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Novel Escapement Mechanism using a Compliant Mechanism and a Piezoelectric Actuator
Novel Escapement Mechanism using a Compliant Mechanism and a
Piezoelectric Actuator
A Thesis
Submitted to the Faculty of the
WORCESTER POLYTECHNIC INSTITUTE
in partial requirement for the
Degree of Master of Science
in
Mechanical Engineering
by:
__________________________________
Girish S. Mali

April 27, 2007
Approved:


_______________________________________________
Professor Robert L. Norton, Advisor


_______________________________________________
Professor Yong-Mo Moon, Thesis Committee Member


_______________________________________________
Professor Mikhail Dimentberg, Thesis Committee Member


___________________________________________________
Professor Cosme Furlong, Graduate Committee Representative i


ABSTRACT

Escapement mechanisms hold back a stream of parts driven either by mechanical or
pneumatic means for a length of time and release a single part as required to an assembly
station. They are used in most automatic multi-component assembly equipment. They
occupy a significant design space and have dynamic characteristics of their own. This
research aimed to develop a novel high speed mechanism for parts escapement that
occupies less design space and contributes less to the dynamic activity of the structure.
Several conceptual mechanisms were generated and evaluated. A compliant mechanism
that amplifies the very small displacement of a piezo actuator was selected for detailed
design. A proof of concept prototype was fabricated and tested. A piezo stack was used to
bend a thin, spring steel, compliant beam. Its deflection was further amplified by
attaching a comparatively rigid beam extension at the end of the compliant section. The
mechanism escapes parts at 16 Hz using constrained layer damping on the beam to
reduce vibrations. The concept is feasible to use on production machinery and provides
advantages in terms of higher operating speeds and compactness. The concept could also
be used where there is a requirement of high speed gating. ii




ACKNOWLEDGEMENTS

I would like to thank Professor Robert L. Norton and Professor Yong-Mo Moon for
providing me with motivation and guidance through out the progress of the research.
Both of them helped me unravel a lot many territories in my ignorance. Thanks to Tim
Sweet for providing with a novel idea to work on and The Gillette Company for
sponsoring the research.

Thanks to people and friends at WPI who helped in manufacture and testing of
mechanism. I would also like to thank my family and friends for providing an
unwavering support in all my endeavors.
iii

CONTENTS

LIST OF FIGURES.....v
LIST OF TABLES.........vii
1. Introduction................................................................................................................ 1
2.
Project
Scope ............................................................................................................. 3
2.1. Goal Statement........................................................................................................ 3
2.2. Project Objective..................................................................................................... 3
2.3. Proposed Approach................................................................................................. 3
3.
Background
Study...................................................................................................... 5
3.1. Compliant Mechanisms .......................................................................................... 5
3.1.1. Advantages of Compliant Mechanisms ......................................................... 5
3.2. Piezoelectricity........................................................................................................ 6
3.2.1. Poling ............................................................................................................. 6
3.2.2. Piezoelectric Properties.................................................................................. 7
3.2.3. Piezoelectric Forces and Stiffness Characteristics......................................... 7
3.2.4. Driving conditions of PZT-elements ............................................................. 9
3.2.5. Piezoelectric Actuators ................................................................................ 12
3.3. Damping Techniques ............................................................................................ 15
3.3.1. Free Layer or Extension Damping............................................................... 16
3.3.2. Constrained Layer Damping ........................................................................ 17
3.4. Literature review................................................................................................... 17
4.

Design Concepts
...................................................................................................... 21
4.1. Concept 1 .............................................................................................................. 21
4.2. Concept 2 .............................................................................................................. 22
4.3. Concept 3 .............................................................................................................. 24
4.4. Concept 4 .............................................................................................................. 25
4.5. Concept 5 .............................................................................................................. 27
4.6. Concept 6 .............................................................................................................. 28
4.7. Concept Selection ................................................................................................. 28
5. Detailed
Design........................................................................................................ 30
5.1. Design Specifications............................................................................................ 30
5.2. Motion Analysis.................................................................................................... 30
5.3. Vibration Analysis ................................................................................................ 33
5.4. Parameter Effects .................................................................................................. 34
5.4.1. Thickness of the Cantilever Beam ............................................................... 35
5.4.2. Width of the Cantilever Beam ..................................................................... 36
5.4.3. Lengths of the compliant and rigid section:................................................. 37 iv
5.4.4. Material for Rigid Part of the Cantilever Beam........................................... 37
5.5. Design Parameters Selected.................................................................................. 39
5.6. Test Bed Requirements ......................................................................................... 42
5.6.1. Set-up Requirements.................................................................................... 43
5.7. Test Bed Description............................................................................................. 43
5.8. Testing Procedure ................................................................................................. 49
6. Experimentation....................................................................................................... 51
6.1. Experiment Setup.................................................................................................. 51
6.1.1. Function Generator ...................................................................................... 51
6.1.2. Dynamic Signal Analyzer ............................................................................ 52
6.1.3. Feeding System............................................................................................ 52
6.1.4. High Speed Camera ..................................................................................... 52
6.2. Dynamic Characteristic of the Cantilever Beam .................................................. 53
6.3. Escapement Experiments with Square Waveform................................................