Method and system for providing variable ramp down control to an electric heater

Abstract

In one aspect, the present disclosure is directed to a method of controlling a temperature of a heater including a resistive heating element, the method including applying power to the resistive heating element at a variable ramp rate to reduce the temperature of the heater to a desired temperature setpoint, the variable ramp rate being set to a desired ramp rate, the method further including monitoring the temperature of the heater to detect a runaway condition, and adjusting the variable ramp rate from the desired ramp rate to an acceptable ramp rate in response to the runaway condition being detected.

Description

Cross - Reference to Related Applications

[0001] This application claims the benefit and priority of U.S. Patent Application No. 62 / 064,523, filed on August 12, 2020. The disclosure of the above - mentioned application is incorporated herein by reference.

Technical Field

[0002] This disclosure relates to controlling the temperature of a heater.

Background Art

[0003] The description in this section merely provides background information related to this disclosure and may not constitute prior art.

[0004] Thermal systems generally include a heater having a resistive heating element and a control system for controlling the power to the heater to generate heat at a temperature set point. In an application example, a semiconductor process system includes a thermal system having a pedestal heater that includes a heating plate with a ceramic substrate and one or more resistive heating elements that define one or more heating zones. The pedestal heater can perform various processes such as heating of semiconductor wafers, cleaning cycles, and other operations and can be heated to different temperature set points.

[0005] To reach the temperature set point, the control system typically ramps up the temperature at a standard ramp rate (e.g., 5 °C / min, 10 °C / min, etc.). The time spent changing the temperature set point typically idles the semiconductor chamber with the heater, which is lost or non - productive manufacturing time. These and other problems related to adjusting the temperature of the heater are addressed by this disclosure.

Summary of the Invention

[0006] This section provides a general summary of the disclosure and is not an all - encompassing disclosure of its full scope or all of its features.

[0007] In one aspect, the present disclosure is directed to a method of controlling the temperature of a heater that includes a resistive heating element, the method including applying power to the resistive heating element at a variable ramp rate to decrease the temperature of the heater to a desired temperature setpoint. The variable ramp rate is set to a desired ramp rate. The method further includes monitoring the temperature of the heater to detect a runaway state and, in response to detecting the runaway state, adjusting the variable ramp rate from the desired ramp rate to a permissible ramp rate.

[0008] In one variation, the runaway state includes a lamp setpoint deviation, and the method further includes determining whether the temperature of the heater deviates from a temperature ramp setpoint by a setpoint deviation threshold. The temperature ramp setpoint is the temperature at which the heater is controlled based on the variable ramp rate such that the temperature of the heater decreases to the desired temperature setpoint.

[0009] In another variation, the method further includes decreasing the variable ramp rate based on the amount of setpoint deviation in response to detecting a lamp setpoint deviation as the runaway state.

[0010] In yet another variation, the runaway state includes a zone floating state, and the method further includes determining whether the power applied to the heater is lower than a nominal power output to detect the zone floating state. The nominal power output is greater than a minimum power output, and the minimum power output is greater than zero volts.

[0011] In one variation, the method further includes decreasing the variable ramp rate to increase the power to the heater to the nominal power output in response to detecting the runaway state and the runaway state being the zone floating state.

[0012] In another variation, the heater includes a plurality of resistive heating elements that define a plurality of zones.

[0013] In yet another variant, the method further includes monitoring the zone temperature for each of a plurality of zones, determining, as a runaway condition, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone shift threshold, and adjusting a variable ramp rate for a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone shift threshold. The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone.

[0014] In one variant, the runaway condition includes at least one of a zone-to-zone shift, a lamp set point shift, a zone floating state, or a combination thereof. The method further includes determining, for the zone-to-zone shift, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone shift threshold. For the lamp set point shift, the method further includes determining whether the temperature of the heater is shifted from a temperature ramping set point by a set point shift threshold, where the temperature ramping set point is the temperature at which the heater is controlled such that the temperature of the heater is lowered to a desired temperature set point based on a variable ramp rate. In the zone floating state, the method further includes determining whether the power applied to the heater is lower than a nominal power output, where the nominal power output is greater than a minimum power output and the minimum power output is greater than zero volts.

[0015] In another variant, the method further includes implementing corrective measures in response to detecting a runaway condition. In response to the runaway condition being an inter-zone deviation, the method further includes, as a corrective measure, adjusting a variable ramp rate of a hot zone, a cool zone, or a combination thereof, where the hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. In response to the runaway condition being a lamp set point deviation, the method further includes, as a corrective measure, reducing the variable ramp rate based on the amount of set point deviation. In response to the runaway condition being a zone floating condition, the method further includes reducing the variable ramp rate and increasing the power to the heater to the nominal power output.

[0016] In yet another variant, the runaway condition includes at least two of an inter-zone deviation, a lamp set point deviation, and a zone floating condition, and the method further includes the following.

[0017] In one variant, the method Runaway (run a way) further includes adjusting the variable ramp rate based on a weighted evaluation of corrective measures associated with the state.

[0018] In one form, the present disclosure is directed 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 include determining an amount of power supplied to the resistive heating element of the heater based on a variable ramp rate to lower the temperature of the heater to a desired temperature set point, where the variable ramp rate is set to a desired ramp rate. The instructions further include monitoring the temperature of the heater to detect a runaway condition and, in response to detecting the runaway condition, adjusting the variable ramp rate from the desired ramp rate to an allowable ramp rate.

[0019] In one modification, the runaway state includes a lamp set point deviation, and the command further includes determining whether the temperature of the heater deviates from the temperature lamp set point by a set point deviation threshold. The temperature ramp set point is the temperature at which the heater is controlled based on a variable ramp rate so that the temperature of the heater decreases to a desired temperature set point.

[0020] In another modification, the command further includes decreasing a variable ramp rate based on an amount of set point deviation in response to the set point deviation being detected as a runaway state.

[0021] In yet another modification, the runaway state includes a zone floating state, and the command further includes determining whether the power applied to the heater is lower than a nominal power output in order to detect the zone floating state. The nominal power output is greater than a minimum power output, and the minimum power output is greater than zero volts.

