JP2013019870A - Mirror cooling dew-point meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mirror cooling dew-point meter which is capable of appropriately correcting a PID constant even if various measurement conditions varies with time.SOLUTION: A mirror cooling dew-point meter 100 comprises a control unit 60 which calculates a manipulation value MV to a temperature control unit 20 to adjust a measured light quantity PV of irradiation light received by a light receiving unit 50 to be a target light quantity SV under PID control, and controls the temperature adjustment unit 20 on the basis of the manipulation value MV. If a result obtained by dividing an amplitude of the manipulation value MV with a proportional gain Kof the PID control is larger than a predetermined amplitude upper limit, the control unit 60 decreases the proportional gain K, and if the result obtained by dividing the amplitude of the manipulation value MV with the proportional gain Kis smaller than a predetermined amplitude lower limit, the control unit 60 increases the proportional gain K.

Description

  The present invention relates to a mirror-cooled dew point meter that cools a mirror surface and measures the dew point temperature.

  The mirror-cooled dew point meter places a reflecting mirror in the gas to be measured, cools the reflecting mirror to cause condensation on the surface, and measures the temperature of the reflecting mirror at this time, thereby measuring the dew point of the gas to be measured. It is a device that measures temperature. Further, when the dew point temperature is continuously measured, it is necessary to maintain a state in which condensation has started while adjusting the temperature of the reflecting mirror. This control is usually performed by PID control. The PID constants of “proportional gain”, “integration time”, and “derivative time” used for PID control are preferably corrected according to the state of the gas to be measured and the state of the sensor in order to maintain high measurement accuracy. .

  In Patent Document 1, a specular cooling type dew point meter is proposed in which a calculation formula for obtaining an appropriate PID constant is created in advance, and the PID constant is calculated by substituting the air temperature or provisional dew point temperature into the calculation formula. . In addition, although not in the technical field of specular cooling dew point meter, Patent Document 2 proposes a PID adjuster having a so-called auto-tuning function for obtaining an appropriate PID constant from a response result when a step signal is added to the system. ing.

JP 2006-126097 A JP-A-11-161301

  However, in the mirror-cooled dew point meter of Patent Document 1, conditions other than the air temperature and the dew point temperature, for example, the gas to be measured itself (the gas to be measured is limited to air in Patent Document 1), gas pressure, and temperature control It is not possible to cope with changes in conditions such as deterioration of parts and changes in flow rate. Further, the correction by the auto tuning of the cited document 2 needs to be performed separately from the measurement of the dew point temperature. When the measurement condition changes every moment, the PID constant cannot be corrected corresponding to the change.

  Accordingly, an object of the present invention is to provide a mirror-cooled dew point meter capable of appropriately correcting the PID constant even when various measurement conditions change every moment.

  A mirror-cooled dew point meter according to an embodiment of the present invention includes a reflecting mirror disposed in a gas to be measured, a temperature control unit that cools the reflecting mirror to cause condensation on the surface, and the temperature of the reflecting mirror. A temperature measuring unit to measure, a light projecting unit that emits irradiation light on the surface of the reflecting mirror, a light receiving unit that receives the irradiation light reflected by the surface of the reflecting mirror, and the light receiving unit receives light by PID control A control unit that calculates an operation amount for the temperature control unit so that a measurement light amount of the irradiation light becomes a target light amount, and controls the temperature control unit based on the operation amount, and the control unit includes the operation unit If the amplitude of the amount divided by the proportional gain of the PID control is larger than a predetermined amplitude upper limit value, the proportional gain is decreased, and the amplitude of the manipulated variable divided by the proportional gain is the predetermined amplitude. If it is less than the lower limit, increase the proportional gain. Make.

  According to such a configuration, instead of directly obtaining the proportional gain based on the change in each measurement condition, the proportional gain is set to an appropriate value based on the change in the operation amount that reflects all the changes in each measurement condition. It can be corrected. For this reason, even when various measurement condition changes including a change in an unexpected condition occur, it can be corrected to an appropriate proportional gain. Furthermore, since the proportional gain can be corrected while measuring the dew point temperature, even when the measurement condition changes every moment, the proportional gain can be corrected according to the change.

  In the above mirror-cooled dew point meter, both the amplitude upper limit value and the amplitude lower limit value may be fixed values.

  In the mirror-cooled dew point meter, when the phase difference between the operation amount and the measured light amount is larger than a phase upper limit value, the control unit decreases the integration time and the differentiation time of the PID control, When the phase difference is smaller than the phase lower limit value, the integration time and the differentiation time may be increased.

