps to restore the spring to its new position.  Higher values cause more rapid adaptation to the target speed and lower values cause slower adaptation to the target speed.  CV56 will have no effect on decoder operation until you program CV57 as described below.  Excessively high CV values programmed to CV56 will tend to let the locomotive to "hunt" around a new desired speed when a change of speed is commanded.  We recommend that you use the lowest CV value in CV56 that gives the desired performance.

CV57 controls the amount of droop or speed loss as load is increased, by limiting the amount of change or compensation that the decoder is allowed to implement.  CV57 controls the droop separately for both regular addresses and consist addresses.  The first hex digit controls the amount of droop in effect when speed is controlled using the standard decoder 2 or 4 digit address.  The second hex digit of the CV value controls the droop in effect when the decoder is part of an advanced consist for speed and direction control. Values for each digit can range from 000 to 015 decimal or x00-x0F hex.

If either digit is 0, speed stabilization is OFF.  A value of 015 or x0F hex is speed stabilization FULL ON.  If the droop CV value is too high, you may see locos jump from one speed to the next if they encounter an obstacle or problem with track work. If the value is too low, there will be very little speed stabilization effect at all. A higher digit makes the droop or speed fall-off less.  A typical value for most locomotives not part of an advanced consist is CV57=005 (same in decimal & hex), but the actual value that is best for a locomotive and train size must be determined by observation and experimentation.  Note that this value of 005 for this example means that no speed compensation is used when this decoder is in an Advanced Consist.

CV55, CV56 and CV57 can be changed while the locomotive is moving using Ops Mode programming. With Ops Mode, you can change the characteristics of the whole train as needed while it is running on the layout.  When you are using scalable speed stabilization, you don't need to program a large CV value for V-start (CV02) to compensate for sluggish motors.  This is because when scalable speed stabilization is in effect, the decoder will automatically try to adjust the motor power up to at least the Vstart setting, to achieve the actual speed commanded. This means that low speeds like 3% or 4% will give best performance when CV02 is programmed to a CV value of 00.

 How to set up a loco with scalable speed stabilization

1.  Install the decoder.

2.  Program CV57 (Droop) to a CV value of 05. This will turn on speed stabilization.

3.  Put the loco on level track and run it at about 20% of full speed.  Beginning with the default value of 128 (x80hex) in CV55 (Static), reprogram the CV value to increasingly higher values until you observe the loco jumping as speed steps increase when you run the locomotive. Finish this step by reprogramming CV55 to the CV value just before the jumping started.  Ops Mode programming on the main line works very well here.

4.  Follow the same procedure with CV56, beginning with the default value of 048 (x30 hex) and increasing it until you notice the loco oscillating, faster-slower, faster-slower, as speed is increased.  Finish this step by programming CV56 to the CV value just before the oscillation started.

5.  Follow the same procedure with CV57, beginning with the value 05 as programmed in step 1. Increase the CV value in this CV until the speed when going up hill is roughly equivalent to the speed on level track. This will yield a best droop consistent with the locomotive characteristics.

Keep notes about the values you program for these 3 scalable speed stabilization control CVs so that you can use them as a starting point for setting up scalable speed stabilization in similar locomotives.