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CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/961,483, filed Sep. 24, 2001, currently pending, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The invention relates to paving machines and, more particularly to a heating control system for a screed of a paving machine. 
     BACKGROUND 
     Paving machines are generally used for laying a heated paving material, such as bituminous aggregate mixtures or asphalt, onto a roadbed and spreading the heated paving material to achieve a road with a uniform, smooth surface. After the heated asphalt is laid it is compacted and cools so that the road becomes passable by vehicles. 
     The paving machines include a screed assembly for spreading the heated material. The screed assembly has one or more screed plates, and these screed plates are at times heated for ease in manipulating the heated paving material. The temperature of the screed plates is important, because if the temperature is too high, the screed may warp or the heated paving material may be damaged. Also, if the temperature is too low, the heated paving material adheres to the screed and is difficult to spread and compact. Generally, the screed plates should be heated to a temperature close to the temperature of the heated asphalt material. 
     Controllers have been developed for controlling the temperature of the screed plates. Published patent application WO 00/47822 is directed to a heating control system that automatically regulates the screed plate temperature by adjusting current flow through resistive heating elements for each of the screed plates. However, when the temperature of a screed plate drops below a predetermined value, the entire screed plate is heated even though only a section of the screed plate may require heating. In addition, several screed plates may be heated simultaneously. Heating the entire screed plate or several screed plates simultaneously drains the power supply, which is used for other functions, such as lighting, of the paving machine, and such general heating is inefficient. 
     The present invention is directed to overcome one or more of the problems as set forth above. 
     SUMMARY OF THE INVENTION 
     A heating control system for heating a screed assembly of a paving machine is provided. The screed assembly includes a screed plate. The heating control system includes a plurality of resistive heating elements attached to the screed assembly and positioned adjacent to the screed plate. An electrical power supply is connected to the plurality of resistive heating elements. A controller is connected to the plurality of resistive heating elements to automatically adjust the temperature of the screed plate by selectively energizing one or more of the plurality of resistive heating elements. 
     A method for heating the screed assembly includes sequentially and automatically energizing one or more of the plurality of resistive heating elements at a time to achieve a predetermined screed plate temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a screed assembly, embodying the present invention, that is towed by an asphalt paving machine; 
         FIG. 2  is a schematic view of a screed heating control system of the present invention; and 
         FIG. 3  is a flow diagram illustrating a method of operation of the heating control system of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A screed assembly  10  for a paving machine  12 , only a portion of which is, is shown in  FIG. 1 . The paving machine  12  includes a heating control system  14  as shown in  FIG. 2 . The screed assembly  10  includes a main screed  16  and left and right screed extensions  18 , 20 . It should be understood that left and right screed extensions  18 , 20  are shown as being front mounted but can be either front or rear mounted extensions. In an extended mode the left and right screed extensions  18 , 20  extend outwardly from either side of the main screed  16 . 
     The main screed  16  is made up of first and second sections  30 , 32  one on each side of a longitudinal central axis  34 . The left and right screed extensions  18 , 20  are slidably mounted to first and second sections  30 , 32  of the main screed  16  respectively and include first and second screed plates  36 ,  38 . The left and right screed extensions  18 , 20  also include left and right screed plates  40 , 42 . 
     The screed assembly  10  functions to spread paving material distributed by the paving machine  12  onto a roadbed. In order to achieve optimum workability of the paving material, the temperature of the screed plates  36 , 38 ,  40 , 42  should be maintained within a predetermined temperature range. This predetermined temperature range is for exemplary purposes, between 250-310° F., however, other temperature ranges may be set. The heating control system  14  automatically controls the temperature of screed plates  36 , 38 ,  40 , 42  and is illustrated in detail in  FIG. 2 . 
     The heating control system  14  includes resistive heating elements  50   a - 50   d ,  52   a - 52   d ,  54   a - 54   d ,  56   a - 56   d , an electrical power supply  60  and a controller  62 . Power supply  60  is any suitable AC or DC power source that is connected, as by wire, to the controller  62 . The power supply  60  shown in  FIG. 2 , utilizes a 24V DC power supply, but other voltages and power sources may be used, as would be known in the art. The controller  62  may include a typical microprocessor and memory, and can be programmed or hard-wired to provide the functions discussed below. 
     As illustrated in  FIG. 2 , the four resistive heating elements  50   a - 50   d,    52   a - 52   d  are disposed on first and second screed plates  36 , 38  respectively, and four resistive heating elements  54   a - 54   d ,  56   a - 56   d  are disposed on the left and right screed plates  40 , 42  respectively. However, the present invention is not limited to this construction and other numbers of resistive heating elements may be provided for each screed plate. 
