US20120216557A1

US20120216557A1 - Vehicle air conditioning control system

Info

Publication number: US20120216557A1
Application number: US13/372,932
Authority: US
Inventors: Isamu Ito; Hideki Hashigaya
Original assignee: Suzuki Motor Corp
Current assignee: Suzuki Motor Corp
Priority date: 2011-02-24
Filing date: 2012-02-14
Publication date: 2012-08-30
Also published as: DE102012101402B4; DE102012101402A1; JP2012171581A; CN102650461B; CN102650461A; JP5640810B2

Abstract

To determine whether or not integral control is performed when a requested compressor output value is limited by an upper or lower limit, based on an evaporator temperature deviation, prevent divergence of an integral value, and make an evaporator temperature follow a target evaporator temperature during the integral control, an evaporator temperature deviation calculation means, integral control means for calculating an integral value based on an evaporator temperature deviation, and output value calculation means for calculating a requested compressor output value based on the integral value and limiting the requested compressor output value to calculate a compressor output value are provided.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent Application No. 2011-038312 filed Feb. 24, 2011.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a vehicle air conditioning control system, and specifically relates to a vehicle air conditioning control system capable of controlling the cooling performance of, e.g., an electric compressor or an external variable capacity compressor by means of a controller.
2. Description of Related Art
In the case of a vehicle air conditioning control system that sets a target value for, e.g., an electric compressor or an external variable capacity compressor by means of a control unit (also referred to as “A/C controller”) to make the compressor operate, in general, feedback control using PI control is used, and in order to bring an evaporator temperature close to a target evaporator temperature, an output value (a rotation speed for the electric compressor or a requested swash plate inclination for the external variable capacity compressor (also referred to as “duty”)) is changed by means of integral control.
As related art documents, JP 2010-964 A and JP 2010-89697 A are known.
In conventional vehicle air conditioning control systems, an output is subject to, e.g., limitations provided by external factors, such as engine rotation speed and vehicle drive power, a limitation according to an upper limit as a countermeasure for operating noise of the electric compressor and limitations according to upper/lower limits, which provide an operation rating for the electric compressor, and where the output is limited by these limitations, a corrected integral value is not reflected in the output, and an evaporator temperature cannot be brought close to a target evaporator temperature. Thus, in order to prevent divergence of an integral value, the integral control is halted (see FIG. 4).
It should be noted that where there is a difference between an output value Neop and a requested output value Nerq (both will be described later) (see determination (B03) in FIG. 4), it is determined that the output value is being limited by the external factors and the system.
However, as a result of the vehicle air conditioning control system halting the integral control, inconveniently, it takes too much time to follow the target value when the limitations are subsequently cancelled.
Furthermore, where there should be no harm in performing the integral control, for example, integral control to decrease the output value when the output value is being limited by an upper limit or integral control to increase the output value when the output value is being limited by a lower limit, the difference between the target evaporator temperature and the evaporator temperature cannot be reduced, that is, a problem inconveniently arises in that the control remains unstable.

SUMMARY OF THE INVENTION

An object of the present invention to determine whether or not integral control is performed when a requested compressor output value is limited by an upper limit or a lower limit, based on an evaporator temperature deviation, prevent divergence of an integral value, and where there is no harm in performing the integral control, make an evaporator temperature follow a target evaporator temperature.
Therefore, in order to eliminate the aforementioned inconveniences, the present invention provides a vehicle air conditioning control system including: evaporator temperature deviation calculation means for calculating a difference between an evaporator temperature and a target evaporator temperature as an evaporator temperature deviation; integral control means for calculating an integral value based on the evaporator temperature deviation calculated by the evaporator temperature deviation calculation means; and output value calculation means for calculating a requested compressor output value based on the integral value calculated by the integral control means and limiting the requested compressor output value to calculate a compressor output value, wherein the integral control means halts integral control when the requested compressor output value is limited, and the integral control means does not halt the integral control if the evaporator temperature deviation is not less than 0 when the requested compressor output value is limited by a lower limit, and does not halt the integral control if the evaporator temperature deviation is less than 0 when the requested compressor output value is limited by an upper limit.
As described in detail above, the present invention provides a vehicle air conditioning control system including: evaporator temperature deviation calculation means for calculating a difference between an evaporator temperature and a target evaporator temperature as an evaporator temperature deviation; integral control means for calculating an integral value based on the evaporator temperature deviation calculated by the evaporator temperature deviation calculation means; and output value calculation means for calculating a requested compressor output value based on the integral value calculated by the integral control means and limiting the requested compressor output value to calculate a compressor output value, wherein the integral control means halts integral control when the requested compressor output value is limited, and the integral control means does not halt the integral control if the evaporator temperature deviation is not less than 0 when the requested compressor output value is limited by a lower limit, and does not halt the integral control if the evaporator temperature deviation is less than 0 when the requested compressor output value is limited by an upper limit.
Accordingly, divergence of the integral value can be prevented, and where there is no harm in performing integral control (where the compressor output value is increased when the compressor output value is limited by a lower limit, or where the compressor output value is decreased when the compressor output value is limited by an upper limit), the evaporator temperature can be made to follow the target evaporator temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of a vehicle air conditioning control system (embodiment);

