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BACKGROUND OF THE INVENTION
This invention relates to a control system for controlling the operation of
a refrigeration system, particularly an automotive air-conditioning
system, to obtain maximum efficiency and to conserve energy.
Air-conditioning systems for automotive vehicles, such as automobiles,
trucks and buses, are subject to widely varying operating conditions (for
example, heat loads and compressor speeds), and are usually inefficient in
that their system capacities do not always match their loads. Such
inefficient operation wastes energy and significantly affects the
vehicle's fuel consumption. The control arrangement of the present
invention, on the other hand, controls the operation of a refrigeration
system in a unique way so that the system capacity is regulated to just
meet the needs of the vehicle compartment to be cooled. By maintaining a
correct balance between system capacity and refrigeration load at all
times, no energy is wasted and fuel economy is enhanced.
SUMMARY OF THE INVENTION
The control system of the invention is incorporated in a refrigeration
system of the type where refrigerant flows through a closed vapor cycle
refrigeration circuit having an evaporator, a controlled displacement
compressor, a condenser and an automatic expansion device. The flow rate
through the evaporator is regulated by the control system in order to
maintain a substantially constant desired temperature in a space to which
cooled air is supplied by the refrigeration system. The control system
comprises means, including a first temperature sensor located in the space
to be temperature controlled, for providing a space temperature signal
representing the actual ambient temperature therein. There are means,
including an adjustable device, for providing a temperature set point
signal representing the desired temperature for the temperature controlled
space. Means respond to the space temperature signal and to the
temperature set point signal to produce a temperature control point signal
which represents a desired evaporator refrigerant outlet temperature,
referenced with respect to a predetermined desired minimum evaporator
outlet temperature. Means, including a second temperature sensor
positioned adjacent to the evaporator outlet, provide an evaporator outlet
temperature signal representing the actual temperature of the refrigerant
at the evaporator outlet. There are means responsive to the temperature
control point signal and to the evaporator outlet temperature signal for
providing an error signal which varies as a function of the difference
between the desired control point temperature and the actual evaporator
outlet temperature. Finally, the control system of the invention comprises
means responsive to the error signal for varying the displacement of the
controlled displacement compressor to modulate the refrigerant flowing
through the evaporator to establish the evaporator outlet temperature at
the desired control point, thereby to maintain the controlled space at the
desired temperature.
DESCRIPTION OF THE DRAWING
The features of the invention which are believed to be novel are set forth
with particularity in the appended claims. The invention, together with
further advantages and features thereof, may best be understood, however,
by reference to the following description in conjunction with the
accompanying drawing in which:
FIG. 1 schematically illustrates a control system, constructed in
accordance with one embodiment of the invention, and the manner in which
it is incorporated in a refrigeration system, shown specifically as an
automotive air-conditioning system, and
FIG. 2 is a characteristic curve that will be helpful in understanding the
operation of the control system.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
The disclosed air-conditioning system includes a controlled displacement
compressor 10, a condenser 12, an automatic expansion device or valve 13
and an evaporator 15, the four components being intercoupled in series to
form a conventional closed vapor cycle refrigeration circuit. Refrigerant
gas is compressed in compressor 10 and supplied to condenser 12 where it
is condensed to liquid refrigerant and delivered to expansion device 13.
The refrigerant expands in device 13 and emerges as a two-phase mixture of
liquid and gas but primarily a liquid. As the two-phase mixture then flows
through evaporator 15, which is in heat exchange relation with air
supplied to the vehicle compartment space to be cooled, heat is
transferred from the air to the refrigerant and the entirety of the
refrigerant vaporizes and assumes its gaseous state. The refrigerant gas
at the evaporator outlet is then passed to the suction inlet of the
compressor. In well-known manner, line 17 provides external pressure
feedback from the evaporator outlet to automatic expansion valve 13 to
maintain a constant evaporator outlet pressure.
In order to control the flow rate of the refrigerant through the
refrigeration circuit, and thus through evaporator 15, the displacement of
compressor 10 is made variable. By changing the displacement, the
compressor capacity is varied and consequently the refrigerant flow.
