Patent Publication Number: US-9889996-B2

Title: Adhesive melter and method having predictive maintenance for exhaust air filter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 13/953,032, filed Jul. 29, 2013, and published as U.S. Patent Application Pub. No. 2015/0027546 on Jan. 29, 2015, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to an adhesive melter used with an adhesive dispensing system, and more particularly, to control components and methods used to monitor and operate a fill system supplying solid adhesive to the adhesive melter. 
     BACKGROUND 
     A conventional dispensing system for supplying heated adhesive (i.e., a hot-melt adhesive dispensing system) generally includes a melter having an inlet for receiving adhesive materials in solid or semi-solid form, a heater grid in communication with the inlet for heating and/or melting the adhesive materials, and an outlet in communication with the heater grid for receiving the heated adhesive from the heated grid. The outlet communicates with a pump for driving and controlling the dispensation of the heated adhesive through the outlet and to downstream equipment, such as dispensing modules. Furthermore, conventional dispensing systems generally include a controller (e.g., a processor and a memory) and input controls electrically connected to the controller to provide a user interface with the dispensing system. The controller is in communication with one or more of the melter, the pump, and other components, such that the controller controls the dispensation of the heated adhesive. 
     Conventional hot-melt adhesive dispensing systems typically operate at ranges of temperatures sufficient to melt the received adhesive and heat the adhesive to an elevated application temperature prior to dispensing the heated adhesive. In order to ensure that the demand for heated adhesive from the gun(s) and module(s) is satisfied, the adhesive dispensing systems are designed with the capability to generate a predetermined maximum flow of molten adhesive. For example, the inlet of the melter communicates with a fill system operated by the controller of the dispensing system. In a typical arrangement, the fill system operates to deliver a stream of solid particulate or pelletized adhesive using a pressurized air flow from a bulk supply or source of the solid adhesive to the inlet of the melter whenever a receiving space (e.g., hopper) above the heater grid requires refilling. In these arrangements, the melter also includes an exhaust outlet with a filter for discharging the pressurized air flow from the fill system or receiving space after that pressurized air flow has delivered the solid adhesive into the receiving space. Thus, each fill system cycle requires the exhausting of pressurized air flow out of the melter. 
     As readily understood, the exhaust air filter will become clogged over time as the fill system is used. This clogging of the exhaust air filter stifles the efficient operation of the fill system because it can limit the amount of pressurized air flow generated through the fill system and the melter. Conventional adhesive melters and dispensing systems do not specifically monitor the use of the exhaust air filter, so there is currently no known mechanism in this field to provide predictive maintenance information to an operator regarding when the exhaust air filter will need to be replaced. Instead, conventional systems typically continue to operate until the exhaust air filter is so clogged that the fill system effectively cannot keep up with the demands for molten adhesive from the melter, such as when the dispensing system requires the predetermined maximum flow of molten adhesive. Alternatively, the fill system may also stop working for other reasons such as a burst hose in the fill system or an obstruction of flow at the source of adhesive. As a result, a shutdown of the fill system occurs, which can eventually lead to the melter running out of adhesive and shutting down as well. Therefore, the adhesive dispensing system undergoes a period of unplanned downtime until maintenance personnel can identify the issue with the clogged exhaust air filter (or the other issues described above, when applicable) and then perform appropriate maintenance, such as a replacement of the exhaust air filter. These unplanned downtimes for the system are undesirable and costly for operators of conventional adhesive melters and dispensing systems. 
     In other pneumatic fields such as HVAC systems, air filters have been monitored using air flow measurement devices and/or pressure detection sensors that provide estimates of how much air flow moves through the air filter. The air filters in these other fields are then replaced after a set amount of air flow has passed through the air filter. While this type of equipment could hypothetically be used in the conventional adhesive melters, this equipment has not been added for multiple reasons. First, the additional air flow measurement devices and/or pressure detection sensors add additional cost to the manufacturing and maintenance of the adhesive melter, and this additional cost may outweigh the benefit of attempting to provide predictive maintenance information about the exhaust air filter at the adhesive melter. Second, these types of predictive maintenance based on total air flow through the exhaust air filter are unreliable in this context because exhaust air filters in adhesive melters are subject to highly variable conditions that may significantly alter the lifespan or total air flow that the exhaust air filter can pass through before clogging. Thus, merely measuring the total air flow through an exhaust air filter at an adhesive melter is not a reliable method for accurately determining when the exhaust air filter will become clogged, and unplanned downtimes for the adhesive melter would likely still occur. 
     The highly variable conditions that subject the exhaust air filters to unpredictable lifespan include the use of different adhesive materials or variable pellet shapes/form factors with filters, as these different materials or form factors can affect the amount of air flow required to move the solid adhesive. In another example, the length of hose used between the source of adhesive for the fill system and the melter may also affect the cycle time for a fill system and the lifespan of an exhaust air filter. In some instances, a more significant source of unpredictability in the lifespan of exhaust air filters is the selective use of powder that may be put on the solid adhesive to prevent tackiness and sticking together of the pellets or particles before delivery to the receiving space. This powder causes more rapid clogging of the exhaust air filter at the melter, thereby shortening the lifespan of the exhaust air filter. Furthermore, the use of powder on certain batches of solid adhesive delivered to the bulk supply from which the fill system draws solid adhesive is unpredictable because not every batch of solid adhesive may include the powder (e.g., the powder may only be used at hotter times of the year when the adhesive supplier and the ambient conditions at the bulk supply may be more prone to pellets sticking together). The amount of powder on the adhesive that will be captured by the exhaust air filter may also vary dramatically even between different batches or fill system cycles. As a result, it is currently impractical to reliably predict when an exhaust air filter in a conventional adhesive melter will require replacement. Furthermore, there is currently no known method for distinguishing reduced performance of the fill system caused by exhaust air filter clogging from reduced performance of the fill system caused by other problems such as burst hoses or adhesive supply obstructions. 
