Method and system for providing variable ramp-up control for an electric heater

Abstract

In one aspect, the present disclosure relates to a method for controlling the temperature of a heater including a resistive heating element. The method includes applying power to the resistive heating element of the heater at a variable ramp rate to raise the temperature of the heater to a desired temperature setpoint. The variable ramp rate is set to the desired ramp rate. The method further includes monitoring a current flowing through the resistive heating element of the heater and, in response to the current being greater than a lower limit of a current limit band, reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate. The upper limit of the current limit band is given as a system current limit.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to and the benefit of U.S. Provisional Application No. 63 / 064,523, filed August 12, 2020, the disclosure of which is incorporated herein by reference.

[0002] The present disclosure relates to temperature control of a heater. [Background technology]

[0003] The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

[0004] A thermal system generally includes a heater having a resistive heating element and a control system for controlling power to the heater to generate heat at a temperature setpoint. In one example application, a semiconductor processing system includes a thermal system having a heating plate with a ceramic substrate and a pedestal heater including one or more resistive heating elements that define one or more heating zones. The pedestal heater can be heated to different temperature setpoints to perform various processes such as semiconductor wafer heating, cleaning cycles, among other processes.

[0005] To reach the temperature setpoint, the control system typically increases the temperature at a standard ramp rate (e.g., 5°C / min, 10°C / min, among others). The time spent changing the temperature setpoint typically causes the semiconductor chamber containing the heater to idle, which is lost time or unproductive manufacturing time. These and other problems with heater temperature regulation are addressed by the present disclosure. Summary of the Invention [Problem to be solved by the invention]

[0006] This section provides a general overview of the disclosure and is not an exhaustive disclosure of its entire scope or all of its features. [Means for solving the problem]

[0007] In one aspect, the present disclosure relates to a method for controlling the temperature of a heater including a resistive heating element, the method including applying power to the resistive heating element of the heater at a variable ramp rate to raise the temperature of the heater to a desired temperature setpoint, the variable ramp rate being set to the desired ramp rate, the method further including monitoring a current flowing through the resistive heating element of the heater, and reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current being greater than a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit.

[0008] In one variation, the step of reducing the variable ramp rate further includes determining a desired ramp rate reduction amount based on a variable reduction factor, the variable reduction factor increasing as the current in the resistive heating element approaches a system current limit, and reducing the variable ramp rate by the reduction amount to an allowable ramp rate.

[0009] In another variation, a variable reduction factor provides a scaled reduction in the variable ramp rate based on the proximity of the current to the system current limit.

[0010] In yet another variation, the reduction amount is determined based on RedAmt=(DesiredRate*%Reduction*RedFactor), where Reduction=1.0-((ZoneCurLim-MeasuredCurrent) / CurrentBand), where "RedAmt" is the reduction amount, "DesiredRate" is the desired ramp rate, "RedFactor" is the amount by which the variable ramp rate is reduced when the current equals the system current limit, "ZoneCurLim" is the maximum current limit in the resistive heating element, "MeasuredCurrent" is the current being measured, and "CurrentBand" is the current limit band.

[0011] In one variation, the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate.

[0012] In another variation, the current in each of the plurality of zones is monitored, and the variable ramp rate is reduced from the desired ramp rate to an allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than a lower limit of a current limit band.

[0013] In yet another variation, the method further includes determining a reduction amount in at least one zone having a current greater than a lower limit of a current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit, and reducing the variable ramp rate for each of the plurality of zones based on the reduction amount to obtain an allowable ramp rate for each of the plurality of zones.

[0014] In one variation, the method further includes monitoring a zone temperature for each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and adjusting a variable ramp rate for the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein the first zone and the second zone having the higher zone temperature are designated as a hot zone and the other of the first zone and the second zone is a cool zone.

[0015] In other variations, to adjust the variable ramp rates, the variable ramp rate in the hot zone is decreased, the variable ramp rate in the cool zone is increased, or a combination thereof.

[0016] In yet another variation, to adjust the variable ramp rate, the method further includes the steps of reducing the variable ramp rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is less than the deviation threshold, and increasing the variable ramp rate of the hot zone in response to the difference being less than the deviation threshold.

[0017] In one variation, the method further includes setting the variable ramp rate to a glide control rate, the glide control rate being less than that of a desired ramp rate, and increasing the variable ramp rate to the desired ramp rate in response to a glide condition being met, the glide condition including the passage of a predetermined time and the temperature of the heater being equal to a glide temperature setpoint that is less than the desired temperature setpoint, or a combination thereof.

[0018] In another variation, the method further includes determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than a desired temperature setpoint, and reducing the variable ramp rate to an approach ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approach ramp rate being less than the desired ramp rate.

[0019] In one aspect, the present disclosure relates to a control system for controlling power to a heater including a resistive heating element, the control system including a processor and a non-transitory computer-readable medium including instructions executable by the processor, the instructions including determining an amount of power to be supplied to the resistive heating element of the heater based on a variable ramp rate to raise a temperature of the heater to a desired temperature setpoint, the variable ramp rate being set to the desired ramp rate, the instructions further including monitoring a current flowing through the resistive heating element of the heater, and reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current being greater than a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit.

[0020] In one variation, the instructions further include determining a desired ramp rate reduction amount based on a variable reduction factor, the variable reduction factor increasing as the current in the resistive heating element approaches a system current limit, and reducing the variable ramp rate by the reduction amount to obtain an acceptable ramp rate.

[0021] In another variation, a variable reduction factor provides a scaled reduction in the variable ramp rate based on the proximity of the current to the system current limit.

[0022] In yet another variation, the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate.

[0023] In one variation, the current in each of a plurality of zones is monitored, and the variable ramp rate is reduced from a desired ramp rate to an allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than a lower limit of a current limit band.

[0024] In another variation, the instructions further include determining a reduction amount in at least one zone having a current greater than a lower limit of a current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit, and reducing the variable ramp rate for each of the plurality of zones based on the reduction amount to obtain an allowable ramp rate for each of the plurality of zones.

[0025] In yet another variation, the instructions further include monitoring a zone temperature for each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; and adjusting a variable ramp rate for the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein the first zone and the second zone having the higher zone temperature is designated as a hot zone and the other of the first zone and the second zone is a cool zone.

