single phase flux switching dc linear...

Journal of Engineering Science and Technology EURECA 2013 Special Issue August (2014) 28 - 39 © School of Engineering, Taylor’s University

28

SINGLE PHASE FLUX SWITCHING DC LINEAR ACTUATOR

ARAVIND CV1,* , KHUMIRA I.

1, R. N. FIRDAUS

2, FAIRUL A.

3

School of Engineering, Taylor’s University, Taylor's Lakeside Campus,

No. 1 Jalan Taylor's, 47500, Subang Jaya, Selangor DE, Malaysia 2Power Electronics and Drives, University Teknikal Melaka, Malaysia

*Corresponding Author: [email protected]

Abstract

A number of variable speed applications require linear actuation such as in linear

compressors, drilling and cutting applications. In this research a flux switching

DC induction actuator with a feedback sensor for operation in closed loop

condition is proposed. The appropriate choice of the permanent magnet

dimensions improves the performance of the PM machines. The linear induction

actuator is chosen by the variations in the height to width of the permanent

magnet through Finite Element Methods. Analysis on the effect of changing the

magnet ratio on the torque and the cogging force is reported in this work. The flux

switching between the adjacent poles is achieved through a control drive circuitry

based on the feedback signal from the sensor. The static characteristic of such a

machine is presented together with the experimental results.

Keywords: linear actuator, Hall sensor, DC machine, Flux switching, Height to

width ratio, Permanent magnet

1. Introduction

Linear actuators are machines that develop the linear force along its length (linear

thrust) [1, 2]. Linear motor covers a wide range of applications from transportations

[3] to its utilization in medical field such as linear actuator syringe pump. Linear

motor is preferred compared to rotary motor due to the relative speed accuracy,

better positioning, and fast acceleration response characteristics [2-3]. This motor

eliminates the mechanical components used to convert the rotation as it requires no

mechanical transmission thereby reducing the flux density. Flux switching

technique is whereby the flux in the coil is switched by varying the operation

through control element thereby changing the direction of the operation through the

feedback element. The control element significantly controls the electro-magnetic

energy inside the machine thereby increasing the thrust. A high power control

Single Phase Flux Switching DC Linear Actuator 29

Journal of Engineering Science and Technology Special Issue 8/2014

module is essential for these types of machines that derive a feedback signal and

then decide the switching pattern.

In this paper the developed linear machine is analysed using Finite Element

methods to choose the best ratio of permanent magnet ratio and then it is analysed

for the static thrust characteristics.

2. Methodology

2.1. Principles of the linear actuator

Linear Induction Actuators are used for short travel and thus their mechanical air-

gap is about 1 mm [3]. Figure 1 shows the flux flow of HDLM with positive,

negative and zero current.

In the unexcited condition, Fig. 1(a), the upper section of the permanent magnet

creates a path for the flux flow like a short circuit in an electrical circuit, so that the

most of the flux flow of the permanent magnet pass through this path and a smaller

flux flow cross from air gap. Whereas the cogging force arises from flux flow of

unexcited condition, in this structure the cogging force is very low. Figures 1(b) and

(c) show the flux flow under the excited condition. With the applied magnetic force

created by the coil energization, the flux of permanent magnets and that of the coils

cross in the air gap. The direction of thrust depends on the direction of magnetic force

developed as shown in Fig. 1(b). The change of the flux direction due to the reverse

polarity excitation, changes the direction of the thrust as shown in Fig. 1(c). The

reverse in polarity by the driver circuit is initiated by the feedback signal so that the

advance excitation is possible thereby improving the thrust value. Thus in this

magnetic circuit, the magnetic fluxes effectively produce thrust at pole (A) and (A’)

with single phase, while the leakage fluxes from the slots are eliminated [4].

Fig. 1. Operational Principle of an HDLM.

N N SS N S N S

N N SS N S N S

N N SS N S N S

(a) unexcited mode

(b) forward excited mode

(c) reverse excited mode

A A’

A A’

A A’

HS HS

HS HS

HSHS

30 Aravind CV et al.


2.2. Design aspects

The overall research design is a quantitative study derived from the reference [5],

where magneto-static analysis is done on the design through finite element methods.

This analysis is accomplished with two significant steps - the first one is to design

three different actuators through the height to width ratio of the permanent magnet and

then using finite element tool to analyse the static characteristics. Results of this step

demonstrate the flux flow, magnetic flux density, magnetic field intensity and its

thrust. Data obtained from the initial design [6] is compared with different height-to-

width ratios of its permanent magnet size to study the effect of varying the parameter

to the motor’s performance characteristic.

