Global view that shows the generation of the induced air flow (the path of the air flow is darkened). 

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... an original algorithm for distributed part differentiation was designed and several distributed synchronous and asynchronous discrete state acquisition algorithms and some stopping criteria were proposed [28]. Fig. 3 shows a general overview of the surface, from which we can see, scalability of this approach can be ensured by adding more systems in order to design a larger test bed. The Smart Blocks project [29, 30] is an extension of smart surface project. This project was funded by the French National Agency for Research (ANR). It has federated four French research laboratories and one Japanese laboratory. Its goal was to create a self-reconfigurable modular conveyor based on a contact-free technology. This conveyor is composed of centimeter-size blocks (1-3 cm), called smart blocks, which are linked together to form the conveying surface (Fig. 4). Re-configurability and scalability are highly considered in this project. Basically, these features come from the distributed nature of the smart conveyor. One can easily add blocks in order to extend the conveyor capabilities. Fig.5 displays basic block motion that can be implemented with block prototypes via electro-permanent (EP) magnets. Konishi and Fujita designed a fluidic micro actuator which has two on-off nozzles and fabricated an array of these micro actuators on an SOI (Silicon On Insulator) substrate for transporting tiny objects. The dimension of each actuator is about 100 μ m * 200 μ m. Each micro actuator can control the direction of air flow by electrostatically closing one of nozzles and can carry a flat object on the flow to the desired direction. Fig. 5 shows the principle of this actuator. The nozzle is normally open. When pressure is supplied from backside, air flows at an angle aslant from nozzles. Electrostatic force between the electrodes and the substrate is used to close nozzles. When the voltage is applied between the electrode and the substrate, the nozzle is squeezed down and closed by electrostatic force. When one nozzle is closed, the air can only flow through the other nozzle. As a result, the direction of the air flow can be controlled. Furthermore, if using a pair of actuators arranged orthogonally each other, then it can convey objects in four directions [31]. With the increasing complexity of industrial transport, advanced tasks such as high precision and manipulating many objects along arbitrary paths are required. A more complicated moving mechanism is invented: aerodynamic- traction, it’s a manipulation of the pressure field to move objects. Ku has presented a system, called ASD (active surface device). It is a new type of automation concept to handle multiple objects with high precision. The concept consists of using a massively parallel micro actuator array to generate a pressure field on a planar surface. The objects can be translated, rotated, and flipped by controlling the pressure field surrounding the objects. It is worth mentioning that, the general architecture of this system is modular, which allows the system to be expanded. As shown in Fig. 6, two types of pressure are used in this system: positive (blowing) pressure and negative (vacuum) pressure. The blowing pressure creates an ascending force to lift the objects above the surface so that the friction between object and surface is decreased. The vacuum pressure creates a descending force, which draws the object to the surface. When ascending force is equal to the descending force, there is a balance and objects can be maintained there. On the other hand, a vacuum tube next to one blowing tube forms airflow together. This produces a lateral force of traction directed from the high pressure area to the low pressure area. Undoubtedly the objects will move with the airflow when the force of traction is larger than the friction force. The precise movement of the objects is the advantage of this system, which is realized in three steps. Firstly, the target position is selected artificially; it is the place where the center of the object should be placed. Then a rectangular working area is defined automatically to reserve all the tubes between the initial position and the target position (Fig. 7). Secondly, two lines of vacuum in vertical and horizontal directions are placed near the object; all the tubes not belonging to the current two lines are set to blow. Then airflow is formed to draw the object toward the desired position. Thirdly, the two lines of vacuum are moved step by step towards the target position until they reach the edges of working area [32]. In the Smart Surface project, another device for manipulating objects was also designed; it has similar structure of Ku’s device, but the nozzles can only blow, not vacuum, so it is simpler. The product is carried on a thin air cushion and transported along the system by induced air fl ows. This induced air fl ow is an indirect effect of strong vertical air jets that pull the surrounding fl uid. As shown in Fig. 8, the surface is consists of 225 holes in total; each one is 0.4 mm in diameter. The holes are divided into to two categories, 113 fixed holes and 112 speci fi c holes, each speci fi c hole connect one nozzle out of two to independent air inlets. The air fl ow spreads over these fixed holes and creates the air cushion under the object; the object is maintained in constant levitation due to the air cushion. The novelty is that the object can be moved on the table by generating strong vertical air jets through the speci fi c nozzles of the surface. Each nozzle is driven by an independent solenoid valve. When a valve is open, a vertical air jet is generated. The air jet creates an induced air fl ow in the surrounding fl uid that pulls the object toward the nozzle [33]. Researchers have done a lot of works based on this device, in order to identify the location of object and control it; A H ∞ robust controller is designed and implemented on the device (see [34]). In order to control the object precisely, distributed control architecture is presented (see [35]). The two projects presented before require a mechanically complex array of actuators, each of which must be individually controllable. Compared to this, Luntz and Moon [36-39] have introduced a simple method of generating distributed manipulation fields in which passive air flow fields is generated at only a small number of discrete points. It applies forces over the entire surface of object floating in the field. A properly established field will manipulate the object to a predictable equilibrium position and orientation. This system levitates objects by air cushion generated from a standard air table, and moves objects by generating the manipulation flow field on the top surface of the object (Fig. 9). The motion of objects is also accomplished in three steps: firstly, the “air palm”, placed over the object, attracts it to its center and orients it using a pre-designed air field generated by flow sinks. Secondly, after a set time, the object is assumed to be oriented, suction increases, lifting the object and holding it firmly against the palm. Thirdly, the palm carries the object away with known orientation [40]. At present, there are also some conveying systems combined with the advantages of different levitation techniques. Air cushion is used for levitation and other techniques for moving. Pister has demonstrated a stable micro fabricated air bearing capable of levitating objects between 2 μ m and 1mm thick with extremely low kinetic friction, and no static friction. In order to minimize friction, the moving elements of their design ride on a cushion of air supplied through nozzles in the fixed surface of the motor. These moving parts, or platforms, are then actuated by electrostatic fields generated by conductors in the surface of the motor [41] (Fig. 10). Dkhil proposes a new approach where objects are manipulated through magnetic fields. The modeling proposed in his paper is illustrated on a magnetic device composed of four electromagnets. The magnetic force produced induces the motion of the micro object on the air interface (see Fig. 11) [42]. Experimental measurements demonstrate the repeatability of the motion for objects larger than 50 μ m (the variation of the permanent position of an object of 100x90x25 μ m 3 for a constant meniscus represents about 1% of its size). But, this device is designed without possible scalability properties. IV. C ONCLUSIONS This paper starts with the acknowledgment of the importance of levitation techniques and scalability and re- configurability properties in modern conveyors. Four different technologies based on non-contact levitation are presented and compared. The air levitation has several advantages, such as clean, magnetic free and generating little heat, simple in design, easy to use and maintenance, and has less demanding on the environment. A detailed analysis of many kinds of air levitation devices is made. According to the difference of moving mechanisms; they are summarized into three categories, i.e. inclined air jet, aerodynamic-traction and other non-flow integrated approaches. This provides a reference for selecting a reliable movement mechanism for future smart conveyors. Possible scalability of the different technique considered is presented. In future work, we plan to design scalable smart conveyors based on smart blocks and air levitation. This work will be a sequel of the Smart Block project. We shall study in particular motion mechanisms of blocks and control laws of the distributed ...