# In-Plane Material Handling: A Systematic Review
## 3 Results
### Answer to RQ2
Lastly, in [82], researchers employed modules with inlet and outlet holes to create local air flows, and in [87], the authors used acoustic pressure fields made up of ultrasonic transducers. After pneumatic technology, it is the turn of variable-morphology surfaces. This class of devices exploits punctual changes in altitude in some areas of the surface to move light mesoscopic and macroscopic parts. The shape of the object does not prevent functioning but may require different motion rules. The main fields of application include research and certain intralogistics tasks, such as sorting [59]. Among the retrieved solutions, the oldest of the group is the \"magic carpet\" [57], i.e., a grid of vertical linear actuators capable of creating a deformed surface for material handling. Along the same line but connecting all the actuators with planes, a similar version was studied in [60]. Additionally, in [70], air jets were incorporated into the design. Conversely, in [10; 37; 59; 66; 67], a modular surface was used, exploiting four inflatable air chambers per module to manipulate objects with a bio-inspired caterpillar motion. The researchers also suggested that their device is a soft table and can be an alternative to contactless conveyance for fragile parts. Similarly, in terms of soft conveyance, in [58] a smaller modular version of the magic carpet using soft pneumatic actuators was proposed. Lastly, in [68], a ferrofluid-based sheet was employed to handle objects through the use of controllable magnetic fields. The last group shown in Figure 6 consists of surfaces with rotors. These systems are the only of the investigated technologies capable of transporting significantly heavy and large objects for intralogistics purposes. Their development was tailored to this task, alongside other applications, such as manipulation for flexible manufacturing [5]. The handled objects must have a planar surface to be supported simultaneously by multiple rotors on the same plane without getting stuck. The solutions retrieved within this research propose two main approaches for the manipulation of material flows, namely fixed-axis rotors and swiveling rotors. As mentioned in Section 3.2, the first approach was initially proposed by [PERSON] and [PERSON] [5; 9; 31; 88]. Their proposal consisted of an array of modules, each containing two motorized omnidirectional wheels positioned with their axes perpendicular to each other. Subsequently, other solutions emerged; in [89], eight wheels per module were arranged in a square layout, alternating the direction of their axes. Similarly, Refs. [7; 90] featured nine and four rotors per module, respectively. More complex designs still following the fixed-axis approach were developed in [4; 23]. The first introduces an E-pattern layout for the rotors in each module, alternating between three smaller horizontal rotors and a large vertical one (with the axes in the surface plane). The second employs a single module that interchanges rollers and pulleys with elastic bands to control actuation in two perpendicular directions. The bands primarily serve to divert the material on top, while the rollers convey it in the desired direction. The most industrialized solution with this approach is the _Cellueyor_, which was analyzed in [2; 3; 8; 91], featuring three omnidirectional wheels per module, each oriented 120\({}^{\circ}\) apart. On the other hand, the second approach uses swiveling driven wheels either in packs [53] or individually arranged for each module [52; 92]. Our own proposal also features a surface with rotors and adopts a similar \"swiveling\" approach [73; 93]. However, it uniquely embraces certain characteristics. For instance, it lacks spinning actuation (i.e., is underactuated), and the continuous swiveling of the axis is replaced by discrete temporary positions. Having now presented all the classes with their characteristics, it is possible to make comparisons and determine which solution is most suitable depending on the application.