JP3620981B2 - Sample temperature control method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a sample temperature control method for raising or lowering a sample according to a predetermined temperature program curve like a sample of a thermal analyzer, and in particular, a low-temperature gas obtained by vaporizing a liquefied refrigerant as a means for cooling a sample. The present invention relates to a method for controlling a sample temperature using the above.
[0002]
[Prior art]
As a method of cooling the sample to a temperature lower than room temperature, there is a method of cooling the sample with a low-temperature gas obtained by vaporizing liquid nitrogen. In this case, it is possible to adjust the amount of vaporization by heating the heater provided in the liquid nitrogen container to adjust the flow rate of the low-temperature gas, thereby adjusting the sample cooling capacity. When a large electric power is supplied to the liquid nitrogen heater, vaporization is promoted and the sample temperature is drastically lowered. When a smaller electric power is applied, the sample temperature is gradually lowered.
[0003]
When the sample is cooled in the thermal analyzer, the sample is cooled, for example, at a constant temperature decrease rate according to a predetermined temperature program curve. However, such precise control is not possible with low-temperature gas cooling from liquid nitrogen, so in addition to cooling with low-temperature gas, feedback control using heating by the heater of the electric furnace in the sample chamber is used in combination. .
[0004]
JP-A-2-105046 discloses a conventional example in which the temperature of a sample is controlled by using both the cooling of the sample by the power control of the liquid nitrogen heater and the heating of the sample by the power control of the electric furnace heater. The following technique (hereinafter referred to as the first conventional technique) is known. In this first prior art, a preliminary experiment is used to determine the power supplied to the liquid nitrogen heater. That is, a constant electric power is supplied to the liquid nitrogen heater, the sample arrival temperature after a predetermined time is measured, and the relationship between the electric power and the sample arrival temperature is experimentally obtained in advance for a plurality of electric powers. If such a preliminary experiment is performed, when an ultimate temperature lower than room temperature is set as the temperature program signal, the supply power corresponding to this ultimate temperature can be obtained from the preliminary experiment result. What is necessary is just to supply this electric power to a liquid nitrogen heater. If it does so, sample temperature will fall to ultimate temperature after predetermined time.
[0005]
In this first prior art, when an ultimate temperature lower than room temperature is set, only one power supply of the liquid nitrogen heater corresponding to this is determined. When the sample is cooled by vaporizing liquid nitrogen with the power supply determined in this way, and the temperature of the sample is feedback controlled using an electric furnace heater, the temperature of the sample can be controlled according to a predetermined temperature program curve. . However, if this method is used to cool the sample to the above-mentioned temperature at a “constant temperature-decreasing rate”, the cooling capacity is insufficient during cooling.
[0006]
As a method for resolving the lack of cooling capacity of the first conventional technique, a conventional technique (hereinafter referred to as a second conventional technique) disclosed in Japanese Patent Laid-Open No. 9-196871 has been developed. In the second prior art, the power of the liquid nitrogen heater is changed to a necessary and sufficient value at any time based on the sample temperature at each time and the temperature lowering rate, thereby eliminating the lack of cooling capacity. It also prevents unnecessary power consumption.
[0007]
[Problems to be solved by the invention]
The second prior art described above functions very effectively when the sample is cooled at a constant temperature-decreasing rate, but control to keep the sample at a constant temperature (hereinafter referred to as isothermal holding control). As a result, it was found that the temperature stability was poor. For example, when there is an isothermal control region at the beginning of the temperature program curve, there is a problem that the holding temperature fluctuates. The cause of this wobbling is that the power of the liquid nitrogen heater is determined depending on the temperature change rate (differential value of temperature) at each time.
[0008]
The present invention has been made to solve the above-described problems of the second prior art, and an object thereof is to generate a low-temperature gas by heating and vaporizing a liquefied refrigerant with a first heater, and using the low-temperature gas as a sample. In the sample temperature control method for feedback control of the temperature of the sample chamber by heating the second heater provided in the electric furnace of the sample chamber while being introduced into the chamber, the temperature control is maintained by devising the voltage control of the first heater. It is an object of the present invention to provide a sample temperature control method capable of following a sample temperature with high accuracy for any temperature program curve including control.
