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Experimental Evaluation of New Magnetic Movement Converter for Linear Oscillatory Actuator

Fumiya KITAYAMA*1, Katsuhiro HIRATA*1, Noboru NIGUCHI*1 and Tatsuro YAMADA*1

In this paper, we propose a new magnetic movement converter for linear oscillatory actuators. The proposed magnetic movement converter can convert a rotational motion into a linear motion, and reduce the load torque on a rotor, and the armature is oscillated with fewer harmonics. First, the basic construction and the operation principle are described. Next, we confirm its characteristics through FEM analysis and measurement on prototypes.

Keywords: Linear oscillatory actuator, magnetic movement converter, load torque reduction, harmonics reduction, FEM. (Received: 24 July 2014)

1. Introduction

Recently, active control engine mounts (ACMs) employing linear oscillatory actuators have been mounted between an engine and an automobile frame to reduce a frame vibration. There are many requirements for linear oscillatory actuators: high thrust, low friction losses, low manufacturing costs, low noises and small size, and various kinds of linear oscillatory actuators have been developed [1-4].

In order to reduce the costs, we proposed a simple linear oscillatory actuator using a DC motor and a magnetic movement converter (MMC). The MMC is a device converting a rotational motion into a linear motion, as shown in Fig. 1 [5,6]. The linear oscillatory actuator oscillates when the DC motor rotates. And, the linear oscillatory actuator can be cheaply manufactured because of the use of the DC motor. However, this actuator had two problems: large load torque and harmonics in the actuator’s oscillation. If a load torque on the DC motor is large, the size of the DC motor has to be increased accordingly, causing an increase in the size of the overall system. Also, due to the harmonics in the actuator’s oscillation, new vibration frequencies will be inadvertently added to the automobile frame. And, these� problems are caused by the MMC.

In this paper, these problems of the conventional MMC are evaluated, and a new MMC are proposed [7].�And, the characteristics of MMCs are verified through FEM analysis and measurement on prototypes in order to clarify the effectiveness of the proposed MMC.

2. Basic Magnetic Movement Converter

As shown in Fig. 1, a conventional MMC which we

first proposed (basic MMC) is composed of an armature, a rotor and springs. The rotor and the armature are faced with an initial air-gap length. The armature is composed of a 4-pole permanent magnet with a yoke and an output shaft, and the output shaft is connected to the springs. The armature is fixed so that it cannot rotate but can only move in a linear direction. The rotor is also composed of a 4-pole permanent magnet with a yoke, and it is connected to a shaft of a motor.

Fig. 2 shows the operational principle of the basic MMC. The armature reciprocates in a linear direction by rotating the rotor due to attractive and repulsion forces between the permanent magnets as shown in Fig.2 (a). However, large load torques is also generated in the

Armature

Output shaft

Magnets with Yokes

Springs

Rotor

Fig. 1. Basic MMC.

SN

S N

S N

S N

SN

Load torques

SN

S N

S N

� zRotor Armature

ThrustAttractive force

SN

S N

S N

+�+ �

� +� +

(a)When thrust is (b) When load torque generated. is generated.

Fig. 2. Operational principle of the basic MMC.

_______________________ Correspondence: F.KITAYAMA, Department of Adaptive

Machine systems, Graduate school of Engineering, OsakaUniversity, Yamadaoka 2-1, Suita, Osaka, Japan email: fumiya.kitayama@ams.eng.osaka-u.ac.jp

*1 Osaka University

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motor when the angle difference between armature magnetic poles and rotor magnetic poles is 45 degrees as shown in Fig.2 (b). In the basic MMC, the air-gap length is changed when the armature is moved, and the thrust is also changed. Then, the thrust variation against the armature position affects the oscillation, and the oscillation contains harmonics.

3. New Magnetic Movement Converter

Fig. 3 shows the newly proposed MMC. The MMC is composed of an armature, a rotor and springs. The armature is placed inside the rotor. At an initial position, the air gap length between the armature and the rotor is 2 mm each. The armature is composed of a 4-pole permanent magnet and an output shaft as shown in Fig. 3 (b), and it is fixed so that it cannot rotate. The output

Springs

ArmatureRotor

(a) Overall.

s

Magnet

Output shaft

(b) Armature.

