Patent Publication Number: US-6990384-B2

Title: Truss plate detector

Description:
This application is a continuation-in-part of, and claims priority from, U.S. Provisional Application Ser. No. 60/328,255 filed Oct. 9, 2000, the contents of which are incorporated herein by reference in their entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an apparatus for detecting truss plates. More specifically, this invention relates to a detector that identifies missing or misaligned truss plates during the truss assembly process. 
   2. Background Art 
   One of the most problematic and recurring quality control issues in a truss mill is that of missing truss (e.g., nail) plates on finished products. The issue exists in nearly all truss mills, whether they are building custom trusses for an expensive home, or simply building the same stock trusses for local lumber yards for days at a time. This problem is common throughout the industry and can be expensive to resolve. 
   During the truss assembly process, builders tap nail plates into place on the top of each lumber component intersection (joint). The builder then pries up the truss to place a companion plate in the same position on the back side. A roller gantry moves down the line and forces each plate partially into the lumber. The truss is then moved off to a final exit roller (press) which is generally set to an exact 1.5″ span. The exit roller presses with enough force to embed the nail plates into the truss and provide the strength that the truss was engineered to have. The truss is then moved down a line and stacked up with other trusses in a job. When the job stack is complete, it is banded together and placed on a truck to be delivered to the customer (e.g., a building contractor). 
   Unfortunately, however, the roller gantry system requires two elements for ideal operation. The first requirement is an attentive builder. If a builder is not fully attentive, or is rushing through production to meet quotas, it is very easy to miss positioning a nail plate, particularly on the bottom side of the truss. Omitting nail plates on the bottom of the truss is a common mistake. The builder must also remain attentive after the truss is complete and is being pressed by the exit rollers. If a truss is missing a nail plate, the builder should catch the error before the truss is stacked with the other trusses. If the builder fails to notice the missing plate, the error will likely not be caught until the truss has already been shipped to the job site. 
   The second important element for ideal operation of the roller gantry is mechanical accuracy of the equipment. If gantry tables are misaligned or out of level, the rolling press action that embeds the nail plates partially into the lumber can be inefficient. As a result, when the truss is popped up to be sent through the exit roller, nail plates may only be loosely connected on a back side of the truss or may not be connected at all. Another problem can be improperly operating conveyance rollers that are used to move the truss to the exit rollers. Bad or damaged rollers can catch poorly embedded nail plates and cause them to fall off or be peeled back and folded up prior to reaching the exit roller press. Either error results in a truss that cannot meet the engineered joint strengths. 
   Generally, when a truss leaves the mill without proper plating, it is the contractor or end customer that finds the defect and reports it to the truss mill management. Since the improperly plated truss does not meet the required engineering standards for the truss design, it is the truss mill&#39;s responsibility to remedy the situation. This can be done by sending a mill employee to the job site with the required nail plate(s) and some special field equipment that allows the employee to press the plate into the lumber with the necessary pressure. The other solution would be to set up and build an entirely new truss to be delivered to the job site. Neither of these solutions are desirable, however, particularly considering that most truss mills deliver their products within a radius of about 100 miles, and sometimes up to 200 miles or more. 
   The time and expense of sending a replacement truss, or an agent and equipment to repair the truss, can be quite substantial, eating into the truss mill&#39;s profit. In addition, negative public relations can result, particularly if the contractor is delayed substantially while waiting for the truss to be repaired, or if missing nail plate mistakes happen frequently. Contractors and customers view this problem as a serious lack of quality control and may consider looking to other sources to fulfill their future truss requirements. 
   The industry would be benefited by a method and apparatus for monitoring trusses during the production process to detect missing plates before they are placed in a stack and shipped off to a job site. The industry would also benefit from a method and apparatus that can detect misaligned nail plates. 
   SUMMARY OF THE INVENTION 
   The principles of the present invention enable a truss plate detector that can detect missing or misaligned truss plates during a truss assembly process. The truss plate detector is preferably reliable, easy to use, and easy to maintain. The truss plate detector can further be configured to use inexpensive and readily replaceable sensor arrays to lower manufacturing and maintenance costs. 
   A truss plate detector preferably includes a first sensor array having a plurality of sensors arranged in a line to detect truss plates on a first side of a truss. A second sensor array includes a plurality of sensors arranged in a line to detect truss plates on a second side of a truss. A control circuit is configured to receive signals from the first and second sensor arrays and to notify a user when one or more truss plates are missing or misaligned. 
