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http://www.iaeme.com/IJMET/index.asp 312 [email protected]
International Journal of Mechanical Engineering and Technology (IJMET)
Volume 9, Issue 4, April 2018, pp. 312–327, Article ID: IJMET_09_04_036
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=9&IType=4
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication Scopus Indexed
FLEXURAL MECHANISMS FOR HIGH PRECISE
SCANNING APPLICATIONS: A REVIEW
Sharad Mulik
Research Scholar, Department of Mechanical Engineering,
Sathyabama Institute of Science & Technology, Chennai, Tamilnadu, India
A. Krishnamoorthy
Professor, Department of Mechanical Engineering,
Sathyabama Institute of Science & Technology, Chennai, Tamilnadu, India
Suhas Deshmukh
Associate Professor, Department of Mechanical Engineering,
Government College of Engineering, Karad, Maharashtra, India
ABSTRACT
In the era of high precision positioning, various mechanisms like piezo based
stages, spring loaded systems and ball screw mechanism have been developed till the
date to fulfil the need of precise scanning applications. Precision position is important
for achieving highly sophisticated manufacturing and measurements. Rigid body
mechanism (spring loaded and ball screw) have large range of motion but operating
at lower speed, positioning accuracy is also low and they suffer with friction and
backlash. Piezo based Nano positioning stages have high speed of operation and high
positioning accuracy but it has smaller scanning range. To overcome these issues,
researchers are favoring flexural mechanisms which exhibit high speed of operation,
large scanning range and a higher degree of positioning accuracy. Flexural
mechanisms offer frictionless, backlash free motion to achieve high degree of
repeatability and precise control. The current article reviews development of flexural
mechanism (both planar and hinge type) by various researchers across the globe. It
presents design aspects, structural aspects and application domains of precision
flexural mechanisms. The current status of the research in designing flexural
mechanisms for high precision applications is also presented. Scope of design and
development of flexural mechanisms for various applications are listed at the end of
the paper.
Keywords: High Precision Positioning, Precise Scanning, Flexural Mechanism,
Frictionless, backlash free motion.
Cite this Article: Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh, Flexural
Mechanisms for High Precise Scanning Applications: A Review, International Journal
of Mechanical Engineering and Technology, 9(4), 2018, pp. 312–327.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=9&IType=4
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 313 [email protected]
1. INTRODUCTION
Precision scanning systems has numerous applications which demands high precision and
repeatability in order to achieve highly sophisticated manufacturing and measurements [1].
The necessity of precision scanning in commercial applications such as scanners used in
biomedical applications, stereo-lithography application for development of prototypes, laser
scanning, micromachining and scanning probe microscopy is gaining interest of researchers
[2]. In the recent years, technological advancements have witnessed fast progression of high
precision Micro-Electro-Mechanical-Systems with significant implications to several
traditional and developing industries like high resolution machining and micro systems
engineering. Hence to achieve precisely guided motion it motivates to model, design and
develop Micro/Nano stages which can be used in widespread applications, such as in various
consumer products [3], aerospace applications [4], precision alignment and actuation
instruments [5-7], scanning probe systems for precision metrology and Nano-manufacturing
[8-11] Nano-shaving and Nano-grafting [12-13], Micro-mirrors [14], Nano-steering systems
[15], multi-stable structures [15-18] and medical devices [18], as Micro/Nano manipulators,
Nanolithography [19], scanning probe systems [20-21], Micro-grippers [7-8], bistable
structures [22-23], energy harvesting devices [24], electrostatic comb drive actuators [25], and
so on.
Positioning accuracy depends on resolution of positioning sensor and precise actuator
which of high cost ultimately increase cost of scanning system. To achieve precision
positioning, various mechanisms are used. Piezo based Nano positioning stages have high
speed of operation and positioning accuracy but it has smaller scanning range. Rigid body
mechanism (spring loaded, ball screw, etc.) suffers with friction and backlash. The roller
bearing, air bearings and magnetic bearings may leads to nonlinear characteristics. Hence
these conventional mechanisms have several constrains such as limited range of scanning,
restricted performance in sense of accuracy, fixed degrees of freedom, reliance of motion on
one another etc. Moreover it is challenging to design a suitable control system and interface it
to endow precise control for desirable working.