MagnetsNon-magnetic material

Yokes

Rotor A Rotor B

(c) Rotor. Fig. 3. Newly proposed actuator.

shaft is fitted with the springs. The rotor is composed of two sets of 4-pole permanent magnets, yokes and non-magnetic material. The rotor permanent magnets are connected to each other through the non-magnetic plates as shown in Fig. 3 (c). The rotor is directly connected to the shaft of a motor.

Outer diameters of permanent magnets and yokes are 18 mm, and inner diameters of permanent magnets and yokes are 10 mm, and heights of permanent magnets and yokes are 3 mm.

Fig. 4 shows the operational principle of the proposed MMC. This principle is the same as that of the basic MMC. In the proposed MMC, load torques on the motor are reduced because torques generated in the rotor A and B are cancelled with each other, as shown in Fig. 4 (b). Also, a total air-gap length of the proposed MMC is constant regardless of the armature position. From this, a constant thrust with respect to the armature position is obtained, and harmonics in the oscillation are also reduced.

4. Evaluation by Analyses

In order to verify the effectiveness of the proposed MMC, the static and dynamic characteristics are investigated through FEM analysis.

First, Static thrusts and static load torques were computed by 3-D FEM [6]. Fig. 5 shows the computed

� z

Rotor(Rotor A)

SN

S N S N

SN S N

SN

�

Rotor(Rotor B)

S N S N SN

Attractive force Repulsive forceThrust

Armature

+� + �+ �

+�

(a) When a thrust is generated to an armature.

SN

S N

S N

S N

SN

S N

SN

Load torques� +

+ �

(b) When load torque are reduced. Fig. 4. Operational principle of the proposed MMC.

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static thrusts and load torques against the rotor angle when the armature position is fixed. As we can see in Fig. 5 (a-i) and (b-i), the thrust of the basic and proposed MMCs at the initial armature position are almost the same at about 40 N. Additionally, the load torque of the proposed MMC is close to 0 although that of the basic MMC is 150 mNm. This is because the

Table 1 Specification of prototype of basic MMC. Spring constant 61.4 N/mm Mass of armature 85.3 g Viscous damping coefficient 23.0 Ns/m

Table 2. Specification of the proposed MMC. Spring constant 58.3 N/mm Mass of armature 82.6 g Viscous damping coefficient 3.6 Ns/m

-160

0

160

-60

0

60

0 45 90 135 180 225 270 315 360

Thrust Load torque

Angle, degree

Thru

st, N

Torque, m

Nm

(a-i) The armature position is 0mm.

-160

0

160

-60

0

60

0 45 90 135 180 225 270 315 360

Thrust Load torque

Angle, degree

Thru

st, N

Torque, m

Nm

(a-ii) The armature position is 1mm. (a) The basic MMC.

-160

0

160

-60

0

60

0 45 90 135 180 225 270 315 360

Thrust Load torqueLoad torque (only rotor A) Load torque (only rotor B)

Angle, degree

Thru

st, N

Torque, m

Nm

(b-i) The armature position is 0mm.

-160

0

160

-60

0

60

0 45 90 135 180 225 270 315 360

Thrust Load torqueLoad torque (only rotor A) Load torque (only rotor B)

Angle, degree

Thru

st, N

Torque, m

Nm

(b-ii) The armature position is 1mm. (b) The proposed MMC.

Fig. 5. Static characteristics vs. rotation angle.

torques of rotor A and B cancel each other out. In comparison with Fig. 5 (a-ii) and (b-ii), the thrust of the proposed MMC are higher than that of the basic MMC, and the load torque of the proposed MMC is smaller that of the basic MMC. However, in the proposed MMC, the load torques on rotors A and B at shifted armature positons do not completely cancel each other out and the load torque on a motor generates because the load torques on rotors A and B are not the same with each other.

