The present invention relates generally to induction heating, and particularly to a method and apparatus for inductively heating a workpiece using a thermocouple to indicate workpiece temperature.
Induction heating is a method of heating a workpiece. Induction heating involves applying an AC electric signal to a conductor adapted to produce a magnetic field, such as a loop or coil. The alternating current in the conductor produces a varying magnetic flux. The conductor is placed near a metallic object to be heated so that the magnetic field passes through the object. Electrical currents are induced in the metal by the magnetic flux. The metal is heated by the flow of electricity induced in the metal by the magnetic field.
Typically, induction heating systems are designed to heat a workpiece to a desired temperature and maintain the workpiece at that temperature for a desired period of time. Temperature feedback devices, such as thermocouples, are used to provide the system with an electrical signal corresponding to the temperature of the workpiece. Thermocouples typically consist of two dissimilar metals that produce a voltage between the two metals that varies according to the temperature of the two metals. The voltage difference between the two metals is used to produce a signal that is representative of the temperature of the workpiece. In an induction heating system, at least one thermocouple is typically placed on a workpiece in close proximity to the area being heated. Electrical conductors are used to couple the thermocouple to a controller that is used to control the operation of the induction heating system. However, the thermocouple and electrical conductors are susceptible to picking up electrical noise and transmitting the noise, as well as the temperature signal produced by the thermocouple, to the controller. The electrical noise distorts the thermocouple signal, which may result in improper heating of the workpiece or in the recordation of incorrect temperature data.
Electrical noise may be produced by several potential sources. For example, electrical noise may be produced by the varying magnetic field produced by an induction coil placed around a workpiece. Additionally, electrical noise may be produced by the power source in the induction heating system. The arc produced by an electric arc welder may also produce electrical noise that may be transmitted to the thermocouple and conductors. Radios in the vicinity of the workpiece may also produce electrical noise that may interfere with the signal produced by a thermocouple.
There is a need therefore for an induction heating system that avoids the problems associated with current temperature sensing means and methods. Specifically, there is a need for an induction heating system that reduces or eliminates electrical noise in the electrical signal generated by a temperature feedback device, such as a thermocouple.
The present technique provides novel inductive heating components, systems, and methods designed to respond to such needs. An induction heating system is featured according to one aspect of the present technique. The induction heating system has an electrical connector that is adapted to electrically couple a temperature feedback device to a system controller. In addition, the electrical connector couples the temperature feedback device to ground via a capacitor circuit. The capacitor circuit shunts electrical noise to ground. However, the capacitor circuit allows temperature signals from the temperature feedback device to be conducted to the controller and a data recorder, if used.
According to another aspect of the present technique, a shielded extension cable is provided to electrically couple a temperature feedback device to an induction heating system. The shielded extension cable has conductive shielding that surrounds a plurality of conductors. The plurality of conductors are used to conduct a signal representative of temperature from the temperature feedback device to the system. The shielding is electrically coupled to ground to conduct electrical noise, such as voltage spikes, to ground.
According to another aspect of the present technique, a shielded extension cable is provided that is operable to electrically couple a plurality of temperature feedback devices, such as thermocouples, to an induction heating system. Each of the temperature feedback devices is coupled through a separate group of conductors. The shielded extension cable has shielding that surrounds each of the separate groups of conductors. The shielding is electrically coupled to ground to conduct electrical noise, such as voltage spikes, to ground.
The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and:
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A step-down transformer 88 is used to couple the AC output from the first inverter circuit 80 to a second rectifier circuit 90, where the AC is converted again to DC. In the illustrated embodiment, the DC output from the second rectifier 90 is, approximately, 600 Volts and 50 Amps. An inductor 92 is used to smooth the rectified DC output from the second rectifier 90. The output of the second rectifier 90 is coupled to a second inverter circuit 94. The second inverter circuit 94 steers the DC output current into high-frequency AC signals. A capacitor 96 is coupled in parallel with the fluid-cooled induction heating cable 56 across the output of the second inverter circuit 94. The fluid-cooled induction heating cable 56, represented schematically as an inductor 98, and capacitor 96 form a resonant tank circuit. The capacitance and inductance of the resonant tank circuit establishes the frequency of the AC current flowing through the fluid-cooled induction heating cable 56. The inductance of the fluid-cooled induction heating cable 56 is influenced by the number of turns of the heating cable 56 around the workpiece 52. The current flowing through the fluid-cooled induction heating cable 56 produces a magnetic field that induces current flow, and thus heat, in the workpiece 52.
