# High-Precision Roller Supported by Active Magnetic Bearings
## 1 Introduction  
Display devices, such as flexible OLED, micro LED, electronic papers, RFID sensors, solar panels and other electronic elements can be mass-produced in a continuous manner similar to newspapers. Thus, the low-cost mass-production of printed electronics technology is attracting considerable attention [1; 2]. In particular, since display devices are composed of fine patterns with features ranging in size from a few \(\upmu\)m to tens of \(\upmu\)m, the roller must have a vibration amplitude smaller than this feature size. Ideally, the vibration amplitude should remain within a few \(\upmu\)m. The vibration of the roller and the application of an ever force during printing are some of the most important factors directly affecting the printing accuracy. Therefore, the roller used for printing requires extremely precise printing surface processing and low rotational vibration. Commonly used printing machines for producing sub-micron patterns include roll-based nanoimprinting and reverse offset printing. In addition, since printing forces significantly affect the patterning mechanism and printing quality [3; 4], printing machines for producing sub-micron patterns must have high-precision printing force control and a uniform printing force. For multi-step continuous roll-to-roll printing technology, the tension control of the web [5], the roll of paper used in offset printing, using the roller and longitudinal displacement control are alsoimportant to reduce printing errors [6]. Furthermore, the implementation of proper roller alignment is one of the most important factors in achieving high printing precision and a uniform printing force. In general, rollers are supported by ball bearings. In the case of printed electronics equipment, the performance improvement and commercialization of printed electronics elements are hindered by the poor rotation accuracy or difficulties during the roller alignment process. Most printing equipment under research/development uses rollers with ball bearings. Ball bearings exhibit significant mechanical wobble, which produces rotational vibrations with at least 10-20 \(\upmu\)m of displacement owing to non-uniform stiffness and clearance [7]. In addition, rollers tend to shift during roll-to-roll printing, which causes misalignment of the printing rollers such that disassembly and re-assembly of the printing rollers is required during the production process. Due to this, it is extremely difficult and time-consuming to implement reliable precision printing. A non-uniform force distribution on the printing surface causes periodic errors due to the printing force, and may cause serious printing defects such as nanoimprinting, microcontact printing, and offset printing [8; 9; 10; 11; 12]. The printing force applied by rollers must be uniform to improve the printing quality. For this, many additional devices are required, including various force and position sensors, pneumatic or hydraulic actuators, linear motors, and voice coil motors, which make the structure of the printing equipment complicated and more expensive while still requiring periodic re-calibration [13]. Magnetic bearings cause constant bearing stiffness with no friction because they support rollers in a non-contact way, while controlling the magnetic force [14]. This makes the rotational vibration of the roller very small. In addition, since the positions of both ends of the roller can be arbitrarily and easily changed within the air gaps between the shaft core and the electromagnets of the magnetic bearings, it is possible to automatically align the roller so that the printing forces at both ends of the roller can act uniformly. Active magnetic bearings have been used for a wide range of applications including rotors [15], turbo blowers [16], control moment gyroscopes [17], gears [18], and for machining [19].