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INSTITUTE OFPHYSICSPUBLISHING SMARTMATERIALS ANDSTRUCTURES

Smart Mater. Struct.13(2004) N1N6 PII: S0964-1726(04)79018-7

TECHNICAL NOTE

Multi-DOF and sub-micrometerpiezoelectric-electrorheological steppermotor

Xiangcheng Chu1, Hongyun Qiu, Longtu Li and Zhilun Gui

Department of Materials Science and Engineering, Tsinghua University, Beijing 100084,

Peoples Republic of China

E-mail: [email protected]

Received 30 July 2003, in final form 11 February 2004PublishedOnline at stacks.iop.org/SMS/13/N1DOI: 10.1088/0964-1726/13/0/N00

AbstractA new type of piezoelectric-electrorheological plane stepper motorcombining the piezoelectric effect with the electrorheological effect isproposed in this paper. Four electrorheological clampers and four multilayerpiezoelectric actuators are designed in the prototype motor. Based on abionic inchworm movement mechanism, when these electrorheological

clampers are combined with piezoelectric actuators in different ways, themovements in thex-direction, the y-direction andz-rotation with a longtravel stroke of 100 and 0.36m resolution can be completed. Themaximum moving speed and driving force of the prototype motor are1.8 mm min1 and 100 gf, respectively. The steady stepper velocity andinstant motion image are measured by a CCD optical measuring systemfrom 0.2 to 23m s1. The motor may be applied in fields such as MEMs,optical manipulator, manipulator in SEM or STM, laser adjustor,micromachining, etc.

Ascii/Word/SMS/sms179018/TECPrinted 8/5/2004

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1. Introduction

Many inchworm actuators have been proposed and investi-

gated. They mostly use piezoelectric material to generate anexpansion to move themselves, but generally utilize differentcontact and clamping mechanisms, such as an inverse piezo-

electric effect, electromagnetic force, electrostatic force andelectrorheological effect. They have different merits:

(a) Electrostatic force is suitable for smaller size devicesbut the contact force is much smaller than in other

mechanisms.(b) Electromagnetic force may generate a larger contact force

but it also yields electromagnetic noise.

(c) Piezoelectric contact force is suitable for smaller sizeand larger force, and can be simply controlled using DCvoltage.

1 Author to whom any correspondence should be addressed.

Moreover, a stepper motor combining the piezoelectric andelectrorheological effects, proposed firstly in 1992 [1], hasshown some unique advantages, such as no friction, no opera-

tional noise, large travel, high resolution, etc. Up to now, therehasbeen much work on such linear or rotarymotors [26]. But,most of them focus on a one-dimensional freedom of motion.

This paperproposes a newpiezoelectric-electrorheological(PE) stepper motor with multi degrees of freedom (x-, y-direction, z-rotation) and sub-micrometer resolution. It useselectrorheological fluid as grippers to eliminate contact noiseand, at the same time, the precise motion attributes to an in-verse piezoelectric effect of multilayer piezoelectric actuators.These devices can be used as an objective stage with a high

resolution motion in the fields of precise instruments, biolog-ical analysis, etc. The structure and operation principles ofthis motor will be introduced, and its characteristics and mo-tion behaviour are investigated experimentally through a CCDoptical system.

0964-1726/04/000001+06$30.00 2004 IOP Publishing Ltd Printed in the UK N1

http://www.paper.edu.cn


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Technical Note

ER fluid Base

Elastic body

Clamping plate Piezoelectric actuator P1

C1 C2

C3C4

P2

P4

P3

Figure 1.The schematic configuration of the multi-DOF PE motor.

2. Structure and operation principle

Electrorheological (ER) fluids are liquids that can be

transformed from the liquid state to approximately the

solid state under an external electric field; it is called the

electrorheological effect and this transition process can be

reversed. The ER fluids are high insulating liquids and a slurry

of solid particles with polarity effect. When a sufficient strong

electricfield is used across them, the dispersed particles in the

fluids can form particle chains or fibres to resist shear force,

and thefluid shows solid-like properties.