     For reasons such as these, an improved adhesive melter and method of operation, including a control process for accurately predicting and alerting an operator when an exhaust air filter requires replacement or maintenance, would be desirable. 
     SUMMARY OF THE INVENTION 
     According to one embodiment of the invention, a method for operating an adhesive melter enables a predictive maintenance of an exhaust air filter used with a fill system associated with the melter. The method includes repeatedly actuating the fill system to perform a fill system cycle that refills a receiving space of the melter with solid adhesive particulate delivered with a pressurized air flow, which must then be exhausted through the exhaust air filter. The duration of each of the fill system cycles is monitored. The method also includes calculating an average duration for a plurality of the fill system cycles and detecting a change in the average duration for the fill system cycles. A user interface operatively coupled to the melter emits an alert if the detected change exceeds a maintenance threshold that is indicative of the exhaust air filter becoming clogged and requiring maintenance. Accordingly, the exhaust air filter may be replaced before the clogging stops operation of the adhesive melter. 
     In one aspect, the method also includes repeatedly sensing with a level sensor at the receiving space a fill level of adhesive located within the receiving space. The level sensor is capable of determining when the fill level within the receiving space crosses multiple thresholds associated with at least a nearly empty state and a nearly full state. In this regard, operation of the fill system starts to deliver solid adhesive particulate into the receiving space when the level sensor senses that the fill level has dropped below a refill threshold. Operation of the fill system stops when the level sensor senses that the fill level has exceeded a full fill threshold. The method also includes determining first and second times when the fill system starts and stops operating, respectively, from the readings of the level sensor. The difference between these first and second times provides the duration of the selected fill system cycle, which is then used to calculate the average durations that control when an alert is emitted. A controller of the adhesive melter performs the calculation of the average durations and detecting a change in the average duration such that the emission of the alert with the user interface is initiated based only on data from the level sensor and the monitoring of the durations of each of the fill system cycles. This controller process avoids false positive indications of the need for exhaust air filter maintenance that may occur when using additional sets of data from other types of sensors or equipment. 
     In another aspect, detecting a change in the average duration for the fill system cycles further includes identifying a predetermined number of most recently calculated average durations for a plurality of the fill system cycles. The predetermined number of most recently calculated average durations is then statistically analyzed to determine a trend line for the most recently calculated average durations. The slope of this trend line corresponds to the change in the average duration for the fill system cycles. Consequently, if the most recently calculated average durations are increasing at a slope greater than the maintenance threshold, the alert will be emitted at the user interface. The fill system is typically configured to shut down when the average duration for a plurality of the fill system cycles exceeds a maximum flow threshold that may indicate that the clogging at the exhaust air filter is preventing the fill system from keeping up with demands for more adhesive at the receiving space. Therefore, the emission of the alert is configured to be initiated before the average duration exceeds the maximum flow threshold, as this will provide a period of time (e.g., preferably a day or more) for maintenance of the exhaust air filter before shut down of the fill system would occur (and a possible shut down of the melter caused by running out of adhesive) due to clogging of the exhaust air filter. The alert can then continue to be emitted until maintenance is performed on the exhaust air filter or the fill system shuts down. 
     To prevent statistical outliers from affecting the analysis of the average durations for fill system cycles, the method includes additional steps for identifying and removing such statistical outliers not caused by clogging of the exhaust air filter. For example, the method may further include statistically analyzing the duration of each of the fill system cycles to identify the individual data outliers that indicate a change in the average duration for reasons unrelated to exhaust air filter clogging (e.g., a burst hose in the fill system, an obstruction in the adhesive source, a change in adhesive material used or the length of hose in the fill system). These individual data outliers are then discarded before calculating the average duration and detecting a change in the average duration for the fill system cycles. In another example, the duration for each of the fill system cycles may be evaluated until the average duration stabilizes after an initial time period following maintenance or replacement of the exhaust air filter. All data for durations of fill system cycles during this initial time period are then discarded before detecting a change in the average duration for the fill system cycles. As such, data outliers caused by events unrelated to gradual filter clogging and data outliers known to occur at the beginning of a filter&#39;s lifespan are not used to control when the alert is emitted to prompt maintenance for the exhaust air filter. 
     In another embodiment, an adhesive melter is configured to provide the predictive maintenance for an exhaust air filter to avoid unplanned shut downs of the fill system. The melter includes a receiving space configured to receive a supply of solid adhesive particulate that is to be melted by a heater unit, a fill system that performs fill system cycles that refill the receiving space, and the exhaust air filter which communicates with the fill system and the receiving space to exhaust pressurized air flow that carries the solid adhesive particulate into the receiving space. The melter also includes a controller that works during operation of the melter to repeatedly actuate the fill system, to monitor the duration of each fill system cycle, to calculate an average duration for a plurality of the fill system cycles, to detect a change in the average duration for the fill system cycles, and to emit an alert with a user interface if the detected change exceeds a maintenance threshold indicative of the exhaust air filter becoming clogged. As a result, maintenance or repair of the clogged exhaust air filter may be conducted before the clogging causes a shutdown of the fill system. 