Preferably, compressor 10 is constructed as illustrated and described in
detail in U.S. Pat. No. 3,861,829, issued Jan. 21, 1975 in the name of
Richard W. Roberts et al., and assigned to the present assignee. In the
compressor shown in that patent, the pressure in a crankcase cavity, which
is vented to the compressor's suction inlet, determines the stroke of a
plurality of pistons which in turn determines the compressor displacement.
The crankcase is pressurized by the leakage of high pressure gas past the
pistons (called piston blow-by) and into the crankcase, and by regulating
the escape of the blow-by gas from the crankcase through the vent to the
suction line, the crankcase pressure may be changed to vary the compressor
displacement and thus the capacity. As described in the Roberts et al
patent, the crankcase pressure is controlled by varying a control valve in
the vent to provide an adjustable restriction or orifice therein. The
greater the restriction in the vent line, the greater the crankcase
pressure and the lower the displacement. In other words, the displacement
varies inversely with the crankcase pressure. As taught in the Roberts et
al patent, the crankcase pressure is preferably between five and ten
percent of the difference between suction and discharge pressure. For
example, when operating at 200 psig discharge and 30 psig suction, the
crankcase pressure should be controlled between 38.5 and 47 psig.
In the illustrated embodiment of the present invention, the crankcase
pressure in compressor 10 is adjusted by means of a solenoid operated
control valve 21, the inlet of which is connected via line 22 to the
crankcase while the outlet of the valve connects through line 23 to the
compressor's suction inlet. By controlling the energization of solenoid
coil 24 (by a pulse width modulated signal in a manner to be described),
control valve 21 may be modulated to effectively interpose a variable
orifice between lines 22 and 23. Coil 24 is alternately energized and
de-energized, namely cycled on and off, in response to the pulse width
modulated signal. The greater the energization of coil 24 (namely the
greater the duty cycle which is the ratio of each energized interval
relative to the time duration or period of a complete cycle), the less
restriction introduced by valve 21 and the lower the crankcase pressure.
Hence, the displacement of compressor 10, and consequently the flow rate
of the refrigerant through the refrigeraton circuit, are directly
proportional to the duty cycle of coil 24.
Before the control system for coil 24 is discussed, it will be helpful to
consider the characteristic curve 25 of FIG. 2 which plots the evaporator
effectiveness or capacity (on the ordinate or y-axis) as a function of the
temperature of the refrigerant at the evaporator outlet (on the abscissa
or x-axis). As illustrated by curve 25, the two characteristics are
inversely related. As the evaporator refrigerant outlet temperature
increases, the capacity of the evaporator reduces. To explain, the heat
transfer coefficient of an evaporator is much greater when heat, from the
air to be cooled, is transferred to a two-phase, liquid/gas refrigerant
mixture than when the heat is transferred to refrigerant gas only. After
the liquid/gas mixture vaporizes completely and becomes all gas as it
travels through the evaporator, any additional heat taken from the air
superheats the gas and the effectiveness of the evaporator is decreased.
The lower the refrigerant flow rate through the evaporator, the greater
the superheat and the greater the evaporator refrigerant outlet
temperature. Hence to maximize the evaporator capacity, the flow rate
should be sufficiently high that the refrigerant remains a two-phase
liquid/gas mixture throughout almost the entire evaporator. Preferably,
the refrigerant should be converted entirely to gas before it reaches the
end of the evaporator and a small minimum amount of superheat should then
be added. The evaporator outlet temperature therefore should not drop
below a predetermined minimum level. Otherwise, refrigerant liquid may
flow through the suction line and into the compressor, causing structural
damage thereof.
Dashed construction line 26 in FIG. 2 designates the desired minimum
evaporator outlet temperature in the illustrated embodiment. Thus, the
intersection of that dashed line with curve 25 indicates the maximum
evaporator effectiveness at which the system will operate. When the flow
rate of the refrigerant is adjusted so that the evaporator outlet
temperature is at the desired minimum, a desired minimum superheat is
maintained in the refrigerant gas at the evaporator outlet. If the outlet
temperature is allowed to drop below the desired minimum temperature
(namely to the left of dashed line 26), insufficient superheat would be
present and refrigerant liquid would flow into the compressor. As will be
seen, the evaporator outlet temperature, at which the refrigeration system
operates, will automatically be selected so that the refrigeration
capacity is matched to the heat load at all times to provide the most
efficient manner of operation and to expend the least energy. For
convenience, the selected desired evaporator outlet temperature along the
abscissa in FIG. 2 will be referred to as the control point temperature.
