Patent Application
20060082034
The present invention relates generally to the field of furnace systems and, in particular, to an automatic temperature control system for a furnace.
Brazing is a commonly used technique for joining metal parts with close fitting joints. Typically, a clad material that has a flux cleaning solution applied is melted within a furnace or oven allowing the clad to flow into the gap between adjacent parts. As the clad flows into the gap, the flux cleaning solution cleans the adjacent parts allowing the clad material to be strongly bonded to the adjacent parts. Many commercial brazing operations are carried out on a continuous conveyor belt that passes through heated sections, or furnaces, of the brazing system. The conveyer belt or carrier belt is designed with a sufficient length to raise the part to be brazed to a brazing temperature, and to maintain the part at the desired temperature for a length of time sufficient to braze the part.
The furnaces are usually fitted with a muffle disposed within a refractory structure. A muffle is a tube that extends along the length of the furnace. The muffle is used to maintain a desired atmosphere surrounding the part to be brazed. The atmosphere is selected to protect the surface from contaminants. For example, when brazing aluminum components the presence of oxygen may result in oxidation or coloration at the surfaces being brazed, which results in an unacceptable braze. Accordingly, the controlled atmosphere when brazing aluminum components is typically maintained by continuously pumping nitrogen into the muffle. Thus, the area within the muffle is maintained at with a positive pressure of the selected atmosphere with respect to the area outside of the muffle.
In a typical brazing process, the part to be brazed is fluxed and the components to be brazed are placed in the desired position in contact with each other. In order to ensure an optimum braze, it is first necessary to eliminate any moisture from the metal parts of the flux. Accordingly, most brazing systems include a dry-off or dehydration oven between the fluxer and the controlled atmosphere furnaces. The purpose of the dry-off oven is to raise the core part temperature sufficiently high to ensure that all moisture has been evaporated from the part. Typically, a dry-off oven will raise the part temperature to about 180° C. (350° F.) in an air atmosphere.
After the components have been dried, the carrier belt carries the components into the muffle area. The muffle is heated by the use of banks of natural gas and/or electric heating elements outside the muffle. The heating elements are located outside of the muffle area to reduce leakage of the controlled atmosphere. The heating elements are in turn located within an outer casing or containment that provides insulation. The insulated casing allows the area within the casing, including the muffle area, to be heated significantly above ambient temperature in a reasonably efficient manner. In a typical aluminum brazing operation, the temperature within the muffle is desired to be about 590° C. (1100° F.).
Thermocouples are typically used to provide indications of the temperature within the muffle and/or temperatures near the heating banks. The indicated temperatures are used to control the external heating elements. If used, thermocouples measuring the actual heating element temperatures are located within thermocouple wells that penetrate the insulated casing and terminate outside of the muffle next to the heating elements. Thermocouples that are provided to obtain an indication of the temperature within the muffle area may be located within wells that penetrate the muffle wall.
As the components enter into and move through the muffle area, the components, along with carrier belt and any required fixturing used to maintain the components in the desired position are heated to the brazing temperature. The foregoing items thus act as heat sinks, absorbing the heat energy within the muffle and causing the temperature within the muffle to be lowered. This cooling effect is not felt evenly throughout the muffle, as the work pieces will typically be at a lower temperature as they enter the first zone. Thus, there is more cooling toward the front or upstream end of the furnace. Accordingly, it is known to locate different banks of heaters in different locations about the muffle and to operate the heater banks independently of each other. Zone heating systems are based upon this type of an approach.
In zone heating, the heater banks are divided into controlled banks along the length of the carrier belt. Each zone is then independently controlled. Thus, more heat can be injected into the upstream zones while maintaining the downstream zones at the desired temperature while injecting less heat into the downstream portion of the furnace.
Of course, the work pieces are typically not symmetrical. Thus, the temperature in the areas nearest to the items or parts of items presenting the largest heat sink are affected more intensely than the temperature in areas that are closer to smaller heat sinks. Therefore, because the heater banks within a zone typically include upper heater banks and lower heater banks it is known to independently control the upper heater banks and the lower heater banks. The independent control of the upper and lower banks, particularly after the work pieces have been heated to some extent, provides additional efficiencies as it is possible to focus any increased heating within the muffle to those zones and areas of zones that are actually cooling the most.
While this method of temperature control is effective, there are problems associated with the method. For example, the brazing operation is very sensitive to fluctuations in heat. Thus, to maintain a minimum temperature within a zone, the zone must be kept above the needed temperature and controlled at that higher set point so that the temperature does not fall too far below the set point. This set point offset is established based upon the ability of the system to respond to a cooling event. Accordingly, the set point offset is a function of the size of components passing through the muffle, the initial temperature of the components, the speed of the carrier belt, the heating capacity of the heating bank, and the delay associated with the use of thermocouples and external heaters. This delay is referred to as temperature lag.