     The adhesive melter also includes a level sensor located at the receiving space for repeatedly sensing a fill level of adhesive located within the receiving space. The level sensor, in one embodiment, includes a plate element with an electrically driven electrode and a ground electrode such that the level sensor measures a dielectric capacitance of air and adhesive acting as dielectric between the driven and ground electrodes. The dielectric capacitance varies with the fill level of the adhesive, so the level sensor can monitor when the fill level passes certain thresholds such as a refill threshold for starting operation of the fill system, or a full fill threshold for stopping operation of the fill system. The information from the level sensor may be used to determine the start and stop times and total durations of time for each fill system cycle. The controller actuates the emission of the alert based solely on data received from the level sensor, thereby avoiding false positive alerts that may occur when additional data is used for predictive maintenance. 
     These and other objects and advantages of the invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an embodiment of the invention and, together with a general description of the invention given above, and the detailed description of the embodiment given below, serve to explain the principles of the invention. 
         FIG. 1  is a schematic block diagram view of an adhesive dispensing system including an adhesive melter and a fill system according to one embodiment of the current invention. 
         FIG. 2  is a cross-sectional front view of the adhesive melter of  FIG. 1 , illustrating additional features such as a level sensor in a receiving space and a cyclonic separator unit defining the inlet and the exhaust for pressurized air flow to and from the adhesive melter. 
         FIG. 3  is a detailed cross-sectional front view of the receiving space and cyclonic separator unit of  FIG. 2 , with an exemplary flow of pressurized air and solid adhesive pellets shown entering the melter and exiting through an exhaust air filter. 
         FIG. 4  is a flowchart describing a sequence of operational steps performed by the fill system and a controller connected to the adhesive melter of  FIG. 1 , to thereby provide predictive maintenance alerts for clogging of the exhaust air filter of  FIG. 3 . 
         FIG. 5  is a time graph showing trends of an average daily fill time obtained using the sequence of operational steps of  FIG. 4 , to explain how the predictive maintenance alerts are triggered. 
         FIG. 6  is a schematic representation of a user interface of the adhesive dispensing system of  FIG. 1 , showing an exemplary maintenance alert for replacing the exhaust air filter. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
     Referring to  FIGS. 1 through 6 , an adhesive dispensing system  10  in accordance with one embodiment of the invention is shown, the system  10  including an adhesive melter  12  configured to perform a method for predictive maintenance of an exhaust air filter  14  associated with the adhesive melter  12 . The adhesive melter  12  operates to monitor a duration of fill system cycles used to refill the adhesive melter  12  with solid adhesive particulate, and these durations of fill system cycles are then used to determine whether an alert should be emitted regarding the immediate need for maintenance or replacement of the exhaust air filter  14 . When an alert is emitted, such as on a user interface  16  operatively connected to the adhesive melter  12 , the operator of the adhesive dispensing system  10  will be provided with a period of time to conduct maintenance on the exhaust air filter  14  before the exhaust air filter  14  becomes so clogged that the adhesive melter  12  cannot function properly. Accordingly, unplanned downtime for the dispensing system  10  that is caused by clogging of the exhaust air filter  14  is minimized or avoided altogether. Advantageously, the adhesive melter  12  and method described in further detail below operate to reliably provide predictive maintenance regardless of variable operating conditions at the adhesive melter  12 , including, but not limited to, selective powdering of the adhesive to prevent tackiness and changes in adhesive form factor. 
     Before describing the detailed operation and functionality associated with the method for providing predictive maintenance (see discussion pertaining to  FIGS. 4 and 5 , below), a description of the exemplary adhesive dispensing system  10  and adhesive melter  12  that perform this process will be helpful to understanding the functionality. With particular reference to  FIG. 1 , the adhesive melter  12  of the adhesive dispensing system  10  includes a receiving space  20  (also referred to as a “hopper” in some embodiments), a level sensor  22 , a heater unit  24  receiving adhesive from the receiving space  20 , and a reservoir  26  receiving adhesive melted and heated by the heater unit  24 . The adhesive melter  12  of this embodiment also includes a fill system  28  operable to deliver solid or semi-solid adhesive particulate to the receiving space  20  using a pressurized air flow to refill the receiving space  20  when necessary. The adhesive melter  12  therefore also includes the exhaust air filter  14 , through which the pressurized air flow from the fill system  28  and the receiving space  20  is discharged from the adhesive melter  12 . This exhaust air filter  14  requires maintenance or replacement over time as a result of clogging, and the adhesive melter  12  and methods described below advantageously enable predictive maintenance of this exhaust air filter  14 . It will be understood that the adhesive melter  12  may include more or fewer elements in other embodiments, including the other elements shown outside the dashed line box representing the adhesive melter  12  in  FIG. 1 , without departing from the scope of the invention. 
     As shown in  FIG. 1 , the adhesive melter  12  further includes a pump  32  configured to deliver heated adhesive from the reservoir  26  to a dispenser gun  34  or module. As the dispenser gun  34  operates to discharge melted adhesive from the dispensing system  10 , adhesive material is removed from the adhesive melter  12 , and this eventually leads to a fill system cycle operated by the fill system  28  to refill the receiving space  20  with more solid adhesive particulate. These fill system cycles are monitored to determine when an alert should be provided on the user interface  16  regarding necessary maintenance for a clogged exhaust air filter  14 . It will be understood that the pump  32 , fill system  28 , and/or other elements may be separated from the melter  12  in some embodiments without departing from the scope of the invention. 
