System and method for closed-loop baking control

Abstract

A control system for operating a heater includes a controller configured to determine an operating power level based on measured performance characteristics of the heater, a power setpoint, and a power control algorithm, determine a toasting power level based on measured leakage current at the heater, a leakage current threshold, and a moisture control algorithm, and select a power level to be applied to the heater. The selected power level is the lower of the operating power level and the toasting power level.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of U.S. Provisional Application No. 62 / 731,373, filed September 14, 2018. The disclosure of the above application is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a thermal system and method for bake-out control of a heater. Background Art

[0004] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0005] Thermal systems are used in a variety of applications and typically include a heater for heating a workpiece and a control system for controlling the performance of the heater. The heater may be a layered heater having multiple resistive heating elements formed by a layered process (e.g., thick film, thin film, thermal spray, sol-gel), a metal sheathed heater, or other suitable heater. The heater may be a low-voltage heater operating at a voltage of approximately 600 V or less, or a medium-voltage heater operating at a voltage level of approximately 600 V to 4 kV.

[0006] Moisture ingress can occur in many types of heaters and is particularly problematic with heaters that have hygroscopic insulation, which allows moisture to enter when the heater is at room temperature. To reduce or remove this moisture, heaters undergo a "bakeout" process, during which power is applied to the heater to remove or reduce the moisture. In some applications, the heater may include a dedicated heater element for the bakeout process, while in other applications, the heater element used to heat the workpiece is controlled to perform the bakeout process.

[0007] Some bake-out processes rely on time-based control, which can result in bake-out periods that are too short or too long. If the bake-out time is too short, moisture remains in the heater, preventing it from operating at full voltage, necessitating a repeat bake-out process. If the bake-out time is too long, the thermal system may operate at high temperatures longer than necessary, resulting in wasted power. The present disclosure addresses these and other issues related to removing moisture from heaters. Summary of the Invention

[0008] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0009] The present disclosure provides a control system for operating a heater, the control system including a controller configured to determine an operating power level based on measured performance characteristics of the heater, a power setpoint, and a power control algorithm. Furthermore, the controller determines a toasting power level based on a measured leakage current at the heater, a leakage current threshold, and a moisture control algorithm, and selects a power level to be applied to the heater. The selected power level is the lower of the operating power level and the toasting power level.

[0010] In one embodiment, the control system further comprises a first sensor configured to measure a performance characteristic of the heater and a second sensor configured to measure a leakage current. In this embodiment, the first sensor may be a discrete current sensor for measuring an operating current of the heater as a performance characteristic.

[0011] In another approach, the heater is a two-wire heater, and the controller is configured to calculate the operating current as a performance characteristic based on the resistance of the heater.

[0012] In another embodiment, the control system further comprises a power regulation circuit configured to be electrically coupled to the heater and to apply the selected power level to the heater. In this embodiment, the power regulation circuit may comprise a power switch operable by the controller to provide adjustable power to the heater.

[0013] In another approach, the power control algorithm and the moisture control algorithm are defined as proportional-integral-derivative (PID) control.

[0014] The present disclosure also provides a thermal system comprising a control system having some or all of the features disclosed above. The thermal system further comprises a heater electrically coupled to the control system, the heater comprising a heating element for heating a workpiece. The control system is configured to apply a desired power level to the heating element. In this embodiment, the heater can be selected from the group consisting of a layered heater, a tubular heater, a cartridge heater, a polymer heater, and a flexible heater.

[0015] The present disclosure also provides a method for controlling a heater. The method includes: measuring a performance characteristic of the heater; measuring a leakage current; determining an operating power level based on the measured performance characteristic, a power set point, and a power control algorithm; determining a toasting power level based on the measured leakage current, a leakage current threshold, and a moisture control algorithm; and applying one of the operating power level or the toasting power level as a selected power level to the heater.

[0016] In one embodiment, the method further includes selecting a lower power level from the operating power level and the toasting power level as the selected power level.

[0017] In another approach, the performance characteristic is the amount of current in the heater.

[0018] In yet another approach, the heater is selected from the group consisting of a layered heater, a tubular heater, a cartridge heater, a polymer heater, and a flexible heater.

[0019] In one approach, the power control algorithm and the moisture control algorithm may be defined as proportional-integral-derivative (PID) control.

