Patent Publication Number: US-2006016994-A1

Title: System and method to prevent cross-talk between a transmitter and a receiver

Description:
TECHNICAL FIELD  
      This invention relates in general to preventing cross-talk between a transmitter and a receiver, and specifically, to preventing cross-talk in an infrared proximity sensor.  
     BACKGROUND OF THE INVENTION  
      Proximity sensors may be found in various applications, from consumer products to commercial and industrial machines. Traditional proximity sensors usually include one transmitter and one receiver that are placed such that their respective transducers both point outward to a detection region. When an object moves in front of the proximity sensor, it reflects light from the transmitter, some of which is picked up by the receiver. When the receiver picks up light from the transmitter, it sends a signal that is interpreted as indicating that an object is present.  
      An issue that often arises with proximity sensors is the phenomenon of cross-talk. Cross-talk, in many traditional applications, may be considered to be when light from a transmitter is detected by a receiver without first having been reflected off of an object in the detection zone. Cross-talk is often associated with stray light and unwanted light reaching the receiver, which may hamper the sensor&#39;s accuracy and degrade performance.  
      A traditional approach for reducing cross-talk is to place a piece of material between the transmitter and the receiver or to try to surround each of the transmitter and receiver with separate light-blocking structures that are not continuous with respect to the piece of material that separates the transmitter and receiver. Further, proximity sensors mounted on Printed Circuit Boards (PCBs) may experience some cross-talk from light that is transmitted through the material of the PCB.  
     BRIEF SUMMARY OF THE INVENTION  
      According to at least one embodiment of the invention, a method includes mounting a transmitting device and a receiving device on a circuit board, wherein the circuit board includes a layer that blocks waves (e.g., light waves) emitted from the transmitting device, and wherein the transmitting device and the receiving device are mounted in an area defined by the layer. The method further includes manipulating a structure to form a first compartment for the transmitting device and a second compartment for the receiving device, wherein the compartments are separated by a folded double wall that is continuous with each compartment. The method further comprises mounting the structure on the circuit board. According to this embodiment, the transmitter and the receiver may be part of a proximity sensor, and the layer that blocks waves, the first and second compartments, and the double wall operate to prevent cross-talk between the transmitter and the receiver.  
      According to another embodiment, an apparatus comprises a transmitter, a receiver, and a Printed Circuit Board (PCB), wherein the PCB includes a layer of material that blocks electromagnetic waves from the transmitter, and wherein the transmitter and receiver are mounted on the PCB in an area defined by the layer. Accordingly, this embodiment may operate to prevent electromagnetic waves from the transmitter from reaching the receiver through the PCB, thereby preventing cross-talk.  
      According to yet another embodiment, an apparatus comprises a piece of material manipulated to form two compartments, a folded double wall separating the two compartments, wherein the double wall is continuous with each of the two compartments, and a transmitter and a receiver, wherein each of the transmitter and receiver are substantially within a volume defined by one of the compartments. The two compartments and the double wall may form a shield, which operates to prevent cross-talk. The continuousness of the double wall and the two compartments may function to ensure that waves from one compartment do not reach the other compartment without first being reflected from an object in the detection zone.  
      The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an illustration of a shield, adapted according to various embodiments, for preventing cross-talk between a receiver and a transmitter;  
       FIG. 2  is an illustration of a manipulating process, adapted according to various embodiments, for forming a shield,  
       FIG. 3  is an illustration of an example system, wherein a shield is mounted on a PCB;  
       FIG. 4  is an illustration of an example proximity sensor unit, adapted according to various embodiments, for preventing cross-talk;  
       FIG. 5  is an illustration of an example proximity sensor unit, adapted according to various embodiments, for preventing cross-talk;  
       FIG. 6  is a flowchart illustrating an example method for preventing cross-talk;  
       FIG. 7  is a flowchart that depicts an example method, according to various embodiments, for preventing cross-talk;  
       FIG. 8  is an illustration that depicts an example application that employs a proximity sensor unit, adapted according to various embodiments.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  is an illustration of shield  100 , adapted according to various embodiments, for preventing cross-talk between a receiver and a transmitter. Shield  100 , in this example, is constructed from material that is manipulated to form compartments  101  and  102 . An example construction is discussed below with regard to  FIG. 2 . Area  105  of shield  101  depicts the separation between compartments  101  and  102 , and, as will be explained below, compartments  101  and  102  are separated by a common wall structure, wherein each of the compartments are continuous with the wall structure. The common wall structure cannot be seen in  FIG. 1 , but an example is seen in  FIG. 2 , as a folded double wall.  