Temperature lag is the time that it takes for a change in temperature to be sensed and corrected. With reference to a cooling event, because the thermocouples are located within wells, the well wall must cool before the thermocouple senses the drop in temperature. Accordingly, as temperature in the muffle cools, a thermocouple reading does not indicate the actual temperature. Rather, the thermocouple will generally indicate that the temperature in the muffle area is warmer than the actual temperature in the muffle area until a steady state value is achieved. Thus, by time a temperature deviation is sensed that triggers activation or increased activation of a heater bank or element, the temperature within the muffle area may have fallen to yet a lower temperature. The temperature lag problem is compounded by the fact that the heat generated by the external heaters must propagate through the muffle before heating the component with radiant heat. Therefore, temperature within the muffle may continue to fall until the component passes out of the area proximate to the heater bank or until the heater bank generates sufficient heat through the muffle to offset the cooling effect of the components.
Therefore, when designing a heater control system, the temperature offset is selected to account for the above factors and maintain a higher temperature than is needed for the actual brazing operation. Of course, the higher temperature introduces additional inefficiency into the system. Moreover, as zone size increases, inefficiencies may further increase since the entire zone is controlled based upon a decreased temperature in one area of the muffle zone.
Accordingly, although zone heat control provides an improvement in efficiency, the system still suffers from inefficiencies related to the increased set point offset within each zone. The established set point may of course be reduced in a number of ways. However, each of these methods has undesired consequences. For example, by lowering the speed of the carrier belt through the muffle, any localized cooling will occur at a slower rate. Thus, the set point may be established at a lower temperature while ensuring that the components do not fall below the minimum brazing temperature. However, this results in an increased brazing time per component and reduced furnace throughput. Alternatively, the amount of heat that can be injected into the zone by the external heating banks can be increased. However, the additional heaters introduce increased capital expenditures.
What is needed therefore is a system and method that allows for a reduced set point offset without increasing brazing time per component. It would be beneficial if the method and system did not require additional heating banks or heating banks of larger heat generating capacity. It would be further beneficial if the method and system incorporated the advantages of zone heating control. It would also be beneficial if the method and system reduced the magnitude of temperature excursions above and/or below the temperature set point for the furnace and/or zone.
In order to address these needs, the present invention contemplates a method and system for controlling a muffle temperature that predicts heat load within heating zones. A heating zone for the muffle is provided with a first and a second thermocouple, the second thermocouple located downstream of the first thermocouple. The temperature in the muffle is sensed by the upstream thermocouple. This indication of temperature is used to control an upstream bank of heaters or heating elements. The temperature indication from the first thermocouple is also used to modify the temperature set point used to control a bank of heaters or a heating element downstream of the first thermocouple.
In one embodiment, when a downward temperature excursion is sensed by the first thermocouple, the set point used to control the downstream bank of heaters is increased. Thus, more heat energy is injected into the muffle area proximate the downstream bank of heaters. Accordingly, by the time the components causing the cooling within the muffle reach the area proximate the second heating bank, the second heating bank is already energized and additional heat energy is being injected into the muffle. Thus, any downward excursion in the temperature within the muffle below the original set point used to control the bank of heaters downstream of the first thermocouple is reduced.
In another embodiment, a first set point is defined for use in controlling the temperature in a first area of a furnace and a second set point is defined for controlling the temperature in a second area of the furnace. A high deviation set point and a low deviation set point are also defined. A controller is programmed to compare sensed temperature in a first area of a furnace to the high deviation set point and a low deviation set point and to increase the second set point if a low deviation condition is sensed in the first area and to decrease the set point if a high deviation condition is sensed in the first area.
One benefit of the present invention is that temperature excursions above and/or below the temperature set point for the furnace and or zone are minimized by increasing or decreasing the amount of energy being introduced into a portion of the furnace in anticipation of an increased or decreased need for energy in that portion of the furnace. Another benefit of the present invention is that the effects of temperature lag in a muffle furnace are minimized. It is a further benefit that the above advantages may be realized without increasing the number or sizes of heating elements.
Other objects and benefits of the present invention will become apparent upon consideration of the following written description, taken together with the accompanying figures.
For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one of ordinary skill in the art to which this invention pertains.
