1 Introduction

Nowadays, due to the advantages of noncontact, zero friction, high precision, light weigh, simple structure, high reliability, fast response, and fewer interfering factors, planar magnetic levitation (maglev) technology has been widely employed in many promising precision positioning systems (such as lithography machines systems, intelligent numerical control systems, scanning probe microscopes, atomic force microscopes, etc.,) to achieve the lubricated 2-D motion on a plane [1]–[3]. Thus, the operation efficiency optimization and structure design optimization of the planar maglev levitator have attracted much more attention. Recently, the research of planar maglev levitator are mainly focused on control strategy optimization to improve the system stability and reliability [4]-[6]. Besides, the levitation magnet and coils optimization design of planar maglev levitator are also carried out to achieve the planar maglev system lubricated operation [7]–[9]. Moreover, the system operating efficiency can be further improved by using the planar maglev levitator (PML) with features of miniaturization and lightweight, simple control system, energy saving, and strong robustness [10]–[12]. Therefore, the new PML technology and device should be investigated to improve the system operation performance.

Generally, maglev technologies can be divided into four types including, electromagnetic suspension (EMS) [13], permanent magnet suspension (PMS) [14], electro dynamic suspension (EDS) [15] and high temperature superconducting pinning suspension (HTSPS) [16]. In contrast to other maglev technologies, HTSPS technology has the excellent characteristics of passive self stabilization levitation, light weight, high vacuum gain, simple structure, energy saving, and no electromagnetic pollution [17]. Hence, HTSPS technology has been well developed in recent years and successfully deployed in many industrial fields such as rail transit [18], flywheels energy storage [19], astrophysics [20], space station [21] and vibration isolation fields [22]. HTSPS technology is based on unique magnetic flux pinning characteristics of the non-ideal type II HTS materials (bulks, tapes, and thin films) to offer the self stabilization levitation. For HTS bulks, they are mainly employed to fabricate the levitator in rail transit fields with supplying the higher linear loading density [23]. Besides, HTS tapes can be stacked as the equivalent bulks to improve the drawbacks of the traditional HTS bulks [24, 25]. But, considering the strict volume and weight restrictions of the PML systems, HTS bulks and stack-based tapes may not offer the reliable levitation operation in mentioned PML systems.

Compared to HTS bulks and HTS tapes, HTS thin films have the features of low profile characteristics and higher current density, thus, it can be employed as PML in the limited volume and weight occasion to offer the levitation operation. Moreover, the double sided HTS thin film can be also used to manipulate the levitation performance by designing the meander-lines in its surface. For the traditionally application of HTS thin films, due to their unique electromagnetic characteristics, they are mainly used in microelectronics fields [26]–[28] and quantum fields [29, 30]. As for electric power applications, their quench resistance generated by phase change from superconducting state to quench state, can be used to limit the fault current and switch the circuit [31, 32]. Additionally, HTS thin films are also employed as the planar device in some emerging fields such as, planar capacitor in wireless power transfer system [33, 34], planar transformer in power electronic converter system [35, 36], to improve the system operation efficiency.

Hence, the HTS thin film PML (HTS-TFPML) can be employed in the planar motor system to isolate the mover from the stator (its role is shown in Fig. 1), it has the potential to achieve the PML miniaturization and lightweight design. Compared to the existing PML technology, the proposed HTS-TFPML is just replaced the levitator plate with an HTS thin film, the other device can also be shared [12]. Especially, due to the levitation force vacuum environment gain and dynamic stability of HTS maglev technology [37,38,39], it can meet the vacuum environment operation and stability requirements of the precision positioning systems, this is the first of its kind for PML application. Moreover, it can be also used to limit the fault current in the planar maglev system, by its self-detection, self-triggering and self-recovery without other external device. Therefore, compared to the conventional PML device and current limiting device, the system compactness, stability, and reliability can be further improved. Meanwhile, to the best of our knowledge, there is no published literature that has focused on the design and application of the double sided YBa2Cu3O7-x (YBCO) thin film device as the planar maglev levitator and fault current limiter.