[0009]
[Means for Solving the Problems]
According to the present invention, a liquefied refrigerant is heated and vaporized by a first heater to generate a low-temperature gas, the low-temperature gas is introduced into the sample chamber, and a second heater provided in an electric furnace of the sample chamber is heated to thereby sample the sample chamber. In the sample temperature control method for feedback control of the temperature, the following control steps (e) to (g) are performed simultaneously after the following preliminary steps (a) to (d). (A) Supplying a constant voltage to the first heater without heating the second heater to introduce the low temperature gas into the sample chamber, and showing the relationship between the elapsed time at that time and the temperature of the sample chamber A step of measuring a curve (hereinafter referred to as a constant voltage cooling curve). (B) changing the value of the constant voltage to obtain a plurality of the constant voltage cooling curves; (C) A step of measuring the temperature of the sample chamber (hereinafter referred to as intermediate attainment temperature) when the slope reaches a predetermined set value for each of the plurality of constant voltage cooling curves. (D) A step of obtaining a combination of a voltage and the intermediate ultimate temperature for a plurality of the constant voltage cooling curves and obtaining a correspondence relationship between the voltage and the temperature based on the combination. (E) A step of obtaining a voltage of the first heater (hereinafter referred to as a predicted voltage) by applying the temperature of the sample chamber at each time to the correspondence relationship. (F) A step of increasing or decreasing the predicted voltage based on a deviation between the temperature of the sample chamber at each time and the target temperature at each time. (G) A step of controlling the voltage of the second heater based on the deviation between the temperature of the sample chamber at each time and the target temperature at each time.
[0010]
In the control step (e) described above, in determining the voltage of the first heater, first, the voltage is subjected to the predictive control based on the temperature of the sample chamber at each time. Since this control does not depend on the deviation between the target temperature (temperature on the temperature program curve) at each time and the temperature of the sample chamber, the control method is open control. In this advance prediction control, the first heater voltage suitable for the temperature of the sample chamber at that time is set in advance regardless of the deviation between the target temperature and the temperature of the sample chamber at each time. Will be.
[0011]
In the control step (f) described above, the voltage increase / decrease amount by the feedback control of the first heater system is added to the predicted voltage obtained in the control step (e). The control stage (f) is feedback control as a control method, and is follow-up control based on a deviation from the target temperature. By adding the feedback control of (F) to the preceding prediction control of (E), the advance control and the follow-up control are used together. There is a defect that cannot remove the steady-state deviation only by the advance prediction control, and there is a defect by the follow-up correction control only by the feedback control. Therefore, the optimum voltage of the first heater is determined by combining the two control defects so as to complement each other.
[0012]
By the way, in the preceding predictive control (e), the target temperature at each time may be used instead of the temperature of the sample chamber at each time. In the advance predictive control, the temperature dependence of the predicted voltage to be used is not so great, and there is no great difference in the control capability regardless of which of the sample chamber temperature and the target temperature is used.
[0013]
While the control steps (e) and (f) described above are voltage control of the first heater, the control step (g) is voltage control for the second heater. By finally feedback controlling the temperature of the sample chamber using this second heater (heater of the electric furnace), the sample temperature can be made to follow the target temperature with high accuracy.
[0014]
The “temperature of the sample chamber” used in the above steps (a), (c), (e), (f), and (g) is the temperature detected by a temperature sensor (for example, a thermocouple) arranged as close as possible to the sample. (This will be referred to as a sample temperature.) Or a temperature detected by a temperature sensor (for example, a thermocouple) disposed on the wall of the electric furnace (this will be referred to as an electric furnace temperature). May be used. However, in the three stages (a), (c), and (e), which are related to the use of the predicted voltage, either the sample temperature or the electric furnace temperature is used as the “sample chamber temperature”. There is a need. In general, when the temperature of the sample chamber is feedback controlled using the second heater (electric furnace heater), the temperature control becomes smoother when the electric furnace temperature is used. It is preferable to use the furnace temperature. In the control of the first heater (liquefied refrigerant heater) system, it is considered that there is no big difference regardless of whether the sample temperature or the electric furnace temperature is used.