  In the mirror-cooled dew point meter, the differential time may be set to be 1/6 of the integration time.

  Moreover, in said mirror surface cooling-type dew point meter, you may comprise so that the said phase lower limit may be 30-40 degree | times and the said phase upper limit may be 140-150 degree | times.

  As described above, according to the mirror-cooled dew point meter according to the present invention, the PID constant can be appropriately corrected even when various measurement conditions change every moment.

It is a block diagram of the specular cooling type dew point meter which concerns on embodiment of this invention. 5 is a flowchart illustrating a method for setting a proportional gain according to an embodiment of the present invention. It is the conceptual diagram which showed the change of the operation amount which concerns on embodiment of this invention, and the change of the measurement light quantity. 5 is a flowchart illustrating a method for setting integration time and differentiation time according to an embodiment of the present invention. It is the figure which showed the change of the measured value of dew point temperature when correction of the PID constant which concerns on embodiment of this invention is not performed, and when it is performed.

  Embodiments according to the present invention will be described below with reference to the drawings. Below, the same code | symbol is attached | subjected to the element which is the same or it corresponds through all the drawings, and the overlapping description is abbreviate | omitted.

(Structure of mirror-cooled dew point meter)
First, the structure of the mirror-cooled dew point meter 100 will be described with reference to FIG. FIG. 1 is a block diagram of a mirror-cooled dew point meter 100. As shown in FIG. 1, the mirror-cooled dew point meter 100 includes a reflecting mirror 10, a temperature control unit 20, a temperature measuring unit 30, a light projecting unit 40, a light receiving unit 50, and a control unit 60. Yes.

  The reflecting mirror 10 is a mirror that reflects the irradiation light emitted from the light projecting unit 40. The reflecting mirror 10 is thin and has a flat surface. Further, the reflecting mirror 10 is disposed inside the measurement container 11. The measurement container 11 is formed with an inlet 12 through which the measurement gas 101 flows and an outlet 13 through which the measurement gas 101 flows out. That is, the measurement container 11 is configured such that the gas to be measured 101 flows through the measurement container 11. As a result, the reflecting mirror 10 is disposed in the measured gas 101 and is exposed to the measured gas 101. The gas to be measured 101 is not particularly limited, and may be a gas other than air.

  The temperature adjustment unit 20 is a part that cools the reflecting mirror 10. By cooling the reflecting mirror 10, moisture in the measurement target gas 101 can be condensed on the surface of the reflecting mirror 10. The temperature adjustment unit 20 is disposed below the reflecting mirror 10. Although the temperature control part 20 which concerns on this embodiment uses the Peltier device, you may use the other apparatus which has a cooling function. A Peltier element is an element that utilizes the “Peltier effect” in which heat is transferred from one metal to the other when a current is passed through a junction of two types of metal. The amount of heat absorption can be changed by adjusting the current value of the current flowing through the Peltier element. Moreover, if the direction of the current passed through the Peltier element is changed, the cooling side and the heat dissipation side can be interchanged. That is, the reflecting mirror 10 can be heated. The temperature adjustment unit 20 is controlled by the control unit 60. When a signal about the operation amount MV of the temperature adjustment unit 20 is transmitted from the control unit 60, the drive circuit 21 receives this signal, and the drive circuit 21 supplies a current corresponding to the operation amount MV to the temperature adjustment unit 20. Supply.

  The temperature measuring unit 30 is a part that measures the temperature of the reflecting mirror 10. The temperature measuring unit 30 is attached to the back side of the reflecting mirror 10. However, since the reflecting mirror 10 of this embodiment is thin, the temperature measuring unit 30 substantially measures the surface temperature of the reflecting mirror 10. Note that the temperature measuring unit 30 is not limited to the one that contacts the back surface of the reflecting mirror 10 and measures the temperature, but may measure the surface temperature of the reflecting mirror 10 in a non-contact manner using, for example, infrared rays. . The temperature measuring unit 30 transmits a signal about the measured temperature to the temperature display unit 31 and also transmits to a PID constant setting unit 62 of the control unit 60 described later. In the temperature display unit 31, the temperature of the reflecting mirror 10 is displayed based on the signal transmitted from the temperature measuring unit 30. As will be described later, the temperature displayed on the temperature display unit 31 is the dew point temperature of the gas 101 to be measured.

