Self-Positioning Rotor Servo Actuator for Field Weakening
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Solution Overview
Problem
Permanent magnet motors are limited in speed due to back electromotive force (bemf) and require high currents for high forces, leading to inefficient and costly motor designs that are not optimized for applications requiring high forces at low speeds and high speeds at low forces, with existing solutions for field weakening being complex and costly.
Innovation Solution
A linear actuator with a field weakening technique that allows the rotor to automatically shift within the rotor-stator assembly, enabling a wider range of torque-speed operation by adjusting the magnetic flux, reducing current requirements and package size, and incorporating a low-cost force monitoring system using a secondary spring and linear position sensor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If permanent magnet motors are designed for high forces, then sufficient force output is achieved, but speed is limited by back electromotive force (bemf)
Solution Approach 1:
The rotor assembly is made axially movable relative to the stator assembly, transitioning from a fixed position to a dynamic position that changes based on operational conditions. This dynamic adjustment allows the motor to optimize the air gap between rotor and stator, enabling field weakening at high speeds while maintaining strong magnetic coupling at low speeds for high force output.
Solution Approach 2:
The invention changes the physical parameter of the air gap distance between rotor and stator by allowing axial movement of the rotor assembly. By varying this parameter, the motor can adjust its magnetic flux density, achieving field weakening to overcome bemf limitations at high speeds while maintaining high torque capability at low speeds.
2Force
If high currents are used to deliver sufficient force, then force requirements are met, but motor and drive size and cost increase
Solution Approach 1:
The dynamic rotor positioning enables the motor to maintain high torque density across a wider speed range. By adjusting the air gap, the motor optimizes its torque constant, reducing the current required to produce the same force at different speeds, thereby reducing the size and cost of motor windings and drive electronics.
Solution Approach 2:
By changing the air gap parameter through rotor displacement, the motor optimizes its electrical characteristics. This allows the motor to operate more efficiently across the full speed range, reducing peak current requirements and allowing for smaller, less costly motor components and drive systems.
3Speed
If field weakening is achieved through conventional methods, then high speed capability is improved, but device complexity and cost increase
Solution Approach 1:
The invention uses the existing axial motion capability of the screw mechanism to dynamically position the rotor, repurposing the linear actuation system already present in the motor. This eliminates the need for separate field weakening mechanisms, reducing device complexity while achieving high speed capability through natural field weakening as the rotor moves axially.
Solution Approach 2:
The screw mechanism serves dual functions: providing the linear output motion required by the application and simultaneously adjusting the rotor position for field weakening. This multi-functionality eliminates the need for separate components dedicated solely to field weakening, reducing overall device complexity.
4Device complexity
If the rotor is fixed relative to the stator, then motor structure is simple, but speed is limited by back electromotive force (bemf)
Solution Approach 1:
The rotor assembly is designed to move axially relative to the stator, transitioning from a fixed static structure to a dynamic structure that can adjust its position. This simple dynamic adjustment changes the air gap and magnetic flux distribution, enabling field weakening that overcomes bemf limitations and extends the motor's speed capability without complicating the overall structure.
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 enables operating speeds twice that of current designs, reduces current requirements by over 50%, and provides precise force feedback without the need for expensive sensors, resulting in smaller, less costly, and more efficient motor components.
Implementation Method 1
A linear actuator with a field weakening technique that allows the rotor to automatically shift within the rotor-stator assembly, enabling a wider range of torque-speed operation by adjusting the magnetic flux
Implementation Method 2
The device will utilize a traditional screw mechanism for converting rotary to linear motion
Implementation Method 3
Permanent magnet motors are typically limited in speed by the back electro-motive force (bemf) generated with respect to the available driving voltage
Data Source
AI summary
A linear actuator is provided with an integral permanent magnet, brushless motor capable of axial, self adjustment of the rotor position within the stator for field weakening. Axial displacement of the rotor within the stator allows for significantly improved speed performance from aligned rotor/stator components during light axial loading. Upon contact with a load, the screw and nut, as an integral part of the motor rotor, self align to fully engage within the stator for generation of high forces with minimal current. This axial shift can further be used as a high precision, low cost measure of the force applied for quality assurance purposes.