[0026] In one variation, to adjust the variable ramp rate, the instructions further include decreasing the variable ramp rate in the hot zone, increasing the variable ramp rate in the cool zone, or a combination thereof.

[0027] In another variation, the instructions further include reducing the variable ramp rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is less than the deviation threshold, and increasing the variable ramp rate in response to the difference being less than the deviation threshold.

[0028] In yet another variation, the instructions further include setting the variable ramp rate to a glide control rate, the glide control rate being less than that of the desired ramp rate, and increasing the variable ramp rate to the desired ramp rate in response to a glide condition being met, the glide condition including the passage of a predetermined time and the temperature of the heater being equal to a glide temperature setpoint that is less than the desired temperature setpoint, or a combination thereof.

[0029] In one variation, the instructions further include determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than a desired temperature setpoint, and reducing the variable ramp rate to an approach ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approach ramp rate being less than the desired ramp rate.

[0030] Further areas of applicability may 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 explanation of the drawings]

[0031] In order that the present disclosure may be fully understood, various aspects of the disclosure will now be described, given by way of example only, with reference to the accompanying drawings, in which:

[0032] [Figure 1] 1 illustrates a thermal system having a heater and control system with variable ramp rate temperature control according to the present disclosure.

[0033] [Figure 2] 1 is a flow chart of an exemplary variable ramp rate temperature control according to the present disclosure.

[0034] [Figure 3] 3 is a flowchart of an example of the variable ramp-up control of FIG. 2.

[0035] [Figure 4] 3 is a flowchart of an example of the variable ramp-down control of FIG. 2.

[0036] [Figure 5A] 10 is a graph of a constant ramp-up control according to the present disclosure.

[0037] [Figure 5B] 4 is a graph of variable ramp-up control according to the present disclosure.

[0038] [Figure 6] 4 is a graph of variable ramp-up control according to the present disclosure.

[0039] [Figure 7A] 10 is a graph of variable ramp-down control according to the present disclosure.

[0040] [Figure 7B] 10 is a graph of variable ramp-down control according to the present disclosure.

[0041] [Figure 8] 10 is a graph of variable ramp-down control for suppressing a zone drift condition according to the present disclosure.

[0042] [Figure 9] 10 is a graph of variable ramp-down control for mitigating a runaway condition according to the present disclosure.

[0043] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way. DETAILED DESCRIPTION OF THE INVENTION

[0044] The following description is merely exemplary in nature and is in no way intended to limit the disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0045] Referring to FIG. 1 , a thermal system 100 includes a pedestal heater 102 and a control system 104 having a controller 106 and a power converter system 108. In one embodiment, the heater 102 includes a heating plate 110 and a support shaft 112 disposed on the bottom surface of the heating plate 110. The heating plate 110 includes a substrate 111 and a plurality of resistive heating elements (not shown) embedded in or disposed along the surface of the substrate 111 (i.e., "plurality" means two or more). In one embodiment, the substrate 111 may be made of ceramic or aluminum. The resistive heating elements are independently controlled by the control system 104 and define a plurality of heating zones 114, as shown by the dashed lines in FIG. 1 . It will be readily understood that the heating zones 114 can have different configurations while remaining within the scope of the present disclosure. Additionally, the pedestal heater 102 may include one or more zones and should not be limited to a multi-zone heater.

[0046] In one form, the heater 102 is a "two-wire" heater in which the resistive heating element functions as a heater and a temperature sensor with only two leads, rather than four, operably connected to the heating element. Such two-wire functionality is disclosed, for example, in U.S. Patent No. 7,196,295, which is commonly assigned and incorporated by reference in its entirety. Generally, in a two-wire system, the resistive heating element is defined by a material whose resistance changes with temperature, such that the average temperature of the resistive heating element is determined based on the change in resistance of the resistive heating element. In one form, the resistance of the resistive heating element is first calculated by measuring the voltage across and the current through the heating element, and then Ohm's Law is used to determine the resistance. Using resistance-to-temperature conversion data (e.g., tables, algorithms, among others), the temperature of the resistive heating element and, therefore, the zone 114 (i.e., the zone temperature) is determined. The resistive heating element may be defined by a relatively high temperature coefficient of resistance (TCR) material, a negative TCR material, or, in other words, a material having a nonlinear TCR.

[0047] The control system 104 is configured to control the operation of the heater 102, and more particularly, to independently control the power to each of the zones 114. In one form, the control system 104 is electrically coupled to the zones 114 via terminals 115, such that each zone 114 is coupled to two terminals that provide power and a sensed temperature.

[0048] In one form, the control system 104 is communicatively coupled (e.g., wirelessly and / or wired) to a computing device 117 having one or more user interfaces such as a display, keyboard, mouse, speakers, touch screen, etc. Using the computing device 117, a user can provide inputs or commands such as temperature setpoints, power setpoints, and / or commands to execute tests or processes stored by the control system 104.

[0049] The control system 104 is electrically coupled to a power source 118 that provides an input voltage (e.g., 240V, 208V) to the power converter system 108 via an interlock 120. The interlock 120 controls power flow between the power source 118 and the power converter system 108 and is operable by the controller 106 as a safety mechanism to shut off power from the power source 118. As shown in FIG. 1, the control system 104 need not include the interlock 120.

[0050] The power converter system 108 regulates the input voltage and outputs an output voltage (V OUT) to the heater 102. In one form, the power converter system 108 includes multiple power converters (not shown) operable to apply adjustable power to the resistive heating elements of the zones 114. An example of such a power converter system is described in U.S. Pat. No. 10,690,705, entitled "POWER CONVERTER FOR A THERMAL SYSTEM," which is commonly owned with this application and incorporated herein by reference in its entirety. In this example, each power converter includes a buck converter operable by the controller 106 to generate a desired output voltage that is less than or equal to an input voltage for one or more heating elements of a given zone 114. Thus, the power converter system 108 is operable to supply a customizable amount of power (i.e., desired power) to each zone 114 of the heater 102. Other power converter systems configured to supply adjustable power to the heater 102 can also be used and should not be limited to the examples provided herein. For example, the power converter system may be an isolated power converter system for supplying an isolated power output to the heater. An example of such a power converter system is described in U.S. Pat. No. 11,038,431, entitled "ISOLATED POWER CONVERTER FOR A THERMAL SYSTEM," which is commonly owned with the present application and is incorporated herein by reference in its entirety.

