EPS Motor Current Rate Limiting Strategy
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Solution Overview
Problem
Existing electric power assisted steering systems face challenges in managing sudden changes in motor current demand, which can lead to battery depletion or voltage drops when the alternator cannot immediately meet the increased demand, causing issues like dimming of vehicle lights.
Innovation Solution
A control strategy that estimates the rate of change of current drawn from the electrical supply and limits it when exceeding a threshold, using pulse width modulation (PWM) drive signals and a PI controller to adjust motor current demand signals, ensuring the battery current gradient does not exceed a predefined limit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If the motor current demand is increased to provide high assistance torque, then the steering assistance performance is improved, but the battery current draw rate increases causing voltage drops and light dimming
Solution Approach 1:
The control system performs preliminary estimation of battery current draw rate using duty ratio and current demand signals before actually applying the current. This allows the system to predict and prevent excessive current draw that would cause voltage drops, while still enabling high assistance torque when needed by managing the rate of current increase.
Solution Approach 2:
The system uses feedback from the estimated battery current draw rate to dynamically adjust the motor current demand. When the estimated draw rate exceeds the threshold, the control system modifies the current demand signal to reduce the rate of increase, thereby maintaining electrical system stability while still providing steering assistance.
2Productivity
If the alternator current output is limited by its response time, then the electrical supply capacity is constrained, but the battery can be depleted during sudden high demand transitions
Solution Approach 1:
The system performs preliminary estimation of the battery current draw rate using duty ratio and current demand signals before actually applying the current. This allows the system to predict when the alternator cannot keep up with demand and prevent excessive battery drain by proactively limiting the rate of current increase during transition periods.
Solution Approach 2:
The control system dynamically adjusts the motor current demand based on real-time estimation of battery current draw rate. During sudden transitions from low to high assistance demand, the system modulates the current ramp-up rate to match what the alternator can supply, optimizing the balance between meeting steering assistance needs and preventing battery depletion.
3Power
If the PWM duty ratio is increased to meet high current demand, then the motor torque output is improved, but the battery voltage drops causing system instability
Solution Approach 1:
The control system performs preliminary estimation of battery current draw rate using duty ratio and current demand signals before actually applying the current. This allows the system to predict when high PWM duty ratios would cause excessive current draw and voltage drops, enabling proactive adjustment of the duty ratio ramp-up rate to maintain voltage stability.
Solution Approach 2:
The system uses feedback from the estimated battery current draw rate to dynamically modulate the PWM duty ratio. When the estimated draw rate approaches the threshold, the control system reduces the rate of duty ratio increase, thereby maintaining battery voltage stability while still enabling high motor power output when needed for steering assistance.
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 strategy effectively prevents battery depletion and voltage drops by managing sudden changes in current demand, maintaining stable vehicle operations and preventing excessive battery drain.
Implementation Method 1
each phase being driven with a cyclic PWM drive signal having a first state and a second state and a duty ratio indicative of the ratio of the time spent in each state in a cycle
Implementation Method 2
an electric motor applies an assistance torque to a part of a steering system
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
A control strategy for an electric motor of an electric power assisted steering system of the kind in which a control means produces motor current demand signals that are fed to a motor drive means, the demand signals being dependent on the amount of assistance torque demanded from the motor, the motor drive means being arranged to cause currents to flow in each phase of the motor as required to meet the demanded assistance torque, the control strategy comprising limiting the rate of change of current that is drawn from the electrical supply of the vehicle by the motor in the event that it would otherwise exceed a threshold rate of change.