JP2000121131A - Air conditioner and controller therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide comfortable air conditioning for residents by varying the switchingover conditions between cooling mode and air supply mode based on the number of switching times. SOLUTION: An outer air intake damper 2 varies the volume of outer air being taken into an air conditioner body 1. A cooling means, i.e., a cooling coil 5, cools or dehumidifies mixture gas of the outer air and the return air. A thermometer 11 measures outer air temperature, a hygrometer 12 measures humidity of outer air, a thermometer 13 measures indoor temperature and a hygrometer 14 measures indoor humidity. A control means 100 controls a two- way valve 6 based on a signal from the thermometer 13 and also controls the outer air intake damper 2, and the like, while interlocking a fan 8 and a return air fan 9. A count means 15 switches the cooling mode and an outer air introduction value set means 16 varies predetermined outer air introduction conditions based on a count. A memory means 17 stores the outer air introduction conditions, i.e., upper and lower limits of outer air temperature and humidity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air conditioner having a mode for saving energy by taking in outside air, and a control device therefor.

[0002]

2. Description of the Related Art FIG.
It is explanatory drawing of the air conditioner control method and air conditioner control apparatus shown in Sanitary Engineering Journal Vol.60 No.7 p13 "Air Conditioning Equipment and Automatic Control (Part 1)". In the figure, 1 takes in outside air,
An air conditioner that performs cooling and heating with cold and hot air, an outside air intake damper that changes the amount of outside air taken into the air conditioner body 1, an exhaust damper that changes the amount of exhaust air from the air conditioner body 1, and a return from the room. A return air damper that changes the mixing ratio of air and outside air, 5 is a cooling coil that cools or dehumidifies the mixture of outside air and return air, 6 is a two-way valve that controls the flow rate of cold and hot water according to the indoor temperature, and 7 is A humidifier that humidifies the air-fuel mixture that has passed through the cooling coil 5, 8 is a blower fan that blows air into the room, 9 is a return air fan that returns air from the room, 11 is a thermometer that measures the temperature of outside air, 12 is a hygrometer for measuring the humidity of the outside air, 13 is a thermometer for measuring the indoor temperature, 14 is a hygrometer for measuring the indoor humidity, 10
Controls the two-way valve 6 based on the signal of the thermometer 13, controls the outside air intake damper 2, the exhaust damper 3, and the return air damper 4, and interlocks the blower fan 8 and the return air fan 9. It is an air conditioner controller.

[0003] The operation of the conventional air conditioner configured as described above will be described with reference to FIG. In FIG. 6, outside air indicated by OA is taken in by the air conditioner main body 1, and the taken-in outside air is reduced in amount by the outside air damper 2. In synchronization with this, the exhaust damper 3
The return air damper 4 is controlled by the air conditioning controller 10.
For example, if the outside air damper 2 has an opening corresponding to an air volume of 30%, the return air damper 4 is controlled to a 70% opening, and the exhaust damper 3 is controlled to a 30% opening. By this damper control, 30% of outside air and 70% of return air are mixed. In order to perform cooling, the air-fuel mixture is cooled by the cooling coil 5 and blown into the room by the blower fan 8. Usually, the temperature of the blown air is about 15 ° C. Conversely, return air from the room is sucked in by the return air fan 9. 70% of the sucked return air is returned to the cooling coil 5 side by the return air damper 4, and 30% is exhausted to the outside from the exhaust damper 3.
In air conditioning, the goal is to keep the temperature and humidity constant. In summer, the air conditioning controller 10
Measures the room temperature with a thermometer 13, opens the two-way valve 6 when the temperature is high, closes the two-way valve 6 when the temperature is low,
It is controlled so that the temperature is always constant.

Next, the outdoor air cooling will be described. With outdoor air cooling, the temperature and humidity conditions of the outside air are
When the temperature is low or the humidity is low, outside air is introduced into the room to save energy required for cooling, thereby achieving energy saving control. The energy saving effect of outdoor air cooling differs depending on the season. For example, since the outside air introduction condition is rarely satisfied during the cooling period in summer, the utility value is low in an office building or the like that requires cooling only during normal daytime. However, in a common area of a hospital or hotel that requires 24-hour cooling, the control is effective early in the morning and late at night. In addition, since outside air introduction conditions are often satisfied in the middle period such as spring and autumn, even office buildings can be cooled without cooling by cooling coils. In winter, outside air cooling is used when the heat load increases in a place where the number of people in the room increases rapidly, such as a department store during heating.