   The sensors in the first and second sensor arrays are preferably arranged directly opposite corresponding sensors in the other array. The control circuit of the truss plate detector is then preferably configured to notify a user when a sensor in the first sensor array detects a truss plate but neither a corresponding sensor nor an adjacent sensor in the second sensor array detects a truss plate. Using a plurality of laterally-arranged, oppositely located sensors, the truss plate detector can detect the presence as well as a lateral location of a truss plate on a truss being fed through the truss plate detector. 
   In one embodiment, the sensors of the sensor arrays are arranged in readily replaceable sensor modules. The replaceable sensor modules are located in first and second sensor array housings. One or both sensor array housing can be configured to rotate about an axis thereof to provide easy access to sensors arranged in the first sensor array. In a most preferred embodiment, the sensors are inductor sensors configured to detect changes in a magnetic field caused by an adjacent metal truss plate. The truss plate detector could also be configured to communicate with a truss press controller to stop or reverse the direction of a truss press when one or more truss plates are missing or misaligned. 
   A method of detecting missing or misaligned truss plates in a truss is also provided. According to this method, a truss is fed through a truss detector having a first sensor array arranged proximal to a first truss surface and a second sensor array arranged proximal to a second truss surface, opposite the first truss surface. The presence and location of truss plates on the first and second truss surfaces are detected using the first and second sensor arrays, respectively. Missing or misaligned truss plates, if any, on the opposite truss surface are then detected using the other sensor array. A user can then be notified when one or more truss plates are missing or misaligned. If no missing or misaligned plates are detected, the truss is fed through a truss press. 
   Detecting missing or misaligned truss plates on a truss surface can be accomplished by determining whether truss plates corresponding to the detected truss plates on an opposite truss surface are present and appropriately located on the truss surface. First and second sensor arrays arranged in proximity to a corresponding truss surface can, for instance, include a plurality of corresponding, oppositely located sensors. In such an embodiment, the user can be notified if a sensor in one of the arrays detects the presence of a truss plate and neither a corresponding sensor nor an adjacent sensor in the opposite sensor array detects a truss plate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and additional objects, features, and advantages of the present invention will become more readily apparent from the following detailed description of preferred embodiments thereof, made with reference to the accompanying figures, in which: 
       FIG. 1  is a schematic block diagram of a truss plate detector system according to a preferred embodiment of the present invention; 
       FIGS. 2A-2C  are schematic plan, top, and side views, respectively, of a truss plate detector according to a more specific embodiment of inventive principles herein; 
       FIGS. 3A-3C  are schematic plan, side, and top views respectively, of a right end frame for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 4A-4C  are schematic plan, side, and top views, respectively, of a left end frame for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 5A and 5B  are schematic plan and side views, respectively, of a shear for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 6A-6C  are schematic bottom, side, and enlarged cross-sectional views, respectively, of a sensor array housing for the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 7A and 7B  are schematic plan and side views, respectively, of a spacer for use in the sensor array housing of  FIGS. 3A-3C ; 
       FIGS. 8A and 8B  are schematic plan and side views, respectively, of a sensor plate for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 9A and 9B  are schematic plan and side views, respectively, of a sensor for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIGS. 10A and 10B  are schematic plan and side views, respectively, of a sensor array and communications elements for use in the truss plate detector of  FIGS. 2A-2C ; 
       FIG. 11  is a schematic block diagram of a sensor board for use in the truss plate detector of  FIG. 1 ; 
       FIGS. 12A-12M  are schematic circuit diagrams illustrating the circuitry of the sensor board of  FIG. 11 ; 
       FIG. 13  is a schematic block diagram of a control board for use in the truss plate detector of  FIG. 1 ; 
       FIGS. 14A-14V  are schematic circuit diagrams illustrating the circuitry of the control board of  FIG. 13 ; and 
       FIG. 15  is a schematic plan view of the truss plate detector of  FIGS. 2A-2C  arranged in operative relationship with a truss press according to one arrangement. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a schematic block diagram of a truss plate detector system  100  according to a preferred embodiment of the present invention. Referring to  FIG. 1 , a truss plate detector system  100  preferably includes top and bottom sensor arrays  102 ,  104  that communicate with a control unit  110  through corresponding ribbon cables  106 ,  108 . The top and bottom sensor arrays  102 ,  104  include a plurality of corresponding, oppositely located sensors (S-T 1  through S-T 11  and S-B 1  through S-B 11 , respectively). 
