Door Ligature Detection with Angle-Adjusted Load Thresholds
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Solution Overview
Problem
Existing ligature detection systems on doors are limited in their sensing ability, unable to detect ligatures at points other than where a switch is directly located, and are prone to false negatives when the door is in motion, as recalibration during motion can miss ligature detection.
Innovation Solution
A controller system that uses load and angle sensors to determine a baseline load value as a function of door angle, adjusting thresholds dynamically to detect ligatures accurately regardless of door motion, with optional integration of accelerometers for power conservation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If recalibration is performed during door motion to maintain detection accuracy, then baseline load values can be updated, but ligature detection reliability deteriorates due to motion-induced load variance
Solution Approach 1:
The system dynamically adjusts the door state determination criteria based on motion detection. When motion is detected, the system transitions from a static closed-door assumption to a dynamic state recognition mode, where baseline load values are not updated even during apparent stationary periods, preventing motion-induced calibration errors
Solution Approach 2:
The system uses feedback from motion state detection to control the baseline update process. By continuously monitoring door motion and using this feedback to determine when calibration should occur, the system ensures baseline values are only updated when the door is truly stationary and closed, eliminating the contradiction between updating frequency and detection reliability
2Measurement precision
If the door is kept closed to ensure reliable ligature detection, then detection accuracy improves, but door usability deteriorates due to frequent false alarms during normal operation
Solution Approach 1:
The detection threshold is dynamically adjusted based on real-time door state assessment. When the door is open or moving, the system recognizes this state and suppresses alarm generation even if load anomalies are detected, allowing normal door operation without false alarms while maintaining high detection accuracy when the door is closed
Solution Approach 2:
The system changes the operational parameters of the detection algorithm based on door state. By modifying the alarm trigger conditions according to whether the door is open, closed, moving, or stationary, the system achieves both high detection accuracy and ease of operation without requiring the door to remain closed
3Device complexity
If a fixed load threshold is used for ligature detection, then system complexity is reduced, but detection accuracy deteriorates due to door angle variations
Solution Approach 1:
The load threshold transitions from a fixed value to a dynamic value that automatically adjusts with door angle. The system calculates the expected load component due to door angle and uses this to dynamically set the detection threshold, maintaining high detection accuracy without adding complex mechanical adjustment mechanisms
Solution Approach 2:
Instead of physically adjusting the threshold mechanism to match door angle, the system creates a computational model (copy) of the expected load variation with angle. This virtual model is used to set the detection threshold, achieving the same effect as physical adjustment without the complexity
Data Source
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AI summary
There is provided a controller, a system, computer software and a method for detecting a ligature on a door mounted within a door frame on mounting means. A load applied by the door to the mounting means is detected by a load sensor. An angle of the door relative to the door frame is detected by an angle sensor. Baseline load data is obtained, the baseline load data being indicative of an expected load applied by the door to the mounting means as a function of the angle of the door. The method comprises determining a threshold load value indicative of a ligature on the door for the measured angle in dependence on the baseline load data and the detected angle of the door; determining whether the detected load exceeds the threshold load value for the detected angle; and outputting an indication of a detected ligature event if the detected load exceeds the threshold load value.