Sensorless FOC – Backward Motion on Startup (Need Help with IPD Integration)

[Mani2]

Hello Everyone,

I’m using sensorless FOC to control a BLDC motor with 3-shunt current sensing for closed-loop control. At startup, the motor slightly moves backward before starting to rotate in the correct (forward) direction. I understand this is likely due to the lack of initial rotor position information. There are no position or speed sensors involved—everything is handled via FOC and current feedback.

I verified that 6-step control on the same hardware supports Initial Position Detection (IPD), although I haven’t tested it yet. That said, I am not switching to 6-step control. I want to stay with FOC and retain the 3-shunt topology for current sensing in closed-loop mode.

My questions:

1. Is there a way to prevent this backward motion using FOC only?

2. If IPD is the only viable solution, how can I integrate IPD into the FOC framework, using my current hardware setup (STM32F401RE + 3-shunt topology)?

3. Can I reuse the 6-step IPD logic just for startup, and then transition into FOC with 3-shunt current sensing?

Any guidance, suggestions, or example implementations to help integrate IPD with FOC would be greatly appreciated.

Thank you!

Best Regards,
Manikanta

[GMA]

Hello @Mani2,

1. You can use an Encoder with absolute position Motor or HALL sensor motor.
2. You can also use HSO algorithm capable board (like STEVAL-PTOOL4A inverter or B-G473E-ZEST1S/STEVAL-LVLP01 combo) that provides PolPulse feature to detect initial motor position. Refer to User Manual documentation available through "Workbench tool">About>Documentations>Documentation>"User manual" tab, "Speed & position feedback sensorless algorithms" link, "4. High Sensitivity Observer (HSO)" chapter.
3. It is not possible to swap dynamically from 6-Step to FOC Sensor less modes.

[Mani2]

Hi @GMA,

Thank you for the response.

I’d like to clarify that I’m planning to go sensorless only, and I’m already committed to using the STM32F401RE MCU. I’ve checked that this hardware does not support HSO in the STM32 Motor Control Workbench, which is why I’m exploring other options.

Specifically, I want to reuse or adapt the 6-step IPD logic just for startup, with necessary modifications to make it compatible with FOC. I’m not planning to switch between 6-step and FOC modes dynamically — the goal is to keep everything within the FOC framework, using the existing 3-shunt hardware setup.

If there's any guidance or example on how to implement IPD purely for startup before switching into FOC with this hardware, that would be very helpful.

Thank you.

Best Regards,
Manikanta

 

[GMA]

Hello @Mani2,

HSO is only available on mentioned boards.
6-Step IPD feature is based on HW topology (BEMF measurement / Single Shunt current measurement, refer to documentation available through "Workbench tool">About>Documentations>Documentation>"User manual" tab "Six-step Firmware Algorithm" link), that is not compatible with FOC Sensorless HW topology.