[0051] With the use of two-wire heaters, the control system 104 includes a sensor circuit 124 for measuring the electrical characteristics (i.e., voltage and / or current) of the resistive heating elements, which are then used to determine performance characteristics of the zones 114, such as resistance, temperature, current, voltage, power, and other suitable information. In one form, a given sensor circuit 124 includes an ammeter 126 and a voltmeter 128 for measuring the current flowing through and the voltage applied to the heating element in the given zone 114, respectively. In another form, voltage and / or current measurements are taken at zero crossings, as described in U.S. Pat. No. 7,196,295.

[0052] Instead of or in addition to a "two-wire heater," the thermal system 100 can include individual sensors for measuring characteristics (e.g., voltage, current, and / or temperature) of the heater 102 and provide respective data to the controller 106. For example, in one form, at least one voltmeter and ammeter can be provided to measure electrical characteristics (e.g., voltage and current) of the zones 114, and at least one temperature sensor can be provided to measure the temperature of the heater and / or the temperature of each zone 114.

[0053] In one form, the controller 106 includes one or more microprocessors and memory for storing computer-readable instructions executed by the microprocessors. In one form, the controller 106 is configured to execute one or more control processes in which the controller 106 determines a desired power to be applied to the zone 114, such as 100% of the input voltage, 90% of the input voltage, etc. Examples of control processes are described in U.S. Pat. No. 10,690,705 (see above) and U.S. Pat. No. 10,908,195, entitled "SYSTEM AND METHOD FOR CONTROLLING POWER TO A HEATER," which are commonly owned with this application and are incorporated herein by reference in their entireties. In one form, the controller 106 executes closed-loop temperature control in which the temperature of the heater is controlled to a temperature setpoint. For example, using the resistance of the resistive heating element and a calibrated resistance-temperature model, the controller 106 determines the temperature of the zone 114 and then adjusts the power to the zone 114 to bring the temperature of the zone 114 closer to the temperature setpoint.

[0054] In one form, the control process also includes variable ramp rate temperature (VRRT) control 130, in which the heater 102 initially undergoes a variable temperature ramp rate to reach a temperature setpoint. Once at the temperature setpoint, the controller provides steady-state closed-loop control to maintain the temperature of the heater 102 at the temperature setpoint. In certain applications, the heater 102 may be controlled to different temperature setpoints for industrial processes, and sometimes the temperature may fluctuate from a first temperature to a second temperature that is much lower than the first temperature.

[0055] In one embodiment, the VRRT control 130 is configured to perform variable ramp-up control to increase the temperature of the heater 102 and variable ramp-down control to decrease the temperature of the heater 102. Although the VRRT control 130 is configured to have both, the VRRT control 130 can include either the variable ramp-up control or the variable ramp-down control, and does not need to have both.

[0056] The variable ramp-up control is configured to supply power to the resistive heating element of the heater 102 at a variable ramp-up rate to raise the temperature of the heater 102 to a temperature setpoint. The variable ramp-up rate is determined based on the current supplied to the heater 102 and, for a multi-zone heater, based on the temperature of the zone 114. More specifically, to prevent damage to components of the thermal system 100, such as power switches, power converters, wiring, and / or fuses, among others, the current applied to the heater 102 is controlled below a system current limit, which may be a zone current limit and / or a heater current limit. For example, for the zones 114, the current to each zone 114 is monitored and controlled below the zone current limit as a system current limit for each respective zone 114. In one form, for a multi-zone heater, the current in one zone can affect the current in other zones. That is, to provide coherent ramping, when a single zone approaches a system current limit, the variable ramp-up control adjusts (e.g., decreases) the variable ramp-up rate for all zones by the same decreasing rate. For a temperature set point, the variable ramp-up control defines a system current limit (ie, the maximum allowable current in the heater and / or zone) and a desired ramp rate, which is the maximum desired ramp rate for the variable ramp-up rate.

[0057] To provide a coherent temperature profile for the multi-zone heater, variable ramp-up control monitors and controls the temperature of the zones 114 so that the temperature difference between any two zones 114 (e.g., a first zone and a second zone) is less than a deviation threshold (zone-to-zone drift / deviation). More specifically, ramping is governed by a travel setpoint (i.e., temperature ramp setpoint (TempRampSP)) that travels at a rate setpoint (RateSP, i.e., variable ramp rate). That is, in one form, the rate setpoint, in °C / min, is the rate at which TempRampSP changes. TempRampSP is the absolute temperature at which the controller 106 holds the measured temperature as it travels, for example, using proportional-integral-derivative (PID) control. The measured temperature is sometimes referred to as the process value (PV). Because TempRampSP is constantly traveling until it reaches the temperature setpoint, the process value should also travel. In one form, the integral time constant in the PID is responsible for building power to match the rate setpoint. In one embodiment, if the process variable of any one zone deviates from the process variables of the other zones 114, the variable ramp-up control adjusts the RateSP of one or more zones to provide coherent temperature control of the heater 102. In one embodiment, the variable ramp-up control can decrease the RateSP of the zones that deviate from the other zones to provide a coherent temperature profile. In another embodiment, the variable ramp-up control can increase the RateSP of the other zones 114 while monitoring the current to those zones 114 to improve the performance of those zones 114.

[0058] For variable ramp-up control, Table 1 gives the control variables used to control the ramp rate based on current and temperature.

[0059] [Table 1]

[0060] In one form, to control the ramp rate based on current, the variable ramp-up control sets a variable ramp rate for a zone based on the measured current in the zone and the total current to the heater. In particular, the variable ramp rate is set high enough to stay below a system current limit (e.g., the zone current limit and / or the zone current limit). The variable ramp rate is initially set to a desired ramp rate, and if the zone current limit is within a current limit band, the variable ramp rate is reduced from the desired ramp rate to an allowable ramp rate based on a calculated reduction amount. In addition to the zone approaching the zone current limit, the variable ramp rates of other zones are reduced by the same reduction amount to provide coherent current control. The reduction amount depends on how close the measured current is to the system current limit, such that the smaller the difference between the measured current and the system current limit, the greater the reduction amount.

