Patent Publication Number: US-7714253-B2

Title: Method and apparatus for the uniform resistance heating of articles

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority of U.S. Provisional Patent Application Ser. No. 60/782,801 filed Mar. 16, 2006, entitled “Method and Apparatus for the Uniform Resistance Heating of Articles,” which is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to resistance heating. More specifically, the invention relates to methods and apparatus for uniformly heating irregularly shaped articles by electrical resistance beating. 
   BACKGROUND OF THE INVENTION 
   Electrical resistance heating is an effective and energy efficient method for heating electrically conductive articles, such as metal workpieces. In this process, an electrical current is flowed through the workpiece. By controlling the magnitude of the current, the amount of heating may be readily regulated. 
   In general, resistance heating methods provide for a reliable and uniform beating of workpieces; however, particular problems can be presented by workpieces which have an irregular shape or include openings, thinned portions, or other features therein. These irregularities can disrupt the current path and prevent uniform heating from occurring. For example, openings can result in a concentration of current proximate their perimeters, and thereby cause hotspots. Likewise, a thinned cross-sectional or transverse portion of an article can concentrate current and create hotspots. Establishing a uniform flow of current through an irregularly shaped workpiece is often difficult, since current tends to take the shortest path through the workpiece; and hence, particular regions may be “shaded” and under heated. This problem is particularly significant when relatively thin workpieces, such as structural components of motor vehicles, are being fabricated. 
   In response to the above stated problem, Arnosky in U.S. Pat. No. 3,737,618 disclosed a two-stage process for heating a slotted tube. In a first stage of the process, four sets of electrodes are attached to the end portions of the tube and energized in order to preheat the end regions. In a second stage, two of the sets of electrodes are removed from the tube so that the end regions and the slot-like area are heated simultaneously. Also, Gomez, in U.S. Pat. No. 6,897,407, discloses a two-stage process in which a first set of electrodes preheat a workpiece while a second set of shunting electrodes allows current to bypass small cross section regions of the workpiece. The first stage involves passing current through the workpiece with the smaller regions conductively bridged so that said regions were heated to a lesser extent. In the second stage, the shunting electrodes are removed and the entire workpiece is heated. 
   Both of the above prior art methods involve multiple stage processing on symmetrical workpieces. Hence, these systems cannot provide the control necessary for beating highly irregular pieces; and, the two-stage nature of the prior art processes complicates their use in high-speed automated processes. Thus, it will be appreciated that there is a need for a one-stage method and apparatus whereby electrical resistance heating may be utilized to uniformly heat workpieces which include irregular features and wherein a measured parameter of the workpiece aids in controlling the heating system. As will be explained in detail hereinbelow, the present invention meets this need. 
   SUMMARY OF THE INVENTION 
   A method for providing a controlled heating profile in an article being heated in a resistance heating process is provided. The method includes contacting the article with a plurality of electrodes, selecting a first correlated group that includes at least two electrodes, and establishing a flow of electrical current between the electrodes of the first correlated group. The method also includes selecting a second correlated group that includes at least two electrodes, the second correlated group differing from the first correlated group. A flow of electrical current is also established between the electrodes of the second correlated group for the purpose of providing a controlled heating profile for an article undergoing resistance heating. 
   The electrical current between the electrodes of the second correlated group can be established after the flow of electrical current is established between the electrodes of the first correlated group. In addition, the flow of electrical current between the electrodes of the first correlated group can be terminated before the flow of the electrical current is established between the electrodes of the second correlated group. The method can also include the further step of selecting a third correlated group comprised of at least two of the said plurality of electrodes and establishing a flow of electrical current between the electrodes of the third correlated group. 
   A step of controlling at least one sequence, duration, and magnitude of current flow between the electrodes of the various correlated groups can be included. In addition, the flow of current between the electrodes of the various correlated groups can be controlled by a preselected program. The flow of current can also be controlled by a response to a measured parameter of the article, for example the temperature and/or electrical resistance of the article. Bridging members can be used to control current flow in the region of openings and/or thinned portions of the workpiece. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a top view of an article with electrodes attached thereto according to the present invention; 
       FIG. 2  is a top view of a corner electrode according to the present invention; 
       FIG. 3  is a schematic diagram of a controller, a series of electrodes, a power supply and a sensor according to the present invention; 
       FIG. 4  is a side cross-sectional view of an article with a plug filler member according to the present invention; 
       FIG. 5  is a side cross-sectional view of an article with a plug filler member according to the present invention; 
       FIG. 6  is a side cross-sectional view of an article with an integral filler member according to the present invention; 
       FIG. 7  is a side cross-sectional view of an article with a recessed portion and a filler member according to the present invention; 
       FIG. 8  is a side cross-sectional view of an article with a recessed portion and a filler member according to the present invention; and 
       FIG. 9  is a side view of an article with a reduced cross-section and wire junctions attached thereto according to the present invention. 