The static torques became maximum when the rotor angle was 45 degree, and the static thrusts became maximum when the rotor angle was 90 degree because magnets are four poles. Fig. 6 shows the maximum load torque and the maximum thrust against the armature position. As we can see in Fig. 6, the load torque of the proposed MMC is reduced by 165 mNm (maximum value) compared with that of the basic MMC at all armature positions. Also, the thrust variation against the position of the proposed MMC is reduced by 80% against that of the basic MMC, because a total air-gap length of the proposed MMC is constant.

A dynamic analyses using Matlab/Simulink and 3-D FEM analysis was conducted to investigate the dynamic characteristics of these MMCs [8]. In analyses, an equat ion of mot ion is ca lcula ted through Matlab/Simulink when the rotor is rotated at a constant speed of about 1800rpm. In this calculation, the thrusts and the load torques of MMCs are obtained using the

0

150

300

-1 -0.5 0 0.5 1

BasicProposed

Position , mm

Torq

ue ,

mN

m

(a) Maximum torque.

0

50

100

-1 -0.5 0 0.5 1

BasicProposed

Position , mm

Thru

st, N

(b) Maximum thrust. Fig. 6. Static characteristics vs. armature positon.

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static characteristics calculated by 3-D FEM. The specifications of the basic and proposed

actuators are shown in Tables 1 and 2. Results of the dynamic analyses are shown in Fig. 7.

As shown in Fig. 7 (a) and (b), the maximum load torque of the proposed MMC is reduced by 59 %, and it is much smaller than the basic MMC. And, harmonics of the proposed MMC was less than the basic actuator because the proposed MMC has a nearly constant thrust with respect to the position.

5. Experimental Evaluation

5.1 Prototype

Prototypes of the basic MMC and proposed MMC

-60

0

60

-0.025 0 0.025

Thru

st ,N

Time, s

BasicProposed

(a) Thrust.

Posi

tion

,mm

Time, s

BasicProposed

-1.5

0

1.5

-0.025 0 0.025

(b) Armature position.

-150

0

150

-0.025 0 0.025

Torq

ue ,m

Nm

Time, s

BasicProposed

(c) Load torque. Fig. 7. Dynamic characteristics.

are manufactured in order to evaluate the characteristics by carrying out measurements. Fig.8 and 9 show our prototypes. These are mainly composed of armatures, rotors, stators and springs. Armatures are composed of

ArmatureRotor SpringsStator

BearingsLinear bearing

(a) Overall. Magnet with Yoke

PTFE partOutput shaft

(b) Armature.

Magnet with YokeShaft

(c) Rotor. Fig. 8. Prototype of the basic MMC.

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ArmatureRotor SpringsStator

BearingsLinear bearing

(a) Overall. Magnet

PTFE partOutput shaft

(b) Armature.

Magnets with Yokes

Shaft

(c) Rotor. Fig. 9. Prototype of the proposed MMC.

permanent magnets, yokes, output shafts and polytetrafluoroethylene parts (PTFE parts), and there are supported by linear bearings. PTFE parts of armatures are a square shape and slide in square shaped stators. Rotors are composed of permanent magnets, yokes and shafts, and there are supported by bearings. Shafts of rotors are connected to those of motors. A resonance frequency of the basic MMC is 133 Hz, and that of the proposed MMC is 135 Hz.

5.2 Experimental Setup An experiment setup is shown in Fig. 10. First, static

load torques and static thrusts were measured by a torque meter and a load cell, respectively, the position and the angle are measured by a displacement meter and a controller. Second, the dynamic armature positon is measured by a displacement meter, when the rotor of the MMC is rotated by the motor at about 1800rpm.

5.3 Experimental Results

Fig. 11 shows the static load torques and thrusts against the rotor angle when the armature positon is 0.5 mm. In the proposed MMC, the maximum load torque was 47 mNm and the maximum thrust was 41 N. The load torques of the proposed MMC is smaller than that of the basic MMC, as same as analyzed results. This is because the torques of rotor A and B cancel each other out in the proposed MMC.