[0020] The present disclosure also provides a method for controlling moisture within a heater. The method includes: operating the heater in a main operating mode to heat a workpiece, wherein an operating power level is applied to the heater in the main operating mode; measuring a leakage current of the heater using a leakage current sensor, wherein the leakage current indicates moisture within the heater; determining a toasting power level based on the measured leakage current, a leakage current threshold, and a moisture control algorithm, wherein the moisture control algorithm is defined as proportional-integral-derivative (PID) control; operating the heater in the toasting mode in response to the toasting power level being less than the operating power level; and operating the heater in the main operating mode in response to the toasting power level being greater than the operating power level.

[0021] In one embodiment, the step of operating the heater in the primary operating mode further comprises measuring a performance characteristic of the heater and determining an operating power level based on the measured performance characteristic, a power set point, and a power control algorithm, wherein the power control algorithm is defined as PID control. In this embodiment, the performance characteristic may be an operating current flowing through the heater.

[0022] In other embodiments, the method further includes calculating an operating current of the heater as the performance characteristic based on the resistance of the heater, and / or measuring the operating current of the heater as the performance characteristic using a discrete current sensor.

[0023] Further areas of applicability will become apparent from the description provided herein.It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order that the present disclosure may be better understood, various aspects thereof will now be described by way of example with reference to the accompanying drawings, in which:

[0025] Figure 1 is a block diagram of a thermal system including a heater and a control system according to the present disclosure;

[0026] FIG2A is a top view of an exemplary layered heater formed by a layering process;

[0027] FIG2B is a representative cross-sectional view of a layered heater.

[0028] FIG3 is a partial cross-sectional view of a cartridge heater.

[0029] Figure 4 yes Figure 1 a circuit diagram of a thermal system illustrating a path for leakage current according to the present disclosure;

[0030] Figure 5 yes Figure 1 A block diagram of the control system; and

[0031] Figure 6 is a flow chart of a heater control routine for controlling moisture removal in a heater according to the present disclosure.

[0032] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. DETAILED DESCRIPTION

[0033] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0034] The present disclosure is directed to a control system for controlling moisture accumulation in a heater during a baking process. Figure 1 In one embodiment, the thermal system 100 includes a heater 102 and a control system 104 configured to control the heater 102 .

[0035] In one embodiment, the heater 102 includes one or more heating elements 106 operable to heat a workpiece 108. For example, referring to Figures 2A and 2B, the heater 102 can be a layered heater 200 comprising a dielectric layer 202, a resistive layer 204 defining one or more heating elements, and a protective layer 206 disposed on a substrate 208. In one embodiment, the heating element formed by the resistive layer 204 is a two-wire heating element operable as both a heater and a temperature sensor to detect one or more electrical characteristics of the heating element. Such a two-wire heating element is disclosed in U.S. Patent No. 7,196,295, which is commonly assigned with the present application and is incorporated herein by reference in its entirety.

[0036] It should be understood that the number of layers and the configuration of the layers of the layered heater 200 are exemplary only, and that various combinations of layers applied to one another without a separate substrate are within the teachings of the present disclosure. For example, such variations are disclosed in U.S. Patent Nos. 7,132,628 and 8,680,443, which are commonly assigned with the present application and are incorporated herein by reference in their entirety. The layers are formed by applying or accumulating materials onto a substrate or another layer using processes related to thick film, thin film, thermal spray, or sol-gel, among others.

[0037] Although the heater 102 is described as a layered heater, the teachings of the present disclosure can be applied to other types of heaters, such as tubular heaters, cartridge heaters, polymer heaters, and flexible heaters, and should not be limited to layered heaters. For example, referring to FIG3 , the heater 102 can be a cartridge heater 300 that includes a resistive heating element 302 (e.g., a metal wire) disposed around a non-conductive portion 304, a sheath 306, a dielectric material 308 (e.g., MgO) disposed between the resistive heating element 302 and the sheath 306, and two pins 310. In one embodiment, the pins 310 are connected to leads (not shown) and extend through the non-conductive portion 304 and are connected to the ends of the resistive heating element 302 to power the resistive heating element.

[0038] During operation, moisture may begin to accumulate within the heater 102, such as within the dielectric layer 202 and / or the protective layer 206 of the layered heater 200. In another example, and specifically with respect to the cartridge heater 300, moisture may begin to accumulate between the ends of the resistive heating element 302 and the leads. Moisture within the heater 102 creates alternative current paths, and the current flowing through these alternative paths is often referred to as leakage current. In some applications, the heater 102 draws more total current when moisture is present than when the heater 102 is dry due to the additional current flowing from hot to ground. Typically, to remove any moisture, the heater 102 undergoes a bake-out process during which one or more heating elements 106 within the heater 102 are activated to remove or "bake out" the moisture.

