Motor Average Current Determination via PWM Duty Ratio
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Solution Overview
Problem
Existing methods for determining the average current drawn by electric motors, particularly in automotive applications like EPAS systems, face challenges due to large fluctuations in drive current, making direct measurement difficult and costly, especially for systems using single current sense series with motor phases.
Innovation Solution
A method involving cyclic pulse width modulated (PWM) drive signals with specific duty ratios is used to determine the average current by combining current measurements from each phase with their respective duty ratios, allowing for accurate average current calculation without additional hardware, and incorporating dead-time compensation for increased accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single current sensor is used to measure motor phase currents, then cost is reduced, but the ability to directly measure battery current is lost due to large fluctuations in drive current
Solution Approach 1:
The patent applies preliminary action by measuring the current at a specific instant before the PWM switching occurs (during the dead time or just before the switch turns on). This timing allows capturing the current value before the large fluctuations begin, enabling accurate average current calculation without needing to measure during the fluctuating PWM period
Solution Approach 2:
The patent uses the duty ratio as an intermediary parameter. By measuring the current at a specific instant and combining it with the known duty ratio, the system can calculate the average battery current without directly measuring the fluctuating drive current. The duty ratio mediates between the instantaneous current measurement and the average current determination
2Measurement precision
If external analogue hardware filtering is used to obtain average current from single current sensor, then average current measurement is achieved, but measurement accuracy deteriorates due to switching dynamics and noise
Solution Approach 1:
The patent replaces the mechanical/analog filtering approach with a digital calculation method. Instead of using analog hardware filters that struggle with switching dynamics and noise, the system uses digital processing to combine the instantaneous current measurement with the duty ratio to calculate the average current, achieving better accuracy without the limitations of analog filtering
Solution Approach 2:
By measuring current at a predetermined instant before PWM switching, the system avoids the switching dynamics and noise that plague analog filtering approaches. This timing strategy captures a clean current value that can be accurately processed to determine average current
3Measurement precision
If modelling approach is used to predict motor current from power consumed, then current information can be obtained, but system complexity increases due to multiple required parameters
Solution Approach 1:
The patent extracts only the essential elements needed for current determination: a single current measurement at a specific instant and the duty ratio. By eliminating the need for complex modeling parameters such as voltage and current phases, resistances, inductances, rotor position, and battery voltage, the system achieves current prediction with minimal complexity
Solution Approach 2:
The patent changes the approach from using multiple varying parameters in a complex model to using a single instantaneous current measurement combined with the duty ratio. This parameter simplification transforms a complex multi-parameter modeling problem into a simple calculation based on readily available data
Data Source
AI summary
A method of determining the average current drawn by an electric motor, the motor comprising a plurality of phases, the method comprising: driving each phase of the motor with cyclic pulse width modulated drive signals having a first state and a second state, and a duty ratio indicative of the ratio of time spent in the first to second states within a cycle, determining the current flowing through each phase at an instant in the cycle; and determining the average current drawn by the motor over at least one cycle by combining the currents flowing through each phase with the duty ratios for each phase. The method may be embodied in a combination of a motor and a drive circuit for the motor, such as can be used in an Electric Power Assisted Steering system.