In a manner to be explained, the refrigerant flow through the evaporator
will automatically be regulated to establish the evaporator outlet
temperature at the appropriate control point, along the abscissa, required
to maintain the vehicle compartment space at the desired space
temperature.
Turning now to the control system, a temperature sensor, in the form of a
thermistor 28, is located in the vehicle compartment space to be air
conditioned. For example, sensor 28 may be located near the vehicle's
dashboard. One terminal of sensor 28 is connected to a ground plane of
reference potential while its other terminal is connected through a
resistor 29 to a source of positive DC voltage labelled V+. Preferably,
the magnitude of that DC voltage is around +10 volts and the ground plane
of reference potential is zero volts. Of course, all of the terminals in
the drawing marked "V+" are tied or connected to the same DC power source.
Thermistor 28 has a negative temperature coefficient so that its
resistance is an inverse function of the actual temperature of the
compartment space which is to be controlled. In other words, if the space
temperature increases, for example, the resistance of thermistor 28
decreases and the voltage at the junction of sensor 28 and resistor 29
decreases.
The junction of temperature sensor 28 and resistor 29 connects through a
resistor 31 to the non-inverting or (+) input of integrated circuit
operational amplifier (IC op amp) 32 which preferably is a type 3401
current mode Norton amplifier. Actually, all of the other IC op amps shown
in the drawing (namely those amplifiers designated by the reference
numbers 39, 51, 57, 58, 59 and 67) are also preferably type 3401
amplifiers. While not specifically illustrated, each of the op amps is
connected to the V+ power supply so that its operating voltage will be +10
volts.
A type 3401 Norton amplifier requires input currents at its inputs rather
than input voltages. For this reason, resistor 31 and all of the other
corresponding or equivalent resistors in series with the inputs to the
operational IC amplifiers are needed to convert from input voltages to
input currents. As is also characteristic of a Norton amplifier, good
signal isolation is provided for summing terms since both of its inputs
are at virtually ground or zero potential. Moreover, a Norton amplifier,
with the addition of a polarized capacitor between its output and its
inverting or (-) input, forms an integrator circuit wherein the output
voltage is always positive with respect to the voltages at the inputs. As
will be appreciated, all of the signal voltages in the control system will
be positive with respect to ground.
Since the voltage at the junction of thermistor 28 and resistor 29 varies
inversely with the actual ambient temperature of the air-conditioned space
in the vehicle, the current signal supplied to the (+) input of amplifier
32 likewise varies as an inverse function of the actual space temperature
and thus represents that space temperature. The desired set point
temperature for the compartment space may be established, usually by the
driver of the vehicle, merely by manipulating an adjustable device in the
form of a potentiometer 34 which, of course, is preferably mounted on the
dashboard. Hence, the combination of thermistor 28 and potentiometer 34
effectively constitutes a thermostat. A current signal will thus be
supplied through series resistor 35 to the inverting or (-) input of
amplifier 32. The amplitude level of this input current (which may be
called the temperature set point signal since it represents the desired
space temperature) will be directly proportional to the set point selected
or thermostat setting. The higher the desired space temperature, the
greater the current flowing into the (-) input.
The Norton type amplifier functions as a differential amplifier in that the
output voltage is proportional to the difference between the two input
current signals. Because of the presence of polarized capacitor 37,
amplifier 32 also serves as an integrator. If the current entering the (+)
input of amplifier 32 is greater than that flowing into the (-) input, the
output voltage increases gradually in a positive direction (namely it
integrates or sweeps upwardly to form a ramp shaped waveform) to an
amplitude level determined by the difference between the two input
signals. If there is then a change in either of the input signals such
that the current supplied to the (-) input becomes greater than the
current into the (+) input, the output voltage decreases gradually in a
negative direction (namely it sweeps or integrates downwardly) toward zero
or ground potential. As mentioned, the output voltage of amplifier 32 can
never drop below the voltage at the two inputs which are essentially at
ground potential.