To overcome these issues, researchers are favoring flexural mechanisms which offer
frictionless, backlash free motion and exhibit high speed of operation, large scanning range
and a higher degree of positioning accuracy [1,3]. Flexures are nothing but the bending
members which deforms in a particular direction on the application of load [26-27]. Flexures
are more suitable due their distributed flexibility in providing the desired motion in the
required direction along with the advantage of absence of development of assembly, no
wear/tear hence no need of greasing/oiling and exclusion of backlash [26-29]. These
mechanisms typically use high resolution non-contact type sensors such as optical encoders in
feedback loop to achieve high order of precision positioning. Next section provides rigorous
review of flexural mechanisms developed by researchers across the globe particularly for high
precision applications.
2. BASIC BUILDING BLOCKS
Flexures being monolithic structures produce precise guided motion by means of elastic
flexibility and deformation of their constitutive compliant elements which are commonly used
in compact precision positioning systems without nonlinear disturbances such as backlash and
friction present in bearings [30]. This leads to smooth and highly repeatable guided motion,
with zero maintenance and potentially infinite life, which are exclusive features of compliant
mechanisms [1]. Flexural mechanisms are commonly contrived by means of unconventional
machining such as wire Electro-Discharge-Machining (EDM) or water jet machining. For
Nano and Micro-scale applications monolithic construction is necessary but for large range
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 314 [email protected]
mechanisms which are significantly big, it turns out to be a costlier process. They offer typical
benefits such as ease of fabrication, cost saving, optimized usage of materials etc. Only a
small number of investigators have recommended assembly of flexure links and apportioned
with the excess of constraining problems challenged for assembly beneficial in designing
more non-monolithic fashioned complex flexure mechanisms [26-28]. Several two axes
planar flexural mechanism exists which facilitates essential precise motion [29-32]. Any
flexure mechanism is composed of various basic building blocks such as cantilever,
parallelogram, double flexure etc. Arrangement of these basic building blocks plays important
role to achieve desired motion and transmission objectives.
2.1. Basic principle of planar flexural mechanism
Design of planar flexural mechanism emphasizes more on reducing some of its limitations
such as high stiffness along degrees of constrains, parasitic errors, vulnerability to
temperature effects and parasitic coupling-actuator cross sensitivity [7, 33-36].
Planar flexural mechanism consists of four rigid stages one is ground or fixed stage, two
are intermediate stages and one is motion stage as shown in figure 1.
Figure 1 Basic Principle of flexural mechanism [26-28]
Motion stage comprises mobility of two with translational motion with reference to fixed
stage. Intermediary stages are required to segregate the two axes motion and seclude the
actuators. Flexural units A, B, C, D have a different stiffness in different axes. Flexural units
A and C have high stiffness in Y-direction and B and D have high stiffness in X-direction.
Intermediate stage 1 befits ultimate position for application of the actuation force in X plane
and stage 2 for actuation force in Y plane. For any deformed configuration, intermediate stage
1 has a pure displacement in X plane whereas intermediate stage 2 has a pure displacement in
Y plane. This fundamental principle is used for designing two axis planar flexural
mechanisms. Further these basic building blocks of flexure mechanisms are discussed and
mechanisms developed using these building blocks are explained with their merits and
demerits as per present application concerned.
2.2. Single Cantilever Beam
When cantilever beam is loaded with end point load it will get deflected. Figure 2 illustrates
the bending of the cantilever beam under point load. Cantilever beam is important building
element in most of the flexural mechanisms. From beam bending analysis, beam tip translates
in Y-direction (δ) as well as rotates (θ) under the point load. A parasitic error exhibits due to
bending in the X plane (€) along with motion in Y plane (δ).
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 315 [email protected]
Figure 2 Cantilever beam with end point load [26-28]
Theoretically, deflection, parasitic motion and angular rotation are calculated by,
(2.1)
Where, L is length of the beam, E is Young’s modulus of the material, I is second moment
of the area of the beam cross-section.
The beam flexure is used for modeling of two axes mechanism with units A, B, C and D is
illustrated in Figure 3 (a). An actuation force in X plane yields very small displacement in Y
plane, and vice versa. The application of actuation force in X plane moved in the Y direction
on Intermediate Stage 1. Also, an application of force in Y plane moved the point of appliance
of the force in X plane. Hence, it was not possible to accomplish isolation of actuator. As a
result of the overhanging motion stage small amount of out of plane stiffness was observed.
Hence geometric symmetry was modeled using mirror design about a diagonal axis illustrated
in figure 3 (b).