[0039] Continue to refer Figure 1To monitor the current flow within the heater 102, the thermal system 100 includes an operating current sensor 110 (e.g., a first current sensor) and a leakage current sensor 112 (e.g., a second current sensor) electrically connected to the heater 102. The number of operating current sensors 110 and leakage current sensors 112 may vary based on the type of heater 102 used. In one embodiment, the operating current sensor 110 is a current transformer that measures the current flowing through the heater 102 (i.e., the current leaving the heater 102 on the intended neutral line), which may be referred to as the operating current of the heater 102 and is an example of a performance characteristic of the heater 102.

[0040] For example, Figure 4 is an exemplary schematic diagram illustrating operating current and leakage current through a heater. In this example, a heater 400 having a heating element 402 receives power from a control system 404, which is configured similarly to control system 104. As described in detail below, control system 404 receives power from a power supply 406 and is configured to regulate the power to a selected voltage applied to heater 400. Arrows A and B illustrate the normal current path of the operating current. When moisture begins to accumulate, a leakage path is created in heater 400, which is illustrated by the dotted line, with arrow C indicating the direction of the leakage current.

[0041] In one approach, if heater 102 is a two-wire system, the operating current is measured based on the change in resistance of heating element 106. That is, this thermal system combines heater design with control of power, resistance, voltage, and current into a customizable feedback control system that limits one or more of these parameters (i.e., power, resistance, voltage, current) while simultaneously controlling another. For example, by calculating the resistance of the heating element and knowing the applied voltage, the operating current through the heating element can be determined without the use of a discrete sensor. Thus, the two-wire system can operate as an operating current sensor.

[0042] In one embodiment, the leakage current sensor 112 is a current transformer that measures the amount of leakage current leaving the heater 102 on, for example, a ground line. The operating current sensor 110 and the leakage current sensor 112 send signals indicative of their respective current measurements to the control system 104, which in turn controls the amount of power applied to the heater 102.

[0043] Continue to refer Figure 1The control system 104 is connected to a power source 114, such as an AC or DC power source, and is configured to apply an adjustable input voltage to the heater 102. The control system 104 includes a combination of electronics (e.g., a microprocessor, memory, a communication interface, a voltage-to-current converter and voltage-to-current measurement circuitry, etc.) and a software program / algorithm stored in the memory and executable by the microprocessor to perform the operations described herein.

[0044] More specifically, in one embodiment, the control system 104 is configured to control the heater 102 during a primary operation period, during which the heater 102 is heating the workpiece 108 according to one or more predetermined performance parameters. In one embodiment, the primary operation of the heater 102 includes different operating states, such as a preheating state, a steady state, and / or a power-off state. Each operating state may include different performance parameters, such as a power set point, for a given state. During the primary operation, the control system 104 monitors moisture within the heater 102 via a measured leakage current from the leakage current sensor 112, and interrupts the primary operation to perform the bake-out process when the leakage current exceeds a leakage current threshold.

[0045] More specifically, based on signals from sensors 110 and 112 and a predefined control algorithm, control system 104 determines the amount of power required to limit leakage current and the amount of power required to meet the power set point for primary operation. The lower of the two power amounts is then applied to heater 102. More specifically, in some applications, leakage current is limited during a bakeout process by applying a low voltage across heater 102 to prevent excessive current to ground, which could damage heater 102 and / or other equipment. As moisture is removed from heater 102, the resistance along the area with moisture increases (e.g., along or within the insulation / dielectric), thereby allowing the voltage to heater 102 to be increased without exceeding a leakage current threshold. In one embodiment, the control algorithm is a proportional-integral-derivative (PID) control.

[0046] refer to Figure 5In one embodiment, the control system 104 includes a controller 500 and a power regulation circuit 501. The controller 500 is configured to include a main operation module 502, a leakage current module 504, and a power module 506. The main operation module 502 determines the operating power level based on the measured operating current from the operating current sensor 110, a power set point, and a power control algorithm. In one embodiment, the power set point is a baseline parameter (i.e., a user-defined set point) that can be set by a user using a user interface for the operating state being executed and / or a predetermined value associated with the operating state. In one embodiment, the power control algorithm is defined as a PID control (i.e., a first PID control or an operating PID control) to calculate the operating power level to be applied to the heater 102 so that the actual power applied to the heater 102 is close to the power set point. For example, in one embodiment, based on the measured operating current and the input voltage applied to the heater 102, the power control algorithm calculates the actual power provided to the heater 102. The power control algorithm determines the difference between the actual power applied and the power set point and determines the required power level (i.e., the operating power level) to minimize the difference between the actual power applied to the heater and the power set point. Thus, through PID control, the main operating module 502 is provided as a closed loop control to adjust the power applied to the heater 102 to meet the power set point.