     The adhesive dispensing system  10  also includes a controller  36  operatively connected to one or more of the fill system  28 , the level sensor  22 , the heater unit  24 , the pump  32 , and the dispenser gun  34 . The controller  36  includes a processor and a memory (not shown), and also program code resident in the memory and configured to be executed by the processor. As described in further detail below, the program code operates to monitor fill levels of adhesive in the receiving space  20 , actuate refilling operations by the fill system  28 , and then monitor these fill system cycles to determine whether an alert should be provided to the operator to prompt repair or replacement of the exhaust air filter  14 . To this end, the controller  36  includes or is connected to a timer  38  configured to measure the elapsed time for fill system cycles. The timer  38  may be a separate time measurement device or a clock device configured to provide the current time to the controller  36  in embodiments where the timer  38  is not incorporated into the controller  36 . The controller  36  then communicates with the user interface  16 , which may be incorporated as part of the adhesive melter  12  or unrelated to the adhesive melter  12  in other embodiments, to initiate the alert for predictive maintenance. It will be understood that the predictive maintenance methods and functionality described below may be used with other types of dispensing systems and melters having a different arrangement of components, without departing from the scope of this invention. 
     The exemplary embodiment of the adhesive melter  12  shown schematically in  FIG. 1  is illustrated in further detail in  FIGS. 2 and 3 . Many of the components of the adhesive melter  12  are also described in co-pending U.S. patent application Ser. No. 13/799,622 to Clark et al., entitled “Adhesive Dispensing Device having Optimized Reservoir and Capacitive Level Sensor,” the disclosure of which is hereby incorporated by reference herein in its entirety. Similarly, the specific flows of solid adhesive particulate and pressurized air flow shown in these FIGS. is also described in co-pending U.S. patent application Ser. No. 13/799,788 to Chau et al., entitled “Adhesive Dispensing Device having Optimized Cyclonic Separator Unit,” the disclosure of which is hereby incorporated by reference herein in its entirety. The following description summarizes these more detailed disclosures with a particular emphasis on the structural components used to perform the predictive maintenance of the exhaust air filter  14 . 
     With reference to  FIG. 2 , the receiving space  20  or hopper is mounted directly above the heater unit  24 , which is in turn mounted directly above the reservoir  26 . Consequently, a gravity-driven flow of adhesive is provided between the receiving space  20  where solid adhesive particulate is initially delivered and the reservoir  26  that communicates heated and melted adhesive to the pump  32  (not shown in  FIGS. 2 and 3 ). The adhesive melter  12  is configured to operate filled with adhesive material so that the heater unit  24  is not exposed to open air for extended periods of time, which could lead to overheating of the heater unit  24  and any remaining adhesive in the melter  12 . More specifically, the adhesive melter  12  is filled during normal operation such that the heater unit  24  and the reservoir  26  are completely filled with adhesive and the receiving space  20  is at least partially filled with solid or semi-solid adhesive. Therefore, the fill level of the adhesive inside the melter  12  is measured by the level sensor  22 , which is located at the receiving space  20 . As the fill level lowers within the receiving space  20 , the controller  36  receives this information from the level sensor  22  and then actuates the fill system  28  to perform a fill system cycle and deliver solid adhesive particulate to refill the receiving space  20 . This process, including sensing the fill level in the receiving space  20  and then actuating refills with the fill system  28 , repeats during normal operation of the melter  12  and is the basis for performing the predictive maintenance of the exhaust air filter  14 . 
     The level sensor  22  of the exemplary embodiment includes a capacitive level sensor in the form of a plate element  42  mounted along one of the peripheral sidewalls  44  of the receiving space  20 . The plate element  42  includes a driven electrode  46  and a ground electrode  48  that is coupled to one or more of the sidewalls  44  of the receiving space  20  with plate fasteners  50  as shown. Therefore, the sidewalls  44  of the receiving space  20  also act as a portion of the ground electrode for the level sensor  22 . The level sensor  22  determines the fill level of adhesive material in the receiving space  20  by detecting with the plate element  42  where the capacitance level changes between the driven electrode  46  and the ground electrode  48 . To this end, open space or air in the receiving space  20  provides a different capacitance than the adhesive material in the receiving space  20 . The level sensor  22  is connected with the controller  36  to provide information corresponding to the fill level passing multiple threshold levels in the receiving space (e.g., a refill threshold level where refill of the receiving space  20  should be actuated immediately and a full fill threshold level when the receiving space  20  has been sufficiently filled by the fill system  28 ). Alternatively, the single level sensor  22  shown in  FIGS. 2 and 3  may be replaced by multiple smaller level sensors (not shown) operable to sense when the fill level in the receiving space  20  passes the relevant thresholds. Accordingly, the level sensor  22  is capable of providing signals to the controller  36  to start and stop fill system cycles with the fill system  28  to keep the receiving space  20  from becoming too empty or overfilled, and these signals can also be used to determine predictive maintenance for the exhaust air filter  14 , as described in further detail below. 
     In this regard, the controller  36  is operatively connected to or includes the timer  38 , which applies a time stamp to each instance when the level sensor  22  senses that the fill level within the receiving space  20  drops below the refill threshold or exceeds the full fill threshold. For each fill system cycle actuated by the controller  36 , the difference between the time when the fill level drops below the refill threshold and the time when the fill level exceeds the full fill threshold is indicative of the duration for the fill system cycle operated by the fill system  28 . As a result, the level sensor  22  and timer  38  provide sufficient data for the controller  36  to record the duration of each fill system cycle. This data is then collected together and analyzed per the methodology described below to determine when the exhaust air filter  14  is becoming clogged and requires maintenance or replacement. This functionality of the controller  36  uses information that is already required to keep the receiving space  20  and melter  12  filled with sufficient adhesive during operation, so no additional air flow or pressure sensors are necessary within the adhesive melter  12 . 
