Field Oriented Control

Field Oriented Control (FOC) (or synonymously referred to as Vector Control) is a well-established method of control for three phase AC machines. FOC aims to directly control the torque producing current, Iq and magnetic field producing current, Id as two decoupled quantities. The FOC algorithm

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Figure 2.13 - Block diagram of the FOC technique.

References (or set-points) for the desired torque producing current, Id are set based on input from

the user (throttle input). The magnetic field producing current, Iq is set to zero for the non-salient

rotor PMSM. This means that the motor is producing the maximum amount of torque for the amount of current that it is drawing.

Measurements of the actual phase currents are taken and transformed into the alpha-beta

stationary reference frame using the Clarke Transformation. Measurements of the rotor position are taken and used for the second reference frame transformation to the direct-quadrature

synchronous reference frame using the Park Transformation. This gives the measured values for Id

and Iq.

The measured values for Id and Iq are compared to those set by the controller as the reference Id and

Iq values. Typically a PI or PID controller is used for each of Id and Iq controllers. The Id and Iq

controllers generate a direct axis voltage, Vd and quadrature axis voltage, Vq that aims to reduce any

difference between measured and reference values of Id and Iq to zero.

The measured rotor position is then used to transform the Vd and Vq voltages to alpha and beta

voltages, Vα and Vβ respectively using the Reverse Park transformation. Vα and Vβ are used directly in

Space Vector Modulation in generating the PWM gate drive signals for each of the three phases in the inverter.

2.3.4.1. Space Vector Modulation

Space Vector Modulation (SVM) may be considered as an extension of the six step control technique detailed in the previous section. We have seen how a voltage vector may be generated that is aligned with any one of the six switching states. Now consider how we might generate a voltage vector with a direction not aligned with these states. An arbitrary voltage vector may lie somewhere in between the directions dictated by the physical distribution of the phase windings. Figure 2.14 shows an arbitrary voltage vector which is produced using a combination of state 1 and state 2 of the 6 possible inverter output states.

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Figure 2.14 - Vector diagram for SVM showing an arbitrary output vector produced in sector ‘S12’ of the SVM scheme.

SVM uses a combination of the two adjacent fixed voltage vectors to generate a voltage vector whose net direction is determined by the ratio of the directions of the two fixed phase vectors used, and whose magnitude is determined by the PWM duty cycle. This effectively allows the controller to place the output voltage vector anywhere inside the shaded region in the above figure. The sectors labelled as ‘S12’, ‘S23’, etc. represent which of the two inverter states are required to produce a voltage vector in that region. For example, a vector which lies in sector ‘S34’ is produced using a combination of state 3 and state 4 of the six active inverter states.

2.3.4.2. Rotor Position Sensing

Recall the Six step control technique places the output voltage vector along the axis formed by the six active vectors. This requires the rotor position to be known to the nearest 60° of electrical rotation and can be measured directly by a set of three Hall Effect sensors. In contrast, FOC is able to place the output voltage vector anywhere in the shaded region of the phasor diagram in order to align the quadrature current perpendicular to the rotor magnet angle at any given instant. Essentially the controller output voltage vector is continuously variable in both direction and magnitude and is dependent on the rotor position. This requires that the rotor position

measurements must be more accurate than the ±30° obtained from the hall sensors. There are numerous options for obtaining higher resolution rotor position measurements:

High Resolution Encoders

Either an absolute encoder or incremental encoder with home position may be used to give high accuracy rotor position measurements suitable for FOC. Unlike the Hall sensor implementation, the absolute encoder input may be read on demand rather than by means of external interrupt

whenever the input changes. The incremental encoder however, requires an external interrupt generated in order to count the input pulses.

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Hall Sensor Extrapolation

This technique involves timing the duration between changes in the Hall sensor input. With this information, the rotor speed can be calculated and updated after each time the input toggles i.e. every 60° of electrical rotation. With the measurement of the rotor speed from the previous 60° of electrical rotation, the current rotor angle may be estimated to a high resolution by extrapolating from the last hall input change.

Sensorless Methods

There are various sensorless methods of measuring the rotor position to a high resolution for FOC. The previously mentioned High Frequency Injection (HFI) may be used or other methods such as ‘Phase Locked Loop (PLL) Observer’ or ‘Cordic Observer’. The reader is encouraged to research these methods separately pursuant to interest.