(a) (b)
Figure 3 Planar flexural mechanism using simple cantilever beam as building block (a) Flexural
Mechanism using simple beam flexure (b) Flexural Mechanism using simple beam flexure [26-28]
As we apply actuation force in X plane, the both sides tend to yield the displacement in Y
direction which opposes each other, and therefore cancel out. Better plane stiffness was
acknowledged due to an upgraded structural loop, given that it was supported from two sides.
But it was observed that the design still underwent from absence of isolation of actuator.
Correspondingly, there were no major enhancements in the parasitic yaw of the motion stage.
Thus, it was concluded that symmetry may assist in relationship of some performance
parameters but does not fetch expansions in others. [26-28]
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 316 [email protected]
2.3. Parallelogram Flexure as a building block
Figure 4 shows parallelogram flexural unit. Analytically parallelogram flexure provides small
resistance to Y-direction relative motion however in X direction it offers very rigid motion
and rotation.
[
]
(2.2)
This unit undergoes adverse parasitic error in X-direction with respect to above stated
analytical expression. The two axis planer flexure mechanism with Flexure Units A, B, C and
D is shown in figure 5 (a).
Figure 4 Parallelogram Flexure [26-28]
Nevertheless undesired motions still occurs in spite of better accuracy. The actuation of
displacements with respect to actuation forces is same of earlier case. Hence complete
actuator isolation was not attained in this case also. Out of plane stiffness was also
comparatively low, as the motion stage was supported only from one side. Geometric
symmetry as shown in Figure 5 (b) shows some improvement in performance in terms of
parasitic error. To eliminate the additional rotational constraints arising from the
parallelogram flexures, the motion stage yaw should be more compact in size. On the
application of an X actuation force, the both sides of the mechanism incline to generates
displacements in Y direction that stand each other, and consequently reduce the cross-axis
coupling errors. Out of plane stiffness also progresses owing to better grip. Complete isolation
of actuator is still not attained in this design.
(a) Flexural Mechanism using parallelogram (b) Flexure parallelogram
Figure 5 Planar flexural mechanism using parallelogram flexure as building block [26-28]
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 317 [email protected]
2.4. Double Parallelogram Flexure as a building block:
Double parallelogram flexure is usually cited as a compound, folded beam or crab-leg flexure
as shown in figure 6. Analysis of the structure showed that displacement and rotation in X
direction is relatively stiff but it allows relative translational motion in Y direction between A
and B. Length contraction due to beam deformation is absorbed by a secondary motion stage
hence the parasitic error along X direction was observed to be considerably smaller. The
rotational parasitic motions may be eliminated by Y direction force in suitable location.
Hence, body A generates translation motion in Y direction with respect to body B on the
application of force in Y direction. This is true only in the absence of X direction forces. The
double parallelogram structure used to construct XY mechanisms as shown in figure 6. In
these cases, cross axis coupling and motion stage yaw is small and actuator isolation is also
being better than previous designs.
[
]
(2.3)
Double parallelogram flexure is more close to the ideal one as compared to other flexural
units. Analytical equation shows that it has zero parasitic error motion. Hence double
parallelogram flexural unit can be used for precise positioning for XY scanning system.
Figure 7 illustrates the planar flexural mechanism using double parallelogram flexural unit.
Hence it is the best suitable for opto-mechanical scanning system in micro-stereo-lithography.
Figure 6 Double parallelogram flexural unit [26-28]
(a) (b) (c)
Figure 7 Planar flexural mechanism using double flexure as building block [26-28]
Design of flexural mechanism is based on basic beam bending equations. Various flexural
elements are used for XY Mechanism and presented here. The review shows clearly that
double parallelogram flexure has zero parasitic error motion and small amount of rotation of
stage. Appropriate placement of actuator can eliminated rotation of stage. XY Flexural
mechanism is further designed using double parallelogram flexure and shown in figure 7.
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 318 [email protected]
Here two or four double parallelogram flexures are used for one direction and these flexures
work as spring in parallel.