The multi-DOF PE motor consists of four actuatorsP1, P2, P3 and P4 based on multilayer piezoelectric material

to produce a stretched or contracted force, and subsequently

generate a linear displacement along the x- or y-direction.

Four electrode plates C1, C2, C3 and C4 are used as electrical

terminals to generate a high electric field to form a clamping

force between the multi-DOF PE motor and the base plate,

and ER fluids are filled between the motor and the base

plate connected to GND. Four electrode plates are joinedQ.1

to each other through a square elastic frame, as shown in

figure 1. Here, the piezo-actuators are driving elements of

the multi-DOF PE motor. There are several small insulators

under the electrode plates as short-circuit protection. When

a high voltage is applied to any electrode plate, the filled ER

fluids under this electrode plate will enter a solid-like state,

namely a clamping state. Otherwise, when the voltage is

removed from the electrode plate, the filled ER fluids will

return to a liquid state, namely a free state. The dimensions

of the device, outside the four piezoelectric actuators, are

454510mm3 (Siemens Inc.). Theelastic frame is stainless

steel and manufactured using a linear cutting machine. The

four multilayer piezoelectric actuators are bonded inside the

elastic frame. The photo of the prototype motor is shown in

figure 2.

Under the control of the computer system, the four piezo-

actuators and four ER clampers of the prototype motor canoperate according to different motion modes, such as the x-

direction,y-direction andz-rotation.

The multi-DOF PE motor travels along the x- or y-

direction with a linear motion mode, shown in figure 3. For

Figure 2.Photograph of the multi-DOF PE motor.

(1) (2)

y

y

y

Figure 3.Motion principle in the y -direction of the multi-DOF PEmotor.

example, when it walks along the y-direction, the operation

steps are:

(1) The ER clampers C3 and C4 are activated under a high

voltage (10003000V mm1)tomaketheER fluids under

plates C3 and C4 enter a clamping state. This is followed

by an extension of the piezo-actuator P2 and P4 to push

the electrode plates C1 and C2 upward.

(2) The ER clampers C1 and C2 enter a clamping state by

supplying an electric field. At the same time, the ER

clampers C3 and C4 are freed by removing an electric

field, and piezo-actuators P2 and P4 shortened to restore

their original length. This also generates a force to pull

the clampers C3 and C4 upward. So the motor can walk

upward by a small distance y, which is equal to the

displacement of the piezo-actuator.

As the operation mentioned above is repeated, the motor

will move upward continuously. If the operation sequence

is reversed, the motor will travel downward.

For the movement between two random points, for

example, if the multi-DOF PE motor travels from the first

point (x0,y0)to the second point(x,y), there are generallytwo simple methods. For thefirst method, the motor travels

from (x0,y0)to (x,y0)in the x-direction, subsequently, it

travels from (x,y0)to (x,y) to complete the whole route.

For the second method, the motor walks along a small zigzag

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Technical Note

2

1

Y

(x0, y0)

(x, y)

X

Figure 4.Zigzag motion of the multi-DOF PE motor between anytwo points.

(1) (2)

y

Figure 5.The principle of rotation motion of a multi-DOF PEmotor.

trajectory to form an approximate linear route, namely

(x0,y0) (x0 +x,y0) (x0 +x,y0 +y)

(x0 + 2x,y0 +y) [x0 +nx,y0

+(m 1)y] [x0 +nx,y0 +my]

wherex= x0+ nx,y = y 0+my,and nand mare integers.

Ifnand mare sufficiently large, this zigzag trajectory will

be similar to a straight line, see figure 4. The whole motion

procedurecan be controlledandeach step of thezigzagmotion

can be adjusted by a computer and power amplifier.

