Power Tool Motor Stall Control With Reverse-Then-Brake Logic
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Solution Overview
Problem
Existing motor control methods for power tools, particularly sensor-less FOC algorithms, fail to achieve consistent stall torque performance due to the number of free rotation cycles affecting the stall torque peak value, leading to potential overheating and damage, and braking the motor limits the number of rotation cycles, making stall torque performance inconsistent.
Innovation Solution
A motor control algorithm that operates the motor in a first stall state for a predetermined duration or number of rotation cycles, allowing it to reverse direction or freely rotate without excitation before transitioning to a second stall state involving braking, ensuring optimal stall torque performance by controlling the motor's stopping position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the motor is braked immediately upon stalling, then the motor stops quickly and is ready for the next operation, but the number of rotation cycles is limited and stall torque performance becomes inconsistent
Solution Approach 1:
The patent implements periodic action by alternating between braking mode and free rotation mode in a controlled cycle. The motor operates in braking mode to quickly reduce speed, then transitions to free rotation mode to allow the rotor to coast and accumulate rotation cycles, creating a periodic pattern that ensures consistent stall torque performance while maintaining quick response capability.
Solution Approach 2:
The patent applies dynamics by making the motor control system adaptable and flexible. The controller dynamically switches between different operating modes (braking and free rotation) based on the current operational state and requirements. This dynamic adjustment allows the system to optimize between quick stopping and consistent stall torque performance, rather than using a fixed control strategy.
2Force
If the motor is allowed to rotate freely for more cycles, then stall torque peak value increases, but the motor takes longer to stop and reduces readiness for next operation
Solution Approach 1:
The patent uses periodic action to alternate between phases of free rotation (to build up stall torque) and braking (to stop the motor quickly). This periodic switching allows the motor to accumulate the necessary rotation cycles for high stall torque peak values while ensuring the motor stops in a timely manner for the next operation, resolving the trade-off between torque peak and stopping time.
Solution Approach 2:
The patent applies preliminary action by allowing the motor to rotate freely and accumulate rotation cycles before the actual working operation begins. This preliminary free rotation phase ensures that the motor reaches optimal conditions for high stall torque performance, and the controller is prepared to apply braking at the appropriate moment to achieve quick stopping when needed.
3Device complexity
If sensor-less FOC is used instead of Hall sensor-based blocking commutation, then component cost and failure points are reduced, but acceleration performance at zero and low speeds deteriorates
Solution Approach 1:
The patent applies mechanics substitution by replacing the mechanical/Hall sensor-based commutation system with a sensor-less FOC control system. The invention compensates for the reduced low-speed acceleration performance by implementing a dual-mode control strategy that uses free rotation to accumulate momentum and then applies controlled braking, effectively substituting the physical sensor-based speed detection with a computational control approach that achieves similar performance benefits.
Data Source
AI summary
A method of controlling a motor of a power tool includes driving the motor such that the motor rotates in a first direction and in response to the power tool entering a stall condition, operating the motor in a first stall state for a predetermined duration or a predetermined number of motor rotation cycles. In the first stall state, either the motor is driven such that the motor rotates in a second direction opposite the first direction or the motor is allowed to reverse direction and freely rotate in the second direction without excitation via a gear box coupled to the motor. The method further includes, upon expiration of the predetermined duration or the predetermined number of motor rotation cycles, operating the motor in a second stall state which includes braking the motor. Also described herein is electronic circuitry for a motor of a power tool.