Digital Isolator STO Control for Selective High-Side PWM Isolation
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Solution Overview
Problem
Elevator systems face challenges in achieving safe torque off (STO) operation, particularly in ensuring compliance with safety integrity levels (SIL) without complex monitoring and testing of actuator status, especially in traditional schemes that block pulse width modulation (PWM) signals.
Innovation Solution
A digital isolator is used to selectively block PWM signals between a digital signal processor and an inverter, ensuring that the motor does not rotate during STO operations, simplifying monitoring and testing for SIL compliance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional PWM signal blocking schemes are used for safe torque off operation, then motor rotation can be prevented, but monitoring and testing complexity increases significantly
Solution Approach 1:
A digital isolator is introduced as an intermediary component between the control system and inverter. This isolator actively monitors PWM signal transmission and can selectively block high-side PWM signals when STO conditions are detected, providing both safety function and simplified monitoring capability in a single integrated component
2Reliability
If complex monitoring and testing schemes are implemented for SIL compliance, then safety integrity level is ensured, but system complexity and cost increase
Solution Approach 1:
The digital isolator performs multiple functions simultaneously: it provides galvanic isolation between control and power circuits, actively monitors PWM signal integrity, enables selective blocking of high-side signals for STO operation, and provides simplified compliance monitoring. This multi-functionality achieves SIL 3 compliance without requiring separate complex monitoring and testing subsystems
3Reliability
If all PWM signals are blocked for safe torque off, then motor rotation is prevented, but control flexibility is reduced
Solution Approach 1:
Instead of uniformly blocking all PWM signals, the digital isolator selectively blocks only the high-side PWM signals when STO conditions are detected, while allowing low-side PWM signals to pass through. This localized blocking approach prevents motor rotation (since high-side switching is essential for motor operation) while maintaining the ability to differentiate between signal types and preserve low-side control functionality
Data Source
AI summary
A control system for safe torque off (STO) operation is provided, including a motor, a digital signal processor (DSP) which outputs three-phase low side pulse width modulation (PWM) signals and three-phase high side PWM signals, an inverter receptive of the three-phase low side PWM signals and the three-phase high side PWM signals, the inverter being configured to combine the three-phase low side PWM signals and the three-phase high side PWM signals into a three-phase high-voltage output to control the motor, and a digital isolator electrically interposed between the DSP and the inverter and configured to selectively block at least a portion of the three-phase high side PWM signals from being received by the inverter.