      In this particular embodiment, shield  100  is designed to prevent cross-talk between a receiver (RX) and a transmitter (TX) (not shown in  FIG. 1 ), when each of the TX and RX are substantially within a volume of one of the compartments. In this example, an RX may be placed in compartment  101 , and a TX may be placed in compartment  102 . That each of the TX and RX are substantially within a volume of one of the compartments means that some, but not necessarily all, of each of the TX and RX apparatuses are located inside the compartments, while being sufficiently located within compartments  101  and  102  such that the common wall structure can act as a barrier to prevent cross-talk. For example, in some embodiments, the transducers of each of the TX and RX will be located in compartments, whereas the wires that are in electrical communication with the transducers to carry signals and power may be routed outside of the compartments. Alternative embodiments may employ other arrangements, and all are within the scope of the invention as long as shield  100  can, by itself or with other components, be used to reduce or prevent cross-talk.  
      Aperture  104  allows the TX to transmit electromagnetic waves outside of shield  100 , and aperture  103  allows the RX to receive electromagnetic waves from outside shield  100 . In this example, the TX and RX may be part of a proximity sensor, such that an object that is in front of apertures  103  and  104  (i.e., in the detection region) will reflect waves from the TX, and the RX will receive at least some of the reflected waves from the object. When the RX receives waves from the TX it signals that an object is near. Accordingly, in many embodiments it is undesirable for the RX to receive waves from the TX that have not been reflected from an object in the detection region because the reception of those waves will trigger a false indication (i.e., cross-talk). Cross-talk may sometimes be referred to as “direct communication” between the transmitter and receiver, though the waves may be reflected from one or more surfaces other than an object in the detection region.  
      Shield  100  acts to prevent cross-talk by blocking waves from the TX to the RX that are not reflected from an object in the detection region. Specifically, the walls of compartments  101  and  102 , including the common wall structure separating the compartments, act to isolate the RX from the waves emitted from the TX, unless, of course, those waves are received through aperture  103 . As explained further below, shield  100  may be combined with a TX and RX and mounted to a circuit board to produce a proximity sensor unit for use in a variety of applications, although applications that use a transmitter and a receiver for other than proximity sensing are within the scope of various embodiments.  
       FIG. 2  is an illustration of manipulating process  200 , adapted according to various embodiments, for forming shield  100 . Manipulating process  200  includes steps  220 ,  230 ,  240 ,  250 ,  260 , and  270 .  
      In step  220 , the structure is a single piece of material that is laid flat, and it can be seen that the material includes sections  201 - 208 . Apertures  103  and  104  may be seen as cut-out areas of sections  201  and  202 , respectively. In step  230 , the material is folded such that sections  201 ,  202 ,  207 , and  208  are bent 90 degrees from sections  203  and  204  and appear as nearly one-dimensional lines. Sections  205  and  206  are folded an additional 90 degrees such that they are directly above sections  203  and  204  in this view. Section  202  is also bent part of the way toward sections  204  and  206 .  
      In step  240 , section  202  is bent the rest of the way toward sections  202  and  204 . Section  201  will be bent in a similar manner toward sections  203  and  205 , as seen in step  250 . It should be noted that the volume defined by sections  202 ,  204 ,  206 , and  208  will be used to form compartment  102 , and the volume defined by sections  201 ,  203 ,  205 , and  207  will be used to form compartment  101 . Also in step  250 , the structure is bent such that sections  207  and  208  form an acute angle. As can be seen in step  250 , sections  207  and  208  are used to form folded double wall  209  that was mentioned with respect to  FIG. 1 . Accordingly, folded double wall  209  may be referred to as a “reverse-bend folded double wall.” In step  260 , the structure is shown with surfaces  201  and  202  facing the viewer. In this step, the structure is bent such that sections  207  and  208  are touching or nearly touching, thus forming folded double wall  209 . Further, the end portions of sections  203 - 206  are bent toward the structure. Step  270  shows the final shape of the structure, which can be recognized as shield  100 . The difference between the result of step  260  and step  270  is that in step  270 , the end portions of sections  203 - 206  are bent to enclose sections  101  and  102 .  