A continuous brazing system 10, shown in
As is typical with most continuous brazing systems, the muffles 14 and 24 provide a controlled atmosphere and include means for maintaining that controlled atmosphere within the interior of the muffle. In an aluminum brazing system, the atmosphere is primarily composed of nitrogen. In order to maintain this controlled atmosphere, the system 10 may be provided with a vestibule (not shown) between the outlet 30 of the pre-heat section 22 and the inlet 18 of the brazing furnace 12. Likewise, a vestibule (not shown) can be provided at the inlet 28 of the pre-heat section 22. The vestibules can be of conventional construction.
With the downstream components of the system 10 described, attention can turn to the dry-off oven 32 at the upstream end of the process. The dry-off oven 32 receives stock material and flux after it has left the fluxer (not shown). The dry-off oven 32 includes an inlet 34 and an outlet 36 that provide a path for the material through the dry-off oven 32. It is understood that the dry-off oven 32, as well as the downstream pre-heat section 22 and brazing furnace 12 can be integrated with a continuous conveyor system extending through the respective inlets and outlets.
The dry-off oven 32 can include an exhaust unit 38 that is operable to exhaust spent gas from the chamber 40 of the oven. The exhaust unit 38 can be of a variety of configurations to exhaust the gases from the oven to the atmosphere. The exhaust unit 38 may comprise one or more rotary fans connected to discharge hoods or shrouds at the ends of the oven 32, or more particularly, shrouds situated around the perimeter of the inlet 34 and the outlet 36. The fans can feed the exhaust gas to one or more exhaust stacks outside the building housing the system 10. Typically, a single exhaust blower is ducted to both the discharge and charge hoods and dampers are used to control flow from each hood.
The brazing furnace 12 in this embodiment includes upper and lower heating elements, each element having a thermocouple associated with the element. With reference to
The brazing system 10 in this embodiment includes a plurality of heating zones, each zone having a plurality of upper and lower heating elements. As shown in
The control circuit 68 also includes the silicon controlled rectifiers (SCRs) 80, 82, 84 and 86. The SCRs 80, 82, 84 and 86 in this embodiment are model TC2000 SCRs commercially available from Eurotherm, Inc. of Leesburg, Va. The SCRs 80, 82, 84 and 86 provide power to the upper upstream heating element 44, the upper downstream heating element 60, the lower upstream heating element 48 and the lower downstream heating element 62, respectively. Power to the SCRs 80, 82, 84 and 86 may be interrupted by the contacts 88, 90, 92 and 94, respectively. The contacts 88 and 90 are controlled by the control instrument 96 which receives input from the upper over-temperature thermocouple 64 while the contacts 92 and 94 are controlled by the control instrument 98 which receives input from the lower over-temperature thermocouple 66. The controller 70 is also connected to the contacts 88, 90, 92 and 94 such that an enable or reset signal may be issued from the controller 70 to shut the contacts 88, 90, 92 and 94.
A user interface that may be used in accordance with one embodiment of the invention is shown in
The navigation buttons 104, 106, 108 and 110 are used to change the subject matter being displayed in the zone control area 114. Thus, a user can select any of the zones 2-9 for display in the zone control area 114. The navigation button 112 is used to navigate to a pan control view of the brazing system 10 from which navigation to control panels for the various portions of the brazing system 10 is possible.
The zone control area 114 shown in
The change value buttons are used to provide various inputs that are used in controlling the heater banks. For example, change value button 124 is used to provide a manual override of the normal PID function of the controller 70. By operation of the change value button 124, a percentage value is input into the control system for use by the controller 70 and the heater bank output is then controlled to the set percentage value. The change value button 124 is typically used in calibrating the output of the controller 70 to the SCRs 80, 82, 84 and 86.
Activation of the change value button 128 causes a keypad to be displayed which may be used to modify the temperature set point used to control the temperature in the displayed zone. Activation of the heat enable button 121 causes the controller 70 to output a signal to the contacts 88, 90, 92 and 94. The signal from the controller 70 is used to shut the contacts 88, 90, 92 and 94 to allow power to be sent to the respective heater elements. For example, the heat enable button 121 is used to reset the contacts 88, 90, 92 and 94 after an over-temperature condition, discussed more fully below, has cleared.
Another control panel that is provided for control of the temperature within the brazing furnace 12 is the automatic control input window 136 shown in
In one embodiment of the invention, the various parameters input by the operator are stored in a memory for use by the controller 70. The memory may be located within the controller or in a memory external to the controller. As shown in
Operation of the control circuit 68 is described with reference to
Continuing with the operational description, at step 166 the value stored in memory slot 148 is read. The value stored in memory slot 148 is the set point temperature established by the operator. The controller 70 performs a first preprogrammed PID function at step 168 using the set point temperature from memory slot 148 and the signal (process variable) indicative of the temperature in the upper upstream portion of the muffle 14 from the input/output module 72 to generate a first control signal C1.