   More particularly, the truss plate detector system  100  preferably comprises a first sensor array  102  having a plurality of sensors (S-T 1  to S-T 11 ) arranged along a line to detect truss plates on a first side  52  of a truss  50 . A second sensor array  104 , also having a plurality of sensors (S-B 1  to S-B 11 ) arranged in a line, is configured to detect truss plates on a second side  54  of a truss  50 . A control circuit (e.g., control unit  110 ) is configured to receive signals from the first and second sensor arrays  102 ,  104  and to notify a user when one or more truss plates are missing or misaligned. 
   The control unit  110  can be configured to receive signals from the sensor arrays  102 ,  104  through respective ribbon cables  106 ,  108 . The control unit  110  is preferably configured to analyze the signals from the sensor arrays  102 ,  104  to determine when a truss plate on either of the truss sides  52 ,  54  is missing or misaligned. When a missing or misaligned truss plate is detected, the control unit  110  preferably generates an alarm condition. 
   Based on the alarm condition, the control unit  110  can control visual status indicators  112  and/or audible alarms  114  to notify an operator of the error. Different alarms can be generated based on whether the plate is missing or misaligned. More detailed information on the error could also be provided to a user through visible or audible indicator means, such as an LCD or other display (not shown) or voice alerts. 
   In operation, a truss (not shown), having truss plates arranged thereon, is fed through the truss plate detector system  100  between the two sensor arrays  102 ,  104 . When a sensor in one of the arrays detects the presence of a truss plate, the opposite sensor array also checks for a truss plate. If neither a corresponding sensor nor an adjacent sensor in the opposite sensor array detects the presence of a truss plate within a predetermined time period, an error signal is generated. For example, if a fifth top sensor (S-T 5 ) detects the presence of a truss plate but neither the fifth bottom sensor (S-B 5 ) nor adjacent bottom sensors (S-B 4  and S-B 6 ) detect a truss plate within a predetermined time period, an error signal is generated and a user is notified. 
   The error signal can trigger an audible alarm (such as through a speaker), a visible alarm (such as through the status indicator LEDs  112 ), or provide any other type of user notification that an alarm condition exists. In addition, the truss plate detector system  100  can include a press control  116 . The press control can be used to stop a corresponding truss press (not shown) during an alarm condition. The truss plate detector system  100  could also communicate with the press control  116  to reverse the truss press and cause it to move in the opposite direction for an appropriate distance when the error signal is generated. 
     FIGS. 2A-2C  are schematic plan, top, and side views, respectively, of a truss plate detector  200  according to a more specific embodiment of various inventive principles herein disclosed. Referring to FIG.  1  and  FIGS. 2A through 2C , a truss plate detector  200  can include a frame having oppositely located end frames  202 A,  202 B. A control box  204 , which houses the control unit  110 , can be mounted to one of the end frames. Top and bottom sensor array housings  206 ,  208 , which house the top and bottom sensor arrays  102 ,  104 , respectively, can be mounted between the end frames  202 A,  202 B. The top and/or bottom sensor array housings  206 ,  208  can be further configured to rotate about a longitudinal axis thereof to provide easy access to sensors arranged in the arrays  102 ,  104 . The opposite end frames  202 A,  202 B are preferably arranged to support opposite ends of the sensor array housings  206 ,  208 . For smaller detectors, a single end frame can suffice. 
   A first (e.g., top) sensor array  102  is arranged in a first sensor array housing  206  and preferably includes a plurality of sensors (S-T 1  to S-T 11 ) arranged along a line to detect truss plates on a first side  52  of a truss  50 . A second (e.g., bottom) sensor array  104  is arranged in a second sensor array housing  208 . The second sensor array  104  also has a plurality of sensors (S-B 1  to S-B 11 ) arranged along a line. The second sensor array  104  is configured to detect truss plates on a second side  54  of a truss  50 . The first and second sensor arrays  102 ,  104  can thereby be configured to detect the presence and lateral location of truss plates on a truss being fed through the truss plate detector. A longitudinal location of the truss plates can be determined (e.g., through a shaft encoder  118 ) based on an amount by which the truss has been fed through the truss plate detector. A two-dimensional location of the truss plate can thereby be determined. 
