Method for manufacturing long force sensors using screen printing technology

[0001] The present application is a national stage entry of PCT Application Number PCT/US06/23578, filed Jun. 16, 2006, which is a continuation-in-part of its priority application, U.S. patent application Ser. No. 11/154,004, filed Jun. 16, 2005, now abandoned herein incorporated by reference.

[0002] The present invention relates to a method for manufacturing long force sensors with a repeated design pattern using screen printing or other repetitive printing technology. Sensors produced according to the method do not have any practical limitation on length.

[0003] Such sensor technology is desirable in situations in which a lengthy sensor construction is needed. For example, in a tennis court, it is desirable to automate line calling, which is the detection as to whether a tennis ball impacts the ground at an in-bounds location or an out-of-bounds location. Flat force detecting sensors may be utilized at the boundaries to make a determination of the point of ball impact. An exemplary use of such sensors is described in the U.S. patent application Ser. No. 11/917,802, herein incorporated by reference.

[0004] Because of the tennis court size, sensors have to be manufactured extremely long (up to 60′ long). In principle, one could simply create and utilize sensors having a length of, e.g., 3′ or, and then arrange such sensors next to one another all the way along the various boundary lines. However, the sensors manufactured with various embodiments of the present inventive technology provide numerous advantages.

[0005] During the installation of such flat sensors, one cannot avoid overlapping the sensors in order to provide a sensing area all the way along the lines. This overlapping leads to surface unevenness. The primary reason for this is that along the perimeter of the sensor, there is typically an area which is not sensitive and which is devoted for adhesive or waterproofing. For short sensors, the overlaps become numerous.

[0006] Additionally, each sensor area requires a cable connecting it to a computer. Again, in a short sensor configuration and considering the size of a tennis court, use of short sensors would require a tremendous amount of cables running across the area, which would make the system very complex, unreliable, and very expensive, relative to a system in which long sensors are used.

[0007] The present invention is directed to a method for manufacturing a force or pressure detecting sensor comprising: designing a repeating conductive trace pattern that can be replicated to produce a consistent conductive trace across more than one adjacent pattern section forming an electrical bus; and printing more than one section of a series of conductive traces on a thin and flexible dielectric backing using the pattern. The invention is also directed to a sensor comprising: a thin and flexible dielectric backing; a repeated pattern of conductive traces printed above the dielectric backing; one or more dielectric layers provided above the conductive traces, the dielectric layers having access regions permitting contact of conductors above the one or more dielectric layers; and a sensor conductor layer printed above the one or more dielectric layers that contacts the conductive traces via at least one of the access regions or regions not covered by the one or more dielectric layers.

[0008] It should be noted that sensors made as long as 60′ still require one to address the effect of thermal expansion and contraction, because of the difference in the coefficients of thermal expansion for plastic (as a part of the sensor) and asphalt or concrete (on or within which the sensor resides). In order to prevent bubbling and separation of the sensor from the ground, one may use a double sided adhesive, contact cement, epoxy or other adhesion means which forms a sufficiently strong bond. Examples could include VHB tape or Dp190 and Dp460 epoxies made by 3M.

[0009] The obvious advantage of printing a multi-layer sensor is that conductive traces do not take up space on the side which minimizes the dead area of the sensor dramatically. For example, if one tried to print a 40′ long sensor and run conductive traces on the sides on an 18″ wide strip of Mylar plastic, the actual sensor width would be reduced to 12″ (30% loss of the area). One could try to reduce the width and separation between the traces, but that would lead to unacceptable increase in resistance, as well as to errors due to screen printing technology tolerance.

[0035] As illustrated in FIG. 1, each sensor 10 comprises sections 20 that are fairly short in length and thus easy to print in a repetitive manner; such a length, for example, may be 1′. Each sensor section 20 may comprise a separate analog output. Separate sensor areas permit one to distinguish between different force or pressure events (for example, a ball impact and foot step) that can happen at the same time on separate areas of one particular sensor. They also allow one to localize the location of an event to within the area of a sensor, and in case of failure of a sensor area, only one small area would be affected. This idea of splitting up a sensor into smaller sensor areas is described in U.S. Pat. No. 3,982,759 (Grant).