  The light projecting unit 40 is a part that emits irradiation light to the surface of the reflecting mirror 10. The light projecting unit 40 is disposed obliquely above the reflecting mirror 10 and is configured to irradiate irradiation light obliquely with respect to the surface of the reflecting mirror 10. Although the light projecting unit 40 according to this embodiment uses an LED, another device that emits light may be used. Further, the light projecting unit 40 is controlled so that the amount of irradiation light emitted therefrom is constant. In order to make the amount of irradiation light irradiated from the light projecting unit 40 constant, for example, the irradiated irradiation light is optically branched, and the amount of light of the branched light is monitored, and the amount of light becomes constant. The light projecting unit 40 may be controlled as described above.

  The light receiving unit 50 is a part that receives irradiation light reflected by the surface of the reflecting mirror 10. The light receiving unit 50 is disposed obliquely above the reflecting mirror 10 and at a position facing the light projecting unit 40. For the light receiving unit 50, for example, a phototransistor or a photodiode can be used. Further, an imaging camera may be used as the light receiving unit 50. In the present embodiment, a regular reflection light detection method in which the light receiving unit 50 receives regular reflection light of the irradiation light emitted from the light projecting unit 40 is employed. However, you may employ | adopt the irregular reflection light detection system in which the light-receiving part 50 receives the irregular reflection light of the irradiation light irradiated from the light projection part 40. FIG. The light receiving unit 50 is configured to transmit a signal regarding the light amount (measured light amount) PV of the received irradiation light to the amplification circuit 51, and the amplification circuit 51 amplifies this signal and transmits it to the control unit 60. When the light receiving unit 50 is an imaging camera, the measured light quantity PV can be obtained by performing image processing.

  The control unit 60 is a part that controls the temperature control unit 20 based on the light amount (measured light amount) PV of the irradiation light received by the light receiving unit 50. As described above, when the reflecting mirror 10 is cooled, condensation occurs on the surface thereof. When dew condensation occurs on the surface of the reflecting mirror 10, the amount of irradiation light reaching the light receiving unit 50 is lower than when no dew condensation occurs. That is, the temperature of the reflecting mirror 10 (the temperature displayed on the temperature display unit 31) when the measured light quantity PV in the light receiving unit 50 starts to decrease becomes the dew point temperature of the gas 101 to be measured. In the case of the irregular reflection light detection method, if condensation occurs on the surface of the reflecting mirror 10, the amount of irradiation light that is irregularly reflected increases. Therefore, the reflecting mirror when the measured light amount PV in the light receiving unit 50 starts to increase. A temperature of 10 is the dew point temperature.

  Furthermore, if the state when the dew condensation starts is maintained, the display temperature of the temperature display unit 31 always indicates the dew point temperature of the gas 101 to be measured. In order to realize this, the value when the measured light amount PV starts to decrease is stored as the target light amount SV, and the temperature adjustment unit 20 may be adjusted so that the target light amount SV and the measured light amount PV coincide. More specifically, when the measured light quantity PV is larger than the target light quantity SV, the cooling intensity of the temperature control unit 20 is increased so that the temperature of the reflecting mirror 10 is lowered, and when the measured light quantity PV is smaller than the target light quantity SV. The cooling intensity of the temperature control unit 20 may be lowered so that the temperature of the reflecting mirror 10 increases.

Here, the control unit 60 includes a CPU or the like, and includes a PID control unit 61 and a PID constant setting unit 62 as functional means. Among these, the PID control unit 61 calculates the operation amount MV of the temperature adjusting unit 20 so that the above-described control, that is, the measured light amount PV coincides with the target light amount SV, and the temperature control is performed based on the operation amount MV. This is a part that controls the unit 20. The PID control unit 61 calculates the operation amount MV using the following equation (1) which is a basic equation of PID control. In the equation (1), “K _P ” indicates a proportional gain, “T _I ” indicates an integration time, and “T _D ” indicates a differentiation time. As described above, these three constants are collectively referred to as “PID constants”. Further, “e” in the equation (1) is a difference (error) between the target light amount SV and the measured light amount PV as shown in the equation (2).

  The PID constant setting unit 62 of the control unit 60 is a part that sets the PID constant to an appropriate value and corrects it if necessary. According to the control by the PID control unit 61, since the cooling strength of the temperature control unit 20 is increased or decreased as described above, the operation amount MV changes with a constant fluctuation width. How the PID constant is set also affects the amplitude of the manipulated variable MV. For example, if the PID constant is not appropriate, the amplitude of the manipulated variable MV may become too large. In this case, the fluctuation width of the temperature of the reflecting mirror 10 (the fluctuation width of the measured dew point temperature) is also increased, and high-precision measurement cannot be performed. On the contrary, the fluctuation range of the manipulated variable MV may become too small. In this case, if a disturbance occurs in the signal indicating the measurement light quantity PV and the value of the manipulated variable MV fluctuates greatly, it takes time to converge, and even in this case, highly accurate measurement cannot be performed. That is, appropriately setting the PID constant avoids the above-described state and leads to highly accurate measurement. Hereinafter, a method for setting the PID constant by the PID constant setting unit 62 will be described.

