Reaction Wheel Attitude Control via Nonlinear Phase Error Torque
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Solution Overview
Problem
Existing spacecraft attitude control systems using reaction wheels face challenges in compensating for frictional forces, particularly at low speeds and during direction changes, leading to attitude errors due to nonlinear frictional forces.
Innovation Solution
A control system that determines phase errors in reaction wheels and applies modified motor torque commands nonlinearly related to these errors to overcome stiction and kinetic friction, ensuring the rotor remains in phase lock and reduces unintentional stops, using a combination of phase-locked loop and stiction-overcoming torque commands.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If closed-loop control systems are used to mitigate friction in reaction wheel bearings, then friction compensation is improved, but the system fails to adequately compensate for frictional forces within a desired amount of time due to nonlinearities, resulting in attitude errors
Solution Approach 1:
The patent applies parameter changes by modifying the torque command based on the rotor's operational state (direction of rotation, speed range). The control system adjusts torque parameters dynamically: applying higher torque when rotation direction changes to overcome static friction, and adjusting torque magnitude based on whether the rotor is accelerating or decelerating. This resolves the contradiction by adapting control parameters to match the nonlinear friction characteristics at different operating conditions.
Solution Approach 2:
The patent implements dynamics by making the control system adaptive to changing rotor states. The controller continuously monitors rotor direction and speed, then dynamically adjusts the torque command accordingly. When the rotor changes direction or operates in different speed ranges, the control parameters are modified in real-time to compensate for varying friction levels, thereby improving both response time and compensation accuracy.
2Adaptability or versatility
If reaction wheels operate at low speeds or change direction, then attitude control flexibility is improved, but bearing friction increases torque requirements and causes unintentional stops
Solution Approach 1:
The patent applies preliminary action by detecting when the rotor is about to change direction or operate at low speeds, then pre-applying sufficient torque to overcome the impending static friction. The control system anticipates direction changes and ensures adequate torque is available before the rotor slows down or reverses, preventing unintentional stops and ensuring smooth transitions while maintaining attitude control flexibility.
Solution Approach 2:
The patent modifies torque parameters based on rotor operational state. When the rotor changes direction or operates at low speeds, the control system increases torque magnitude to overcome elevated friction forces. This dynamic parameter adjustment ensures that sufficient force is available when needed (during direction changes and low-speed operation) without continuously applying excessive torque during normal operation.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively mitigates the effects of friction, reducing attitude errors and response times, allowing the reaction wheels to maintain desired orientations with improved accuracy and efficiency.
Implementation Method 1
A reaction wheel typically consists of a rotor (or wheel) and an electric motor, wherein the electric motor is operated to apply torque to increase or decrease the angular velocity of the rotor
Implementation Method 2
The bearings of reaction wheels exhibit friction, which impair operation of reaction wheels at low speeds or when a reaction wheel changes its direction of rotation
Data Source
AI summary
Methods and systems are provided for controlling attitude of a vehicle using a reaction wheel onboard the vehicle. One exemplary method involves receiving a torque command for adjusting the attitude of the vehicle using the reaction wheel, determining a phase error of the reaction wheel based at least in part on the torque command, and determining a motor torque command for the reaction wheel based on the phase error. The motor torque command is provided to an electric motor of the reaction wheel to apply a corresponding torque to the rotor of the reaction wheel. The relationship between the magnitude of the motor torque command and the magnitude of the phase error is nonlinear. In exemplary embodiments, the magnitude of the motor torque command exceeds the stiction torque, at least instantaneously, when the reaction wheel has fallen behind an expected position by more than a threshold amount.