Brushless Motor Phase Current Correction for Steering Assist
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Solution Overview
Problem
Conventional electric power steering devices with three-phase brushless motors experience instability and poor steering feel when one phase fails, leading to insufficient assist during steering transitions and a feeling of the steering wheel being caught due to uneven torque distribution and external load effects.
Innovation Solution
The system includes a star-connected three-phase brushless motor with a steering torque detection section and a motor control section that adjusts phase current instruction values to maintain continuous operation using the remaining phases, with an acceleration region correction mechanism to prevent braking torque and enhance steering feel by intentionally accelerating the motor in the steering direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If the motor torque is increased just before reaching the angular area where constant torque cannot be obtained, then the motor rotation is accelerated in the positive direction, but the motor torque is reduced immediately after passing over the angular area, causing the motor to easily stop in the angular area where constant torque is not obtained
Solution Approach 1:
The system performs preliminary acceleration by increasing motor torque before the motor reaches the angular area where constant torque cannot be obtained. This preliminary action builds up rotational momentum that carries the motor through the unstable angular area, preventing the motor from stopping in regions where constant torque is not available.
Solution Approach 2:
The motor control section dynamically adjusts the torque characteristics based on the motor's rotational state and position. By changing torque characteristics according to whether the motor is accelerating or decelerating, the system maintains reliable continuous rotation while passing through angular areas where constant torque cannot be obtained.
2Speed
If the motor torque is decreased just before reaching the angular area where constant torque is not obtained, then the motor rotation is accelerated in the negative direction, but the motor torque is increased immediately after passing over the angular area, creating a braking torque that causes the motor to stop in the angular area where constant torque cannot be obtained
Solution Approach 1:
The system performs preliminary deceleration by decreasing motor torque before the motor reaches the angular area where constant torque cannot be obtained. This preliminary action reduces the opposing torque effect that would otherwise occur after passing through the unstable angular area, preventing braking torque from causing the motor to stop.
3Power
If the electric current limit is applied to the motor, then the motor torque cannot be maintained constant in the angular area where the electric current limit is reached, but the driver feels as if the steering wheel were caught by something, deteriorating the steering feeling
Solution Approach 1:
The system dynamically changes torque characteristics based on the motor's operational state. When the motor is accelerating, the system allows torque to increase even when current limits are reached. When the motor is decelerating, the system reduces torque to avoid braking effects. This dynamic adaptation maintains smooth steering feeling while respecting current limits.
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
This approach enables continuous driving of the motor using the remaining phases, efficiently passing over unstable torque regions and improving steering feel by preventing braking torque and maintaining stable motor torque characteristics.
Implementation Method 1
a three-phase brushless motor 12 having respective phase coils 12L to 12W connected in a star connection for applying a steering assist force to a steering system
Data Source
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AI summary
There is provided an electric power steering device which, when an abnormality occurs in one phase of a three-phase brushless motor, permits continuous driving of the motor using the remaining two phases without deteriorating the steering feeling. When a power distribution abnormality of a drive system of one phase of respective phase coils has been detected by an abnormality detection section 27, an abnormal-time phase current instruction value for using the remaining two phase coils is calculated based on a steering assist electric current instruction value Iref, and an electric motor 12 is driven based on that abnormal-time phase current instruction value. At that time, when an electrical angle θe falls within an acceleration region, the abnormal-time phase current instruction value is increased or decreased for correction, so as for the rotation of the three-phase brushless motor 12 to be accelerated in a steering direction.