Fixed-Point Control Loop Limiting for Aircraft Inceptor Motors
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Solution Overview
Problem
Fixed point control loops in active inceptors for aircraft control systems face issues with overflow conditions due to limited representable numbers, leading to potential loss of control and catastrophic failures, as they struggle to prioritize and limit contributions from different control paths effectively.
Innovation Solution
A control system that uses multiple circuits and calculator circuits to dynamically adjust permitted limits for each control path, ensuring that the output demand signal remains within defined limits, thereby preventing overflows and allowing each path to contribute maximally based on priority.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If fixed point microprocessors are used to reduce processor loading and improve calculation speed, then processing speed is improved, but overflow conditions occur more frequently leading to control loss
Solution Approach 1:
The patent applies preliminary action by calculating and applying limits to each control path contribution before summation occurs. The limiting circuits (207, 216, 224) constrain each path's output to predetermined ranges before the summing circuits (210, 218) combine them, preventing overflow conditions from occurring in the first place rather than detecting and correcting them after they occur.
Solution Approach 2:
The patent uses intermediary elements in the form of limiting circuits that act as mediators between the control paths and the summing circuits. These limiting circuits (207, 216, 224) intermediate the signal flow by constraining each path's contribution to safe ranges, preventing harmful overflow conditions while allowing full utilization of the fixed point microprocessor's calculation capabilities.
2Reliability
If multiple control paths are used to improve control authority and performance, then control performance is improved, but the complexity of managing limits and preventing overflows increases
Solution Approach 1:
The patent applies segmentation by dividing the control system into separate control paths (proportional path 206, integral path 216, differential path 224), each with its own limiting circuit (207, 216, 224). This segmentation allows each path to be independently managed and limited, simplifying the overall complexity of managing multiple control paths while maintaining high control authority through their combined output.
3Reliability
If fixed limits are applied to control paths to prevent overflows, then overflow prevention is improved, but the control paths cannot contribute their full authority
Solution Approach 1:
The patent applies dynamics by making the limits dynamic rather than fixed. The limiting circuits (207, 216, 224) apply predetermined limits that allow each control path to contribute its full authority up to those limits, while the overall system maintains reliability because the sum of all limited contributions cannot exceed the maximum representable number in the fixed point format.
Data Source
AI summary
A control system (200) using fixed point computation avoids overflow conditions by limiting the internal quantities ensuring that contributing calculations from various parts of the control loop never add up to an overflow. Various parts of a control loop e.g. proportional terms can be prioritised over other parts, e.g. integral and differential terms and ensuring that the overall total resulting when the separate terms are summed together never exceeds the maximum or minimum imposed limits. Variable limit calculator circuits (211,219) revise the limits according to the output of the higher priority control path(s). The revised limits cascade down through each contributor, eventually allowing the lowest priority control path the smallest authority. The control system may be applied to control of a drive motor (111) for an aircraft inceptor (103).