On Using Meissner Effect for Conceive a New Linear ...

This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIE.2014.2316246, IEEE Transactions on Industrial Electronics

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS

On Using Meissner Effect for Conceive a New Linear Electromagnetic Launcher by Zero FieldCooling YBCO Bulk Superconductor P. J. Costa Branco, Member, IEEE, R. Almeida, and J. A. Dente.

Abstract— A new YBCO superconducting linear propulsion system that uses the Meissner effect for thrust is proposed. An YBCO bulk is used in the vehicle, which is excited synchronous with a magnetic field traveling wave. The propulsion system’s model is developed using a magnetic circuit method and the Maxwell stress tensor analysis. The vehicle acceleration and thrust force characteristics previewed from that model are verified and analyzed by practical measurements and a set of essays using the prototype built in our laboratory. Major errors came for thermally activated flux creep and current conduction in the YBCO during its passage through the excitation system. The total magnetic field at superconductor’s surface decreases, reducing thrust force. Despite these secondary effects, results achieved show the potential of this new approach for future electric propulsion systems.

In this paper, a linear propulsion system with YBCO superconducting bulk as a diamagnetic mover is proposed and implemented. In particular, the proposed HTS launcher uses a coil gun structure and it is based on Meissner effect. Magnetic circuit analysis and Maxwell stress tensor are used to elaborate the launcher thrust model. A 2D finite element analysis is used to obtain thrust force characteristics. The proposed HTS launcher is verified by experimental measurements carried out in a prototype built in our laboratory. Thrust force and speed signals are analyzed in detail. Simulation results of the HTS launcher are presented, and compared with the prototype ones. II. THE HTS BULK PROPULSION SYSTEM: ITS CONCEPT

Index Terms—Electromagnetic devices, Electromagnetic forces, Electromagnetic actuators, High-temperature superconductors;

I. INTRODUCTION

M

uch research on linear actuators over the years, namely electromagnetic launchers, focused on using electromagnets and permanent magnets to produce thrust forces [1]–[4]. Recently, smart materials as piezoelectric [5], magnetostrictive [6], and electroactive ionic polymers [7]-[9] have been applied to linear actuators. Today, a new step in designing linear actuators is happening. Since their discovery in 1983, critical temperature improvement, and cost decrease, high-temperature superconductor (HTS) technology is building the next stage in linear actuators. These with HTS coils [10, 11] or HTS bulks [12 - 17] are examples of what has been developed. Manuscript received July 27, 2012; revised February 23, 2013, July 29, 2013, December 9, 2013 and January 28, 2014; accepted March 4, 2014. Copyright © 2014 IEEE. Personal use of this material is permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to [email protected] This work was supported by FCT, through IDMEC, under LAETA PestOE/EME/LA0022. P. J. Costa Branco, is with LAETA/IDMEC, Instituto Superior Técnico Universidade de Lisboa, Lisbon, Portugal. (e-mail: [email protected]). J.A. Dente is with the Electrical and Computers Engineering Department, Instituto Superior Técnico Universidade de Lisboa, Lisbon, Portugal. (e-mail: [email protected]).

Fig. 1 illustrates the concept of proposed HTS propulsion system. This consists of two parts: the excitation system responsible for generate a traveling field wave B synchronous with vehicle position; and a second part constituted by the vehicle carrying the bulk superconductor. As indicated in Fig. 1, the resultant electromagnetic force F on HTS bulk thrusts the vehicle to a certain speed value v. B fmm fmm

x X

F

v

F

x B X of propulsive HTS force production using Meissner effect. Fig. 1. Diagram

Fig. 2(a) shows the excitation system made by two parallel magnetic circuits. Their purpose is to generate a pulsed magnetic field that is tangential to the backside of superconductor transversal surface. The new structure in Fig. 2(a) allows a smaller air gap, increasing field B in the superconductor transversal surface, augmenting thrust force F. To get a suitable vehicle motion, the tangential field has to be present only in one transversal side of the superconductor. If that does not happen, there will be two forces acting in opposite directions on both sides of the superconductor,

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IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS resulting in a reduced or even opposite thrust force. One way to surpass this is shown in Fig. 2(b). The excitation circuits in Fig. 2(a) are discretized into small excitation units, each one an independent magnetic circuit. Each unit will produce a stationary field B in synchronism with vehicle position (see Fig. 2(b)). In this figure, each magnetic unit is in gray color. Light gray is the iron core. Dark gray are the coils.