     The resistive heating elements  50 , 52 , 54 , 56  with subscript a &amp; b are disposed on forward portions of the screed plates  36 , 38 , 40 , 42  and the resistive heating elements  50 , 52 , 54 , 56  with subscript c &amp; d are disposed on rearward portions of the screed plates  36 , 38 , 40 , 42 . As used herein throughout, forward refers to the side of the screed plate that is closest in proximity to the paving machine  12 , while rearward refers to the farthest away from the paving machine  12 . In use, the screed assembly  10  is pulled in the forward direction as indicated by arrow  58  in  FIG. 1 . 
     The heating control system  14  also includes inputs to and outputs from the controller  62 . An on/off switch  70 , a both/rear-only switch  72  and a warm-up switch  74  provide one set of inputs. Each of the switches  70 , 72  and  74  selectively connect the electrical power supply  60  to respective input connectors of the controller  62 . Temperature switches  80 , 82 , 84 , 86 , which are normally open, provide another set of inputs. The function and operation of each of these switches is provided in further detail below. 
     The temperature switches  80 , 82 , 84 , 86  are provided, one for the first and second screed plates  36 , 38  of the main screed  16 , and one for the left and right screed plate  40 , 42  of the left and right screed extensions  18 , 20 . A temperature sensor  90 , 92 , 94 , 96  is provided with each temperature switch  80 , 82 , 84 , 86  to detect the temperature of the corresponding screed plate  36 , 38 , 40 , 42 . Temperature switches  80 , 82 , 84 , 86  will close when the detected temperature falls below a first predetermined temperature Ta, e.g., Ta=250° F., and will open when the temperature reaches a second predetermined temperature Tb, e.g., Tb=310° F. 
     As shown in  FIG. 2 , eight outputs (1-8) from the controller  62  are provided respectively to eight solenoid relay switches, i.e., four forward output switches  100 , 102 , 104 , 106  and four rearward output switches  110 , 112 , 114 , 116 . Two output switches ( 100 , 110 ;  102 , 112 ) are assigned for each respective section  30 , 32  of the main screed plate  16 . Similarly, two output switches ( 104 , 114 ;  106 , 116 ) are provided respectively for the left and right screed extensions  18 , 20 . The output switches are normally open, and the output switches are closed in response to signals provided from the controller  62 . When closed, the switches connect an AC power generator  118  to the respective resistive heating elements  50   a - 50   d ,  52   a - 52   d ,  54   a - 54   d ,  56   a - 56   d , thus selectively heating the screed plates  36 , 38 , 40 , 42  in response to the temperature being below a threshold temperature Ta. Similarly, when the temperature then increases above the second threshold temperature Tb, the closed switch opens and AC power is disconnected from the heating element. 
     Each temperature switch  80 , 82 , 84 , 86  controls two corresponding output switches of the associated screed plate. As shown in  FIG. 2 , temperature switch  80  corresponds to the forward and rearward output switches  100 , 110  of the first screed plate  36 , the forward output switch  100  corresponds to the forward resistive heating elements  50   a ,  50   b  and the rearward output switch  110  corresponds to the rearward resistive heating elements  50   c ,  50   d . Thus, the heating of the forward and rearward resistive heating elements on the first screed section  30  is separately controlled by the two respective output switches  100 ,  110 . Similarly, temperature switches  82 , 84 , 86  correspond to the second screed section  32  and the left and right screed extensions  18 , 20  respectively. 
       FIG. 3  is a flow chart illustrating a control diagram for the present invention. First, the paving machine is turned on (S 1 ), and either the auto mode or the warm-up mode is turned on (S 2 ). If the warm-up mode is turned on, it is determined whether the rear-only mode is turned on (S 3 ). If yes, all of the rearward heating elements are energized simultaneously (S 4 ) until the screed plates reach a predetermined temperature Tb, e.g., 310° F. If no, all of the heating elements are energized simultaneously (S 5 ) until the screed plates reach a predetermined temperature Tb, e.g., 310° F. After all of the desired heating elements have been heated so that the screed plates reach a predetermined temperature Tb, the controller is automatically set to the auto mode. 
     In the auto-mode, it is determined whether the rear-only mode is turned on (S 7 ). If yes, only the rearward heating elements are to be heated. Each rearward heating element is heated in sequence so that the screed plates are heated in sequence. Of course, if only one screed plate is present, this step may refer to only one rearward heating element being heated at a time, whereas if multiple screed plates are present, those rearward heating elements associated with one screed plate are heated together, followed by the rearward heating elements of a second screed plate, etc. 