Fig. 1
figure 1

HTS-TFPML deployed in the planar maglev system

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In addition, the proposed HTS-TFPML device has the potential to be employed in the planar precision positioning systems with offering the 2-D levitation motion on a plane, and limiting the fault current in the planar maglev system with the multifunctional integrated design. Hence, the compactness, stability, reliability, miniaturization and lightweight can be further improved by employing the HTS-TFPML. In this paper, a double sided YBCO thin film based HTS thin film device as planar maglev levitator and fault current limiter was firstly designed and tested in our laboratory. Its maglev levitation and fault current limiting performance was also verified through the preliminary experiment.

This paper is structured as follows. Section 2 presents the operation principle analysis of the HTS-TFPML. Section 3 presents the design and fabrication of the HTS-TFPML device. Section 4 presents the levitation force characteristics and fault current limiting test of the HTS-TFPML. Finally, Section 5 concludes this paper.

2 Levitation and fault current limiting operation principle analysis of the HTS-TFPML

As we know, the levitation characteristics of the HTS thin film device is based on magnetic flux pinning theory, which are the interaction between the non-ideal type II HTS superconductor and non-uniform external magnetic field. While its fault current limiting function is based on phase change from superconducting state to quench state. Considering the levitation and fault current limiting characteristics are different operation mode of the HTS-TFPML, thus, we analyzed its levitation and fault current limiting operation principle, respectively. The levitation characteristics illustration of HTS-TFPML is shown in Fig, 2, and its levitation characteristics are still based on Maxwell's equations, as following:

$$\nabla \times H = J + \frac{\partial D}{{\partial t}}$$
(1)
$$\nabla \times E = - \frac{\partial B}{{\partial t}}$$
(2)
$$\nabla B = 0$$
(3)
$$\nabla D = \rho$$
(4)
Fig. 2
figure 2

Levitation operation principle schematics of the proposed HTS-TFPML

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Here, J is the conduction current density (A/m2), D (C/m2) is electric displacement vector, ρ (C/m3) is the free charge volume density. Besides, the electromagnetic characteristics analysis of HTS-TFPML should also consider the charge conservation law and electromagnetic medium equations, as following:

$$\nabla J = - \frac{\partial \rho }{{\partial t}}$$
(5)
$$D = \varepsilon E$$
(6)
$$B = \mu H$$
(7)
$$J = \sigma E$$
(8)

where ε (F/m) denotes the dielectric constant, μ (H/m) denotes the permeability, σ (S/m) denotes the conductivity. Only under the conditions of linear and isotropic materials, can the mentioned parameters be constant. As we all know, HTS materials (tapes, bulks, and thin films) are the anisotropic materials, thus the tensor should be introduced to solve the HTS-TFPML electromagnetic characteristics equations.

To analyze the HTS-TFPML electromagnetic characteristics, it is assumed that the HTS-TFPML is generally in quasi-static motion, thus the electric displacement vector in (1) can be ignored, so the full current law can be simplified as:

$$\nabla \times H = J$$
(9)

The E-J power law of HTS thin film materials is as following:

$$E = \rho J = \frac{{{E_c}J}}{{J_c}}{\left( {\frac{\left| J \right|}{{J_c}}} \right)^n}$$
(10)

It can be found from (10) that the E-J power law of HTS materials is obvious nonlinearity relationship. By inserting (7), (9) and (10) into (2), we can obtain:

$$\nabla \times (\rho \nabla \times H) = - \frac{\partial \mu H}{{\partial t}} = - \frac{{\partial ({\mu_0}{\mu_r}H)}}{\partial t}$$
(11)

It can be found from (11) that, it is a partial differential equation and is the electromagnetic control equation of HTS-TFPML device. Meanwhile, the current density distribution in HTS-TFPML can be also obtained by solving (11). The electromagnetic field boundary condition and initial conditions of (11) are respectively as following:

$$\mathop n\limits^\wedge \left\langle {\left. D \right\rangle } \right. = {q_s},\mathop n\limits^\wedge \times \left\langle {\left. E \right\rangle } \right. = 0$$
(12)
$$\mathop n\limits^\wedge \left\langle {\left. B \right\rangle } \right. = 0,\mathop n\limits^\wedge \times \left\langle {\left. H \right\rangle } \right. = {J_s}$$
(13)

where qs (C/m2) is the surface charge distribution density, Js (A/m2) is the current distribution density, and \(\mathop n\limits^\wedge\) is the normal direction of interface, respectively.

$$D{|_{t = 0}} = {D_0},\,E{|_{t = 0}} = {E_0}$$
(14)
$$B{|_{t = 0}} = {B_0},\,H{|_{t = 0}} = {H_0}$$
(15)
$$J{|_{t = 0}} = {J_0}$$
(16)

Then, the electromagnetic forces of HTS-TFPML in inhomogeneous external magnetic field can be obtained by:

$$F = \int {J \times BdV}$$
(17)

Here, when the magnetic induction intensity is a horizontal magnetic field component, we can obtain the levitation force of HTS-TFPML.