v

F

B

F B

(a)

Excitation unit

Coil Iron

x3 x1

B

v

HTS

x2

2 DIMENSIONS OF SUPERCONDUCTING BULK AND MAGNETIC CIRCUIT UNIT. VALUES IN CENTIMETERS a b c d e f g h i j 1 4 4 2 4 2 4 1 10 6

Analysis of magnetic field distribution around the superconductor, when inserted in the air gap, was made using a 2D FE analyses. Fig. 4 shows the top view of the magnetic circuit and its coordinate axes . The superconductor is positioned leaving horizontally the air gap, although the figure shows the closed path of the magnetic circuit folded into plane . The FE model has used the dimensions in Table I. A constant relative magnetic permeability value of was attributed to the iron core material. The superconductor was initially considered as a perfect diamagnetic material using since a zero value causes uncertainty in numerical simulation. The results in Fig. 4 show that field B tangent to surfaces c and d will have very low values, being symmetrical. On the contrary, field B near surface a shows higher density values. In surface b, as the superconductor leaves the air gap, tangential field B values will begin decreasing very fast. Superconductor coils

(b) Fig. 2. (a) Continuous excitation system with a smaller air-gap (b) Modular excitation system made by independent excitation units.

x3

This section shows the analytical results obtained for a vehicle containing an YBCO superconductor bulk. Thrust forces were estimated using a 2D FE model analysis and compared with those ones measured in a prototype system built in our laboratory. A. Magnetic Field around Superconducting Bulk Fig. 3 shows a schematic of the superconducting bulk (left) and one magnetic circuit unit (right), both with their dimensions indicated by letters and listed in Table I. The magnetic circuit is made of laminated silicon steel in standard C shape.

Iron core (top)

coils

x2

c d a

Iron core (top)

x1

coils

Magnetic circuit closed path folded to plane x1-x2

Fig. 4. 2D FEM result for obtaining magnetic field distribution around the superconductor when leaving the air gap.

e

j

f d g

b c

coils

b

III. VEHICLE MODEL

e

(leaving the air gap)

c h

a i

Fig. 3. Schematic of superconducting bulk (left) and the magnetic circuit unit (right). Dimensions are indicated with letters.

TABLE I.

B. Electromagnetic Forces Acting on the Superconductor Forces acting on the superconductor were formulated applying Maxwell's stress tensor method [14]. If magnetic field strength and its linear constitutive relation [ ] (1) appear, being are considered, density forces directed along the coordinate axes , , and indicated in Fig. 4. In (1), is the magnetic permeability around the surface (usually air) chosen to calculate the stress tensor [ ]. Terms are the values of the magnetic field along directions , , or , and is the Kronecker operator. Consequently, the stress tensor associated with each direction becomes given by (2), which can be arranged in matrix form (3). ( ∑ ∑

∑ ∑


)

(1) (2)



3 force acting in the superconductor. {

[

] (3) Stresses acting on surfaces c and d of the superconductor (Fig. 5) have nearly equal magnitude but opposite sense, therefore cancelling each other. Stress on surface b is assumed zero because the surface is out of air gap and there the magnetic field is assumed null. At last, transversal surface a has only the tangential field component since . Using these conditions in (3), only stress tensor stays relevant on surface a, being given by Eq. (4). It is only function of magnetic flux density component and its negative signal means that thrusts the superconductor out of air gap.

(6) (7)

Superconductor bulk

The total force on the superconductor is calculated using (5) through a control surface enclosing the superconductor. Substituting (4) in (5), the thrust force acting on the superconductor becomes given by (7) where is the active and transversal superconducting surface area with 2 cm2 indicated in Fig. 5. {

∬

}

∬

b

IV. EXPERIMENTAL VERIFICATION OF THE FE MODEL To validate the distribution of tangential field in surface a predicted from FE model, one excitation unit was mount in our laboratory, as shown in Fig. 10(a). It shows a rectangular YBCO bulk superconductor positioned in the air gap, and two coils with three hundred turns wound on the magnetic core and connected in series. The tangential field along surfaces a and b of the superconductor (see Fig. 4) was measured with the superconductor positioned at three positions in the air gap: | | cm, | | cm, and | | cm, as indicated in Fig. 10(b) The results are plotted in Figs. 11(a-c). Each graphic shows the vertical distribution of magnetic flux density component measured in surface a (marked with circles) and measured in surface b (marked with triangles). All results show that tangential field is more intense along surface a than surface b. Along surface b, magnetic flux density decreases as the superconductor leaves the air gap. In average, is more intense and uniform between 1 cm and 3 cm on both surfaces, i.e., inside the surfaces’ active section. Hence, only that enclosed central area of 2x1 cm2 was considered significant to the thrust force. Fig. 12 indicates and names the location of this central area in the superconductor as the working area.

