Coating apparatus – Control means responsive to a randomly occurring sensed... – Responsive to attribute – absence or presence of work
Reexamination Certificate
1999-01-08
2001-11-20
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Control means responsive to a randomly occurring sensed...
Responsive to attribute, absence or presence of work
C118S671000
Reexamination Certificate
active
06319323
ABSTRACT:
TECHNICAL FIELD
The invention relates to automatic adjustment of working distance between a tool and a work surface.
BACKGROUND
It is often necessary or desired to provide a coating of a particular substrate. For example, in the video electronics industry it is often desired to coat panels which will serve as flat panel displays (FPD) to be incorporated into television sets, computer monitors and the like. It is important in such applications to ensure the accuracy and consistency of coating thicknesses across the panel.
A commonly employed method of coating flat panel displays is to have a stationary head extruding fluid at a particular rate over linearly moving panels however, moving heads are also used. Using such a configuration, the coating consistency is affected by a number of parameters such as the gap between the head and the panel surface, the variation in this gap as the panel moves, the dimensional consistency of the panel, the mechanical tolerances of the slot in the dispensing head, the pump characteristics and the fluid flow in the gap between the slot and the panel.
In prior art systems, variation in the height of the dispenser or extrusion head with respect to the panel can cause breaking of the coating bead and/or variation in coating thickness. The causes of such height variation include part dimension variation, part placement error, and gradual drift in machine dimensions over time. Accordingly, there is a need in the art for a system and method for ensuring constant height or “head gap” of a dispenser or extrusion head over a substrate being coated.
An independent contributor to possible variation in the height of the dispenser or dispensing head with respect to the substrate stems in fact from variation in machine dimensions rather than just variations in part placement and part dimensions. This variation in machine dimensions can result from a slow drift in mechanical dimensions over time, such as from the gradual bending of metal parts, or wearing of certain surfaces. The variation may also arise from thermal cycling causing expansion and contraction, and “settling” of surfaces/bearings under gravity.
Yet another possible contributor to possible variation in the height of a dispenser over the substrate to be coated could be variation in vertical position of the chuck holding the substrate along the direction of relative travel between the dispenser and the substrate.
Two separate problems should be distinguished here. There is the initial problem of establishing an accurate head gap when preparing for a coating operation. It is one problem in the art that variation in this initial head gap can exist between different coating runs. Separately, once this head gap has been initially established, there is the problem of accurately maintaining this head gap throughout a single coating process.
Prior art methods for solving the stated problems have involved manually inserting shims of varying thicknesses to set the head gap. This approach is deficient for several reasons. Although a head might be accurately established when the shims are first inserted, gradual drift in the dimensions of machine parts and the connections between them can cause the head gap to gradually change. It is important to note here that head gap changes of a few microns can adversely affect the coating bead.
A further problem with the prior art shim method is that even if the head is initially established with sufficient accuracy, the shims cannot compensate for subsequently introduced head gap variation arising from dimensional variation in the substrate or the chuck, part misplacement, or imperfect leveling of the substrate or supporting equipment. Therefore the manually inserted shims do not address the issue of maintaining the head gap throughout a coating process.
Therefore there is a need in the art for a system and method for accurately establishing the initial head gap in a coating apparatus prior to beginning a coating operation.
There is a further need in the art for a system and method for establishing the head gap which is independent of gradual change in the dimension and position of parts of the coating apparatus over time.
There is a still further need in the art for a system and method for accurately maintaining a properly established initial head gap throughout the performance of a coating operation.
There is a still further need in the art for a system and method for accurately maintaining a head gap throughout a coating operation independent of variation in dimensions of the part to be coated, error in part placement, and imperfect leveling of the structure supporting the part to be coated.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method which provides for real time sensing and adjustment of the height of the dispensing head with respect to the substrate being coated. This approach permits the inventive method to actively compensate for error in the dispenser independently of the source of such error.
The following discussion refers primarily within the context of adjusting the working height of dispenser, dispensing head, or extrusion head, above the surface of a substrate to be coated, the surrounding machinery being a coating apparatus. The principles discussed herein however, are applicable to the adjustment of a working distance, in any direction including various dimensions of angular motion, of any object to any other object within an apparatus where precise adjustment of the linear or angular distance between the two objects is important to the performance of said apparatus. Although the two objects will often be, respectively, a variety of tool and a workpiece, this is not always the case.
The problem of head gap variation is addressed by employing height sensing means feeding information to a control system which activates a motor or other drive means to drive the dispensing head higher or lower as the sensing data dictates. The height sensor is taught an appropriate “zero” or reference point representing the correct height of the dispensing head. Subsequent deviation from that point results in an error signal causing the control system and motor to correct the dispensing head's height. Preferably, the rate or velocity of adjustment in the height of dispensing head is tempered so as to ensure that the bead of the fluid being dispensed will not be harmed or broken. Sensing means available for this purpose include but are not limited to mechanical contact sensing, optical, air cushion, electromagnetic, hall effect, sonic, and ultrasonic.
It is noted that the dispenser could have several possible configurations including but not limited to an extrusion head with a precisely machined linear slot cut in it for extruding coating fluid in a precisely controllable manner.
In a preferred embodiment the drive means for controlling the height of the dispenser above the substrate to be coated is the same as that which provides larger scale motion of much of the structure holding the dispensing head, such as a shuttle mechanism or other transport system which moves the dispenser along the coating direction over the substrate. This arrangement is economical in that it obviates the need for a second drive means to move the dispenser with respect to the substrate.
Alternatively, a second drive means could be utilized, such as might be attached in between the shuttle mechanism and the dispenser. Preferably, with such an alternate arrangement, the vertical axis of the head gap mechanism or other dispenser mounting apparatus would remain mostly stationary during the coating operation permitting what is preferably a highly precise second drive means to conduct the head gap maintenance operation.
In either of the configurations of drive means discussed, the dispenser may either be attached to the shuttle mechanism or other dispenser mounting apparatus in such a manner as to be readily and rapidly removable therefrom, or it may be more permanently attached to the shuttle mechanism. Sensing the height
Frerking Darwin R.
Gibson Gregory M.
Hawes John E.
Poonawala Altaf A.
Wang Zi-Qin
FAStar, Ltd.
Fulbright & Jaworski L.L.P.
Lamb Brenda A.
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