# A Review of Innovative Electromagnetic Technologies for a Totally Artificial Heart
## 5 Unconventional Electromagnetic Actuators
### Linear Rotary Actuator (LiRA)  
Linear rotary motion can be obtained by two different approaches: magnetic conversion (MC-LiRA) from a rotating motion impressed by a conventional electric motor into a linear one by using PMs and possibly ferromagnetic parts (passive configurations), and straightforward generation of a roto-translating field in a single device (conventional LiRA) using supplied windings (active configurations). MC-LiRAs have been conceived for different applications that require a short stroke and a high thrust [83]. The Magscrew TAH project has already implemented such an actuator [84]. The operating principle is based on the concept of a magnetic screw-nut (Figure 13a). Surface PMs with helicoidal distribution mounted on the back of the screw and on the nut move in synchronism, driven by the corresponding pole interaction. Motion control is in charge of a prime rotating motor, which must produce a clockwise and counterclockwise torque \(T_{n}\) to develop a reciprocating thrust force \(F_{t}\). The relation to be fulfilled between the torque and force is \(T_{n}=\tau_{p}/\pi\cdot F_{t}\) (\(\tau_{p}\): pole pitch). With Halbach magnetization and properly sized PMs, with an outer nut diameter of 17 mm and mover length of 12 mm, a thrust higher than 100 N is achieved. An actuator based on this concept, implemented in in vivo experiments, achieved a blood flow greater than 8 l/min and a maximum pressure of 100 mm Hg, with a device beat rate ranging from 157 to 249 ppm [85]. A field-modulated magnetic screw, proposed in [86], is based on the concept of a magnetic screw and a linear magnetic gear. An intermediate translated set of helically ferromagnetic pole pieces modulate the magnetic field produced by outer static helically shaped PM arrays (Figure 13b). Therefore, its interaction with the inner side rotating PMs enables the conversion of the torque applied to the rotor PM screw into thrust by acting on the intermediate ferromagnetic translator. The increase of the thrust to torque ratio by the gearing effect relies on satisfying the relation \(n_{s}=p_{r}+p_{s}\), with \(n_{s}\) number of ferromagnetic pole pieces, \(p_{r}\) and \(p_{s}\) number of pole-pairs on the rotor, and stationary screws, respectively. Such actuators can achieve a force density of up to 20 N/cm\({}^{3}\), consistent with TAH requirements, even if their placement must consider the presence of the prime motor and the troublesome manufacturing of very small helicoidal parts. Conventional LiRAs with supplied stator winding are attractive in modern machine tools and robotics because of their ability to provide motion with two degrees of freedom. However, their inherent tubular configuration makes them suitable for TAH application; moreover, because of the possibility of the independent management of rotation and translation, their control is more straightforward and flexible than in MC-LiRAs. In this field, VR PM-free configurations are also proposed. The most significant ones rely on rotary-linear switched reluctance motors (RL-SRMs), as they benefit from simple but robust construction, low manufacturing and maintenance costs, high reliability and fault tolerance, and easy control [87]. They can present a multi-stack assembly of classical SRMs with suitably displaced rotors, where an adequate supplying sequence of the stator coils can provide the rotation or linear movement. Alternatively, they can be provided with two separate sections: a rotating section for clockwise-counterclockwise motions in the middle and a linear section for the forward-backward movements at the actuator sides. Despite its attractiveness for some applications (pick and place machines, robotics, and automotive), RL-SRMs suffer from the main limitations of its LOA counterpart in terms of thrust density (\(\sigma_{f}\cong 0.02\) N/cm\({}^{3}\) in [88]), making its application for a TAH unattractive. Figure 13c shows a basic PM LiRA.