4.2 Sensorless Field Oriented Control
The phase currents, Ia and Ib, are used as feedback signals to implement the Sensorless FOC technique.
Since the PMSM has a balanced three-phase winding,we know that Ia + Ib + Ic = 0. Therefore, we can derive the third-phase current, Ic from Ia and Ib. The three-phase currents are first converted to a two-phase stator system by using Clarke transformation before being converted to a two-phase rotor system by using Park transformation. This conversion provides two computed current components: Id and Iq. The magnetizing flux is a function of the current Id and the rotor torque is a function of the current Iq.
A position estimator estimates the rotor position and speed information. The motor model uses voltages and currents to estimate the position. The motor model essentially has a position observer to indirectly derive the rotor position. The PMSM model is based on a DC motor model.
After the speed is determined by mathematical estimation, the error between the desired speed and the estimated speed is fed to the speed compensator. The speed compensator produces an output that acts as a reference to the Iq compensator. For a surface mounted permanent magnet synchronous motor, the reference to the Id compensator is zero value. The PI controllers for Iq and Id compensate errors in the torque and flux, thereby producing Vd and Vq as the output signals respectively.
The Inverse Park transformation and Space Vector Modulation (SVM) techniques are applied to generate the duty cycle for the Insulated Gate Bipolar Transistors (IGBTs). The motor control PWM module is used to generate PWM pulses.
Refer to application note AN1078, Sensorless Field Oriented Control of PMSM Motors (DS01078), for information about how to design, implement, and tune the compensator.
The implementation details and the hardware configuration details required to develop the integrated system are discussed in the following sections.