In the initial design the height to width ratio is 2.5 with the magnet dimensions

as 15 mm height and width of 6 mm. In this research, the ratio is varied to 2.7 and

2.9 by keeping the original width as it is and varying the height of the pole

magnet. This is to increase the torque density accumulations at the surface of the

mover to be higher. The design methodology employed in this investigation is as

shown in Fig. 2. Three different design structures with varied h/w ratio as shown

in Table 1 are analysed. Magneto-static analysis is applied to each design. The

main objective of the analysis is to study the machine static characteristics for

each of the incremental 1 mm distance along its linear movement. The same

simulation is repeated for three different current values; 0 A, 1 A and 5 A. The

thrust values obtained are plotted and compared so that the design with the best

sinusoidal waveform can be sent for fabrication.

Fig. 2. Design Methodology.

Table 1. Dimension of Permanent Magnet.

Specification of the

Machine

Design of Machine

Finite Element

Analysis

Results

Magnetostatic

Analysis

Modelling Tool

Simulation Tool

Flux Flow, Flux

Density, Field

Intensity and Thrust

Design Xplorer

Initial Condition

Forward and Reverse

Condition

Static Thrust

Characteristics

Stack Length = 92.5 mm

Height to Width Ratio

2.5

(initial) 2.7 2.9

Height of permanent magnet, mm 15.00 16.25 17.50

Height of moving yoke, mm 6.50 5.50 4.50

Width of permanent magnet, mm 6.00 6.00 6.00

Width of moving yoke, mm 6.00 6.00 6.00



2.2.1. Structural design

Figure 3 is the structural configuration of the linear actuator brushless type with a

required electronic commutation circuit for controlling the flux switching

operation of the motor [7]. Hence a hall sensor is utilized that usually give signal

for the control circuit. The advancements in the microelectronic field s high

efficient digital driver with MOSFET and controller is utilized.

Moving Part SensorStator Yoke

Permanent

MagnetMoving

Yoke

Fig. 3. Structural Configuration.

Table 2 shows the materials used in the design. The stator yoke and the

moving element is made from silicon core steel. High performance of high energy

rare-earth materials, an NdFeB permanent magnet with a remnant flux density of

1.05T, a coercive force of 750 kA/m, and a relative permeability of 1.08 is

chosen. This minimizes the energy required and maximizes the flux. This help to

achieve a high force per unit volume of the magnet. When a magnet is used as a

field source it becomes biased at an operating point (Bm, Hm) on its

demagnetization curve. The operating point depends on the circuit in which it is

used. It can be determined from the load line of the circuit. This intersects the

demagnetization curve at the operating point (Bm, Hm) as shown Fig. 4.

Table 2. Material Specifications of LFSIA.

Fig. 4. Permanent Magnet Operating Point.

Section Item Material

Stator Stator Yoke Silicon Core Iron Coils Copper Alloy Coil Case Teflon

Mover Permanent Magnet NdFeB Moving yoke Silicon Core Iron Shaft SS304

H -Hc

B

-Hm

Bm

Br

0

Operation point

Demagnetization curve

Load line



2.2.2. Static analysis

In order to analyse the variation in the machine dimensions the ratio of the

height to width is investigated. The rationale on this is to increase the height of

the permanent magnet and keeping the width as it thereby increase the flux

accumulation and thereby increase the torque density. The parameters varied in

the design are as shown in Fig. 5. Numerous simulations on the ratio are done

however the restrictions on two deviations from the original model are

presented in this work. A detailed analysis on the design variations through

finite element method is available in [8, 9].

Cross-section of

Permanent Magnet

Height

Width

HeightCross-section of

Moving Yoke

Fig. 5. Parameters Considered for Thrust Improvement.

In order to derive the torque characteristics of the double rotor reluctance

machine it is constructed using the FEA tool [10]. Finite element analysis is a

numerical method of solving linear and non-linear partial differential equations.

FEA tool is used to obtain the magnetic vector potential values due to the

presence of complex magnetic circuit geometry and non-linear properties of the

magnetic materials. The force on the object in the magneto-static field is

calculated from Maxwell’s equation stress [11, 12]:

(1)

(2)

(3)

where H is the magnetic field intensity, J is the source current density, B is the

magnetic flux density, and ν is the magnetic reluctivity. The divergence-free field

B introduces a magnetic vector potential A .

(4)

( ) (5)

In two-dimensional analysis, we can assume that the current density J has only

a z-direction component. Likewise, the magnetic vector potential A has only a z-

direction component. Then, we obtain the following Poisson equation.