Amplifier 32 thus compares the space temperature signal and the temperature
set point signal to produce an output voltage signal which varies as a
function of the difference between the actual and desired space
temperatures. This voltage signal is converted by series resistor 38 to a
current signal for application to the (-) input of amplifier 39. As will
be made apparent, the current signal flowing through resistor 38 and into
the (-) input effectively represents the desired evaporator refrigerant
outlet temperature, referenced with respect to the predetermined desired
minimum evaporator outlet temperature, and may thus be called the
temperature control point signal since it indicates the desired control
point on the abscissa of FIG. 2 where the system should operate in order
to properly cool the controlled space. In other words, the control point
temperature is a function of the output of amplifier 32.
A temperature sensor, in the form of a thermistor 41, is physically
attached to or positioned adjacent to the evaporator outlet in heat
exchange relation so as to monitor the temperature of the refrigerant
after it leaves the evaporator. Sensor 41 is similar to sensor 28 in that
it also has a negative temperature coefficient so that its resistance is
inversely proportional to the evaporator refrigerant outlet temperature.
If the temperature of the refrigerant decreases, for example, the
resistance of thermistor 41 increases and the voltage at the junction of
resistor 42 and temperature sensor 41 increases. The voltage signal
produced at the junction thus represents the actual temperature of the
refrigerant at the evaporator outlet and the amplitude of this voltage
signal varies inversely with the temperature. By virtue of series resistor
43, the voltage signal is converted to a current signal and supplied to
the (-) input of amplifier 39. The current flowing through resistor 43 may
therefore be referred to as the evaporator outlet temperature signal.
Since there are two signals fed to the (-) input of amplifier 39, a
summation or addition of those signals occurs at the input. As will be
appreciated, if the compartment space becomes colder than desired or if
the evaporator outlet temperature becomes colder than the desired control
point, the effect on amplifier 39 will be the same.
The voltage divider comprising resistors 44 and 45 provides a reference
voltage, at the junction of those resistors, which is converted by series
resistor 46 to an input current for the (+) input of amplifier 39. The
current signal serves as a reference signal whose amplitude represents the
desired minimum temperature for the refrigerant at the evaporator outlet.
In the illustrated case, the level of the reference signal is such that
during normal operation of the air-conditioning system the refrigerant
temperature at the evaporator outlet is prevented from dropping below the
desired minimum denoted by dashed line 26 in FIG. 2.
Amplifier 39 functions primarily as a differential amplifier, its response
characteristics being determined by feedback resistor 47. Except when the
control system is operating at the control point denoted by dashed line
26, the amplifier's output voltage, which is always a positive voltage
between 0 and +10 volts and may be called the error voltage signal, varies
above (in a positive direction) and below (in a negative direction) with
respect to a reference level depending on the difference between the input
currents. The error voltage signal therefore varies as a function of the
difference between the desired control point temperature, the actual
evaporator outlet temperature and the desired minimum evaporator outlet
temperature. The reference level of the error signal at the output of
amplifier 39 varies as the control point temperature changes, so each time
a new control point is selected along the abscissa in FIG. 2 the error
signal stabilizes around a new reference level.
When the control system is operating at the control point indicated by
dashed line 26, the input signals to amplifier 39 will be such that the
reference level will have its maximum amplitude and the error signal will
be limited to that amplitude. At other times when the control system is
operating at some control point to the right of dashed line 26, the error
signal is permitted to vary above and below the reference level.
Under steady state conditions, the input currents supplied to amplifier 39
will be constant and have a fixed relationship to hold the error signal at
the required reference level. If the current entering the (-) input of
amplifier 39 then increases, the error signal decreases below its
reference level. On the other hand, if the input current at the (-) input
decreases, the output voltage of amplifier 39 increases above the
reference level.
A pair of series-connected resistors 48 and 49 convert the error voltage
signal to an error current signal for application to the inverting or (-)
input of amplifier 51 which, due to the inclusion of polarized capacitor
52, serves as an integrator. Current is fed into the (+) input by means of
resistors 53, 54, 55 and 56 to set the bias level for amplifier 51. The
output of amplifier 51 varies from essentially zero potential to V+ (+10
volts in the illustrated case) as determined by the amplitude of the error
signal supplied to the amplifier's (-) input. Of course, since amplifier
51 is an integrator, anytime there is a change in the magnitude of the
error signal the output of the amplifier does not change abruptly but
rather increases or decreases gradually.
