Hello,
I suppose that you actually mean ST Motor Control Workbench, from ST MCSDK and not STM32CubeMx as the sensorless modes are not visible from CubeMx, AFAIK.
First, HFI+Observer is not supported anymore. Indeed, the choice is still possible in the UI of the Workbench but this is a bug that has been fixed with MCSDK version 6.0.0.
The Observer observes the back-EMF of the motor from the measurement of the currents flowing through the phases. It then reconstructs the rotor speed and its electrical angle from the observed back-EMF. This reconstruction can be done either with a PLL or with a CORDIC. The choice is mainly a matter of user's preferences. On STM32G4xx MCUs, the Observer+CORDIC may be preferred as it uses the hardware CORDIC IP embedded in these devices, that is faster than the software implementation.
Now, the Observer is observing the back-EMF. The slower the rotor speed, the smaller the back-EMF and the more sensitive it gets to current measurements noise. When the speed goes too low, the current measurement gets so noisy that the observed back-EMF and, as a consequence, the reconstructed speed and angle are not reliable anymore.
The implementation of the Observer provided with the SDK features a mechanism that assesses the reliability of the angle and that triggers a Speed Feedback Error and the motor is stopped.
Two tracks that can be investigated to address this issue. Both imply hardware changes:
- Switching to a sensorred solution: a quadrature encoder or Hall effect sensors can be used with the SDK to provide the position and the speed of the rotor.
- reworking some aspects of the hardware to improve the quality of the measured signals.
I hope it can help!
Best Regards,
Fred
