JP3047685B2 - Combustion control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a combustion control device for oil-fired appliances and the like.

[0002]

2. Description of the Related Art Conventionally, in this type of combustion control device, as shown in FIG. 12, a heater 51 is energized at the start of operation, and when a vaporizing cylinder 52 reaches a set temperature, a burner fan 54 is driven to communicate with the outdoor. Outdoor air is taken in from the air supply pipe and supplied to the burner section, the fuel pump 55 is energized, the ignition device performs an ignition operation, and transitions to combustion. When the combustion is started, the burner motor 56 and the fuel pump 55 are operated at the air-fuel ratio set in advance by the combustion control unit 57 according to the combustion amount to perform the combustion.

[0003]

However, in the above-described conventional configuration, the primary air that governs the combustion state is supplied from an air supply pipe that communicates with the outside of the room. Combustion continues.
At this time, if the exhaust pipe is detached or a hole is formed in the middle of the exhaust path, the exhaust gas fills the room, further promoting a decrease in the indoor oxygen concentration, and leads to an unsafe state. That is, in the above-described conventional configuration, since combustion air is taken in from the outside of the room, combustion continues normally even if the indoor oxygen concentration decreases, and exhaust gas is filled into the room due to disconnection of the exhaust pipe or leakage from the exhaust path. Nor could it detect this.

In addition, the set value of the air-fuel ratio must be set in consideration of the balance of characteristics such as combustion efficiency, thermal problems, NOx emissions, etc. over the entire variable width of the combustion amount. Could not be taken too large.

[0005] Generally, the combustion state is determined by the air-fuel ratio, that is, the amount of primary air. If the air-fuel ratio is set to around 1 in order to increase the thermal efficiency, the flame temperature increases, and there is a problem in flashback and heat resistance of parts. In a region where the amount of combustion occurs or the amount of combustion is small, dew condensation occurs due to a decrease in exhaust gas temperature, so that the amount of combustion cannot be reduced to an extremely small amount. There is also a problem that the amount of NOx emission becomes extremely large. On the other hand, if the air-fuel ratio is set to about 1.6 in order to lower the flame temperature, the NOx emission will be reduced and the thermal problem will be reduced. The problem of disappearance occurred, and the combustion amount could not be reduced to an extremely small amount.

Therefore, in the conventional configuration, the air-fuel ratio is set to an intermediate value of about 1.4, and the variable range of the combustion amount is set in a combustion range where the above-mentioned problem does not occur. Therefore, the variable width of the combustion amount was extremely small. In addition, the air-fuel ratio is a fixed value even when the combustion air amount fluctuates due to fluctuations in the flow rate of the electromagnetic pump, motor rotation speed, fan variations, and clogging of the air supply path, which are factors that affect the combustion state. Appropriate combustion can be ensured only within the permissible range of the burner capacity, and if the fluctuation amount exceeds that, abnormal combustion occurs.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its first object to improve the safety by enabling detection of the oxygen concentration in a room, and to set an optimum air-fuel ratio in accordance with the combustion amount. The second object is to enlarge the variable width of the combustion amount.

[0008]

SUMMARY OF THE INVENTION In order to solve the above-mentioned conventional problems, the present invention provides a burner, an air supply path switching means for switching an air supply path for taking in combustion air to the burner between the outside and the inside of the room, An oxygen sensor provided in the exhaust path of the burner, a combustion control unit for controlling the amount of combustion of the burner, and comparing the output from the oxygen sensor with an air-fuel ratio target value of an air-fuel ratio target setting unit to determine the amount of combustion air or A control output unit for outputting at least one control amount of fuel, and an output or control output unit from the oxygen sensor when the air supply path switching unit switches the air supply path so as to supply air from the indoor side. An oxygen deficiency detection unit that determines whether or not the oxygen deficiency state is present based on the output change of the combustion detection unit or the output change of the combustion detection unit and sends a combustion stop signal to the combustion control unit when the oxygen deficiency occurs. Constitution A self-correction unit that corrects the sensor output due to deterioration of the characteristics of the oxygen sensor, and a K value correction unit that corrects the air-fuel ratio target value based on the combustion amount control output from the combustion control unit. The air-fuel ratio target value is set based on the output from the correction unit.