[0022] In one modification, the command further includes decreasing a variable ramp rate to increase the power to the heater to the nominal power output in response to the runaway state being detected and the runaway state being a zone floating state.

[0023] In another modification, the heater includes a plurality of resistive heating elements that define a plurality of zones, and the command further includes monitoring the zone temperature of each of the plurality of zones, determining, as a runaway state, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold, and adjusting a variable ramp rate of a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone deviation threshold. The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone.

[0024] In yet another variation, the heater includes a plurality of resistive heating elements that define a plurality of zones, and the runaway condition includes at least one of an inter-zone shift, a lamp set point shift, a zone floating condition, or a combination thereof. For an inter-zone shift, the instructions further include determining whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than an inter-zone shift threshold. For a lamp set point shift, the instructions further include determining whether the temperature of the heater is shifted from a temperature ramping set point by a set point shift threshold, where the temperature ramping set point is a temperature at which the heater is controlled such that, based on a variable ramp rate, the temperature of the heater decreases to a desired temperature set point. For a zone floating condition, the instructions further include determining whether the power applied to the heater is lower than a nominal power output, where the nominal power output is greater than a minimum power output and the minimum power output is greater than zero volts.

[0025] In one variation, the instructions further include at least one of: adjusting a variable ramp rate for a hot zone, a cool zone, or a combination thereof in response to the runaway condition being an inter-zone shift, where the hot zone is the one of the first zone or the second zone having the higher zone temperature and the cool zone is the other of the first zone or the second zone, decreasing the variable ramp rate based on a set point shift in response to the runaway condition being a lamp set point shift, or decreasing the variable ramp rate such that the power to the heater is increased to the nominal power output in response to the runaway condition being a zone floating condition.

[0026] In one variation, the runaway condition includes at least two of: an inter-zone shift, a lamp set point shift, and a zone floating condition, and the instructions Runaway (run a way) further include adjusting the variable ramp rate based on a weighted evaluation of corrective measures for the condition.

[0027] In one form, the present disclosure is directed to a thermal system that includes a heater having a plurality of resistive heating elements and the described control system, the plurality of resistive heating elements defining a plurality of zones. The instructions include monitoring a zone temperature for each of the plurality of zones and, as a runaway condition, 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 zone deviation threshold, and adjusting a variable ramp rate of a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone deviation threshold. The hot zone is the zone having the higher temperature of the first zone or the second zone, and the cool zone is the other of the first zone or the second zone.

[0028] Additional applicable areas will become apparent from the description provided herein. It is to be understood that the description and specific examples are for illustrative purposes only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] To enable a full understanding of the present disclosure, various forms thereof given by way of example will be described with reference to the accompanying drawings.

[0030]

Figure 1

[0031]

Figure 2

[0032]

Figure 3

[0033]

Figure 4

[0034]

Figure 5A

[0035]

Figure 5B

[0036]

Figure 6

[0037]

Figure 7A

Figure 7B

[0038]

Figure 8

[0039]

Figure 9

[0040] The drawings described in this specification are for illustrative purposes only and are not intended to limit the scope of the present disclosure.

Mode for Carrying Out the Invention

[0041] The following description is merely exemplary in nature and is not intended to limit the present disclosure, its application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0042] Referring to FIG. 1, the 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 form, 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 form, the substrate 111 can 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 indicated by the dashed line in FIG. 1. It is readily understood that the heating zones 114 can take different configurations while remaining within the scope of the present disclosure. Further, the pedestal heater 102 can include one or more zones and should not be limited to a multi-zone heater.

[0043] 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 in which only two lead wires, rather than four, are operably connected to the heating element. Such two-wire functionality is disclosed, for example, in U.S. Patent No. 7,196,295, which is assigned to and incorporated herein by reference in its entirety in connection with this application. Typically, in a two-wire system, the resistive heating element is defined by a material that exhibits a change in resistance as the temperature changes such that the average temperature of the resistive heating element is determined based on the change in the resistance of the resistive heating element. In one form, the resistance of the resistive heating element is calculated by first measuring the voltage across the heating element and the current through the heating element, and then using Ohm's law to determine the resistance. Resistance temperature conversion data (e.g., tables, algorithms, etc.) is used to determine the temperature of the resistive heating element and thus the zone 114 (i.e., the zone temperature). The resistive heating element can be defined by a material having a relatively high temperature coefficient of resistance (TCR), a negative TCR material, or, in other words, a material having a non-linear TCR.

[0044] The control system 104 controls the operation of the heater 102 and is configured to independently control the power to each zone 114. In one form, the control system 104 is electrically coupled to the zones 114 via the terminals 115 such that each zone 114 is connected to two terminals that provide power and sense temperature.

[0045] In one form, the control system 104 is communicatively coupled to a computing device 117 having one or more user interfaces such as a display, keyboard, mouse, speaker, touch screen, etc. (e.g., wireless and / or wired communication). Using the computing device 117, a user can provide inputs or commands such as temperature set points, power set points, and / or commands to execute tests or processes stored by the control system 104.

[0046] The control system 104 is electrically coupled to a power supply 118 that supplies an input voltage (e.g., 240V, 208V) to the power converter system 108 via an interlock 120. The interlock 120 controls the power flowing between the power supply 118 and the power converter system 108 and is operable by the controller 106 as a safety mechanism to cut off the power from the power supply 118. As shown in FIG. 1, the control system 104 may not include the interlock 120.

[0047] The power converter system 108 regulates the input voltage and outputs a voltage (V OUT) to apply. In one form, the power converter system 108 includes a plurality of power converters (not shown) operable to apply adjustable power to the resistive heating elements of zone 114. An example of such a power converter system is described in U.S. Patent No. 10,690,705 entitled "POWER CONVERTER FOR A THERMAL SYSTEM", which is commonly owned with this application and its contents and is hereby incorporated 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 below the input voltage to one or more heating elements of a given zone 114. Thus, the power converter system 108 is operable to provide a customizable amount of power (i.e., the desired power) to each zone 114 of the heater 102. Other power converter systems configured to provide adjustable power to the heater 102 may 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 providing an isolated power output to the heater. An example of such a power converter system is described in U.S. Patent No. 11,038,431 entitled "ISOLATED POWER CONVERTER FOR A THERMAL SYSTEM", which is commonly owned with this application and the entire contents of which are hereby incorporated by reference.

