Digitrax

From the Digitrax Decoder Manual 4/2004.

Note:  Values given below in "ZZZ/xYY" format are "Decimal/Hex" values.

6.15 Scaleable Speed Stabilization (Back EMF): CV55, 56 & 57 


Some Digitrax decoders offer scaleable speed stabilization or back EMF con- trol to help smooth out low speed operation. This feature is especially benefi- cial for switching. Speed stabilization is a decoder feature that can be used with any DCC system. Digitrax FX3 decoders are shipped with CV57, the Scaleable Speed Stabilization CV set at a value of 006/x06 and CV55 & CV56 set at 000/x00. These values are beneficial to the operation of most locomotives. Other Digitrax decoders that support this feature are shipped with scaleable speed stabilization turned off so, to use it with those decoders, you will need to program the following 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. CV values can range from 000/x00-255/xFF. Higher values give a more intense reaction and lower values give less intense reactions. The factory default setting for CV55 is 128/x80. It 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. CV values can range from 000/x00 to 255/xFF. Higher values cause more rapid adaptation to the target speed and lower values cause slower adapta- tion to the target speed. The factory default setting for CV56 is 048/x30. It will have no effect on decoder operation until you pro- gram CV57 as described below. Excessively high CV values pro- grammed to CV56 will tend to let the locomotive "hunt" around a new desired speed when a change of speed is commanded. We rec- ommend that you use the lowest CV value in CV56 that gives the desired performance. 

CV57 control the amount of intensity or speed loss as load is increased, by limiting the amount of change or compensation that the decoder is allowed to implement.
ADVANCED CONSISTING NOTE:
CV57 also allows one to tuned the intensity for normal operation and when part of an Advanced consist independently.  Unfortunately that makes CV57 is a complex CV because it contain two independent numbers that must be combined into a single number for programming.

The lower first hex digit/number controls the intensity in effect when the decoder is operating normally.

The
upper second hex digit/number controls the intensity in effect when the decoder is part of an advanced consist .


In decimal, the range of values for each digit is 0 to 15.  In hex the range is 0x0 to 0xF.

In decimal, the default value of CV57 is 5 for standard operation and 0 for Advanced Consisting.  In hex, CV57 = 0x05.
To have them both be the same value of 5 (0x55 in hex), program a decimal value of 85 into CV57. 

If either digit is 00, speed stabilization OFF.  A value of 015/x0F is speed stabilization FULL ON.  If the intensity 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 intensity or speed fall-off less.  A typical value for most locomotives not part of an advanced consist is CV57=005/x05, 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 operating characteristics of the decoder while it is running on the layout.  When you are using Scaleable Speed Stabilization (Back EMF), you don't need to program a large CV value for V-start (CV02) to compensate for sluggish motors.  This is because when scaleable speed stabilization is in effect, the decoder will auto- matically 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% of full speed will give best performance when CV02 is programmed to a CV value of 00. 

How to set up a loco with Scaleable Speed Stabilization (Back EMF) 

1. Install the decoder. 

2. Program CV57 (Intensity) to a CV value of 005/x05. 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/x80 in CV55 (Static), repro- gram the CV value to increasingly higher values until you observe the loco jumping as speed steps increase when you run the locomo- tive. 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 and increasing it until you notice the loco oscillat- ing, faster-slower, faster-slower, as speed is increased. Finish this step by programming CV56 to the CV value just before the oscilla- tion 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 intensity consistent with the locomotive characteristics. 

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