FIG. 2 is a diagram of a schematic configuration of a vehicle air conditioning control system (embodiment);

FIG. 3 is a flowchart for determination of a limitation of air conditioning by a vehicle air conditioning control system (embodiment);

FIG. 4 is a flowchart of feedback integral control before change (related art); and

FIG. 5 is a flowchart of feedback integral control after change (embodiment).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detail with reference to the drawings.
FIGS. 1 to 5 illustrate an embodiment of the present invention.
In FIGS. 1 and 2, reference numeral 1 denotes a vehicle air conditioning control system.
As illustrated in FIG. 2, the vehicle air conditioning control system 1 includes an external air introduction port 3 and an inner air circulation port 4 on the upstream side of an air conditioning passage 2, and switches between the external air introduction port 3 and the inner air circulation port 4 by means of an internal and external air switching door (also referred to as “inlet actuator”) 5.
Furthermore, a blower fan (also referred to as “air conditioning fan”) 6 is provided downstream of the internal and external air switching door 5 to blow an air to the downstream side of the air conditioning passage 2 by means of the blower fan 6.
Furthermore, at a part of the air conditioning passage 2 downstream of the blower fan 6, an evaporator (also referred to as “evaporator core”) 7 is provided, and an HVAC unit 8 for cooling, heating and air conditioning is provided downstream of the evaporator 7.
The HVAC unit 8 includes an air mixing door (also referred to as “A/M actuator”) 9 that switches between the air conditioning passage 2 for cooling and that for heating, and also includes a heater core 10 provided at a part thereof used for heating.
Furthermore, at a part of the air conditioning passage 2 downstream of the HVAC unit 8, a defroster duct 12, which provides a defroster outlet 11, a ventilation duct 14, which provides a ventilation outlet 13, and a foot duct 16, which provides a foot outlet 15.
A first outlet switching door 17 that switches between the defroster outlet 11 of the defroster duct 12 and the ventilation outlet 13 of the ventilation duct 14 is provided, while a second outlet switching door 18 that opens and closes the foot outlet 15 of the foot duct 16 is provided.
The first outlet switching door 17 and the second outlet switching door 18 are also referred to as “mode actuators”.
The vehicle air conditioning control system 1 is an air conditioning control system for a vehicle including a motor (not illustrated) for driving the vehicle (not illustrated) mounted therein, and includes a main controller 19 and an A/C controller (also referred to as “control unit”) 20.
Here, the main controller 19 is referred to as an “EV controller” in the case of an EV, an “HEV (hybrid vehicle) controller” in the case of an HEV and an “engine controller” in the case of an engine vehicle. In the case of an engine vehicle, rotation speed control is also performed.
As illustrated in FIGS. 1 and 2, the A/C controller 20 is connected to the main controller 19.
The A/C controller 20 connects an incorporable A/C panel 21, an evaporator temperature sensor 22 arranged downstream of the evaporator 7, and a sensor detecting device 23 required for A/C control.
The A/C controller 20 is further connected to the air mixing door 9, the first and second outlet switching doors 17 and 18, and an electric compressor (or “external variable capacity compressor”) 24 to output control signals to them.
As illustrated in FIG. 1, the A/C controller 20 includes target evaporator temperature calculation means 25, evaporator temperature deviation calculation means 26, proportional control means 27, integral control means 28, upper and lower limit value limitation means 29 and output value calculation means 30.
The target evaporator temperature calculation means 25 calculates a target evaporator temperature based on values detected by respective sensors and value set via the panel, which are input from an A/C system 31.
The evaporator temperature deviation calculation means 26 calculates a difference between an evaporator temperature from the A/C system 31 and the target evaporator temperature from the target evaporator temperature calculation means 25 as an evaporator temperature deviation.
The proportional control means 27 calculates a proportional value based on the evaporator temperature deviation calculated by the evaporator temperature deviation calculation means 26.
The integral control means 28 calculates an integral value based on the evaporator temperature deviation calculated by the evaporator temperature deviation calculation means 26.
The output value calculation means 30 calculates a requested compressor output value based on the proportional value calculated by the proportional control means 27 and the integral value calculated by the integral control means 28, and limits the requested compressor output value to calculate a compressor output value.
Then, the vehicle air conditioning control system 1 halts integral control by means of the integral control means 28 when the requested compressor output value is limited.
Here, the integral control means 28 does not halt the integral control if the evaporator temperature deviation is not less than 0 when the requested compressor output value is limited by a lower limit, and does not halt the integral control if the evaporator temperature deviation is less than 0 when the requested compressor output value is limited by an upper limit.
More specifically, control performed by the vehicle air conditioning control system 1 is provided by addition of a condition that integral control used for calculation of an availability (output) signal output from the A/C controller 20 to the electric compressor 24 in the system including the A/C controller 20 that controls HVAC, the A/C panel 21 operated by a passenger, the HVAC unit 8 that performs cooling and heating and the electric compressor 24 whose availability can be controlled by the A/C controller 20, which are illustrated in FIG. 2, is performed even when an output is limited, in order to enhance the capability to follow a target evaporator temperature.
Conventionally, the integral control is forcibly halted for prevention of divergence when the output is being limited.
However, as indicated in Table 1 below, when the requested compressor output value is limited by a lower limit, if the evaporator temperature deviation is not less than 0, the integral control is not halted by the integral control means 28 and is held in an “actuated” state, and when the requested compressor output value is limited by an upper limit, if the evaporator temperature deviation is less than 0, the integral control is not halted by the integral control means 28 and is held in an “actuated” state.