A pulse width modulated signal is produced having a waveshape dependent on
the output of amplifier 51. To explain, such a signal is rectangular
shaped, containing periodically recurring positive-going pulse components
with intervening negative-going pulse components. The frequency will be
constant but the relative widths of the positive and negative pulse
components will vary depending on the output signal of amplifier 51. As
the width or duration of each positive pulse component increases, each
negative pulse component decreases proportionately, and vice-versa. In
other words, since the period or time duration of a complete cycle is
constant, when the duration of a positive pulse component changes in one
sense the width of the immediately succeeding negative pulse component
must change in the opposite sense. The pulse width modulated signal has a
duty cycle characteristic which is the ratio of the width of each
positive-going pulse compared to the duration of a complete cycle. As will
be made apparent, the duty cycle of the pulse width modulated signal is
the same as the energizing or operating duty cycle of solenoid coil 24.
The pulse width modulated signal is developed at the output of amplifier 57
which functions as a comparator. Amplifiers 58 and 59, and their
associated circuit elements, form a well-known triangular wave generator
or oscillator for supplying a triangular shaped current signal through
series resistor 61 to the (-) input of amplifier 57. Preferably, the
frequency of the signal is around four cycles per second or hertz. In
addition, the voltage signal at the output of amplifier 51 is applied, via
resistor 62, as a current signal to the (-) input. Summation of the two
current signals occurs at the (-) input. In other words, the triangular
wave is essentially superimposed on the output signal from amplifier 51. A
fixed reference level is established at the (+) input of amplifier 57. The
net current flowing into the (-) input varies alternately (at the
frequency of the triangular wave) above and below the level of the
reference current entering the (+) input. Each time the input current at
the (-) input drops below the input current at the (+) input, the output
voltage of amplifier 57 abruptly switches from ground or 0 to V+ or +10
volts, where it remains until the current at the (-) input becomes greater
than the reference current at the (+) input. At that instant, the output
voltage switches from its high level back to its low level or zero. The
greater the current flowing from the output of amplifier 51, the greater
the time intervals during which the output of amplifier 57 is established
at zero potential, and the smaller the time intervals when the output is
at its high potential level. In this way, the output of amplifier 57
provides a pulse width modulated, rectangular shaped signal having a 10
volt peak-to-peak amplitude, the relative widths of the alternating
positive-going and negative-going pulses being modulated under the control
of amplifier 51. The duty cycle of the pulse width modulated signal is the
ratio of the time interval of one positive pulse component compared to a
complete cycle, namely the total time duration of one positive pulse
component and one negative pulse component.
The pulse width modulated signal operates the solenoid driver, comprising
transistors 64 and 65 and their associated circuit elements, to
effectively apply that signal to solenoid coil 24. Preferably, the +12
volts at the left terminal of coil 24 is derived from the vehicle's
voltage regulator. During each positive-going pulse when the output of
amplifier 57 is established at its high level, transistors 64 and 65
conduct and the right terminal of coil 24 will be essentially grounded,
thereby applying a full 12 volts DC across the coil. During the
intervening negative-going pulses, when the output of amplifier 57 is at
its low or zero level, transistors 64 and 65 will be non-conductive and
coil 24 will be de-energized. Since coil 24 is energized only by the
positive-going pulses, it is apparent that the duty cycle of coil 24 is
the same as, and is determined by, the duty cycle of the pulse width
modulated signal. The greater the duty cycle, the less the restriction
introduced by valve 21 between lines 22 and 23, the lower the crankcase
pressure, and the greater the compressor displacement. Since the duration
of the intervals, when the output of amplifier 57 is at its high level, is
inversely proportional to the output signal of amplifier 51, the duty
cycle, and consequently the compressor displacement, likewise vary
inversely with the output of amplifier 51.