     As shown in  FIGS. 2 and 3 , a cyclonic separator unit  52  may be mounted on top of the receiving space  20  in the exemplary embodiment of the adhesive melter  12 . The cyclonic separator unit  52  receives adhesive pellets  54  or other solid adhesive particulate driven by a pressurized air flow through an inlet hose (not shown) leading to the fill system  28 . To this end, the inlet hose connects to a tangential inlet pipe  56  that communicates with a generally cylindrical pipe  58  extending from the tangential inlet pipe  56  to an opening  60  at the top of the receiving space  20 . The generally cylindrical pipe  58  may include a mounting plate  62  configured to be coupled to the sidewalls  44  of the receiving space  20  using bolt fasteners  64  or other connecting elements, as shown. Consequently, the incoming flow of adhesive pellets  54  and pressurized air flow is directed to spiral downwardly through the generally cylindrical pipe  58  towards the receiving space  20  as shown by first flow arrows  66  in  FIG. 3 . It will be understood that the cyclonic separator unit  52  decelerates the speed of the pressurized air flow and the incoming adhesive pellets  54  before those adhesive pellets  54  are delivered into the receiving space  20 , and this reduction in speed minimizes any splashing of liquid adhesive that may be within the receiving space  20 . The receiving space  20  is sealed from the environment upstream from the heater unit  24  but for the connection to the cyclonic separator unit  52 , so the cyclonic separator unit  52  also includes an exhaust pipe  68  proximate to the tangential inlet pipe  56  for removing the pressurized air flow from the receiving space  20 . 
     To this end, the tangential inlet pipe  56  defines the inlet into the receiving space  20 , and the exhaust pipe  68  defines the outlet from the receiving space  20 . The exhaust pipe  68  therefore defines an internal passage  70  sized to receive the exhaust air filter  14  used with the exemplary embodiment of the adhesive melter  12 . The incoming flow of air and pellets  54  shown by the first flow arrows  56  is separated at or near the receiving space  20  such that the adhesive pellets  54  drop into the receiving space  20  as shown by second flow arrows  72  and the pressurized air flow reverses direction and flows upwardly within the generally cylindrical pipe  58  and through the exhaust pipe  68  and exhaust air filter  14  back to the surrounding environment, as shown by third flow arrows  74 . In this regard, all of the pressurized air flow exiting the receiving space  20  and the adhesive melter  12  passes through the exhaust air filter  14  such that any adhesive vapors, powder, or other contaminants may be removed from the outgoing exhaust flow. 
     The amount of contaminants that must be removed with the exhaust air filter  14  can vary significantly between fill system cycles as a result of various factors, including the form factor or shape defined by the solid adhesive particulate and whether the solid adhesive particulate is powdered to avoid sticking together upstream from the fill system  28 . The operator of the adhesive melter  12  likely has very little or no control over these varying operating conditions, so it is difficult to predict how quickly the exhaust air filter  14  will clog over time. However, the predictive maintenance enabled by the process described below automatically adjusts to the varying operating conditions, thereby overcoming the problems previously encountered when using exhaust air filters  14  that unexpectedly clog and cause unplanned downtime for the adhesive melter  12 . More particularly, an alert is provided on a user interface  16 , either located at the melter  12  or some other convenient location, to prompt the operator to repair or replace the exhaust air filter  14  before the clogging causes an unplanned shutdown of the fill system  28  (and also potentially a later shutdown of the melter  12 ). 
     To summarize the functionality, the adhesive melter  12  operates by having the controller  36  actuate heating and melting of adhesive with at least one heater element  80  located in sidewalls  82  and/or partitions  84  of the heater unit  24  and with at least one heater element  86  located in sidewalls  88  and/or fins/partitions  90  of the reservoir  26 . As the heated adhesive is drawn out of the reservoir  26  by the pump  32 , the level sensor  22  detects the need to refill the receiving space  20  and the controller  36  actuates the fill system  28  to provide more solid adhesive particulate through the cyclonic separator unit  52 . The pressurized air flow generated during a fill system cycle is then exhausted through the cyclonic separator unit  52  and the exhaust air filter  14 . The controller  36  uses information from the level sensor  22  and the timer  38  to statistically analyze the data regarding fill system cycle durations and thereby determine any change in the average duration for fill system cycles, which provides the information necessary to determine when maintenance of the exhaust air filter  14  will be required. For example, the “change” that is determined may include changes in duration over multiple cycles or the rate of change of such changes in duration (e.g., a second derivative analysis) in some embodiments. One specific method programmed into the controller  36  for performing this analysis and predictive maintenance is described in further detail below, but it will be understood that the exemplary embodiment of the adhesive melter  12  shown in  FIGS. 1 through 3  may be modified in other embodiments without departing from the scope of the invention. 
     Now turning to  FIG. 4 , the controller  36  is configured to perform the series of operations defining the predictive maintenance process according to one embodiment of the invention, this series of operations being labeled with reference number  400  in the Figure. The series of operations begins by starting the timer  38  at a time t=0 (block  402 ). This variable t will be used to time stamp the beginning and end of fill system cycles as briefly described above. The controller  36  operates the melter  12  to provide hot melt adhesive to the remainder of the adhesive dispensing system  10  (block  404 ). This adhesive melter operation includes the heating and melting of solid particulate adhesive as the heated adhesive is removed using the pump  32  or the dispenser gun  34 . After a period of operation, the fill level of adhesive within the receiving space  20  will drop below a refill threshold that indicates a refill is necessary to avoid uncovering the heater unit  24 . Once the level sensor  22  detects that the fill level within the receiving space  20  has dropped below this refill threshold, the controller  36  actuates the fill system  28  to perform a fill system cycle and thereby refill the receiving space  20  with more adhesive (block  406 ). The fill system  28  will deliver adhesive pellets  54  in a pressurized air flow into the receiving space  20  as described in detail above. 