(Proportional gain setting method)
First, referring to FIGS. 2 and 3, the procedure for setting the proportional gain K _P of the PID constants. Figure 2 is a flowchart illustrating a method of setting the proportional gain K _P. FIG. 3 is a conceptual diagram showing changes in the manipulated variable MV and changes in the measured light quantity PV. The control described below is performed by the PID constant setting unit 62 of the control unit 60.

First, as shown in FIG. 2, the proportional gain K _P is set to an initial value (step 201). At this time, set to an initial value also to the integration time T _I and the differentiation time T _D. And the temperature control part 20 is controlled for a while by the PID control part 61 in the state which set the PID constant to the initial value. The initial value of the PID constant is determined by the temperature of the reflecting mirror 10. Specifically, an appropriate PID constant corresponding to the temperature of the reflecting mirror 10 is stored in advance, and the value of the PID constant is determined based on the actual temperature of the reflecting mirror 10. The temperature of the reflecting mirror 10 can be calculated based on a signal sent from the temperature measuring unit 30. Note that the initial value of the PID constant decreases as the temperature of the reflecting mirror 10 increases.

  The initial values of the above PID constants are appropriate under the conditions that the gas to be measured 101 is air, the atmospheric pressure is standard atmospheric pressure, the temperature is 24 ° C., and the flow rate is 1 liter per minute. Therefore, when the gas to be measured is a gas other than air or when the atmospheric pressure is not the standard atmospheric pressure, the value of the PID constant set here is not necessarily appropriate. Also, when the Peltier element used in the temperature control unit 20 deteriorates, the value of the PID constant may not be appropriate.

  Subsequently, an average value of the amplitude (vibration width) of the manipulated variable MV is obtained (step 202). When the measurement light quantity PV is relatively stable, as shown in FIG. 3, the manipulated variable MV fluctuates with a constant fluctuation width (amplitude), so that the amplitude of the manipulated variable MV can be easily obtained. If it is difficult to measure the amplitude, processing such as smoothing may be performed. In addition, when the manipulated variable MV suddenly fluctuates greatly due to disturbance, or when the measured light quantity PV deviates significantly from the target light quantity SV, the average value may be obtained by eliminating the manipulated variable MV at that time. Good. In addition, any method may be used as a calculation method for obtaining the amplitude.

Subsequently, the average value of the amplitude calculated in step 202 is divided by the proportional gain K _P set at that time and normalized (step 203). The reason for normalization will be described later. Hereinafter, a value obtained by normalizing the average value of the amplitude is referred to as a “normalized value”.

  Subsequently, it is determined whether or not the normalized value is larger than the amplitude upper limit value (step 204). This amplitude upper limit value is a fixed value set in advance, and is an upper limit value of a normalized value that enables highly accurate measurement. If the normalized value is larger than the amplitude upper limit value (YES in step 204), the proportional gain is corrected to a value reduced by a certain amount from the current value (step 205). As a result, the amplitude of the manipulated variable MV decreases. When the normalized value is smaller than the amplitude upper limit value (NO in step 203), the process proceeds to step 206.

  In step 206, it is determined whether or not the normalized value is smaller than the amplitude lower limit value. This amplitude lower limit value is a fixed value set in advance, and is a lower limit value of a normalized value that allows highly accurate measurement. If the normalized value is smaller than the amplitude lower limit value (YES in step 206), the proportional gain is corrected to a value increased by a certain amount from the current value (step 207). Thereby, the amplitude of the manipulated variable MV increases. When the normalized value is larger than the amplitude lower limit value (NO in step 206), the process returns to step 202 and the above steps are repeated.

  As described above, by repeating the above steps, the normalized value enters the range from the amplitude lower limit value to the amplitude upper limit value, and the state within this range can be maintained. When the normalized value falls within this range, highly accurate measurement can be performed.