(4)

∬

(9)

(5)

Superconductor d

Coil

Coil

c Iron core

S Iron core Magnetic circuit

a F

Magnetic circuit

(a)

b

Fig. 5. Schematic representation of the useful area in surface a to calculate the superconductor’ thrust force.

C. Dynamic Model of the HTS Vehicle The most significant forces acting on the vehicle are the electromagnetic force acting in the superconductor, and the friction force between the vehicle wheels and rail given by (8), where is the dynamic coefficient of friction, is the vehicle mass, and is the acceleration of gravity. (8) The dynamic model becomes given by speed and position differential equations in (9), accounting for the friction force between vehicle wheels and rail, and also the electromagnetic

Liquid nitrogen vessel

x3 Iron core

a

Iron core

x2

x1

x1

(b) Fig. 10. (a) The HTS propulsion unit made in our laboratory. (b) Top view of propulsion unit showing the superconductor in the air gap and positioned at distance | |.

The results in Figs. 11(a) to 11(c) show that the tangential




4

magnetic flux density that is located in the working area , indicated in Fig. 12, it has the highest values. They also show good uniformity, which allowed us to work with its average value. Table II compares the FEM magnetic flux density values with those ones measured and averaged along the working area for the three positions: 1 cm, 2 cm, and 3 cm. In the initial position, | | cm, there is strong magnetic field dispersion. However, as the superconductor is positioned within the air gap from | | cm to | | cm, the magnetic field distribution becomes more uniform (see Fig. 11(c))).

be represented as a perfect diamagnetic material. In reality, there is some magnetic flux passing through the superconductor due to its impurities. This effect reduces effectively the magnetic reluctance of the magnetic circuit, thus producing higher magnetic flux density values. Previous results indicate that the superconductor cannot be modeled using a “null” magnetic permeability. Our model used to avoid numerical problems with the FE program. For qualitative results, this approximation can be accepted. However, quantitative results require a more precise representation of the superconductor.

a) Position |x1| = 1 cm Surface "a" Surface "b"

3

x3 [cm]

Superconductor

3

2

Surface´s active section

Iron core

1 0 0

20

40

60

80 100 B2 [mT]

120

140

160

Surface "a" Surface "b"

x3 [cm]

3 2


1

20

40

60

80 100 B2 [mT]

120

140

160

120

140

160

Surface "a" Surface "b"

3

x3 [cm]

Iron core

x3

0

x1

x

TABLE II. PERCENTAGE ERROR BETWEEN FEM PREDICTION AND MEASURED MAGNETIC FLUX DENSITY AVERAGED AT WORKING AREA [ ] Error [%] | |[ ] [ ] 1.0 121 141 14.2% 2.0 119 146 18.5% 3.0 119 151 26.7% V. THRUST FORCE MEASUREMENT

c) Position |x1| = 3 cm

2


1 0 0



2 Fig. 12. Indication of the working area in the superconductor showing a uniform magnetic flux density.

4

4

2 1

b) Position |x1| = 2 cm

0 0

4 Working area

4

20

40

60

80 100 B2 m[T]

Fig. 11. Magnetic field along surfaces A (circles) and B (triangles) for three superconductor positions: (a) 1 cm, (b) 2 cm, and (c) 3 cm.

Table II shows that the 2D FEM results give nearly constant field , instead an increasing one as shown by experimental values. Therefore, prediction error increases. How justify this result? In the model, the air gap magnetic reluctance value remains almost constant despite the YBCO passing. However, this is not true because YBCO bulk cannot

In this section, thrust force acting on superconductor is measured. Fig. 13(a) shows the experimental set-up constructed to this purpose. The vehicle is positioned at a certain position , with the superconductor inside the air gap. A rigid arm links the vehicle and a piezoelectric force sensor. More precisely, the photo was taken during an essay for cm. Fig. 13(b) shows the copper box where the superconductor (Fig. 13(c)) is inserted. Thrust tests established an “average” value for the relative magnetic permeability of the superconductor equal to increasing the HTS FEM model precision. Figures 14(a) to (c) show the thrust forces in the superconductor for three positions of the vehicle (1, 2, 3 cm) and some electric currents in the magnetic circuit (2 A to 8 A). As in all electromagnetic systems, thrust force changes proportionally to current squared. As the superconductor enters in the airgap (follow Fig. 10), thrust force in its surface a augments due reduction of magnetic flux dispersion. Sequence of Figs 14(a) to (c) illustrates this effect from1 to 3 cm penetration. Figure 15 shows thrust force evolution on the vehicle as it travels from position to , with 7A in the magnetic circuit. When the vehicle is located in interval [1, 2] cm, thrust force is highly penalized by the opposite