3. OVERVIEW OF DESIGN AND STRUCTURAL ASPECTS
FLEXURAL MECHANISMS
Assumed the comprehensive benefits of flexural mechanisms, it persist a generous area of
design considerations. However flexural mechanism design has been commonly established
on artistic interpretation and technical insight; analytical software can support in design
development, assessment and optimization method. Flexure based mechanisms can be driven
by piezoelectric actuators which are used in ultra-precision positioning, e.g. atomic force
Microscopes (AFM), scanning probe Microscope (SPM), laser-based confocal Microscope,
by utilizing their inverse piezoelectric effect [37-39]. For a small output motion range, a
piezoelectric actuator can be directly used to achieve the output displacements. One such
study is carried out regarding flexure design using low stiffness actuators such as Lorentz
actuators [40]. Here, leaf spring flexures were selected to realize a low stiffness. The
advantage of this method is to maximize second resonance frequency with respect to first
resonance frequency which enables to a high control band width. A positioning system is
illustrated in figure 8. Lorentz actuators are used to generate the force Fz for the actuation in
Z plane. Due to utilization the Lorentz force, these actuators have no mechanical stiffness
between the moving mass and the fixed frame [43], which result into a low-stiffness actuator.
The positioning system was modeled with single leaf spring flexures as shown in figure 9 to
derive first resonance frequency and multiple leaf-spring flexures was modeled as shown in
figure 2.3 to derive the high resonant frequencies.
Figure 8 illustration of
positioning system [40]
Figure 9 Model of a single leaf-
spring flexure to derive first
resonant frequency of positioning
system. [40]
Figure 10 Model of multiple leaf-
spring flexures to derive the high
resonant frequencies [40]
The flexure-based mechanism can also be integrated with the voice coil motor (VCM) for
actuation purpose. A similar attempt is made for design of nanometer level resolution and
millimeter level operational ranged high accuracy XY-scanner [41-43]. The XY-scanner is
actuated with the integration of double compound linear spring flexure guide mechanism with
voice coil motor [44]. Leaf spring mechanisms having minor widths were used for escalation
of the working range of XY scanner to the millimeter level. It was designed to enhance the
first resonant frequency of the XY scanner to raise the response speed. The XY scanner has
position resolution of 10 nm and working range of 2 mm. The scanner comprises of a double
compound linear spring flexure guide mechanisms and a voice coil motor (VCM) to fulfill
travel range specifications & accuracy and to reduce parasitic motion during scanning. Due to
symmetric structure of double compound linear spring mechanism shown in figure 11 (a), it is
free from parasitic motions and free from effects of heat deformation. The conceptual design
of one axis scanning using the double compound linear spring mechanism integrated with a
voice coil motor is illustrated in figure 11 (b). [44]
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 319 [email protected]
Figure 11 Schematic of scanner: (a) leaf spring based double compound linear spring guide
mechanism; and (b) guide mechanism combined with VCM [44].
A modular mechanism design consists of monolithic 1 degree of freedom flexure
mechanism as shown in figure 12 (a). The separate modules can be integrated in sequence to
develop multiple degrees of freedom mechanism. Each of this module is driven by symmetric
kinematic arms and piezoelectric actuators. The input and output faces of each module
includes mounting holes in it which permits it to couple together in several configurations of
each arm consisting of two flexures. The experimental 2-DOF (XY) manipulator was
developed as shown in figure 12 (b). A webbed mounting bracket was designed and analyzed
to limit coupling between X and Y modules. Further the system is controlled through a tele-
operated haptic control scheme by coupling the master to the slave device. A scaled forced
pollution controller and a passivity controller were utilized to attain transparency and stability
of the controller. It provides significant design flexibility of flexure mechanism increasing
range of motion and allowing DOF. Haptic tele-operation control scheme enables in natural
performance of manipulation task [45]
(a) Planar view of 1-DOF module (b) Conceptual 2-DOF system design
Figure 12 Modular mechanism design features. [45]
A similar study on assembling of flexural mechanism is presented here [46]. The problem
is proposed to have mobility to be exactly zero while determining the number of dowel pins.
While defining a new concept called “half joint”, two additional rules are needed to be
introduced as proposed. This design of novel manipulator facilitates coupled mechanism to
meet task specific requirements. It provides significant design flexibility of flexure
mechanism increasing range of motion and allowing DOF. Haptic tele-operation control
scheme enables in natural performance of manipulation task. The system can be expanded
into DOFs through the coupling of additional stages and for this future work is intended to
investigate enhanced transparency and stability. According to Grubler’s criteria required
number of locating pins and their locations has been determined [46].
