Sensorless Vector Control for PMSM Across Full Speed Range: Pre-positioning + IF + SMO

This article introduces a sensorless control strategy for Permanent Magnet Synchronous Motors (PMSM) covering the full speed range, using a three-stage approach of pre-positioning + IF + SMO for initial position detection, IF strong-dragging to certain speed, and then switching to SMO observer. After theoretical introduction, simulation implementation verifies the effectiveness of this strategy.

1. Research Background of Sensorless Control for PM Motors

The research of sensorless control is mainly conducted to address requirements due to external condition limitations and cost considerations. The main methods are divided into high-frequency signal injection method and model-based method. PMSM differs from induction motors – due to motor characteristics at different speed ranges, different estimation methods need to be adopted.

Figure 1.1 Classification of sensorless control technologies for PMSM

Besides the methods mentioned above, there also exist single-observer sensorless control strategies covering the full speed range, such as nonlinear flux observer method, current estimation method based on dq-axis, etc.

2. Pre-positioning

High-performance control of PMSM requires precise rotor position information. Starting with known rotor position enables obtaining maximum starting torque. Starting from unknown rotor position may cause excessive starting current, reverse rotation or starting failure of the motor, which is unacceptable in many applications.

For example, when the angle between stator magnetic field and permanent magnet field is 180° or 0°, the rotor torque is 0 and the motor cannot start normally – this position is called the motor’s starting dead point. Conversely, when the stator armature magnetic field is perpendicular to the permanent magnet field, the motor achieves maximum starting torque at this moment. However, since the angle between stator field and permanent magnet field still has 90° and 270° distinction (two opposite positions): if it’s 90°, the motor starts with maximum torque; if it’s 270°, the motor starts with maximum torque in the opposite direction. Therefore, the rotor’s N and S poles still need to be determined to ensure successful PMSM starting.

Existing initial rotor position detection methods for PMSM can be divided into two categories: those based on motor’s own magnetic circuit structural saliency effect and those based on nonlinear saturation characteristics of stator iron core. Interior PMSM exhibits structural saliency due to asymmetric rotor magnetic circuit structure resulting in different d-axis and q-axis inductances. Normally, surface-mounted PMSM has equal d-axis and q-axis inductances. When current is injected into stator windings and the generated magnetic field direction aligns with permanent magnet field direction, it causes d-axis magnetic path saturation, making d-axis inductance smaller than q-axis inductance, thus presenting saturation saliency.

This section mainly introduces the pre-positioning method, which makes the motor rotor shaft rotate to a specified position. In practical applications, it’s commonly used in fan and pump applications.

Implementation approach of pre-positioning: Apply fixed torque current iq, set id=0, apply different theta angles, and the motor shaft will rotate to the position corresponding to the angle.

3. IF Control

Figure 3.1 IF control block diagram

The IF control method controls the current and is used in the starting stage of medium-high speed sensorless control. IF control can achieve smooth motor starting without current overshoot during the starting process. However, the basic IF control method is an open-loop scheme where current amplitude and frequency cannot be automatically adjusted, and it has disadvantages like being prone to step loss and speed being easily disturbed. It’s not very suitable for motor steady-state operation and can only be used for assisted starting.

4. Simulation Implementation and Verification

This system simulation is a per-unit simulation system, realizing full-speed-range sensorless control in three steps:

• Perform pre-positioning to determine PMSM’s initial rotor position
• IF strong-dragging to specified speed (since PMSM’s back-EMF is proportional to speed, observers have difficulty estimating accurate back-EMF in low-speed regions, causing system instability or even divergence)
• Switch from IF to sliding mode observer (SMO)

The contents of sliding mode observer (SMO) and phase-locked loop (PLL) have been introduced in previous articles and won’t be elaborated here.

Figure 4.1 Pre-positioning process

Figure 4.2 IF control process

Figure 4.3 Angle variation during IF strong-dragging

Figure 4.4 Synchronous angle variation

From Figures (4.3) and (4.4), we can see it first pre-positions to 1.5π position, then undergoes IF strong-dragging, and finally switches to SMO.

Figure 4.5 Speed tracking variation

Figure 4.6 Stator current waveform

Figure (4.5) shows speed estimation variation: first IF strong-drags to 800rpm, then switches to SMO observer and changes to 2000rpm. During the switching process, estimated speed fluctuates significantly. This is caused by the selection of switching function. To improve this, this part needs modification. 

5. Discussion Questions

5.1 In practical applications, what methods are generally used for initial position detection?
5.2 How to select the switching strategy from IF to SMO to reduce the impact of q-axis current mutation on control?