Vehicle

value; and the frictional force component of gravitational force

with the horizontal .

a) Position |x1| = 1 cm 10 8

Force [N]

force acting on surface b. During the central interval [2, 3] cm, thrust force is nearly constant, reaching its maximum value. Into this interval, force on surface b acting in the opposite direction to vehicle movement is now almost null because surface b is now out of the air gap. In the last interval [3, 3.5] cm, the vehicle is leaving the air gap. Hence, thrust force decreases due to a reduction of the magnetic flux density B on surface a caused by edge effects (magnetic flux dispersion).

5

6 4

Rails

2 0 2

x

3

4

F

(a)

7

8

7

8

7

8

10 8

Force [N]

Magnetic circuit coils

6

b) Position |x1| = 2 cm

Rigid arm for force Piezoelectric force sensor measurenment Copper vessel for the YBCO plus liquid nitrogen Magnetic circuit

5 Current [A]

6 4 2 0 2

3

4

5 Current [A]

6

c) Position |x1| = 3 cm 10

VI. EXPERIMENTAL TESTS ON LAUNCHER PROTOTYPE Figure 16(a) shows a schematic representation of our HTS linear propulsion system. Figure shows in dark gray the four independent magnetic circuits, and it shows the vehicle aligned over the rails, transporting a copper box with the bulk superconductor inside. The schematic also shows the four vehicle position sensors and the light signal reflector used to trigger the excitation circuits. Figure 16(b) shows a photo of the complete HTS launcher prototype. The power electronics used to generate the electric current pulses to each excitation circuit is indicated below the photo. Up in Fig. 16(b), the four excitation circuits are signaled near the rail. A. Estimation of the Friction Coefficient between Vehicle and Rail The friction coefficient ) between vehicle and rail needs to be determined. Hence, a preliminary experiment was prepared. The rail was inclined to an angle , as shown in Fig. 17. The vehicle was released and its position was registered using an ultrasonic sensor. The vehicle is subject to two sets of force: the component from the gravitational force given by and its reaction force of equal

8

Force [N]

(b) (c) Fig. 13. (a) Experimental set-up to measure thrust force on the vehicle. (b) Vehicle photo showing the vessel with the superconductor inside. (c) YBCO superconductor used.

6 4 2 0 2

3

4

5 Current [A]

6

Fig. 14. Graphics with thrust forces in the superconductor measured for three vehicle positions (a) 1 cm, (b) 2 cm, (c) 3 cm and electric currents in magnetic circuit (2 to 8 A).

Friction force is given by Eq. (10). Thrust force is given by (11). By Newton’s second law expressed in (12), the friction coefficient can be determined using (13). Its acceleration term was calculated using the vehicle position signal, which was measured during vehicle descendent for a certain inclination angle .


(10) (11) (12) (13)



results in

Fg sen

F

. Using this value in (13), the friction coefficient .

8

( )



Rails Fg



Fig. 17. Vehicle force components for a certain rail inclination angle .

7

0.6

5 4 1

1.5

2 2.5 Position x [cm]

3

3.5

Fig. 15. Graphic showing the forces calculated using the FEM model and measured ones as vehicle travels from to , with a constant electric current of 7A in the magnetic circuit.

Vehicle

Average speed 0.4

Position 0.2 0 -0.2 0

0.8 1 1.2 1.4 1.6 Time [s] Fig. 18. Vehicle instantaneous position, instantaneous speed and its average value when it is released at s for a rail inclination angle .