In addition, through justifying the time sequence of the

driving voltages for the clamping plates and piezo-actuators,

the PE motor can also rotate at its central point. The operating

sequence is:

1, (C1, P1, C2); 2, (C2, P2, C3);

3, (C3, P3, C4); 4, (C4, P4, C1).

As in the above steps, each motion combination is

regarded asa linearmotion. If thefour piezo-actuatorsproduce

continuous clockwise or counterclockwise displacement, the

motor can make a clockwise rotation or counterclockwise

rotation. Figure 5 shows the principle of rotation motion of

a multi-DOF PE motor.

Besides the above motion modes, the piezo-actuator P1

contracts and then extends, whereas the other P3 operates in a

reverse way, that is, it extends and then contracts. The piezo-

actuatorsP2 and P4 stay free thewhole time. These operations

will result in the turning of the motor towardsthe +ydirection,

Clamping

position

(2)(1)

Clamping

position

Figure 6.The principle of turning motion of the multi-DOF PEmotor.

Driving circuit

Computer

Control system

Video collector

Computer

Video data encoder

Light source

Focusing unit

CCD

Pattern recognition

Trajectory analysis

microscope

Optical resolution 1 m, image rate 30 Frames/s

PE motor

Figure 7.CCD optical measuring system to analyse the motioncharacteristics of the motor.

seefigure 6. If the time sequence of the driving voltage for

the piezo-actuators P1 and P3 is reversed, the motor will turntowards the y -direction.

3. CCD system and image decode principle

The motor was observed using a CCD optical system to

measure the video image of multi-DOF motion. This is a non-

contact measurement method, which can track and describe

the plane motion trajectory of the motor. Figure 7 illustrates a

CCD optical measuring system used to analyse the motion

characteristics of the motor. This optical system consists

of a head-up objective lens, lens cone, prism, light source,

condensing unit, CCD, video collector and computer. Theoptical unit is similar to the reflected pattern microscope, in

which the moving image of the motor can be reflected into the

CCD and then transformed to electric signals. Subsequently,

these signals are obtained by the video collector, and analysed

by the computer to form a video image and datafile for further

analysis.

An identifiable mark needs to be put on the surface of the

motor to measure linear motion, while two marks for rotation

motion. First, the motion image of the motor is amplified

onto the CCD, and then amplified onto the display screen of

the computer. For these two amplification procedures, the

enlargementfactorof theoptical systemis theproductbetween

the enlargement factor of the objective lens and the ratio of

computer screen/CCD screen, e.g. for objective lens of45

and CCD of 1/3 feet, the whole enlargement factor is 2025.

The operating rate of the image obtained by the optical system

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Technical Note

FRAME1 00:00:00:01 FRAME2 00:00:00:04 FRAME1 and 2 and 3FRAME3 00:00:00:07

FRAME2 00:00:00:07FRAME1 00:00:00:01 FRAME 1 and 2

(b)

(a)

Figure 8.Trajectory description of the PE-motor. (a) Trajectory description of the mark points of a pure linear motion. (b) Rotational angledescription of the mark points of a rotary or turning motion.

is 30 frames s1. When a corrective scale plane with a 5 m

grating is placed under the CCD system, points with 1 m

distance can be identified on the computer screen. It is enough

for a PE motor with a 10 Hz driving frequency, and its movingtrajectory can be observed.

Due to the AVI or BMP file format and around 2030

frames storage, managing these massive data is difficult. To

overcome this difficulty, the approach to pattern recognition

should be applied to track the motion trajectory. For a linear

or zigzag motion from any point to another point, only one

mark is required on the image. Through comparing the mark

points, each frame of themotion trajectory canbe described, as

shown infigure 8(a). But for the rotation and turning motion,

two marks on the motor are necessary. To evaluate the slope

ratio of the motion trajectory on each frame, the turning angle

can be obtained, as shown infigure 8(b).

Figure 9 shows the gripped transient images during themotion procedure, that is (a) (b) (c). The reference

object is an electronic chip on the surface of the moving motor.