[0048] By using a two-wire heater, the control system 104 includes a sensor circuit 124 for measuring the electrical characteristics (i.e., voltage and / or current) of the resistive heating element. The electrical characteristics are used to determine the performance characteristics of zone 114, such as resistance, temperature, current, voltage, power, and other appropriate information. In one form, a given sensor circuit 124 includes an ammeter 126 and a voltmeter 128 that measure the current flowing through the heating element(s) within a given zone 114 and the applied voltage, respectively. In another form, as described in U.S. Patent No. 7,196,295, the measured values of voltage and / or current can be acquired at zero crossings.

[0049] Instead of or in addition to the "two-wire heater", the thermal system 100 can include separate sensors for measuring the characteristics of the heater 102 (e.g., voltage, current, and / or temperature), and can provide each data to the controller 106. For example, in one form, at least one of a voltmeter and an ammeter can be provided for measuring the electrical characteristics (e.g., voltage and current) of zone 114, and at least one temperature sensor can be provided for measuring the temperature of the heater and / or the temperature of each zone 114.

[0050] In one form, controller 106 includes one or more microprocessors and memory for storing computer-readable instructions executed by the microprocessors. In one form, controller 106 is configured to execute one or more control processes in which controller 106 determines a desired power to be applied to zone 114, such as 100% of the input voltage, 90% of the input voltage. Examples of control processes are described in U.S. Patent No. 10,690,705 (referenced above), and U.S. Patent No. 10,908,195 entitled "SYSTEM AND METHOD FOR CONTROLLING POWER TO A HEATER", which is commonly owned with this application and the entire contents of which are incorporated herein by reference. In one form, controller 106 performs 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, controller 106 determines the temperature of zone 114 and adjusts the power to zone 114 to bring the temperature of zone 114 closer to the temperature setpoint.

[0051] In one form, the control process also includes variable ramp rate temperature (VRRT) control 130, in which the heater 102 first receives a variable temperature ramp rate to reach the temperature setpoint. At the temperature setpoint, the controller provides steady-state closed-loop control to maintain the temperature of the heater at the temperature setpoint. In certain applications, heater 102 can be controlled to different temperature setpoints for an industrial process, and sometimes the temperature can vary, changing from a first temperature to a second temperature much lower than the first temperature.

[0052] In one form, VRRT control 130 is configured to provide variable ramp-up control to raise the temperature of heater 102 and variable ramp-down control to lower the temperature of heater 102. VRRT control 130 is provided as having both, but VRRT control 130 may include one of variable ramp-up control and variable ramp-down control and is not required to have both.

[0053] The variable ramp-up control is configured to supply power to the resistive heating element of heater 102 at a variable ramp-up rate to raise the temperature of heater 102 to the temperature set point. The variable ramp-up rate is defined based on the current supplied to heater 102 and, for a multi-zone heater, based on the temperature of zone 114. More specifically, in particular, to suppress damage to components of the thermal system 100, such as the power switch, power converter, wiring, and / or fuse, the current applied to heater 102 is controlled to be less than the system current limit, which can be a zone current limit and / or a heater current limit. For example, in zone 114, the current to each zone 114 is monitored and controlled to be less than the zone current limit, which is the system current limit for each zone 114. In one form, in a multi-zone heater, the current in one zone can affect the current in other zones. That is, to provide coherent ramping, when one zone approaches the system current limit, the variable ramp-up control adjusts (e.g., reduces) the variable ramp-up rate of all zones at the same rate of decrease. At the temperature set point, the variable ramp-up control defines the desired ramp rate, which is the system current limit (i.e., the maximum allowable current to the heater and / or zone) and the desired maximum ramp rate of the variable ramp-up rate.

[0054] To provide a coherent temperature profile to the multi-zone heater, the variable ramp-up control monitors and controls the temperature of the zones 114 such that the temperature difference between any two zones 114 (e.g., the first zone and the second zone) is less than the deviation threshold (inter-zone drift / deviation). More specifically, the ramping is managed by a setpoint (i.e., the temperature ramping setpoint (TempRampSP)) that moves at a rate setpoint (RateSP, i.e., the variable ramp rate). That is, in one form, the rate setpoint is in °C per minute and is the rate at which the TempRampSP changes. The TempRampSP is the absolute temperature that holds the measured temperature when the controller 106 moves using, for example, proportional integral derivative (PID) control. The measured temperature may be referred to as the process value (PV). Since the TempRampSP is always moving until the temperature setpoint is reached, the process value also moves. In one form, the integral time constant in the PID responds to building power to correspond to the rate setpoint. In one form, when the process variable of any one zone deviates from the process variable(s) of the other zone(s) 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 form, the variable ramp-up control may decrease the RateSP of the zone that deviates from the other zones to provide a coherent temperature profile. In another form, the variable ramp-up control increases the RateSP of the other zone(s) while monitoring the current to the zones 114 to enhance the performance of those zones 114.

[0055] Regarding the variable ramp-up control, Table 1 provides the control variables used to control the ramp rate based on current and temperature:

Table 1

[0056] In one form, to control the ramp rate based on current, the variable ramp up control sets the variable ramp rate for a zone based on the current measured for the zone and the total current to the heater. In particular, the variable ramp rate is set high enough to stay below the system current limit (e.g., zone current limit and / or system current limit). The variable ramp rate is initially set to the desired ramp rate, and if the zone current limit is within the current limit band, the variable ramp rate is decreased from the desired ramp rate to the allowed ramp rate based on the calculated decrease amount. In addition to the zone approaching the zone current limit, the variable ramp rates of other zones are decreased by the same amount of decrease to provide coherent current control. The amount of decrease depends on how close the measured current is to the system current limit such that the amount of decrease increases as the difference between the measured current and the system current limit is smaller.

