DC Brush, Brushless DC (BLDC) and step motors are the three most commonly used motor types for positioning and velocity motion control applications. Of these, BLDC and step motors are 'multi-phase', meaning they require some type of external coil excitation to keep the motor moving. This deep dive article will examine the most popular techniques for multi-phase motor control, with an eye toward determining what control techniques, including field-oriented control (FOC), work best for a given application in positioning and high-speed spinning applications.
A tail of two vectors
For Brushless DC motors, magnetic fields are generated by magnets mounted directly on the rotor and by coils in the stator. The stator windings generally come in a 3-phase configuration and are arranged to be 120 electrical degrees apart from each other. It is the sum of the force generated by these three phases that will ultimately generate useable motor rotation.
Depending on how the individual magnetic coils are phased, they can interact to create force that does not generate rotational torque, or they can create force which does drive rotation. These two different kinds of force are known as quadrature (Q) and direct (D), with the useful quadrature forces (not to be confused with quadrature encoding scheme for position feedback devices) running perpendicular to the pole axis of the rotor, and the non-torque generating direct forces running parallel to the rotor's pole axis (Fig. 1).
Figure 1: Three Phase Brushless DC motor
The trick to generating rotation is to maximize Q (quadrature) while minimizing D (direct) torque generation. If the rotor angle is measured using a Hall sensor or position encoder, the direction of the magnetic field from the rotor is known.
Six step commutation is a simple technique that reads Hall sensors and excites the coils in a specific sequence. The downside to this technique is that for many motors it gives up some efficiency and is not as smooth as more advanced techniques. This is because the output control signal for each coil changes abruptly when a new Hall state is read, which occurs every 60 electrical degrees.
That kind of performance is fine for simple spinning applications, or applications where the motor is geared way down. But for systems that need smoother motion and higher performance, two advanced techniques: sinusoidal control and field oriented control (FOC), provide a jump in performance.
Field oriented control (FOC)
Field oriented control (FOC) is an important control approach for Brushless DC motors. It resembles sinusoidal commutation, but adds a major mathematical twist.
Figure 2: Sinusoidal Commutation
Figure 3: Field Oriented Control
Figure 3 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 keeps the actual winding current at the desired current value.
An important characteristic of this approach is that as the frequency of motor rotation increases, so does the challenge of maintaining the desired current. This is because the current loop is affected by the rotation frequency. Lag in the current loop, 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 field oriented control (FOC) 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 current loops, one for the Q torque and another 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 is math-intensive transform operations that convert the vectorized phase angle to, and from, the de-referenced D and Q reference frame. Known as Park and Clarke transforms, their practical implementation in Brushless DC and AC Induction drives is now commonplace due to the availability of low-cost, high-performance DSPs and microprocessors.
Why can't I fry an egg on this motor?
So what does all this wizardy add up to besides lifetime employment for math majors? The answer is... higher top speed and, often just as importantly, higher motor drive efficiency.
Motor controllers which adopt an FOC approach can drive the motor more efficiently, as high as 97 % in certain applications. This advantage is particularly pronounced at higher speeds.
As it turns out, field-oriented control techniques can also benefit the top speeds of step motors, but this technique for step motors is nowhere near as common as for BLDC motors. There are many reasons for this but perhaps it is primarily due to the fact that if you want to spin very fast, you probably won't be using a step motor in the first place. Few step motors are designed to spin above 5,000 RPM, while specialized BLDC motors routinely spin at 30,000 RPM or even higher.
Where the B-Field hits the road
The following chart shows some common applications that benefit most from FOC:
To the laboratory!
The diagram below illustrates a simple demonstration set-up in PMD Corp.'s lab of the performance improvement from FOC versus Hall-based commutation. In this application, a high speed blower has an internal high speed BLDC motor, and is connected to a Performance Motion Devices' ION Digital Drive.
Figure 3: Illustration of apparatus set-up for videos below
The fun in the lab videos below show a visual difference in motor efficiency between Field-oriented Control and Hall-based techniques. The first video shows the blower driving the car using field oriented control. The second video shows the exact same setup with a Hall-based commutation.
Field oriented control: Car driven by wind from blower
Hall-based control: Car driven by wind from blower
Using Hall and FOC control modes as shown above, the ION digital drive was given a simple command to drive the motor as fast as it could. In each case the supply voltage was the same, @ 24V.
Under these conditions, the Hall-based technique drove the blower spindle at 17,895 RPM, and the FOC technique drove the blower at 29,310 RPM - about 64% faster!
These are actually larger differences in performance than one would normally expect, which may be because of the fact that the motor is not doing a lot of work (other than blowing a toy car). But the principle remains that at high speed in particular, Field Oriented Control (FOC) can provide significant performance advantages over Hall-based and sinusoidal commutation techniques.
Chuck Lewin, CEO
Performance Motion Devices, Inc.