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The Evolution of Model Railroad DCC Decoder Motor Control
When a prototype engine starts out to pull a load, it does so with a smooth start and acceleration. The smooth start is easy to do when you have a locomotive that weighs many tons. Trying to emulate this with a loco that only weighs a few ounces is not quite the same. Over the years many attempts have been made to improve the starting and low speed performance of our model locomotives. In the DC days, flywheels and pulse power were a great improvement in motor performance. Over the years motor drive technology has continued to improved but some techniques were not practical to implement in a DC layout environment. With DCC, we can now being in all the latest motor drive technology to help us start and run our locomotive motors even better. Just like DC power packs evolved, DCC decoders have evolved in BASIC motor drive control technology. Below is some history of DCC decoders improving motor control.
1st Generation DCC Decoder: Low Frequency Pulse Drive
This acted like a DC power pack with "pulse power" turned on for low speed performance. This gave good low speed performance, but had a mechanical noisy buzz when the motor ran. The Decoder motor drive frequencies used were about 30 to 200 Hz. This level simply got the decoder on par with the most advanced DC power packs available.
However these low motor driver pulse frequencies were not friendly to coreless motors. Coreless motor are designed to be run on pure smooth DC power and not pulses.. The low frequency pulses would cause the coreless motor to overheat and go bad depending on how long and hard you ran it. Ticking time bomb.
2nd Generation DCC Decoder: High Frequency Pulse Drive
Next came the quiet drive/silent running high frequency drive (15kHz to as high as 43kHz) that acted like "pure DC" from a Power Pack with pulse power turned off. This was needed as the motor buzz noise detracted from a realistic operational experience especially in sound equipped locomotives. The problem with the high frequency power was a loss of low end speed performance due to the loss of low end motor torque.
This version of decoders will work correctly with coreless motors. The high frequency motor pulses are high enough to fool the motor into thinking it getting pure DC power resulting in little heat generation. Safe to operate.
3rd Generation DCC Decoder: High Frequency Pulse Drive with Torque Compensation/Dither
Some decoder manufacturers skipped 2nd Generation and went straight to this version. Torque compensation is a trick of carefully combining/blending low frequency pulses in with the high frequency drive so there is minimal or no noticeable noise. The result is the restoration of the slow speed performance offered in the 1st Generation decoders without the buzz!
Coreless motor work well with this type of decoder given the high frequency motor drive. See 2nd Generation.
4th Generation DCC Decoder: 1st generation basic Back-EMF (BEMF) Drive
The latest form of motor control is a very sophisticated system that uses the power of the microprocessor in the decoder to monitor and control motor speed. These decoders use a property of all DC motors called called BEMF or Back Electro Motive Force where the word "Back" refers to something "coming from the motor" and the word "Force" is referring voltage. This term means nothing more than recognizing that all DC motors can function as DC generators too.
A BEMF decoder controls the motor a bit like the thermostat regulating the room temperature controls the furnace. You set a thermostat temperature and the thermostat then controls the off/on cycles of the furnace to hold the room to the set temperature. In the decoder's case, you set a train speed and the microprocessor then controls the motor speed to hold it at the train speed you set. If motor speed drops or increases, the microprocessor senses the change speed by monitoring the motor's Back EMF voltage and adjusts the motor drive to counter the change in train speed to get you back to where you were. The exact way this is done varies with different decoder manufacturers. The decoders had minimal BEMF optimization CV's with most decoders only offering a simple on/off control. To learn more about BEMF, go here: Back-EMF (BEMF)
NOTE: BEMF can help a poor running locomotive, but will not fix one that has a serious running problem. It does not matter if you use DC or DCC, if the locomotive is NOT lubricated properly or NOT broken in enough or has a mechanical problem, it will not run well period!
There were two downsides with the 1st generation BEMF decoders:
a) Consisting. If you have more than one engine with a BEMF decoder installed part of a consist, you can have under extreme condition locomotives fighting each other for control of the train speed. To learn more about the problem go here: BEMF & Consisting
b) Variation in BEMF performance with different DC motors. Not all motors are made the same and BEMF performance can suffer such that one get disappointed results. The lack of being able to access key BEMF parameter settings prevented one being able to fix the problem.
Coreless motors work well with all BEMF decoders. The motor drive adapts to the coreless motor operating characteristics resulting in it driving it more efficiently (less heat). Safe to operate.
5th Generation DCC Decoder: 2nd Generation BEMF Drive with PID & Consisting Control
To address the consistent motor performance and consisting short comings with the 1st generation BEMF decoders, these decoders added better control features and corresponding CV's to address them.
PID is a 3 letter acronym (TLA) for Proportional, Integral and Differential. All this means is you now have control of the BEMF parameter settings that define the motor performance. In other words, you now have access to the parameters that allow you can get the most performance out of motor in your given engine. Stated another way, there are no more fixed settings. However access to this level of tuning control also comes at the price of figuring out how to go about doing the tuning. What should you adjust first and so on to get the best results. It can get very involved.
To learn more about how to do this, go here: Decoder Motor Tuning
Consist performance improvement simply mean the PID parameters could dynamic change as a function of speed and/or consisting mode you are using. The basic idea is to back off on the strength of the BEMF when the engine is part of consist and do so as the speed increases. This allows the consisted engines to cooperate more with each other when they start to get up to normal train speeds. The fighting is minimized.
To learn more about how to do this optimization methods, go here: BEMF & Consisting
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