[0057] More specifically, the variable ramp up control defines a scaled amount of decrease for the current limit band based on the percentage of the reduction factor and the difference between the measured current and a system current limit such as the zone current limit. That is, in the application example, the scaled decrease is based on the proximity of the current to the system current limit. For example, the amount of decrease is determined using Equations 1 and 2, where “%Reduction” is provided as a variable reduction factor that increases as the measured current of the resistive heating element of the zone approaches the zone current limit. Equation 1 RedAmt = (DesiredRate ×%Reduction × RedFactor) Equation 2 %Reduction = 1.0 - ((ZoneCurLim - MeasuredCurrent) / CurrentBand)

[0058] As provided in Equation 2, the variable reduction factor provides a scaled reduction such that the reduction parameter is 0% when the measured current is below the current limit band, 0 - 100% when the measured current is within the current limit band, 100% when the measured current is equal to the system current limit, and greater than 100% when the measured current is greater than the system current limit, such that the reduction factor further decreases. In one form, when the measured current exceeds the zone current limit, the variable ramp rate continues to decrease to a nominal rate such as 1 °C / min or other suitable value to prevent stalling.

[0059] In one form, to control the ramp rate based on temperature, the variable ramp-up control measures the temperature of each zone, first sets the temperature ramp set point of each zone to the respective measured temperature value, and suppresses temperature jumps. From this point, the temperature begins to rise towards the temperature set point. The temperature of the zone is measured periodically, and if the temperature of the zone starts to deviate from other zones (i.e., is too high or too low), the variable ramp-up rate is adjusted to provide a coherent temperature. In one form, the variable ramp-up control reduces the ramp rate of the zone closest to the temperature set point (i.e., the hot zone), allowing other zones (i.e., cool zones (s)) to catch up to the temperature ramping set point of the hot zone. The amount of reduction is selected to provide a responsive reduction, but is not as aggressive in reducing the heating operation. For example, the ramp rate can be reduced by 5 - 15% for each degree of deviation. In another form, while monitoring the current to the heater and zones, the variable ramp-up control increases the ramp rate of the cool zone(s) to allow the cool zone(s) to catch up to the temperature ramp set point of the hot zone. For example, the ramp rate of the cool zone(s) can be increased at a set increase amount (e.g., an increase of 1 °C / min, 2 °C / min, 0.5 °C / min). In this boost method, the variable ramp-up control reduces the ramp rate of the hot zone, either reducing the rate of temperature rise of the hot zone or holding the temperature of the hot zone at the current ramp rate reduction or the current temperature ramping set point until other zones approach or reach the measured temperature of the hot zone.

[0060] In one form, at the start of control, the variable ramp-up control may provide a slew control that controls the speed at which the ramp rate changes, and an approach control when approaching the temperature setpoint to reduce or suppress temperature spikes. More specifically, the ramp rate is set to a slew control rate that is significantly lower than the desired ramp rate (e.g., slew control rate = 1.0 °C / min). In one form, the ramp rate is maintained at the slew control rate until the slew condition is met, which may include, for example, a predetermined time and / or reaching the desired temperature ramp setpoint (i.e., the slew temperature setpoint). Thereafter, the variable ramp rate is increased to the desired ramp rate. In one form, the slew control rate is applied each time the ramp rate changes to manage the acceleration of the ramp rate.

[0061] The approach control is configured to reduce the ramp rate to an approach ramp rate when the measured temperature is within a defined distance / range (i.e., the temperature approach threshold) from the final temperature setpoint. The ramp rate is reduced so that the heater can reach the temperature setpoint without overshooting the temperature setpoint. In one form, the approach control is applied to provide an integration time to wind in an appropriate value to the temperature setpoint when approaching the temperature setpoint (e.g., during ramp-up or ramp-down). For example, if the factor is 1.0, the reduction starts at a rate a few degrees away from the temperature setpoint. Thus, a 10 °C / min reduction starts 10 °C away from the temperature setpoint.

[0062] Variable ramp-down control is configured to provide coherent cooling of the heater to a temperature setpoint lower than the measured temperature. In semiconductor processes, the rate at which the heater cools is a function of the chamber, and the rate can decrease as the temperature drops and / or as the chamber walls are heated. In a multi-zone heater, when power is removed or significantly reduced, different zones of the heater can cool at different rates. To reduce the temperature difference between zones, variable ramp-down control is configured to maintain the variable ramp rate at or above the natural decay rate (i.e., the rate of decrease without power).

[0063] In one form, variable ramp-down control decreases the temperature of the zone at a variable cooling ramp rate such that the temperature setpoint decreases continuously at a defined rate. For example, in one form, the variable cooling 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.

[0064] To provide a coherent thermal profile, variable ramp-down control determines whether one or more of the following runaway conditions exist: namely, inter-zone drift, ramp setpoint deviation, and / or zone float. If a runaway condition is detected, variable ramp-down control executes a corrective action.

[0065] In inter-zone drift, variable ramp-down control determines whether a zone is cooling faster or slower than other zones. Specifically, in one form, variable ramp-down control determines whether the temperature of the zone of interest is within a zone deviation threshold from other zones. If the zone of interest is deviated from one or more other zones, as a corrective action, the variable ramp rates of all zones are adjusted to reduce the deviation and provide a coherent ramp-down.

[0066] In the case of lamp set point deviation, the variable lamp down control determines whether a zone lags too much from the temperature lamp set point during lamp down. Specifically, during lamp down, the temperature ramp set point continuously decreases according to the variable lamp rate. If the temperature of the target zone lags (i.e., is not cooled fast enough), the lamp rate is adjusted so that the temperature of the target zone continues to decrease while allowing the target zone to catch up with the temperature ramp set point. In form, to detect lamp set point deviation, the variable lamp down control determines whether the temperature of the target zone deviates from the temperature ramp set point by a value greater than or equal to a set point deviation threshold (i.e., deviation threshold). If so, a state where the lamp set point is deviated is detected.

