Brushless Motor Control for Pulsating Voltage in Electric Tools
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Solution Overview
Problem
Electric tools with brushless motors driven by full-wave rectified AC power without smoothing condensers experience rotational variations due to pulsating drive voltage, making it difficult to accurately perform single-shot mode functions, such as stopping the motor after a predetermined number of strikes.
Innovation Solution
An electric tool with a control system that includes a PWM control unit to adjust the duty ratio of the PWM signal, a correction parameter generator to reduce rotation speed variations caused by voltage pulsation, and a rotation speed condition determiner to ensure the motor meets predetermined conditions, allowing for accurate detection and execution of single-shot mode functions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If full-wave rectified AC power is supplied to the brushless motor without a smoothing condenser, then the device complexity is reduced and cost is lowered, but rotational variation occurs due to voltage pulsation making it difficult to accurately detect striking
Solution Approach 1:
The control board detects the rotation number of the motor and compares it against a threshold to determine whether striking has occurred. This feedback mechanism allows the system to accurately detect striking conditions despite the rotational variation caused by voltage pulsation, resolving the contradiction between simplified power supply and accurate detection.
Solution Approach 2:
The system dynamically adjusts the threshold rotation number based on the detected rotation number and striking detection results. By changing the threshold parameter adaptively, the system maintains accurate striking detection precision even when rotational variation occurs due to full-wave rectified voltage pulsation.
2Measurement precision
If the threshold rotation number is increased to detect striking, then striking detection sensitivity is improved, but rotational variation due to voltage pulsation causes erroneous detection
Solution Approach 1:
The threshold rotation number is not fixed but is dynamically adjusted based on the detected rotation number and striking detection results. This dynamic adjustment allows the threshold to adapt to changing operating conditions, maintaining high detection sensitivity while avoiding erroneous detection caused by voltage pulsation.
Solution Approach 2:
The system uses feedback from the detected rotation number and striking detection results to adjust the threshold rotation number. This closed-loop control ensures that the threshold remains optimal for detecting actual striking events while filtering out false positives caused by voltage pulsation.
3Reliability
If the threshold rotation number is decreased to avoid voltage pulsation interference, then false detection is reduced, but actual striking may be overlooked
Solution Approach 1:
The threshold rotation number is dynamically adjusted based on real-time detection results rather than being set to a fixed low value. This allows the system to maintain high reliability by avoiding false positives from voltage pulsation while preserving the sensitivity needed to detect actual striking events.
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
The solution effectively reduces rotation speed variations due to voltage pulsation, enabling precise detection and execution of single-shot mode functions, thereby improving the accuracy of screw fastening operations.
Implementation Method 1
A PWM control unit configured to control switching elements of the motor drive circuit by a PWM signal
Implementation Method 2
a brushless motor which is driven by a motor drive circuit converting the full-wave rectified voltage to three-phase voltages to rotate a tip tool
Data Source
Figure 1
Figure 2
Figure 3
AI summary
An electric tool (1) in which a pulsating input voltage is inputted to a drive circuit of a motor (3), characterized in that the electric tool includes: a control part (8) configured to vary output power or output voltage supplied to the motor from the drive circuit in accordance with the pulsation of the input voltage inputted to the drive circuit.