Sensorless BLDC control removes a Hall sensor, encoder or resolver by estimating rotor position from motor voltages and currents. That can reduce wiring, package constraints and sensor exposure. It also makes startup, low-speed operation and signal quality part of the control problem.
The correct question is not whether a motor can spin without a sensor. It is whether the estimator can produce enough reliable position information through every required start, load, speed, temperature and fault condition.
Why sensorless control is difficult at standstill
Back EMF is generated as the rotor moves, and its magnitude increases with speed. At standstill, conventional back-EMF sensing has little or no position information. At low speed, the useful signal may be comparable to PWM switching noise, offset, bus ripple and inductive ringing.
Microchip’s motor-control feedback note identifies this low-speed limitation. A controller therefore needs a startup strategy before a back-EMF estimator becomes trustworthy.
Possible strategies include rotor alignment followed by an open-loop commutation ramp, initial-position detection, high-frequency injection for a suitable salient motor, or a physical sensor. Each has constraints. Alignment can move the load. An open-loop ramp can lose synchronization. Injection adds measurement and model requirements. A sensor adds hardware but may reduce total system risk.
Sensorless six-step commutation
In common six-step operation, two phases conduct while the third is left undriven. The floating phase can be observed for back-EMF zero crossing. The zero crossing occurs within the commutation sector; the controller then schedules the next transition after an electrical-angle delay.
Microchip’s sensorless six-step guide describes the 60-degree sectors, floating phase and practical interference from PWM and inductive ringing. ST’s sensorless back-EMF application note compares sampling approaches and their implementation tradeoffs.
What must be measured
The controller may compare the floating phase against a reconstructed neutral or bus midpoint using comparators or ADC sampling. The measurement network must tolerate the full phase voltage and settle within the available observation window. Divider tolerance, common-mode range, filter delay, leakage and input protection affect commutation timing.
Blanking and filtering
A commutation event produces switching edges and winding-current redistribution. Detecting the first comparator transition after switching can create a false zero crossing. Blanking suppresses the interval known to be unreliable. Analog or digital filtering rejects noise, but excessive delay shifts commutation timing. The filter should be designed around measured waveforms at speed, current, voltage and temperature corners.
PWM strategy
Which device is modulated, where the phase floats and when the sample is taken determine measurement quality. On-time, off-time and synchronous-rectification states create different observable voltages. The control and sampling strategy must be developed together.
Sensorless FOC
Sensorless field-oriented control estimates rotor electrical angle from measured currents, commanded or measured voltages and a motor model. Back-EMF, flux and state observers can provide continuous angle information once the motor is in an observable region.
The estimator depends on current-measurement accuracy, DC-bus knowledge, voltage reconstruction, stator resistance, inductance, flux linkage and timing. Some parameter errors matter more at low speed; others dominate at high current or field weakening. The estimator and current controller cannot be tuned independently of the motor and inverter.
NXP’s sensorless BLDC resources and Microchip’s motor-control documentation show the range from zero-crossing methods to observer-based sensorless FOC.
Startup is a controlled handoff
A common observer-based sequence is:
- Preflight. Calibrate current offsets, verify bus voltage and temperature, and confirm that the motor is not already moving unexpectedly.
- Alignment or initial-position step. Establish a known electrical angle or estimate the initial position using a method compatible with load motion.
- Open-loop acceleration. Advance commanded electrical angle while controlling current so the rotor follows without excessive slip.
- Estimator qualification. Wait until estimated speed, angle consistency and signal magnitude meet defined criteria.
- Blend or transfer. Hand control from open-loop angle to estimated angle without a torque discontinuity.
- Closed-loop supervision. Monitor estimator confidence and define behavior if lock is lost.
Microchip’s sensorless FOC startup sequence illustrates alignment, open-loop ramp and observer transition. The exact current, acceleration and transition thresholds must be derived from the actual motor and load.
Load determines sensorless feasibility
A fan or pump with a predictable monotonic load may tolerate an open-loop ramp. A high-friction mechanism, unknown gravity load, compressor against pressure, or actuator that must reverse through zero speed can make the same method unreliable.
Important questions include:
- Can the load move during alignment?
- What is the maximum breakaway and startup torque?
- Can the rotor be back-driven before enable?
- Must the drive produce holding torque at zero speed?
- Can speed cross zero under closed-loop control?
- What happens if startup fails repeatedly?
- Is absolute mechanical position needed at power-up?
If the answer requires known angle or controlled torque at standstill, a physical sensor may be the more robust architecture.
Measurement-chain design
Current sensing
Observer-based control needs phase-current information with enough bandwidth, accuracy and timing. Shunt topology determines measurable PWM windows. Amplifier settling, common-mode transients, offset and ADC timing can appear as angle error or torque ripple.
Voltage reconstruction
The controller may infer applied phase voltage from duty cycle and bus voltage, but dead time, MOSFET and diode drops, switching delay and bus ripple create error. Direct phase-voltage sensing adds hardware and its own filtering and common-mode limitations.
Computation and timing
Current sampling, observer update, control law and PWM update need a deterministic schedule. The relevant delay is the complete signal and execution path, not only processor instruction time. Jitter and delayed samples reduce phase margin and estimator quality.
Failure detection and recovery
Sensorless control needs explicit detection for failed start, wrong direction, stalled rotor and loss of estimator lock. Useful checks may compare commanded and estimated speed, current demand, acceleration, phase consistency, back-EMF magnitude and timeout state.
Recovery should not repeatedly apply high alignment or stall current. Define retry count, delay, thermal budget and whether the system enters a latched fault. Record bus voltage, current, temperature, state and estimator values so field failures can be distinguished from mechanical blockage or power limitations.
Bench-validation matrix
- minimum and maximum bus voltage;
- minimum, nominal and maximum breakaway load;
- cold and hot motor resistance;
- every allowed initial rotor position;
- forward, reverse and windmilling startup where applicable;
- rapid load changes and speed commands;
- operation near the estimator’s low-speed boundary;
- current-sensor offset and gain limits;
- phase open/short, blocked rotor and interrupted power;
- production motor and component variation.
Select sensorless control for a reason
Sensorless control is valuable when removing the sensor and harness creates real product benefit and the required operating envelope is observable. It is not automatically lower risk or lower cost after startup development, parameter control, fault handling and validation are included.
Outer Reef develops sensorless BLDC control systems and custom motor-controller hardware and firmware around the motor, load, power stage, measurement chain and required startup behavior.