[0067] To mitigate inter-zone drift and / or lamp set point deviation, as a corrective measure, the variable lamp down control reduces the variable lamp rate to a value smaller than the desired lamp rate (e.g., from 10 °C / min to 5 °C / min). In one form, the variable lamp down control determines a reduction amount (i.e., lamp cooling reduction amount (RCoolRedAmt)) based on the amount of deviation between the temperature of a zone and the temperature of other zones and / or the temperature ramp set point. For example, in one form, the reduction amount is determined using Equations 3 - 5. Here, PVH is the measured temperature of the hot zone. PVL is the measured temperature of the cool zone. WeightPara1 is a weighted parameter for the delta measured temperature and is provided as the reduction amount per degree of deviation (e.g., 10% / °C). WeightPara2 is a weighted parameter for the difference between the cool zone and the temperature ramp set point and is provided as the reduction amount per degree of deviation (e.g., 5% / °C). Once determined, the lamp cooling reduction amount is applied to each zone of the zonator. Equation 3: RCoolRedAmt = Zone Deviation Reduction + Set Point Deviation Reduction Equation 4: Zone Deviation Reduction = |(PVH - PVL)| × WeightPara1 Setpoint Deviation Reduction for Equation 5 = |(PVL - TempRampSP)| × WeightPara2

[0068] In a variant, the amount of ramp cooling reduction is based on one of the zone deviation reduction or the setpoint deviation reduction (i.e., the amount of setpoint deviation). For example, if only inter-zone drift exists, setpoint deviation reduction may not be required. Alternatively, if both deviation states exist, the variable ramp-down control may first reduce the deviation of the inter-zone drift based on the zone deviation reduction and until the inter-zone deviation is within the threshold. Thereafter, the amount of ramp cooling reduction is determined using both the zone deviation reduction and the setpoint deviation reduction provided in Equation 3. It should be readily understood that the numerical values provided herein are for illustrative purposes only and may take any suitable value.

[0069] In another form, when the temperature of at least one zone begins to deviate from other zones, the temperature ramping setpoint of the cool zone(s) is set to the measured temperature of the hot zone. That is, the variable ramp-down control increases the power to the zone with the lower temperature to raise the temperature of that zone to the temperature of the zone with the higher temperature. Thus, the variable ramp-down control can keep the temperatures of the zones together or within a deviation threshold (e.g., ±5 °C) and flatten the temperature ramping setpoint curve before the zones approach the temperature setpoint.

[0070] In the zone floating state, the variable ramp-down control determines whether the zone is floating or wandering. More specifically, as the power to the zone(s) decreases, it can become difficult to accurately measure the process value (e.g., temperature), and depending on the situation, the power can become very low such that the zone becomes uncontrollable (e.g., the power is greater than zero volts but is at a minimum power level / output insufficient to control the zone). That is, the temperature of the zone may start to deviate from the temperature ramping set point, and in the case of multiple zones, the temperature of a zone may start to deviate from another zone. To control the ramp-down during the zone floating state, the variable ramp-down control is configured to increase the power to the floating zone to a nominal power output (e.g., 2% power, 5% power) greater than the minimum power level (i.e., minimum power output) required to obtain control of the zone while reducing the temperature of the zone. In one form, the power increases by decreasing the variable ramp set point until the power is reapplied at the nominal power output. The nominal power output applied to the zone to suppress the floating state of the zone can be defined based on testing and can slightly exceed the minimum power level (e.g., the nominal power output exceeds 5V).

[0071] When one or more runaway conditions are detected, the amount of reduction is a weighted combination of the amounts of reduction for the detected deviation conditions. In one form, the weight assigned to each deviation condition can be based on which stage of the cooling process the heater is in. That is, typically, the lamp setpoint deviation occurs earlier with the cooling of the heater than the inter-zone drift that can occur as the heater cools. Thus, a higher weight is assigned to the amount of reduction associated with the lamp setpoint deviation than the amount of reduction associated with the inter-zone drift when the heater first begins to cool. After a while and / or after the temperature of the heater reaches a selected temperature setpoint that is higher than the desired temperature setpoint, the variable ramp-down control can assign a higher weight to the amount of reduction associated with the inter-zone drift than the lamp setpoint deviation. When the temperature is low, a minimum amount of power can be applied to suppress the floating state of the zone that can be prioritized over the inter-zone drift and the lamp setpoint deviation because the power to the heater may no longer be needed. Thus, the weighting factor can be assigned based on the stage of the heater during cooling and the heater itself (i.e., the responsiveness of the heater).

[0072] It should be readily understood that the variable ramp-down control can be configured to monitor one or more runaway conditions and does not need to monitor all of them. For example, in a single-zone heater, inter-zone drift is not required.

[0073] Referring to FIG. 2, an exemplary VRRT control routine 200 is provided, and the control of the temperature of the heater to one or more temperature set points is performed by the control system. At 202, the control system obtains the temperature set point of the heater from a defined state mode that provides, for example, the temperature set point and duration of the heater. At 204, it is determined whether the temperature set point is lower than the current temperature of the heater. If the temperature set point is high, the control system performs variable ramp-up control at 206. On the other hand, if the temperature is low, the control system performs variable ramp-down control at 208. When the temperature set point is reached, the control system returns to routine 200 and, at 210, uses a temperature control model (e.g., PID control) to maintain the temperature at the temperature set point and, at 212, determines whether there is a new temperature set point. If there is a new temperature set point, the control system returns to 202. In one form, the temperature set point may include the nominal set point when the heater is subsequently turned off.