[0015]
Further, in the step (e) described above, instead of “applying the temperature of the sample chamber at each time to the correspondence”, “the target temperature at each time may be applied to the correspondence”.
[0016]
In the above-mentioned stage (c), the meaning of the intermediate reached temperature is as follows. When a constant voltage cooling curve is measured by supplying a constant voltage to the first heater and cooling the sample with a low-temperature gas, the slope of the curve approaches zero as long as a long time elapses. The temperature of the sample chamber settles to a substantially constant value (hereinafter referred to as the final reached temperature). This final reached temperature depends on the voltage applied to the first heater. If the correspondence between the final temperature and the voltage is obtained in advance, the voltage of the first heater suitable for the temperature of the sample chamber at that time can be obtained from the above correspondence during the actual temperature control of the sample. it can. However, when such a correspondence is used, a “just-fit” voltage that can finally reach the sample temperature at that time is obtained. In this case, the voltage used for the advance prediction control is insufficient. This is because the voltage supplied to the first heater is slightly higher, so that the sample temperature tends to be slightly lower than the target temperature, and the amount is compensated for by heating the sample with the second heater, thereby enabling feedback control. This is because it is important to make it function. Therefore, it is appropriate to use the intermediate ultimate temperature instead of using the final ultimate temperature. This intermediate temperature can be defined as the temperature of the sample chamber at the time when the slope of the constant voltage cooling curve reaches a predetermined set value. What value to set the slope becomes a real problem, but according to the researches of the inventors, the set value is a numerical value within the range of minus 0.1 ° C. per minute to minus 3 ° C. per minute. Is appropriate.
[0017]
Note that when “power control” of the first heater and the second heater is actually performed, the “applied voltage” is controlled as a control amount in the control circuit. Therefore, in the present invention, voltage control is used as an expression in each control stage. Eventually, the power supplied to the heater is controlled.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a configuration diagram of an example of a thermal analyzer that performs the sample temperature control method of the present invention. This thermal analyzer is an example of a differential scanning calorimeter, and a cylindrical electric furnace 12 is arranged inside a cylindrical sample chamber 10. The internal space of the sample chamber 10 is partitioned by a cylindrical partition wall 14, and low temperature gas is introduced into an annular space 16 between the partition wall 14 and the inner wall surface of the sample chamber 10. The outer peripheral surface of the electric furnace 12 is in close contact with the inner wall surface of the central portion of the partition wall 14 in the height direction. A sample container and a reference container are placed on a soaking plate fixed to the inner wall of the electric furnace 12. A second heater 18 is embedded in the electric furnace 12. The second heater 18 is connected to a second heater power source 19. The sample temperature is measured with a thermocouple 20 bonded to a soaking plate. Further, the temperature of the electric furnace 12 is measured by a thermocouple 22 embedded in the furnace body. The second heater power source 19 and the thermocouples 20 and 22 are connected to the temperature control device 24.
[0019]
Liquid nitrogen 28 is accommodated in the liquid nitrogen container 26. A first heater 30 is disposed inside the liquid nitrogen container 26. When a voltage is applied to the first heater 30 from the first heater power source 32, the heater 30 is heated, the vaporization of the liquid nitrogen 28 is promoted, and the amount of low-temperature gas generated increases. A low temperature gas pipe 34 is connected above the liquid nitrogen container 26, and the low temperature gas pipe 34 is connected to the annular space 16 of the sample chamber 10. The low temperature gas that has entered the annular space 16 of the sample chamber 10 from the low temperature gas pipe 34 fills the annular space 16, cools the partition wall 14 and the electric furnace 12, and exits from the outlet 36.
[0020]
FIG. 2 is a block diagram showing an example of a signal flow in the sample temperature control method of the present invention. A voltage command is output from the temperature control device 24 to the first heater power source and the second heater power source. The temperature controller 24 receives the electric furnace temperature Tf detected by the thermocouple 22 shown in FIG. 1 and the sample temperature Ts detected by the thermocouple 20 shown in FIG. A desired temperature program curve can be set in the temperature program setter, and a target temperature (temperature on the temperature program curve) at each time point is output from the temperature program setter during temperature control.
