# Study and Simulation of an Under-Actuated Smart Surface for Material Flow Handling
## 1 Introduction  
The material transport inside factories and warehouses is a very studied topic. However, companies are always searching for new, cheaper solutions while keeping an eye on efficiency and flexibility [1; 2; 3; 4]. Starting with the most basic conveyors, many alternative systems [5; 6; 7; 8; 9; 10; 11; 12] have been developed in order to move, sort, orient, and handle the material flow. More recently, the focus of investigation has shifted towards modular devices and surfaces that allow high handling capability, adaptability, and reconfigurability [3; 4; 13]. This happens because the market increasingly demands a flexible industry, and this is reflected in all production levels, even the internal transportation. These devices are often called smart surfaces because they are controllable and reprogrammable with computers. Thanks to these characteristics, these new systems permit achieving their goals without structural changes [4; 13]. In addition, the new devices are capable of identifying a part with sensors and acting in diverse ways, according to their programming. As a result, supervision by an operator is no longer required for the decision process and the system becomes autonomous, promising to reduce errors and costs [14]. The most relevant solutions among the smart surfaces, according to the current state of the art and classified by the operating principle, are: micro electro-mechanical systems, vibrating surfaces, ciliary motion, variable morphology surfaces, pneumatic surfaces, surfaces with rotors, and mobile platforms. Micro electro-mechanical systems (MEMS) [6; 15; 16] are an array of microscopic cantilevers or tilting planes, actuated electrically, that generateforces to transport the object in contact. The second category of the list are vibrating surfaces. These consist of vibrating plates on which a sequence of supply frequencies is applied to generate a two-dimensional force field for the handling tasks [5,17]. In [12, 18, 19, 20], ciliary motion is proposed to move and manipulate objects, taking advantage of an array of controllable cilia for contact conveyance. Another class are variable morphology surfaces [9, 21, 22, 23]. For these devices, gravity together with inclined planes and vertical actuators at different altitudes are used to create a preferential path for the movement and rotation of an object. Additionally, pneumatic surfaces [7, 24, 25, 26, 27, 28, 29, 30] were introduced to handle materials without direct contact. The working principle for the latter is to have modules with nozzles to move parts by directing the air flow below them. On the other hand, surfaces with rotors take advantage of contact forces made by actively driven wheels to manipulate the material flow [3, 11, 13, 14, 31, 32, 33, 34, 35]. Finally, mobile platforms [8, 36, 37, 38, 39] consist of mobile pallets, not connected to fixed axes and free to move on a plane, transporting objects on top. Despite the categorization, not all of these are practically used for macroscopic intralogistics purposes (object size and displacement \(>\) cm). In fact, as summarized in Figure 1, some of them, such as MEMS, together with some mobile platforms [8, 37, 38, 39] and ciliary devices [12, 18, 19], are more suitable for microscopic transport (object size and displacement \(<\) mm). In contrast, pneumatic systems, vibrating surfaces, surfaces with rotors, variable morphology surfaces, some mobile platforms [36], and even tilted brushes (Cilia) [20] can be used to move macroscopic and mesoscopic (object size and displacement \(>\) mm, \(<\)cm) objects. Referring to the previous classification, the system proposed in this paper can be included in the surfaces with rotors category. The literature review conducted on this category identified two main types of modular smart surfaces: systems where each module has only one degree of freedom [3, 11, 13, 35] and systems where each module has two [14, 34].