[0061] More specifically, the variable ramp-up control defines a scaled reduction in the current limit band based on a percentage reduction factor and the difference between the measured current and a system current limit, such as a zone current limit. That is, in an application, the scaled reduction is based on the proximity of the current to the system current limit. For example, the reduction is determined using Equation 1 and Equation 2, where "%Reduction" is provided as a variable reduction factor that increases as the measured current of the resistive heating element in the zone approaches the zone current limit.

[0062] Formula 1....RedAmt=(DesiredRate*%Reduction*RedFactor) Formula 2....%Reduction=1.0-((ZoneCurLim-MeasuredCurrent) / CurrentBand)

[0063] The variable ramp down factor is configured to provide a scaled ramp down parameter of 0% when the measured current is below the current limit band, 0-100% when the measured current is within the current band, 100% when the measured current is equal to the system current limit, and greater than 100% when the measured current exceeds the system current limit, resulting in an even greater ramp down than the ramp down factor, as given by Equation 2. In one form, when the measured current is above the zone current limit, the variable ramp rate continues to ramp down to a nominal rate, such as 1°C / min or other value suitable to prevent stall-out.

[0064] In one embodiment, to control the ramp rate based on temperature, the variable ramp-up control measures the temperature of each zone and initially sets the zone's temperature ramp setpoint to the measured temperature value to suppress temperature jumps. From this point, the temperature begins to rise toward the temperature setpoint. The zone's temperature is routinely measured, and if the zone's temperature begins to deviate from the other zones (i.e., too high or too low), the variable ramp-up rate is adjusted to provide coherent temperatures. In one embodiment, the variable ramp-up control reduces the ramp rate of the zone closest to the temperature setpoint (i.e., the hot zone) to allow the other zones (i.e., the cool zones) to catch up to the hot zone's temperature ramp setpoint. The amount of reduction is selected to provide a responsive reduction, but not overly aggressive to reduce heating operation. For example, the ramp rate may be reduced by 5-15% for each degree of deviation. In another embodiment, while monitoring the current to the heater and zone, the variable ramp-up control increases the ramp rate of the cool zone to allow it to catch up to the hot zone's temperature ramp setpoint. For example, the ramp rate in the cool zone can be increased in set increments (e.g., 1°C / min, 2°C / min, 0.5°C / min increments). In this boost method, the variable ramp-up control can also slow the ramp rate of the hot zone or hold the hot zone temperature at its current temperature ramp set point until the other zones are at or near the measured temperature of the hot zone.

[0065] In one embodiment, at the start of the control, the variable ramp-up control can provide glide control to control how quickly the ramp rate changes and approach control when approaching the temperature setpoint to reduce or suppress temperature spikes. More specifically, the ramp rate is set to a glide control rate that is significantly lower than the desired ramp rate (e.g., glide control rate = 1.0°C / min). In one embodiment, the ramp rate is maintained at the glide control rate until a glide condition is met, which may include, for example, a predetermined time and / or reaching a desired temperature ramp setpoint (i.e., glide temperature setpoint). Thereafter, the variable ramp rate is increased to the desired ramp rate. In one embodiment, a glide control rate is applied whenever the ramp rate is changed to manage the acceleration of the ramp rate.

[0066] The approach control is configured to slow the ramp rate to an approach ramp rate when the measured temperature is a specified distance / range (i.e., a temperature approach threshold) from the final temperature setpoint. The ramp rate is slowed to allow the heater to reach the temperature setpoint without exceeding it. In one form, the approach control is applied when approaching the temperature setpoint (e.g., during ramp-up or ramp-down) to provide an integration time to wind down to a value appropriate for the temperature setpoint. For example, if the coefficient is 1.0, the ramp will begin at a rate several degrees away from the temperature setpoint. Thus, a 10°C / min ramp will begin ramping 10°C away from the temperature setpoint.

[0067] The variable ramp-down control is configured to provide coherent cooling of the heater to a temperature setpoint that is lower than the measured temperature. For semiconductor processes, the rate at which the heater cools may be a function of the chamber, and may slow as the temperature drops and / or as the chamber walls heat up. For multi-zone heaters, different zones of the heater may cool at different rates when power is removed or significantly reduced. To reduce temperature differences between zones, the variable ramp-down control is configured to maintain a variable ramp rate above the natural fall rate (i.e., the rate of decrease without power).

[0068] In one embodiment, the variable ramp-down control reduces the temperature of the zone at a cooling variable ramp rate such that the temperature set point is continuously reduced at a predetermined rate. For example, in one embodiment, the cooling variable ramp rate is initially set to a desired cooling ramp rate, such as 10°C / min, and the temperature of the zone is monitored to maintain a coherent thermal profile of the heater during cooling.

[0069] To provide a coherent thermal profile, the variable ramp-down control determines whether one or more of the following runaway conditions exist: zone-to-zone drift, ramp set point deviation, and / or zone floating condition. If a runaway condition is detected, the variable ramp-down control implements corrective action.

[0070] With respect to zone-to-zone drift, the variable ramp-down control determines whether a zone is cooling faster or slower than other zones. Specifically, in one form, the variable ramp-down control determines whether the temperature of the target zone is within a zone deviation threshold from the other zones. If the target zone deviates from one or more other zones, the variable ramp rates of all zones are adjusted as a corrective action to reduce the deviation and provide a coherent ramp-down.

[0071] With respect to ramp set point deviation, the variable ramp down control determines whether a zone is falling too far behind its temperature ramp set point during ramp down. Specifically, during ramp down, the temperature ramp set point is continuously decreased according to a variable ramp rate. If the temperature of the target zone is falling behind (i.e., not cooling fast enough), the ramp rate is adjusted so that the temperature of the target zone continues to decrease while allowing the target zone to catch up to the temperature ramp set point. Formally, to detect ramp set point deviation, the variable ramp down control determines whether the temperature of the target zone deviates from the temperature ramp set point by a value equal to or greater than the set point deviation threshold (i.e., deviation threshold). If so, a ramp set point deviation condition is detected.