   The control unit  110  is configured to receive signals from the first and second sensor arrays  102 ,  104  and to notify a user when one or more truss plates are missing or misaligned. The truss plate detector  200 , for instance, can be configured to notify a user when a sensor in one of the arrays  102 ,  104  detects a truss plate but neither a corresponding sensor nor an adjacent sensor in the other sensor array detects a truss plate. The truss plate detector  200  can be further configured to communicate with a press controller  116  through the control unit  110 . The press controller  116  can be instructed to stop a truss press (not shown) when one or more truss plates are missing or misaligned. The truss press controller  116  could be further configured to cause the truss press to reverse directions by an amount sufficient to enable the missing or misaligned truss plate to be added or adjusted. 
   Top and bottom shears  212 A,  212 B are also preferably arranged between the end frames  202 A,  202 B to align the truss between the sensor arrays  102 ,  104  and to reduce an amount of vibration of the truss  50  as it is fed through the truss plate detector  200 . The shears  212 A,  212 B are most preferably arranged approximately one-quarter inch from first and second truss surfaces  52 ,  54 , and are separated from each other by about two inches overall. The first and second sensor arrays  102 ,  104  are preferably arranged approximately one-half inch from the first and second truss surfaces  52 ,  54 , respectively. 
     FIGS. 3A-3C  are schematic plan, side, and top views, respectively, of a right end frame  202 A of the truss plate detector  200  of  FIGS. 2A-2C .  FIGS. 4A-4C  are schematic plan, side, and top views, respectively, of a left end frame  202 B for use in the truss plate detector  200  of  FIGS. 2A-2C . Referring to  FIGS. 3A-4C , the right and left end frames  202 A,  202 B of this embodiment are substantially mirror images of each other. Each end frame  202 A,  202 B preferably comprises a vertical frame member  216  that extends vertically from a footing  214  located at a base of the vertical frame member  216 . Sensor array housing support members  218  (e.g., Stauff clamps) are preferably located at an appropriate vertical distance from the footing  214 . The sensor array housing support members  218  are preferably configured to receive support tubes  220  from the sensor array housings  206 ,  208 . Although any other mechanism for attaching the sensor array housings to the vertical frame members is also acceptable, tubular members are most preferred because of their ability to carry sensor cables therein. 
   Shear attachment members  210  can also be provided. Shears  212 A,  212 B can be arranged on the attachment members  210  to more efficiently direct the truss between the sensor arrays and to reduce an amount of truss vibration. The shears  212 A,  212 B can be attached to the shear attachment members  210  through any appropriate physical attachment mechanism, such as bolts, welding, or other types of mechanical attachment. Although one specific shear design is described below, many alternative shear designs could also be used. 
     FIGS. 5A and 5B  are schematic plan and side views, respectively, of a shear  212  for use in the truss plate detector of  FIGS. 2A-2C . As shown in  FIGS. 2A-2C , a top shear  212 A and a bottom shear  212 B are preferably positioned on opposite sides of a truss opening to appropriately direct the truss into the truss plate detector and reduce vibration thereof. Referring now to  FIGS. 2A-2C ,  5 A, and  5 B, each of the shears  212 A,  212 B preferably comprises a substantially planar member  213  that extends longitudinally across a length of the truss plate detector  200 . A forward portion  213 A of a top planar member is bent upwards to form a guide that will direct a truss being fed into the truss plate detector between the sensor array housings  206 ,  208 . Similarly, a forward portion  213 A of a lower planar member is bent downwards to form a guide to appropriately direct the truss between the sensor array housings  206 ,  208 . 
     FIGS. 6A-6C  are schematic bottom, side, and enlarged cross-sectional views, respectively, of a top sensor array housing  206  of the truss plate detector  200  shown in  FIGS. 2A-2C . A bottom sensor array housing  208  preferably has a similar construction. Referring to  FIGS. 6A-6C , the sensor array housing  206  comprises an outer wall having a substantially triangular cross-section. An opening  209  is arranged in a bottom of the outer wall  207  to receive sensor modules  300  (see FIGS.  10 A and  10 B). A spacer  310  (see  FIGS. 7A and 7B ) can also be arranged in the opening  209 . Support members  220  (e.g. support tubes) are preferably arranged at opposite ends of the sensor array housing  206  to support the housing on the frame (see FIGS.  2 A- 2 C). The support members  220  can be rods, bolts, hinges, or any other mechanical structure that attaches the housing to the frame but are preferably tubular members to carry sensor cables  106 ,  108  out to the control unit  110 . Most preferably, the support members  220  are configured to permit rotation of the sensor array housing  206  about a longitudinal axis  221  defined through the housing  206 . Rotation of the housing  206  from an operation to a maintenance position allows easy access to the sensor modules  300  for repair or replacement. A locking mechanism can be provided to maintain the housing  206  in a selected one of the operating or the maintenance positions. 