[0021]
First, the control system of the second heater will be described. In the comparator 38, a deviation ΔT = Tr−Tf between the target temperature Tr output from the temperature program setter and the electric furnace temperature Tf is calculated. This deviation ΔT is input to the PID controller 40 for the second heater. The PID controller 40 outputs a voltage command to the second heater power source in such a direction that the deviation ΔT becomes zero. For example, if only proportional control (P component) of PID control is described as an example, if ΔT is positive (the electric furnace temperature Tf is lower than the target temperature Tr), it is proportional to ΔT from the PID controller 40. A voltage command is output to the second heater power supply. The voltage command increases as ΔT increases. When ΔT becomes negative, the voltage command for the second heater power supply becomes zero. In this way, feedback control is performed so that the electric furnace temperature Tf follows the target temperature Tr.
[0022]
Next, the control system of the first heater will be described. The prediction controller outputs a prediction voltage command Ea based on the sample temperature Ts. The predicted voltage command Ea will be described later. On the other hand, the above-described deviation ΔT is input to the PID controller 42 for the first heater. Then, the voltage command Eb is output to the first heater power source in the direction in which the deviation ΔT becomes zero. For example, if only the proportional component of the PID control is described as an example, the PID controller 42 outputs a voltage command Eb having an absolute value proportional to ΔT and having an opposite sign. That is, when ΔT is positive, the voltage command Eb is negative, and when ΔT is negative, the voltage command Eb is positive. That is, Eb = −k · ΔT (where k is a positive proportionality coefficient). The adder 44 adds the voltage command Eb from the PID controller 42 for the first heater to the voltage command Ea from the prediction controller. That is, the predicted voltage Ea predicted by the prediction controller is increased or decreased by the voltage command Eb based on the deviation ΔT. The voltage command Ec (= Ea + Eb) thus obtained is input to the delay element. This delay element is a first-order delay filter (digital filter) connected in one to four stages, and delays and outputs the voltage command Ec. In this embodiment, four stages of first-order lag filters are connected so that the delay time constant is about 3 seconds. The output of this delay element finally becomes the voltage command for the first heater.
[0023]
In this embodiment, the PID controller 42 for the first heater performs the following proportional control. The above-mentioned proportionality coefficient k = 1.5 V / ° C. from the start of the temperature control of the sample to the first one and a half minutes (when the ejection of the vaporized gas starts), and after that, k = 0 .6 V / ° C.
[0024]
Although there are two PID controllers 40 and 42 in FIG. 2, not all of the proportional component (P), integral component (I), and differential component (D) are required in these PID controllers. If necessary, one or two of these components may be adopted and controlled. In particular, the PID controller 42 for the first heater does not require as strict control as the PID controller 40 for the second heater (temperature control with vaporized gas cannot provide highly accurate feedback control in the first place). In addition, the PID controller 42 may perform only proportional control.
[0025]
Next, the function of the prediction controller will be described. FIG. 3 is a graph of a constant voltage cooling curve, in which the horizontal axis represents time, and the vertical axis represents sample temperature (or electric furnace temperature). This constant voltage cooling curve is obtained by measuring a time change when the sample temperature is lowered by supplying a constant voltage to the first heater after the sample temperature is set to 200 ° C. The voltage of the second heater is off. In this embodiment, constant voltage cooling curves are measured for five types of voltages, and each voltage is E1 = 20V, E2 = 40V, E3 = 60V, E4 = 80V, E5 = 100V. When each voltage is applied to the first heater, currents of 1A, 2A, 3A, 4A, and 5A flow through the first heater, and the electric power supplied to the first heater is 20 W, 80 W, 180 W, 320 W, and 500 W, respectively. It becomes.