As for fault current limiting function of HTS-TFPML, it is based on phase change from superconducting state to quench state of HTS superconducting materials. In normal operation, the current is less than the critical current of HTS-TFPML, it is in superconducting state, hence it has almost no influence on the system. While the fault current exceeds the critical current of HTS-TFPML, it can be limited by the quench resistance of HTS-TFPML. The resistance characteristics of HTS-TFPML can be obtained by:

$$R = {v_{\text{p}}}\frac{\rho }{S}t$$
(18)

where \(\rho\)(\(\Omega {\text{m}}\)) denotes the resistivity of HTS-TFPML in quench state, \(S\)(m2) denotes the cross-section area of HTS-TFPML, \({v_{\text{p}}}\)(m/s) denotes the quench propagation velocity, it can be also obtained by:

$${v_{\text{p}}}(J = {J_{\text{c}}}) = \frac{{{J_{\text{c}}}}}{{{c_{\text{p}}}}}\sqrt {\frac{2\rho k}{{{T_{\text{c}}} - {T_0}}}}$$
(19)

Here, \({J_{\text{c}}}\) (A/m2) denotes the critical current density of the HTS-TFPML, \({c_{\text{p}}}\)(\(J/(kg \cdot K)\)) denotes the specific heat capacity per unit, \(k\)(\(W/(m \cdot K)\)) denotes the thermal conductivity, \({T_{\text{c}}}\)(K) denotes the critical temperature of the HTS-TFPML, \({T_0}\)(K) denotes the reference temperature of the HTS-TFPML.

Besides, the resistance characteristics of the HTS-TFPML can be also expressed by the index model:

$$R = {R_{{\text{sc}}}}[1 - {e^{ - \frac{t}{\tau }}}]$$
(20)

where \({R_{{\text{sc}}}}\) is the quench resistance of the HTS-TFPML, \(\tau\) is the time constant. It can be found from the mentioned fault current limiting principle analysis of the HTS-TFPML that, its resistance in normal operation can be ignored, but its quench resistance can be used to limit the fault current during fault conditon.

3 Design and fabrication of the HTS-TFPML Device

The proposed HTS-TFPML device is based on double sided YBCO thin film, which is fabricated by magnetron sputtering technology, and its layout is shown in Fig. 3. The diameter of YBCO thin film is 50.8 mm, its substrate material is sapphire (Al2O3). It can be found from Fig. 3 that, the layers of the double sided YBCO thin film is Au layer (150 nm-thick), YBCO layer (300 nm-thick), CeO2 (40 nm-thick), respectively, they are symmetrical distribution about the Al2O3 substrate (0.5 mm-thick). The Au layer is the shunt layer to prevent the hot point and oxidation issues of the thin film, and CeO2 layer is the buffer layer to isolate the substrate and YBCO layer.

Fig. 3
figure 3

Layout of the double sided YBCO thin film

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Besides, the meander-lines of HTS-TFPML is generated by photo-etching technology, the machining process is consist of the following five parts as shown in Fig. 4.

Fig. 4
figure 4

Photo-etching machining process of the meander-lines in HTS-TFPML

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1) Applying glue: applying the photoresist on the surface of HTS thin film;

2) Exposure: the light is irradiated onto the surface of HTS thin film through a mask plate, causing the chemical change in the photoresist of the illuminated portion, generating different chemical properties of the photoresist in the exposed and non-exposed areas;

3) Development: the exposed and non-exposed areas in the processing flow 2) is revealed by using development technology. The photoresist in the exposed and non-exposed areas is dissolved by using positive and negative photoresist techniques, respectively, then the meander-lines pattern can be transferred from the mask plate to the photoresist;

4) Etching: the areas in HTS thin film surface without covering the photoresist will be removed by the etching method, then the meander-lines with same pattern as the photoresist can be obtained on the surface of HTS thin film;

5) Removing glue: dissolving and removing all photoresist on the HTS thin film surface.

