# A Review of Innovative Electromagnetic Technologies for a Totally Artificial Heart
## 4 Linear Electromagnetic Actuators
### PM-LOAs  
Even if various potentially applicable LOA typologies could easily be borrowed from numerous short-stroke industry applications (compressors, pumps, vibrators, etc. ), PM synchronous motors, regardless of the variable reluctance (VR), are the best suited to meet the requirements of high efficiency and force density for TAHs [63,64]. It is worth pointing out that the installation in a chamber separated from blood sacs avoids any contact, and then no biocompatibility issues arise with the actuator materials (e.g., PM material). Demagnetization issues can be avoided by adopting suitable NdFeB grades and PM thickness at the design stage. PM-free LOA are also considered for an LVAD actuator [65]. Their simple and inexpensive manufacturing as well as their straightforward control strategy represent significant features for possible TAH application. However, the production of high reluctance force requires a very high current density and a very small air-gap length, which is difficult to implement in miniaturized devices. As a result, \(\sigma_{f}\) tends to be too low for a TAH application. For example, the rated thrust density of the linear switched reluctance actuator analyzed in [65] is \(\sigma_{f}\cong\) 0.06 N/cm\({}^{3}\) at 10 A/mm\({}^{2}\) and with 0.2 mm air-gap length. Tubular PM configurations are preferred because they best fit the available volume between the ventricle chambers. The most effective quadrature condition between coil m.m.f. and PM magnetic field can hardly be achieved with ordinary single-phase supply, taking into consideration the short stroke and other electromagnetic effects (e.g., longitudinal end effects, magnetic saturation, and cogging). A proper design, possibly involving the magnetic core shape, the PM magnetization pattern, and a suitable supply current switching strategy can mitigate force ripple, providing an adequate thrust average value at the same time. PM-type LOAs can be divided into coreless moving coils (MC-LOA) and moving PMs (MM-LOA), provided or not with back iron, according to the PM magnetization pattern. In MC-LOAs, the supplied coils slide along stationary PMs, mounted on a back-iron core, or enclosed between two core flux concentrators (Figure 7a). The confinement of PM flux generally requires rather bulky cores; moreover, the moving coils should be divided into sections supplied separately with a proper sequential current control to equalize the thrust profile and limit winding losses, since the Lorenz force is proportional to the ampere-turns of the motor coil portion passed through by the PM magnetic flux [32]. In MM-LOAs, the PM mover slides alongside the stator core with a C- or E-shaped cross section, enclosing a single- or a double-coil winding. With respect to MC-LOA, the stator coil supply benefits from a faster dynamic response and lower ohmic losses. Moreover, slotted configurations can take advantage of the cogging force in the forward stroke, resulting in an effective reduction of ohmic losses, although possibly impairing thebackward stroke motion. Electromagnetic interference with other devices because of the PM flux leakage at the stroke ends and the eddy current losses in the ferromagnetic cores are other possible issues. Alternatively, both the PMs and the coils could be placed on the stator and could interact with a passive VR mover. Such a geometry complicates small-sized stator manufacturing, and it requires very low air-gap length (0.3-0.5 mm) to enhance the modulation effect of the coil m.m.f.. However, it enables adequate thrust density and PM flux confinement. For example, in [66] a hollow-type mover is proposed that slides between two U-shaped stator rings (Figure 7b). Cogging force minimization calls for a suitable mover slotting design based on a half-period shift between the inner and outer parts. The supply of the stator coil pair by 90\({}^{\circ}\) phase displacement yields \(\sigma_{f}=0.57\) N/cm\({}^{3}\).