The first preprogrammed PID function operates in a typical manner to bring the sensed temperature to the set point temperature, and to then maintain the sensed temperature at the set point temperature as is understood by those of ordinary skill in the relevant art. The first control signal C1 corresponds to a percentage output power that should be supplied to the upper upstream heating element 44 in order to reach or maintain the set point temperature stored in the memory slot 148. The first control signal C1accomplishes this by controlling the duty cycle of the SCR 80 when the first control signal C1 is output to the SCR 80 at step 170. The duty cycle corresponds to a percentage output power that is subsequently supplied to the muffle 14 from the upper upstream heating element 44.
The controller 70 at step 172 then determines if the temperature sensed by the upper upstream thermocouple 42 is greater than the set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 plus the high temperature deviation value stored in the memory slot 150 (the deviation high set point). If the sensed temperature is greater than the deviation high set point, then at step 174 the high temperature deviation set point reduction value stored in the memory slot 152 is subtracted from the set point temperature stored in the memory slot 148 with the remainder being the current set point. If the sensed temperature is not greater than the deviation high set point, then at step 176 the sensed temperature is compared to the remainder of the set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 minus the low temperature deviation value stored in the memory slot 154 (the deviation low set point). If the sensed temperature is less than the deviation low set point, then at step 178 the low temperature deviation set point increase value stored in the memory slot 156 is added to the set point temperature stored in the memory slot 148 with the sum being the current set point. If neither of the conditions of step 172 or 176 is met, then the set point temperature stored in the memory slot 148 is the current set point.
At step 180, the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of the temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56. The controller 70 then performs the second preprogrammed PID function at step 182 using the current set point temperature and the signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C2. At step 184, the second control signal C2 is output to the SCR 82. The second control signal C2 and the SCR 82 operate in a manner similar to that described above with respect to the first control signal C1 and the SCR 80 so as to control the power output of the upper downstream heating element 60.
At step 186 the control cycle 158 ends until the next wake-up call. Upon receiving a wakeup call, the control cycle 158 begins again at step 162.
The operation of the controller 70 in controlling the lower heating elements is similar to the process described above with respect to the upper heating elements. Moreover, although discussed above in the context of a single zone, the controller 70 may be programmed and connected to other equipment so as to similarly control multiple zones of the brazing furnace 12. Generally, by controlling one or two zones in the upstream area of a furnace, incoming cold work pieces can be quickly brought to a stable brazing temperature. Thus, the potential for low temperature excursions is relatively low in the latter zones. However, the chance for high temperature excursions in the latter zones is not reduced by the same mechanism. Moreover, if desired, both an upper bank and a lower bank of heaters may be controlled using a single thermocouple. All of the above variations are within the scope of the present invention.
The simplified operation of the control circuit 68 is described below as a response to a hypothetical high sensed temperature in the upper upstream portion of the brazing furnace 12 which occurs as a result of the following hypothetical scenario. The brazing furnace 12 has been operating and the memory slots 148-156 have been previously populated with the values shown in
When the next wake-up call is issued, the controller 70 begins the control cycle 158 at step 162. At step 164 the temperature in the upper upstream portion of the muffle 14 is read as 1116 degrees F. At step 166 the set point temperature established by the operator in memory slot 148 is read as 1100 degrees F. The controller 70 at step 168 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from memory slot 148 and the signal (process variable) indicative of the sensed 1116 degrees F. temperature in the upper upstream portion of the muffle 14 to generate a first control signal C1. Because the sensed temperature is greater than the set point temperature, the first control signal C1 is generated and output at step 170 so as to shorten the duty cycle of the SCR 80, resulting in a lower power output from the upper upstream heating element 44. This will lead to a lowering of the temperature in the upper upstream portion of the muffle 14 as a result of normal heat loss from the brazing furnace 12.
The controller 70 at step 172 then determines that the 1116 degrees F. temperature sensed by the upper upstream thermocouple 42 is greater than the 1115 degrees F. deviation high set point defined by the 1100 degrees F. set point temperature stored in the memory slot 148 for the upper upstream portion of the muffle 14 plus the 5 degrees F. high deviation value stored in the memory slot 150. Therefore, at step 174 the high temperature deviation set point reduction value of “10” stored in the memory slot 152 is subtracted from the set point temperature stored in the memory slot 148 and the remainder of 1090 degrees F. is selected as the current set point.
At step 180 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56. The sensed temperature is still 1100 degrees F. because the downstream portion of the muffle 14 has not yet been affected by the reduction in heat load. Of course, in some situations the sensed temperature may be higher, or an increase in temperature may not yet have been sensed due to temperature lag. The actual situation will vary as a result of design and operational considerations.
