Page 196 - Mechanics of Microelectromechanical Systems

Chapter 4

MICROTRANSDUCTION: ACTUATION AND
SENSING

1. INTRODUCTION

This chapter analyzes the main forms of MEMS actuation and sensing‚
also known together as transduction methods. The actuation basically
converts a form of energy‚ such as electric or thermal‚ into mechanical
motion by various means. The performance of a specific actuation method is
usually qualified through mechanical amounts‚ such as force/moment or
linear/rotary displacement. Conversely‚ in sensing‚ an already-existing
mechanical motion or the effects of the microdevice interaction with its
environment need to be evaluated by transforming the mechanical energy
corresponding to motion into another type of energy‚ which can directly be
measured.
On the other hand‚ one microdevice can be used as either actuator or
sensor. An electrostatic comb drive‚ for instance‚ can function as an actuator
when is supplied with electric energy and sets into motion a microdevice‚ but‚
equally‚ it can be used as a sensor in a microdevice that is actuated by a
different source in order to measure displacements by quantifying electric
capacitance changes. However‚ the relationship between mechanical
displacement and capacitance variation is unique. In other words‚ a unique
equation governs a specific transduction form‚ which can be used
conveniently to describe either actuation or sensing‚ through calculation of
the corresponding output amount in terms of the input quantities.
Studied in this chapter are transduction methods such as thermal‚
electrostatic‚ electromagnetic and magnetic‚ piezoelectric‚ piezomagnetic‚ by
means of shape memory alloys‚ and through bimorphs and multimorphs.
Transduction is energetically imperfect‚ as one form of energy cannot
convert into another form of energy without losses (which can be substantial
at times)‚ but this subject‚ which needs further research‚ especially regarding
the mechanisms that are involved in energy conversion‚ is not approached
here.