   

   DETAILED DESCRIPTION OF TEE INVENTION 
   As detailed above, it is frequently difficult to establish a uniform current flow through irregularly shaped workpieces. The present invention provides a multi-electrode system and method wherein a plurality of electrodes are affixed to a workpiece and correlated groups of these electrodes are energized in accord with various sequences to uniformly beat the workpiece. 
   Referring now to  FIG. 1 , there is shown one particular implementation of the present invention, as used to heat an irregularly shaped workpiece  10 . The workpiece is contacted by a series of electrodes  12   a - 12   i  which are affixed to the perimeter of the workpiece  10 . Each of the electrodes  12   a - 12   i  is selectively connectable to a source of electrical current and/or a ground. The selectable connection may be established through a switchboard, a controller, or the like, as will be apparent to those of skill in the art. 
   In the operation of the present invention, groups of these electrodes, such groups termed herein “correlated groups,” are selectively energized so that current flow through the workpiece may be controlled and directed. For example, electrodes  12   a  and  12   f  may be energized so as to cause an electrical current to flow along substantially all of the width of the workpiece  10 . This current path may not adequately heat the corner of the workpiece in the region of electrodes  12   c  and  12   b , and this area may be heated by a current established between electrodes  12   b  and/or  12   c  and  12   f . Similarly, this region could be heated by a current passing between electrodes  12   i  and  12   d . In some instances, a current path may be established by utilizing more than two electrodes. For example, a current path between electrode  12   b  and each of  12   i  and  12   h  could be utilized to heat the region in that corner of the workpiece. In the alternative an electrode designed to fit corners such as a corner electrode  11   a  and/or  11   b  shown in  FIG. 2  can be utilized to heat a corner region of the workpiece. In the use of the present invention, various combinations of electrodes may be energized in sequence, and in some instances simultaneously, so as to quickly and efficiently bring the workpiece  10  to a uniform temperature. Control of the timing, duration and magnitude of the current pulses applied to the workpiece may be adjusted in accord with measured parameters, or a stored program such as to provide and maintain a uniform temperature in the workpiece. It is appreciated from  FIG. 1  that the electrodes  12   a - 12   i  remain in contact with the workpiece  10  during an energizing sequence. 
   Referring now to  FIG. 3 , there is shown another feature of the present invention comprising a controller  14  which is in communication with a plurality of electrodes  12   a - 12   e  and a power supply  16 . The controller  14  operates to selectively energize various groups of the electrodes  12   a - 12   e  via the power supply  16  so as to carry out the method of the present invention and can be operable to control at least one of the sequence, duration and/or magnitude of current flow between the electrodes of the various/correlated groups. The controller may also energize the electrodes in accord with a preselected program stored therein. 
   In other instances, the system further includes a sensor, such as a temperature sensor  18  which is also in communication with the controller. The temperature sensor may be an optical pyrometer, an infrared sensitive camera, a thermocouple array, or any other such device operative to sense the temperature of the workpiece and provide a control signal which enables the controller to appropriately control the energization of the electrodes. In particular instances, the temperature sensor provides a signal indicative of the distribution of thermal energy in the workpiece. This signal carries information indicative of hotspots and cool areas in the workpiece, and by appropriately reading this signal, the controller can selectively energize and de-energize particular groups of electrodes so as to provide and maintain a desired temperature profile across the device. Typically, this profile will be a uniform temperature profile; however, in some instances, there may be a need for maintaining a preselected non-uniform profile, and such may be accomplished by the present system. 
   As is known in the art, a number of parameters or physical properties of a body of metal will vary as a function of the temperature of that metal, and such parameters may be sensed and used as part of the control system of the present invention. For example, metals exhibit an increase in electrical resistance as their temperature increases. The spatial profile of the electrical resistance of the workpiece may be readily measured and used to control heating. For example, the aforementioned electrodes may also function as probes for measuring resistance in the workpiece and thus function as a sensor array. Magnetic properties of iron and other ferromagnetic alloys also vary as a function of their temperature, and these properties may also be measured and similarly used to provide a control signal. 
