Brushless Power Tool Motor Control With Single-Current-Sensor FOC
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Solution Overview
Problem
Conventional brushless DC motors require multiple sensors and shunts for precise phase control, leading to complex and inefficient motor control topologies that lack dynamic and steady-state performance.
Innovation Solution
Implementing sensored field-oriented control (sFOC) in power tools using a single current sensor and Hall effect sensors to eliminate the need for shunts on the voltage source inverter legs, allowing for independent control of motor speed and torque without additional current sensing components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional brushless DC motors use multiple sensors and shunts for phase control, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent extracts and eliminates the shunt resistors from the conventional motor control topology. By using the existing current sensor to measure bus current at specific commutation points, the system obtains phase current information without requiring shunts on each inverter leg, thereby reducing device complexity while maintaining measurement precision through selective sampling
Solution Approach 2:
The single current sensor serves multiple functions: it measures bus current for FOC control, provides over-current protection, and enables reconstruction of all three-phase currents through coordinate transformation. This multi-functionality eliminates the need for separate shunts and sensors for each phase, reducing overall device complexity while maintaining comprehensive measurement capability
2Measurement precision
If shunt resistors are added on VSI phases for current sensing, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The patent removes shunt resistors from the VSI phase legs and instead uses a single current sensor on the DC bus. By sampling the bus current at specific commutation intervals and using coordinate transformation algorithms, the system reconstructs phase current information without requiring physical shunts on each phase, thereby reducing sensing component quantity while maintaining current sensing accuracy
Solution Approach 2:
The patent introduces a mathematical intermediary (coordinate transformation algorithm) that converts the single bus current measurement into three-phase current information. This intermediary enables the system to obtain complete phase current data from a single sensor measurement, eliminating the need for multiple shunt resistors and reducing overall device complexity
3Ease of manufacture
If classic block commutation control is used in power tools, then ease of manufacture is improved, but dynamic performance deteriorates
Solution Approach 1:
The patent implements field-oriented control (FOC) which dynamically adjusts stator flux to remain orthogonal to rotor flux throughout operation. This dynamic control approach, as opposed to static block commutation, enables continuous optimization of motor performance across varying speed and load conditions, significantly improving dynamic response speed while maintaining ease of manufacture through standard FOC implementation
4Reliability
If additional current sensing components are added for over-current protection, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent makes the single current sensor multi-functional by using it for both FOC control and over-current protection. The same sensor that measures bus current for flux control also provides over-current protection by monitoring current magnitude, eliminating the need for separate protection sensors and reducing device complexity while maintaining or improving reliability through integrated monitoring
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
sFOC enhances dynamic and steady-state performance, reduces hardware complexity, and improves controllability by aligning stator flux orthogonally to rotor flux, eliminating the need for shunts and additional current sensing components.
Implementation Method 1
one or more Hall effect sensors disposed adjacent to the brushless motor, the one or more Hall effect sensors configured to generate output signals corresponding to a rotational position of the brushless motor
Implementation Method 2
a stator and a rotor configured to rotate with respect to the stator by a magnetic field generated in one or more phases of the stator
Data Source
AI summary
Power tools described herein include a housing, a battery pack receptacle, a brushless motor, one or more Hall effect sensors, a power switching circuit, a single current sensor, and an electronic controller. The battery pack receptacle is configured to receive a battery pack. A motor shaft is arranged to produce a rotational output to a drive mechanism. The one or more Hall effect sensors are configured to generate output signals corresponding to a rotational position of the brushless motor. The power switching circuit is configured to provide a supply of power from the battery pack to the brushless motor. The single current sensor is disposed between the battery pack and the brushless motor. The single current sensor is configured to measure the supply of power from the battery pack to the brushless motor. The electronic controller is configured to implement field-oriented control (“FOC”) of the brushless motor.