[0036] Because of the desired length of the long sensors 10, they can only be printed if the artwork or layout design has a repeating pattern. The following discussion and references to the Figures illustrate how this is done.

[0037] First, a series of conductive traces 12 are printed on a thin and flexible dielectric backing. Given the excellent conductivity characteristics of silver, its use would be beneficial in the present design, although other known conductive materials may be used. Mylar plastic is an ideal dielectric backing that has the desired characteristics of being thin and flexible.

[0038] The pattern for the conductive traces may utilize a trace width of approximately 50 mils, with an appertaining separation 14 between the traces being approximately 50 mils as well. Of course, the widths and distances can easily be modified by one of skill in the art to values that are suitable for any particular application. The values chosen can depend on a length of the sensor, a number of wires to be printed, as well as on a size of a printing screen. An exemplary screen pattern is shown in FIGS. 2 and 6. It can be seen that the pattern consists of a continuous common trace which is thicker than the other traces 12. This common trace is shared by all of the sensor areas on a sensor. Additionally, one trace 12 is printed for each sensor area on the sensor. These traces take one step up or down after each print, forming a cascading pattern. This pattern is printed repetitively until the required length is achieved. Because the traces cascade, each sensor area ends up being connected to just one trace on the bus (discounting the common trace).

[0039] The printed trace section 20 is printed in a repeated manner, as illustrated in FIG. 2. It can be seen that repeating the patter shown in FIG. 1 permits a conductive trace pattern to span more than one printed section 20. Such a pattern can be repeatedly printed to a desired length, limited only by the amount of raw materials available.

[0040] FIG. 3 illustrates the next step, in which a tail 30 is printed to the left which connects the sensors with cables from various electronics and/or computer systems used to acquire sensor readings. (Note that tail is printed on the same plastic as the sensor, therefore there is no connection point at an installation surface, such as the playing area of the tennis court).

[0041] As can be seen in FIG. 4, once the conductive traces 12 are printed, they are covered with one or more layers of a dielectric 40. Each print of the dielectric layer may have vias 42, which are holes that allow traces below 12 to interconnect with traces that are printed above 50 in the following step. Also, the dielectric layer does not cover tips from the bus, on top of which the final layer of conductive print will be applied. These tips also interconnect with traces that are printed above 50 in the following step. By way of these interconnections, the next layer printed 50 which is the layer that does the sensing, is electrically connected to appropriate traces 12 on the bus.

[0042] FIG. 5 illustrates the final layer that is applied on top of the dielectric layer 40, and comprises interdigitated fingers 50 that are used to contact portions of the conductive traces 12 lying below. This interdigitated finger 50 technique is a standard technique which is well known in the art and is described in U.S. Pat. No. 4,314,227 (Eventoff).

[0043] The sensor layout illustrated in FIGS. 1-5 is ideally designed and suited for detecting whether a tennis ball impact with the ground occurred “in” or “out” of a particular boundary line in which such sensors 10 have been placed, i.e., on the sidelines, baseline, and service lines of a tennis court.

[0044] In an embodiment of the sensor illustrated in FIGS. 6 and 20, it can be seen that an asymmetrical pattern (with regards to a longitudinal dividing line) is provided. Such a pattern may be utilized in, e.g., a center line of a tennis court for detecting whether a tennis ball landed to the left, right, or directly under the center line between two service courts.

[0045] The ideal pattern illustrated in the following figures is different due to the fact that players change the direction of the serve after each point. Thus, the sensor needs to have three positions with respect to the boundary line between two service courts, the position to the left, right, and directly under the center line between two service courts. The position directly under the center line always registers an IN bounce while the other two positions can register either OUT or IN depending on the direction of serve. The asymmetry of the trace pattern for the three position sensor is due to the fact that three sets of trace and a common trace need to be run to the three sets of sensor sections.

