Recursive hexapod system and method for multiaxial mechanical testing
Inventors
Michopoulos, John G. • Hermanson, John C. • Iliopoulos, Athanasios
Assignees
US GOVERNMENT IN NAME OF SECRETARY OF NAVY • US Department of Agriculture USDA • US Department of Navy
Publication Number
US-8978480-B2
Publication Date
2015-03-17
Expiration Date
2032-08-02
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Abstract
A recursive hexapod testing machine for mechanical testing of deformable material specimens under six degrees of freedom multiaxial loading. The machine includes a fixed base, a movable base connected to the fixed base at by a hexapod arrangement of six linearly extendable actuator linkages, with an end of each linkages attached to the movable base and the other end of the linkage attached to the fixed base. Each actuator linkage includes a hydraulic cylinder and piston, a digitally-controlled electro-hydraulic servo-valve for actuating the hydraulic cylinder in response to a digital command string, a load cell in line between the piston and the universal joint attached to the movable base, a linear variable displacement transducer for measuring extension of the piston, and a magneto-resistive position transducer for measuring a position of the piston. The servo-valves extend each hydraulic cylinder a predetermined amount to effect a desired motion of the material specimen.
Core Innovation
The invention is a recursive hexapod testing machine designed for mechanical testing of deformable material specimens under six degrees of freedom multiaxial loading. The machine includes a fixed base and a movable base connected by six linearly extendable actuator linkages arranged in a hexapod configuration. Each actuator linkage includes a hydraulic cylinder and piston, a digitally controlled electro-hydraulic servo-valve actuated by a digital command string, a load cell positioned in line between the piston and the universal joint attached to the movable base, a linear variable displacement transducer to measure piston extension, and a magneto-resistive position transducer to measure piston position.
The machine operates by having the servo-valves extend each hydraulic cylinder a predetermined amount to effect desired motion of the material specimen. Positions measured by the six magneto-restrictive displacement transducers provide feedback for closed loop control of actuator position. Additional features include a specimen holding fixture with lower and upper grips, the fixture base connected to the fixed base by a hexapod arrangement of frame support members equipped with load cells to measure deformation or strain, and digital video cameras positioned to capture images of the specimen during testing.
The system is structured with multiple layers of hexapod recursions, including the main actuator hexapod, load cell arrangements, displacement transducers, and positioning sensors, each contributing to precise control and measurement of specimen deformation under complex multi-dimensional loading paths. The recursive design accommodates specimen sizes using rigid adapters of varying heights, facilitating testing of diverse specimens under controlled and repeatable six-degree-of-freedom movements.
Claims Coverage
The patent discloses fourteen main inventive features spanning one independent claim and dependent claims.
Hexapod arrangement with linearly extendable actuator linkages for multiaxial specimen testing
A materials testing machine comprising a fixed base and a movable base connected by six linearly extendable actuator linkages arranged in a hexapod configuration, where each actuator linkage includes a hydraulic cylinder and piston, a digitally controlled electro-hydraulic servo-valve responsive to digital commands, a load cell inline with the piston and universal joint on the movable base, a linear displacement transducer measuring piston extension, and a magneto-resistive position transducer measuring piston position, wherein the servo-valves extend hydraulic cylinders to effect desired specimen motion.
Universal joint attachment at both ends of actuator linkages
Each actuator linkage is attached to the movable base and fixed base by universal joints at the first and second ends, respectively, allowing multi-directional movement.
Closed loop control using magneto-restrictive displacement transducers
Positions measured by the six magneto-restrictive displacement transducers are used by controllers to regulate the electro-hydraulic servo-valves for closed loop actuator position control.
Fixture with hexapod support and load cells for specimen holding
A fixture for holding a specimen includes a lower grip attached to a rigid fixture base and an upper grip, with the fixture base connected to the fixed base by a hexapod arrangement of six rigid frame support members. Each support member is equipped with a load cell to measure deformation or strain.
Distinct load cell types for frame supports and actuator linkages
Load cells in the frame support members are strain-gauge based, while the load cells in the actuator linkages are charge-controlled piezoelectric load cells.
Digital video camera pairs for specimen imaging
Two pairs of digital video cameras are attached to the fixture base, positioned to image the specimen during testing.
Adjustable rigid adapter piece for accommodating specimen size variability
A rigid adapter piece affixed to the movable frame and the upper grip enables testing specimens of different sizes by varying the adapter's height.
Plurality of rigid adapter pieces with varying heights
Multiple adapter pieces of different heights allow accommodation of different specimen lengths while maintaining constant overall machine height.
Double I-beam construction for rigid adapter piece
The rigid adapter piece is formed from a length of double I-beam with flanges positioned for affixation to the movable base and upper grip, offering structural rigidity.
Hexapod extendable linkages with displacement transducers between upper grip base and fixture base
A hexapod arrangement of six extendable linkages each includes a linear displacement transducer measuring the position of the upper grip base with respect to the fixture base.
Controller for servo-valve operation based on piston position and predefined loading paths
A controller receives piston position information from magneto-resistive transducers and controls servo-valves according to computer-based instructions for predetermined specimen loading paths.
Controller feedback from multiple sensors for enhanced servo-valve control
Control of servo-valves is based in part on position data from actuator linear displacement transducers, upper grip position information from extendable linkage transducers, and strain or displacement from load cells.
Use of magneto-restrictive linear position transducers
Magneto-restrictive position transducers specifically comprise magneto-restrictive linear position transducers measuring piston extension for control feedback.
Servo-valve operation responsive to piston extension data from magneto-restrictive transducer
Servo-valves extend hydraulic cylinders in response to piston extension information measured by the magneto-restrictive linear position transducer to effect desired specimen motion.
The claims collectively cover a multi-degree-of-freedom materials testing machine incorporating a hexapod arrangement of hydraulically actuated linkages with integrated multi-sensor feedback for precise control, a specimen holding fixture with hexapod support and load measurement, video imaging for specimen observation, and modular adaptors for specimen size accommodation, operating through computer-controlled servo-valves to follow predetermined multidimensional loading paths.
Stated Advantages
Allows precise control of specimen path and loading profile through digital control of actuator valves.
Equipped with multiple cameras enabling three-dimensional determination of displacement and strain using a marked specimen system.
Capable of rapidly and precisely loading, gripping, moving, and releasing specimens with different six-dimensional movement paths.
Multiple hexapod arrangements provide precise position information improving actuator control signal refinement.
Documented Applications
Mechanical testing and characterization of deformable material specimens under six degrees of freedom multiaxial loading.
Testing isotropic and anisotropic materials, specifically laminates such as carbon-graphite epoxy based composite materials.
Measurement of full field displacement and strain during deformation test using digital video cameras and marker tracking.
Constituent material parameter identification using multidimensional constitutive response characterization methods.
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