So the gripped images of the electronic chip is shown on the

computer screen. According to the recognition principle in

figure 8, only one mark inside a small region on the chip can

be tracked to obtain its motion resolution. The direction of the

motion is shown using an arrow mark in the figure.

4. Experiments

Using the CCD optical system, the motion characteristics

of the multi-DOF PE motor were investigated. The piezo-actuators used in the motor consist of multilayer piezoelectric

material and electrode material in parallel, and its outer size is

5 5 20 mm3 (supplied by Siemens Co.) with an allowable

voltage range of 0100 V DC. The size of each ER clamping

plate is 3030mm2, and a maximum static clamping force of

1 kgfcanbe obtainedwhen theER clampingplate isactivated

by a high voltage.

The electromechanical characteristics of the piezo-

actuators have a great impact on the velocity and motion

stability of the motor. To obtain its displacement, the tough

needle of a micrometer gauge is pressed onto one side of the

piezo-actuator and the other side is bonded to a base. The

dynamic behaviourof the piezo-actuator is shown in figure 10.

It can be seen that the displacement of the piezo-actuator

is a function of the duty ratio of the impulse, amplitude of

driving voltage and operating frequency. In figures 10(a)

(a) Frame 1

(b) Frame 2

(c) Frame 3

Figure 9.Three frames of gripped images using the CCD opticalmeasuring system.

and (b), the displacement varies with driving frequency, and

the maximum displacement is 28 m under 2.6Hz and 24 m

under 5.2 Hz. Fromfigures 10(c) and (d), there is a great

difference under free and preload boundary conditions. In

order to obtain a larger output force, the piezoelectric stack

actuators are sandwiched inside the framed structures, which

supply a pressure as a preload. For a free boundary condition,

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Technical Note

10 20 30 40 50 60 70 80

0

5

10

15

20

25

30

f=2.6Hz

Displacement(m)

Voltage(V)

10 20 30 40 50 60 70 80

0

5

10

15

20

25

f=5.2Hz

Dis

placement(m)

Voltage (V)

0 10 20 30 4 500

0

5

10

15

20

25

30

35

40

Displacement(m)

Frequency(Hz)

0 5 10 15 20 25 30 35 4068

10

12

14

16

18

20

22

24

26

Displacement(m)

Frequency (Hz)

(a)

(b)

(c)

(d)

Figure 10.Dynamic behaviour of the piezo-actuator used in themulti-DOF PE motor. (a) Duty ratio 10.5%, frequency 2.6 Hz. (b)Duty ratio 10.5%, frequency 5.2 Hz. (c) 76.0 V, duty ratio 50% andfree boundary condition. (d) 72.8 V, duty ratio 50% and preloadcondition.

the maximum displacement of 36 m can be obtained at a

frequency of 18.2 Hz under 76 V and 50% duty ratio. In

contrast, for a preload boundary condition, the maximum

displacement of 24.5 m can be obtained at a frequency of

23 Hz under 72.8 V and 50% duty ratio.

The multi-DOF PE motor was measured under the

following conditions: 76.0 V for the piezo-actuator, 300 V for

the ER clampers, travel stroke 100 m. Figure 11 shows the

2 4 6 8 10 12 14 16

4

8

12

16

20

24

Velocity

Displacement/step

Frequency (Hz)

Velocity(m/s)

0

1

2

3

4

Displacement/step(m)

Figure 11.Velocity and displacement/step versus frequency of themulti-DOF PE motor.

2 4 6 8 10 12

0

10

20

30

40

50

60

Load(gf)

Velocity (m/s)

1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0 2.1

0

10

20

30

40

50

60

Load(gf)

Velocity (m/s)

(a)

(b)

Figure 12.Load versus velocity under 76 V for the piezo-actuatorand 300 V for the ER clamps. (a) Frequency 3.5 Hz. (b) Frequency10 Hz.

average results of speed and displacement/step characteristicsof the multi-DOF PE motor. It can beseen that the velocityand

displacement per step are a function of the driving frequency.