[0072] To mitigate zone-to-zone drift and / or ramp setpoint deviations, the variable ramp-down control reduces the variable ramp rate to a value less than the desired ramp rate (e.g., 10°C / min to 5°C / min) as a corrective measure. In one embodiment, the variable ramp-down control determines a decrement (i.e., ramp cooling decrement (RCoolRedAmt)) based on the amount of deviation between the temperature of a zone and the temperature ramp setpoint of other zones. For example, in one embodiment, the decrement is determined using Equations 3-5, where PVH is the measured temperature of the hot zone, PVL is the measured temperature of the cool zone, WeightPara1 is a weighting parameter in the delta measured temperature, given as the decrement per degree of deviation (e.g., 10% / °C), and WeightPara2 is a weighting parameter in the difference between the cool zone and the temperature ramp setpoint, given as the decrement per degree of deviation (e.g., 5% / °C). Once determined, the ramp cooling decrement is applied to each zone of the zoner.

[0073] Equation 3.....RCoolRedAmt = Zone Deviation Reduction + Setpoint Deviation Reduction Equation 4: Zone Deviation Reduction = |(PVH-PVL)|*WeightPara1 Equation 5....Set point deviation reduction=|(PVL-TempRampSP)|*WeightPara2

[0074] In one variation, the ramp cooling reduction amount is based on either the zone deviation reduction or the setpoint deviation reduction (i.e., the setpoint deviation amount). For example, if only inter-zone drift is present, no setpoint deviation reduction may be necessary. Alternatively, if both deviation conditions exist, the variable ramp-down control may first reduce the deviation of the inter-zone drift based on the zone deviation reduction until the inter-zone deviation falls within a threshold. The ramp cooling reduction amount is then determined using both the zone deviation reduction and the setpoint deviation reduction as given in Equation 3. It should be readily understood that the numerical values ​​given herein are for illustrative purposes only and can be any suitable values.

[0075] In another form, if the temperature of at least one zone begins to deviate from the other zones, the temperature ramp setpoint of the cool zone is set to the measured temperature of the hot zone. That is, the variable ramp-down control increases power to the zone with the lower temperature to raise the temperature of the zone with the higher temperature. Thus, the variable ramp-down control keeps the temperatures of the zones together or within a deviation threshold (e.g., ±5°C) that may flatten the temperature ramp setpoint curve before the zones approach their temperature setpoints.

[0076] In a zone floating state, the variable ramp-down control determines whether the zone is floating or wandering. More specifically, as power is reduced to a zone, it may be difficult to accurately measure a process value (e.g., temperature), and in some circumstances, the power may be too low, causing the zone to become uncontrollable (e.g., the power is at a minimum power level / output above 0 volts but insufficient to control the zone). That is, the temperature of the zone may begin to deviate from the temperature ramp setpoint, and if there are multiple zones, the temperature of the zone may begin to deviate from the other zones. To control the ramp-down during a zone floating state, the variable ramp-down control is configured to increase power to the zone experiencing the floating state to a nominal power output (e.g., 2% power, 5% power) greater than the minimum power level (i.e., minimum power output) to gain control of the zone while still reducing the zone's temperature. In one form, power is increased by decreasing the variable ramp setpoint until power is again applied at the nominal power output. The nominal power output applied to the zone to suppress the zone floating condition can be specified based on testing and can be just above a minimum power level (eg, nominal power output greater than 5V).

[0077] If multiple runaway conditions are detected, the decrement amount is a weighted combination of the decrement amounts for the detected deviation conditions. In one form, the weight assigned to each deviation condition can be based on the stage the heater is in during the cool-down process. That is, ramp setpoint deviations generally occur earlier in the heater's cool-down than inter-zone drift, which can occur as the heater cools. Therefore, decrement amounts associated with ramp setpoint deviations are assigned a higher weight than decrement amounts associated with inter-zone drift when the heater first begins to cool. After some time, and / or after the heater temperature reaches a selected temperature setpoint higher than the desired temperature setpoint, the variable ramp-down control can assign a higher weight to decrement amounts associated with inter-zone drift than to ramp setpoint deviations. At lower temperatures, power to the heater may no longer be needed, so a minimal amount of power can be applied to prevent zone-floating conditions that may override inter-zone drift and ramp setpoint deviations. Therefore, weighting factors can be assigned based on the stage of the heater during cool-down and the heater itself (i.e., the heater's responsiveness).

[0078] It should be readily understood that the variable ramp-down control can be configured to monitor one or more runaway conditions, and need not monitor all of them. For example, in the case of a single-zone heater, zone-to-zone drift is not required.

[0079] Referring to FIG. 2, an example of a VRRT control routine 200 is provided, which is executed by a control system to control the temperature of a heater to one or more temperature setpoints. At 202, the control system obtains a temperature setpoint for the heater, for example, from a predefined state mode that provides the heater's temperature setpoint and duration. At 204, it determines whether the temperature setpoint is less than the heater's current temperature. If the temperature setpoint is higher, the control system performs variable ramp-up control at 206. On the other hand, if the temperature is lower, the control system performs variable ramp-down control at 208. Once the temperature setpoint is reached, the control system returns to routine 200, maintains the temperature at the temperature setpoint using a temperature control model (e.g., PID control) at 210, and determines whether there is a new temperature setpoint at 212. If there is a new temperature setpoint, the control system returns to 202. In one form, the temperature setpoint may include a nominal setpoint at which the heater is turned off.

[0080] Referring to FIG. 3, an example of a variable ramp-up control 300 is provided. At 302, the control system sets the variable ramp rate to a desired ramp rate defined based on a temperature setpoint and / or system current limits and supplies power to the heater to achieve the desired ramp rate. At 304, the control system monitors the current flowing through the resistive heating element of the zone and the temperature of each zone. At 306, the control system determines whether the measured current in each zone is below the current limit band. If so, the control system proceeds to 310. If not, the control system determines a reduction factor at 308 and reduces the variable ramp rate in each zone based on the reduction factor. Specifically, using the methodology described above, the control system determines a reduction factor that correlates to how close the measured current is to the system current limit and reduces the variable ramp rate in each zone by the reduction amount to obtain an acceptable ramp rate. At 310, the control system determines whether the temperatures of adjacent zones are within a deviation threshold to maintain a coherent temperature profile of the heater. If the temperatures are within the deviation threshold, the control system proceeds to 314. If at least one zone is deviating, the control system reduces the variable ramp rate of the zone with the higher temperature at 312, as described above. Alternatively, the control system may be configured to increase power to other zones while monitoring the current in the zone. At 314, the control system determines whether the zone is at its temperature setpoint. If not, the control system returns to 304. If the zone is at its temperature setpoint, the control system returns to routine 200 of FIG. 2.