[0009]

According to the present invention, by switching the air supply path to the indoor side, the present invention can detect the oxygen concentration in the room, confirm the presence or absence of an abnormality in the indoor environment, and stop the combustion operation in the event of an abnormality. In the case where the self-correction unit and the K-value correction unit are provided, the air-fuel ratio target value is corrected according to the combustion amount determined by the difference between the room temperature and the set temperature, and the self-correction unit uses Since the air-fuel ratio target value is corrected according to the change in the output characteristics to set the air-fuel ratio target value, an optimal combustion state can be ensured for each combustion amount, and the variable width of the combustion amount can be increased. .

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. First, the configuration will be described with reference to FIG.
Reference numeral 1 denotes a burner fan for supplying combustion air, and a motor 1A
And fan 2. Reference numeral 3 denotes a burner having a vaporizing cylinder 3A for vaporizing fuel and mixing it with combustion air.
A has a heater 4 and a temperature detection sensor 8 embedded therein. Reference numeral 5 denotes a fuel pump for supplying fuel into the vaporizing cylinder 3A, reference numeral 6 denotes a heat exchanger for exchanging heat of the combustion gas, and reference numeral 7 denotes an exhaust path for discharging the heat-exchanged combustion gas.

Reference numeral 9 denotes a limiting current type oxygen sensor which is attached to the exhaust passage 7 and measures the oxygen concentration in the combustion gas. Reference numeral 10 denotes a heater power supply of the oxygen sensor for heating a heater built in the oxygen sensor.
10A sets the heater applied voltage based on a predetermined sequence,
A voltage switching unit 11 for switching is a power supply for the sensor, and is a power supply for generating a limit current according to the oxygen concentration. Reference numeral 12 denotes an adjustment resistor for adjusting the output of the oxygen sensor 9. An amplifier 13 amplifies the output of the oxygen sensor 9 so as to be input to a control unit.

Reference numeral 14 denotes a target setting unit for the air-fuel ratio (m value);
Is a correction calculation unit that calculates a correction value by comparing the output of the oxygen sensor 9 with an air-fuel ratio target value. Reference numeral 16 denotes a burner motor rotation speed correction unit that determines an output value of the burner motor rotation speed based on the calculation result of the correction calculation unit 15.
The correction output unit 15 and the correction operation unit 15 constitute a control output unit 16A. Reference numeral 17 denotes a limiter which stops when the output value of the burner motor rotation speed is out of a predetermined range.
Reference numeral 18 denotes a burner motor drive circuit, and 19 denotes a vaporizing cylinder temperature setting unit.

Reference numeral 20 denotes a K value correction unit for correcting the target value of the air-fuel ratio in accordance with the amount of combustion, and corrects and calculates the target value of the air-fuel ratio based on a signal from the combustion control unit. Reference numeral 21 denotes a self-correction unit that detects the initial characteristics of the oxygen sensor and corrects the air-fuel ratio target value according to the characteristics. Reference numeral 22 denotes a limiter which limits a result of correcting the target value of the air-fuel ratio when the value is out of a predetermined range.
Stop. 23 is an electromagnetic pump drive circuit, 24 is a combustion control unit that sets the combustion amount based on the room temperature and the set temperature, 2
Reference numeral 5 denotes an air-fuel ratio control unit constituted by the above elements.

Reference numeral 26 denotes a first air supply passage communicating with the burner 3, and supplies outdoor air as combustion air. 27 is a second air supply path, which is a path for taking in indoor air to monitor the indoor environment. Reference numeral 28 denotes a damper for switching the first and second air supply paths, which serves as a means for switching the air supply path. Reference numeral 29 denotes a switch control unit of the damper, which switches the damper 28 before the start of combustion and at predetermined intervals by an output from the combustion control unit 24. Reference numeral 30 denotes an oxygen deficiency detection unit which compares the oxygen sensor output read at the time of taking in the indoor air supply with a preset oxygen deficiency level and sends a signal to the combustion control unit 24 to stop or continue combustion. ing.

The operation of the combustion control device having the above configuration will be described with reference to the flowcharts of FIGS. First, a predetermined voltage is applied from the power supplies 10 and 11 to the sensor heater and the sensor when the operation is started. The power supply 10 of the sensor heater switches the voltage level after a predetermined time has elapsed by the power supply switching unit 10A. This is done to improve the initial response of the sensor. In this state, it waits until the sensor output is stabilized.