[0074] Referring to FIG. 3, an exemplary variable ramp-up control 300 is provided. At 302, the control system sets a variable ramp rate to a desired ramp rate defined based on a temperature set point and / or a system current limit, and supplies power to the heater to reach 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 current measured for each zone is less than the current limit band. If so, the control system proceeds to 310. Otherwise, at 308, the control system determines a reduction factor and reduces the variable ramp rate of each zone based on the reduction factor. In particular, using the above methodology, the control system determines a reduction factor that correlates to how close the measured current is to the system current limit, reduces the variable ramp rate by the amount of reduction, and obtains an allowable ramp rate as the variable ramp rate of each zone. At 310, the control system determines whether the temperature of an adjacent zone is within a deviation threshold to maintain a coherent temperature profile of the heater. If the temperature is within the deviation threshold, the control system proceeds to 314. If at least one zone is deviated, at 312, the control system reduces the variable ramp rate of the zone having a higher temperature as described above. Alternatively, the control system may be configured to boost the power to other zones while monitoring the current of the zone. At 314, the control system determines whether the zone is at the temperature set point. If not, the control system returns to 304. If the zone is at the temperature set point, the control system returns to the routine 200 of FIG. 2.

[0075] Referring to FIG. 4, an exemplary variable ramp down control 400 is provided. At 402, the control system sets a 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 difference in temperature between zones is within a deviation threshold so as to provide a coherent temperature profile when the heater cools to the temperature set point. For example, the control system determines whether the difference in temperature between adjacent zones is greater than the deviation threshold. If the difference in temperature is within the deviation threshold, the control system proceeds to 410. If at least one zone is deviated, the control system, at 408, sets the temperature ramp set point 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), and increases the power of the cool zone to reach the new temperature ramping set point. At 410, the control system determines whether the zone is at the temperature set point. If not, the control system returns to 404. When the heater is at the temperature set point, the control system returns to routine 200 of FIG. 2.

[0076] Routines 200, 300, and 400 can be configured in various suitable ways and it should be readily understood that they should not be limited to the steps described herein. For example, if the heater is a single-zone heater, the control system can skip the steps related to providing a coherent temperature profile in routine 300 and can omit the variable ramp down routine. In another example, VRRT control can also include glide speed control and / or approach control to provide a smooth transition to the desired ramp rate and temperature set point respectively. In yet another example, in variable ramp down control, instead of setting a cooling ramp rate, the control turns off the power to the heater, monitors the temperature of the zone, and reduces the temperature that may deviate.

[0077] Figures 5A through 9 illustrate the characteristics of the VRRT control of the present disclosure. Specifically, FIG. 5A shows a ramp-up operation with a constant lamp rate (e.g., 20 °C / min), and FIG. 5B shows a ramp-up operation using the VRRT control of the present disclosure. In both cases, the current is maintained below 30 A, but the constant ramp-up rate of FIG. 5A takes longer to reach 600 °C than the VRRT control of FIG. 5B. In VRRT control, the lamp rate starts at 28 °C / min and decreases as the current approaches 30 A. That is, when the measured current enters within the current limit band (e.g., 25 - 30 A), the lamp rate decreases to control the current applied to the heater while allowing the heater to reach the temperature set point.

[0078] FIG. 6 is a graph showing a ramp-up control in which the lamp rate is controlled from the glide speed to a desired lamp rate and then, when the measured temperature approaches the temperature set point, is controlled to an approach control rate.

[0079] FIGS. 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 each other, which can cause thermal stress. In FIG. 7B, the heater has a coherent temperature profile by addressing the deviated temperatures.

[0080] FIG. 8 shows the variable ramp-down control of VRRT control, in which power is supplied to the heater at a level slightly higher than the minimum amount (e.g., 5% of the supplied power) to suppress the zone floating state. By supplying a small amount of power to the heater, the temperature of the heater is continuously monitored and decreased to the temperature set point.

[0081] FIG. 9 shows variable ramp-down control in which a runaway condition is reduced or suppressed by controlling the lamp rate and / or power. In the figure, the filament temperature representing 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 reduction in the optimal ramp process variation (PV) is the amount of reduction due to the large deviation of various zones, the optimal lamp bottom reduction is the amount of reduction due to the power obtained being too low (floating), the optimal lamp net ramp set point (SP) gain is the weighted sum of three corrective measures (for example, the net gain is a multiplier from 0.0 to 1.0 of the ramp SP for reducing the lamp rate, where 1.0 is no reduction and 0.5 is a 50% reduction), and the optimal lamp set point (SP) reduction that may spike first due to the zone(s) deviating from the lamp SP.

[0082] As used herein, the term deviation threshold can generally take various possible thresholds defined for comparing the difference between a measured value (e.g., zone temperature, heater temperature) and another value (e.g., temperature set point, temperature of another zone, etc.). In one form, in variable ramp-up control and variable ramp-down control, the deviation thresholds used to monitor inter-zone drift / deviation can be the same or different thresholds. In one form, in variable ramp-down control, the deviation thresholds for inter-zone drift and lamp set point deviation may be the same or different. Further, the deviation threshold can be provided as a single absolute value (e.g., 5 °C) or as a range (e.g., ±5 °C). The actual value of the deviation threshold is based on a particular application and is thus not limited to any specific numerical values provided herein.

[0083] Unless otherwise expressly indicated herein, all numerical values indicating mechanical / thermal properties, compositional percentages, dimensions and / or tolerances, or other properties are to be understood as being modified by the terms "about" or "approximately" when describing the scope of the present disclosure. This modification is desired for various reasons including industrial practices, materials, manufacturing, assembly tolerances, and test capabilities.

[0084] As used herein, the phrase "at least one of A, B, and C" should be interpreted to mean a logical (A OR B OR C) using non-exclusive logical OR and should not be interpreted to mean "at least one A, at least one B, and at least one C".

[0085] In the present application, the term "controller" may be replaced by the term "circuit". The term "controller" refers to, or is part of, or may include, an Application Specific Integrated Circuit (ASIC), a digital, analog, or analog / digital mixed discrete circuit, a digital, analog, or analog / digital mixed 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 executed by the processor circuit, other suitable hardware components that provide the described functionality, or some or all combinations thereof such as a system on chip.

[0086] The term "code" can refer to programs, routines, functions, classes, data structures, and / or objects that may include software, firmware, and / or microcode. The term "memory circuit" is a subset of the term "computer-readable medium". As used herein, the term "computer-readable medium" does not include transient electrical or electromagnetic signals propagated through a medium (such as a carrier wave). Thus, the term "computer-readable medium" can be considered tangible and non-transitory.