[0005] The operation of the outside air cooling will be described with reference to FIGS. 7 and 8. FIG. 7 shows a psychrometric chart with temperature and humidity as axes as the air state. Enthalpy is used as an index indicating the energy of a certain volume of air. For example, even with air at the same temperature, the enthalpy is higher when the humidity is higher. This is because the specific gravity of dry air and the specific gravity of water vapor are different. FIG. 8 is a flowchart showing the operation of the outside air cooling. The outside air enthalpy is compared with the inside air enthalpy in the room, and when the outside air enthalpy is smaller than the inside air enthalpy, the outside air is introduced to reduce the cooling load.

In FIG. 8, in step S1, the outside air condition is measured from the signals of the outside air thermometer 11 and the outside air hygrometer 12, and the outside air enthalpy is calculated from the values. Step S
At 2, the inside air condition is similarly measured from the signals of the room thermometer 13 and the indoor hygrometer 14, and the inside air enthalpy is calculated. Next, in step S3, the outside air enthalpy is compared with the inside air enthalpy. If the outside air enthalpy <the inside air enthalpy, one of the conditions for the outside air cooling is satisfied, and the process proceeds to the next step S4. If the condition in step S3 is not satisfied, the process proceeds to step S6 in which outside air cooling is turned off.

In step S4, the outside air introduction condition is examined in consideration of the occupant's comfort in addition to the condition satisfied in step S3. Specifically, an upper limit value Th, a lower limit value Tl, an upper limit value Hh, and a lower limit value Hl of the outside air temperature indicated by a hatched portion 20 in FIG. 7 are determined, and it is checked whether the outside air is within this range. . If the condition of step S4 is satisfied, outside air cooling is executed in step S5. Specifically, 100% of the outside air damper 2 is fully opened and the exhaust damper 3
Also, the return air damper 4 is fully closed, the two-way valve is fully closed, and energy is saved in a ventilation mode using outside air cooling. If the condition of step S4 is not satisfied, the control proceeds to step S6 to stop the outside air cooling control and return to the normal cooling coil cooling control. Further, in step S7, after waiting for a timer for a fixed time, the control to return to step S1 is repeated.

[0008]

Since the conventional air-conditioning control device is configured as described above, if the outside air introduction range shown by the hatched portion 20 in FIG. 7 is narrowed, the outside air in the middle period between spring and autumn can be reduced. This is effective when the conditions are met or when the temperature rises sharply in winter, but in summer, there is no air blowing mode using outside air cooling. Conversely, if the outside air introduction range is widened, the occupants may feel hot and uncomfortable, and if the cooling coils are not used, the occupants' comfort will be impaired. On the other hand, when the enthalpy of the outside air is in the boundary region of the hatched portion 20 in FIG. 7, there is a problem that the air blowing mode by the outside air cooling and the cooling mode by the cooling coil hunt due to changes in the temperature and humidity of the outside air. SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has as its object to provide an air conditioner control in which the outside air introduction range is appropriate and the occupants can be in a comfortable air conditioning state.

[0009]

In an air conditioner according to the present invention, an outside air intake damper for changing the amount of air taken in from the outside, cooling means for cooling air taken in from the room, Measuring means for measuring the state of indoor air, a cooling mode in which the cooling means uses the energy to cool the indoor air, and a blowing mode in which the outdoor air is taken in to save energy consumed by the cooling means. And storage means for storing the switching condition, and the cooling mode or the air blowing mode is selected based on the state of the air measured by the measuring means and the switching condition, and based on the selection, the cooling means and Control means for controlling the damper; detecting a number of times of switching between the cooling mode and the air blowing mode; Te, in which and a changing means for changing the switching condition.