When a predetermined time has elapsed, the damper switching control unit 2
At 9, the damper 28 is operated to switch the air supply path to the indoor side 27. When the switching of the air supply path is completed, the motor 1A is driven for a predetermined time, and the indoor air is supplied by the fan 2 to the oxygen sensor 9 via the burner 3, the heat exchanger 6, and the exhaust path 7, and the supply of the supplied air is performed. Oxygen concentration amplifier 13
Is read into the oxygen deficiency detection unit 30 via the. The oxygen deficiency detection unit 30 compares the read oxygen sensor output data with a preset oxygen deficiency detection level. If the oxygen sensor output data is equal to or less than the oxygen deficiency level, the oxygen deficiency detection unit 30 determines that the room is in an oxygen deficiency state and performs an operation. Stop. If the read oxygen sensor output data is equal to or higher than the oxygen deficiency level, it is determined to be normal, and the damper switching control unit 29 operates the damper 28 to switch the air supply path to the outdoor side 26.

When the switching of the damper 28 is completed, the initial setting of the air-fuel ratio target value is performed according to the flowchart of FIG. Self-correction is performed, and if it is within the limiter range, the air-fuel ratio target value is updated. In addition, if it is out of the limiter range, it is stopped. That is, the correction is performed for the purpose of correcting the deviation of the output characteristic due to deterioration or the like, and the air-fuel ratio initial value D is corrected based on the slope D / C of the standard characteristic and the sensor read value E as shown in FIG. Find points. Without the self-correction operation, the oxygen concentration in the combustion exhaust gas controlled at the air-fuel ratio point D with the output characteristics deviated increases. In other words, the carbon dioxide setting results in low-lift combustion. Therefore, the combustion is performed at the normal oxygen concentration by correcting the air-fuel ratio to the point F.

Next, the combustion controller 24 starts a combustion operation in a predetermined combustion sequence, and thereafter performs combustion under predetermined conditions until the combustion is stabilized. When a predetermined time has elapsed and the K value control operation for controlling the combustion amount based on the difference between the set temperature and the room temperature is started, the air-fuel ratio target value is further corrected to a value suitable for the combustion amount, and set as the air-fuel ratio target value. . At the same time, the sensor output is read and compared with the air-fuel ratio target value to perform a correction operation.

The correction operation will be described with reference to FIG. 4. The correction is performed by a predetermined width (comparison amount X) with respect to the air-fuel ratio target value.
Air-fuel ratio target value-comparison amount X
In the case of, + Y% (2%) is set as the correction amount, and conversely, in the case of the air-fuel ratio target value + the comparison amount X, the correction amount is -Y%
(2%), and when the read value of the sensor output is within the range of the comparison amount, the correction amount is set to 0%. This cycle of setting the correction amount is repeated for a predetermined time (400 ms), and when a predetermined time (2 seconds) elapses, the correction amount is determined. The correction amount calculation in the correction amount setting cycle is performed by an integrating method, and the correction is performed by averaging the combustion state for two seconds.

Next, when the correction amount is determined, the limiter 17
It is checked whether the correction amount is within a preset allowable range. If the correction amount is within the allowable range, the output is sent to the drive circuit, the motor or the fuel pump is controlled, and the combustion is adjusted so as to burn at the set air-fuel ratio. If it is out of tolerance, it is regarded as an abnormal use state and is stopped. When the series of correction operations is completed, the sensor output is read again and the same correction operation is repeated.

FIG. 6 shows the relationship between the sensor characteristics and the carbon dioxide characteristics. By controlling the air-fuel ratio by the target value ± comparison amount X, it becomes possible to control carbon dioxide with a specified set width in any combustion condition fluctuation.

By repeating the above operations, the change in sensor characteristics is corrected, the optimum air-fuel ratio is set according to the amount of combustion, the variable width of the amount of combustion is expanded, and the influence of each factor on combustion is corrected. The combustion state at each combustion amount is optimally controlled. Also, the indoor environment is monitored by checking the indoor oxygen concentration by switching the air supply path, and the safety is maintained.