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 320 [email protected]
Researchers have also been concentrating on designing compact flexural mechanisms,
wherein monolithic and parallel manipulators are area of interest of many researches. Along
with the compacting of mechanism the errors encountered in flexural motion need also to be
reduced. One such research is carried out on a 2-legged XY parallel flexure motion stage with
minimised parasitic rotation shown in figure 13 (a) [47]. A XY compliant parallel manipulator
(CPM) has been proposed using the stiffness center based approach. This innovative design
approach makes all of the stiffness centers, associated with the passive prismatic (P) modules.
The proposed XY CPM has a millimeter-level motion range of 4 mm per direction and can
well deal with the issue of actuator isolation. In comparison with the emerging monolithic XY
CPMs obtained from the configuration of 4-PP kinematically decoupled translational parallel
manipulator (TPM) as shown in figure 13 (b), the present XY CPM mainly has a smaller size,
simpler modelling as well as smaller lost motion due to the use of only two legs. The study
takes into consideration 3-legged XY CPM (see figure 14) and two legged stacked CPM with
reduced size [47].
Figure 13 XY parallel manipulators: (a) 4-PP (Prismatic) kinematically decoupled TPM; (b) 2-PP
kinematically decoupled Translational parallel manipulator (TPM) [47]
Figure 14 A 3-legged XY CPM with minimised parasitic rotation [47]
Figure 15 Two-legged stacked XY CPM design I using basic parallelogram module as passive P joint [47].
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 321 [email protected]
Figure 16 Two-legged stacked XY CPM design II using double parallelogram module as passive P
joint [47].
Compared with the existing design of XY compliant parallel manipulators which are
obtained by using 4-legged mirror-symmetric constraint arrangement, the proposed design of
XY compliant parallel manipulators based on stiffness centre approach largely benefits from
fewer legs resulting in reduced size, simpler modeling as well as smaller lost motion.
Comparing with the existing 2-legged designs with the conventional arrangement, the
presented design has minor parasitic rotation is shown in figure 15 and 16, which has been
proved from the finite element analysis results [47].
One study is regarding a two-dimensional parallel Piezoelectric-actuator -driven Nano
positioner with a novel mechatronic structure of a large workspace and a high-natural
frequency. The parallel kinematic XY flexural mechanism provides good geometric
segregation. The proposed design has a large work space and high bandwidth which is
verified by FEA. The analysis shows that Nano positioner has a large workspace more than
200 μm and a high-natural frequency of 760 Hz. Furthermore, the dynamic model of the Nano
positioner, including the dynamics of the PZT actuators, is also generated from the
perspective of transfer functions and the parameters are identified by frequency-response
analysis, which can be used for Nano precision servo mechanism [48-49].
4. OVERVIEW OF CURRENT INVESTIGATIONS
A current requirement of precision scanning application is to design and develop a low cost
flexural mechanism and control system with high precision positioning accuracy. Researchers
across the world addressed the problem with monolithic design of flexural mechanisms and
used a high resolution position sensor such as optical encoders or piezo based position
sensors. These sensors are further used as feedback element for precision scanning. Flexural
mechanism has an advantage of zero backlashes, high order of repeatability and frictionless
operation is achieved by using a non-contact type actuator such as voice coil actuator. All
precision scanning mechanisms use high resolution sensor in feedback control system. Sensor
resolution plays a very important role to achieve high precision scanning. Optical sensors are
most commonly used in feedback loop and needs high degree of alignment and manufacturing
accuracy. Small misalignment leads to erratic variations in scanning system. Cost of the
sensor is also one of the important issues that need to be addressed carefully during mass
manufacturing. It is clear from the above discussion that accuracy of positioning needs a
feedback and following are the main disadvantages of feedback sensors in precision scanning
applications: 1. Mounting/alignment of the sensors into flexural mechanism is major problem.
2. Costly sensors are used in feedback and overall cost of the mechanism becomes high. XY
flexural mechanisms developed till date have following major limitations like:
Range of scanning is limited.
Positioning Resolution is limited due to type of sensor in feedback loop.
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 322 [email protected]
Mostly monolithic design and it complicate mounting of actuator and sensors for
scanning operations.
To achieve high resolution non-contact type optical encoders are to be used.
In case of flexural mechanism movement of the motion stage is mostly predictable and
predetermined. These mechanisms have linear characteristics within operational range and it
is very much possible to develop a model which predicts the behavior of such mechanisms.