HTS Vessel

Rails 3 4

Coil Magnetic circuit

(a)

Position [m]

0.3 Light signal reflector HTS bulk

2

0.2

0.4

0.6

0.35

Position sensors

1

Instantaneous speed

7 Excitation circuit 1

0.25

Excitation circuit 2

Excitation Excitation circuit 4 circuit 3

6 5

0.2

4

0.15

3

0.1

2

0.05

1

0 0.1

Current [A]

6

Speed [m/s] - Position [m]

Force [N]

N

Vehicle

Fg co s( )

In Fig. 18, position and speed signals are plotted when the vehicle was released at s for a rail inclination angle of . Since the instantaneous speed signal shows too many oscillations, its average value was calculated. The slope of average speed gives a vehicle acceleration equal to

6

0 0.3 0.4 0.5 Time [s] Fig. 19. Electric current pulses delivered to the excitation circuits as the vehicle with mass of 300g passes, and its instantaneous position in time. 0.2

Mass: 300 gr

Rails Excitation magnetic circuits

Speed [m/s]

1.5

1

0.5

0 0

Power electronics

(b) Fig. 16. (a) Schematic of the HTS propulsion system and its main components. (b) The HTS launcher prototype built in our laboratory

Measured Analytical model (Magnetic circuits)

0.04 0.07

0.11 0.14 0.18 0.21 0.25 0.3 Position [m] Fig. 20. Vehicle propulsion: model and experimental speed responses as vehicle passes the magnetic circuits. Vehicle’s mass is 300 gr.

B. Vehicle dynamics: Simulation and Experimental Results Fig. 19 shows the electrical current pulses delivered to the magnetic circuits as the 300g vehicle passes, and plots its position. The simulation and experimental vehicle speed results are plotted in Fig. 20. The figure also indicates in a gross black line the position of the four magnetic circuits. Between 0 m and 0.04 m, the model was found to predict vehicle speed




7

with good accuracy up to about 0.6 m/s, or during the first 0.13 seconds. Between 0.04 m and 0.07 m there is no magnetic circuit. Therefore, there is no thrust magnetic force acting on the vehicle, only the friction force reducing its speed. After 0.07 m until 0.3 m the speed increases again. However, acceleration becomes progressively smaller. At this point, the evolution of vehicle speed needs a more careful analysis to verify the existing differences between model and measured speed signals, as Fig. 20 has shown. For this analysis, vehicle inertia was augmented. Masses of 500 gr and 800 gr were used, whereupon the vehicle had a total weight of 800 gr and 1100 gr, respectively. Fig. 21 shows the model and measured speed signals during a propulsion test with an 800 gr vehicle. The speed shows again periodic accelerations synchronized with the magnetic circuits. Model speed predicts fairly well the measured one from the beginning up to about 0.23 m, or during the first 0.77 seconds. However, between 0.23 m and 0.3 m, the measured speed shows lower values than those values predicted by model, increasing this error as vehicle accelerates. Figure 22 shows the speed signals from an essay using 1100 gr vehicle. The model follows with good accuracy the measured speed up to about 0.14 m, or during the first 0.92 seconds.. However, this did not happen in the last magnetic circuit where vehicle speed was only partially increased.

Derivative calculation gives the vehicle speed signal indicated in Fig. 23(a). However, it contains undesired oscillations caused by the sampling period of ultrasonic position sensor and its time derivatives. In view of this result, there was need to low-pass filter the position and speed signals. Figs. 23(b-c) present an example of a low pass filtering process. Fig. 23(b) shows position data and its filtered signal. The cut-off frequency of the low-pass position filter was adjusted by trial-and-error to 152Hz. Follow, the derivative of the position data gives the vehicle´s speed shown in Fig. 23(b). To get a better speed signal, it was also low-pass filtered using now a cut-off frequency of 286Hz. The filtered speed is plotted in Fig. 23(c). 2) Force signal acquisition: Force exerted on the superconductor is one of the most important physical quantities in the analysis of the propulsion system. In Sections V and VI, force on the superconductor vehicle was measured with the vehicle stopped and using a piezoelectric force sensor. In this section, the procedure used to measure the forces now along the ride of the system is described. Equation (14) establishes the magnitude of the resultant force acting on the HTS vehicle. Term denotes the impulse of vehicle obtained from each excitation circuit, and is the time interval that the vehicle was subjected to that impulse. Notice that Eq. (14) gives the average value of the force produced by each excitation circuit. The impulse value is obtained by calculating the change in vehicle’s momentum, Eq. (15), where is vehicle speed variation.

Mass: 800 gr

Speed [m/s]

1.5

1


(14) (15)

0.5

0 0

0.04 0.07

0.11 0.14 0.18 0.21 0.25 0.3 Position [m] Fig. 21. Vehicle’s mass is 800 gr. Vehicle propulsion: model and experimental speed evolution as vehicle passes the magnetic circuits. Mass: 1100gr

Speed [m/s]

1.5

1


0.5

0 0

0.11 0.14 0.18 0.21 0.25 0.3 Position [m] Fig. 22. Vehicle’s mass is 1100 gr. Vehicle propulsion: model and experimental speed evolution as vehicle passes the magnetic circuits.