Fig. 12 shows the maximum load torque and the maximum thrust against the armature position. As we can see in Fig. 12, the maximum load torque of the proposed MMC is lower, and the maximum thrust of the proposed MMC was more constant with respect to the armature position than that of the basic MMC, as same as analyzed results. This is because a total air-gap length of the proposed MMC is constant.

Next, the armature position was measured when the rotor was rotated by the motor. The results are shown in Fig. 13. In the proposed MMC, the amplitude is 1.6 mm and the drive frequency is 60 Hz, and harmonics in the oscillation of the proposed MMC is smaller than those of the basic MMC because the static and maximum thrust of the proposed MMC was nearly constant with respect to the armature position, as same as analyzed results.

Finally, the measured results almost agreed well with the analyzed results. Errors between the measured and the analyzed results are due to manufacturing errors of prototypes.

Motor Torque meter MMC Displacementmeter

Fig. 10. Experiment setup.

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Thru

st ,N

Angle, degree

Torque, mN

m

Torque (Basic)Torque (Proposed)Thrust (Basic)Thrust (Proposed)

Fig. 11. Static characteristics vs. rotation angle.

0

70

140

-1 0 1

BasicProposed

Position , mm

Torq

ue ,

mN

m

(a) Maximum torque.

0

50

100

-1 0 1

BasicProposed

Position , mm

Thru

st, N

(b) Maximum thrust. Fig. 12. Static characteristics vs. armature positon.

5. Conclusion

In this paper, we proposed a new MMC that an armature is placed inside a rotor. From the analysis and the measurement, we could clearly see that the proposed MMC has lower load torques, more constant thrusts against the armature position and fewer harmonics.

From these results, it could think that a linear oscillatory actuator using the new MMC and a dc motor is effective to vibration control devices because the

�

-1.5

0

1.5

-0.025 0 0.025

Posi

tion

,mm

Time, s

BasicProposed

Fig. 13. Dynamic characteristics.

actuator can oscillate with fewer harmonics. And, in the actuator, we could use a low-power and small dc motor because the new MMC did not generate high load torques.

In the future, we will manufacture a new actuator by connecting the new MMC with a dc motor, and apply this actuator to vibration control devices.

References

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[2] H. Matsuoka, T. Mikasa and H. Nemoto, “NV counter-measure technology for a cylinder-on-demand engine - Development of active control engine mount,” SAETransactions 2004-01-0413, 2004.

[3] B. H. Lee and C. W. Lee, “Model Based Feed-forward Control of Electromagnetic Type Active Control Engine-Mount System,” Journal of Sound And Vibration.,Vol.323, pp.574-593, 2009.

[4] F. Kitayama, K. Hirata, and M. Sakai, “Proposal of a Two Movers Linear Oscillatory Actuator for Active Control Engine Mounts,” IEEE Trans. Magn., Vol.49, No.5, pp.2237-2240, 2013.

[5] T. Yamaguchi, Y. Kawase, S. Suzuki, K. Hirata, T.Ota, and Y.Hasegawa, “Dynamic Analysis of Linear Resonant Actuator Driven by DC Motor Taking into Account Con-tact Resistance Between Brush and Commutator,” Trans.IEEE Trans. Magn., Vol.44, No. 6, pp.1510-1513, 2008.

[6] T. Ota, K. Hirata, T. Yamaguchi, and Y. Kawase, “New Linear Oscillatory Actuator Using DC Motor,” Trans.IEE Jpn., Vol.126-D, No. 8, pp.1156-1160, 2006.

[7] F. Kitayama, K. Hirata, M. Sakai and T.Yamada, “Linear Oscillatory Actuator Using New Magnetic Movement Converter,” in Proceedings of IEEE International Con-ference on Mechatronics and Automation 2013, pp.431-436, 2013.

[8] K. Hirata, Y. Ichii, and Y.Arikawa, “Linear oscillatory actuator with dynamic vibration control,” Trans. IEE Jpn.,Vol.122-D, No. 4, pp.519-351, 2002, (in Japanese).