The controller 70 then performs the second preprogrammed PID function at step 182 using the 1090 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C2. Because the sensed temperature is greater than the current set point temperature, the second control signal C2 causes the SCR 82 to use a shorter duty cycle, resulting in less power output by the upper downstream heating element 60. At step 186 the control cycle 158 ends until the next wake-up call.
The above process is repeated with the control signals being modified in accordance with sensed changes in temperature until the condition set in step 172 is no longer met or until an over-temperature condition is detected by the control instrument 96 resulting in power to the heater banks being interrupted. Thus, under normal conditions, when the sensed temperature at step 164 reaches 1105 degrees F., then at step 172 the sensed temperature will not be greater than the deviation high set point of 1115 degrees F. Therefore, the controller 70 will proceed to step 176. At step 176, the sensed temperature is compared to the deviation low set point. Because the sensed temperature of 1105 degrees F. is greater than the deviation low set point of 1096 degrees F., the current set point temperature for the downstream portion of the zone is selected as the normal set point temperature stored in the memory slot 148. Thus, at step 182, the second PID function will be performed by the controller using the present temperature as sensed by the upper downstream thermocouple 56 and the 1100 degree F. current set point.
Those of skill in the art will appreciate that in accordance with the forgoing example, the upper downstream portion of the muffle 14 may still experience a temperature excursion above 1100 degrees F. However, the excursion in the upper downstream portion of the muffle 14 will be less than the excursion in the upper upstream portion of the muffle 14. The lessened excursion results since the energy injected into the muffle by the upper downstream heating element 60 was reduced when the event causing the high temperature excursion was affecting the temperature in the upper upstream portion of the muffle 14. Thus, the output of the downstream heating elements may be reduced before the temperature in the downstream portion of the muffle 14 is first affected.
Continuing with the above simplified example, after once again achieving a steady state temperature of 1100 degrees F. in both the upstream and downstream portion of the zone 54, a cooling excursion may be initiated by subsequent loading of the conveyor belt. Accordingly, as the work pieces begin to enter the upstream portion of the muffle 14, the temperature in the upstream portion of the muffle 14 begins to decrease. For purposes of this hypothetical, the temperature decreases to 1095 degrees F. after a control cycle 158 has ended.
Subsequently, a wake-up call is issued and the controller 70 begins the control cycle 158 at step 162. At step 164 the temperature in the upper upstream portion of the muffle 14 is read as 1095 degrees F. At step 166 the set point temperature established by the operator in the memory slot 148 is read as 1100 degrees F. Therefore, at step 168 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from the memory slot 148 and the signal (process variable) indicative of the 1095 degrees F. temperature in the upper upstream portion of the muffle 14 to generate a first control signal C1. Because the sensed temperature is less than the set point temperature, the first control signal C1 is generated so as to increase the duty cycle of the SCR 80, resulting in a higher power output from the upper upstream heating element 44. This will lead to an increase of the temperature in the upper upstream portion of the muffle 14 or a reduced rate of cooling, depending on the capacity of the heaters and the size of the work pieces.
At step 172 the controller 70 then determines that the 1095 degrees F. temperature sensed by the upper upstream thermocouple 42 is not greater than the 1115 degrees F. deviation high set point. Accordingly, the controller 70 proceeds to step 176 and determines that the 1095 degrees F. temperature sensed by the upper upstream thermocouple 42 is less than the 1096 degrees F. deviation low set point. The controller 70 then proceeds to step 178 and adds the 1100 degrees F. set point temperature stored in the memory slot 148 for the upper upstream area of the muffle 14 and the 10 degrees F. low deviation value stored in the memory slot 156 to define the current set point as 1110 degrees F. At step 182 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the muffle 14 as sensed by the upper downstream thermocouple 56.
The controller 70 then performs the second preprogrammed PID function at step 184 using the 1110 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the muffle 14 to generate a second control signal C2. Because the sensed temperature is less than the current set point temperature, the second control signal C2 output at step 184 causes the SCR 82 to use a longer duty cycle, resulting in more power output by the upper downstream heating element 60. At step 186 the control cycle 158 ends until the next wake-up call.
Accordingly, the energy injected into the muffle 14 by the upper downstream heating element 60 is increased such that when the newly loaded work pieces on the conveyer belt 52 approach the downstream portion of the zone 54, the downstream portion of the zone 54 will have been “pre-heated” and will not experience the same temperature excursion below the normal set point temperature as was experienced in the upstream portion of the zone 54. Of course, depending upon various design choices, the actual temperature in the zone 54 may not have increased, but the amount of heat being injected into the muffle 14 will have increased.