Referring generally to
In the illustrated embodiment, an output cable 108 is connected to the output block 106. The output cable 108 couples cooling fluid and electrical current to the extension cable 62. The extension cable 62, in turn, couples cooling fluid 104 and electrical current 64 to the fluid-cooled induction heating cable 56. In the illustrated embodiment, cooling fluid 104 flows from the output block 106 to the fluid-cooled induction heating cable 56 along a supply path 110 through the output cable 108 and the extension cable 62. The cooling fluid 104 returns to the output block 106 from the fluid-cooled induction heating cable 56 along a return path 112 through the extension cable 62 and the output cable 108. AC electric current 64 also flows along the supply and return paths. The AC electric current 64 produces a magnetic field that induces current, and thus heat, in the workpiece 52. Heat in the heating cable 56, produced either from the workpiece 52 or by the AC electrical current flowing through conductors in the heating cable 56, is carried away from the heating cable 56 by the cooling fluid 104. Additionally, the insulation blanket 58 forms a barrier to reduce the transfer of heat from the workpiece 52 to the heating cable 56.
Referring generally to
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The control unit 252 is pre-programmed with operational control instructions that control how the control unit 252 responds to the programming instructions. There are a number of control schemes that may be used to control the application of heat to the workpiece. An on-off controller maintains a constant supply of power to the workpiece until the desired temperature is reached, then the controller turns off. However, this can result in temperature overshoots in which the workpiece is heated to a much higher temperature than is desired. In proportional control, the controller controls power in proportion to the temperature difference between the desired temperature and the actual temperature of the workpiece. A proportional controller will reduce power as the workpiece temperature approaches the desired temperature. The magnitude of overshoots is lessened with proportional control in comparison to on-off controllers. However, the time that it takes for the workpiece to achieve the desired temperature is increased. Other types of control schemes include proportional-integral control and proportional-derivative control. Preferably, the control unit 252 is pre-programmed as a proportional-integral-derivative (PID) controller. The integral term provides a positive feedback to increase the output of the system near the desired temperature. The derivative term looks at the rate of change of the workpiece temperature and adjusts the output based on the rate of change to prevent overshoot. Accordingly, the control unit 252 may comprise a processor and memory, such as RAM.
The control unit 252 provides two output signals to the power source 70 via the connector cable 102. The power source 70 receives the two signals and operates in response to the two signals. The first signal is a contact closure signal 262 that energizes contacts in the power source 70 to enable the power source 70 to apply power to the induction heating cable 56. The second signal is a command signal 264 that establishes the percentage of available power for the power source 70 to apply to the induction heating cable 56. The voltage of the command signal 264 is proportional to the amount of available power that is to be applied. The greater the voltage of the command signal 264, the greater the amount of power supplied by the power source. In this embodiment, a variable voltage was used. However, a variable current may also be used to control the amount of power supplied by the power source 70.
In the illustrated embodiment, the electrical switches that provide signals to the control unit 252 include a run button 266, a hold button 268, and a stop button 270. In addition, a power switch 272 is provided to control the supply of power to the controller 72. The run button 266 directs the control unit 252 to begin operating in accordance with the programming instructions. When closed, the run button 266 couples power through the power switch 272 to the control unit 252. In addition, a first relay 274 and a second relay 276 are energized. When energized, the first relay closes first contacts 278 and the second relay 276 closes second contacts 280. The relays and contacts maintain power coupled to the control unit 252 after the run button 266 is released.
The hold button 268 stops the timing feature of the controller 72 and directs the control unit 252 to maintain the workpiece at the current target temperature. The hold button 268 enables the system 50 to continue operating while new programming instructions are provided to the controller 72. When operated, the hold button 268 opens, removing power from the first relay 274 and opening the first contacts 278. This directs the controller to remain at the current point in the heating cycle so that the heating cycle begins right where it was in the cycle when operation returns to normal. Additionally, the second relay 276 remains energized, maintaining the second contacts 280 closed to allow the power supply to continue to provide power to the induction heating coil 56. The run button 266 is re-operated to redirect the control unit 252 to resume operation in accordance with the programming instructions. When re-operated, the first relay 274 is re-energized and the first contacts 278 are closed.
