Magnetic levitation (Maglev) technology has aroused extensive attention due to its advantages of being friction-free, lubrication-free, and maintenance-free, and it has been used in many industrial applications, such as railway systems [
1,
2,
3,
4,
5,
6], rotating machinery [
7,
8,
9,
10,
11,
12], ultra-clean assembly line, and artificial heart pumps [
13,
14], which are the typical applications of maglev. In addition to these, there are some novel applications of maglev, such as maglev gyro [
15], maglev globe [
16], maglev lateral vibration attenuation system [
17], permanent maglev system [
18,
19], and zero-power maglev system [
20,
21]. Similarly, in our previous research, to cater to the testing requirements for the parts and materials working in special environments, we applied maglev to tension and bending testing devices [
22,
23]. Special environments include vacuum, special gases, and special liquids, as well as environments with abnormal air pressure or temperature, which may greatly influence the mechanical performance of the parts or components; therefore, it is necessary to facilitate the mechanical test in special environments. E.g., high-purity aluminum [
24], which is widely used in satellites, spacecraft, and other aerospace devices, has to work in vacuum environments because, in normal air, a layer of oxide film will form on its surface, impairing its mechanical performance, corrosion resistance, and adhesion. In addition, gas-lubricated bearings [
25] need to work in helium environments to reduce air resistance and better heat dissipation during high-speed rotation because helium has a lower viscosity and higher thermal conductivity compared with ordinary air. Furthermore, silicon-based photonic chips [
26], which are typically used in optical communication and sensing, need to work in nitrogen environments because nitrogen can help silicon-based photonic chips maintain optical performance and stability. Previously, if we wanted to test these parts or materials’ mechanical properties in their original working environment, the entire material testing device had to be placed in a container in which the special environment was. On the contrary, with maglev technology, the specimens in the devices [
22,
23] can be applied with load in a noncontact way; as a result, only the specimens and a few parts need to be placed in the container where the special environment can be created, which avoids exposing sensitive components of the testing device (e.g., circuits in force sensors and actuators are sensitive to environment humidity) to the special environment, facilitating the conduct of the testing. The research [
22,
23] was inspired by the research [
27] held by Okayama University. In the research [
27], Naoya Tada and Hiroyasu Masago developed a device in which a noncontact tension force can be applied to one end of a specimen using permanent magnets. As the tension force can be transmitted to the specimen in a noncontact way, the specimen can be isolated from the fixture at the end, and the specimen can be put in a container where a liquid environment can be created, which will be helpful to liquid and humid environment’s testing. However, the device in the research [
27] only allows one end of the specimen to realize noncontact force; the other end still remains in contact force. Therefore, it is difficult to seal the specimen in a closed container, but a completely closed container is necessary to create a vacuum and gas environment. That is to say, this device is only helpful for testing in a liquid environment and not really helpful for testing in a gas or vacuum environment. To address this issue, in the research [
22,
23], we developed a completely noncontact tension testing device and a completely noncontact bending device using maglev technology. In addition, by designing various maglev mechanisms, various types of noncontact material testing devices can be developed. The Torsion test is one of the most common mechanical tests; it can directly measure the shear strength of materials, which is a very important parameter in some applications. The Torsion test is applicable to a wide variety of materials, such as metals, polymers, ceramics, and composites, and it is a short test that can usually be finished in a few minutes. Due to these advantages, it is widely used in metal processing, material research, medical equipment, and the automobile industry. Given the status and superiority of the Torsion test, in this paper, we focus on developing a noncontact torsion testing device using maglev technology.
Primarily, it is necessary to consider a control scheme for global levitation stability. Generally, there are six levitation degrees of freedom (DoF) for a rigid levitated object. If many levitation DoFs can be inherently stable, i.e., passively stable, a lot of hardware, such as displacement sensors, electromagnets, amplifiers, and controllers can be saved. Therefore, we want the designed structure to have as many passively stable levitation DoFs as possible. However, according to Earnshaw’s theorem [
28], it is necessary that at least one levitation DoF be actively controlled for complete non-contact. In this paper, to maximize passively stable levitation DoFs, a structure with four passively stable levitation DoFs was designed. In the structure, the passive stability is mainly realized by four attractive-type permanent magnetic bearings; each attractive-type permanent magnetic bearing consists of two axially magnetized permanent magnet rings. Furthermore, to produce noncontact torque, we employed one permanent magnetic gear, which consists of two radially magnetized permanent magnets.
Furthermore, there are still two levitation DoFs to be actively controlled. We used typical attraction-type electromagnetic levitation for these two DoFs. Generally, a PD-controller is enough for ordinary electromagnetic levitation, and PD-gains are usually determined based on the plant model derived from the electromagnetic force model. However, in this case, since the attractive-type permanent magnetic bearings are involved in the work, the plant model will be affected by the permanent magnetic force. To obtain proper PD-gains, it is necessary to consider the effect caused by the permanent magnetic force. Therefore, before building the plant model, FEM analysis was conducted on the attractive-type permanent magnetic bearings to obtain their support characteristics. Then, taking the support characteristics into account, a plant model was built. With the plant model, two PD-controllers were designed.