The above process is repeated with the control signals being modified in accordance with sensed changes in temperature until the condition set in step 176 is no longer met. Thus, when the sensed temperature at step 164 reaches 1096 degrees F., then at step 176 the sensed temperature will no longer be less than the 1096 degrees F. deviation low set point. Accordingly, the current set point temperature for the downstream portion of the zone 54 is selected as the normal set point temperature stored in the memory slot 148. Accordingly, at step 182, the second PID function will be performed by the controller using the present temperature as sensed by the upper downstream thermocouple 56 and the 1100 degree F. current set point. Thus, the set point temperature for the downstream portion of the zone 54 is lowered once the temperature in the upstream portion has been brought back into the normal operating band.
During the time that the control cycle 158 is generating control signals to control the power passing through the SCRs 80 and 82, the control circuit 68 is also performing over-temperature checks using the upper over-temperature thermocouple 64 and control instrument 96 which operate independently of the controller 70. This independent circuit is provided to protect the zone 54 from an extreme over-temperature condition caused, for example, by a faulty thermocouple or some other device. In the event the temperature in the upper area of the muffle 14 is too high, the high temperature is sensed by the over-temperature thermocouple 64 and a signal indicative of the over-temperature condition is sent to the control instrument 96. In response, the control instrument 96 opens the contacts 88 and 90 in the power supply line to the SCRs 80 and 82, respectively. Accordingly, the energy output of the upper upstream heating element 44 and the upper downstream heating element 60 goes to zero regardless of the first and second control signals generated by the controller 70. The lower over-temperature thermocouple 66 and control instrument 98 work similarly.
Those of skill in the relevant art will appreciate that the present invention may be adapted for use in a number of ways. By way of example, but not of limitation, it may be desired to program the controller such that the downstream temperature current set point is not modified until after the sensed upstream temperature has more closely approached, or even reached, the normal temperature set point for the upstream portion of the zone. One such embodiment which further allows for the normal upstream and downstream temperature set points to be individually controlled is discussed with reference to
The memory slot 194 is used to store the normal set point used by a second PID function in the controller 70 in controlling a downstream heater bank. The memory slot 195 is used to store a value that is added to the set point value in the memory slot 191 in determining if an upstream temperature in the brazing furnace 12 is too hot (the deviation high set point). The memory slot 196 is used to store a set point used by the second PID in controlling the downstream heater bank if the upstream temperature in the zone 54 exceeds the deviation high set point. Accordingly, the value stored in the memory slot 196 will typically be less than the value stored in the memory slot 191.
The memory slot 197 is used to store a value that is subtracted from the set point value in the memory slot 191 in determining if an upstream temperature in the zone 54 is too cold (the deviation low set point). The memory slot 198 is used to store a set point used by the second PID in controlling a downstream heater bank if the upstream temperature in the zone 54 is less than the deviation low set point. Accordingly, the value stored in the memory slot 198 will typically be greater than the value stored in the memory slot 191.
Operation of an alternatively programmed control circuit 68 is described with reference to
At step 208, the value stored in the memory slot 191 is read. The value stored in the memory slot 191 is the set point temperature established by the operator for the upper upstream portion of the zone 54. The controller 70 at step 210 then checks to see if a valid override output value has been stored in memory slot 192. If there is no valid value stored in the memory slot 192, then at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature from memory slot 191 and the signal (process variable) indicative of the temperature in the upper upstream portion of the zone 54 from the input/output module 72 to generate a first control signal C1.
The first preprogrammed PID function in this embodiment also operates to bring the sensed temperature to the set point temperature, and to then maintain the sensed temperature at the set point temperature in a manner understood by those of ordinary skill in the relevant art. The first control signal C1 corresponds to a percentage output power that should be supplied to the upper upstream heating element 44 in order to reach or maintain the set point temperature stored in the memory slot 191. The first control signal C1 accomplishes this by controlling the duty cycle of the SCR 80 when the first control signal C1 is output to the SCR 80 at step 214. The duty cycle corresponds to a percentage output power that is subsequently supplied to the muffle 14 from the upper upstream heating element 44.
Alternatively, returning to step 210, if a valid value is stored in the memory slot 192, then at step 216 that value is read from the memory slot 192 and is set as the first control signal C1 which is output at step 214 to the SCR 80.