[0087] The description of the present disclosure is illustrative in nature. Accordingly, variations that do not depart from the content 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. The invention described in the original claims of the present application is appended below. [1] A method for controlling the temperature of a heater including a resistive heating element, applying power to the resistive heating element at a variable ramp rate to decrease the temperature of the heater to a desired temperature set point, wherein the variable ramp rate is set to a desired ramp rate, monitoring the temperature of the heater to detect a runaway condition, and adjusting the variable ramp rate from the desired ramp rate to an allowable ramp rate in response to detecting the runaway condition. [2] The runaway condition includes a lamp set point deviation, and the method further includes determining whether the temperature of the heater deviates from a temperature ramping set point by a set point deviation threshold, wherein the temperature ramping set point is the temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater decreases to the desired temperature set point. The method of [1]. [3] The method of [2], further comprising decreasing the variable ramp rate based on the amount of set point deviation in response to detecting the lamp set point deviation as the runaway condition. The method of [2]. [4] The runaway condition includes a zone floating state, and the method further includes determining whether the power applied to the heater is lower than a nominal power output to detect the zone floating state, where the nominal power output is greater than a minimum power output, and the minimum power output is greater than zero volts. The method of [1]. [5] The method of [4], further comprising decreasing the variable ramp rate to increase the power to the heater to the nominal power output in response to detecting the runaway condition and the runaway condition being the zone floating state. [6] The method of [1], wherein the heater includes a plurality of resistive heating elements defining a plurality of zones. [7] The method further includes monitoring the zone temperature of each of the plurality of zones, and determining, as the runaway condition, whether the difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold. further comprising adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof in response to the difference being greater than the zone shift threshold; wherein the hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone; The method of [6]. [8] wherein the runaway state includes at least one of an inter-zone shift, a lamp set point shift, a zone floating state, or a combination thereof; The method comprises: for the inter-zone shift, determining whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone shift threshold; for the lamp set point shift, determining whether the temperature of the heater is shifted from a temperature ramping set point by a set point shift threshold; wherein the temperature ramping set point is a temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater is lowered to the desired temperature set point; for the zone floating state, determining whether the power applied to the heater is lower than a nominal power output; wherein the nominal power output is greater than a minimum power output; wherein the minimum power output is greater than zero volts; further comprising; The method of [8]. [9] The method further comprises performing a corrective action in response to detecting the runaway state; in response to the runaway state being an inter-zone shift, as the corrective action, adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof, wherein the hot zone is the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone; in response to the runaway state being the lamp set point shift, as the corrective action, decreasing the variable ramp rate based on the amount of the set point shift; in response to the runaway state being the zone floating state, decreasing the variable ramp rate by increasing the power to the heater to the nominal power output; The method of [8].

[10] The runaway state includes at least two of the zone deviation, the lamp set point deviation, and the zone floating state, The method further comprises adjusting the variable ramp rate based on a weighted evaluation of the corrective measures related to the runway state. The method of [9].

[11] A control system for controlling power to a heater including a resistive heating element, The control system comprises a processor, and a non-transitory computer-readable medium including instructions executable by the processor, The instructions determine an amount of power supplied to the resistive heating element of the heater based on a variable ramp rate to reduce the temperature of the heater to a desired temperature set point; The variable ramp rate is set to a desired ramp rate, monitor the temperature of the heater to detect a runaway state; and in response to detecting the runaway state, adjust the variable ramp rate from the desired ramp rate to an allowable ramp rate. Control system.

[12] The runaway state includes a lamp set point deviation, The instructions further comprise determining whether the temperature of the heater deviates from the temperature ramp set point by a set point deviation threshold. The temperature ramp set point is the temperature at which the heater is controlled based on the variable ramp rate as the temperature of the heater decreases to the desired temperature set point. The control system of

[11] .

[13] The instructions further comprise decreasing the variable ramp rate based on the amount of set point deviation in response to detecting the lamp set point deviation as the runaway state. The control system of

[12] .

[14] The runaway state includes a zone floating state, The instructions further comprise determining whether the power applied to the heater is lower than a nominal power output to detect the zone floating state. The nominal power output is greater than a minimum power output, The minimum power output is greater than zero volts. The control system of

[11] .

[15] The instructions further comprise decreasing the variable ramp rate to increase the power to the heater to the nominal power output in response to detecting the runaway state and the runaway state being the zone floating state. The control system of

[14] .

[16] The heater includes a plurality of resistive heating elements that define a plurality of zones, The instructions, monitor the zone temperature of each of the plurality of zones, as the runaway state, determine whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold, further include adjusting the variable ramp rate of a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone deviation threshold, The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone, The control system of

[11] .

[17] The heater includes a plurality of resistive heating elements that define a plurality of zones, The runaway state includes at least one of a zone shift, a lamp set point shift, a zone floating state, or a combination thereof, The instructions, for the zone shift, determine whether a difference between a first zone temperature of a first zone from among the plurality of zones and a second zone temperature of a second zone from among the plurality of zones is greater than a zone deviation threshold, for the lamp set point shift, determine whether the temperature of the heater is shifted from a temperature ramping set point by a set point deviation threshold, The temperature ramping set point is the temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater is decreased to the desired temperature set point, further include, for the zone floating state, determining whether the power applied to the heater is lower than a nominal power output, The nominal power output is greater than a minimum power output, The minimum power output is greater than zero volts, The control system of

[11] .

[18] The instructions further include performing a corrective action in response to the runaway state being detected, In response to the runaway state being the zone deviation, the command further includes, as the corrective measure, adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof, where the hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. In response to the runaway state being the lamp set point deviation, the command further includes, as the corrective measure, reducing the variable ramp rate based on the set point deviation. In response to the runaway state being the zone floating state, the command further includes, as the corrective measure, reducing the variable ramp rate to increase the power to the heater to the nominal power output. The control system of

[17] .