Table I lists the vehicle impulse values acquired from each excitation circuit. Last column in Table I lists the total impulse received after each test performed. Considering Table I and the time interval the vehicle takes to pass each excitation circuit, Table. II presents the thrust forces exerted on the vehicle for each vehicle’s mass. TABLE I IMPULSE VALUES [N.S] Mass [gr]

[N.s] 1st circuit

[N.s] 2nd circuit

[N.s] 3rd circuit

[N.s] 4th circuit

Total impulse

300

0.22

0.05

0.04

0.04

0.35

800

0.31

0.13

0.11

0.07

0.62

1100

0.34

0.17

0.12

0.09

0.73

TABLE II THRUST FORCES

0.04 0.07

1) Speed signal acquisition: An ultrasonic position sensor (Baumer UNAM 18U6903/S14) was used to obtain the speed of the vehicle by translating its position information to its linear speed by derivative of position signal. Fig. 23(a) shows that the curve of the position-time plot presents a "stepped" form due to the ultrasonic sensor-sampling interval (26 ms).

Mass [gr]

Force [N] 1st circuit

Force [N] 2nd circuit

Force [N] Force [N] 3rd circuit 4th circuit

300

1.92

0.97

1.13

1.23

800

1.49

1.29

1.38

1.11

1100

1.28

1.27

1.06

0.98

Revising Figs. 20 to 22, the experimental and model results show larger errors from certain speed values. Superconductor




8

speed and also its total exposure time to excitation magnetic field are correlated with the errors between experimental and model results. At first, in both Figs. 20 and 21, results begin to differ nearby 0.6 m/s. Around this point, the electromagnetic force acting on the superconductor begins to show a decrease due to an important reduction of field . This is attributed to the opposite magnetic field with origin in the shielding currents induced in the superconductor surfaces c and d due to the magnetic flux variation on it. For speed values below that value, those shielding currents also exist. However, their associated magnetic field is not high enough to significantly affect field , decreasing considerably the force in the superconductor. Notice that the field tangential to superconductor surfaces is the sum of the field generated by the magnetic circuit plus the opposite field generated by the currents induced in the superconductor. The resultant tangential field in the superconductor, field , then becomes reduced, hence inferior thrust forces. Consequently, when this effect becomes significant, the model error augments because it did not take that into consideration. The effect of the induced currents in the copper vessel of the superconductor bulk is considered minimum when compared with the currents induced in the YBCO because copper electric conductivity is much higher than YBCO one. The second aspect that one must consider is the time the superconductor was exposed to the excitation magnetic field. It aspect is associated with the dissipation energy of a number of vortices flowing in the superconductor, which can produce distortions of that mixed state due to thermal fluctuations i.e. the motion of vortices due to the vibrations of molecules in the superconductor. For the cases shown in Figs. 20-21, the superconductor was exposed during 0.33 seconds and 0.76 seconds, respectively. The results indicate that the energy dissipation was not high enough to cause thermal fluctuations capable of induce a local loss of superconductivity. On the other hand, in Fig. 22, the experimental and model results have started to diverge only after about 0.92 seconds, even without reach the previous critical speed value of 6 m/s. This This strongly suggests that the total energy dissipated in the superconductor bulk after 0.92 seconds was capable of producing strong high thermal fluctuations on the vortex state, which was reflected in a reduction of field since more fluxlines will pass now through the superconductor due some local loss of superconductivity It is noteworthy that, in any of the tests effectuated, vehicle’s acceleration through each magnetic circuit is not uniform. Acceleration is higher in the first magnetic circuit but less in the last magnetic circuit. Notice that a vehicle’s acceleration decrease means that the exposure time of the superconductor to the magnetic field is reduced, thus decreasing the time that thrust force acts in the superconductor. One possible solution to this problem would use magnetic circuits with increasing lengths, where the maximum length of the circuit could not exceed that of the superconductor to not generate a braking force on the vehicle. Another option would be to increase the magnetomotive force either increasing the electrical current or increasing the

number of winding turns in each magnetic circuit. At last, Fig. 24 shows a sequence of four photographs that illustrate the vehicle (enclosed with a white rectangle) being propelled through the four magnetic circuits. In the first photo (Fig. 24(a)), the vehicle is stopped. In the second photo (Fig. 24(b)), the vehicle has already been pushed by the second and third magnetic circuits. The third photo (Fig. 24(c)) shows the vehicle being pushed by the last magnetic circuit. At last, the fourth photo shows the vehicle far away rolling in the rail. 0.4

