Actuator Force-Ripple Compensation in Multi-Axis Stages
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Solution Overview
Problem
Electromagnetic actuators, such as linear and planar motors, experience significant challenges in achieving precise motion control due to force-ripple and side-forces, which are position-dependent and can vary over time, leading to accuracy and fidelity issues in applications like microlithography systems.
Innovation Solution
The method involves on-machine calibration and compensation using a compensation matrix to correct force-ripple and side-forces by determining position-dependent compensation ratios through displacement trajectories and force-command data, allowing for real-time adjustment of actuator performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If electromagnetic actuators are used to produce force in a principal stroke direction, then motion along that axis is achieved, but force-ripple and side-forces are generated that degrade positioning accuracy
Solution Approach 1:
The patent converts the harmful force-ripple and side-forces into beneficial compensation signals by measuring their effects and applying inverse compensation. The force-ripple map and side-force map, which initially represent disturbances, are transformed into correction data that is fed back into the control system to cancel out these forces, thereby achieving precise positioning despite the inherent actuator imperfections
Solution Approach 2:
The patent implements feedback by measuring the actual force output of the actuator, comparing it to the desired force, and using the difference to generate compensation commands. The force-ripple compensation and side-force compensation are continuously adjusted based on measured position and force data, creating a closed-loop system that adapts to changing conditions and maintains positioning accuracy
2Manufacturing precision
If force-ripple and side-forces are compensated using pre-characterization maps, then positioning accuracy improves, but the system cannot adapt to changes in actuator performance over time
Solution Approach 1:
The patent transitions from static pre-characterization maps to dynamic compensation that adapts in real-time. The system continuously updates the force-ripple compensation and side-force compensation based on current operating conditions, allowing the actuator to adapt to changes in performance over time while maintaining positioning accuracy
Solution Approach 2:
The system performs self-characterization by measuring its own force output and generating compensation maps during normal operation. Rather than relying solely on manufacturer-provided maps, the actuator system characterizes its own behavior and updates its compensation parameters autonomously, enabling continuous adaptation to changing conditions
3Adaptability or versatility
If compensation maps are updated frequently to adapt to changing actuator performance, then adaptability improves, but computational load and processing time increase
Solution Approach 1:
The patent performs preliminary actions by pre-calculating and storing compensation data in lookup tables during system initialization or during low-demand periods. The force-ripple compensation and side-force compensation are prepared in advance and stored as lookup tables that can be quickly queried during real-time operation, reducing the computational burden while maintaining adaptability
Solution Approach 2:
The system changes parameters by adjusting the frequency and intensity of compensation map updates based on operating conditions. During normal operation, the system uses pre-computed lookup tables with minimal real-time calculation. When changes in actuator performance are detected, the system updates the compensation parameters selectively, balancing adaptability with computational efficiency
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 enables electromagnetic actuators to produce independent and precise actuation forces without significant force-ripple or side-forces, improving the accuracy and reliability of motion control in multi-axis systems.
Implementation Method 1
The magnetic fields produced by the magnet array interact with magnetic fields produced by electrical current flowing in the coil array to impart a linearly translational force to the moving member relative to the stationary member along the principal stroke axis
Implementation Method 2
The movable member and the counter-mass are movably engaged with respect to each other via the linear actuators, with air bearings providing frictionless or low-friction motion
Data Source
AI summary
Methods, apparatus, and systems are disclosed for identifying force-ripple and/or side-forces in actuators used for moving a multiple-axis stage. The identified force-ripple and/or side-forces can be mapped, and maps of corresponding position-dependent compensation ratios useful for correcting same are obtained. The methods are especially useful for stages providing motion in at least one degree of freedom using multiple (redundant) actuators. In an exemplary method a stage member is displaced, using at least one selected actuator, multiple times over a set distance in the range of motion of the subject actuator(s). Each displacement has a predetermined trajectory and respective starting point in the range. For each displacement, respective section force-command(s) are extracted and normalized to a reference section force-command to define a section compensation-ratio. Multiple section compensation-ratios are assembled, as functions of displacement in the range, to provide a map of compensation ratios for the actuator(s) throughout the range.