[0047] Based on the measured leakage current from the leakage current sensor 112, the leakage current threshold, and the moisture control algorithm, the leakage current module 504 determines the baking power level. The leakage current threshold is a preset value that is the level of leakage current allowed (e.g., 30 mA or other value), thereby indicating the amount of moisture allowed. The moisture control algorithm in one embodiment is defined as a PID control (i.e., a second PID control or a baking PID control) to calculate the baking power level to reduce the leakage current to a value equal to or below the leakage current threshold. For example, in one embodiment, the moisture control algorithm determines the difference between the measured leakage current and the leakage current threshold and calculates the required power level (i.e., the baking power level) to reduce the actual leakage current level to a value less than or equal to the leakage current threshold. Therefore, using PID control, the leakage current module 112 is a closed-loop control to adjust the power applied to the heater 102 to quickly bake out moisture in the heater 102 (i.e., reduce leakage current).

[0048] The power module 506 selects a power level from the operating power level and the baking power level and sends a control signal to the power regulation circuit to apply the selected power level (i.e., input voltage). In one embodiment, the power module 506 is configured to select the lower power level from the operating power level and the baking power level as the selected power level.

[0049] In one embodiment, the power regulating circuit 501 is configured to regulate the power from the power supply 114 to a selected power level and apply the regulated power to the heater 102. The power regulating circuit 501 may include a thyristor, a voltage divider, a voltage converter, a transformer, a power switch and / or other suitable electronic components. For example, in one embodiment, the power regulating circuit 501 is configured to use low phase angle switching or zero-crossing switching to regulate the voltage from the power supply. In another example, the power supply 114 may include a high voltage source for operating the power level and a low voltage source for baking the power level, and the power regulating circuit 501 is configured to switch between the two power supplies based on a control signal from the power module 506. In yet another example, the power regulating circuit 501 is configured to provide high current and low current through an autotransformer. In another example, the power regulating circuit 501 is configured to a power converter including a rectifier and a buck converter. Such a power converter system is described in U.S. Application Serial No. 15 / 624,060, filed on June 15, 2017, entitled "POWER CONVERTER FOR THERMAL SYSTEM," which is commonly owned with the present application and is incorporated herein by reference in its entirety. In another example, the power conditioning circuit 501 is a DC power supply. It should be readily understood that the controller is configured to operate the power conditioning circuit 501 and may include different circuits and software applications based on the power conditioning circuit 501.

[0050] In operation, the main operation module 502 controls the power applied to the heater 102 during a given operating state to heat a workpiece. During main operation, the leakage current module 504 monitors the leakage current within the heater 102. Specifically, as long as the measured leakage current is below a leakage current threshold, the leakage current module 504 outputs a bake power level that is greater than the operating power level. Once the measured leakage current is greater than or equal to the leakage current threshold, the leakage current module 504, with the moisture control algorithm, outputs a power level that is lower than the operating power level to initiate bake control.

[0051] By having both operational PID control and bake PID control, the control system of the present disclosure is operable to reduce bake time, thereby removing moisture from the heater, by only applying the time required to reduce leakage current. More specifically, instead of discrete time periods and set power levels, the PID control of the moisture control algorithm is a ramp-up algorithm that continues to ramp up the voltage until the leakage current drops below a leakage current threshold. For example, in one embodiment, the leakage current threshold can be set to zero amperes or approximately zero amperes, so that once leakage current is detected, a bake-out operation is performed to remove moisture. Thus, PID control reduces the time and total power required to dry out the heater.

[0052] The control system can be configured to include additional operating features while remaining within the scope of the present disclosure. For example, the control system can be configured to communicate with one or more external devices to output data regarding the operation of the heater and / or receive user input. In another example, the control system can perform diagnostic routines to evaluate whether the thermal system is operating within predetermined parameters to detect possible anomalies.

[0053] refer to Figure 6 , provides an example of a heater control routine 600. In one embodiment, the heater control routine 600 is executed by the control system when power is applied to the heater. At 602, the control system operates the heater according to the selected heater operation, and at 604, obtains an operating current (IOP) and a leakage current (ILK) from an operating current sensor and a leakage current sensor, respectively.

