Compliant bistable mechanisms

SRMA 2011

ISBN 978-86-82631-59-0

*

Faculty of Mechanical Engineering Niš, Aleksandra Medvedeva 14, Serbia, [email protected]

Compliant bistable mechanisms

Andrija Milojević1,*

1

Faculty of Mechanical Engineering Niš

Bistable mechanisms are mechanisms that have two stable equilibrium positions within their

range of motion. Its advantages include the ability to stay in two positions without power input and

despite small external disturbances. Because of this ability bistable mechanisms have application in many

devices, such as switches, valves, closures, and clasps. Mechanically bistable behavior results from the

storage and release of energy, typically in springs, with stable positions occurring at local minima of

stored energy. Compliant bistable mechanisms are a particular class of bistable mechanisms which use

deflections of their members to gain motion, rather than relying solely on traditional rigid-body joints.

Compliant mechanisms represent an elegant way to achieve bistable behavior because their flexible

members allow both motion and energy storage to be incorporated into one element. Interest in compliant

bistable mechanisms has also recently increased because of the advantages of bistable behavior in many

micro-electromechanical systems (MEMS). Design of compliant bistable mechanisms typically requires

solving nonlinear differential equations and simultaneous consideration of both energy storage and

motion requirements. In this paper the example is presented to demonstrate the ease of design made

possible by the pseudo-rigid-body model that predicts the non-linear deflections of many different flexible

members.

Keywords: Compliant bistable mechanisms, pseudo-rigid-body model, bistable closure.

1. INTRODUCTION

A bistable mechanism is a mechanism

which has two stable equilibrium states within

its range of motion, i.e. bistable mechanisms

tend toward one of their two stable equilibrium

positions. At these states, the mechanism

requires no input power to remain in position,

and the mechanism will return to its stable

position after small disturbances. Because of

their ability to stay in position without power

input and regardless of external disturbances,

bistable mechanisms have application in many

devices, such as light switches, self-closing

gates, cabinet hinges, three-ring binders, valves,

closures, and clasps. Mechanically bistable

behavior results from the storage and release of

energy, typically in springs, with stable

positions occurring at local minima of stored

energy.

Compliant bistable mechanisms are a

particular class of bistable mechanisms which

use deflections of their members to gain motion,

rather than relying solely on traditional rigid￾body joints. Compliant mechanisms represent

an elegant way to achieve bistable behavior

because the flexible members allow both motion

and energy storage to be incorporated into one

element. In addition, compliance offers several

other advantages, such as reduction in part￾count, reduced friction, and less backlash and

wear [1].

Interest in compliant bistable

mechanisms has also recently increased because

of the advantages of bistable behavior in many

micro electromechanical systems (MEMS).

Bistable mechanisms can allow MEMS to be

designed with increased energy efficiency and

improved accuracy and precision in positioning.

The energy efficiency effect may be especially

critical in autonomous applications which must

produce or store their own energy, such as

devices which use micro-batteries as a power

source. Bistable micro-mechanisms could also

be used as non-volatile memory, micro-valves,

or micro-positioners with two repeatable

positions [2].

However, the design of compliant

bistable mechanisms is often not

straightforward or easy, requiring the

simultaneous analysis of both the motion and

energy storage of the mechanism. Because

bistable mechanisms store and release energy

during their motion flexible segments must

usually undergo large, nonlinear deflections,

introducing high stresses and difficult nonlinear

analysis. Nonlinear analysis usually includes

solving nonlinear differential equations for

accurate prediction of flexible segments motion.