Dual-Motor Steering Drive Phasing for Cogging Torque Cancellation
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Solution Overview
Problem
Conventional electric motors in steer-by-wire systems suffer from torque ripple and cogging torque issues, particularly at low speeds, which can lead to undesirable fluctuations in output torque and affect the steering mechanism's performance.
Innovation Solution
A dual motor drive assembly is designed with identical permanent magnet motors having the same number of poles and stators, where the relative phasing of the motors is fixed to cancel out the cogging torque and torque ripple by offsetting the stators or rotors by specific angles, such as 10 degrees for a 9:6 motor topology, to achieve optimal cancellation of harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single permanent magnet motor is used in the steer-by-wire system, then the system structure is simple and cost-effective, but torque ripple and cogging torque cause undesirable fluctuations in output torque especially at low speeds
Solution Approach 1:
The patent combines two permanent magnet motors in parallel to form a dual-motor drive assembly. Both motors drive the same steering shaft through gear mechanisms, allowing their torque outputs to combine while their cogging torque patterns cancel each other out due to being 180 degrees out of phase, thereby reducing overall torque ripple and cogging torque in the steering system
Solution Approach 2:
The patent introduces asymmetry in the phase alignment of the two motors, specifically positioning them 180 degrees out of phase relative to each other. This asymmetric phase relationship causes the cogging torque harmonics from each motor to oppose and cancel each other, transforming the harmful periodic torque fluctuations into a smoother combined output
2Reliability
If two motors are used to provide redundancy and reduce gear rattle, then system reliability improves, but the device complexity and size increase
Solution Approach 1:
The dual-motor assembly serves multiple functions simultaneously: it provides redundancy for fail-safe operation, reduces gear rattle through opposing torques, and minimizes cogging torque through phase cancellation. This multi-functionality justifies the increased complexity by delivering multiple benefits from a single integrated design
Solution Approach 2:
The patent divides the steering drive function into two separate motor-gear units that operate in parallel. Each motor-gear assembly is a self-contained module that can independently drive the steering shaft, providing modular redundancy and facilitating easier maintenance or replacement of individual modules
3Speed
If motors are operated at low speeds in steer-by-wire systems, then precise steering control is achieved, but cogging torque becomes more observable and causes torque fluctuations
Solution Approach 1:
The patent uses the second motor as a counterbalancing element whose cogging torque is 180 degrees out of phase with the first motor. At low speeds where cogging torque is most noticeable, the opposing phase relationship causes the cogging torque from one motor to counteract and cancel the cogging torque from the other motor, providing smooth low-speed steering control
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 dual motor drive assembly significantly reduces overall cogging torque and torque ripple, improving the steering mechanism's stability and reducing the size and cost of the assembly by optimizing the phase offset between the motors.
Implementation Method 1
first and second motors (10, 11), each having an output driving a respective output gear (6, 7)... each motor comprises a permanent magnet motor
Implementation Method 2
a rotor including magnetically hard material which acts as a set of permanent magnets. The permanent magnets may therefore form part of the rotor, and the stator comprises magnetically soft material surrounded by coils of wire
Data Source
AI summary
A dual motor drive assembly comprises a housing, a shaft rotatably mounted with respect to the housing, a first gear connected to and configured to rotate with the shaft, and first and second motors, each having an output driving a respective output gear, the output gears being engaged with the first gear, in which each motor comprises a permanent magnet motor that has the same number of poles and stators such that each motor produces the same pattern of cogging torque over a complete mechanical revolution of the motor.