Field of oriented control dreams

Field oriented control has become an important drive/commutation approach for brushless DC motors, and is becoming that as well for AC induction motors, because it delivers a wide range of usable motor speeds. It is instructive to compare FOC to the previously most common method for brushless DC motors, sinusoidal commutation.

Figure 2 shows control schemes for both sinusoidal commutation and field oriented control. In the sinusoidal control approach, the torque command is “vectorized” through a sinusoidal lookup table, thereby developing a separate command for each winding of the motor. As the rotor advances, the lookup angle advances in kind. Once the vectorized phase command is generated, it is passed on to a current loop, one for each winding, which attempts to keep the actual winding current at the desired current value.

An important feature of this approach is that as the frequency of motor rotation increases, so does the difficulty of maintaining the desired current. This is because the current loop directly “sees” the rotation frequency, and any lag in the current loop, a certain amount of which is inevitable, results in an error between the desired stator torque, and the actual. This lag, insignificant at low rotation speeds, generates increasing amounts of D (unwanted) torque at higher rotation speeds, resulting in a reduction of available torque.

The control scheme for a field oriented control approach differs in that the current loop occurs de-referenced from the motor’s rotation. That is, independent of the motor’s rotation. In the FOC approach there are two actual current loops, one for the Q torque, and one for the D torque. The Q torque loop is driven with the user’s desired torque from the servo controller. The D loop is driven with an input command of zero, so as to minimize the unwanted direct torque component.

The trick to making all of this work are math-intensive transform operations known as Park and Clarke transforms that convert the vectorized phase angle into the de-referenced D and Q reference frame. This is done twice, once to convert the output of the D and Q control loops into the 3-phase motor command, and once to convert the measurement of the rotor’s angle back into the D and Q frame. While these transforms have been known about for years, their practical implementation in brushless DC and AC induction drives has awaited the availability of cheap, high performance DSPs and microprocessors.

Now that these are available, AC induction motors which utilize an FOC approach can develop motor efficiencies of 85+%, compared to around 60% for non field-oriented approaches. brushless motors which adopt an FOC approach, by comparison, can achieve even higher efficiencies of up to 95%. Sinusoidal commutation for brushless DC motors also works very efficiently, but is not as efficient as FOC at the very highest speed range of the motor.