Under normal conditions, the input current at the (+) input of amplifier 39
prevents the refrigerant at the evaporator outlet from decreasing below
the desired minimum indicated by dashed line 26 in FIG. 2. Since the
compressor in an automotive air-conditioning system is usually driven or
rotated by the vehicle's engine, during high speed operation (for example,
during downshift conditions when there is a rapid increase in engine
speed) the refrigerant flow rate will increase and the temperature at the
evaporator outlet may drop below the desired minimum. In order to prevent
the temperature from dropping so low that liquid refrigerant is fed into
the suction inlet of the compressor, a protection circuit is included in
the control system. More particularly, an amplifier 67, which operates as
a comparator, has its non-inverting or (+) input connected through series
resistor 68 to receive the evaporator outlet temperature signal. Resistors
71, 72 and 73 supply to the inverting or (-) input a reference current
signal which represents an absolute minimum level allowed for the
refrigerant at the evaporator outlet. This temperature will, of course, be
below the desired minimum, and thus will be to the left of dashed line 26
in FIG. 2, but it will still be high enough so that all of the refrigerant
liquid flowing through the evaporator vaporizes.
Under normal conditions, the current entering the (+) input of amplifier 67
will be less than the reference current flowing into the (-) input and the
output of the amplifier will be at essentially zero or ground potential.
If the evaporator outlet temperature drops to the absolute minimum level,
the current into the (+) input will then be greater than that fed into the
(-) input and the output voltage of amplifier 67 will abruptly switch from
0 to V+ or +10 volts. This output voltage is applied through resistors 74
and 75 to the bases of transistors 76 and 77, respectively. Resistors 78,
79 and 81 convert the output voltage to an input current for the (+) input
of amplifier 51. When the output of amplifier 67 is V+, transistors 76 and
77 will be rendered conductive thereby grounding the junction of resistors
55 and 56 and also the junction of resistors 48 and 49. At the same time,
current is supplied to the (+) input of amplifier 51 of an amplitude
sufficient to cause the output of the amplifier to integrate upwardly (or
positively) to V+ where it levels off.
In describing the operation of the control system it will be assumed that
when the air-conditioning apparatus is initially turned on the ambient
temperature in the vehicle compartment space to be air-conditioned is
substantially higher than the desired set point temperature established by
the thermostat setting, namely by the adjustment of potentiometer 34. At
this time, the current entering the (-) input of amplifier 32 will be
substantially greater than the current into the (+) input, thereby causing
the output of the amplifier to remain at essentially zero voltage.
Meanwhile, since the refrigerant temperature at the evaporator outlet is
relatively warm at start-up the voltage at the junction of temperature
sensor 41 and resistor 42 will be relatively low. Hence, both the
temperature control point signal (flowing through resistor 38) and the
evaporator outlet temperature signal (flowing through resistor 43) will be
of low amplitude and substantially less than the reference current (which
represents the desired minimum evaporator outlet temperature) entering the
(+) input of amplifier 39. As a result, the error voltage signal produced
at the output of amplifier 39 will be established at its maximum level,
thereby causing the error current signal entering the (-) input of
amplifier 51 to be substantially greater than the current into the (+)
input. Of course, at this time transistors 76 and 77 are non-conductive
since the output voltage of amplifier 67 will be zero. The output of
amplifier 51 thus remains at zero potential, as a consequence of which the
pulse width modulated signal developed at the output of amplifier 57 will
exhibit its maximum duty cycle which in turn causes compressor 10 to
operate at its maximum displacement, thereby to maximize the refrigerant
flow through the closed vapor cycle refrigeration circuit. The high
refrigerant flow rate causes the evaporator outlet temperature to decrease
until it reaches the desired minimum, as denoted by dashed line 26. When
that occurs, the current entering the (-) input of amplifier 39
automatically adjusts in order to hold the evaporator outlet temperature
at the desired minimum. If that temperature tends to become colder than
the desired minimum, the input current at the (-) input increases causing
the error signal to decrease and reduce the duty cycle as required to
return the evaporator outlet temperature to the desired minimum.
The control system will continue to operate at the control point indicated
by dashed line 26 and cooling will be imparted to the air delivered to the
controlled space until that space cools down to the desired set point
temperature. This cooling down period is sometimes called the "pull-down
period". When the desired space temperature is eventually reached, the two
input currents to amplifier 32 will be equal and the output will be zero
at that instant. However, as the controlled space then becomes slightly
colder, the current entering the (+) input of amplifier 32 will exceed
that into the (-) input and the output integrates upwardly, thereby
increasing the current flowing through resistor 38 and into the (-) input
of amplifier 39. The error voltage signal at the output of amplifier 39
therefore decreases to a new reference level, causing the duty cycle to
decrease and the refrigerant flow rate to drop so that the controlled
space does not become colder than the set point.
