Motor Control Safety Switch Protection via FET Current Testing
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Solution Overview
Problem
Existing electromechanical motor vehicle steering systems face challenges in protecting safety switches from damage due to excessive current flow during electrical faults, which can lead to the destruction of semiconductor switches.
Innovation Solution
A method is introduced that involves activating and deactivating FETs in a controlled test switching pattern to measure current flow, allowing safety switches to be opened only when the current is below a predetermined threshold, thus preventing damage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If safety switches are opened to interrupt phase lines in the event of electrical faults, then steering assistance loss is prevented, but semiconductor switches can be destroyed due to excessive current flow
Solution Approach 1:
The patent applies preliminary action by determining the optimal switching-off time before opening the safety switches. The control unit calculates the time required for current to decay to a safe level based on stored electrical parameters (inductance, resistance, capacitance) of the motor and circuit, ensuring switches are opened only when current is below damaging thresholds.
Solution Approach 2:
The patent implements beforehand cushioning by storing electrical parameters (inductance L, resistance R, capacitance C) in advance and using them to calculate safe switching-off times. This pre-computed timing information acts as a cushion against the harmful effect of excessive current, allowing safe interruption of phase lines without damaging the semiconductor switches.
2Reliability
If phase lines are interrupted by opening safety switches, then braking torque generation is prevented, but excessive voltage can destroy the semiconductor switches
Solution Approach 1:
The control unit performs preliminary calculations using stored electrical parameters to determine the precise moment when voltage and current have decayed to safe levels. This advance preparation ensures that safety switches are opened only when voltage stress is below the maximum blocking voltage threshold of the semiconductor switches.
Solution Approach 2:
The patent uses pre-stored electrical parameters (inductance, resistance, capacitance) to compute timing information that cushions against voltage spikes. By opening switches at the calculated optimal time, the system prevents excessive voltage from destroying the semiconductor switches while still achieving fault protection.
3Measurement precision
If current measurement and evaluation are performed for each FET, then fault detection precision is improved, but processing time increases
Solution Approach 1:
The patent implements periodic action by executing test switching patterns at regular intervals rather than continuously monitoring every FET at all times. The control unit periodically activates FETs in sequence, measures current, and evaluates fault conditions, reducing processing load while maintaining adequate fault detection capability for safety-critical applications.
Solution Approach 2:
The patent applies partial action by focusing measurements only on the currently activated FET during each test cycle, rather than simultaneously measuring all FETs. This sequential approach reduces the total measurement burden while still achieving comprehensive fault detection through repeated periodic testing of each component.
Data Source
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AI summary
The invention relates to a method for protecting safety switches (Q7, Q8, Q9) of a control unit (11) of an electric motor, wherein the control unit (11) has a microcontroller (12) and a gate driver (13), which control a driver circuit (14) having 2n FETs (Q1, Q2, Q3, Q4, Q5, Q6), n being the number of phase windings of the electric motor, and the FETs being arranged in half bridges, and the safety switch being arranged in the middle tap of any half bridge, between the driver circuit (14) and the phase winding of the electric motor, wherein the safety switches are FETs, which are designed to break the electrical connection between the driver circuit (14) and any phase winding in the event of fault, and wherein the method, in the event of an electrical malfunction in the driver circuit (14), comprises the following steps: (a) activating one of the 2n FETs (Q1, Q2, Q3, Q4, Q5, Q6) of the driver circuit (14), (b) measuring the current flow in a supply line (19-) of the driver circuit (14) and detecting a current value, (c) deactivating the activated FET (Q1, Q2, Q3, Q4, Q5, Q6) of the driver circuit (14), (d) successively carrying out a test switching pattern in accordance with steps (a) to (c) for the remaining 2n-1 FETs (Q1, Q2, Q3, Q4, Q5, Q6), (e) evaluating the detected current values for failure to reach a specified threshold value, the highest detected current value being ignored, (f) in the event that the threshold value is not reached, breaking the electrical connection between the driver circuit (14) and at least one of the phase windings (u, v, w) by switching the safety switches (Q7, Q8, Q9). The invention further relates to a electromechanical motor vehicle steering system having a control unit, which is designed to carry out a method of this kind.