Most of the flexural mechanisms use various actuators such as piezo, voice coil motors etc.
piezo type actuators offers high precision but has very low range of the motion and becomes
difficult to apply for large scanning applications. Whereas, Voice coil Actuator (VCM) can be
in the range of millimeters and also offers better precision and control of the position which
entirely depends on type of the driver used for control of the position. Till date VCM is used
for scanning purpose only, and none of the researchers have used VCM as sensor as well as
actuator simultaneously.
XY Flexural mechanisms developed by various researchers are based on flexibility of
structures to achieve the desired motion. Precision positioning is achieved in XY flexural
mechanism is possible using feedback control system which typically uses high resolution
position sensors (optical encoder, piezo sensors) and further needs contact-less actuation (e.g.
voice coil motor) to avoid friction during motion. Voice coil Actuator (VCA) generates a
force which can further be applied on to motion stage of the mechanism. It is further observed
that VCM has linear characteristics (force is directly proportional to current in the Voice Coil
Motor (VCM)) over the scanning range. This clearly shows that monitoring the current
supplied to VCM will help to exactly estimate the force generated by VCM. VCM is further
connected to Flexural stage which generates motion by application of the force. All flexural
mechanisms work in elastic range and have linear characteristics. Ultimately monitoring the
current drawn by VCM will estimate the position of the motion stage itself. This quality of
VCM (which work as actuator as well as sensor) provides us idea of position sensing by
monitoring a current and voltage drawn by VCM during motion. Here, VCM work as actuator
as well as sensor and in actual practice we need not to use a separate sensor for feedback in
precision positioning applications. Entire research work is planned to develop a position
estimator algorithm, validation of position estimator and real time implementation of position
estimator algorithm on flexural motion stage. Further, the demonstration of sensor-less
operation of flexural motion stage and its precision positioning at high speed of scanning is
presented [50]
Recently, a novel position estimator algorithm was designed by researchers for voice coil
motor actuators which will work as precision sensor instead of actual high cost, high
resolution non-contact type sensors. Further, it eliminates the use of actual sensor in feedback
loop and offers more flexibility and simplicity in design and development XY planar flexural
mechanism for precision scanning with low cost of complete system. The proposed position
estimator algorithm for VCM and XY scanning mechanism estimates position of motion stage
by measurement of current and voltage across actuator (voice coil motor) in position estimator
algorithm. Proposed work is to demonstrate high precision scanning of XY flexural
mechanism with and without position estimator (i.e stroke estimator) via due theoretical and
experimental investigations [51-54]
Another invention related to design and fabrication voice coil motor for high precision
applications is also proposed. It is integrated with dSPACE DS1104 R&D controller. PID
Control algorithm is developed in MATLAB Simulink and it is applied on the voice coil
motor. Static characteristic such as stiffness was found to be around 8.2 N/mm and dynamic
characteristics like damping factor and frequency response are found to be 0.06381 and
0.0102 respectively. PID parameters are tuned using Ziegler Nichols tuning method.
Sharad Mulik, A. Krishnamoorthy and Suhas Deshmukh
http://www.iaeme.com/IJMET/index.asp 323 [email protected]
Accuracy of less than 5 μm is achieved at low speeds and amplitude of 500μm. Further as we
increase the amplitude and frequency of operation, error increases at it is a progressive error.
State space model was prepared for the VCM and its response was compared with
experimental results via implementation of LQR control. Voice coil motor involves smooth
and frictionless motion that can be further applied to laser scanner and precise positioning
stages for different applications like Atomic Force Microscopy, Stereo lithography and so on
[53-56]
The double flexural mechanism is developed by one of the researchers and it incorporated
to dSPACE DS11004 R&D controller. The characterization of DFM is carried out on the
basis of two distinct fields - (1) Static analysis is executed for finding out force deflection
attributes for the range of entire displacement and (2) Dynamic analysis is accomplished using
frequency response which gives characteristics of system with different frequency inputs.
This frequency response is further utilized to accomplish experimental modelling of DFM.
Empirical model of with the help of frequency response is found out using constrained
minimization approach. Evaluated empirical model is further used for PID control algorithm
implementation. PID control attributes (i.e. proportional gain, integral gain and derivative
gain) are adjusted using Ziegler Nichols approach. Empirical model at the outset was
examined and accuracy of lower than 1 micron was attained. PID algorithm was applied using
dSPACE DS1104 R and D controller and Control Desk GUI environment. Actual positioning
accuracy of lower than 2 microns is accomplished. PID control exhibited good disturbance
rejection and the least amount of error when being used for tracking at different frequencies.