[19] The runaway state includes at least two of the zone deviation, the lamp set point deviation, and the zone floating state. The command further includes adjusting the variable ramp rate based on a weighted evaluation of the corrective measures related to the runway state. The control system of

[18] .

[20] The thermal system is a heater having a plurality of resistive heating elements, wherein the plurality of resistive heating elements define a plurality of zones, and a control system of

[11] . The command further includes monitoring the zone temperature of each of the plurality of zones, determining, as the runaway state, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold, and in response to the difference being greater than the zone deviation threshold, adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof, where the hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. Thermal system.

Claims

1. A method for controlling the temperature of a heater including a resistive heating element, comprising: applying power to the resistive heating element at a variable ramp rate to decrease the temperature of the heater to a desired temperature set point; the variable ramp rate being set to a desired ramp rate; monitoring the temperature of the heater to detect a runaway condition; and adjusting the variable ramp rate from the desired ramp rate to a permissible ramp rate in response to the runaway condition being detected.

2. The runaway condition includes a lamp set point deviation; the method further comprising: determining whether the temperature of the heater deviates from a temperature ramping set point by a set point deviation threshold; the temperature ramping set point being the temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater decreases to the desired temperature set point; The method of Claim 1.

3. The method of Claim 2, further comprising decreasing the variable ramp rate based on a set point deviation amount in response to the lamp set point deviation being detected as the runaway condition. The method of Claim 2.

4. The runaway condition includes a zone floating condition; the method further comprising determining whether the power applied to the heater is lower than a nominal power output to detect the zone floating condition, the nominal power output being greater than a minimum power output, and the minimum power output being greater than zero volts; The method of Claim 1.

5. The method of Claim 4, further comprising decreasing the variable ramp rate to increase the power to the heater to the nominal power output in response to the runaway condition being detected and the runaway condition being the zone floating condition.

6. The method further comprises: monitoring the zone temperature of each of a plurality of zones, the heater including a plurality of resistive heating elements defining the plurality of zones; determining whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold as the runaway condition; In response to the difference being greater than the zone shift threshold, further comprising adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof. The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. The method of claim 1.

7. The runaway state includes at least one of a zone shift, a lamp set point shift, a zone floating state, or a combination thereof. The method comprises: For the zone shift, determining whether a difference between a first zone temperature of a first zone among a plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone shift threshold, wherein the heater includes a plurality of resistive heating elements defining the plurality of zones. For the lamp set point shift, determining whether the temperature of the heater is shifted from a temperature ramp set point by a set point shift threshold. The temperature ramp set point is a temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater is lowered to the desired temperature set point. For the zone floating state, determining whether the power applied to the heater is lower than a nominal power output. The nominal power output is greater than a minimum power output. The minimum power output is greater than zero volts. Further comprising. The method of claim 1.

8. The method further comprises implementing a corrective measure in response to detecting the runaway state. In response to the runaway state being a zone shift, as the corrective measure, adjusting the variable ramp rate of the hot zone, the cool zone, or a combination thereof, wherein the hot zone is the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. In response to the runaway state being the lamp set point shift, as the corrective measure, reducing the variable ramp rate based on the amount of the set point shift. In response to the runaway state being a zone floating state, reducing the variable ramp rate by increasing the power to the heater to the nominal power output. The method of claim 7.

9. The runaway state includes at least two of the zone deviation, the lamp set point deviation, and the zone floating state. The method Further comprising adjusting the variable ramp rate based on a weighted evaluation of the corrective measures related to the runaway state. The method of claim 8.

10. A control system for controlling the power to a heater including a resistive heating element, The control system A processor and A non-transitory computer-readable medium including instructions executable by the processor, and The instructions Determining the amount of power supplied to the resistive heating element of the heater based on a variable ramp rate to reduce the temperature of the heater to a desired temperature set point. The variable ramp rate is set to a desired ramp rate. Monitoring the temperature of the heater to detect a runaway state. In response to the runaway state being detected, adjusting the variable ramp rate from the desired ramp rate to an allowable ramp rate. Control system.

11. The runaway state includes a lamp set point deviation. The instructions Further including determining whether the temperature of the heater deviates from the temperature ramp set point by a set point deviation threshold. The temperature ramp set point is the temperature at which the heater is controlled based on the variable ramp rate when the temperature of the heater decreases to the desired temperature set point. In response to detecting the lamp set point deviation as the runaway state, the variable ramp rate is decreased based on the amount of set point deviation. The control system of claim 10.

12. The runaway state includes a zone floating state. The instructions are to determine whether the power applied to the heater is lower than the nominal power output to detect the zone floating state. The nominal power output is greater than the minimum power output. The minimum power output is greater than zero volts. Further comprising reducing the variable ramp rate to increase the power to the heater to the nominal power output in response to the runaway state being detected and the runaway state being the zone floating state. The control system of claim 10.

13. The heater includes a plurality of resistive heating elements that define a plurality of zones. The instructions monitoring the zone temperature of each of the plurality of zones; determining, as the runaway state, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold; further comprising adjusting the variable ramp rate of a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone deviation threshold. The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. The control system of claim 10.

14. The runaway state includes at least two of a zone shift, a lamp set point shift, and a zone floating state. The instructions further include adjusting the variable ramp rate based on a weighted evaluation of corrective measures related to the runaway state. The control system of claim 13.

15. A thermal system has a heater having a plurality of resistive heating elements, the plurality of resistive heating elements define a plurality of zones, and the control system of claim 10, The instructions further monitor the zone temperature of each of the plurality of zones; determine, as the runaway state, whether a difference between a first zone temperature of a first zone among the plurality of zones and a second zone temperature of a second zone among the plurality of zones is greater than a zone deviation threshold; further comprising adjusting the variable ramp rate of a hot zone, a cool zone, or a combination thereof in response to the difference being greater than the zone deviation threshold. The hot zone is one of the first zone or the second zone having a higher zone temperature, and the cool zone is the other of the first zone or the second zone. Thermal system.

Images