TABLE 1

Table 1 Integral control operation under rotation speed and required
swash plate inclination limitation

		Evaporator
		temperature	Integral
No	Limitation	deviation	control	Remarks

1	Lower	Not less than 0	Actuated	Since the evaporator temperature is
	limit	(target evaporator		higher than the target evaporator
		temperature ≦		temperature, the output value is
		evaporator		increased by the integral calculation
		temperature)		even when the requested compressor
				output value is being limited by a lower
				limit, in order to decrease the
				evaporator temperature, i.e., the integral
				control operation can be performed
				even under limitation.
2		Less than 0	Halted	Since the evaporator temperature is
		(target evaporator		lower than the target evaporator
		temperature >		temperature, the integral calculation is
		evaporator		halted to suppress variation of the
		temperature)		output value.
3	Upper	Not less than 0	Halted	Although the evaporator temperature is
	limit	(target evaporator		higher than the target evaporator
		temperature ≦		temperature, the integral calculation is
		evaporator		halted because the requested
		temperature)		compressor output value is being
				limited by an upper limit, in order to
				suppress an increase in the integral
				value

4		Less than 0	Actuated	Since the evaporator temperature is
		(target evaporator		lower than the target evaporator
		temperature >		temperature, the output value is
		evaporator		decreased even when the requested
		temperature)		compressor output value is being
				limited by an upper limit, in order to
				bring the evaporator temperature close
				to the target temperature, i.e., the
				integral control operation can be
				performed even under the limitation.
5	No	— (No effects)	Actuated	Since no limitations are provided, there
	limitations			are no effects on the integral control

