Three-Phase Drive Torque Limiting via PWM Current Reconstruction
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Solution Overview
Problem
Existing motor-driven shafts in automated machinery and robots lack effective methods to safely limit torque, which is essential for preventing damage to personnel and equipment while meeting stringent safety standards like IEC62061 and ISO 13849.
Innovation Solution
A torque-limiting safety circuit for AC permanent magnet motors is implemented, utilizing a three-phase inverter bridge, current sensors, and a drive control circuit to create a two-channel measurement system for safely limiting torque. This system includes a current vector re-constructor circuit and safety processors that compute motor torque and shut down the drive when limit values are exceeded.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If current sensors are placed in series with each low-side IGBT to measure phase currents, then cost is reduced and level translation is eliminated, but current measurements are only valid when all three low-side IGBTs are ON simultaneously, limiting continuous current monitoring capability
Solution Approach 1:
The patent dynamically modifies the PWM switching pattern to ensure that all three low-side IGBTs are ON simultaneously for a minimum dwell time during each PWM cycle. This dynamic adjustment of switching时序 allows the current sensors to capture valid current measurements periodically, which are then held and interpolated to provide continuous current monitoring capability for both torque control and safety functions.
2Measurement precision
If the minimum dwell time for PWM switching states is increased to allow valid current measurements, then current measurement accuracy improves, but the response speed of the drive system decreases
Solution Approach 1:
The patent applies partial action by inserting minimum dwell times only when required for valid current measurements, rather than continuously extending switching durations. The dwell time is applied selectively during PWM cycles when current measurement is needed, allowing the system to maintain high response speed for non-critical operations while ensuring accurate current measurements are obtained periodically for torque calculation and safety monitoring.
3Reliability
If a two-channel safety system with Hardware Fault Tolerance is implemented to meet SIL1 safety standards, then safety integrity is improved, but device complexity increases
Solution Approach 1:
The patent implements multi-functionality by using the same current measurement infrastructure and processing logic for both torque control and safety monitoring functions. The modified PWM switching pattern serves dual purposes: enabling accurate current measurements for torque calculation and providing valid measurement data for the two-channel safety system. This universal approach allows the safety system to meet SIL1 requirements without proportionally increasing complexity, as the measurement and processing hardware serves multiple functions.
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
The solution effectively limits torque in a fail-safe manner, ensuring the safety of personnel and equipment while meeting the requirements of stringent safety standards, thereby enhancing the reliability and safety of motor-driven systems.
Implementation Method 1
The drive control circuit controls the six pulse-width modulated gate drive signals for the three-phase inverter bridge
Implementation Method 2
In a permanent magnet synchronous motor, torque is produced by the product of the vector of current and the vector of flux
Data Source
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Figure 5~7
AI summary
A torque-limiting safety circuit servo drive for AC permanent magnet motors (804) including a three-phase inverter bridge (801), a first current sensor in series with a first motor phase (807), a second current sensor in series with a second motor phase (808), a third current sensor in series with the DC bus (806), and a drive control circuit that controls the six pulse-width modulated gate drive signals for the three-phase inverter bridge (844). The drive circuit has first (845) and second safety channel STO (847) inputs whereby either channel can shut down the three-phase inverter bridge, emits a signal set to represent the switching state of the three-phase inverter bridge, and modifies the switching pattern of the PWM (851) to ensure the dwell times of PWM is sufficiently long to allow a valid measurement of phase current using the bus current sensor. First and second safety processors (842, 843) controls the first and second safety channel STO inputs, respectively.