[0010] The measuring means includes an outside air measuring means for measuring the enthalpy of the outdoor air, and an inside air measuring means for measuring the enthalpy of the indoor air, and the storage means includes: The information which specifies the upper limit value and the lower limit value of the enthalpy in the blowing mode is stored, and the control unit is configured such that the enthalpy of the outdoor air is lower than the enthalpy of the indoor air, and the enthalpy of the outdoor air is When between the upper limit and the lower limit in the blowing mode, select the blowing mode,
The changing means reduces the upper limit of the enthalpy when the number of times of switching from the cooling mode to the air blowing mode is equal to or more than a predetermined number.

An outside air intake damper for changing the amount of air taken in from outside; cooling means for cooling the air taken in from the room; measuring means for measuring the state of the outside air and the air in the room; A storage unit configured to store switching conditions between a cooling mode in which the cooling unit cools the indoor air using energy and a blowing mode in which the outdoor unit takes in the outdoor air to save energy consumed by the cooling unit; Control means for selecting the cooling mode or the air blowing mode based on the state of air measured by the measuring means and the switching condition, and controlling the cooling means and the damper based on the selection; and And alarm output means for detecting the number of times of switching between the blow mode and outputting an alarm based on the number of times of switching. It is intended.

Further, in the outdoor unit control apparatus according to the present invention, there are provided a measuring means for measuring a state of the outdoor air and a state of the indoor air, a cooling mode for cooling the indoor air using energy, A storage unit for storing a switching condition for a blowing mode for saving energy by taking in outdoor air, and selecting the cooling mode or the blowing mode based on the state of the air measured by the measuring unit and the switching condition. A control unit that controls a cooling unit that cools the indoor air based on the selection, detects a number of times of switching between the cooling mode and the blowing mode, and changes the switching condition based on the number of times of switching. Change means for performing the operation.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a functional block diagram showing an air conditioner according to Embodiment 1 of the present invention. In FIG. 1, reference numeral 1 denotes an air conditioner body that takes in outside air and performs cooling and heating with cold air and hot air, 2 denotes an outside air intake damper that changes the amount of outside air taken into the air conditioner body 1,
Is an exhaust damper that changes the amount of exhaust air from the air conditioner body 1, 4 is a return air damper that changes the mixture ratio of return air from the room and outside air, and 5 is a cooling or dehumidifying air-fuel mixture in which outside air and return air are mixed. 6 is a two-way valve that controls the flow rate of cold and hot water according to the room temperature, 7 is a humidifier that humidifies the air-fuel mixture that has passed through the cooling coil 5, and 8 is a blower fan that supplies air blown into the room. , 9 is a return air fan that returns air from the room, 11 is a thermometer that measures the temperature of the outside air, 12 is a hygrometer that measures the humidity of the outside air, 13 is a thermometer that measures the temperature of the room, and 14 is a room that measures the temperature of the room. A hygrometer 100 controls the two-way valve 6 based on a signal from a thermometer 13, and measures the outside air intake damper 2 and the exhaust damper 3.
And an air conditioner controller which is a control device that controls the return air damper 4 and performs an interlock with the blower fan 8 and the return air fan 9. 15 is an air conditioner controller 10
0 is a cooling mode switching count means provided inside 0, 16 is an outside air introduction value setting means which is a change means for changing a predetermined outside air introduction condition based on the count value, and 17 is an outside air introduction condition, that is, outside air. Upper limit value Th, lower limit value Tl of temperature, upper limit value Hh of ambient air humidity, lower limit value H
This is storage means for storing l. Here, the thermometer 11 and the hygrometer 12 are outside air measuring means, and the thermometer 13 and the hygrometer 14 are inside air measuring means. These outside air measuring means and inside air measuring means are measuring means for measuring the state of the outdoor air and the indoor air. The outside air introduction value setting means 16
Is a change means. The outside air introduction condition is a switching condition.

The operation of the air conditioner configured as described above will be described with reference to FIG. FIG. 2 is a flowchart illustrating a process of switching between the cooling mode and the ventilation mode by control device 100. First, in step S0, the cooling mode switching counting means 15 clears the register N storing the frequency to zero. Next, in step S1, the state of the outdoor air (that is, the outside air) is measured from the signals of the outside air thermometer 11 and the outside air hygrometer 12, and the outside air enthalpy is calculated from the measured value. Subsequently, in step S2, the state of the indoor air (that is, the inside air) is similarly measured from the signals of the room thermometer 13 and the room hygrometer 14, and the inside air enthalpy is calculated. Next, in step S3, the outside air enthalpy is compared with the inside air enthalpy. If the outside air enthalpy <the inside air enthalpy, one of the conditions for the outside air cooling is satisfied, and the process proceeds to the next step S4. When the condition of step S3 is not satisfied, the process proceeds to step S8.

