Multifunction Decoder Motor Control
Summary: How a multifunction decoder controls the vehicle's motor
Digital Command Control brings a new level of realism to model railroading with its integration of motor control technologies.
Okay, so we've explained how the multifunction decoder creates a DC for its needs, but if we applied this directly to the motor, it would go in one direction really fast. To account for this, the output of the diode matrix does not connect directly to the motor.
Between the onboard DC power source (the matrix) and the motor is a series of transistors. Transistors function as a valve, where a small current can control a large one. With a special arrangement of power transistors (meant to control large amounts of power) called an H-Bridge, the decoder can make the train go in either direction.
As shown, the H-Bridge circuit has four Field Effect Transistors (FETs) arranged in two pairs, with the motor connected in the middle. They can be switched rapidly on and off, making control of a motor or other device possible. The micorcontroller's software will generate signals which switch the pairs on and off, which determines the speed and direction of the vehicle. The method of driving the H-Bridge is known as Pulse Width Modulation.
By controlling the pair switched, as well as the on/off period, speed and direction can be controlled. Another benefit of PWM is the ability to read the Back EMF generated within the motor for better control of speed and torque. The mass of the rotor creates enough inertia that the motor keeps spinning. By careful manipulation of the PWM signal, very slow speeds are also possible. By switching at a high frequency, any noise from the motor can be moved into the region above human hearing. See High-Frequency Decoders for more details.
With full power to the motor, and ultimately to the wheels, this helps the locomotive run so much better on DCC than with DC. Unlike Analog controllers, the multifunction decoder has complete control of the motor, as well as the capability to monitor and modify the drive signals as needed for exception levels of control.
The H-Bridge shown driving the motor is the same implementation used in a booster, just replace the motor with the Rail A and B connections. In this application, the operation is slightly different as both pairs will be employed.
When driven by a DCC signal, for the first half of the period on pair is on, then during the second half, the other. Current flows from one FET to Rail A through the load, returning via Rail B to the power supply through the second FET. When the DCC signal changes state, the other pair are switched on, allowing current to flow from Rail B through the load and back the power supply via Rail A. This creates the familiar waveform seen on an oscilloscope, even though there are no negative voltages present. The direction of current flow through the load creates that illusion.