Next, the third configuration will be described with reference to the control block diagram of FIG. The same parts are given the same numbers and their explanation is omitted. The third configuration is the combustion control unit 24
The combustion control is switched from the air-fuel ratio control to the fixed combustion control set for the oxygen deficiency check at a predetermined cycle during combustion by the timer provided in, and the oxygen deficiency state is monitored by the oxygen sensor provided in the exhaust passage. It is.

Reference numeral 24A denotes an operation control unit which turns ON / OFF the operation by a signal from an operation switch, a temperature detector or the like. 24B
Is an ignition detection unit, and 24C is a timer, which operates after the ignition operation and outputs a signal for starting oxygen deficiency check at a predetermined cycle during combustion. Reference numeral 24D denotes an oxygen deficiency combustion control unit which sets the oxygen deficiency detection unit 30 to a preset oxygen deficiency check combustion condition based on the signal of the timer. 24E is the ventilation notice section,
When the output of the oxygen deficiency detection unit 30 drops below the ventilation notice level, the ventilation notice signal is output. Reference numeral 24F denotes a combustion stop unit that outputs a combustion stop signal when the output of the oxygen deficiency detection unit 30 falls below the oxygen deficiency detection level. 24A to 24F are incorporated in the combustion control unit 24. Reference numeral 25D denotes an A / D input unit, which is an A / D input unit for performing data processing on the output of the oxygen sensor 9.
A D conversion section, 31 is a combustion section, and 32 is an alarm circuit, which emits a ventilation display and an alarm sound according to a signal from the ventilation notice section 24E.

In the above configuration, first, before the start of combustion, the oxygen-deficient combustion control unit 24D supplies signals to the damper switching control unit 29 and the oxygen-deficient detection unit 30 to check the oxygen concentration in the indoor air supply path. Next, at this time, when the output of the oxygen deficiency detection unit 30 is equal to or lower than the predetermined level, the operation is stopped. When the output is equal to or higher than the predetermined level, the air supply path is switched to the outdoor path and the combustion operation is started.

When the combustion operation is started, the timer 24C is operated, and after a lapse of a predetermined time, the air-fuel ratio control is switched to the fixed combustion control for checking for oxygen deficiency. At the same time, the oxygen-deficient combustion control unit 24
A signal is supplied to the damper switching control unit 29 and the oxygen deficiency detection unit 30 via D, the air supply path is switched to the indoor path, and the output of the oxygen sensor 9 is read into the oxygen deficiency detection unit 30 to check the indoor oxygen concentration. . When the comparison result in the oxygen deficiency detection unit 30 is equal to or lower than the ventilation notice level, a signal is sent to the alarm circuit 32 through the ventilation notification unit 24E to perform ventilation notice. A combustion stop signal is sent to the oxygen-deficient combustion control unit 24D to stop the combustion. When the output of the oxygen deficiency detection unit 30 is normal, the air supply path is switched to the outdoor side again to continue the combustion. By performing the above operation at a cycle determined by the timer 24C, the indoor oxygen concentration during combustion can be confirmed, and a safe and convenient combustor can be provided.

Next, a fourth configuration will be described with reference to FIGS. 8, 9 and 10. FIG. The same parts are given the same numbers and their explanation is omitted. In the fourth configuration, the oxygen concentration in the room is checked while the combustion by the air-fuel ratio control is continued to monitor the oxygen deficiency state. The oxygen deficiency detection unit 30 outputs the control output unit 16A. An oxygen deficiency state is detected based on whether the value is within a normal range. The operation will be described next. Since the operation for checking the state of oxygen deficiency in the room before the start of combustion is the same as that of the third configuration, only the operation flow is shown in FIG. 9 and the description is omitted. About Figure 1
Explanation will be made using 0.

First, when the combustion operation is started by the operation control unit 24A, a timer 24C is operated to perform a predetermined air-fuel ratio control operation as shown by Y in FIG. A signal is supplied to the output unit 16A, and the operation of the damper 28 switches the air supply path to the indoor side.