   In this embodiment, support tubes  220  are arranged in opposite ends of the sensor array housing  206 . The support tubes  220  are retained in clamps  218  located at a proper vertical position along the vertical support members  216  of the end frames  202 A,  202 B (see FIGS.  2 A- 4 C). The support tubes  220  provide the sensor housing  206  with the ability to rotate about a longitudinal axis  221  of the sensor housing  206  defined along a central longitudinal axis of the support tubes  220 . 
     FIGS. 7A and 7B  are schematic plan and side views, respectively, of a spacer  310  for use in the sensor array housing  206  of  FIGS. 6A-6C . Referring to  FIGS. 7A and 7B , the spacer  310  is preferably shaped to fit within the opening  209  in the housing  206  (see  FIGS. 6A-6C ) in order to maintain a proper spacing between opposite sides of the opening  209 . The spacer  310  preferably has recessed edges  311  and a raised center  312 . When positioned in the housing  206 , the raised center  312  engages opposite sides of the opening  209 . Through holes  313  are arranged in the recessed edges to mate with holes  205 A arranged in the bottom wall  205  of the sensor array housing  206 . Bolts or other appropriate mechanical connection mechanisms can then be inserted through the mating holes to secure the spacer  310  to the housing  206 . 
     FIGS. 8A and 8B  are schematic plan and side views, respectively, of a sensor plate  320  configured to hold a plurality of sensors (not shown) in the sensor array housing  206  of the truss plate detector  200  of  FIGS. 2A-2C . Referring to  FIGS. 8A and 8B , the sensor plate  320  has a similar construction to the spacer  310  in  FIGS. 7A and 7B . More particularly, the sensor plate  320  includes a raised central portion  322  and recessed edges  321  to facilitate proper positioning of the sensor plate  320  within the opening  209  in the sensor array housing  206  (see FIGS.  6 A- 6 C). The sensor plate  320  can also include attachment holes  323  to mate with holes  205 A in the bottom wall  205  of the sensor array housing  206 , to attach the sensor plate  320  to the housing  206 . 
   Unlike the spacer  310 , however, the sensor plate  320  further includes sensor openings  325  for receiving induction sensors  350  (see FIGS.  9 A and  9 B). Each sensor plate  320  is preferably configured to receive a plurality of sensors  350  (four in this embodiment), but can be configured to hold a single sensor  350 . The sensor plates  320 , in combination with the sensors  350 , form sensor modules  300  (see  FIGS. 10A and 10B ) that can be readily maintained or replaced as necessary in the sensor array housings  206 ,  208 . 
   Each sensor  350  preferably comprises an inductor sensor configured to detect changes in a magnetic field caused by an adjacent metal truss plate.  FIGS. 9A and 9B  are schematic plan and side views, respectively, of a sensor  350  for use in the truss plate detector  200  of  FIGS. 2A-2C . Referring to  FIGS. 9A and 9B , a sensor  350 , preferably an induction sensor, is configured having a substantially rectangular body  352  having rounded edges  354 . The sensor  350  is preferably sized and shaped to fit within the sensor openings  325  in the sensor plate of  FIGS. 8A and 8B . In a most preferred embodiment, the dimensions of the sensor are approximately three and one-half inches long by about one and three-quarters inches wide and about one-quarter inch thick. 
   The inductor sensors  350  are preferably configured to detect changes in a magnetic field caused by a truss plate being located in proximity thereto. As can be seen from  FIG. 9B , the sensors  350  preferably include a groove  356 . The groove is most preferably about one-tenth of an inch wide. Sensing wire (not shown) can be wound around the sensor  350  in the groove  356  to provide the ability to sense changes in a magnetic field. 
