Magnetostrictive actuator with differential displacement amplification mechanism.
Lu, Quanguo ; Nie, Qin ; Jiang, Xiaoyang 等
1. Introduction
The giant magnetostrictive material (GMM) has been developed in
recent years as a new type of functional material [1]. It has large
strain, high energy density, a large output force, and a high response
[2]. The actuators, which use GMM as a drive resource (GMA), are very
precise and have a rapid response speed, but the output displacement is
only on the micron level [3]. Therefore, this type of actuator cannot
meet the demands of large output displacement [4]. To remove this
restriction, a GMM with a flexure hinge mechanism was designed. This
actuator has a compact structure, large displacement, clear transmission
relation, no transmission idle stroke and no friction. It is especially
suitable for use with a precision drive structure [5].
In order to make the output displacement large and precise, Cuihong
Li of Tsinghua University [6] used GMA as the driving source and
piezoelectric ceramic as a clamp. Based on the creeping principle, the
piezoelectric ceramic was embedded in the axial symmetric flexible
hinge, however, the entire structure was complex and it was difficult to
determine the peristaltic step. Yang Bintang [7] of Shanghai Jiao Tong
University devised a type of GMA which had large stroke and high
resolution. The design was based on the telescopic driving principle of
GMM and the flexible hinge amplification mechanism. However, it required
a large number of actuators and was expensive to design. Furthermore,
the design did not optimize the structure of the actuator. Japan's
Ibaraki University EDA Hong [8], in conjunction with the Toshiba Co.,
Kobavashi designed a GMA whose positioning accuracy could reach micron
level, and has been successfully applied to the GMA for a micro feeding
device with a large optical diamond lathe [9].
For this study, a GMA was created using differential displacement
amplification, giant magnetostriction material and the flexible hinge
amplification mechanism. Based on the fundamental characteristics of
magnetostrictive material, this study analyzed the actuator prototype
and created a new design for GMA including a drive coil, preloading
mechanism and displacement amplification mechanism. This research
primarily analyzed the influence of the input current and pre-pressure
on the output displacement [10]. Moreover, the reasonable matching
problem of displacement amplification mechanisms and the GMA was
analyzed [11].
2. Materials and methods
2.1. Design of differential amplifying magnetostrictive actuator
As shown in Fig. 1, the structure of the differential amplification
micro actuator was primarily composed of GMA and a displacement
amplification mechanism. Micro displacement produced by the actuator was
transformed into large displacement in the output end of the structure
through the transmission in the amplification mechanism.
[FIGURE 1 OMITTED]
The output shaft of the micro actuator and the output end of the
displacement amplification mechanism were on the same axis, and moved in
the same direction so as to ensure the stability and same-phase of the
displacement amplification.
2.2. Design of micro GMA
GMA is a type of magnetic-mechanical coupling converter, and its
design relates to the selection of magneto-strictive material and the
devise of the coil, the magnetic circuit and the preloading mechanism.
In this article, as shown in Figure 2, the structure of the actuator is
composed of a GMM rod, sleeve, coil, coil skeleton, output shaft,
commutator end bearing bracket, drive end bearing bracket, shell,
preloading nut, preloading spring, linear bearing, as well as other
components. The GMM rod, output shaft, shell and commutator end bearing
bracket consists of a closed magnetic circuit, therefore reducing
magnetic leakage and improving the strength of the magnetic field. The
output shaft, drive end bearing bracket, preloading spring and
preloading nut consists of a preloading mechanism. By applying the
appropriate preloading pressure, the GMM rod exhibits a large axial
strain while simultaneously increasing the output displacement and
force. The GMM rod is made of brittle material, which can resist
pressure, but not tension. Therefore, imposing a certain preloading
stress can make the GMM rod work in the compression state.
[FIGURE 2 OMITTED]
2.3. Design of preloading mechanism
In order to fully utilize the magnetostrictive properties of GMM,
this research is based on a design that allows the preloading mechanism
to put suitable pre-pressure onto the GMM rod [12]. If the pre-pressure
applied is either too large or too small, the magnetostrictive effect of
the GMM rod will be hindered. Therefore, the selection of the amount of
the pre-pressure should be based on the optimal energy conversion
efficiency of the differential amplifying micro actuator; which for this
research, 7 ~ 10 MPa was chosen.