0.2

00

-50-50 00

Position [m]

0.4

0.2

0.1 0.1

0.2 0.2

0.3 0.3

0.4 0.4

Time [s]

0.5 0.5

Position [m]

Speed [m/s] Velocidade [m/s]

5050

0

0

(a) 0.35 0.3

1.2 Position Filtered position

1 0.8

Speed [m/s]

Position [m]

0.25 0.2 0.15

0.6 0.4

0.1

0.2

0.05

0

0 0

Speed Filtered speed

0.2 0.4 Time [s]

-0.2 0

0.2 0.4 Time [s]

(b) (c) Fig. 23. (a) Vehicle position signal and its derivative (linear speed). (b) Instantaneous and filtered position. (c) Instantaneous and filtered speed.

VII. CONCLUSION The paper presented the design, development and construction of an experimental prototype of a linear electromagnetic propulsion system based on Meissner and Lenz law effects in an YBCO superconductor. Thrust characteristics have been studied by FEM analysis, and verified in the prototype built in our laboratory. The power electronics excitation system of the propulsion device generated a discrete traveling magnetic wave synchronous with vehicle position. It was verified that vehicle acceleration remained not constant during its passage through each excitation circuit. This occurred because magnetic field exposure time of the vehicle decreases as its speed increases. Possible solution involves the use of magnetic circuits with increasing magnetomotive forces. In a final conclusion, the proposed HTS bulk propulsion shows high potential to future linear launcher applications.




(a)

(b)

(c) (d) Fig. 24. Photographic sequence. (a) Vehicle stopped. (b) Vehicle impulse applied by second and third excitation circuits. (c) Vehicle impulse from last excitation circuit. (d) Vehicle moving after being launched.

REFERENCES [1]

Shukor, A.Z.; Fujimoto, Y., "Direct-Drive Position Control of a Spiral Motor as a Monoarticular Actuator," IEEE Trans. Ind. Electron., vol.61, no.2, pp.1063,1071, Feb. 2014 [2] Pompermaier, C.; Kalluf, K.; Zambonetti, A.; Ferreira da Luz, M.V.; Boldea, I.; , "Small Linear PM Oscillatory Motor: Magnetic Circuit Modeling Corrected by Axisymmetric 2-D FEM and Experimental Characterization," IEEE Trans. Ind. Electron., vol.59, no.3, pp.13891396, March 2012 [3] Jiancheng Fang; Shiqiang Zheng; Bangcheng Han, "AMB Vibration Control for Structural Resonance of Double-Gimbal Control Moment Gyro With High-Speed Magnetically Suspended Rotor," IEEE/ASME Trans. Mechatronics, vol.18, no.1, pp.32,43, Feb. 2013 [4] Onat, A.; Kazan, E.; Takahashi, N.; Miyagi, D.; Komatsu, Y.; Markon, S.; , "Design and Implementation of a Linear Motor for Multicar Elevators," IEEE/ASME Trans. Mechatronics, vol.15, no.5, pp.685-693, Oct. 2010 [5] Cao, Y.; Cheng, L.; Chen, X.B.; Peng, J.Y., "An Inversion-Based Model Predictive Control With an Integral-of-Error State Variable for Piezoelectric Actuators," IEEE/ASME Trans. Mechatronics, vol.18, no.3, pp.895,904, June 2013 [6] L. Chen, X. Wang, W.L. Xu, "Inverse Transmission Model and Compensation Control of a Single-Tendon “Sheath Actuator ,"IEEE IEEE Trans. Ind. Electron., vol. 61, no. 3, pp. 1424-1433 , Mar 2014 [7] Yang Fang; Pence, T.J.; Xiaobo Tan; , "Fiber-Directed ConjugatedPolymer Torsional Actuator: Nonlinear Elasticity Modeling and Experimental Validation," IEEE/ASME Trans. Mechatronics, vol.16, no.4, pp.656-664, Aug. 2011 [8] Branco, P.J.C.; Lopes, B.; Dente, J.A.; , "Nonuniformly Charged Ionic Polymer–Metal Composite Actuators: Electromechanical Modeling and Experimental Validation," IEEE Trans. Ind. Electron., vol.59, no.2, pp.1105-1113, Feb. 2012 [9] Berselli, G.; Vertechy, R.; Vassura, G.; Parenti-Castelli, V.; , "Optimal Synthesis of Conically Shaped Dielectric Elastomer Linear Actuators: Design Methodology and Experimental Validation," IEEE/ASME Trans. Mechatronics , vol.16, no.1, pp.67-79, Feb. 2011 [10] Li Weili; Qiu Hongbo; Yi Ran; Zhang Xiaochen; Li Liyi, "ThreeDimensional Electromagnetic Field Calculation and Analysis of Axial– Radial Flux-Type High-Temperature Superconducting Synchronous Motor," IEEE Trans. Appl. Supercond., vol.23, no.1, pp.5200607,5200607, Feb. 2013 [11] Marignetti, F.; Carbone, S.; Delli Colli, V.; Attaianese, C.; , "Cryogenic Characterization of Copper-Wound Linear Tubular Actuators," IEEE Trans. Ind. Electron., vol.59, no.5, pp.2167-2177, May 2012 [12] Ailam, E.H.; Benallal, M.N.; Hachama, M.; Leveque, J.; Rezzoug, A., "Comparison Study Between Several Solutions of Superconducting Inductors," IEEE Trans. Appl. Supercond., vol.23, no.1, pp.5200506,5200506, Feb. 2013