Steady state conditions have now been reached and the control system is in
balance. The output voltage of amplifier 32 has integrated upwardly from
zero to a constant positive level where it will remain as long as the
actual space temperature equals the desired space temperature. Since the
refrigerant flow rate through the evaporator is now less than that which
prevailed during the pull-down period, the evaporator outlet temperature
increases and the control point now shifts to the right along the abscissa
in FIG. 2. For illustrative purposes it will be assumed that the new
control point temperature is that which is indicated by dashed
construction line 84. The output of amplifier 32 thus effectively
represents the desired evaporator refrigerant outlet temperature,
referenced with respect to the predetermined desired minimum evaporator
temperature, since the actual evaporator outlet temperature is a function
of the amplitude of the output voltage produced by amplifier 32.
Accordingly, the output signal of amplifier 32 may be referred to as the
temperature control point signal.
The new control point (dashed line 84) matches the heat load requirements
and the control system will stabilize around that control point to
automatically hold the controlled space at the desired set point
temperature, while at the same time maintaining the compressor capacity
and refrigerant flow only as high as necessary to satisfy the heat load.
Hence, energy will be conserved and the vehicle's fuel consumption,
attributable to powering the air conditioning system, will be minimized.
As long as the heat load is constant, and the thermostat remains at the
same setting, the output voltage of amplifier 32 will be constant and the
error signal will remain at the same reference level. If anything tends to
upset or unbalance the steady state conditions, the control system
automatically re-adjusts itself to maintain those conditions.
If there is now an increase in the heat load (assume the outside
temperature increases) and the space tends to become warmer than desired,
the output voltage of amplifier 32 begins to decrease gradually (namely it
integrates downwardly) and the error signal at the output of amplifier 39
increases to a new reference level, thereby to increase the duty cycle and
consequently the flow rate of the refrigerant. This lowers the evaporator
outlet temperature to return the controlled space to the desired
temperature. The control point will now be established to the left of
dashed line 84 and the positive output voltage of amplifier 32 will level
off at a new amplitude in order to hold the evaporator outlet at the
necessary control point. Of course, the same sequence would occur if the
driver of the vehicle lowered the thermostat.
On the other hand, if there is a decrease in the heat load, or the driver
increases the thermostat setting, the output of amplifier 32 integrates
upwardly to a new amplitude level and the refrigerant flow decreases to
increase the evaporator outlet temperature (moving the control point to
the right) and provide less cooling for the controlled space, as a
consequence of which the space will be maintained at the desired
temperature.
During steady state conditions when the thermostat setting is not changed
and the heat load remains constant, the temperature control point signal
produced by amplifier 32 thereby remaining constant, the evaporator outlet
temperature will be held fixed at the desired control point. This is
achieved in the control system by regulating the refrigerant flow so that
the control point always remains fixed if the information received from
amplifier 32 is constant. If the evaporator outlet temperature tends to
increase, for example, the current flowing through resistor 43 and into
the (-) input of amplifier 39 decreases and the output of that amplifier
increases to increase the refrigerant flow and maintain the evaporator
outlet temperature at the desired control point. Likewise, if the
evaporator outlet tends to become too cold, the output of amplifier 39
decreases to lower the refrigerant flow so that the evaporator outlet
remains at the desired control point temperature. Hence, when the heat
load requirements that must be satisfied by the evaporator are essentially
constant, the control system automatically maintains both the evaporator
outlet temperature and the space temperature at constant desired levels,
the compressor capacity and refrigerant flow rate being only as high as
necessary to maintain these constant temperatures.
Of course, since compressor 10 is usually rotated by the vehicle's engine,
the RPM of the compressor will be a function of engine speed and the
refrigerant flow rate will tend to change as the engine speed varies. The
control system, however, automatically compensates for any RPM change. As
the refrigerant flow tends to increase, for example, the evaporator outlet
temperature decreases and this causes the error signal, at the output of
amplifier 39, to decrease, the result of which is that the compressor
displacement reduces to the extent necessary to decrease the flow rate to
a level that will hold the space temperature at the desired set point.
Conversely, in the presence of a reduction in engine speed, the compressor
is automatically controlled in | | |