In step S4, the outside air introduction condition is checked in addition to the condition satisfied in step S3 in consideration of the occupant's comfort. Specifically, the upper limit value Th, the lower limit value Tl, the upper limit value Hh, and the lower limit value Hl of the outside air temperature stored in the storage unit 17 and indicated by the hatched portion 20 in FIG. (That is, the shaded area 2
Check if it is within 0). If the condition of step S4 is satisfied, outside air cooling is executed in step S5. That is, control in the air blowing mode is performed. Specifically, the control device 100 controls the outside air damper 2 to be fully opened, the exhaust damper 3 to be fully opened, the return air damper 4 to be fully closed, and the two-way valve to be fully closed. In the air blowing mode, energy consumed through the cooling coil 5 (for example, electric energy consumed by a not-shown cooler that creates cold and hot water) is saved. Note that this energy may be energy that can cool air, for example, the two-way valve 6.
May be a type in which ice is previously stored in a tank connected to the end, that is, a type in which thermal energy is stored. Next, the process proceeds to step S7.

If the condition of step S4 is not satisfied, it is checked in step S8 whether the outside air cooling is ON. If the outside air cooling is OFF, the process proceeds to step S7. In step S7, after waiting for a timer for a fixed time, the control to return to step S1 is repeated.
On the other hand, if it is determined in step S8 that the outside air cooling is in the ON state, the process proceeds to step S6, in which the outside air cooling control is stopped and the cooling control is returned to the normal cooling coil. Perform control.
Specifically, for example, the opening degree of the outside air damper 2 is controlled to an opening corresponding to the air volume of 30%, the return air damper 4 is controlled to an opening of 70%, and the exhaust damper 3 is controlled to an opening of 30%. Further, control is performed to open the two-way valve 6 at an opening required for cooling, and the air blown by the blower fan 8 is cooled using the cooling coil 5. At this time, energy (for example, electric energy consumed by a not-shown cooler) is consumed through the cooling coil 5 to cool the air. Next, step S9
Then, the cooling mode switching count means 15 sets the value of the register N to N = N + 1 in order to count the cooling mode switching frequency N.

Next, in step S10, the outside air introduction condition value setting means 16 changes the outside air introduction condition based on the frequency value N stored in the register N. FIG. 3 shows details of step S10.
Are shown in steps S100 to S105. In the flowchart of FIG. 3, it is checked in step S100 whether the frequency value N is greater than a predetermined value L. N <=
In the case of L, the process proceeds to step S103. If N> L, it indicates that the frequency of switching between the air blowing mode and the cooling mode is high, so that the maximum values Th and Hh of the outside air introduction range indicated by the hatched portion 20 in FIG. Decrease. Specifically, the outside air introduction condition value setting unit 16 reads the maximum values Th and Hh of the outside air introduction range stored in the storage unit 17, and determines the predetermined values Δt1 and Δh1 from the respective Th and Hh values. Are respectively subtracted, and the values of the maximum values Th and Hh after the subtraction are written in the storage means 17 again.

That is, the set value is represented by the following equation. However,
In order to prevent extreme correction, the minimum value by this correction is determined in advance. Th = Th−Δt1, where Th <Tmin Th = Tmin Hh = Hh−Δh1, but Hh <Hmin Hh = Hmin Δt1: Temperature decrease when the switching frequency exceeds a certain value Δh1: Switching frequency Humidity decrease when a certain value is exceeded