     The fill system  28  is configured to continue delivering adhesive pellets  54  and pressurized air flow until one of two conditions occur: the fill system  28  has been running for a maximum cycle time (e.g., 10 seconds in some embodiments), or the level sensor  22  detects that the receiving space  20  is filled. To this end, the level sensor  22  also senses when the fill level of adhesive within the receiving space  20  exceeds a full fill threshold, at which point the controller  36  knows the receiving space  20  is filled and the operation of the fill system  28  can be stopped. While the controller  36  has been actuating the fill system cycle to start and stop, the timer  38  has been applying a time stamp based on the time t when the level sensor  22  detected the fill level dropping below the refill threshold (i.e., when the fill system cycle started) and the time t when the level sensor  22  detected the fill level exceeding the full fill threshold (i.e., when the fill system cycle stopped). As discussed above, this example of a running timer  38  may instead be replaced with a timing device internal to the controller  36  or a global clock that provides current time information in other embodiments of the invention. The controller  36  receives these data from the timer  38  and determines an elapsed cycle time or “duration” for the fill system cycle, this elapsed cycle time being the difference between these time values monitored by the timer  38  (block  408 ). The fill system cycle and its duration are stored as a data point in the memory associated with the controller  36 , and the specific time t when the fill system cycle was operated may also be stored as a part of this data point (block  410 ). Therefore, over the course of operation, these steps at blocks  406  through  410  can be reused to monitor and store the duration of each fill system cycle. 
     In the exemplary embodiment shown in  FIG. 4 , the controller  36  may be operated in a plurality of modes, including, but not limited to, a startup mode and a monitoring mode. The startup mode is used to calibrate the readings for fill system cycle durations during an initial period of time such as a few days or a week following the installation of a new exhaust air filter  14 . This startup mode ensures that the fill system cycle durations have stabilized and also ensures that sufficient data is collected for the analysis described below. The monitoring mode follows the startup mode and includes this analysis of the fill system cycle durations. It will be understood that the controller  36  may perform the foregoing and following functions without specifically operating in distinct modes such as these in other embodiments, but these modes will assist in understanding the operation of the adhesive melter  12 . 
     Thus, in the exemplary embodiment the controller  36  next determines whether the startup mode is active (block  412 ). If the startup mode is active, then the controller  36  determines a total number of fill system cycles that have been run during the startup mode (block  414 ). This total number should be equivalent to the number of data points stored during this mode. The controller  36  determines if this number of fill system cycles provides sufficient data to generate an average duration for the fill system cycles (block  416 ). This determination may be based on prior testing that determines how many fill system cycles generally need to be performed before the cycle duration stabilizes from the initial unpredictability caused early in the lifespan of the exhaust air filter  14 . For example, the first few days of fill system cycles may be required before a reliable average duration for a plurality of the fill system cycles can be calculated. This sufficient data may be a predetermined set number of data points or a set period of time t that the melter  12  has to be operated during the startup mode. Thus, if sufficient data has not been collected at step  416 , the process returns to block  406  to actuate the fill system  28  again once the level sensor  22  detects that a refill of the receiving space  20  is required. This collection of data repeats until sufficient data has been collected. 
     Once the controller  36  determines at step  416  that sufficient data has been collected during the startup mode, the controller  36  proceeds to remove any data outliers that fall outside a predetermined deviation (such as one or more standard deviations) from the remainder of the stored data (block  418 ). This identification of data outliers is conducted using known statistical analysis methods such as the calculation of a standard deviation and a determination of which data points fall outside the standard deviation. In addition to statistical outliers caused by occurrences unrelated to filter clogging (e.g., caused by a burst hose in the fill system, an obstruction in the adhesive source, a change in adhesive material used or the length of hose in the fill system), a predetermined number of the initial fill system cycles may also be removed during this process to avoid the use of unreliable data known to occur during the first few days of operation with a new exhaust air filter  14 . In another example, a series of consecutive fill system cycles having maximum duration may be discarded because this likely indicates an initial filling of the melter  12  from an empty condition. The statistical analysis performed on the data in step  418  is programmed and tailored to leave only those data points which will be reliable and helpful in determining the gradual clogging of the exhaust air filter  14 . 
     With the remaining data from the startup mode, the controller  36  calculates the average duration for a plurality of fill system cycles (block  420 ). This average duration represents a baseline that will change over time as the exhaust air filter  14  becomes more clogged, as the fill system  28  will not be able to generate and exhaust as much pressurized air flow as the exhaust air filter  14  becomes more clogged. To this end, the average duration for the plurality of fill system cycles is ready to be analyzed over time and further fill system cycles to determine when the clogging of the exhaust air filter  14  is adversely affecting the operation of the fill system cycles. Following this initial calculation of the average duration, the controller  36  ends the startup mode and begins the monitoring mode (block  422 ), at least in those embodiments having distinct modes of operation. The controller  36  then returns to step  402  to reset the timer  38  back to zero for the monitoring mode. 