Pre-pressure can be applied in various models, usually by devices
such as a cylindrical helical spring, disk spring, and elastic plates.
The device can be relatively simplified by using a combination of a
cylindrical spiral spring with a preloading nut, so it can conveniently
adjust the pre-pressure and ensure the direction of pre-pressure in
parallel with the axis of the GMM rod. The pre-pressure, if evenly
distributed, aids the GMM rod from bending.
For this research, the cylindrical spiral spring and displacement
amplification mechanism was used to apply axial pre-pressure. This
method was used because it can adjust the compression degree of the
cylindrical helical spring by rotating preloading nut, as well as adjust
the deformation of the displacement amplification mechanism under an
initial state. Furthermore, the output shaft of the displacement
amplification mechanism can simultaneously apply pre-pressure to the GMM
rod by counterforce.
2.4. Design of displacement amplification mechanism
The displacement amplification mechanism is the core component of
the differential amplifying micro actuator. The main function of the
displacement amplification mechanism is to enlarge the output
displacement to a certain extent. Its performance determines the
performance of the differential amplifying micro actuator. The
realization of micro displacement amplification has many forms, such as
lever amplification, gear combination, ellipse amplification, and
hydraulic amplification [13]. For this study, differential lever
amplification combined with the flexible hinge structure was used. A
principle diagram of differential lever amplification is shown in Fig.
3. According to the different input directions, differential lever
amplification can be divided into two types, synthetic drive and reverse
drive. The amplification effect of these two types is equal. In order to
acquire a rational arrangement of structure, synthetic drive was used.
[FIGURE 3 OMITTED]
Ignoring the elastic deformation of the lever and the resistance of
the hinge, this study assumed that the lever was a rigid body, the hinge
was an ideal hinge, and the input displacement was [[delta].sub.in].
According to the principle of lever, the output displacement was:
[[delta].sub.out] = [[L.sub.4]/[L.sub.3]([L.sub.2]/[L.sub.1] +
[L.sub.6]/[L.sub.5]) + [L.sub.6]/[L.sub.5])[[delta].sub.in]. (1)
The definition of magnification was the ratio of output
displacement and input displacement. It is an important index to measure
the amplification ability of the amplification mechanism. In this study,
magnification of the differential amplification mechanism was:
A = [[delta].sub.out]/[[delta].sub.in] =
[[L.sub.4]/[L.sub.3]([L.sub.2]/[L.sub.1] + [L.sub.6]/[L.sub.5]) +
[L.sub.6]/[L.sub.5]]. (2)
The displacement amplification mechanism is shown in Fig. 4. The
output displacement of the GMM rod acted on the input end of the
displacement amplification mechanism, and the input displacement was
located at the output terminal after amplified in the displacement
amplification mechanism. The realization of movement transmission relied
on the micro rotational deformation of the flexible hinge, therefore,
the flexible hinge needed to have good elastic recovery ability.
Researchers selected 60Si2Mn as the material of the displacement
amplification mechanism [13]. As a result, the displacement
amplification mechanism had the advantage of a simple structure, compact
use of space, no friction, no abrasion, no clearance, no backlash,
strong recovery ability and high motion sensitivity.
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
Fixed installation of the displacement amplification mechanism was
a key problem. If there were three fixed ends that were not placed
solidly, the displacement would be lost during the transfer process.
Also, there would be a decrease in performance of the displacement
amplification mechanism. In order to avoid installation problems, the
assembly structure of displacement amplification mechanism, a splint was
made in a convex shape and was used for clamping the three fixed ends of
the displacement amplification mechanism (Fig. 5). An abutment was
placed on the front end and was used to hold the splint. High strength
bolts connected the splint and the abutment, therefore, the displacement
amplification mechanism had been fixed by the friction of contact
surfaces.
3. Experiment
On the basis of design analysis, researchers trial-produced the
prototype of the differential amplification micro actuator and a process
performance test was executed.