     Next, the temperatures of the screed plates are monitored (S 9 ) and if it is determined that the temperature has fallen below a predetermined temperature Ta, e.g., 250° F., the rearward heating elements of that screed plate are energized (S 11 ). If the temperature is within an acceptable range, the monitoring of the screed plate temperature is continued (S 9 ) until it requires heating. 
     A similar method is followed if the rear-only mode is not on in the auto mode. Specifically, all of the heating elements for all of the screed plates are heated in sequence (S 12 ). Again, if only one screed plate is present, only one of the heating elements may be heated at a time. If multiple screed plates are present, those heating elements associated with one screed plate may be heated together. Still further, as with the main screed plate that is divided into two sections, those heating elements associated with one section of the screed plate may be energized first, followed by the energizing of the heating elements of the other section of the screed plate. 
     After the screed plates have reached their predetermined temperatures, the screed plate temperatures are monitored (S 13 ). If a screed plate temperature falls below the predetermined temperature Ta (S 14 ), e.g., 250° F., the temperature switch of that screed plate closes and the controller notifies the associated output to energize the corresponding heating elements (S 15 ). While these heating elements are being energized, other temperature switches may close as other screed plate temperatures fall below 250° F. However, these screed plates are not heated until the heating of the previous screed plate is completed. If more than one temperature switch closes at the same time, the controller activates the outputs for only one of the screed plates at a time, in a random order. 
     According to the present invention, the controller can be programmed according to the operator&#39;s need or the requirements of a particular paving operation. For example, the controller can be programmed to alternately heat each screed plate, or section of a screed plate. Further, the controller can be programmed to heat first one plate, than two plates, or any pre-selected combination of plates in order to provide optimum heating conditions while conserving power. 
     INDUSTRIAL APPLICABILITY 
     In operation, the on/off switch  70  on the controller  62  selectively connects the 24V power supply  60  to the controller  62 . When the on/off switch  70  is positioned in an ON position, an automatic mode of the heating control system  14  is initiated. In the automatic mode, the controller  62  is programmed to limit the number of plates or sections that can be activated at a time. For example, only one of two output switches that control heaters in the first and second screed sections  18 , 20  or left and right screed extensions  30 , 32  is on at a time. Moreover, the controller  62  can be programmed to allow only one of the output switches that controls right or left hand side heaters of both the extension and main screed plates to be on at one time, i.e., only one extending screed plate or section of the main screed plate can be heated at a time. This control can be variably programmed to have one or more, but fewer than all of the heaters to be on at a time, as would be understood by one skilled in the art. 
     The warm-up switch  74  may be activated if it is desirable to heat more than screed extension plate  40 , 42  or first and second section plate  36 , 38  of the main screed plate  16  simultaneously. Such simultaneous heating may be desirable during initial start up of the paving machine  12 . When the warm-up switch  74  is activated, the controller  62  turns on all the outputs switches  100 , 102 , 104 , 106 , 110 , 112 , 114 , 116  at the same time if all the temperature switches  80 , 82 , 84 , 86  are closed. Once the screed plates  36 , 38 , 40 , 42  have reached their predetermined temperatures, the temperature switches  80 , 82 , 84 , 86  open, and the controller  62  returns to automatic mode. 
     Also, the warm-up switch  74  overrides the automatic mode. Thus, when the automatic mode is not providing the type of surface finish desired, due to inadequate heating, the warm up mode can be activated. After all the temperature switches open once, the controller  62  may go back to the automatic mode. Alternatively, the warm-up mode may be manually controlled. For example, the warm-up switch  74  also can act as an override for the warm-up mode. Thus, if the operator determines that the appropriate surface finish is being achieved, the warm-up switch  74  can be operated again, and the controller  62  will go back to automatic mode. 
     During normal operation the screed plates  36 , 38 , 40 , 42  will cool down and the temperature switches  80 , 82 , 84 , 86  will close. The temperature sensors  90 , 92 , 94 , 96  detect the temperature of the corresponding screed plate  36 , 38 , 40 , 42  and when the temperature falls below a predetermined level, the temperature switch  80 , 82 , 84 , 86  closes. The controller  62  receives a signal that the temperature switch  80 , 82 , 84 , 86  has closed and that the corresponding screed plate  36 , 38 , 40 , 42  should be heated. The controller  62  will energize the resistive heating element or elements corresponding to that screed plate  36 , 38 , 40 , 42  in the time sequenced order in which the temperature switches close. For example, if temperature switch  80  closes first, outputs  100 ,  110  turn on, and then when another temperature switch  84  closes, the controller  62  detects the closing but it waits for outputs  100 , 110  to turn off before issuing a signal to the appropriate relay to turn on the next outputs  104 , 114 . If multiple temperature switches close while an output is on, then the next outputs associated with a particular screed plate or section will be turned on in the order in which the temperature switch is closed. 
