Electric Power Steering Torque Control for Stability and Response
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Solution Overview
Problem
The electric power steering system faces a trade-off between stability, disturbance suppression, and responsiveness, making it difficult to improve the steering feeling for the driver, as each element's adjustment is challenging due to their interrelated characteristics.
Innovation Solution
A control device with a reaction force controller and an assist controller that generates correction torque based on a nominal model, constraining the transfer function of the control target to match the nominal model's transfer function in specific frequency bands, thereby improving the steering feeling by compensating for modeling errors and disturbances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If conventional control methods are used to maintain stability, then system stability is preserved, but responsiveness and disturbance suppression deteriorate
Solution Approach 1:
The control device is divided into two independent controllers: a reaction force controller that ensures system stability by managing baseline torque, and an assist controller that enhances responsiveness and disturbance suppression through correction torque. This segmentation allows each controller to optimize its specific function without compromising the other, resolving the trade-off between stability and responsiveness.
Solution Approach 2:
A nominal model serves as an intermediary between the actual control target and the control inputs. The assist controller uses this nominal model to generate correction torque that compensates for modeling errors and disturbances. This intermediary approach allows the system to maintain stability through the reaction force controller while achieving improved responsiveness through the model-based correction, effectively bridging the gap between stability and performance requirements.
2Object-affected harmful factors
If conventional control methods are used to suppress disturbances, then disturbance suppression is improved, but steering responsiveness deteriorates
Solution Approach 1:
The control system separates disturbance suppression functionality into the assist controller that operates independently from the reaction force controller. The assist controller specifically targets disturbance compensation using the nominal model to generate correction torque, while the reaction force controller maintains the primary steering response characteristics. This segmentation enables disturbance suppression without degrading steering responsiveness.
Solution Approach 2:
The nominal model is used to predict and compensate for disturbances before they significantly affect the system. The assist controller continuously generates correction torque based on the nominal model's predictions of modeling errors and expected disturbances, allowing the system to proactively counteract disturbances rather than reactively responding to them, thus maintaining responsiveness while suppressing disturbances.
3Speed
If conventional control methods are used to improve responsiveness, then steering responsiveness is enhanced, but system stability deteriorates
Solution Approach 1:
The control device segments responsiveness enhancement from stability maintenance by assigning these functions to different controllers. The assist controller focuses on improving responsiveness through model-based correction torque, while the reaction force controller ensures system stability through proven control methods. This segmentation allows responsiveness to be enhanced without compromising stability.
Solution Approach 2:
The assist controller implements feedback using the nominal model to continuously monitor and correct deviations from desired behavior. By comparing actual system performance with the nominal model predictions, the assist controller generates correction torque that improves responsiveness while the reaction force controller maintains stability through traditional feedback mechanisms. This dual feedback approach enables responsiveness enhancement without stability loss.
Data Source
AI summary
A control device that controls a control target including at least a motor in a steering mechanism including an input shaft to which a steering wheel is connected, an output shaft connected to the input shaft via a torsion bar, and the motor connected to the output shaft. The control device includes a reaction force controller to generate an input torque input to the control target based on a torsion bar torque generated in the torsion bar and to control a reaction force transmitted from the steering wheel to the steering person, and an assist controller to generate a correction torque to correct the input torque based on an output of the control target and a nominal model. The assist controller is configured or programmed such that a transfer function of the control target is constrained by a transfer function of the nominal model in a frequency band.