In this state, the output from the control output unit 16A, that is, the correction amount of the air-fuel ratio control is compared with a preset oxygen deficiency level as shown by Z in FIG. In the case of, the combustion is stopped assuming that the room is in an oxygen-deficient state. If the correction amount is greater than the oxygen deficiency level 1 (ventilation notice), a ventilation notice signal is output to continue combustion by air-fuel ratio control. When the correction amount is smaller than the oxygen deficiency level 1, the combustion is considered to be normal and the combustion is continued. The confirmation of the oxygen deficiency state based on the correction amount is performed until the preset oxygen deficiency confirmation time elapses. If the oxygen deficiency confirmation time is normal, the air supply path is switched to the outdoor side again and the normal combustion operation is continued. By performing the above operation at a cycle determined by the timer 24C, it is possible to confirm the indoor oxygen concentration during combustion. In addition, since the oxygen-deficient state can be confirmed while continuing the combustion by the air-fuel ratio control, the combustion can be reliably stopped without causing abnormal combustion even in the event of an oxygen-deficient state due to a disconnection of the exhaust pipe or the like. .

Next, a fifth configuration will be described with reference to FIG. The same parts are given the same numbers and their explanation is omitted. The fifth configuration is to check the state of oxygen deficiency by using a signal from a combustion detection unit using a flame rod in a state where combustion by air-fuel ratio control is continued. 24G is a combustion control circuit which performs combustion control in a predetermined sequence.
Reference numeral 3 denotes a frame rod, and reference numeral 34 denotes a combustion detection unit, which is a part for checking a combustion state based on a signal from the frame rod 33.

In the case of this configuration, the operation of checking the indoor oxygen concentration before the start of combustion is the same as that of the third configuration, and the description is omitted. When the combustion starts, the combustion by the predetermined air-fuel ratio control is continued by the outdoor air supply path until the timer 24C times out, and when the predetermined time has elapsed, a signal is sent from the timer 24C to the combustion control circuit 24G, and the damper switching control unit 29 An instruction to switch the damper 28 and read the output of the combustion detection unit 34 is issued to the oxygen deficiency detection unit 30. Thus, the combustion by the indoor air supply is started, and the state of the indoor oxygen deficiency is confirmed by observing the output of the combustion detector at that time.
As described above, in the fifth configuration, while performing combustion by air-fuel ratio control, in the combustion state confirmation method, the oxygen deficiency state is confirmed by a method using a proven flame rod.
A highly reliable oxygen deficiency state can be confirmed with a simple configuration.

[0032]

As described above, the combustion control apparatus according to the present invention measures the oxygen concentration in the room by switching the air supply path with one oxygen sensor attached to the exhaust path, and makes the room oxygen-deficient. It is possible to check whether or not the air conditioner is not operating, and also to check the indoor oxygen deficiency state while continuing the combustion by the air-fuel ratio control, so that a safe and easy-to-use combustor can be provided.

When the amount of combustion is fixed and oxygen deficiency is confirmed, the detection accuracy of oxygen deficiency is stable, and when oxygen deficiency is confirmed by the change width of the control output, oxygen deficiency is confirmed by the output from the combustion detection unit. In the case of missing confirmation, there is an effect that high reliability can be obtained.

In the air-fuel ratio target value self-correction based on the output of the oxygen sensor and the K value correction based on the combustion amount, the combustion can be controlled with the optimum air-fuel ratio target value for each combustion amount. As a result, the variable width of the combustion amount can be expanded, and even when the combustion amount and the combustion air amount fluctuate due to various factors, the combustion state at each combustion amount can be optimally controlled. Variation,
It is possible to reduce a combustion trouble due to a change in use environment conditions and a factor suitable for aging, and to provide a highly reliable combustor.

[Brief description of the drawings]

FIG. 1 is a block diagram of a combustion control device according to an embodiment of the present invention.

FIG. 2 is a flowchart showing an indoor environment monitoring operation of the apparatus.

FIG. 3 is a flowchart showing a correction operation of the apparatus.

FIG. 4 is a flowchart showing a correction operation of the apparatus.

FIG. 5 is a characteristic diagram of an oxygen sensor used in the apparatus.

FIG. 6 is a characteristic diagram showing a relationship between a sensor output and carbon dioxide by the device.

FIG. 7 is a block diagram of a combustion control device according to another embodiment of the present invention.

FIG. 8 is a block diagram of a combustion control device according to another embodiment of the present invention.

