Technology

Compliant Systems Background and Design

Conventionally engineered mechanisms connected by various joints are designed to be strong and stiff. Nature prefers strength combined with compliance. By exploiting compliant design principles significantly reduces part count.

Engineered components typically have fixed geometry and the systems are generally optimized for peak performance at a specific operating condition thereby compromising performance at off-design points. Designs in nature, through their capacity to vary geometry, customize their performance for different conditions and environmental fluctuations. Compliant Design approach enables variable geometry systems with customized performance rather than a compromised solution.

A compliant structure is a monolithic joint-less mechanism that exploits the elasticity of material to produce a desired functionality, such as force or motion transmission, motion guidance, shape morphing, or energy storage and release. A compliant system has actuators and sensors integrated with the compliant structure or transmission. The arrangement of the material within the compliant mechanism is optimized so that compliance is distributed through small strains in order to produce large deformations, as illustrated in a compliant Iris mechanism (patent pending) shown below.

Advantages of Compliant Mechanisms:
• No joints means no joint friction, backlash, or need for lubrication
• Easily coupled with modern actuators such as piezoelectric, electromechanical, etc.
• Scalable: micro-, meso-, or macro-sized
• Materials friendly: Wide range of applicable materials including steel, aluminum, titanium, polymers, GFRP, CFRP, and metal-matrix composites.
• Materials friendly: FlexSys has designed and built structures using metals, composites, polymers, etc.
• Weight reduction: no need for restoring springs or bulky hinges
• Fatigue resistant

Common Misconceptions about compliant mechanisms

Myth: Flexible structures are prone to fatigue failure with few cycles.
Fact: Life cycle requirement is a paramount design consideration in the compliant structural optimization. FlexSys has developed structures capable of 220 million morphing cycles while exposed to significant air loads and 600gs of inertial loads. Sandia National Labs tested a MEMS actuator (designed by Dr. S. Kota) structure to 10 billion cycles with no failure (pictured below).


MEMS scale device (110 microns in longest dimension) U.S. Patent 6,175,170


Myth: Flexible structures are not scalable and able to support realistic loads.
Fact: Compliant mechanisms are scalable, as demonstrated by FlexSys shape morphing technology (dimensions in meters) and micro-scale devices (dimensions in microns).


Myth: Compliant systems are difficult to manufacture.
Fact: FlexSys’ design approach and the associated software take into account a desired manufacturing method during structural design and optimization. These mass-manufacturing methods include stamping, casting, extrusion, injection molding, composite molding, etc.


Myth: The FlexSys technology is no different than monolithic mechanisms decades old.
Fact: Employing elastic strain to generate motions is not new and can be found in household products like shampoo-bottle lids where the lid and the cap are relatively rigid and the flexion is concentrated in a thin flexure hinge (living hinge). Such designs, called Lumped Compliance, result in stress concentrations, limited fatigue life and are not suitable for high load bearing applications. The FlexSys approach utilizes Distributed Compliance, spreading small elastic strain over a large region of the mechanism in a manner that all elements share the load resulting in large deformations and high fatigue life. An illustration of this difference is below.

 

Simulation and Analysis

FlexSys uses a proprietary in-house developed software tool for determining the optimum configuration, shape and size of structural elements while ensuring the manufacturability, reliability and energy efficiency. To verify the performance of a compliant system design, FlexSys maintains a suite of computer-aided engineering software, including ANSYS, Matlab, and Unigraphics, to simulate three- and four-dimensional non-linear elasticity, structure-to-structure contact, and structural dynamics.


Optimized Structures for Function Forming



Nonlinear Finite Element Analysis Tools. US Patent 6,557,436 and others pending

Fabrication, Testing, and Evaluation

FlexSys has the capability and experience necessary to fabricate sophisticated compliant system prototypes and evaluate their performance using the appropriate instrumentation and measurement techniques. FlexSys maintains close working relationships with several machine shops in Southeast Michigan, including specialists in CNC machining, abrasive waterjet, wire-EDM, and composites fabrication.

Initial testing of all FlexSys compliant systems is done in-house. The FlexSys facility includes testing equipment, sensors, and data analysis tools including:

• LabView data acquisition software
• National Instruments data acquisition devices
• Matlab numerical analysis software
• Digital power supplies to power and control conventional and smart materials actuators
• Optical measurement system for two-dimensional shape measurement
• Test stand to cyclically load and deform structures to perform fatigue testing