   In those instances where a batch of similarly configured workpieces are sequentially processed, a set of control profiles may be established on the basis of analyzing one, or a relatively small number of, workpieces, and these profiles can be stored in the controller and used for processing subsequent workpieces without requiring discreet temperature measurements for each. 
   The presence of a thinned or otherwise narrowed portion of a hole or other opening in a workpiece can present some particular problems in a resistive heating process. For example, workpiece  10  of  FIG. 1  includes a hole  20  therein. Current flowing across the region having the hole  20  defined therein tends to concentrate at various points along the perimeter of the hole, thereby producing hotspots in some areas. In some instances, this problem can be alleviated by use of appropriate electrode groups which effectively “steer” current around the opening. However, in some instances, this approach is difficult or impractical. Therefore, in accord with a further aspect of the present invention, an electrically conductive filler member is disposed so as to shunt current across the opening and thereby provide for a uniform flux and consequent uniform heating. 
   Referring now to  FIG. 4 , there is shown a portion of the workpiece  10  with a filler member  22  disposed so as to fill the hole  20  and provide for current to flow there across. The filler member  22  may be made from any electrically conductive material, and preferably should be non-reactive with the workpiece. A filler member may be made to be thicker than the workpiece; although, relatively large filler members can cause heat sinking which may affect the temperature uniformity of the workpiece, and for this reason the heat capacity of the filler member should be taken into account in designing systems of the present invention. The filler member need not contact the entire periphery of the workpiece in all instances. The degree of contact will depend upon the direction of current flow and the nature of the opening itself. As illustrated in  FIG. 4 , the filler member  22  is shown as being in the form of a plug which may be raised and lowered to contact workpieces as they are introduced into the apparatus. As shown, the plug fits directly into the opening; however, in other instances, the plug may include a rim or other feature to aid in positioning and establishment of contact. For example,  FIG. 5  shows a system in which the plug  24  is a tapered member. 
   According to the present invention, the filler member may be inserted into the workpiece before it is put into the apparatus, or it may be inserted after placing the workpiece in the apparatus. In some instances, the filler member is automatically placed and withdrawn from the workpiece by the apparatus. In yet other instances, the filler member may actually be a portion of the workpiece itself. In such instances, the filler member may be defined by a weakened portion of the workpiece held to the remainder of the workpiece by a thin, and in some instances, perforated web. Following heat treatment, this integral filler member may be removed by a punching operation or the like, and in some instances, this punching operation is preferably carried out while the workpiece is still at an elevated temperature, while in other instances, it may be carried out after hardening operations or the like.  FIG. 6  shows a cross-section of a workpiece  40  having an integral filler member  42  retained thereupon by a thinned web portion  44 . 
   In some instances an article to be heated by resistance heating will have a recessed or cavity area within said article that creates a thinned region wherein non-uniform heating can occur. The present invention affords for uniform heating in such an area as shown in  FIG. 7 . An article  50  with a recessed area  54  is afforded uniform resistance heating proximate said recessed area  54  by insertion of a filler member  52 . The filler member  52  is similar to the filler members  22  and  24  mentioned above and affords for electrical current to pass therethrough. 
   The recessed area  54  of the article  50  can also be bridged with a filler member  56  as shown in  FIG. 8 . The filler member  56  contains a plurality of wires or sheets  58  which span the distance, area and/or volume of the recessed region  54 . The filler member  52  does likewise. 
   Turning to  FIG. 9 , a different embodiment of the present invention is shown wherein an article  60  has a reduction in cross section from a thick section  62  to a thin section  64 . For example, and for illustrative purposes only, article  60  can be representative of a bolt, nail set punch or mechanical tensile sample undergoing a resistance heating heat treatment. The present invention affords for uniform heating along the article  60  by attaching a filler member comprising a plurality of bridging electrodes  68  to the area  66 . The electrodes  68  attached to the area  66  can be in connection with the controller  14 . One or more of the electrodes  68  can also serve as a sensor  18  ( FIG. 3 ) wherein the temperature and/or electrical resistance of the area  66  is monitored. 
   In view of the teaching presented herein, it will be appreciated that other modifications and variations of this invention will be apparent to those of skill in the art. The foregoing drawings, discussion and description are illustrative of specific embodiments, but are not meant to be limitations upon the practice thereof. It is the following claims, including the equivalents, which define the scope of the invention.