     With this automatic mode, only one screed plate  36 , 38 , 40 , 42  can be heated at a time, thus, conserving power for other operations of the paving machine  12 . For example, with conventional heating systems several screed plates may be heated simultaneously. This depletes power from other functions, for example, the paving machine speed may slow down, thus slowing down paving operations and affecting road quality. With the present invention, only a limited amount of power is used to heat the screed plates at any given time, and thus other functions of the paving machines are not affected during normal operation. 
     In general, only certain screed plates require heating due to the manner in which the asphalt material interacts with the screed plates. For example, during paving operations, high temperature asphalt material is conveyed from the paving machine  12  onto the roadbed surface. This high temperature asphalt material reaches the center portions (i.e., main screed  16 ) of the screed assembly  10  first, since the material is conveyed from a central portion of the paving machine  12 , and then tends to spread toward the left and right screed extensions  18 , 20 . Since the asphalt material temperature lowers from the time it contacts the main screed  16  to the time it contacts the left and right screed extensions  18 , 20 , the left and right screed plates  40 , 42  are influenced less by the asphalt temperature. Thus, the left and right screed plates  40 , 42  tend to require more heating than the first and second screed plates  36 , 38 . 
     The both/rear only switch  72  distinguishes the forward outputs  100 , 102 , 104 , 106  from the rearward outputs  110 , 112 , 114 , 116 . When the both/rear only switch  72  is off, i.e., “both” is activated, all outputs, forward  100 , 102 , 104 , 106  and rearward  110 ,  112 , 114 , 116 , are enabled. However, when the both/rear-only switch  72  is on, i.e., “rear-only” is activated, only the rear output switches  110 ,  112 , 114 , 116  can be closed. 
     For example, if the rear-only switch  72  is on, only the temperature switch  116  for the right screed plate  42  can close, and thus only the rearward resistive heating elements  56   c , 56   d  associated with the temperature switch  116  are energized accordingly. 
     This feature provides for a more efficient use of the power supply  60  under certain conditions. Namely, those conditions are where only the rearward portion of the screed plate needs to be heated, making it inefficient to energize the forward resistive heating elements. This condition may be preferred due to the manner in which the heated asphalt material interacts with the screed plates. For instance, when the asphalt material is first conveyed from the paving machine  12  onto the roadbed it has a high temperature. This high temperature asphalt contacts the front portions of the screed plates  36 , 38 , 40 , 42 , and then reaches the back portions of the screed plates  36 , 38 , 40 , 42  as the screed assembly  10  travels over the asphalt material. However, the temperature of the asphalt material lowers by the time it reaches the back portions of the screed plates  36 , 38 , 40 , 42 . Thus, the rear portions of the screed plates  36 , 38 , 40 , 42  are not as influenced by the heated temperatures of the asphalt material, as compared to the front portions of the screed plates  36 , 38 , 40 , 42 . Thus, under certain conditions, it may be desirable to only energize the rearward resistive elements in order to conserve power of the paving machine. 
     During initial start-up of the paving machine  12 , all of the temperature switches  80 , 82 , 84 , 86  will probably be closed since the screed plate temperatures will be low from lack of use. If the controller  62  is set to automatic mode, there is no priority of turning on the output switches  100 ,  102 , 104 , 106 , 110 , 112 , 114 , 116 , and they can be randomly activated. Alternatively, the controller  62  can be programmed to heat the screed plates  36 , 38 , 40 , 42  in a predetermined order. 
     Of course, automatic mode can be overridden by initiating the warm-up mode in which case all the outputs would be activated at the same time as described earlier. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. For instance, additional outputs may be provided to more specifically control each individual heating element, or more or less heating elements may be provided to more specifically control the heating of each screed plate. Thus, it is intended that the present invention cover the modifications and variations of the invention provided they come within the scope of the appended claims and their equivalents. 
     Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure, and the appended claims.

Summary:
A control system for heating a screed assembly of a paving machine is provided. The control system includes a plurality of resistive heating elements positioned adjacent to a screed plate of the screed assembly. An electrical power supply is selectively connected with the resistive heating elements via switches operated by a controller. The controller is responsive to inputs from at least manual switches and temperature sensors. The controller is configured to automatically adjust the temperature of the screed plate by selectively energizing fewer than all of the resistive heating elements at a time.