[0026]
In each curve, the temperature at which the slope reaches α (the temperature at a location in contact with the line segment of the slope α may be obtained; hereinafter referred to as the intermediate reached temperature) is obtained. For example, when α = −2 ° C./min is set, the intermediate temperature T1 = 20 ° C. is obtained in the curve of the voltage E1. When the intermediate temperature T2 to T5 is obtained for constant voltage cooling curves of other voltages E2 to E5 with the same slope α, T2 = −64 ° C., T3 = −109 ° C., T4 = −130 ° C., T5 = −145 It becomes ℃. Thus, intermediate temperatures T1 to T5 at the voltages E1 to E5 were obtained. In order to measure the constant voltage cooling curve, the sample temperature is raised to about 200 ° C. and then cooled. However, if the temperature is higher than 200 ° C., the temperature reaches the intermediate reached temperature no matter which temperature is cooled. There is not much difference.
[0027]
FIG. 4 is a graph in which the correspondence relationship between the voltage and the intermediate temperature reached in the constant voltage cooling curve in FIG. 3 is plotted on the coordinate axis of the temperature on the horizontal axis and the voltage on the vertical axis. This is called a predicted voltage curve. The black circles on this graph plot the combinations of the above five points (E1, T1) to (E5, T5). These points are connected by a smooth curve. In this embodiment, the voltage E is connected by an approximate curve that is a cubic function of the temperature T. That is, if this graph is expressed by E = f (T), the function form of f is a cubic expression of the temperature T. Strictly speaking, a quartic expression of T is required to pass completely through five points, but not so much strictness is required, so a curve approximation of about a cubic expression is sufficient. Even if a larger number of combinations of (E, T) are measured and plotted, it is realistic to approximate them with a curve of about a cubic equation. Of course, to obtain the predicted voltage curve, a polygonal line approximation (becomes linear interpolation) may be employed, or an arbitrary multi-order function of a quadratic function or higher may be employed.
[0028]
Up to the generation of the predicted voltage curve is a preliminary stage. In order to perform actual sample temperature control, this predicted voltage curve prepared in advance is used. That is, the prediction controller of FIG. 2 stores the above-described prediction voltage curve, and can calculate the prediction voltage Ea from the sample temperature Ts at that time as shown in FIG. The predicted voltage Ea represents a voltage slightly higher than the first heater voltage necessary for obtaining the sample temperature Ts at that time. In other words, the value is a little higher than the required voltage. The degree of the addition is determined by the inclination α when determining the intermediate temperature in FIG. 3 described above. Increasing the slope α increases the additional voltage, and decreasing the slope α decreases the additional voltage. In this embodiment, −2 ° C./min is adopted as α. However, practically, any value in the range of about −0.1 ° C./min to −3 ° C./min is sufficient. is there.
[0029]
Next, the operation of the block diagram of FIG. 2 will be described with specific numerical examples. In FIG. 2, the target temperature Tr on the temperature program curve is −120 ° C. at a certain time (a certain point when one and a half minutes have passed since the temperature control started), and both the electric furnace temperature Tf and the sample temperature Ts are Assume that it was −109 ° C. In the prediction controller, the sample temperature Ts = −109 ° C. is applied to the prediction voltage curve of FIG. 4 to obtain the first heater voltage Ea = 60V. On the other hand, in the PID controller 42 for the first heater, Eb = −k · ΔT = −0.6 × (−11) = 6.6V is obtained based on the deviation ΔT = Tr−Ts = −11 ° C. . Therefore, Ec = Ea + Eb = 66.6V is obtained in FIG. This voltage is applied to the first heater power source via a delay element. In the PID controller 40 for the second heater power supply, since the deviation ΔT = Tr−Ts = −11 ° C. is negative, the voltage command for the second heater power supply is zero.
[0030]
FIG. 5 is a block diagram of the second embodiment of the present invention. FIG. 5 differs from FIG. 2 in that the electric furnace temperature is input to the prediction controller instead of the sample temperature. FIG. 6 is a block diagram of the third embodiment of the present invention. FIG. 6 differs from FIG. 2 in that the target temperature is input to the prediction controller instead of the sample temperature. FIG. 7 is a block diagram of the fourth embodiment of the present invention. FIG. 7 differs from FIG. 2 in that a deviation between the target temperature Tr and the sample temperature Ts is input to the PID controller 42 for the first heater.