3.1. Introduction of experimental system
The experimental system of the differential amplification micro
actuator was composed of the micro actuator, vibration isolation
platform, temperature controller, constant current source, displacement
measuring system, constant temperature water cooling system and magnetic
field detection system.
The experimental system adopted the PC machine as the controller.
The digital signal sent by the PC machine was converted into an analog
voltage signal through PCI8333D/A. Then, the voltage signal was used as
the input into the controllable constant current source. The constant
current source output corresponding currents to the drive coil, and the
coil produced a magnetic field. In this magnetic field, the GMM rod
produced magnetostrictive deformation. The deformation output was
located at the output end of the displacement amplification mechanism.
After being amplified, the output displacement signal was sent to the PC
through use of a LVDT micrometer. Temperature controller measured the
temperature of cooling water in the constant temperature water tank
through the temperature sensor. The cooling water, whose function was to
dissipate the heat generated by the drive coil, flowed in a circular
motion between the water cooling cavity and the constant temperature
water tank, so as to ensure the GMM rod would work in the state of
constant temperature. A flux meter was used for measuring the driving
magnetic strength distributed in the GMM rod. In order to reduce the
effect of the external environment on the data measurement, the actuator
was placed on the isolation platform.
3.2. Experiment on the characteristics of output displacement
In order to measure the output displacement of the amplification
mechanism and determine the balance position, the displacement sensor
was located on the output end of the displacement amplification
mechanism, and the output displacement was measured using a micrometer
LVDT. After pre-pressure was applied to the GMM rod, the displacement
amplification mechanism produced deformation due to the reaction force.
The output end produced a certain displacement, which was defined as the
initial displacement.
As a result, the pre-pressure was weighed by the initial
displacement and the greater the initial displacement, the greater the
pre-pressure. To accurately measure the initial displacement, the
determination of the equilibrium position was key, therefore, the
equilibrium position was determined by the LVDT micrometer.
4. Results analysis
The initial displacement was 234 mm and the input current was 0 ~ 4
A to the differential amplification micro actuator. The result of the
output displacement is shown in Fig. 6. The output displacement
increased as the current increased from 0 ~ 4 A. On the contrary, the
output displacement decreased as the current decreased from 4 A ~ 0.
However, in the return process, the output displacement displayed a
hysteresis phenomenon, and could not return to the lift stroke curve
along with the current decrease. The phenomenon is termed the hysteresis
effect.
[FIGURE 6 OMITTED]
e = [max [absolute value of ([y.sub.2i] -
[y.sub.1i])]/max([y.sub.i])] x 100% (3)
Hysteresis can be calculated using the equation shown above. In
which, [y.sub.1] represents output displacement in the lift stroke,
[y.sub.1i] represents output displacement in the lift stroke when the
corresponding current is I, [y.sub.2i] represents output displacement in
the return stroke when the corresponding current is I.
In the situation of current strength changing from 0 ~ 4 A ~ 0, and
when the input current is 1.4 A, the difference of output displacement
between return stroke and lift stroke reaches the maximum, and max
[absolute value of [y.sub.2i] - [y.sub.1i]] = 2.15 [micro]m,
max([y.sub.1]) = 158.4 [micro]m, and calculated by Eq. (3), the
hysteresis e = 13.35%.
The hysteresis is different under the effect of different electric
current strength. When the current changing from 0 ~ 1 A ~ 0, 0 ~ 2 A ~
0, 0 ~ 3 A ~ 0, 0 ~ 4 a ~ 0, the corresponding hysteresis is
respectively 13.35%, 15.76%, 19.22%, 20.6%, the greater the current is,
the more serious the hysteresis phenomenon is. As can be seen from Fig.
7, lift curve is basically coincident together, while the return curve
is different for the reason of hysteresis phenomenon.
[FIGURE 7 OMITTED]
The displacement amplification mechanism, whose amplification
ability determines the maximum output displacement, was the core
component of the differential amplification micro actuator. The
magnification was used as an index for measuring amplification ability.
This experiment was designed to indirectly test the magnification of the
displacement amplification mechanism by measuring the input displacement
and output displacement.