Next, in step S102, the cooling mode switching counting means 15 clears the frequency count value N of the register to zero. As a result, the frequency of switching between the air blowing mode by the outside air cooling and the cooling mode by the cooling coil is suppressed, and the outside air introduction condition is optimized. However, in the above algorithm, since the set value is moved only in a direction in which hunting does not occur, logic for increasing the set value is necessary. Therefore, it is checked in step S103 whether the frequency value N is smaller than a predetermined value M. N>
If M = M, the process returns to step S7 in FIG. If N <M, it indicates that the frequency of switching between the air blowing mode using outside air cooling and the cooling mode using normal cooling is smaller than the reference, and therefore, in step S104, the outside air introduction condition value setting means 16 stores the storage means 16 The maximum values Th and Hh (that is, the maximum values of the hatched portion 20 in FIG. 7) of the outside air introduction range stored in the storage area are slightly increased. That is, the set value is represented by the following equation. However, in order to prevent extreme correction, the maximum value by this correction is determined in advance. Th = Th + Δt2 where Th> Tmax
Th = Tmax Hh = Hh + Δh2 where Hh> Hmax
Hh = Hmax Δt2: Temperature increase when the switching frequency exceeds a certain value Δh2: Humidity increase when the switching frequency exceeds a certain value

Next, in step S105, the cooling mode switching counting means 15 clears the frequency count value N of the register to zero. As described above, when the frequency value N is M <N <L, the setting value is not changed. Therefore, while the frequency value is within this range, the control system is stabilized without a change in the set value. Further, FIG. 7 showing the minimum value of the outside air introduction condition
Temperature minimum value Tl and humidity minimum value Hl
Since hunting occurs less frequently in the middle period such as autumn, it is sufficient that the adjuster or the like changes the set value at the time of periodic maintenance adjustment.

The method of changing the outside air introduction condition according to the switching frequency of the cooling mode has been described above. Next, a method of setting a cycle for measuring the frequency will be described. In the flowchart of FIG.
Specify with. For example, when it is desired to measure the frequency in a cycle of one day, one day is set in a timer (not shown), and the control is transferred to step S20 using an interrupt. In step S20, the control device 100 jumps to the process in step S0, and sets the recorded value of the frequency to 0 in step S0 as described above.
And the subsequent processing is continued as described above. This timer cycle is supposed to be one day, but any cycle can be set. In the above example, the set values are automatically updated, but the operator sets new initial values for periodic maintenance and the like,
It is also possible to make automatic settings from there. The above configuration may be a dedicated H / W, or may be executed by a program using a CPU and a memory.

As described above, according to the first embodiment, the outside air introduction range can always be appropriately maintained by the frequency of switching between the blowing mode using the outside air cooling and the cooling mode using the cooling coil. Hunting which repeats the cooling mode and the cooling mode by the cooling coil can be prevented, and a state in which the outside air cooling hardly occurs can be prevented. Therefore, it is possible to provide air-conditioning control that enables the occupant to be in a comfortable air-conditioning state.

Embodiment 2 FIG. FIG. 4 is an explanatory diagram showing Embodiment 2 of the present invention. 4, the same reference numerals as those in FIG. 1 denote the same or corresponding parts. Reference numeral 18 denotes an alarm generating unit that is provided inside the control device 101 and generates a maintenance alarm based on the count frequency counted by the cooling mode switching counting unit 15. 101 is an air conditioner controller 100 shown in FIG.
Is an air conditioner controller provided with an alarm generating means 18 instead of the outside air introduction condition value setting means. Here, the alarm generation means 18 is an alarm output means, and the air conditioner controller 101 is a control device.

Next, the operation will be described. Control device 1
01 is the same as that of FIG. However, step S10 is different. FIG. 5 shows a control device 1 according to the second embodiment.
11 is a flowchart showing the operation of the second embodiment, and shows processing performed in the second embodiment instead of step S10 in FIG. Accordingly, since S0 to S9 are basically the same as those in FIG. 2, the description will be omitted. Step S in FIG.
When the processing of N = N + 1 is completed in step S9, step S in FIG.
The processing moves to 100. In step S100, similarly to FIG. 3, it is checked whether N> L, and if not N> L, the process returns to step S7 in FIG. On the other hand, if N> L, the process moves to step S110.

Step S110 in FIG. 5 is a process for generating an alarm. Instead of slightly changing the set value, the alarm generating means 18 outputs an alarm for frequently switching the outside air cooling. After the alarm is output, the switching process between the cooling mode and the air blowing mode is stopped, and the mode is locked so as not to shift to the air blowing mode. If the maintenance person can immediately correct the maximum values Th and Hh of the outside air introduction range, the process may shift to step S7 after outputting the alarm.