FIG. 9 is a flowchart showing an indoor environment monitoring operation of the apparatus.

FIG. 10 is a flowchart showing a correction operation of the apparatus.

FIG. 11 is a block diagram of a combustion control device according to another embodiment of the present invention.

FIG. 12 is a block diagram of a conventional combustion control device.

[Explanation of symbols]

 Reference Signs List 1 burner fan 3 burner 5 fuel pump 7 exhaust path 9 oxygen sensor 14 air-fuel ratio target setting section 16A control output section 20 K value correction section 21 self-correction section 24 combustion control section 24C timer 26 outdoor air supply path 27 indoor air supply path 28 Air supply path switching means (damper) 29 switching controller 30 oxygen deficiency detector

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F23N 5/24 107 F23N 1/02 F23N 5/00 F23N 5/26 101

Claims (6)

(57) [Claims] 1. A burner, an air supply path switching means for switching an air supply path for taking in combustion air to the burner between an outdoor side and an indoor side, an oxygen sensor provided in an exhaust path of the burner, and a combustion amount of the burner And a control output unit for comparing an output from the oxygen sensor with an air-fuel ratio target value of an air-fuel ratio target setting unit to output at least one of a combustion air amount and a fuel amount. When the air supply path switching means switches the air supply path so as to supply air from the indoor side, it is determined whether or not there is an oxygen deficiency state based on the output from the oxygen sensor. A combustion control device comprising an oxygen deficiency detection unit for sending a combustion stop signal. 2. A burner, an air supply path switching means for switching an air supply path for taking in combustion air to the burner between an outside side and an indoor side, an oxygen sensor provided in an exhaust path of the burner, An oxygen-deficient combustion control unit that drives the air supply path switching means to switch to indoor air supply and interlocks with this to fix and maintain the combustion amount of the burner at a predetermined value;
A control output unit for comparing the output from the oxygen sensor with an air-fuel ratio target value of an air-fuel ratio target setting unit and outputting a control output of at least one of a combustion air amount and a fuel amount;
When the air supply path switching means switches the air supply path so as to supply air from the indoor side, it is determined whether or not an oxygen deficiency state is present based on the output from the oxygen sensor. A combustion control device comprising an oxygen deficiency detection unit that sends a signal. 3. A burner, air supply path switching means for switching a supply path for taking in combustion air into the burner between the outside and the inside of the burner, an oxygen sensor provided in an exhaust path of the burner, and a combustion amount of the burner And a control output unit for comparing an output from the oxygen sensor with an air-fuel ratio target value of an air-fuel ratio target setting unit to output at least one of a combustion air amount and a fuel amount. When the air supply path switching means switches the air supply path so as to supply air from the indoor side, the control output of the control output unit determines whether or not the oxygen deficiency state is within a normal range, and the oxygen deficiency state is determined. A combustion control unit that sends a combustion stop signal to the combustion control unit when the determination is made. 4. A burner, an air supply path switching means for switching a supply path for taking in combustion air to the burner between the outside and the inside of the burner, an oxygen sensor provided in an exhaust path of the burner, and a combustion amount of the burner And a control output unit for comparing an output from the oxygen sensor with an air-fuel ratio target value of an air-fuel ratio target setting unit to output at least one of a combustion air amount and a fuel amount. A combustion detection unit that detects the combustion state of the burner with a frame rod to check the combustion state, and a combustion detection unit when the air supply path switching unit switches the air supply path so as to supply air from the indoor side. A combustion control device comprising: an oxygen deficiency detection unit that determines whether an oxygen deficiency state is present based on an output from the combustion control unit and sends a combustion stop signal to the combustion control unit when oxygen deficiency occurs. 5. The combustion control apparatus according to claim 1, further comprising a timer for driving the air supply path switching means at a predetermined cycle to switch the air supply path to indoor air supply. 6. A self-correction unit for correcting a sensor output due to deterioration of characteristics of an oxygen sensor, and a K value correction unit for correcting an air-fuel ratio target value based on a combustion amount control output from a combustion control unit. 5. The combustion control device according to claim 1, wherein the fuel ratio target setting unit sets an air-fuel ratio target value based on outputs from the self-correction unit and the K value correction unit.