In the world of machinery, there are various transport systems which are required to carry substrates or articles from one station to another in order to have a succession of operations performed on them. For example, in congruent area color printing (CAP), a paper substrate is moved from one fixed printing station to the next for the successive deposition of colors onto the paper, with the color dots making up the image being brought into precise registration at each successive printing station. Registration accuracy is required because the different colored inks which reside on top of one another act collectively as multiple filters which can produce a wide range of color hues when superimposed thusly. Typically, for a 50 .mu.m.sup.2 printing dot, it would be desirable to maintain registration to an accuracy in the order of .+-.5 .mu.m.
Printing is not the only area in which the accurate transport of articles through successive workstations is of concern. For example, in integrated circuit manufacturing processes, semiconductor wafers have to be transported very precisely to successive fixed exposure, masking, etching and lead frame attaching stations in order to create the finished products. Any mispositioning of the wafers relative to the workstation results in wastage which can be quite costly to the manufacturer.
Depending upon the particular application, e.g., printing, masking, etc., the substrate carrier may be a belt or a cylinder. The carrier may even be a so-called moving floor type of conveyor in which one carrier section reciprocates relative to a stationary section with the latter section preventing any back sliding of the conveyed article during rearward motion of the moving section.
Classically, the requirement to accurately transport and position is substrates and other articles has been solved using expensive and cumbersome precision fixating mechanisms which usually include mechanical grippers or vacuum holders to assure the retention of the substrate or other article on its carrier. Carrier devices such as the paper cylinders used in the printing industry are not only complicated and costly because of their elaborate internal gripping mechanisms, but also they limit system speed because the mechanical grippers with their actuators and cams impose serious cycle time restrictions. A further disadvantage of these classical devices is their tendency to damage the substrates or other articles which they are gripping or clamping. Additionally, both mechanically actuated retention mechanisms as well as vacuum operated suction devices are quite noisy and thus they tend to disturb personnel working in the vicinity of the machines.
In addition to holding the paper or other article to its carrier, the prior apparatus invariably also include a tracking mechanism for controlling the position of the moving carrier relative to the fixed workstation. In the case of a belt-type carrier, this usually involves sensing the lateral position of the belt relative to a reference position, and steering the belt toward the reference position using some kind of servo mechanism, all of which further complicates the overall system.
Due to the high positioning registration accuracy required for color printing applications, such belt-type transports are usually not used to advance the paper or other printing substrate from one printing station to the next because the belts still tend to wander laterally despite the presence of the aforesaid belt positioning servo mechanism. It would be desirable, therefore, to be able to provide a belt-type transport system which can achieve good positioning registration accuracy at printing or other work stations served by the transport.
There is also a need to be able to safely and reliably transport and position a variety of substrates such as paper and plastic sheets, thin foils and other usually planar media using minimum power and at operating speeds substantially in excess of the presently attainable speeds.