As described above, according to the second embodiment,
The frequency of switching between the ventilation mode using outside air cooling and the cooling mode using the cooling coil is measured, and a maintenance alarm is generated only when the frequency exceeds a certain value, so that the outside air introduction range can be adjusted by maintenance personnel. In addition, the occupants can be in a comfortable air-conditioning state.

[0027]

As described above, according to the present invention, the outside air introduction range is set to an appropriate range, and there is an effect that hunting between the air blowing mode by the outside air cooling and the cooling mode by the cooling coil can be suppressed.

[Brief description of the drawings]

FIG. 1 is a functional block diagram illustrating an air conditioner according to a first embodiment of the present invention.

FIG. 2 is a flowchart showing an operation of the control device according to the first embodiment of the present invention.

FIG. 3 is a flowchart illustrating an operation of the control device according to the first embodiment of the present invention.

FIG. 4 is a functional block diagram illustrating an air conditioner according to a second embodiment of the present invention.

FIG. 5 is a flowchart showing an operation of the control device according to the second embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a conventional air conditioner.

FIG. 7 is a psychrometric chart showing the state of air.

FIG. 8 is a flowchart showing an operation of a conventional control device.

[Explanation of symbols]

1 air conditioner, 2 outside air intake damper, 3 exhaust damper, 4 return air damper, 5 cooling coil, 6 2-way valve, 7 humidifier, 8 blower fan, 9 return air fan, 1
0, 100, 101 Air conditioner controller, 11 thermometer, 12 hygrometer, 13 thermometer, 14 hygrometer, 15
Cooling mode switching counting means, 16 outside air introduction condition changing means, 17 storage means, 18 alarm generation means.

Claims (4)

[Claims] An outside air intake damper for changing an amount of air taken in from outside; cooling means for cooling air taken in from room; measuring means for measuring a state of the outside air and air in the room; Storage means for storing switching conditions between a cooling mode in which cooling means uses energy to cool the indoor air and a blowing mode in which the outdoor means takes in air to save energy consumed by the cooling means; and Control means for selecting the cooling mode or the air blowing mode based on the state of the air measured by the means and the switching condition, and controlling the cooling means and the damper based on the selection; and Changing means for detecting the number of times of switching during the air blowing mode, and changing the switching condition based on the number of times of switching; For example was the air conditioner. 2. The method according to claim 1, wherein the measuring unit includes an outside air measuring unit that measures an enthalpy of the outdoor air, and an inside air measuring unit that measures an enthalpy of the indoor air. The information which specifies the upper limit value and the lower limit value of the enthalpy in the blowing mode is stored, and the control means is such that the enthalpy of the outdoor air is lower than the enthalpy of the indoor air, and the enthalpy of the outdoor air is In the case of being between the upper limit value and the lower limit value in the air blowing mode, selecting the air blowing mode, the changing means, when the number of switching from the cooling mode to the air blowing mode is equal to or more than a predetermined number of times The air conditioner according to claim 1, wherein an upper limit value of the enthalpy is reduced. An outside air intake damper for changing an amount of air taken in from outside; a cooling unit for cooling the air taken in from the room; a measuring unit for measuring states of the outside air and the room air; A storage unit that stores a switching condition between a cooling mode in which the cooling unit cools the indoor air using energy and a blowing mode in which the outdoor unit takes in the outdoor air to save energy consumed by the cooling unit; Control means for selecting the cooling mode or the air blowing mode based on the state of air measured by the measuring means and the switching condition, and controlling the cooling means and the damper based on the selection; and the cooling mode And alarm output means for detecting the number of times of switching between the blow mode and outputting an alarm based on the number of times of switching. Conditioner. 4. A measuring means for measuring a state of the outdoor air and a state of the indoor air, a cooling mode for cooling the indoor air by using energy, and a blowing mode for saving energy by taking in the outdoor air. Storage means for storing the switching conditions, and the cooling mode or the air blowing mode is selected based on the state of the air measured by the measuring means and the switching conditions, and based on the selection, the air in the room is determined. A control device for an air conditioner, comprising: control means for controlling a cooling means for cooling; detecting means for detecting the number of times of switching between the cooling mode and the air blowing mode; and changing the switching condition based on the number of times of switching. .