In addition to the above, in Table 1, when the requested compressor output value is limited by a lower limit, if the evaporator temperature deviation is not less than 0, which is a first state, the integral control is not halted but is held in an “actuated” state. In other words, since the evaporator temperature is higher than the target evaporator temperature, the output value is increased by the integral calculation even when the requested compressor output value is limited by the lower limit, in order to decrease the evaporator temperature. Thus, the integral control can be performed even under the limitation.
Furthermore, in Table 1, when the requested compressor output value is limited by a lower limit, if the evaporator temperature deviation is less than 0, which is a second state, the integral control is halted. In other words, since the evaporator temperature is lower than the target evaporator temperature, the integral calculation is halted to suppress variation of the output value.
Furthermore, in Table 1, when the requested compressor output value is limited by an upper limit, if the evaporator temperature deviation is not less than 0, which is a third state, the integral control is halted. In other words, although the evaporator temperature is higher than target evaporator temperature, the integral value calculation is halted because the requested compressor output value is being limited by the upper limit, in order to suppress an increase in the output value.
Furthermore, in Table 1, when the requested compressor output value is limited by an upper limit, if the evaporator temperature deviation is less than 0, which is a fourth state, the integral control is not halted but is held in an “actuated” state. In other words, since the evaporator temperature is lower than the target evaporator temperature, the output value is lowered by the integral calculation even when the requested compressor output value being limited by the upper limit, in order to bring the evaporator temperature close to the target. Thus, the integral control can be performed even under the limitation.
Furthermore, in Table 1, when no limitations are provided, which is a fifth state, since there are no effects, the integral control is not halted and is held in an “actuated” state. In other words, because of no limitations provided, there are no effects on the integral control.
Consequently, in the case of
target evaporator temperature≦evaporator temperature
in which the output value is increased when the output value is being limited by a lower limit value, and in the case of
target evaporator temperature>evaporator temperature
in which the output value is decreased when the output value is being limited by an upper limit, the evaporator temperature can be made to follow the target evaporator temperature by means of integral control.
Even where the output value is unintentionally limited by an upper and a lower limit in a state that does not fall under any of the states described herein, also, the evaporator temperature can be made to follow the target evaporator temperature by means of integral control, providing wider control response.
Accordingly, divergence of the integral value can be prevented, and when there are no effects in actuating the integral control (when the compressor output value is increased when the compressor output value is limited by a lower limit, or when the compressor output value is decreased when the compressor output value is limited by an upper limit), the evaporator temperature can be made to follow the target evaporator temperature.
Next, an operation will be described according to a flowchart for determination of a limitation of air conditioning by the vehicle air conditioning control system 1, which is illustrated in FIG. 3, will be described.
The output value arithmetic operation described below is a mere example, and various applications and alterations are possible.
Upon start of a program for determining a limitation of air conditioning by the vehicle air conditioning control system 1 (A01), the A/C controller 20 proceeds to processing (A02) for acquiring values detected by the respective sensors and an evaporator temperature and values set via the A/C panel from the A/C system 31.
Then, after the processing (A02), the A/C controller 20 proceeds to processing (A03) for calculating a target evaporator temperature, which is a target of cooling performance of the air conditioning system, by means of the target evaporator temperature calculation means 25 in the A/C controller 20.
Calculation of the target evaporator temperature is a known technique that is generally employed, and thus, description thereof will be omitted.
After the processing (A03) for calculating a target evaporator temperature, the A/C controller 20 proceeds to processing (A04) for obtaining an evaporator temperature deviation from the target evaporator temperature and the evaporator temperature, by means of the evaporator temperature deviation calculation means 26 in the A/C controller 20.
Here, the state of target evaporator temperature>evaporator temperature means that the air conditioning system is in an excessive cooling state or the external air temperature is low, and the state of target evaporator temperature<evaporator temperature means that the performance of the air conditioning system is insufficient.
Then, after the processing (A04) for obtaining an evaporator temperature deviation from the target evaporator temperature and the evaporator temperature, the A/C controller 20 proceeds to processing (A05) for performing an arithmetic operation for feedback proportional control, by means of the proportional control means 27 in the A/C controller 20.
In the arithmetic operation for feedback proportional control in the processing (A05),
proportional control calculation value P=f(evaporator temperature deviation).
The aforementioned arithmetic operation for proportional control is also control that is performed in general, and thus, description thereof will be omitted.
Furthermore, after the processing (A05) for performing an arithmetic operation for feedback proportional control, the A/C controller 20 proceeds to processing (A06) for performing an arithmetic operation for feedback integral control by means of the integral control means 28 in the A/C controller 20.
In the arithmetic operation for feedback integral control in the processing (A06),
integral control calculation value I=f(evaporator temperature deviation).
Then, after the processing (A06) for performing an arithmetic operation for feedback integral control, the A/C controller 20 proceeds to processing (A07) for performing an arithmetic operation to obtain a requested output value Nerq.
In the processing (A07), the requested output value Nerq is obtained by summing up the proportional value and the integral value as indicated by the following expression:
requested output value Nerq=P+I.
After the processing (A07) for performing an arithmetic operation to obtain the requested output value Nerq, the A/C controller 20 proceeds to processing (A08) for performing an arithmetic operation to obtain an output value Neop.
In the processing (A08), the output value Neop is obtained by the following expression:
output value Neop=min(max(Nerq, lower limit value), upper limit value).
In other words, in the processing (A08), where the output is limited by an external limitation, if the limitation is a lower limit (such as, e.g., an operating rating of the compressor), the compressor provides an output (Neop) equal or higher than the lower limit value, and if the limitation is an upper limit (such as, e.g., an upper limit rating as a countermeasure for operating noise of the compressor, an operation rating of the compressor, a limitation of power consumption requested on the driving side or a limitation of the engine rotation speed) or both, the compressor provides an output (Neop) equal or lower than the upper limit.