Then, the meander-lines of the HTS-TFPML can be obtained, it can be employed as the planar levitation unit and fault current limiting unit. In this paper, the proposed meander-lines is shown in Fig. 5, and its structure parameters is 2 mm wide, 31 cm long, respectively. The critical current density distribution of the double sided YBCO thin film can be inductively measured by using the jc-scan Leipzig system (as shown in Fig. 6a), and the measurement result is shown in Fig. 6b. It can be found from Fig. 6 that, the maximum value of critical current density exceeds 2.8 MA/cm2, the average critical current density exceeds 2.0 MA/cm2, and its distribution is almost uniformly in the surface of the YBCO thin film.

Fig. 5
figure 5

Sample of the HTS-TFPML based on double sided YBCO thin film

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Fig. 6
figure 6

Critical current density measurement and distribution of the double sided YBCO thin film. a Critical current density distribution measurement platform (Jc-scan Leipzig system). b Critical current density distribution of the double sided YBCO thin film

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4 Preliminary verification experiments and results analysis

The preliminary verify experiment section of the proposed HTS-TFPML consists of two parts, levitation force characteristics measurement experiment and fault current limiting characteristic test experiment. Thus, we also designed and built the laboratory scale experimental platform, to carry out the mentioned experiments. The levitation force characteristics measurement experiment is to explore and analyze the levitation force characteristics of the HTS-TFPML, including the magnetic filed measurement of the Halbach permanent magnet guideway, levitation force test of the HTS-TFPML under field cooling (FC) condition and zero field cooling (ZFC) condition. Moreover, the fault current limiting performance experiment is to analyze the current limiting characteristics of the HTS-TFPML, including the critical current test of the HTS-TFPML, fault current limiting response test of the HTS-TFPML and quench resistance characteristic of the HTS-TFPML.

4.1 Facilities and overview of experimental platform

Due to the proposed planar magnetic levitator and fault current limiter is the first exploration and attempt, thus we just carried out the preliminary equivalent experiment to test its operation performance. The levitation force measurement platform is established in our laboratory (as shown in Fig. 7a) to investigate the levitation force characteristics of the HTS-TFPML. In this levitation force measurement platform, its measurement range of levitation force is ± 10 N, its vertical maximum displacement is 150 mm, its horizontal maximum displacement is 60 mm, its force sensor accuracy is 1 mN, its displacement accuracy is ± 0.01 mm and its minimum test height is 2 mm, respectively. As shown in Fig. 7a, the levitation height is controlled by the stepping motor and hand wheel to offer the accurate travel regulation and micro adjusting, respectively. It can be found from Fig. 7b that the levitation force measurement strategy is as follows, parameters input is to determine the levitation height; then the microcontroller will control the motor in vertical direction movement; after that the levitation force of the HTS–TFPML will be obtained by the force sensor, and the test data is obtained by the real time data acquisition system and stored in the computer.

Fig. 7
figure 7

Levitation force experimental measurement platform and strategy. a Levitation force experimental measurement platform. b Levitation force experimental measurement strategy

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As mentioned, the levitation characteristics of the HTS-TFPML are based on the interaction between the HTS–TFPML and non-uniform external magnetic field (Halbach permanent magnet guideway), thus, we also measured the magnetic induction intensity of the Halbach permanent magnet guideway. The magnetic induction intensity measurement results of the Halbach permanent magnet guideway in levitation force measurement platform is shown in Fig. 8. It can be found from Fig. 8 that, the magnetic induction intensity under different levitation height is also different, its maximum can reach more than 700 mT (h = 10 mm). Meanwhile, we can also find that the larger the levitation height is, the smaller the magnetic induction intensity is, its value is near to zero when the height is more than 40 mm. Hence, the ZFC height in the experiment is chosen as 60 mm, to ensure thin film enter superconducting state without external magnetic field. Additionally, we can also find from Fig. 8, the magnetic field in higher height is much more smooth than that in the lower height. Thus, considering the magnetic field distribution and the test conditions, the lowest test height is set as 10 mm.

