Monostable Multi-Element Spring for Wider Negative-Stiffness Stroke
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Solution Overview
Problem
Existing electroactive polymer actuators with bi-stable springs have a limited range of operation and displacement due to their force-displacement curve being linear with a negative slope, making them unsuitable for applications requiring a wider range of operation with high efficiency, such as in aortic pumps.
Innovation Solution
A spring comprising a plurality of elements with a rigid portion and a flexible beam that has a single stable position, allowing for deformation and return to that position when pressure is applied, providing negative stiffness over a wider range and pure radial deformation, eliminating external abutments that limit displacement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Force
If a bi-stable spring with two opposing flexure arms is used to achieve negative stiffness, then the force-displacement curve becomes linear with negative slope, but the operating range and maximal displacement are limited
Solution Approach 1:
The spring is divided into multiple identical elements (at least two) connected in series, where each element consists of a rigid portion and a flexible beam. This segmentation allows the overall spring to accumulate displacement across multiple elements while each element maintains the negative stiffness characteristic, thereby extending the total operating range beyond what a single bi-stable element could provide.
Solution Approach 2:
The flexible beam is designed with a single stable position that allows dynamic deformation when pressure is applied between its extremities in the direction of the rigid portion. The beam returns to its single stable position when pressure is released, creating a dynamic system that can operate over a wider range compared to static bi-stable configurations.
2Length of moving object
If a pre-stretching spring with positive stiffness is used to counteract electroactive polymer contraction, then the actuator can achieve greater displacement, but the force output becomes non-uniform throughout the stroke
Solution Approach 1:
The invention changes the stiffness parameter from positive to negative by designing the flexible beam with a single stable position that deforms under pressure and returns when pressure is released. This negative stiffness characteristic compensates for the electroactive polymer's force variation, enabling both greater displacement and more uniform force output throughout the actuator's stroke.
3Device complexity
If external abutments are used to limit displacement in bi-stable springs, then the operating range is restricted, but the device complexity is reduced
Solution Approach 1:
The flexible beam's inherent single stable position design provides self-limiting behavior without requiring external abutments. The beam naturally returns to its stable position when pressure is released, eliminating the need for additional components to define the operating range, thereby maintaining structural simplicity while achieving greater displacement.
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
The monostable spring design achieves a negative force-displacement factor over a larger range, allowing for increased displacement and control of stiffness, making it more suitable for applications like aortic pumps and artificial sphincters.
Implementation Method 1
an electroactive polymer film 1 whose dimensions are controlled by applying a voltage to electrodes 2 on each side of the film. The reduction in thickness, illustrated on FIG. 1B is used to pump a liquid.
Implementation Method 2
the flexible beam having a single stable position, so that the flexible beam can be deformed when a pressure is exerted between said extremities in the direction of the rigid portion, and returns from itself to said single stable position when the pressure is released
Data Source
AI summary
A spring (3, 3′) comprising a plurality of elements (30), each element (3) comprising a rigid portion (31) and a flexible beam (32), the extremities (320, 321) of the flexible beam being supported by the rigid portion (31), the flexible beam (32) having a single stable position, so that the flexible beam can be deformed when a pressure is exerted between said extremities in the direction of the rigid portion (31), and returns to said single stable position when the pressure is released, and wherein the rigid portion (31) of at least one element (30) is in contact with the flexible beam (32) of the next element between said extremities (320, 321) of the flexible beam (32), so that the spring has a negative stiffness over an operating range. The arrangement ensures a pure radial compression/expansion of the spring.