[0081] Referring to FIG. 4, an example of a variable ramp-down control 400 is provided. At 402, the control system sets the variable ramp rate to a cooling ramp rate (e.g., a second variable ramp rate) and controls the zones based on the cooling ramp rate. At 404, the control system monitors the temperature of each zone, and at 406, the control system determines whether the temperature difference between the zones is within a deviation threshold to provide a coherent temperature profile as the heater cools to the temperature setpoint. For example, the control system determines whether the temperature difference between adjacent zones is greater than the deviation threshold. If the temperature difference is within the deviation threshold, the control system proceeds to 410. If at least one zone is deviating, at 408, the control system sets the temperature ramp setpoint of the zone with the lower temperature (i.e., the cool zone) to the measured temperature of the zone with the higher temperature (the hot zone), thus increasing the power to the cool zone to achieve the new temperature ramp setpoint. At 410, the control system determines whether the zone is at its temperature setpoint. If not, the control system returns to 404. If the heater is at the temperature setpoint, the control system returns to routine 200 of FIG.

[0082] It should be readily understood that routines 200, 300, and 400 can be configured in a variety of suitable ways and should not be limited to the steps described herein. For example, if the heater is a single-zone heater, the control system can skip steps associated with providing a coherent temperature profile in routine 300 and omit the variable ramp-down routine. In other examples, the VRRT control also includes glide rate control and / or approach control to provide a smooth transition to the desired ramp rate and temperature setpoint, respectively. In yet other examples, in the case of variable ramp-down control, instead of setting a cooling ramp rate, the control turns off power to the heater and monitors the temperature of the zone to mitigate possible temperature excursions.

[0083] 5A-9 illustrate the characteristics of the VRRT control of the present disclosure. Specifically, FIG. 5A illustrates a ramp-up operation with a constant ramp rate (e.g., 20°C / min), while FIG. 5B illustrates a ramp-up operation using the VRRT control of the present disclosure. In both cases, the current is maintained at 30 A or less, but the constant ramp-up rate of FIG. 5A takes longer to reach 600°C than the VRRT control of FIG. 5B. With VRRT control, the ramp rate starts at 28°C / min and decreases as the current approaches 30 A. That is, as the measured current falls within the current limit band (e.g., 25-30 A), the ramp rate decreases, controlling the current applied to the heater while allowing the heater to reach the temperature setpoint.

[0084] FIG. 6 is a graph illustrating ramp-up control in which the ramp rate is controlled from the glide rate to the desired ramp rate and then to the approach control rate as the measured temperature approaches the temperature set point.

[0085] 7A and 7B are graphs showing the cooling of a two-zone heater without and with variable ramp-down control, respectively. As shown in FIG. 7A, the zone temperatures begin to deviate from one another, which can cause thermal stress, while in FIG. 7B, the heater has a coherent temperature profile by addressing the deviating temperatures.

[0086] Figure 8 shows a variable ramp-down control of the VRRT control where the heater is powered at a level just above a minimum amount (e.g., 5% power is supplied) to suppress a zone floating condition. By supplying a small amount of power to the heater, the heater temperature is continuously monitored and still reduced to the temperature setpoint.

[0087] FIG. 9 illustrates variable ramp-down control, as previously described, which reduces or suppresses a runaway condition by controlling the ramp rate and / or power. In the figure, the filament temperature, which represents the heater temperature, and the temperature ramp set point (i.e., the ramp set point (SP) in FIG. 9) are substantially the same during ramp-down. In the figure, the optimal ramping process variation (PV) reduction is the amount of reduction due to different zones deviating too much, the optimal ramp bottom reduction is the amount of reduction due to the power going too low (floating), and the optimal ramp net ramp set point (SP) gain is the weighted sum of three corrective actions (e.g., the net gain is a multiplier from 0.0 to 1.0 of the desired ramp SP to reduce the ramp rate, where 1.0 is no reduction and 0.5 is a 50% reduction). The optimal ramp set point (SP) reduction may initially spike as zones deviate from the ramp SP.

[0088] As used herein, the term deviation threshold generally captures a variety of possible thresholds defined for comparing the difference between a measurement (e.g., zone temperature, heater temperature) and another value (e.g., temperature setpoint, temperature of another zone, etc.). In one form, the deviation thresholds used to monitor zone-to-zone drift / deviation in variable ramp-up control and variable ramp-down control may be the same or different thresholds. In one form, for variable ramp-down control, the deviation thresholds for zone-to-zone drift and ramp setpoint deviation may be the same or different. Also, the deviation threshold may be given as a single absolute value (e.g., 5°C) or as a range (e.g., ±5°C). The actual value of the deviation threshold will be based on the particular application and therefore is not limited to the specific numerical values ​​provided herein.

[0089] Unless otherwise expressly indicated herein, all numerical values ​​expressing mechanical / thermal properties, composition percentages, dimensions and / or tolerances, or other properties, when describing the scope of this disclosure, should be understood to be modified by the word "about" or "approximately." This variation is desirable for various reasons, including industrial practices, material, manufacturing, and assembly tolerances, and testing capabilities.

[0090] As used herein, the phrase at least one of A, B, and C should be interpreted to mean the logical (A OR B OR C) using a non-exclusive logical OR, and not to mean "at least one of A, at least one of B, and at least one of C."

[0091] In this application, the term "controller" may be interchanged with the term "circuitry." The term "controller" may refer to or include an application specific integrated circuit (ASIC), a digital, analog, or mixed analog / digital discrete circuit; a digital, analog, or mixed analog / digital integrated circuit; a combinational logic circuit; a field programmable gate array (FPGA); a processor circuit (shared, dedicated, or group) that executes code; a memory circuit (shared, dedicated, or group) that stores code to be executed by the processor circuit; other suitable hardware components that provide the described functionality; or a combination of some or all of the above, such as part of a system-on-chip.