The stop button 270 directs the control unit 252 to stop heating operations. As the stop button 270 is operated, power is removed from both the first and second relays, opening the first and second contacts and removing power from the power source contactors. In the illustrated embodiment, a circuit 281 is completed when the stop button 270 is fully depressed. The circuit 281 directs the control unit 252 to be reset to the first segment of the heating cycle.
The I/O unit 254 receives data from the power source 70 and couples it to the control unit 252 and/or the parameter display 256. The data may be a fault condition recognized by the power source 70 or various operating parameters of the power source 70, such as the voltage, current, frequency, and power of the signal being provided by the power source 70 to the flexible inductive heating cable 56. The I/O unit 254 receives the data from the power source 70 via the connector cable 102.
In the illustrated embodiment, the I/O unit 254 also receives an input from a flow switch 282. The flow switch 282 is closed when there is adequate cooling flow returning from the flexible inductive heating cable 56. When fluid flow through the flow switch 282 drops below the required flow rate, flow switch 282 opens and the I/O unit 254 provides a signal 284 to the control unit 252, causing the control unit 252 to direct the power source 70 to discontinue supplying power to the induction heating cable 56. Additionally, the flow switch 282 is located downstream, rather than upstream, of the flexible inductive heating cable 56 so that any problems with coolant flow, such as a leak in the flexible inductive heating cable 56, are detected more quickly. A power source selector switch 286 is provided to enable a user to select the appropriate maximum available power of the power source. For example, the absolute maximum power that a power source may provide may be 50 KW. The power selector switch 286 may be operated to establish a lower output power, 25 KW for example, as the maximum available power.
The controller 72 also has a plurality of visual indicators to provide a user with information. One indicator is a heating light 288 to indicate when current is being applied to the fluid-cooled induction heating cable 56. Another indicator is a fault light 290 to indicate to a user when a problem exists. The fault light may be lit when there is an actual fault, such as a loss of coolant flow, or when an operational limit has been reached, such as a power or current limit.
Referring generally to
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The connector portion 608 of the thermocouple 60 has a positive prong 612 and a negative prong 614. A DC voltage proportional to temperature is produced at the junction of the thermocouple wires 600 and transmitted to the two prongs of the connector portion 608. In the illustrated embodiment, the receptacle end 606 of the extension 602 has three jacks: a positive voltage jack 616, a negative voltage jack 618, and a ground jack 620. The positive voltage jack 616 is adapted to receive the positive prong 612 and the negative voltage jack 618 is adapted to receive the negative prong 614. The plug end 610 of the extension 602 has three prongs: a positive voltage prong 622, a negative voltage prong 624, and a ground prong 626.
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The multiple extension 654 has a female connector assembly 656 at one end that is electrically coupled through the multiple extension 654 to a male connector assembly 658 at the opposite end of the multiple extension 654. The female connector assembly 656 has a plurality of positive voltage jacks 616, negative voltage jacks 618, and ground jacks 620 to enable the multiple extension 654 to electrically couple a plurality of thermocouples 60. The positive voltage jacks 616 are adapted to receive the positive prongs 612 and the negative voltage jacks 618 are adapted to receive the negative prong 614. The male connector assembly 658 has a plurality of positive voltage prongs 622, negative voltage prongs 624, and ground prongs 626 to enable the male connector assembly 658 to connect to a plurality of connector assemblies 604 on the controller 72.
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It will be understood that the foregoing description is of preferred exemplary embodiments of this invention, and that the invention is not limited to the specific forms shown. For example, the noise reduction system may be used to reduce the noise from temperature feedback devices other than thermocouples, such as an RTD. Additionally, the specific configuration of the electrical connectors, i.e., male or female, may be changed without altering the features of the system. These and other modifications may be made in the design and arrangement of the elements without departing from the scope of the invention as expressed in the appended claims.
This application is a divisional of application Ser. No. 09/995,106, filed on Nov. 26, 2001 is now U.S. Pat. No. 6,713,737.
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Number | Date | Country | |
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20040164072 A1 | Aug 2004 | US |
Number | Date | Country | |
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Parent | 09995106 | Nov 2001 | US |
Child | 10784421 | US |