     While in the monitoring mode, the controller  36  will determine at step  412  (following another detection and storage of an elapsed cycle time for a fill system cycle) that the startup mode is not active. In this circumstance, the controller  36  proceeds by identifying a group of the stored data points for testing whether a significant change in the average duration for fill system cycles has occurred (block  424 ). This identified group of data may include a predetermined number of the most recently stored data in the memory, for example. In other words, the controller  36  may have access to monitored durations for fill system cycles extending back to the beginning of use for the exhaust air filter  14 , but trends or changes in the average duration for fill system cycles will be best revealed when analyzing only a set number of more recent data. The identification of which data to use in the following analysis may be modified in other embodiments as well depending on the preferences of the operator. 
     Once the group of data for the test has been identified, the controller  36  removes any data outliers that fall outside a predetermined deviation from the remainder of the group of data (block  426 ). This removal typically follows similar statistical analysis rules as those described above with reference to step  418 . In another example of removing such data outliers, the data may indicate a change from a plurality of fill system cycles with a stable average duration about 3.0 seconds to another plurality of fill system cycles with a stable average duration of about 5.0 seconds. Such a change is likely caused by factors unrelated to filter clogging, including a change in hose length between an adhesive source and the melter  12  or a change in adhesive material used, so the statistical analysis would disregard the older fill system cycles with the stable average duration of about 3.0 seconds in step  426  for this example. The controller  36  then statistically analyzes the remaining data and formulates a trend line for the data (block  428 ). The formulation of a “trend line” is described for exemplary purposes only, as the controller  36  does not necessarily need to plot all of the data onto a graph to identify any trends in the duration data over time. If the controller  36  did produce a plot of the data on a graph, a sample of such a plotting of data (without a trend line) is shown in  FIG. 5 , which is discussed in further detail below. The trend line defines a slope that will correspond to “a change” in the average duration of fill system cycles over the selected predetermined number of most recently performed fill system cycles. This detected “change” could be the general increase of average duration from cycle to cycle, the rate of change of the changing average duration (e.g., a second derivative analysis), or some similar indicator of hampered performance by filter clogging. The specific statistical analysis chosen for step  428  may be chosen based on the desires of the operator or end user. Regardless of how the “change” is defined, the controller  36  calculates the slope of the trend line relative to the average duration and sets this slope as a variable Δ (block  430 ). 
     The controller  36  then determines whether the variable Δ is greater than or equal to a predetermined maintenance threshold value that indicates clogging of the exhaust air filter  14  and an imminent need to replace or perform maintenance on the exhaust air filter  14  (block  432 ). The predetermined threshold value is set based on a plurality of factors, such as previous test data that shows the typical increase in fill system cycle duration over time. This maintenance threshold also depends on the type of statistical analysis being performed to identify the change in the durations of fill system cycles. For example, the change in the average duration may be required to exceed a 4-5% increase per fill system cycle in one embodiment, although it will be understood that a slope or variable Δ of greater than 1% per fill system cycle may be sufficient to determine significant filter clogging. Regardless of what criteria is used to set the predetermined threshold value, the detection of whether the variable Δ exceeds this value is tailored to provide an early indication of when the exhaust air filter  14  requires maintenance, thereby identifying a potential problem in advance of an automatic shutdown of the fill system  28 . 
     If the variable Δ does not exceed the predetermined threshold value at step  432 , then the controller  36  returns to step  406  to begin another fill system cycle when a refill is again required in the receiving space  20 . The process of monitoring the duration of the next fill system cycle and detecting a change in the average duration of a plurality of fill system cycles repeats until clogging at the exhaust air filter  14  is determined by this process. In this regard, if the variable Δ does exceed the predetermined threshold value at step  432 , then the controller  36  initiates an alert  522  on a display screen  520  of the user interface  16  (see  FIG. 6  and discussion below) that informs an operator of the need to replace the exhaust air filter  14  (block  434 ). Generally, the alert  522  is maintained until the clogging of the exhaust air filter  14  causes an automatic shutdown of the fill system  28  or the operator conducts maintenance on the exhaust air filter  14 , typically by replacement of the exhaust air filter  14 . Once the maintenance occurs, the controller  36  ends the monitoring mode and begins the startup mode again (block  436 ), and then returns to step  402  to reset the timer  38  to zero for the new startup mode. This process continuously cycles between the modes as each exhaust air filter  14  is installed and used to the point where the clogging significantly affects the ability of the fill system  28  to sufficiently refill the adhesive melter  12 . 
     Accordingly, the series of operations included in the process  400  shown in  FIG. 4  is one embodiment that enables predictive maintenance alerts to be generated for clogging of the exhaust air filter  14 . Furthermore, these predictive maintenance alerts are based solely on the time measured for the plurality of fill system cycles operated by the fill system  28 , as this information is sufficient by itself to reliably determine when the exhaust air filter  14  is clogging to the extent of deteriorating performance for the fill system  28 . Therefore, additional sensors do not need to be added to the adhesive melter  12 , and in fact, such additional sensors would be undesirable because the analysis using those sensors may lead to more false positive tests for clogging (e.g., resulting in filters being replaced before being clogged at the end of a lifespan). The information provided by the level sensor  22  and the timer  38  is sufficient for the controller  36  to make a reliable determination regarding when the exhaust air filter  14  is becoming clogged and needs replacement. It will be understood that the process  400  described above may be modified in several aspects without departing from the scope of the invention, so long as the adhesive melter  12  continues to provide the predictive maintenance alerts. 