Note that, in step 202, what dividing the average value of the amplitude in the proportional gain K _P is to evaluate the amplitude regardless of the value of the proportional gain K _P. For example, when the proportional gain K _P is large, the amplitude is large even if it is an appropriate value. Therefore, it is not possible to ignore the magnitude of the proportional gain K _P to evaluate uniformly amplitude with the same upper and lower limit values. By normalizing in this way, the amplitude upper limit value and the amplitude lower limit value can be fixed values regardless of the magnitude of the proportional gain K _P.

However, it is possible to evaluate an average value of amplitudes that are not normalized. What this case, multiplied by the proportional gain K proportional amplitude upper limit value in proportion to _P (as the amplitude upper limit value multiplied by a proportional gain K _P) and the proportional amplitude lower limit (proportional gain K _P to the amplitude lower limit ) And set it as a standard. In other words, the determination of whether or not the normalized value is larger than the amplitude upper limit value in step 204 is synonymous with the determination of whether or not the average value of the amplitude of the manipulated variable MV is larger than the proportional amplitude upper limit value. In step 206, the determination as to whether the normalized value is smaller than the amplitude lower limit value is synonymous with the determination as to whether the average value of the amplitude of the manipulated variable MV is smaller than the proportional amplitude lower limit value.

(Integration time and derivative time setting method)
Next, a method for setting the integration time T _I and the differentiation time T _D among the PID constants will be described with reference to FIGS. Figure 4 is a flowchart illustrating a method of setting the integration time T _I and the differentiation time T _D. The control described below is performed by the PID constant setting unit 62 of the control unit 60.

First, as shown in FIG. 4, to set the integration time _{T I} and the differentiation time _{T D} to the initial value (step 401). At this time, the proportional gain K _P is also set to an initial value. And the temperature control part 20 is controlled for a while by the PID control part 61 in the state which set the PID constant to the initial value. The method for determining the initial value of the PID constant is as described in the description of step 201 in FIG. In the present embodiment, the ratio between the integration time T _I and the differentiation time T _D is set to be 6: 1. That is, the derivative time _{T D} is set so as to be one-sixth of the integration time _{T I.}

  Subsequently, the average of the phase difference between the manipulated variable MV and the measured light quantity PV that periodically changes is obtained (step 402). A phase difference as shown in FIG. 3 occurs between the change in the manipulated variable MV and the change in the measured light quantity PV. This phase difference is the time (dead time) from when the PID control unit 61 transmits a signal indicating the manipulated variable MV until the measured light quantity PV starts to change, and from when the measured light quantity PV starts to change until it finishes changing. This is due to time (first order lag).

Subsequently, it is determined whether or not the average of the phase differences is larger than the phase upper limit value (step 403). This phase upper limit value is a fixed value set in advance, and is an upper limit value of the phase difference that allows highly accurate measurement. If the average of the phase differences is larger than the phase upper limit value (YES in step 403), the integration time T _I and the differentiation time T _D are corrected to values that are reduced by a certain amount from the current values (step 404). As described above, the ratio of the integration time T _I and the differentiation time T _D is set to be 6: 1. By decreasing the integration time T _I and the differentiation time T _D as described above, the phase difference between the variation of the measured light amount PV and variation of the manipulated variable MV is reduced. On the other hand, if the average of the phase differences is smaller than the phase upper limit value (NO in step 403), the process proceeds to step 405. The phase upper limit value is desirably 140 to 150 degrees, and more desirably 144 degrees. This is because if the phase difference exceeds 144 degrees, the response of the measurement light quantity PV to the change in the manipulated variable MV becomes slow and appropriate control cannot be performed, resulting in a decrease in measurement accuracy.

In step 405, it is determined whether or not the average of the phase differences is smaller than the phase lower limit value. If the average of the phase difference is smaller than the phase limit value (YES at step 405), it corrects to a value increased by a predetermined amount the integration time T _I and the differentiation time T _D from the current value (step 406). Integration time to increase the T _I and the derivative time T _D, the phase difference between the variation of the measured light amount PV and variation of the manipulated variable MV is increased. The phase lower limit value is desirably 30 to 40 degrees, and more desirably 36 degrees. This is because if the phase difference is less than 36 degrees, the stability is lowered and proper control cannot be performed, resulting in a decrease in measurement accuracy. On the other hand, when the average of the phase differences is larger than the phase lower limit value (NO in step 405), the process returns to step 402 and the above steps are repeated.

  In this way, by repeating the above steps, the phase difference between the change in the manipulated variable MV and the change in the measured light quantity PV enters the range from the phase lower limit value to the phase upper limit value, and remains in this range. can do. When the above phase difference falls within this range, highly accurate measurement can be performed.