[0046] FIG. 6 illustrates the sensor 10 layout pattern according to this embodiment in which conductive traces are asymmetrically provided around a horizontal longitudinal line. FIGS. 7 and 21 illustrate the appertaining dielectric 40 layer pattern that is utilized, including the holes 42. The hole 42 placement allows each of the three sensor sections to electrically connect with an appropriate trace from each of the three sets of traces.

[0047] FIGS. 8 and 23 illustrate the interdigitating finger pattern 50 that is utilized in the sensor 10 of this embodiment.

[0048] Finally, FIG. 9 illustrates all of the layers of this second embodiment combined, after they are applied in sequence, as described above.

[0049] FIG. 10 illustrates a printing of dielectric dots 62 on top of the interdigitating finger layer 50 with an adhesive on top, as well as, for example, 0.5″ 3M VHB (very high bond double sided tape) 60 across the perimeter of the plastic. On top of the dot pattern, a top layer of plastic is typically attached which has an FSR layer that faces the interdigitating fingers 10. The FSR layer conducts electricity in a manner approximately proportionally to the force that is used to compress the top and bottom layer of the sensor together. In such a way, a long force or pressure sensor can be created. The dot pattern serves both to adhere the bottom and top layer together and to separate them so they do not touch when no force at all is applied. The tape serves to further reinforce the attachment between the top and bottom layers. Although a dot pattern is shown and a particular exemplary tape type described, one of skill in the art would recognize that the pattern could be varied and a perimeter adhesive of any workable type could be employed.

[0050] Because an assembled sensor can be damaged by excessive bending, it is advantageous to ship the top and bottom layer rolled up separately on spools to an installation site and to attach them together on site. Assembly of the top and bottom layer can be done easily by running the two layers simultaneously through a device such as a laminator. The laminator can be run in this way without laminating film, in which case the top and bottom layers would simply be joined together. However, by applying laminating film at the same time as the sensors are run through the laminator, the sensors can be hermetically sealed and waterproofed all in the same step. Furthermore, the lamination, helps in keeping dust out of the sensor, and further increasing the attachment strength between the top and bottom layers.

[0051] The printing of the adhesive on top of the dots as well as attaching VHB strips along the perimeter is optional and depends on the application of the sensor 10. In case the sensors 10 are to be used indoors, for example under Teraflex carpet made by Gerflor, one can avoid permanent attachment of the top layer and the bottom layer using adhesive but instead could laminate top and bottom with a laminating film that would keep dust out but also could be peeled off easily, as needed, to create a portable sensor 10 that can be rolled and re-used at different location or later on at the same location.

[0052] For example, some businesses use indoor facilities for hockey in the winter time and for tennis in the summer time. Therefore these businesses should be able to remove the sensors 10 from the courts after the tennis season is over, and install them back for the next season. When the sensors 10 are permanently assembled (using the adhesive and VHB, as described above) they can not be rolled or folded since that would lead to plastic distortion, and delamination, thereby damaging the sensors 10. Because the sensors 10 are extremely long, without the ability to separate the top and bottom and roll them, it would be problematic and expensive to store them over the winter period, or to transport them from one location to the other.

[0053] For the purposes of promoting an understanding of the principles of the invention, reference has been made to the preferred embodiments illustrated in the drawings, and specific language has been used to describe these embodiments. However, no limitation of the scope of the invention is intended by this specific language, and the invention should be construed to encompass all embodiments that would normally occur to one of ordinary skill in the art. The present invention may be described in terms of functional block components and various processing steps. Such functional blocks may be realized by any number of hardware components configured to perform the specified functions. The particular implementations shown and described herein are illustrative examples of the invention and are not intended to otherwise limit the scope of the invention in any way. For the sake of brevity, conventional aspects may not be described in detail. Furthermore, the connecting lines, or connectors shown in the various figures presented are intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements. It should be noted that many alternative or additional functional relationships, physical connections or logical connections may be present in a practical device. Moreover, no item or component is essential to the practice of the invention unless the element is specifically described as “essential” or “critical”. Numerous modifications and adaptations will be readily apparent to those skilled in this art without departing from the spirit and scope of the present invention.