Brushless Power Tool Restart Control with Timed Pre-Start Braking
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Solution Overview
Problem
Brushless motors without sensors face delays in restarting due to inaccurate back-EMF zero-crossing detection when rotating slowly or coasting to stop, especially noticeable in power tools that are frequently turned on and off.
Innovation Solution
A power tool controller that detects power-on and power-off signals, measures time intervals or motor rotational speeds, and brakes the motor before start-up if the interval is within specific thresholds or the speed is below a predetermined level, allowing for quicker rotor position determination and reduced restart delays.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the power tool is restarted immediately after power-off, then the restart response should be fast, but the back-EMF zero-crossing detection becomes inaccurate due to slow rotor rotation
Solution Approach 1:
The controller performs preliminary braking action before the intended start-up when the time interval is within the first threshold range. This preliminary braking stops the rotor quickly, creating proper conditions for accurate back-EMF zero-crossing detection to occur, thereby enabling fast and accurate restart without sensor assistance
Solution Approach 2:
The controller changes the motor operating state by applying braking force to transition the rotor from slow rotation to complete stop. This parameter change (from rotating to stationary) creates the necessary conditions for accurate position detection, allowing the system to resolve the contradiction between fast restart and accurate detection
2Use of energy by moving object
If the rotor rotates slowly or coasts to stop, then energy consumption is reduced, but the back-EMF zero-crossing method cannot accurately detect rotor position
Solution Approach 1:
The controller detects slow rotation conditions and performs preliminary braking action before normal operation begins. This ensures the rotor is properly positioned and stationary when needed, enabling accurate detection without requiring continuous high-speed rotation that would increase energy consumption
Solution Approach 2:
The system uses the motor's own electromagnetic characteristics during braking to achieve precise rotor positioning. The braking process itself generates detectable electrical signals that provide the necessary information for accurate position determination, eliminating the need for external sensors while maintaining low energy consumption
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 solution significantly reduces restart delays in power tools by braking the motor to stop it quickly and determine the rotor position more efficiently, improving user experience by minimizing the time required for the motor to accelerate after braking.
Implementation Method 1
The movement of the rotor through the magnetic field of the motor, however, is analogous to the behavior of a generator, and consequently the motor not only receives an applied voltage but also generates its own voltage. This voltage is referred to as back electromotive force, and it is proportional to the rotational speed of the motor.
Implementation Method 2
determine whether the time interval is greater than or equal to a first time threshold and less than a second time threshold, and brake the brushless motor before start-up if the time interval is greater than or equal to the first time threshold and less than the second time threshold
Data Source
AI summary
A power tool includes a housing, a brushless motor disposed inside the housing, and a controller. The controller is configured to receive power-off signals and power-on signals. Upon receiving a power-on signal, the controller is further configured to receive a time signal indicative of a time interval from the last power-off signal to the current power-on signal. The controller is further configured to determine whether the time interval is greater than or equal to a first time threshold and less than a second time threshold, and brake the motor before start-up if the time interval is greater than or equal to the first time threshold and less than the second time threshold.