Sensorless Brushless Motor Rotor Position Detection
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Solution Overview
Problem
Existing sensorless brushless motor startup techniques face challenges in determining rotor position at low speeds, particularly at standstill, due to the reliance on back electromotive force (BEMF) detection, which is unreliable at zero speed, and require complex and costly current sensing or magnetic circuit saturation methods that introduce latency and sensitivity issues.
Innovation Solution
A simplified sampling circuit that induces voltage on a non-excited winding by transformer effect, allowing for feedback signal generation to synchronize stator winding switchings, applicable to both star-connected and polygon-connected motors, which reduces noise interference and eliminates the need for direct current measurements, enabling efficient startup and transition to BEMF-closed-loop mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If BEMF detection is used for rotor position determination, then rotor position can be detected during rotation, but it becomes unreliable at low speeds and standstill since BEMF amplitude is proportional to speed
Solution Approach 1:
The patent changes the detection parameter from BEMF (which depends on speed) to inductance (which is speed-independent). By measuring the inductance of phase windings at standstill and during startup, the system can determine rotor position without relying on speed-proportional BEMF signals, thus resolving the contradiction between measurement precision and speed dependency
Solution Approach 2:
The patent replaces the electrical measurement method (BEMF detection) with a magnetic property measurement method (inductance sensing). This substitution allows rotor position detection to work at any speed including zero, eliminating the speed limitation inherent in BEMF-based methods
2Speed
If magnetic circuit saturation method is used for rotor position determination at low speed, then rotor position can be determined at standstill, but it introduces long latency times (hundreds of microseconds) and requires large currents
Solution Approach 1:
The patent applies partial saturation by using moderate voltage steps rather than large currents required for full saturation. This partial action is sufficient to detect rotor position through inductance changes while avoiding the excessive currents and long latency times associated with complete magnetic circuit saturation
Solution Approach 2:
The patent uses simple voltage step excitations and basic current sensing that can be implemented with inexpensive existing circuitry, replacing complex and costly dedicated rotor position sensing systems. The method uses readily available motor phase windings and control circuits rather than requiring additional expensive sensors
3Measurement precision
If current sensing circuits are used for measuring phase winding currents, then accurate rotor position can be determined, but it increases system cost and complexity
Solution Approach 1:
The patent makes the existing motor control circuitry perform dual functions: both driving the motor and sensing rotor position. The phase winding currents used for motor control also serve as the sensing signal for inductance measurement, eliminating the need for separate current sensors and reducing overall system complexity
Solution Approach 2:
The motor's own phase windings and control circuits provide the sensing capability. The system uses its existing current paths and control signals to measure inductance changes, making the motor self-sufficient for rotor position detection without requiring external sensing components
4Speed
If inductive sensing with voltage step application is used, then rotor position can be determined at standstill, but it requires accurate current sensing that adds cost and complexity
Solution Approach 1:
The patent enables the motor control circuit to perform both motor driving and rotor position sensing functions. The same circuitry that controls phase winding excitation also measures the current response to voltage steps for inductance calculation, eliminating dedicated sensing requirements and reducing system complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method allows for accurate rotor position determination and efficient startup with reduced latency, improved speed control, and automatic mode transitions from startup to BEMF-closed loop and active braking, enhancing motor performance and reliability.
Implementation Method 1
sampling on a currently non-excited stator winding a voltage induced thereon by transformer effect
Data Source
AI summary
The method of synchronizing sequential phase switchings in driving stator windings of a multiphase sensorless brushless motor with a reconstructed information on the current angular position of a permanent magnet rotor, includes sampling on a currently non-conductive stator winding a voltage induced thereon by the resultant magnetic field produced by the drive current forced through currently conductive stator windings that inverts its sign when the rotor transitions across a plurality of significant angular positions, at which orthogonality between the resultant magnetic field and a magnetic axis of the non-excited winding verifies. The sign of the sampled voltage induced on the currently non-excited winding is compared with the sign that is expected upon transiting across the angular position of inversion by the moving rotor for the current phase drive configuration to sequentially switch to the next phase drive configuration upon verifying conformity of the sign of the sampled voltage with the expected sign.