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Robotics

Six-Step Commutation

ActivePublished: 21 October 2026Updated: 21 October 2026Published
Key innovation
The simplest electronic BLDC control scheme: six discrete switching states per electrical revolution triggered directly from Hall sensors, without coordinate transforms or continuous current-vector regulation.
Category
Robotics
Abstraction level
Pattern
Operation level
Robot control
Use cases
Fan and pump drivesDrills and power toolsESCs for drones and RC modelsLow-cost wheel and propeller drivesApplications with moderate torque-smoothness requirements

How it works

1) Three Hall sensors (or back-EMF sensing on the floating phase) report the current rotor-position sector — one of six 60° electrical sectors. 2) A commutation table maps each sector to a three-phase bridge switching state: one phase high, one low, one off. 3) As a result a DC current flows through two phases in each step, creating a stator field vector roughly perpendicular to the rotor field. 4) When the rotor advances 60° electrical, the sensors flag a new sector and the drive switches to the next of the six states. 5) Current (hence torque) is usually regulated by PWM on the active switches. Six switches per electrical revolution produce a field that rotates in discrete steps, so torque is not perfectly smooth — it ripples at each commutation.

Problem solved

How to switch current in BLDC windings cheaply and simply so the stator field follows the rotor and produces torque, without the compute cost of vector control. It yields a working drive with a minimal sensor count (three Hall sensors) and simple switching logic.

Components

Three-phase inverter bridgeDelivers current to the two active phases in each of the six steps

Six transistors (3 half-bridge legs) switching the motor phases between the DC+ and DC- rails.

Hall-effect sensorsReport the rotor-position sector that triggers commutation

Three sensors spaced 120° (or 60°) apart, returning six valid signal combinations per electrical revolution.

Back-EMF sensing (sensorless)Instead of Hall sensors, detects back-EMF zero-crossing in the floating phase; complicates standstill startup.

Official

Commutation logicSelects which two phases to energize in each step

A table mapping the six position sectors to the six bridge states; implemented in a microcontroller or a dedicated IC.

Implementation

Implementation pitfalls
Commutation torque rippleMedium

Stepwise phase switching every 60° electrical causes periodic torque dips/pulsations and acoustic noise.

Fix:Use sinusoidal control or FOC where smooth torque is required; shape current around commutation instants.
Sensorless startup from standstillMedium

In the sensorless variant there is no back-EMF at standstill, so startup needs an open-loop align-and-go sequence.

Fix:Use a forced open-loop startup ramp or Hall sensors for reliable starting.

Evolution

1962
Solid-state commutation of a DC machine
Inflection point

T.G. Wilson and P.H. Trickey demonstrate electronic (solid-state) commutation of a DC machine — the basis of BLDC drives and six-step control.

1969
BLDC drives become practical

The development of solid-state electronics in the 1960s makes practical block (six-step) control of brushless motors possible.