[0054] As described above, at 606, using operational PID control, the control system calculates the operating power level and, at 608, calculates the toasting power level. At 610, the control system determines whether the operating power level is less than or equal to the toasting power level. If the operating power level is less than the toasting power level, the main operation is maintained, and at 612, the control system applies the operating power level to the heater and returns to the top of the routine to operate the heater. Conversely, if the operating power level is greater than the toasting power level, the main operation is interrupted to perform the toasting operation. Therefore, at 614, the control system applies the toasting power level to the heater and returns to 604 to obtain a current measurement. Routine 600 may terminate when the main switch to the control system is closed and power is no longer being applied to the heater, when an abnormal condition is detected within the thermal system, and / or other suitable conditions.

[0055] The routines / methods described herein can be implemented in a computer-readable medium. The term "computer-readable medium" includes a single medium or multiple media, such as a centralized or distributed database, and / or associated caches and servers that store one or more sets of instructions. The term "computer-readable medium" should also include any medium that can store, encode, or carry a set of instructions for execution by a processor or cause a computer system to perform any one or more of the methods or operations disclosed herein.

[0056] It should be readily understood that, although specific example diagrams are provided for a control system, the system may include additional components not described in detail in the diagrams. For example, the control system includes components (e.g., a primary controller and an auxiliary controller) that operate at a lower voltage than the power converters of the regional control circuits. Therefore, the control system includes a low-power voltage source (e.g., 3-5V) for powering the low-voltage components. In addition, to protect the low-voltage components from high voltages, the control system includes electronic components that isolate the low-voltage components from the high-voltage components while still allowing the components to exchange signals.

[0057] As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR, and should not be construed to mean "at least one of A, at least one of B, and at least one of C."

[0058] The description of the present disclosure is merely exemplary in nature and, thus, variations that do not depart from the essence of the present disclosure are intended to be within the scope of the present disclosure. Such variations should not be regarded as a departure from the spirit and scope of the present disclosure.

Claims

1. A control system for operating a heater, the control system comprising: A controller configured to: determining an operating power level based on measured performance characteristics of the heater, a power set point, and a power control algorithm, determining a toasting power level based on a measured leakage current at the heater, a leakage current threshold, and a moisture control algorithm, and A power level to be applied to the heater is selected, wherein the selected power level is a lower power level of the operating power level and the toasting power level.

2. The control system according to claim 1, further comprising: a first sensor configured to measure a performance characteristic of the heater; and A second sensor is configured to measure the leakage current.

3. The control system according to claim 2, wherein: The first sensor is a discrete current sensor for measuring an operating current of the heater as the performance characteristic.

4. The control system according to claim 1, wherein: The heater is a two-wire heater, and the controller is configured to calculate an operating current as the performance characteristic based on a resistance of the heater. 5 . The control system of claim 1 , further comprising a power regulation circuit configured to electrically couple to the heater and apply the selected power level to the heater.

6. The control system according to claim 5, wherein: The power regulation circuit includes a power switch operable by the controller to provide adjustable power to the heater.

7. The control system according to claim 1, wherein: The power control algorithm and the moisture control algorithm are defined as proportional-integral-derivative (PID) control.

8. A thermal system comprising: The control system according to claim 1, and The heater is electrically coupled to the control system and includes a heating element for heating a workpiece, wherein the control system is configured to apply the selected power level to the heating element. 9 . The system of claim 8 , the heater being a two-wire heater, and the controller of the control system being configured to calculate an operating current as the performance characteristic based on a resistance of the heater.

10. The system according to claim 8, wherein: The heater is selected from the group consisting of a layered heater, a tubular heater, a cartridge heater, a polymer heater, and a flexible heater.

11. A method for controlling a heater, comprising: measuring performance characteristics of the heater; Measure leakage current; determining an operating power level based on the measured performance characteristics, a power set point, and a power control algorithm; determining a toasting power level based on the measured leakage current, the leakage current threshold, and a moisture control algorithm; as well as One of the operating power level or the toasting power level is applied to the heater as a selected power level, wherein the selected power level is a lower power level of the operating power level and the toasting power level.

12. The method according to claim 11, further comprising: A lower power level is selected from the operating power level and the toasting power level as the selected power level.

13. The method according to claim 11, wherein The performance characteristic is the amount of current in the heater.

14. The method according to claim 11, wherein The heater is selected from the group consisting of a layered heater, a tubular heater, a cartridge heater, a polymer heater, and a flexible heater.

15. The method according to claim 11, wherein The power control algorithm and the moisture control algorithm are defined as proportional-integral-derivative (PID) control.

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