BLDC Power Tool Control for Low-Speed Sensorless Commutation
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Solution Overview
Problem
Existing sensorless control techniques for brushless direct-current (BLDC) motors in power tools are inadequate for low-speed, high-torque applications, particularly when users reverse the motor rotation, and lack effective methods for variable conduction band and advance angle control, leading to defects and errors due to electromechanical faults or software bugs.
Innovation Solution
A sensorless control scheme for BLDC motors in power tools using motor back-EMF for rotor position detection, combined with a secondary controller for speed and direction determination, and a method for low-speed commutation control that includes initial position detection and variable conduction band/advance angle control to manage rotor rotation and prevent overspeed or incorrect direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If sensorless control using back-EMF is used for high-speed motor operation, then the control simplicity and cost are improved, but the control accuracy and reliability deteriorate at low speeds
Solution Approach 1:
The patent implements dynamic switching between two control modes: at high speeds, it uses sensorless back-EMF detection for simplicity; at low speeds, it transitions to Hall sensor-based control for reliability. This dynamic adaptation resolves the contradiction by optimizing the control method according to operating conditions.
Solution Approach 2:
The control system is segmented into two distinct operational modes with different control strategies. The first mode uses sensorless control for high-speed operation, while the second mode uses Hall sensors for low-speed operation. This segmentation allows each mode to be optimized independently for its specific speed range.
2Power
If variable conduction band control is implemented to increase motor power output, then the motor power is improved, but the control complexity increases
Solution Approach 1:
The patent changes the conduction band parameter dynamically based on operating conditions. By adjusting the conduction band width, the system optimizes motor power output without requiring a completely new control architecture. This parameter adjustment resolves the contradiction by achieving higher power within the existing control framework.
3Measurement precision
If Hall sensors are added to detect rotor position for accurate control, then the control precision is improved, but the device complexity and cost increase
Solution Approach 1:
The system dynamically selects the appropriate sensing method based on speed requirements. At high speeds, it uses sensorless control without Hall sensors. At low speeds where precision is critical, it activates Hall sensors. This dynamic approach resolves the contradiction by using sensors only when necessary.
4Power
If the conduction band is increased beyond 120 electrical degrees to overlap phases, then the motor power output is improved, but the control complexity and risk of faults increase
Solution Approach 1:
The patent implements feedback mechanisms using Hall sensors to monitor rotor position and provide feedback to the control system. This feedback allows the system to safely operate with extended conduction bands by continuously verifying that phase switching occurs at the correct moments, preventing faults while maintaining high power output.
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 low-speed operation with accurate rotor position detection and protection against incorrect rotation, reducing defects and enhancing motor performance and safety in power tools.
Implementation Method 1
monitoring the motor induced voltage generated by the back-electromotive force (back-EMF) of the motor in the motor windings to detect a rotational position of the motor
Implementation Method 2
a rotor including a series of magnets that magnetically interact with the stator windings. As the phases of the windings are sequentially energized, they cause rotation of the rotor
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
A power tool is provided including a brushless motor having a stator defining a plurality of phases, a rotor rotatable relative to the stator, and power terminals electrically connected to the phases of the motor. A power unit is provided including power switches. A control unit is interfaced with the power unit to output a drive signal to one or more of the motor switches to drive the phases of the motor over a series of sectors of the rotor rotation. The control unit is configured detect incorrect rotation of the rotor by applying a first series of voltage pulses to a present sector and a second series of voltage pulses to a previous sector, measuring motor currents associated with the first and second series of voltage pulses, and comparing corresponding motor current measurements to detect a transition from the present sector to the previous sector.