[0031]
The present invention is not limited to the above-described embodiment, and the following modifications are possible. (1) Although an example of a differential scanning calorimeter is shown in FIG. 1, the present invention can be applied to a sample temperature control method in other thermal analyzers, and can also be applied to sample temperature control other than thermal analysis. (2) Liquid nitrogen is optimal as the liquefied refrigerant, but other liquefied refrigerants may be used.
[0032]
【The invention's effect】
In the sample temperature control method of the present invention, the voltage applied to the first heater (liquefied refrigerant heater) is obtained by using both the preceding prediction control and the feedback control, so that highly accurate temperature control is possible. As a result, stable control can be realized not only in constant speed temperature rise and constant temperature drop temperature control but also in isothermal holding control.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an example of a thermal analysis apparatus that performs a sample temperature control method of the present invention.
FIG. 2 is a block diagram showing an example of a signal flow in the sample temperature control method of the present invention.
FIG. 3 is a graph of a constant voltage cooling curve.
FIG. 4 is a graph of a predicted voltage curve in which a correspondence relationship between a voltage and an intermediate reached temperature in a constant voltage cooling curve is plotted.
FIG. 5 is a block diagram of a second embodiment of the present invention.
FIG. 6 is a block diagram of a third embodiment of the present invention.
FIG. 7 is a block diagram of a fourth embodiment of the present invention.
[Explanation of symbols]
10 Sample Chamber 12 Electric Furnace 18 Second Heater 19 Second Heater Power Supply 20 Sample Temperature Thermocouple 22 Electric Furnace Temperature Thermocouple 24 Temperature Control Device 26 Liquid Nitrogen Container 28 Liquid Nitrogen 30 First Heater 32 First Heater Power Supply 38 Comparison 44 Adder

Claims (6)

The liquefied refrigerant is heated and vaporized by the first heater to generate a low temperature gas, and the low temperature gas is introduced into the sample chamber, and the second heater provided in the electric furnace of the sample chamber is heated to feed back the temperature of the sample chamber. In the sample temperature control method to be controlled, the sample temperature control method is characterized in that the control steps (e) to (g) are simultaneously performed after the following preliminary steps (a) to (d).
(A) A constant voltage is applied to the first heater without heating the second heater to introduce the low temperature gas into the sample chamber, and the relationship between the elapsed time and the temperature of the sample chamber is shown. A step of measuring a curve (hereinafter referred to as a constant voltage cooling curve).
(B) changing the value of the constant voltage to obtain a plurality of the constant voltage cooling curves;
(C) A step of measuring the temperature of the sample chamber (hereinafter referred to as intermediate attainment temperature) when the slope reaches a predetermined set value for each of the plurality of constant voltage cooling curves.
(D) A step of obtaining a combination of a voltage and the intermediate ultimate temperature for a plurality of the constant voltage cooling curves and obtaining a correspondence relationship between the voltage and the temperature based on the combination.
(E) A step of obtaining a voltage of the first heater (hereinafter referred to as a predicted voltage) by applying the temperature of the sample chamber at each time to the correspondence relationship.
(F) A step of increasing or decreasing the predicted voltage based on a deviation between the temperature of the sample chamber at each time and the target temperature at each time.
(G) A step of controlling the voltage of the second heater based on the deviation between the temperature of the sample chamber at each time and the target temperature at each time. 2. The method according to claim 1, wherein the sample temperature is used as the temperature of the sample chamber in the step (e), and the electric furnace temperature is used as the temperature of the sample chamber in the steps (f) and (g). A method characterized by. 2. The method according to claim 1, wherein the sample temperature is used as the temperature of the sample chamber in the steps (e) and (f), and the electric furnace temperature is used as the temperature of the sample chamber in the step (g). A method characterized by. The method according to any one of claims 1 to 3, wherein the voltage obtained in the step (f) is supplied to the first heater via a delay element. 5. The method according to claim 1, wherein the setting value of the slope in the step (c) is set to a value within a range of minus 0.1 ° C./min to minus 3 ° C./min. A method characterized by that. The method of claim 1, a method according to feature the use of "target temperature at each time" instead of the "temperature of the sample chamber at each time" the in step of (e).

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