     A sample representation of the data collected during the beneficial operation of the adhesive melter  12 , while using the series of operations shown in  FIG. 4  is illustrated in graphical form in  FIG. 5 . To this end,  FIG. 5  illustrates a graphical plot  500  of average daily fill times (shown as daily averages rather than raw data on each fill system cycle, for simplicity) against the number of days since the exhaust air filter  14  was most recently replaced, which corresponds to the current lifespan of the exhaust air filter  14 . These average daily fill times provide a typical series of data that may be encountered when performing the analysis process with the controller  36  as described above. As described above, the controller  36  does not necessarily generate such a plot or graph during the statistical analysis, but this plot is helpful in understanding how the statistical analysis identifies trends that appear to be caused by filter clogging. It will be understood that these data points are provided from sample lab testing for descriptive purposes only and shall not be deemed to limit the invention in any substantial way. 
     As shown in  FIG. 5 , the plot  500  of the average daily fill times begins with a series of average durations that are relatively high (about 5-6 seconds) at point  502 . Then average durations then drop to a relatively stable value over the next few days of about 2-3 seconds. This initial higher set of average durations are a result of the startup period as described above, and these data should be removed from the analysis when the controller  36  begins monitoring whether the average durations are changing in such a way to indicate filter clogging. Although the average daily fill times change from day to day, the average durations calculated by the controller  36  remain largely within the 2-3 second window over a period of about 60 days. One exception is a spike at point  504 , but this data point  504  stands out as a statistical outlier when reviewing the plot  500  of data as a whole. Therefore, using the statistical analysis described above, the data point at  504  would be eliminated from the repeated evaluation of whether the fill system cycles are changing in duration. More specifically, the data point  504  is likely an aberration caused by factors other than clogging of the exhaust air filter  14 , as such clogging over time does not tend to have an immediate effect like that data point  504  would show. For example, the spike in duration may be caused by a hose bursting at the fill system  28  or a temporary running out or obstruction of adhesive at the source feeding the fill system  28  (these types of events may also shut down the fill system  28 , but not for reasons related to filter clogging). As such, the controller  36  will determine that the change in average durations for fill system cycles is not significant in this first 60 day window. 
     However, after day 60 the average daily fill times begin to increase relatively rapidly over the remainder of the lifespan of the exhaust air filter  14 . After a few consecutive increases in the average duration for the fill system cycles, such as at point  506 , the variable Δ for the slope of the trend line would exceed the corresponding predetermined threshold value as a result of the deteriorating performance shown in the average daily fill times. In the example shown in  FIG. 5 , a small series of 3-5 most recent average daily fill times may be considered in the detection of changing average durations. However, it will be understood that the average duration may be calculated after each fill system cycle, and a higher number (e.g., when the adhesive melter  12  performs 20 refills per hour or more) of these average durations could be analyzed in other embodiments of the invention to detect a sufficient increase or change in average durations. In the example shown in  FIG. 5 , the slope or variable Δ that the controller  36  may test for is an increase of more than 1 second per day in the average daily durations over 3 or more days. Such a predetermined threshold will cause initiation of the alert  522  at a relatively early point like  506  in the clogging of the exhaust air filter  14 , and this would provide a period of a few days before the clogging led to an average daily fill time of over 10 seconds at data point  508 , at which point an automatic shutdown of the fill system  28  would occur. To this end, the sample data shown in  FIG. 5  would provide up to 5 days of alert or notification to change the exhaust air filter  14  before an unplanned downtime would occur. That time period advantageously provides predictive maintenance for the exhaust air filter  14  in a reliable and repeatable manner. 
     With reference to  FIG. 6 , the user interface  16  according to the exemplary embodiment of the adhesive dispensing system  10  is shown. As discussed above, this user interface  16  may include a display screen  520  located at the adhesive melter  12  itself or at some other convenient location such as a control room. The user interface  16  will provide multiple pieces of information pertaining to operational data and settings being used for the components of the melter  12  and the dispensing system  10 , so this is also a convenient location for the alert  522  to be displayed to an operator. As shown in  FIG. 6 , the alert  522  is tailored to draw the immediate attention of the operator, and the alert  522  could be presented in a different font, color, or with a flashing display to further enhance the visibility of the alert  522 . In addition, an audible signal may also be emitted if desired. An operator interacting with the display screen  520  will readily understand from the clear statement of the alert  522  what maintenance needs to occur. To this end, the maintenance alerts for the exhaust air filter  14  may be formatting similarly to other component maintenance alerts that occur after a certain period of operational cycles (such as for a pump  32 ) or a similar monitoring metric. The process and dispensing system  10  described above advantageously provide predictive maintenance for these exhaust air filters  14 , which is a feature not provided in conventional adhesive systems. 
     In addition, the predictive maintenance enabled by the process and dispensing system  10  of this invention operates reliably regardless of changing operational conditions present in most adhesive dispensing systems  10 . More particularly, during a hotter time of year when more powdering of solid adhesive particulate is done by suppliers, the lifespan of the exhaust air filter  14  will shorten significantly, but it will still exhibit a period of time where the average durations of fill system cycles stays about the same followed by a period of time with a discernable steady increase in the average durations of fill system cycles as the exhaust air filter  14  becomes more clogged. Therefore, no matter whether the total lifespan of the exhaust air filter  14  is 30 days or 90 days, the increase in average durations for fill system cycles will be detected and an alert emitted in advance of the automatic shutdown of the fill system  28  caused by excessive clogging of the exhaust air filter  14 . The adhesive dispensing system  10  therefore enables predictive maintenance of exhaust air filters  14  that minimizes or eliminates unplanned downtime that are caused by clogging of these filters in conventional systems. 
     While the present invention has been illustrated by a description of several embodiments, and while those embodiments have been described in considerable detail, there is no intention to restrict, or in any way limit, the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broadest aspects is not limited to the specific details shown and described. The various features disclosed herein may be used in any combination necessary or desired for a particular application. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims which follow.