The controller 70 at step 218 then checks to see if a valid override output value has been stored in memory slot 193. If there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is equal to the set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. If the two values are equal, then at step 222 the current set point pointer is set to memory slot 194 wherein the normal downstream set point temperature is stored. If the two values are not equal, then at step 224 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is greater than the deviation high set point temperature defined as the temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the high deviation value stored in the memory slot 195. If the temperature sensed by the upper upstream thermocouple 42 is greater than the deviation high set point temperature, then at step 226 the current set point pointer is set to the memory slot 96 wherein the current set point temperature for a deviation high condition is stored.
If the temperature sensed by the upper upstream thermocouple 42 is not greater than the deviation high set point temperature, then at step 228 the controller 70 determines if the temperature sensed by the upper upstream thermocouple 42 is less than the deviation low set point temperature defined by the set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 minus the low deviation value stored in the memory slot 197. If the temperature sensed by the upper upstream thermocouple 42 is less than the deviation low set point temperature, then at step 230 the current set point pointer is set to the memory slot 198 wherein the current set point temperature for a deviation low condition is stored.
At step 232, the controller 70 reads the value in the memory slot to which the current set point pointer has been set as the current set point temperature for a second preprogrammed PID function. The current set point temperature will thus be one of the normal set point temperature in the memory slot 194, the current set point temperature for a deviation low condition in the memory slot 196 or the current set point temperature for a deviation high condition in the memory slot 198. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of the temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.
The controller 70 then performs the second preprogrammed PID function at step 236 using the current set point temperature for the sensed condition and the signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C2. Alternatively, if at step 218 a valid value is stored in the memory slot 193, then at step 238 that value is read from the memory slot 193 and is set as the second control signal C2. At step 240, the second control signal C2 is output to the SCR 82. The second control signal C2 and the SCR 82 operate in a manner similar to that described above with respect to the first control signal C1 and the SCR 80 so as to control the power output of the upper downstream heating element 60.
At step 242 the control cycle 200 ends until the next wake-up call. Upon receiving a wakeup call, the control cycle 200 begins again at step 204.
The simplified operation of the alternatively programmed control circuit 68 is described below as a response to a hypothetical high sensed temperature in the upper upstream portion of the brazing furnace which occurs as a result of the following hypothetical scenario. The brazing furnace 12 has been operating and the memory slots 148-162 have been previously populated with the values shown in
When the next wake-up call is issued, the controller 70 begins the control cycle 200 at step 204. At step 206 the temperature in the upper upstream portion of the zone 54 is read as 1111 degrees F. At step 208 the set point temperature established by the operator in memory slot 191 is read as 1100 degrees F. The controller 70 at step 210 determines that a valid override output value has not been stored in memory slot 192. Therefore, at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from memory slot 191 and the signal (process variable) indicative of the sensed 1111 degrees F. temperature in the upper upstream portion of the zone 54 to generate a first control signal C1. Because the sensed temperature is greater than the set point temperature, the first control signal C1 is generated so as to shorten the duty cycle of the SCR 80, resulting in a lower power output from the upper upstream heating element 44. This will lead to a lowering of the temperature in the upper upstream portion of the zone 54 as a result of normal heat loss from the brazing furnace 12.
The controller 70 at step 218 then checks to see if a valid override output value has been stored in the memory slot 193. Because there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines that the 1111 degrees F. temperature sensed by the upper upstream thermocouple 42 is not equal to the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. Accordingly, the controller 70 proceeds to step 224 and determines that the 1111 degrees F. temperature sensed by the upper upstream thermocouple 42 is greater than the 1110 degrees F. deviation high set point temperature defined by the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the 10 degrees F. high deviation value stored in the memory slot 195. Therefore, at step 226 the current set point pointer is set to the memory slot 196 wherein the 1090 degrees F. current set point for a deviation high condition is stored.
At step 232, the controller 70 reads the 1090 degrees F. set point in the memory slot 196 as the current set point temperature for a second preprogrammed PID function. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.
The controller 70 then performs the second preprogrammed PID function at step 236 using the 1090 degrees F. current set point for a high deviation condition and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C2. Because the sensed temperature is greater than the current set point temperature, the second control signal C2 causes the SCR 82 to use a shorter duty cycle, resulting in less power output by the upper downstream heating element 60. At step 242 the control cycle 200 ends until the next wake-up call.
Accordingly, the energy injected into the zone 54 by the upper downstream heating element 60 is reduced such that when the empty portion of the conveyer belt 52 approaches the downstream portion of the zone, the downstream portion of the zone 54 will not experience the same temperature excursion above the normal set point (the set point temperature in memory slot 194) as was experienced in the upstream portion of the zone 54.