Fig. 8
figure 8

Magnetic induction intensity measurement results of the Halbach permanent magnet guideway at different levitation height. X denotes the horizontal direction of Halbach permanent magnet guideway, Y denotes vertical direction of Halbach permanent magnet guideway

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Meanwhile, to test the fault current limiting performance of the proposed HTS-TFPML, a preliminary test was carried out in our laboratory, the equivalent circuit of the fault current limiting test is shown in Fig. 9. As shown in Fig. 9, S1 and S2 are solid relays, in normal operation, S2 is in open state, while in fault condition, S2 is in closed state, they are controlled by the field-programmable gate array controller (FPGA), and their control time sequence is shown in Fig. 10 (high level is valid). US (0 ~ 300 VAC, 50 Hz) is the power source, RP = 1.5 Ω, is the protective resistance to protect the HTS-TFPML, RL = 10 Ω, is the load resistance, respectively, the experimental test results are obtained by the NI data acquisition system.

Fig. 9
figure 9

Equivalent circuit of the fault current limiting experiment

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Fig. 10
figure 10

Control time sequence signal of the solid relays

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5 Levitation force characteristics measurement experiment of the hts-tfpml

Based on the mentioned experimental test platform, according to the measurement strategy shown in Fig. 7b, we carried out the levitation force characteristics test of the proposed HTS-TFPML. The levitation force experimental test platform is shown in Fig. 11 and the force characteristics test results are shown in Fig. 12. It can be found from Fig. 12 that, the smaller the levitation gap is, the higher the levitation force is, this is because the magnetic induction intensity value in Halbach permanent magnet guideway surface is larger (as shown in Fig. 8), hence, according to (17) the levitation force is also higher closed to the guideway surface. Besides, due to the magnetic field variation of HTS-TFPML under ZFC condition (from 60 mm–10 mm–60 mm) is larger than that in FC condition (30 mm–10 mm–30 mm), the levitation force maximum value under ZFC condition (more than 400 mN) is much larger than that under FC condition (200 mN).

Fig. 11
figure 11

Overview of levitation force experimental test platform

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Fig. 12
figure 12

Levitation force measurement results of the HTS-TFPML. a Levitation force measurement result of the HTS-TFPML under FC condition. b Levitation force measurement result of the HTS-TFPML under ZFC condition. FC condition (from 30 mm–10 mm–30 mm) denotes that the thin film enters superconducting state with external magnetic field, ZFC condition (from 60 mm–10 mm–60 mm) denotes that the thin film enters superconducting state without external magnetic field

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Considering the low profile characteristics of the planar maglev system, we also analyzed the levitation force maximum normalization characteristics of the HTS-TFPML, as shown in Fig. 13. We can find from Fig. 13 that, under FC condition, the single one HTS-TFPML can offer the levitation force more than 200 kN/m3, 50 N/kg, while under ZFC condition, it can offer the levitation force more than 400 kN/m3, 100 N/kg, respectively. Thus, the proposed HTS-TFPML has the advantages of low profile, miniaturization and lightweight, it can be employed as the planar maglev levitator to improve the compactness of the whole system.

Fig. 13
figure 13

Levitation force maximum normalization results of the HTS-TFPML

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In addition, based on the levitation force characteristics of single one thin film, we also analyzed the levitation force maximum of two thin films with different distance, as shown in Fig. 14. We can find from Fig. 14 that, the levitation force maximum of two thin films is much higher than that of single one thin film, the reason is that the superconducting layer thickness of the former is thicker than that of the latter. The levitation force maximum can be reached more than 1400 mN and 920 mN under ZFC condition and FC condition, respectively. Besides, the levitation force maximum is positively correlated with the thickness of superconducting layer. Thus, the levitation force can be further increased by adding the quantity of thin film or the thickness of superconducting layer (Fig. 15).

