Sensorless BLDC Motor Stopping by Back-EMF Speed Reduction
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Solution Overview
Problem
Brushless DC motors used in motor vehicle wiping devices face challenges in controlling speed and stopping without position sensors, leading to potential damage from current peaks when short-circuiting the inverter branches at nominal speed, which increases costs by requiring oversized transistors.
Innovation Solution
A method for controlling a brushless and sensorless DC motor that reduces the rotation speed to a predetermined level where electromotive forces can be measured to determine the rotor position, allowing the motor to be stopped by short-circuiting the inverter branches at a reduced speed, minimizing current peaks and avoiding transistor damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the inverter branches are short-circuited to stop the motor at nominal speed, then the motor stops quickly in the desired position, but significant current peaks are generated that can damage the transistors
Solution Approach 1:
The control unit performs preliminary speed reduction before the stopping action. When the stop command is received, the control unit first reduces the PWM duty cycle to decrease the motor speed from nominal speed to a lower speed (between 5-15% of nominal speed) within a predetermined time, and only then activates the short-circuiting of inverter branches to achieve the final position stop. This preliminary speed reduction prevents excessive current peaks while maintaining accurate positioning.
2Ease of manufacture
If position sensors are used to determine rotor position, then the motor can be controlled without rotating at high speed, but the cost of the electric motor increases significantly
Solution Approach 1:
The invention extracts and eliminates the position sensor component from the motor system. Instead of using Hall effect sensors or other position detection devices, the control unit determines the rotor position by measuring the electromotive forces (back-EMF) generated in the motor phases during operation. This sensorless approach using back-EMF measurement reduces the bill of materials cost while maintaining the capability to control motor stopping at the desired position.
3Measurement precision
If the motor rotates at high speed to generate detectable electromotive forces, then the rotor position can be determined without sensors, but the motor cannot stop accurately at low speeds
Solution Approach 1:
The invention applies dynamic control by adjusting the PWM duty cycle over time during the stopping process. The control unit dynamically reduces the duty cycle in stages: first from nominal speed to an intermediate speed, then to a very low speed (5-15% of nominal) just before the final position stop. This dynamic speed adjustment ensures that electromotive forces remain detectable for position determination throughout the stopping sequence, enabling accurate position sensing even as the motor approaches its final stop position at low speed.
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 the motor to stop accurately in the desired position without damaging transistors, reducing the need for oversized components and lowering overall costs by managing current peaks through controlled speed reduction.
Implementation Method 1
the position of the rotor is determined from a measurement of electromotive forces at the phases of the electric motor
Data Source
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AI summary
A method for controlling a brushless and sensorless, direct current electric motor (3) for motor vehicle equipment, wherein the electric motor (3) comprises a rotor and phases (A, B, C) powered by a pulse width modulation applied to a power inverter (1) of the electric motor (3), and wherein, beyond a minimum threshold (Smin) of the rotation speed of the rotor, the position of the rotor is determined from a measurement of electromotive forces at the phases (A, B, C) of the electric motor (3), said control method is characterised in that, in the event of a command (104) to stop the electric motor (3), the speed of rotation of the rotor is reduced (105) from a nominal speed to a predetermined rotation speed (V1) within a range between the minimum threshold (Smin) and 10% above said minimum threshold (Smin) by modifying the pulse width modulation, then the electric motor (3) is stopped (106) in a predetermined position by short-circuiting the branches (A, B, C) of the inverter (1) when the predetermined position is reached.