The series of curves illustrate that there is an optimum value

of frequency required to obtain a maximum velocity. In

this case, it is found to be 7.6 Hz for a maximum velocity

23 m s1. The measured frequency band of the motor is

from 2.3 to 14.8 Hz. The displacement per step decreases

with the frequency increase. There is a gentle decrease below

7.8 Hz, whereas there is a large decrease above this value.

The displacement per step is 3.22 m at driving frequency

of 2.3 Hz, and 0.36 m at driving frequency of 14.8 Hz.

Moreover, it verifies that ERfluids have a slow response timeand the velocity of the PE motor is limited by the frequency

response of the ERfluids. To obtainexcellentcharacteristicsof

the PE motor, the ERfluids performance should be improved

in the future.

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Technical Note

Furthermore, the relationship between load and velocity

is shown infigure 12, which plots no load against velocity for

driving frequencies of 3.5 and 10 Hz. It is shown that the load

is 53 gf at a frequency of 3.5 Hz and velocityof 3m s1, and

the same value of 53 gf at a frequency of 10 Hz and velocity

1.32m s1.

The piezo-actuator can obtain maximum velocity at itsresonant frequency. However, due to the ER fluids, the

operating frequency of the multi-DOF PE motor differs from

that of the piezo-actuator. Due to the absorption of resonant

vibrationsof piezoelectric actuators, theER fluids will weaken

the resonant behaviour of the motor.

5. Summary

A planar stepper motor combining the piezoelectric effect

with the electrorheological effect has been developed. This

motor prototype can walk in both the x- and y-directions

with large travel and high resolution of less than 0.36 m

on the base plate. The speed of the motor can be controlledby an input voltage applied to piezo-actuators and operational

frequency. The ERfluid is a key functional material for the

motor. The maximum moving speed and driving force of the

prototype motor is 1.8 mm min1 and 100 gf, respectively.

The steady stepper velocity and instant motion image are

measured by a CCD optical measuring system from 0.2 to

23 m s1. The ERfluids with quicker response to electric

field and higher yield strength should be developed in the

future. The motor can be applied in thefields such as MEMs,

optical manipulator, manipulator in SEM and STM, laser

adjustor, micro machining, etc.

Acknowledgments

Theauthors aregrateful forthe financial support of theNational

Natural Science Foundation of China, grant no. 50235010and appreciate Dr Shuxiang Dong for initial work on the

configuration and principles of the multi-DOF PE motor.

References

[1] Dong S X and Li L T 1992 A piezoelectric-electrorheologicallinear stepper motorChinese Patent Specification92105232.4

[2] Maruyama M, Nakamura K and Ueha S 1995 Ultrasonic motorusing electrorheologicalfluidReport of the Meeting of theAcoustical Society of Japanp 1063

[3] Maruyama M, Ueha S and Nakamura K 1995 Improvement inthe characteristics by modifying the structure and thematerial,-ultrasonic motor using electrorheological fluid (2)Report of the Meeting of the Acoustical Society of Japanp 1145 Q.2

[4] Maruyama M 1996Ultrasonics342614[5] Kay E W C and Portington E C Design characteristics of a

piezoelectric/ERfluid motorProc. 2nd Int. Conf. onMechtronic & Machine Vision in Practice M/SUP 2/VIP.95p 175 Q.3

[6] Dong S X and Li L T 1995 A new type of linear piezoelectricstepper motorIEEE Trans. Compon. Packag. Manuf.Technol.A18 Q.4

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Queries for IOP paper 179018

Journal: SMS

Author: X Chu et al

Short title: Technical Note

Page 2

Query 1:Author: Please expand GND

Page 6

Query 2:

Author: [3] Article title correct?

Query 3:-

Author: [2, 3, 5]: Any more details?

Query 4:-

Author: [6]: Please provide pagenumber.


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