( )

( ) (6)



The above equation is solved using the finite element method. From FEA

simulation, the path of flux flow, magnetic flux density, the torque characteristics

are derived. Figure 6 shows the magnetic flux flow, flux intensity inside the of

the stator yoke and mover configuration for a chosen ratio of 2.5. The results

derived from the simulation are presented as next section. A comprehensive

numerical analysis through the finite element is documented in [8]. The three

different models are analysed for their magneto-static analysis and the

comparison is presented in the following section.

(a)

(b)

Fig. 6. Flux Flow inside the Machine.

(a) Flux Density (b) Flux Intensity

2.3. Driver circuit

The operation of the single phase linear actuator required a power electronic drive

system that enables the sequence of operations. Figure 7 shows the flowchart on

the sequence of operation and Fig. 8 shows the circuit configuration on the design.

A comprehensive approach on the drive circuit design is presented in [6, 9]. The

drive circuit comprises an H bridge MOSFET with driver unit that can switch

bidirectional and thereby the forward reverse movement is achieved. The

controller used is the Atmega series and a voltage regulator to power up the unit is

used in the design.

Hall sensor reads current position

Yes

Switching Circuit Reversed

NoDetect

Initiate Drive Circuit

Fig. 7. Drive Control Circuit Flowchart.



H-Bridge Circuit Driver Driver

(a)

(c)

(b)

Fig. 8. Driver Controller Circuit.

2.4. Experimental setup

Figure 9 shows the experimental setup of the flux switching actuator with a fan

load to derive the thrust characteristics of the machine under investigations. An

LCR meter is used to calculate the passive parameters of the machine. A DC

power supply is used that give controlled voltage based on the switching of the

device. A recording unit through a computer interface using LABVIEW

instrumentation (not shown in picture) is used to capture the data in real time.

Linear

Acutator

Oscilloscope

DC Power

Supply

DriverMultimeter

LCR

Meter

Load

Fig. 9. Experimental Setup.



3. Results and Discussions

3.1. Thrust and cogging characteristics

From Figs. 10(a), (b) and (c), the thrust characteristics of three different ratio

under investigations. The design with 2.9 h/w ratio has the most stable sinusoidal

waveform at all current conditions simulated. All designs at all current conditions

showed a normal pattern of sinusoidal waveform with different thrust values

according to their h/w ratio as well as their current condition. However, design

with h/w ratio of 2.5 at current condition 5 A shows unexpected results at its

1 mm displacement where its thrust dropped significantly.

Fig. 10. Thrust Characteristics of the h/w Ratio, (a) 2.5 (b) 2.7 (c) 2.9.

Figure 11 shows a comparison of all the three designs to determine which

design has the least cogging force. Cogging force is a main contributor of force

ripple. Cogging force is the retention magnetic field contained in the actuator

when the current is 0 A. To increase the performance of the actuator, researchers

have been optimizing the actuators structure dimension to reduce the cogging

force. In Fig. 11 it shows that Design 3 has the least cogging force compared to

the other two designs.

Fig. 11. Cogging Thrust Characteristics of the h/w Ratio, (a) 2.5 (b) 2.7 (c) 2.9.

3.2. Electro-mechanical characteristics

Figure 12 shows the pulsed waveforms at the various voltage levels applied to the

actuator to study the performance of the test machine.

Displacement (mm)5-5 -4 -3 -2 -1 0 1 2 3 4

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

Thru

st (

N)

-

120

-80

-40

0

40

80

120

Thru

st (

N)

-

120

-80

-40

0

40

80

120

Thru

st (

N)

-

120

-80

-40

0

40

80

120

Thru

st (

N)

-

240

-

160

-80

0

80

160

240

Thru

st (

N)

-

240

-

160

-80

0

80

160

240

Thru

st (

N)

-

240

-

160

-80

0

80

160

240

1 A 5 A

(c)(a) (b)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-140-120-100-80-60-40-200

20406080

100120

Th

rust

(N

)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-100

-80

-60

-40

-20

0

20

40

60

80

100

Th

rust

(N

)

-5 -4 -3 -2 -1 0 1 2 3 4 5Displacement (mm)

-100

-80

-60

-40

-20

0

20

40

60

80

100

Th

rust

(N

)

(a) (b) (c)



Fig. 12. Pulsed Waveforms from the Oscilloscope.

The voltage to displacement is shown in Fig. 12. A simple vibration analysis

is also performed to study the impact of use with the load application. With less

than 18 V given to the actuator, it is vibrating very slow and emits slow noise

from the motor. However, as the input voltage increases, the displacement of the

shaft’s position is linearly increases. It also produces louder sound and stronger

vibration as the effect of longer linear motion when high input voltage is given to

the actuator.



