# Development of Piezoelectric Inertial Rotary Motor for Free-Space Optical Communication Systems
## 2 Motor Design and Operation Principle  
The motor is composed of a stator that is clamped, via stator supports, between two printed circuit boards that are additionally connected to each other by three supports. The top and bottom surfaces of the stator have three alumina oxide contacts, each of which is used to preload disc-shaped rotors to the stator. The preload force of the rotors to the alumina oxide contacts is created by the spring, whose stiffness is controlled by changing the longitudinal position of the spring clamper on the axis of the motor. The design of the motor is represented in Figure 1.  
Figure 1: Design of the motor: (**a**)—assembled view; (**b**)—exploited view; (1)—support of printed circuit boards; (2)—stator; (3)—clamping bolts; (4)—printed circuit boards; (5)—bottom disc-shaped rotor; (6)—top disc-shaped rotor; (7)—preload spring; (8)—axis; (9)—spring clamper; (10)—spring clamper fixing bolts. Therefore, according to Figure 1, the design of the motor can be found to be simple and compact. The total volume of the motor is 443 cm\({}^{3}\), while the footprint is 13.84 cm\({}^{2}\) and the mass is 28.14 g. Therefore, considering these parameters, the motor has good physical characteristics for space applications. However, to fully represent the motor, a detailed stator design is shown in Figure 2.  
As can be found in Figure 2, the stator is composed of three elastic rectangular plates on both surfaces of which are placed piezoceramic plates and, by this way, piezoelectric bimorphs are composed. The piezoelectric bimorph plates are composed to one structure via the connecting cylinder, i.e., one of the ends of the piezoelectric bimorph plates is connected to the cylinder while the angular distribution of the plates, around the vertical axis of the cylinder, is equal. Moreover, cuts are made at free surfaces of the connecting cylinder, which reduces the structural stiffness of the whole stator structure and enables the possibility to obtain higher displacement amplitudes. On the top and bottom surfaces of the stator, six alumina oxide contacts are used to transfer, via friction force, displacements of the stator to the rotors. The position of alumina oxide contacts is chosen in such a way that their vertical axis of symmetry coincides with the antinode of the first, out-of-plane, bending mode of the piezoelectric bimorph plates. Such a position of the contacts allows the transfer of the highest displacement amplitudes of the piezoelectric bimorph plates to the rotors. Finally, the supports, which are used to clamp the stator to the PCB, are placed on the top and bottom surfaces of the stator in a such way that their vertical axis of symmetry is in coincidence with the node of the first, out-of-plane, bending mode of the piezoelectric bimorph plates. Such a position of the supports allows us to reduce the structural stiffness of the whole structure. A detailed sketch of the stator and geometrical characteristics of it are given in Figure 3 and Table 1.  
Figure 2: Detailed design of the stator: (**a**)—assembled view; (**b**)—exploited view; (**1**)—support of the stator; (**2**)—elastic plate; (**3**)—cut in the connecting cylinder; (**4**)—alumina oxide contacts; (**5**)—connecting cylinder; (**6**)— piezoceramic plates. Therefore, operation of the motor is based on the inertial stick-slip principle, which is inducted by excitation of the first, out-of-plane, bending modes of the piezoelectric bimorph plates by a single sawtooth signal with a frequency equal or close to the resonance