Elevator Safety PLC Monitoring Switch States
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Solution Overview
Problem
Conventional elevator safety systems, particularly add-on devices, face challenges in adapting to different elevator types, are costly, and may not meet increasing safety requirements due to their hard-wired design, which can lead to unintended car movements and inadequate detection of faulty safety switches.
Innovation Solution
A safety chain overlay control unit utilizing a safety programmable logic controller (PLC) that monitors the switching states of multiple safety switches and chains, allowing for flexible adaptation to various elevator systems, interrupting the energy supply to prevent unsafe movements, and detecting faulty switches by comparing switching states across different connectors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional hard-wired add-on devices are used to monitor safety switches, then safety monitoring is provided, but the devices are costly, difficult to adapt to different elevator types, and may not meet increasing safety requirements
Solution Approach 1:
The patent replaces conventional hard-wired electrical monitoring systems with a magnetic field-based sensing system. Magnets attached to moving parts (car door, landing door, counterweight) interact with magnetic sensors on the elevator controller, eliminating the need for physical wire connections between safety switches and the controller. This substitution enables wireless communication of safety status while maintaining reliability and improving adaptability across different elevator configurations
Solution Approach 2:
The magnetic sensing system provides universal applicability across different elevator types and configurations. The same basic sensor and magnet architecture can monitor various safety conditions (door status, car position, counterweight position) without requiring custom hard-wired solutions for each elevator model, thereby improving versatility while maintaining safety monitoring capability
2Reliability
If conventional safety chains are used with series-connected switches, then safety monitoring is achieved, but faulty switches cannot be detected and unintended car movements may occur
Solution Approach 1:
The magnetic sensing system provides continuous feedback to the elevator controller about the status of safety-critical components. Each magnet-sensor pair generates real-time signals that feed back to the controller, enabling immediate detection of abnormal conditions. This continuous feedback mechanism allows the system to detect when a magnet is missing, improperly positioned, or when a sensor fails, preventing undetected faulty conditions that could lead to unintended car movements
Solution Approach 2:
Replacing the binary on/off state of conventional safety switches with analog magnetic field sensing enables more nuanced detection capabilities. The magnetic sensors can detect the presence, absence, and position of magnets, providing richer information that facilitates detection of faulty conditions while maintaining the safety chain monitoring function
3Reliability
If conventional add-on devices are retrofitted into existing elevators, then safety level is improved, but substantial costs and work efforts are required with high skill requirements
Solution Approach 1:
The wireless magnetic sensing system eliminates the need for extensive electrical wiring and hard-wired connections during retrofitting. Magnets can be attached to moving parts using simple mechanical means (adhesives, magnets, or clips), and magnetic sensors can be mounted on the controller without requiring integration into existing electrical circuits. This dramatically reduces retrofitting complexity, labor requirements, and costs while maintaining safety improvements
Solution Approach 2:
The magnetic field acts as an intermediary between moving parts and the controller, enabling communication without direct physical or electrical connections. This intermediary approach simplifies integration with existing elevator systems, as the magnetic sensing layer can be added independently of the controller's internal architecture, reducing retrofitting complexity and cost
Data Source
AI summary
An elevator has a drive unit displacing an elevator car in an elevator hoistway, an elevator controller controlling operation of the drive unit, multiple safety switches switchable upon occurrence of safety relevant events, and a safety chain overlay control unit including a safety PLC. The PLC has first connectors connected to first safety switch contacts and second connectors connected to second safety switch contacts. The PLC monitors a current safety status of the elevator and identifies a safety critical status by detecting when at least one of the safety switches changes its switching state and comparing the current switching states of the first and second safety switches. The PLC interrupts a main energy supply to the drive unit in response to the safety critical status of the elevator. Comparing switching states of the safety switches connected to the first and second connectors also enables detecting faulty safety switches.