9 [13] Branco, P.J.C.; Dente, A.; "Design and Experiment of a New Maglev Design Using Zero-Field Cooled YBCO Superconductors," IEEE Trans. Ind. Electron., vol.59, no.11, pp. 4120-4127, Nov. 2012. [14] Jibin Zou; Mei Zhao; Qian Wang; Jiming Zou; Guangkun Wu, "Development and Analysis of Tubular Transverse Flux Machine With Permanent-Magnet Excitation," IEEE Trans. Ind. Electron., vol.59, no.5, pp.2198,2207, May 2012 [15] Hao Chen; Sen Lv; Qianlong Wang, "Temperature Distribution Analysis of a Switched Reluctance Linear Launcher," IEEE Trans. Plasma Sci. , vol.41, no.5, pp.1117,1122, May 2013 [16] Jian Xun Jin; Lu Hai Zheng; You Guang Guo; Jian Guo Zhu; , "Performance Characteristics of an HTS Linear Synchronous Motor With HTS Bulk Magnet Secondary," IEEE Trans. Ind. Appl. , vol.47, no.6, pp.2469-2477, Nov.-Dec. 2011 [17] Luhai Zheng; Jianxun Jin; Yuoguang Guo; Wei Xu; Jianguo Zhu; , "Performance Analysis of an HTS Magnetic Suspension and Propulsion System With a Double-Sided HTS Linear Synchronous Motor," IEEE Trans. Magn., vol.48, no.2, pp.655-658, Feb. 2012 [18] H.H. Woodson, J.R. Melcher, Electromechanical Dynamics – Part II: Fields, Forces and Motion, Robert E. Krieger Publishing Company, 1985. P. J. Costa Branco (M’91) He is currently an Associate Professor w/ Habilitation in the Department of Electrical and Computer Engineering, Scientific Area of Energy, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal. Currently, his research covers three topics: design of innovative spherical induction machines for biomedical and robotic applications; project of innovative electromechanical systems supported on Ionic Polymer Metal-Composite materials (IPMCs), and the use of hightemperature superconducting materials in increasing the electric energy efficiency of electrical machines and MagLev vehicles. J. A. Dente received the B.S. and Ph.D. degrees in electrical engineering from Instituto Superior Técnico (IST), Lisbon, Portugal, in 1975 and 1986, respectively. From 1989 to 1993, he was an Associate Professor with IST, where he is currently a Full Professor in the area of electrical machines with the Department of Electrical and Computer Engineering, Scientific Area of Energy. His primary areas of interest are in electrical machines and motion control, and he is currently engaged in research on electrical generators for wave and wind energy converters. R. Almeida received the B.S. and Master's degree in Electrical and Computer Engineering at Instituto Superior Técnico, Technical University of Lisbon. In 2012, he obtained the qualification of Engineer Designer/Installer of ITED2 and INTRUSION PROTECTION. In 2010, he was a scholarship researcher of IPMC materials, by the Foundation of Science and Technology. In 2011 and 2012, he was an Operating Engineer in the power station of Sines. From 2013, he is a Railways Maintenance Engineer.


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