   As indicated previously, the sensors in each sensor array can be arranged in readily replaceable sensor modules, with each sensor module preferably comprising two or more sensors.  FIGS. 10A and 10B  are schematic plan and side views, respectively, of sensor modules  300  and corresponding communications circuitry for use in the truss plate detector  200  of  FIGS. 2A-2C . Referring to  FIGS. 10A and 10B , a sensor module  300  includes a plurality of sensors  350  arranged in a sensor plate  320 . Two modules  300  are shown in FIG.  10 A. Circuit boards  302  are arranged in proximity to the each module  300 , and ribbon cables  106  communicate between the circuit boards and a control unit  110  (see FIG.  1 ). When the sensors  350  of the first second sensor array  102  are arranged in readily replaceable sensor modules  300 , corresponding sensor modules  300  are preferably arranged in the second sensor array  104 , opposite to the modules  300  in the first sensor array  102 . 
     FIGS. 11 ,  12 A through  12 M,  13 , and  14 A through  14 V provide both block and schematic circuit diagrams of various circuit elements used to generate, communicate, receive, and analyze data from the sensor arrays  102 ,  104  (see  FIG. 1 ) and to generate an error signal under appropriate conditions. More specifically,  FIG. 11  is a schematic block diagram of a sensor circuit for use in a sensor circuit board  302  (see  FIGS. 10A and 10B ) of the truss plate detector  200  of  FIGS. 2A-2C .  FIGS. 12A-12M  are more detailed schematic circuit diagrams of the circuitry of the sensor circuit board  302  of FIG.  11 . 
     FIG. 13  is a schematic block diagram of a control circuit for use in the control unit  110  of the truss plate detector system  100  of FIG.  1 . And  FIGS. 14A-14V  are more detailed schematic circuit diagrams of the circuitry of the control board of FIG.  13 . The specific operation of these circuits will be apparent to those skilled in the art from the foregoing diagrams illustrating the construction thereof. A detailed description thereof will therefore not be provided. Various modifications to the depicted circuitry will also be apparent to those skilled in the art and all such modifications fall within the spirit and scope of this invention. 
     FIG. 15  is a schematic top plan view of the truss plate detector  200  of  FIGS. 2A-2C , shown in operative relationship with a truss press  60  according to one arrangement. Referring to  FIG. 15 , the truss plate detector  200  is preferably arranged on an input side of the truss press  60 , before the plates are pressed completely into the truss  50 . In this manner, truss defects including missing or misaligned plates  55  can be detected and corrected before feeding the truss  50  through the exit roller press  60 . 
   The truss plate detector  100  could also, however, be arranged on an output side of the truss press  60 . When arranged on the output side, however, the truss plate detector  200  is unable to detect defects until after the truss  50  has already been fed through the press  60 . Accordingly, when defects are found, the press  60  must either be reversed so that the error can be corrected, or the entire truss  50  must be refed through the press  60  after a plate has been appropriately positioned on the truss  50 . 
   Referring to  FIGS. 1 ,  2 A- 2 C, and  15 , in operation, a truss  50  includes a plurality of boards  51  attached together via truss (or nail) plates  55 . The truss  50  is fed through a truss detector  200  having a first sensor array  102  arranged proximal to a first truss surface  52  and a second sensor array  104  arranged proximal to a second truss surface  54 . The presence and location of truss plates  55  on the first truss surface  52  is detected using the first sensor array  102 . The presence and location of truss plates  55  on the second truss surface  54  are detected using the second sensor array  104 . A user is notified when one or more truss plates  55  on either truss surface are missing or misaligned. If no missing or misaligned truss plates  55  are detected, the truss  50  is fed through the press  60 . 
   Missing or misaligned truss plates on the truss surfaces are preferably detected by determining whether truss plates corresponding to the detected truss plates on one truss surface are present and appropriately located on the opposite truss surface. The first and second sensor arrays preferably comprise a plurality of corresponding, oppositely located sensors. The user can then be notified if a sensor in one sensor array detects the presence of a truss plate and neither a corresponding sensor or an adjacent sensor in the opposite sensor array detects a truss plate. An error signal can further be communicated to a truss press when one or more truss plates on the truss surfaces are missing or misaligned. The press can then be stopped in response to the error signal. 
   Having described and illustrated the principles of the invention in preferred embodiments thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. All such modifications and variations come within the spirit and scope of the following claims.