Fig. 14
figure 14

Levitation force maximum of two YBCO thin films. d denotes the distance between the two YBCO thin films, d is 0 mm, 0.5 mm, 1 mm, 1.5 mm and 2 mm, respectively

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Fig. 15
figure 15

Volt-ampere characteristic curve of the HTS-TFPML with the meander-lines

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6 Fault current limiting performance experiment of the HTS-TFPML

According to the mentioned photo-etching technology shown in Fig. 4, we designed and fabricated the Archimedes spiral based meander-lines on the surface of the HTS-TFPML as shown in Fig. 5. Meanwhile, we also measured the critical current of the HTS-TFPML with meander-lines, according to the quench criterion “1 μV/cm,” its critical current is 17.21 A. To test the fault current limiting response of the HTS-TFPML, a preliminary verification test platform was established in our laboratory, as shown in Fig. 16. Due to the HTS-TFPML combined with the functions of maglev levitation levitator and fault current limiter in this paper is the first attempt and exploration, thus we just test the current limiting response by using single-sided thin film, and its fault current limiting performance is shown in Fig. 17.

Fig. 16
figure 16

Overview of the fault current experiment test platform

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Fig. 17
figure 17

Fault current limiting response of the HTS-TFPML under 55 VAC, 50 Hz. I denotes the current of HTS-TFPML, V denotes the voltage of HTS-TFPML, respectively

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It can be found from Fig, 17 that, the HTS-TFPML is in superconducting state under normal operation, its voltage is near to zero, thus it has almost no influence on the system. While under short-circuit condition, the fault current exceeds the critical current of HTS-TFPML, it will transition from superconducting state to quench state by its self detection and self triggering functions to limit the fault current. Therefore, the fault current in circuit can be effectively limited by the HTS-TFPML without other current limiting strategy.

As analyzed, the fault current in circuit is limited by the quench resistance of the HTS-TFPML, thus we also calculated its quench response characteristics, as shown in Fig. 18. It can be found from Fig. 18 that, under normal operation, the resistance value of HTS-TFPML is also closed to zero, thus its influence on system can be ignored. While under short-circuit condition, the HTS-TFPML can generate the quench resistance (more than 15 Ω) rapidly to limit the fault current. The quench resistance characteristics are consistent with the fault current limiting response as depicted in Fig. 17. Additionally, the quench resistance will be also increased with the quench propagation. Meanwhile, we can also find from the quench resistance characteristics shown in Fig. 18 that, due to the relationship between voltage and current of HTS-TFPML is nonlinear, its resistance characteristics is also nonlinear power law. The unique electromagnetic characteristics of HTS-TFPML can be also employed to limit the fault current in the planar maglev system without other external current limiting device.

Fig. 18
figure 18

Quench resistance characteristic of the HTS-TFPML

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7 Conclusion

In this paper, a novel HTS thin film device as the planar maglev levitator and fault current limiter was proposed and tested for the first time. Since the HTS maglev technology has the advantages of passive self stabilization levitation, light weight, high vacuum gain, simple structure, energy saving, and no electromagnetic pollution, etc., meanwhile, the double sided HTS thin film has the low profile, miniaturization and lightweight characteristics, thus the double sided HTS thin film can be used to design the planar maglev levitator. In order to verify the levitation force characteristics of the proposed HTS-TFPML, a HTS-TFPML prototype was developed in our laboratory. We can find from the experimental results that, the HTS-TFPML can offer the levitation force more than 200 mN (under FC condition) and 400 mN (under ZFC condition), respectively. Besides, as for single one HTS-TFPML, under FC condition, it can offer levitation force more than 200 kN/m3, 50 N/kg; while under ZFC condition, it can offer levitation force more than 400 kN/m3, 100 N/kg, respectively. Hence, the HTS-TFPML can be employed to improve the compactness of the planar maglev levitation system. Furthermore, the fault current limiting response of the HTS-TFPML was also carried out by the preliminary verification experiment. It can be found from the verification experimental results that, the fault current can be limited effectively based on its phase change from superconducting state to quench state. The quench resistance of HTS-TFPML generated by the superconducting phase change is more than 15 Ω. This is the first attempt and exploration to combine the maglev levitator and fault current limiter in one double sided HTS thin film device, and we can find from the test results that it can operate as expected. The combination of planar maglev levitator and fault current limiter is perspective for the future applications, to improve the system compactness, stability and reliability, it has the potential application prospect in the planar maglev system and other maglev fields. In future work, the optimal analysis of the HTS-TFPML with different parameters and its application in a dynamical maglev system should be investigated and explored.