      Two items should be noted regarding shield  100  as formed in steps  220 - 270 . First, shield  100  may be formed such that folded double wall  209  is continuous with compartments  101  and  102 . A result is that there are no gaps where sections  203  and  205  join section  207 , and the same can be said for sections  204 ,  206 , and  208 . The continuousness of the material that results in the lack of gaps and the folded double wall provides a more complete separation of the TX and RX, and helps to assure that in some embodiments, no waves (or very few waves) will penetrate compartment  101  from compartment  102 , such that cross-talk is prevented. That wall  209  is a double wall helps to ensure that the material is thick enough to stop all (or nearly all) waves from passing directly from compartment  102  to compartment  101 . The continuousness also means that shield  100  can be formed from a single piece of material, as shown in  FIG. 2 .  
      The second item that should be noted is that step  270  provides a view of shield  100  from the top (or detection area) only, and that the bottom of shield  100  is not enclosed. In other words, each compartment is open both at its respective aperture  103  or  104  and at its bottom side. As explained further below, various embodiments may employ one or more techniques to prevent cross-talk occurring through the bottom side when mounting shield  100  on a Printed Circuit Board (PCB).  
      In an example embodiment, the length of compartments  101  and  102  together is about 7 mm, while the height and width are each about 3 mm. The length of compartment  101 , in this embodiment, is 4 mm, while the length of compartment  102  is 3 mm. Compartment  102  may be larger than compartment  101  in order to accommodate an RX that is slightly larger than a corresponding TX. Thus, a proximity sensor contained in shield  100  is of a small size and may be deployed in various consumer, business, and industrial applications without occupying a large volume. Also, in this example embodiment, the material may be constructed of stainless steel that is 0.1 mm thick. Such a construction may provide adequate cross-talk shielding for a variety of proximity sensors, including proximity sensors that operate in the infrared (IR) frequency band. Other embodiments may employ different materials and/or thicknesses, and all are within the scope of embodiments as long as the qualities chosen provide adequate shielding for the intensity and frequency of the waves used in the particular application. For example, alternate embodiments may use metals other than stainless steel or may use plastics or ceramics. However, stainless offers many advantages not offered by some other materials, such as resistance to corrosion, hardness, stiffness, and the ability to retain its shape after folding.  
      Although the previous example (along with other examples below) illustrates an embodiment wherein shield  100  is a single-piece structure with a folded double wall, alternative embodiments may employ other forms for shield  100  that include a common wall structure, wherein each compartment is continuous with at least part of the wall structure. For example, an alternative embodiment may be similar to that depicted in  FIG. 2 , but with the fold in wall  209  cut such that the wall (and shield  100  itself) are split in two separate pieces. In this example embodiment, wall  209  is still a common wall structure, since both halves are included in shield  100 , and each compartment is still continuous with its corresponding half of wall  209 . In another alternative embodiment, wall  209  may be a quadruple wall from having extra material that is folded twice, rather than once (notice it is folded once on step  250 ). Numerous other embodiments not specifically disclosed herein, are also within the scope of various embodiments of the present invention.  
       FIG. 3  is an illustration of example system  300 , wherein shield  100  is mounted on PCB  304 . Notice that the walls of shield  100  (including double wall  209 ) extend through a portion of the depth of PCB  304 . In this example, TX  302  and RX  301  are substantially within a volume defined by shield  100 , and also mounted on PCB  304 . Further, TX  302  and RX  301  may be part of single-piece proximity sensor, wherein TX  302  and RX  301  are in a single package and are coupled to one another such that one TX/RX component is mounted on PCB  304 , rather than each of TX  302  and RX  301  being mounted separately. A layer of molding  303  is on top of PCB  304 , and it functions to physically hold TX  302  and RX  301  in place while also collimating the light waves for better performance. In this example, it may be made of an epoxy-based plastic.  
      In many practical applications, a proximity sensor (or other type of TX and RX) will be mounted on a PCB, similar to other components that make up an electronic device. Such an arrangement may allow the proximity sensor to interface with the power and control systems of the device while benefiting from the structural support offered by the PCB. Wires to send and receive signals by TX  302  and RX  301  are not shown in this example for simplicity; however, practical applications may employ a number of hard-wired connections that extend through at least a portion of PCB  304 , and those applications are within the scope of various embodiments. PCBs, such as PCB  304 , are usually made out of fiberglass, and are, therefore, usually light-conductive. It is the light-conductive property of PCB  304  that facilitates cross-talk in example system  300 . Thus, IR light wave  305  travels through PCB  304  from TX  302  to RX  301 , reflecting on the inside of shield  100  and the bottom of PCB  304 , thereby causing RX  301  to indicate (falsely) that an object is nearby.  