LQR control could only be used for regulation purpose whereas LQI control attained good
disturbance rejection during regulation. To improve tracking, use of feed forward control with
LQI and with other control strategies and implement is suggested [52-60]
The XY mechanism that utilizes fundamental building elements such as beam of
cantilever structure, double parallelogram and parallelogram flexural structures with respect
to their execution aspects like parasitic error and stiffness was proposed. Distinguishing
amongst bi-flex mechanisms is done in view of various parameters like stiffness, parasitic
flaw and angular movement of deformation stage. Theoretical and finite element analysis is
done for this and it is noted that double parallelogram flexure gives great execution results
with respect to other flexural building structures. XY flexural system utilizing double flexural
mechanism (DFM) is further fabricated and test experiments are implemented. Experimental,
FEA and theoretical results show acceptable degree of accuracy. Load in weight pane
(maximum 35 N) is given such that maximum of 7.5 mm displacement is achieved. Maximum
displacement of 25 mm was observed and some variations were seen due to surface distortion
and flaws. Therefore, it is noted that there is no parasitic error takes place in Y-axis when
motion stage is moving in X-axis [53-56]
One research was carried on design, analysis and modeling of XY flexure mechanism
which is based on double parallelogram flexure (DFM). The XY flexural mechanism for
displacement developed using double parallelogram flexure module. The theoretical results
are verified using FEA results. The force-deflection curve found to be linear. The slope of this
curve shows stiffness which is constant. It is observed that error between FEA and theoretical
results is less than 3%. Mechanism has stiffness of 4.8986 N/mm. The mechanism has range
of ± 5 mm for a force of 25 N. The design parameters are optimized by using parametric
analysis. Mechanism presented in this paper can be used in various precision applications
such as atomic force microscope (AFM), laser cutting, laser surgery and scanning probe
microscope [55].
Six types of flexural joints in terms of stiffness, stress, deflection for the influence of
geometric parameters on the performance of hinges were evaluated in one of the
Flexural Mechanisms for High Precise Scanning Applications: A Review
http://www.iaeme.com/IJMET/index.asp 324 [email protected]
investigation. Further the operating range of each joint is stated within the considered
parametric range in hinge length and minimum hinge thickness. Guiding accuracies defining
the accuracy of motion are also derived. A catalog of design charts based on the parametric
modeling using FEA tool ANSYS® Workbench™ 14.5, characterizing the joints are
presented, allowing for rapid sizing of the joints for custom performance. XY planar scanning
mechanism employing elliptical flexure is designed to have a long travel range up to 5 mm in
both X- and Y-directions, while having a size of 300mm × 300mm × 3 mm. In the proposed
stage system, the stage would be driven by PZT (Piezo-Electric Amplifier) at amplifier legs
considering the driving force in the range of 20 to 35 N. The experimental measurements
validate the large travel range of the mechanism. Errors in motion direction displacement and
off axis displacement are near to 11 % and 15 % respectively. Analysis of flexural joints
providing actual numbers would require some normalization of parameters and is an interest
to be considered in future research. Ongoing work includes dynamic analysis to determine
natural frequency and mode shapes of the XY planar scanning mechanism was presented [61]
5. CONCLUSION
The current article reviews development of flexural mechanism (both planar and hinge type)
by various researchers across the globe. It presents design aspects, structural aspects and
application domains of precision flexural mechanisms. Precision position is important for
achieving highly sophisticated manufacturing and measurements. These precision positioning
systems generate precisely guided motion. To fulfil the need of precise scanning applications
various mechanisms like piezo based stages, spring loaded systems and ball screw mechanism
have been developed till the date. To overcome limitations of traditional mechanisms,
researchers are favoring flexural mechanisms which exhibit high speed of operation, large
scanning range and a higher degree of positioning accuracy. Flexural mechanisms offer
frictionless, backlash free motion to achieve high degree of repeatability and precise control.
The current status of the research in designing flexural mechanisms for high precision
applications is presented in the paper. From the rigorous literature review, authors have
proposed a novel position estimator algorithm for voice coil motor actuators which will work
as precision sensor instead of actual high cost, high resolution non-contact type sensors. This
eliminates the use of actual sensor in feedback loop and offers more flexibility and simplicity
in design and development XY planar flexural mechanism for precision scanning with low
cost of complete system. Scope of design and development of flexural mechanisms for
various applications are also conserved in the paper.
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