Green Motor Tech provides a one-stop global solution for electric vehicle (EV) conversion drive systems. Our drive system adopts advanced Field-Oriented Control (FOC) technology, offering efficient and smooth motor control performance. This significantly enhances electric vehicles’ overall driving efficiency and response speed. With this technology, we help clients optimize vehicle performance, achieve more precise power control, and extend driving range to meet the demands of various application scenarios.
In addition, our team offers comprehensive technical support and after-sales service to ensure a seamless and reliable experience throughout the electrification conversion process. Our solutions are not limited to standard products but can also be customized according to specific customer needs, ensuring that the most suitable drive system is provided to help clients deliver more professional and efficient services to their markets. In this article, we will have a brief introduction to FOC.
Field-oriented control (FOC), also known as vector control, is a technique used to control Permanent Magnet Synchronous Motor (PMSM) and AC induction motors (ACIM). FOC provides good control capability over the full torque and speed ranges. The FOC implementation requires a transformation of stator currents from the stationary reference frame to the rotor flux reference frame (also known as the d–q reference frame).
Speed control and torque control are the most commonly used control modes of FOC. The position control mode is less common. Most of the traction applications use the torque control mode in which the motor control system follows a reference torque value. In the speed control mode, the motor controller follows a reference speed value and generates a torque reference for the torque control that forms an inner subsystem. In the position control mode, the speed controller forms the inner subsystem.
FOC algorithm implementation requires real-time feedback of the currents and rotor position. Measure the current and position by using sensors. You can also use sensor-less techniques that use the estimated feedback values instead of the actual sensor-based measurements.
Permanent Magnet Synchronous Motor (PMSM)
This figure shows the FOC architecture for a PMSM. For a detailed set of equations and assumptions that Motor Control Blockset uses to implement the FOC of a PMSM.
AC Induction Motor (ACIM)
This figure shows the FOC architecture for an AC induction motor (ACIM). For a detailed set of equations and assumptions that Motor Control Blockset uses to implement the FOC of an induction motor, see the Mathematical Model of Induction Motor.
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, Brushless DC and step motors are ‘multi-phase’, meaning they require some type of external coil excitation to keep the motor moving.
This deep dive will examine the most popular motion control techniques, including field-oriented control (FOC), for multi-phase motor control, to determine what control techniques work best for positioning and high-speed 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 driven, they can interact to create a force that does not generate rotational torque, or they can create a force that does generate 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 (Figure 1).
Figure 1: Quadrature and Direct Forces
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. Both of these phenomena can be seen in Figure 2, which shows the torque reduction that stems from having only six measurable vector angles per electrical rotation.
Figure 2: Torque Reduction vs Hall-based Sensing
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 a sinusoidal commutation but adds a major mathematical twist.
Figure 3a: Sinusoidal Commutation
Figure 3b: Field-Oriented Control
Figure 3a 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 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 FOC, Figure 3b, 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 drives is now commonplace due to the availability of low-cost, high-performance DSPs and microprocessors.
Motor controllers that 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, FOC techniques can also benefit the top speeds of step motors, particularly if the step motor is driven using a closer loop stepper technique (also sometimes called stepper servo). Although step motors are generally two-phase rather than three-phase devices, all the same D and Q force concepts discussed above apply. And since the closed loop stepper drives the step motor using a variable torque command servo technique rather than with a fixed torque command, dramatic reductions in heat output in the step motor are possible.