Then, after the processing (A08) for performing an arithmetic operation to obtain the output value Neop, the A/C controller 20 returns to the processing (A02) for obtaining values detected by the respective sensors, an evaporator temperature and values set via the A/C panel.
An operation will be described according to a flowchart of feedback integral control before change in FIG. 4.
Although the flowchart of feedback integral control before change, which is illustrated in FIG. 4, is used for description of the related art, provision of a description with reference to a flowchart of feedback integral control after change, which is illustrated in FIG. 5, following the description of the related art with reference to FIG. 4 clarifies differences therebetween, and thus, the description is provided in this position.
Upon start of a program for feedback integral control before change (B01), the program proceeds to determination (B02) of whether or not a certain period of time has passed, by means of an integral control timer t.
If the result of the determination (B02) is NO, the determination (B02) is repeated until the result of the determination (B02) becomes YES.
If the result of the determination (B02) is YES, the program proceeds to determination (B03) of whether or not an output value Neop is equal to a requested output value Nerq, that is,
output value Neop=requested output value Nerq.
If the result of the determination (B03) is NO, the program proceeds to determination (B02) of whether or not a certain period of time has passed, by means of the integral control timer t.
If the result of the determination (B03) is YES, the program proceeds to processing (B04) for calculating an integral control calculation value I according to the following expression:
integral control calculation value I=I(n−1)+f(evaporator temperature deviation),
and subsequently proceeds to the determination (B02) of whether or not a certain period of time has passed, by means of the integral control timer t.
An operation will be described with reference to a flowchart of feedback integral control after change in FIG. 5.
Upon the start (C01) of a program of feedback integral control after change, the program proceeds to determination (C02) of whether or not a certain period of time has passed, by means of the integral control timer t. Here, integral control is performed after elapse of driving time of a timer for performing the integral control.
If the result of the determination (C02) is NO, the determination (C02) is repeated until the result of the determination (C02) becomes YES.
If the result of the determination (C02) is YES, the program proceeds to determination (C03) of whether or not an output value Neop is equal to a requested output value Nerq, that is,
output value Neop=requested output value Nerq.
If the result of the determination (C03) is YES, it can be determined that no limitations are provided, and in order to perform calculation for integral control, the program proceeds to processing (C04) for calculating an integral control calculation value I according to the following expression,
integral control calculation value I=I(n−1)+f(evaporator temperature deviation),
and subsequently proceeds to determination (C02) of whether or not a certain period of time has passed, by means of the integral control timer t.
If the result of determination (C03) of whether or not the output value Neop is equal to the requested output value Nerq, that is,
output value Neop=requested output value Nerq
is NO, it can be determined that the output is limited, and the program proceeds to determination (C05) of whether or not the output value Neop is larger than the requested output value Nerq, that is,
output value Neop>requested output value Nerq.
Then, if the result of the determination (C05) is YES, it can be determined that the output value Neop is limited by a lower limit, and thus, the program proceeds to determination (C06) of whether or not the evaporator temperature deviation is not less than 0, that is,
evaporator temperature deviation≧0.
If the result of the determination (C05) is NO, it can be determined that the output value Neop is limited by an upper limit, and thus, the program proceeds to determination (C07) of whether or not the evaporator temperature deviation is less than 0, that is,
evaporator temperature deviation<0.
In the determination (C06) of whether or not the evaporator temperature deviation is not less than 0, that is,
evaporator temperature deviation≧0,
if the result of the determination (C06) is YES, that is, if
evaporator temperature deviation≧0
when the output value Neop is being limited by the lower limit, the evaporator temperature has not reached the target temperature even though only the lower limit is provided, and then, the program proceeds to the processing (C04) for calculating the integral control calculation value I according to the following expression:
integral control calculation value I=I(n−1)+f(evaporator temperature deviation)
in order to increase the output value by means of integral calculation so as to make the evaporator temperature follow the target temperature.
If the result of the determination (C06) is NO, the program directly returns to the determination (C02) of whether or not a certain period of time has passed, by means of the integral control timer t.
In the determination (C07) of whether or not the evaporator temperature deviation is less than 0, that is,
evaporator temperature deviation<0,
if the result of the determination (C07) is YES, that is, if
evaporator temperature deviation<0
when the output value Neop is limited by the upper limit, the output value is excessively large, and thus, the program proceeds to the processing (C04) for calculating the integral control calculation value I according to the following expression,
integral control calculation value I=I(n−1)+f(evaporator temperature deviation)
in order to decrease the output value by means of integral calculation so as to make the evaporator temperature follow the target temperature.
If the result of the determination (C07) is NO, a corrected integral value is not reflected in the output, resulting in divergence of the integral value, and thus, no integral calculation is performed and the program directly returns to the determination (C02) of whether or not a certain period of time has passed, by means of the integral control timer t.
The present invention is not limited to the above embodiment, and various applications and alterations are possible.
For example, the present invention can provide a special configuration applicable to control performed by a system for controlling cooling performance of a compressor (an electric compressor or an external variable capacity compressor) in terms of an engine rotation speed, in which a limitation is provided on the engine rotation speed and feedback is used in integral control.
Furthermore, the present invention can provide a special configuration in which determination of whether or not a limitation is provided is made not by comparison between the output value Neop and the requested output value Nerq, but by directly reading the content of a limitation and determining whether or not a limitation is provided based on the data.
Furthermore, the present invention can provide a special configuration in which the calculation of an output value in limiting air conditioning in FIG. 3 is performed based not only on PI (proportional and integral) control, but also on a feed-forward value (a basic output value) determined by the target evaporator temperature and environmental conditions (e.g., an external air temperature and an air volume).

