Brushless Motor Rotor Position Detection Under Variable PWM Duty
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Solution Overview
Problem
Existing sensorless motor position detection methods for brushless DC motors face challenges in accurately detecting rotor position at low speeds and varying duty ratios, particularly under PWM control with 120-degree energization, leading to limited controllable motor torque and inefficient operation.
Innovation Solution
A field magnet position detection method for brushless DC motors that utilizes a control unit to perform PWM control with 120-degree energization, including an off cycle and A/D conversion to measure energization and non-energization phase voltages, selecting an optimum duty ratio for sensing, and applying driving voltage in on-duty intervals to reliably detect the rotor position even at varying duty ratios.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If 120-degree energization with PWM control is used for sensorless motor operation, then cost is reduced and durability is improved by eliminating position sensors, but rotor position detection accuracy deteriorates at low speeds and varying duty ratios
Solution Approach 1:
The patent applies preliminary action by detecting rotor position at specific predetermined timing points (when phase current becomes zero) before commutation switching occurs. This timing-based detection approach allows the system to acquire position information in advance, enabling accurate position detection even during PWM control with varying duty ratios and at low speeds where continuous position sensing is challenging.
Solution Approach 2:
The patent changes the detection parameter from continuous voltage monitoring to sampling at specific timing conditions (when phase current equals zero). By transforming the detection approach from continuous to event-triggered sampling, the system achieves accurate position detection across varying duty ratios without requiring complex continuous sensing hardware.
2Speed
If high current is applied for motor activation to overcome ramp start limitations, then positioning speed is improved, but energy consumption increases and efficiency deteriorates
Solution Approach 1:
The patent implements feedback by continuously monitoring phase current to detect when it becomes zero, using this information to determine the optimal timing for position detection and commutation switching. This feedback mechanism enables the system to achieve fast positioning at low currents by intelligently timing the detection and switching operations, eliminating the need for high-current activation methods.
3Measurement precision
If inductance difference method is used for rotor position detection, then position detection accuracy is improved in stopped state, but device complexity increases due to large scale hardware and software requirements
Solution Approach 1:
The patent extracts only the essential position detection function by monitoring when phase current becomes zero during commutation, rather than implementing complex inductance measurement systems. This extraction approach isolates the critical timing event for position detection, achieving accurate rotor positioning with minimal hardware and software complexity.
Solution Approach 2:
The patent uses simple, low-cost current sensing that leverages existing commutation timing information rather than expensive dedicated position sensing hardware. By reusing existing current measurement capabilities at specific moments in the commutation cycle, the system achieves position detection functionality without investing in complex permanent magnet position sensors or large-scale measurement systems.
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
Enables reliable detection of the field magnet position at low speeds, allowing smooth continuous operation of the motor without sensing noise or electromagnetic interference, improving efficiency and controllability across a wide range of duty ratios.
Implementation Method 1
a control unit configured to perform PWM control on a coil output
Implementation Method 2
a measurement unit configured to perform A/D conversion on the three-phase coil voltage
Data Source
AI summary
When a three-phase brushless motor is operated by sensorless driving under PWM control with 120-degree energization, an operation can be performed with a field magnet position of an electric motor operating reliably detected even when a duty ratio of driving voltage is varied. When performing an operation with a duty ratio of driving voltage applied to a three-phase coil varied through an invertor circuit (52), an MPU (51) selects an optimum duty ratio for sensing non-energization phase coil voltage for predetermined driving voltage, applies the driving voltage in an on-duty interval corresponding to the optimum duty ratio when the driving is performed with the optimum duty ratio, and when applying the driving voltage based on a duty ratio higher than the optimum duty ratio, applies the driving voltage with an on-duty interval of a single cycle divided into a plurality of on-duty intervals including the optimum duty ratio.