The above process is repeated with the control signals being modified in accordance with sensed changes in temperature and the current set point pointer used in obtaining the set point temperature for the second PID function will continue to point at the memory slot 196 until one of the conditions set in step 220 and 228 are met or until a manual override is detected. Thus, when the sensed temperature at step 206 reaches 1100 degrees F., then at step 220 the sensed temperature will equal the set point temperature for the upstream upper portion of the zone 54. Accordingly, the controller will proceed to step 222 and set the current set point pointer to the memory slot 194 which has the 1100 degrees F. set point value that is stored for use in normal conditions. Therefore, the set point temperature for the downstream portion of the zone will return to the normal set point temperature once the temperature in the upstream portion has been normalized.
The embodiment of
Continuing with the above simplified example, after re-establishing steady state temperature within the muffle at 1100 degrees F., a cooling excursion may be initiated by subsequent loading of the conveyor belt. Accordingly, as the work pieces begin to enter the upstream portion of the zone 54, the temperature in the upstream portion of the zone 54 begins to decrease. For purposes of this hypothetical, the temperature decreases to 1094 degrees F. after a control cycle 200 has ended.
Subsequently, a wake-up call is issued and the controller 70 begins the control cycle 200 at step 204. At step 206 the temperature in the upper upstream portion of the zone 54 is read as 1094 degrees F. At step 208 the set point temperature established by the operator in the memory slot 191 is read as 1100 degrees F. The controller 70 at step 210 determines that a valid override output value has not been stored in the memory slot 192. Therefore, at step 212 the controller 70 performs a first preprogrammed PID function using the set point temperature of 1100 degrees F. from the memory slot 191 and the signal (process variable) indicative of the 1094 degrees F. temperature in the upper upstream portion of the zone 54 to generate a first control signal C1. Because the sensed temperature is less than the set point temperature, the first control signal C1 is generated so as to increase the duty cycle of the SCR 80, resulting in a higher power output from the upper upstream heating element 44. This will lead to an increase of the temperature in the upper upstream portion of the zone 54.
The controller 70 at step 218 then checks to see if a valid override output value has been stored in the memory slot 193. Because there is no valid value stored in the memory slot 193, then at step 220 the controller 70 determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is not equal to the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54. Accordingly, the controller 70 proceeds to step 224 and determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is not greater than the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 plus the 10 degrees F. high deviation value stored in the memory slot 195. The controller 70 then proceeds to step 228 and determines that the 1096 degrees F. temperature sensed by the upper upstream thermocouple 42 is less than the 1100 degrees F. set point temperature stored in the memory slot 191 for the upper upstream area of the zone 54 minus the 5 degrees F. high deviation value stored in the memory slot 197. Therefore, at step 230 the current set point pointer is set to the memory slot 198 wherein the 1110 degrees F. current set point for a sensed deviation low condition is stored.
At step 232, the controller 70 reads the 1110 degrees F. value in the memory slot 198 as the current set point temperature for the second preprogrammed PID function. At step 234 the value of the signal generated by input/output module 74 is read. The signal generated by input/output module 74 is indicative of an 1100 degrees F. temperature in the upper downstream portion of the zone 54 as sensed by the upper downstream thermocouple 56.
The controller 70 then performs the second preprogrammed PID function at step 236 using the 1110 degrees F. current set point temperature and the 1100 degrees F. signal (process variable) indicative of the temperature in the upper downstream portion of the zone 54 to generate a second control signal C2. Because the sensed temperature is less than the current set point temperature, the second control signal C2 causes the SCR 82 to use a longer duty cycle, resulting in more power output by the upper downstream heating element 60. At step 242 the control cycle 200 ends until the next wake-up call.
Accordingly, the energy injected into the zone 54 by the upper downstream heating element 60 is increased such that when the newly loaded work pieces on the conveyer belt 52 approach the downstream portion of the zone 54, the downstream portion of the zone 54 will have been “pre-heated” and will not experience the same temperature excursion below the normal set point temperature as was experienced in the upstream portion of the zone 54.
The above process is repeated with the control signals being modified in accordance with sensed changes in temperature and the current set point pointer used in obtaining the set point temperature for the second PID function will continue to point at the memory slot 198 until one of the conditions set in step 220 and 224 are met or until a manual override is detected. Thus, when the sensed temperature at step 206 reaches 1100 degrees F., then at step 220 the sensed temperature will equal the set point temperature for the upstream upper portion of the zone 54. Accordingly, the controller will proceed to step 222 and set the current set point pointer to the memory slot 194 which has the 1100 degrees F. set point temperature that is stored for use in normal conditions. Therefore, the set point temperature for the downstream portion of the zone 54 is lowered once the temperature in the upstream portion has been normalized.
While the invention has been illustrated and described in detail in the drawings and the foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.