EXPLANATION OF REFERENCE SYMBOLS

1 vehicle air conditioning control system
2 air conditioning passage
3 external air introduction port
4 inner air circulation port
6 blower fan (or also referred to as “air conditioning fan”)
7 evaporator (“evaporator core”)
8 HVAC unit
10 heater core
11 defroster outlet
13 ventilation outlet
15 foot outlet
19 main controller
20 A/C controller (or also referred to as “control unit”)
21 A/C panel
22 evaporator temperature sensor
23 sensor detecting device
24 electric compressor (or external variable capacity compressor)
25 target evaporator temperature calculation means
26 evaporator temperature deviation calculation means
27 proportional control means
28 integral control means
29 upper and lower limit value limitation means
30 output value calculation means

Claims

1. A vehicle air conditioning control system comprising:

evaporator temperature deviation calculation means for calculating a difference between an evaporator temperature and a target evaporator temperature as an evaporator temperature deviation;

integral control means for calculating an integral value based on the evaporator temperature deviation calculated by the evaporator temperature deviation calculation means; and

output value calculation means for calculating a requested compressor output value based on the integral value calculated by the integral control means and limiting the requested compressor output value to calculate a compressor output value, wherein

the integral control means halts integral control when the requested compressor output value is limited, and

the integral control means does not halt the integral control if the evaporator temperature deviation is not less than 0 when the requested compressor output value is limited by a lower limit, and does not halt the integral control if the evaporator temperature deviation is less than 0 when the requested compressor output value is limited by an upper limit.