[FIGURE 8 OMITTED]
The GMM rod produced magnetostrictive deformation when the input
current to the differential amplification micro actuator was 0 ~ 4 A.
The relationship between current and input displacement is shown in Fig.
8, a. The input displacement increased with the increasing current, and
under 1 ~ 4 A current strength, the linearity degree of relationship
curve was high enough.
The relationship between the current and the output displacement is
shown in Fig. 8, b. The output displacement increased with the
increasing current, and when current strength was in the range of 1 ~ 4
A, the linearity degree of relationship curve was high enough.
The linearity degree of output displacement and input displacement
were both high enough, therefore could be determined as a linear
relationship, with the magnification of displacement amplification
mechanism being 7.14.
As it can be seen from the Table 1, the bridge amplification is
characterized by a high amplify magnification, but is limited by the
fact that the input displacement and output displacement are in
different directions. The magnification of the lever amplification
mechanism is large enough, but the displacement loss becomes larger when
there is a load. The eight rod symmetric linkage has a larger amplify
magnification compared to the lever amplification, but the structure is
large and complicated. Differential amplification can obtain a larger
displacement output with the relatively small structure; however, the
magnification is not ideal. In this magnetostrictive actuator, the
differential principle and the lever principle are combined to achieve
the largest possible displacement magnification, resulting in the
realization of the synthetic drive, small displacement loss, small
counter force and compact structure.
5. Conclusions
This study designed a new type of differential amplification micro
actuator based on the use of giant magnetostrictive material and
flexible hinges. The maximum displacement can reach up to 158.4 pm, and
the magnification can reach up to 7.14. This experiment not only
enlarges the output displacement of the micro actuator, but also
improves its performance, and therefore leading to the potential of
broad applications. Future research for this study can include ways to
optimize the structure of the displacement amplifier to make it more
precise and compact. Also, research on the characteristics of
micro-displacement amplification actuator under the load action can be
analyzed.
Acknowledgments
The authors would like to acknowledge the financial support by the
National Natural Science Foundation of China (Grant No.51165035).
Science and Technology Fund of Jiangxi Province of Higher Education
(KJLD14094), Youth Science Fund of Jiangxi Province (20133BAB21004), and
the Young scientist cultivation plan of Jiangxi Province
(20112BCB23025).
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Received September 15, 2015
Accepted July 04, 2016
Quanguo Lu, Institute of Micro/Nano Actuation and Control, Nanchang
Institute of Technology, Nanchang, China, 330099, E-mail:
Luqg2010@126.com
Qin Nie Institute of Micro/Nano Actuation and Control, Nanchang
Institute of Technology, Nanchang, China, 330099, E-mail:
1134598717@qq.com,
Xiaoyang Jiang, Institute of Intelligent Manufacturing and Control,
Wuhan University of Technology, Wuhan, China, 430063, E-mail:
18575535576@163.com
Qinghua Cao, Institute of Micro/Nano Actuation and Control,
Nanchang Institute of Technology, Nanchang, China, 330099, E-mail:
QH9863@126.com
Dingfang Chen, Institute of Intelligent Manufacturing and Control,
Wuhan University of Technology, Wuhan, China, 430063, E-mail:
cadcs@126.com
http://dx.doi.org/10.5755/j01.mech.22.4.16166
Table 1
Comparison of displacement amplification mechanisms
No. Name Affiliation
or Author
1 Micro-displacement Northwestern
Amplifier for GMA Polytechnical
[14] University
2 Micro-displacement Henan Coal
Amplifier for Scientific Research
Piezoelectric Institute Co., Ltd.
Bimorph [15]
3 Mechanically Juuti J, Kordas K
amplified large and Lon-nakko R
Displacement
Piezoelectric
Actuators [16]
4 Flexible Hinge China Shipbuilding
Displacement Industry
Amplification Corporation(CSIS)
Mechanism [17]
5 Differential This article
Displacement
Amplification
Mechanism
No. Amplify Principle Amplify
Magnification
1 Lever Principle 3.8
2 Eight Rod Symmetric 6.2
Linkage
3 Bridge Amplification 16~20
4 Differential 1.8
Amplification
5 Differential and 7.14
Lever Principle