      Accordingly, to further prevent cross-talk, it may be desirable to implement another light-blocking structure to further isolate RX  301  from waves that travel through PCB  304  from TX  302 .  FIG. 4  is an illustration of example proximity sensor unit  400 , adapted according to various embodiments, for preventing cross-talk. Proximity sensor unit  400  is similar to system  300  in that TX  302 , RX  301 , and shield  100  are mounted on PCB  304 . Proximity sensor unit  400 , however, adds copper layers  401  and  402 . Copper layer  401  acts as a light-blocking layer to reduce cross-talk by further isolating RX  301  from the electromagnetic waves from TX  302 . TX  302  and RX  301  are mounted on PCB  304  in an area defined by copper layer  401 . In this example embodiment, copper layers  401  and  402  may extend in any direction as far as PCB  304  extends, as long as copper layer  401  contains the footprint of shield  100 .  
      In this example, copper layer  401  runs between two layers of PCB  304 , and shield  100  is mounted such that its walls (including double wall  209 ) extend below copper layer  401 . In this way, compartment  101 , folded double wall  209 , and copper layer  401  act to surround RX  301 , and compartment  102 , folded double wall  209 , and copper layer  401  act to surround TX  302 . Accordingly, in this example, IR waves  403  and  404  are not able to pass through PCB  304  from TX  302  to RX  301 . Instead, IR waves  403  and  404  are blocked by copper layer  401  and reflected such that they exit shield  100  at aperture  104  ( FIG. 1 ). This helps to ensure that the waves received by RX  301  are reflected from an object in the detection range, rather than from cross-talk.  
      Many PCBs are sold on the market with a thin copper layer on both the top and the bottom. Accordingly, an example technique to make proximity sensor unit  400  may include acquiring two PCBs, each with two copper layers. The PCBs are then bolted together, such that there is a single copper layer on top, then a PCB layer below that, then two copper layers below the PCB layer, then the bottom PCB layer, which has a single layer of copper on its bottom surface. The topmost and bottommost copper layers may then be etched to form circuits, leaving a PCB-mounted circuit, which includes two copper layers sandwiched between two PCB layers.  
      Accordingly, in such an example embodiment, copper layer  401  may actually be two copper layers, and copper layer  402  may be etched to form circuits. In example unit  400 , the topmost copper layer is not shown because it has been etched to form circuits, while copper layer  402  has not been etched. In the example embodiment depicted as system  400 , copper layer  401  is 20-30 microns thick, which is adequate to block light from some IR transmitters. Alternative embodiments may use other thicknesses, number of layers, or other types of waves in the electromagnetic spectrum, and all are within the scope of embodiments, as long as the light-blocking layer blocks light in an adequate manner for the application in which it is disposed.  
      Using one or more copper layers as a light-blocking structure, in some embodiments, may have both desirable and undesirable effects. For instance, while copper is a good light-blocking substance for some applications, it may produce extra cost in the manufacturing process by dulling blades used to cut the boards. It should be noted that boards may be cut during manufacture to produce desired sizes, and also during the mounting process to accommodate components which must extend below the surface of the topmost layer. Further, one or more copper layers in a board may fail to cut in a smooth manner, thereby leaving jagged edges (called “burrs”) which may unintentionally make electrical contact with elements mounted on the board. Accordingly, another light-blocking material may be desirable in some applications.  
       FIG. 5  is an illustration of example proximity sensor unit  500 , adapted according to various embodiments, for preventing cross-talk. Proximity sensor unit  500  is similar to system  300  and unit  400  in that all mount TX  302 , RX  301 , and shield  100  on PCB  304 . Example unit  500  is different from example unit  400 , because system  500  includes light-blocking soldermask layers  501  and  502  and omits copper layers.  
      In this example embodiment, shield  100  is mounted such that the walls of shield  100  (including double wall  209 ) extend below layer  502 . In this manner, compartment  101 , folded double wall  209 , and layer  502  act to surround RX  301 , and compartment  102 , folded double wall  209 , and layer  502  act to surround TX  302 . Accordingly, IR waves from TX  302  are prevented from reaching RX  301  through PCB  304  because they are reflected from layer  502  and shield  100 . Thus, cross-talk is prevented.  
      In various PCB applications, soldermask may be used as an insulator that is applied to the circuits on an etched PCB in order to protect those circuits from electrical contact with other conductors. There are various varieties of soldermask, including dry-film and liquid. In this example embodiment, wherein TX  302  emits IR light, it is important that the soldermask chosen for the light-blocking layer be able to block IR light. Similarly, for applications that use other electromagnetic frequencies, it is important to select a soldermask for the light-blocking layer that is operable to block light in that frequency range. Soldermask layer  501  is optional, but may be applied as a redundant mechanism to block any waves which might otherwise penetrate PCB  304  and layer  402 .  