According to the setting method of the PID constants (proportional gain K _P , integration time T _I , and differential time T _D ) according to the present embodiment, when the above-described normalized value or phase difference is out of a predetermined range, Regardless of what causes it, the PID constant is modified. For this reason, even if various measurement condition changes including a change in conditions that are not assumed, it can be corrected to an appropriate PID constant. In addition, according to the method for setting the PID constant according to the present embodiment, the PID constant can be corrected while measuring the dew point temperature. It can be modified to a PID constant.

(Effects of PID constant correction)
Next, the effect of correcting the PID constant according to the present embodiment will be described with reference to FIG. FIG. 5 is a diagram showing a change over time of the temperature of the reflecting mirror 10 measured by the temperature measuring unit 30. That is, it is a diagram showing the change over time in the dew point temperature measured by the mirror-cooled dew point meter 100. Note that the dew point temperature of the gas to be measured 101 used in the test does not actually change. The fluctuation of the dew point temperature is due to PID control. In FIG. 5, the portion of 10 to 30 seconds corresponding to the left side of the graph shows the result when the above-described PID constant is not corrected, and the portion of 30 to 80 seconds corresponding to the right side of the graph is the above-described PID constant. Shows the results when corrections are made. As described above, in this embodiment, the initial value of the PID constant is measured under the measurement conditions in which the gas to be measured 101 is air, the atmospheric pressure is standard atmospheric pressure, the temperature is 24 ° C., and the flow rate is 1 liter per minute. Is set to a value that is appropriate.

  In this situation, when the flow rate of the gas to be measured 101 is 2 liters per minute, the measured dew point temperature fluctuates relatively large as shown on the left side of the graph. This is because the state of the gas to be measured 101 is different from the planned measurement conditions (flow rate conditions). However, when the PID constant is corrected according to the present embodiment, as shown on the right side of the graph of FIG. 5, the measured dew point temperature is stabilized as compared to the case where the PID constant is not corrected. From the experimental results as described above, it can be seen that the above-described correction of the PID constant is very effective in measuring the dew point temperature with high accuracy.

  As described above, the embodiments of the present invention have been described with reference to the drawings. However, the specific configuration is not limited to these embodiments, and even if there is a design change or the like without departing from the gist of the present invention. It is included in the present invention. For example, a mirror-cooled dew point meter that does not have the temperature display unit 31 and stores the temperature data (dew point temperature data) of the reflecting mirror 10 in the storage device is also included in the present invention.

  According to the mirror-cooled dew point meter according to the present invention, the PID constant can be appropriately corrected even when various measurement conditions change every moment. Therefore, it is useful in the technical field of specular cooling type dew point meter.

DESCRIPTION OF SYMBOLS 10 Reflecting mirror 20 Temperature control part 30 Temperature measuring part 40 Light projection part 50 Light receiving part 60 Control part 100 Mirror surface cooling-type dew point meter 101 Gas to be measured

Claims (5)

A reflector disposed in the gas to be measured;
A temperature control unit that cools the reflecting mirror and causes condensation on the surface;
A temperature measuring unit for measuring the temperature of the reflecting mirror;
A light projecting unit for emitting irradiation light to the surface of the reflecting mirror;
A light receiving unit for receiving the irradiation light reflected by the surface of the reflecting mirror;
A control unit that calculates an operation amount for the temperature control unit by PID control so that a measurement light amount of the irradiation light received by the light receiving unit becomes a target light amount, and controls the temperature control unit based on the operation amount; With
The control unit reduces the proportional gain when the amplitude of the manipulated variable divided by the proportional gain of the PID control is greater than a predetermined amplitude upper limit value, and reduces the amplitude of the manipulated variable by the proportional gain. A mirror-cooled dew point meter that increases the proportional gain when the divided value is smaller than a predetermined lower limit amplitude.   The specular cooling dew point meter according to claim 1, wherein both the upper limit value of amplitude and the lower limit value of amplitude are fixed values.   The control unit decreases the integration time and the differentiation time of the PID control when the phase difference between the operation amount and the measured light amount is larger than the phase upper limit value, and when the phase difference is smaller than the phase lower limit value. The specular cooling dew point meter according to claim 1 or 2, wherein increases the integration time and the derivative time.   The specular cooling dew point meter according to claim 3, wherein the differential time is set to be 1/6 of the integration time.   The specular cooling dew point meter according to claim 3 or 4, wherein the phase lower limit value is 30 to 40 degrees, and the phase upper limit value is 140 to 150 degrees.

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