Fig. 12. Comparison of the Proposed Structure.

The voltage current characteristic of the machine is as shown in Fig. 13 to see

the effect of manipulation of Vin to the behaviour of the current. The data of

current is acquired from the operating motor when input voltage is varied. The

graph plotted below shows that as the input voltage value is increased, the value

of current is linearly increases as well.


3.2. Thrust characteristics comparison

Figure 14 shows the investigation on the same force by comparing conventional

design with proposed design. As can be seen the thrust characteristics is

improvised and near to sinusoidal. In other words the cogging force of the model

is reduced. However an optimisation procedure on the machine could further

improve the thrust characteristics value. Figure 15 shows the improvement in the

design through the ratio being 2.9 better than the original design ratio of 2.5 [6].

Hence the performance characteristic of such a machine is improved through the

variations in the magnetic circuit inside the machine.

1st Trial 2nd Trial 3rd Trial Average Displacement

5 10 15 20 25 30 35 40

Voltage Input (V)

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Dis

pla

cem

ent

(mm

)

1st Trial 2nd Trial 3rd Trial Average Current

5 10 15 20 25 30 35 40

Voltage Input (V)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Cu

rren

t (A

)



Fig. 14. Static Thrust Characteristics Comparison of the Model.


4. Conclusions

Single phase flux switching DC pulsed actuator is investigated to improve the

thrust characteristics from its conventional design. A comparison on the original

design with the height to width ratio of 2.5 is done with other variations through

numerical tool. Three different ratios including the original design are used for

comparative investigations. Induction actuator with height to width ratio of 2.9

has the best thrust characteristics where it is shown through the almost perfect

sinusoidal. Further work involves in the use of optimization tool through either

numeric and analytical is to be presented in its continuing research.

References

1. Boldea, I.; and Nasar S.A. (2005). Linear electric actuators and generators.

Cambridge University Press.

2. Nasar, S.A.; and Boldea, I. (1976). Linear motion electric machines. (2nd

Ed.)

John Wiley & Sons Inc.

Design 1 (original)

Design 2

Design 3 (proposed)

-5 -4 -3 -2 -1 0 1 2 3 4 5

Displacement (mm)

-200

-150

-100

-50

0

50

100

150

200

Thru

st (

N)

-5 -4 -3 -2 -1 0 1 2 3 4 5

Displacement (mm)

-150

-100

-50

0

50

100

150

Th

rust

(N

)

Proposed (numerical)

Initial Model

Proposed (experimental)



3. Gieras, J.F.; and Piech, Z.J.; and Tomczuk, B. (2011). Linear synchronous

motors: Transportation and automation systems. (2th

Ed.) CRC Press.

4. Osawa, S.; Wada, M.; Karita, M.; Ebihara, D.; and Yokoi, T. (1992). Light-

weight type linear induction motor and its characteristics. IEEE Transactions

on Magnetics, 28(2), 3003-3005.

5. Takano, Y.; Yaezaki, S.; Matsumoto, K.; Nishizawa, N.; and Yamada, H.

(1997). Thrust simulation of linear oscillatory actuator. IEEE Transactions

on Magnetics, 33(2), 2085-2088.

6. Fairul, A. (2008). Design of linear oscillatory actuator for oil palm

mechanical cutter. Master of Science Thesis, University Putra Malaysia.

7. Chen, Y.-R.; Wu, J.; and Cheung, N.C. (2003). Lyapunov’s stability theory-

based model reference adaptive control for permanent magnet linear motor

drives. Journal of South China University of Technology, 31(6), 31-35.

8. Khumira, I. (2013). Linear flux switching induction actuator. Bachelors

Thesis, School of Engineering, Taylor’s University, Malaysia.

9. Khumira, I.; Aravind, CV.; Raja, R.N.; and Fairul, A. (2013). Computations

of thrust characteristics of flux switching induction actuator with different

height to width ratio. Proceedings of Engineering Undergraduate Research

Catalyst Conference (eureca 2013), School of Engineering, Taylor’s

University, Kuala Lumpur, Malaysia.

10. Binns, K.; Lawrenson, P.J.; and C.W. Trowbridge (1992). The analytical

and numerical solution of electric and magnetic fields. John Wiley & Sons.

11. Kameari, A. (1993). Local force calculation in 3D FEM with edge elements.

International Journal of Applied Electromagnetics in Materials, 3(1), 231-240.

12. Kameari, A.; and Niikura, S. (1993). Magnetic force calculation by nodal

force method in FEM using edge elements. Proceedings of Compumag

Conference, Part A, 2-13.

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