Electric Power Steering Capacitor Discharge via Motor Relay
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Solution Overview
Problem
Conventional electric power steering apparatuses face increased consumption current and cost due to the need for dedicated circuits to discharge current ripples, with either large resistors or additional switches, which also lead to unnecessary motor rotation and steering assist power application.
Innovation Solution
The electric power steering apparatus incorporates a brushless motor with a motor driving circuit using switching elements and a drive control portion that directs electric charge from a capacitor into a motor current path or exciting coil for discharge after power supply switch deactivation, eliminating the need for dedicated discharge circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a dedicated discharge circuit with resistor or switch is provided, then the electric charge accumulated in the capacitor can be discharged, but the device complexity and cost increase
Solution Approach 1:
The patent merges the discharge function with the existing motor driving circuit by keeping the motor relay turned on after power supply relay is turned off, allowing the capacitor to discharge through the motor winding. This eliminates the need for a separate discharge circuit while utilizing existing circuit components.
Solution Approach 2:
The motor relay and motor driving circuit are designed to serve dual functions: driving the motor during operation and discharging the capacitor after power supply is turned off. The motor winding acts as both the motor coil and the discharge path, providing multi-functionality to existing components.
2Reliability
If a large resistor is used for discharge, then the discharge function is achieved, but the consumption current during operation increases
Solution Approach 1:
The patent uses the motor winding as a temporary discharge path only when needed (after power supply is turned off), rather than using a resistor that would continuously consume current during operation. The motor winding serves as a disposable discharge path that is activated only when required.
3Use of energy by moving object
If a switch is added to the discharge circuit, then the consumption current during operation is reduced, but the device complexity and cost increase
Solution Approach 1:
The patent combines the discharge control function with the existing power supply relay control logic. The motor relay is kept on after the power supply relay is turned off, using the existing relay control mechanism to achieve discharge without adding extra switches.
4Device complexity
If the motor relay is turned off immediately after power supply relay, then the device complexity is reduced, but the capacitor cannot be discharged properly
Solution Approach 1:
The patent performs preliminary action by keeping the motor relay turned on in advance after the power supply relay is turned off, creating a discharge path before the discharge is actually needed. This timing arrangement ensures the capacitor can be discharged properly without adding complex control circuits.
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 reduces the size and cost of the apparatus while preventing unnecessary motor rotation and steering assist power application, achieving efficient discharge of current ripples without additional circuits.
Implementation Method 1
a capacitor 92 is provided between two power supply lines. The capacitor 92 accumulates electric charge and discharges the electric charge thus accumulated when the current flowing into the motor driving circuit 93 from the power supply is insufficient, whereby the current ripple can be absorbed
Implementation Method 2
The six MOS-FETs contained in the motor driving circuit 93 are controlled by a PWM (Pulse Width Modulation) signal output from a drive control portion (not shown), and the motor driving circuit 93 outputs three-phase driving currents which phases differ by 2n/3 from one another and each changes in a sine wave manner
Implementation Method 3
a three-phase brushless motor 90 is employed in order to generate a steering assist power to be applied to the steering mechanism of a vehicle
Data Source
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AI summary
When stopping operation of an electric power steering apparatus, a motor relay is kept in an on-state even after a power supply relay is in an off-state. A drive control portion sets a target value of a d-axis current to a value other than zero and sets a target value of a q-axis current to zero, and performs a processing same as that at the time of rotating a motor. MOS-FETs contained in a motor driving circuit are controlled such that each of driving currents of the two phases or more is not zero and the brushless motor does not rotate even supplied with these driving currents. Electric charge accumulated in a capacitor is discharged via the MOS-FETs each in an on-state, a motor relay and the windings of the brushless motor. The electric charge accumulated in the capacitor may be discharged via the excitation coil of the motor relay.