Hoist Motor Current Limiting With Closed-Loop/Open-Loop Switching
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Solution Overview
Problem
Existing helicopter-hoist systems face challenges in controlling current and speed without a current loop, particularly in BLDC motors with low inductance, and often lack three-phase current sensors, leading to potential speed oscillations and difficulty in monitoring and limiting DC-link current.
Innovation Solution
A hoist application system with a motor drive unit, closed-loop and open-loop control units, and a switch logic unit that transitions between closed-loop and open-loop duty commands based on DC system information to manage DC-link current, using a fuzzy map algorithm for open-loop duty calculation and gradual time-based reductions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If closed-loop control is used to maintain speed control, then speed stability is improved, but DC-link current may exceed threshold values causing harmful effects
Solution Approach 1:
The system continuously monitors DC-link current and provides feedback to the switch logic unit. When current exceeds the threshold, the feedback mechanism triggers a transition from closed-loop to open-loop control, thereby preventing harmful current levels while maintaining speed stability during normal operation
Solution Approach 2:
The control system dynamically switches between closed-loop and open-loop modes based on real-time current conditions. This dynamic adaptation allows the system to optimize speed control when current is acceptable and prevent harmful current excursions when thresholds are approached
2Object-affected harmful factors
If open-loop control is used to limit DC-link current, then current control is improved, but speed oscillations occur
Solution Approach 1:
The system applies periodic duty cycle reductions in open-loop mode rather than continuous limiting. This periodic action allows the motor to operate in pulses, limiting average current while maintaining speed through the periodic nature of the control, thereby reducing oscillations compared to continuous open-loop limiting
3Measurement precision
If three-phase current sensors are installed to monitor current, then measurement precision is improved, but device complexity and cost increase
Solution Approach 1:
The system uses an intermediary approach by monitoring DC-link current instead of directly measuring three-phase currents. This intermediate measurement point provides sufficient current information for control purposes without requiring complex three-phase sensor installations, thereby reducing system complexity while maintaining adequate measurement capability
Solution Approach 2:
The system employs a simpler, more cost-effective current monitoring approach using DC-link current sensors rather than expensive three-phase current sensors. This cheaper measurement method provides adequate control information without the high cost and complexity of three-phase sensing
Data Source
AI summary
A hoist application system is provided. The hoist application system includes a motor drive unit, which is receptive of a final duty command and which drives a motor in accordance with the final duty command, a closed-loop control unit configured to generate a closed-loop duty command, an open-loop control unit configured to generate an open-loop duty command and a switch logic unit. The switch logic unit is receptive of direct current (DC) system information, the closed-loop duty command and the open-loop duty command, and is configured to generate the final duty command from one of the closed-loop duty command and the open-loop duty command based on the DC system information.