[0092] The term code may include software, firmware, and / or microcode, and may refer to programs, routines, functions, classes, data structures, and / or objects. The term memory circuit is a subset of the term computer-readable medium. As used herein, the term computer-readable medium does not encompass transitory electrical or electromagnetic signals propagating through a medium (such as on a carrier wave); thus, the term computer-readable medium may be considered tangible and non-transitory.

[0093] The description of the present disclosure is merely exemplary in nature and, thus, variations that do not depart from the content of the disclosure are intended to be within the scope of the disclosure. Such variations should not be considered a departure from the spirit and scope of the disclosure. The inventions described in the claims at the time of filing of this application are as follows: [1] 1. A method of controlling the temperature of a heater including a resistive heating element, comprising: applying power to the resistive heating element of the heater at a variable ramp rate to raise the temperature of the heater to a desired temperature set point, the variable ramp rate being set to a desired ramp rate; monitoring the current flowing through the resistive heating element of the heater; reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current exceeding a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit; A method comprising: [2] The step of decreasing the variable ramp rate comprises: determining a decrease in the desired ramp rate based on a variable decrease factor, the variable decrease factor increasing as the current in the resistive heating element approaches the system current limit; decreasing the variable ramp rate by the decrease amount to the permissible ramp rate; The method according to [1], further comprising: [3] 10. The method of claim 2, wherein the variable reduction factor provides a scaled reduction in the variable ramp rate based on the proximity of the current to the system current limit. [4] The amount of decrease is RedAmt=(DesiredRate*%Reduction*RedFactor) Formula 2....%Reduction=1.0-((ZoneCurLim-MeasuredCurrent) / CurrentBand) is determined based on During the ceremony, "RedAmt" is the amount of decrease, "DesiredRate" is the desired ramp rate, "RedFactor" is the amount by which the variable ramp rate is reduced when the current is equal to the system current limit; "ZoneCurLim" is the maximum current limit for the resistive heating element; "MeasuredCurrent" is the current being measured, "CurrentBand" is the current limit band; [2] The method described in [2]. [5] 10. The method of claim 1, wherein the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate. [6] the current in each of the plurality of zones is monitored; the variable ramp rate is reduced from the desired ramp rate to the allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than the lower limit of the current limit band. [5] The method described in [5]. [7] determining a reduction in the at least one zone having the current greater than a lower limit of the current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit; decreasing the variable ramp rate for each of the plurality of zones based on the decrease amount to obtain the allowable ramp rate for each of the plurality of zones; The method according to [6], further comprising: [8] monitoring a zone temperature for each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; adjusting the variable ramp rate for the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein one of the first zone and the second zone having a higher zone temperature is designated as a hot zone and the other of the first zone and the second zone is a cool zone; The method according to [5], further comprising: [9] To adjust the variable ramp rate, the method includes: reducing the variable ramp rate in the hot zone; increasing the variable ramp rate of the cool zone; or Combinations of these, The method according to [8], further comprising:

[10] To adjust the variable ramp rate, the method includes: reducing the variable ramp rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is equal to or less than the deviation threshold; increasing the variable ramp rate of the hot zone in response to the difference being less than the deviation threshold; The method according to [8], further comprising:

[11] setting the variable ramp rate to a glide control rate, the glide control rate being less than that of the desired ramp rate; and increasing the variable ramp rate to the desired ramp rate in response to a glide condition being satisfied, the glide condition including a predetermined time lapse and the temperature of the heater being equal to a glide temperature set point that is lower than the desired temperature set point, or a combination thereof; The method according to [1], further comprising:

[12] determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than the desired temperature setpoint; reducing the variable ramp rate to an approaching ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approaching ramp rate being less than the desired ramp rate; The method according to [1], further comprising:

[13] 1. A control system for controlling power to a heater including a resistive heating element, comprising: a processor; a non-transitory computer-readable medium containing instructions executable by said processor; Equipped with The instruction: determining an amount of power to be supplied to the resistive heating element of the heater based on a variable ramp rate to raise the temperature of the heater to a desired temperature set point, the variable ramp rate being set to the desired ramp rate; monitoring the current flowing through the resistive heating element of the heater; reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current exceeding a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit; Including, Control system.

[14] The instruction: determining a decrease in the desired ramp rate based on a variable decrease factor, the variable decrease factor increasing as the current in the resistive heating element approaches the system current limit; decreasing the variable ramp rate by the decrease amount to obtain the allowable ramp rate; The control system according to

[13] , further comprising:

[15]

[14] The control system of

[14] , wherein the variable reduction factor provides a scaled reduction in the variable ramp rate based on the proximity of the current to the system current limit.

[16] 13. The control system of claim 12, wherein the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate.

[17] the current in each of the plurality of zones is monitored, and the variable ramp rate is reduced from the desired ramp rate to the allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than the lower limit of the current limit band.

[16] The control system described in

[16] .

[18] The instruction: determining a reduction in the at least one zone having the current greater than a lower limit of the current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit; decreasing the variable ramp rate for each of the plurality of zones based on the decrease amount to obtain the allowable ramp rate for each of the plurality of zones;

[17] The control system according to

[17] , further comprising:

[19] The instruction: monitoring a zone temperature for each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; adjusting the variable ramp rate for the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein one of the first zone and the second zone having a higher zone temperature is designated as a hot zone and the other of the first zone and the second zone is a cool zone;

[17] The control system according to

[17] , further comprising:

[20] To adjust the variable ramp rate, the instructions include: reducing the variable ramp rate in the hot zone; increasing the variable ramp rate of the cool zone; or Combinations of these,

[19] The control system according to

[19] , further comprising:

[21] The instruction: reducing the variable ramp rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is equal to or less than the deviation threshold; increasing the variable ramp rate in response to the difference being less than the deviation threshold;

[19] The control system according to

[19] , further comprising:

[22] The instruction: setting the variable ramp rate to a glide control rate, the glide control rate being less than that of the desired ramp rate; and increasing the variable ramp rate to the desired ramp rate in response to a glide condition being satisfied, the glide condition including a predetermined time lapse and the temperature of the heater being equal to a glide temperature set point that is lower than the desired temperature set point, or a combination thereof; The control system according to

[13] , further comprising:

[23] The instruction: determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than the desired temperature setpoint; reducing the variable ramp rate to an approaching ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approaching ramp rate being less than the desired ramp rate; The control system according to

[13] , further comprising:

Claims

1. 1. A method of controlling the temperature of a heater including a resistive heating element, comprising: applying power to the resistive heating element of the heater at a variable ramp rate to raise the temperature of the heater to a desired temperature set point, the variable ramp rate being set to a desired ramp rate; monitoring the current flowing through the resistive heating element of the heater; reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current exceeding a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit; A method comprising:

2. The step of decreasing the variable ramp rate comprises: determining a decrease in the desired ramp rate based on a variable decrease factor; the variable reduction factor increasing as the current in the resistive heating element approaches the system current limit; decreasing the variable ramp rate by the decrease amount to the permissible ramp rate; The method of claim 1 further comprising:

3. The amount of decrease is RedAmt=(DesiredRate*%Reduction*RedFactor) Formula 2. .. .. .. .. %Reduction=1.0-((ZoneCurLim-MeasuredCurrent) / CurrentBand) is determined based on During the ceremony, "RedAmt" is the amount of reduction, "DesiredRate" is the desired ramp rate, "RedFactor" is the amount by which the variable ramp rate is reduced when the current is equal to the system current limit; "ZoneCurLim" is the maximum current limit in the resistive heating element; "MeasuredCurrent" is the current that is measured, "CurrentBand" is the current limit band; The method of claim 2.

4. the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate; the current in each of the plurality of zones is monitored; the variable ramp rate is reduced from the desired ramp rate to the allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than the lower limit of the current limit band; determining a reduction amount in the at least one zone having the current greater than a lower limit of the current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit; decreasing the variable ramp rate for each of the plurality of zones based on the decrease amount to obtain the allowable ramp rate for each of the plurality of zones; The method of claim 1 further comprising:

5. setting the variable ramp rate to a glide control rate, the glide control rate being less than that of the desired ramp rate; and increasing the variable ramp rate from the glide control rate to the desired ramp rate in response to a glide condition being met, the glide condition including at least one of the glide condition having expired for a predetermined time and the temperature of the heater reaching a glide temperature setpoint that is lower than the desired temperature setpoint; The method of claim 1 further comprising:

6. determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than the desired temperature setpoint; reducing the variable ramp rate to an approaching ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approaching ramp rate being less than the desired ramp rate; The method of claim 1 further comprising:

7. 1. A control system for controlling power to a heater including a resistive heating element, comprising: a processor; a non-transitory computer-readable medium containing instructions executable by said processor; Equipped with The instruction: determining an amount of power to be supplied to the resistive heating element of the heater based on a variable ramp rate to raise the temperature of the heater to a desired temperature set point, the variable ramp rate being set to the desired ramp rate; monitoring the current flowing through the resistive heating element of the heater; reducing the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to the current exceeding a lower limit of a current limit band, the upper limit of the current limit band being given as a system current limit; Including, Control system.

8. The instruction: determining a decrease in the desired ramp rate based on a variable decrease factor, the variable decrease factor increasing as the current in the resistive heating element approaches the system current limit; decreasing the variable ramp rate by the decrease amount to obtain the allowable ramp rate; The control system of claim 7 further comprising:

9. The control system of claim 8 , wherein the variable reduction factor provides a scaled reduction in the variable ramp rate based on the proximity of the current to the system current limit.

10. 8. The control system of claim 7, wherein the heater includes a plurality of resistive heating elements defining a plurality of zones, each of the plurality of zones having a defined variable ramp rate.

11. the current in each of the plurality of zones is monitored, and the variable ramp rate is reduced from the desired ramp rate to the allowable ramp rate in response to at least one zone of the plurality of zones having a current greater than the lower limit of the current limit band. The control system of claim 10.

12. The instruction: determining a reduction amount in the at least one zone having the current greater than a lower limit of the current limit band based on a variable reduction factor, the variable reduction factor increasing as the current approaches the system current limit; decreasing the variable ramp rate for each of the plurality of zones based on the decrease amount to obtain the allowable ramp rate for each of the plurality of zones; The control system of claim 11 further comprising:

13. The instruction: monitoring a zone temperature for each of the plurality of zones; determining whether a difference between a first zone temperature of a first zone of the plurality of zones and a second zone temperature of a second zone of the plurality of zones is greater than a deviation threshold; adjusting the variable ramp rate for the first zone, the second zone, or a combination thereof in response to the difference being greater than the deviation threshold, wherein one of the first zone and the second zone having a higher zone temperature is designated as a hot zone and the other of the first zone and the second zone is a cool zone; The control system of claim 11 further comprising:

14. To adjust the variable ramp rate, the instructions include: reducing the variable ramp rate in the hot zone; increasing the variable ramp rate of the cool zone; or Combinations of these, The control system of claim 13 further comprising:

15. The instruction: reducing the variable ramp rate of the hot zone to zero to maintain the zone temperature of the hot zone until the difference is equal to or less than the deviation threshold; increasing the variable ramp rate in response to the difference being less than the deviation threshold; The control system of claim 13 further comprising:

16. The instruction: setting the variable ramp rate to a glide control rate, the glide control rate being less than that of the desired ramp rate; and increasing the variable ramp rate from the glide control rate to the desired ramp rate in response to a glide condition being met, the glide condition including at least one of the glide condition having expired for a predetermined time and the temperature of the heater reaching a glide temperature setpoint that is lower than the desired temperature setpoint; The control system of claim 7 further comprising:

17. The instruction: determining whether the temperature of the heater is at a temperature approach threshold, the temperature approach threshold being less than the desired temperature setpoint; reducing the variable ramp rate to an approaching ramp rate in response to the temperature of the heater being at the temperature approach threshold, the approaching ramp rate being less than the desired ramp rate; The control system of claim 7 further comprising:

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