      In some applications, soldermask may provide one or more desirable qualities. For instance, it may be an excellent light-blocking material, even when applied in thin layers. Further, it may avoid dulling cutting blades or producing burrs, as with copper layers. Additionally, the cost of soldermask may make it an economically attractive material for the manufacture of such applications.  
       FIGS. 4 and 5  both depict systems for preventing cross-talk using one or more light-blocking layers applied to a PCB. Alternative embodiments may utilize other materials, thicknesses, and electromagnetic frequencies. It should be noted that the various embodiments herein are not limited to the specific embodiments disclosed. For instance, an embodiment which uses opaque materials, rather than reflective materials, to block light is within the scope of the embodiments. Further, an embodiment which uses a material other than copper or soldermask to block light, or an embodiment which uses radio waves, microwaves, or the like also falls within the scope of the embodiments.  
       FIG. 6  is a flowchart illustrating example method  600  for preventing cross-talk. In block  601 , a transmitting device and a receiving device are mounted on a circuit board, wherein the circuit board includes a layer that blocks waves emitted from the transmitting device, and wherein the transmitting device and the receiving device are mounted in an area defined by the layer. Various techniques for mounting the transmitting device and the receiving device are within the scope of various embodiments, including manual mounting. The circuit board may be any of a variety of circuit boards, including, for example, a PCB. In an example embodiment, the transmitting device and the receiving device are IR-frequency devices and are included as part of a proximity sensor.  
      In block  602 , a structure is manipulated to form a first compartment for the transmitting device and a second compartment for the receiving device, wherein the compartments are separated by a folded double wall that is continuous with each compartment. In an example embodiment, after the manipulating, the structure is similar to shield  100  ( FIG. 1 ), and the folded double wall is similar to wall  209  ( FIG. 2 ). The manipulating may include, for example, folding, bending, creasing, and the like, as long as the particular technique is suitable for forming the compartments from the material used to construct the structure.  
      In block  603 , the structure is mounted on the circuit board. The structure may be mounted on the circuit board with any of a variety of techniques suitable for the mounting, such as a customized pick and place technique utilizing glue to attach the shield to the PCB. As in the embodiments depicted in  FIGS. 4 and 5 , the structure may be mounted such that its footprint is entirely within the area defined by the light-blocking layer and wherein each compartment is aligned with the TX or RX, respectively. Further, the structure may be mounted before or after the transmitting and receiving devices are mounted, depending on the particular manufacturing technique that is chosen.  
      Mounting the structure as described above, with a folded double wall that is continuous with each of two compartments, may be advantageous compared to mounting a structure wherein the compartments are separate. For example, the continuousness of the material may help to eliminate some alignment issues that would be present in mounting a shield wherein the compartments are separate. Also, as explained above, the continuousness of the metal helps to eliminate gaps that might let electromagnetic waves penetrate the shield and cause cross-talk.  
       FIG. 7  is a flowchart that depicts method  700 , according to various embodiments, for preventing cross-talk. In block  701 , a transmitter is operated, wherein the transmitter is located in a first compartment, wherein a receiver is located in a second compartment separated from the first compartment by a wall structure, and wherein each compartment is continuous with at least part of the wall structure. In an example embodiment, the wall structure is a folded double wall structure, and the compartments form a shield, as seen in  FIG. 2 . Further, the transmitter may be operated, for example, by causing it to emit electromagnetic waves, such as IR waves.  
      In block  702 , the compartments and a layer in a substrate block waves from the transmitter, thereby preventing cross-talk. In an example embodiment, the substrate is a PCB, and the layer is soldermask layer that is operable to block IR light waves. The compartments may form a shield that is mounted on the PCB. While waves are blocked in this example, some waves may still reach the receiver from the transmitter by being reflected off of an object in the detection zone.  
       FIG. 8  depicts an example application that employs a proximity sensor, adapted according to various embodiments. Water faucet  800  is an automatic, touchless faucet, similar to those found in washrooms worldwide. However, faucet  800  employs a proximity sensor unit, adapted according to various embodiments, which is disposed behind protective, IR-transparent cover  803 .  
      The elimination of cross-talk provided by shield  100  ( FIG. 1 ) may facilitate the use of a more sensitive (and, therefore, more precise) proximity sensor. A control system (not shown) can be programmed to control water flow  802  from opening  801  according the presence or absence of hand  804 . Eliminating water flow  802  when hand  804  is not under opening  801  may help conserve water and protect the environment.  
      Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.