U.S. patent number RE42,006 [Application Number 10/760,173] was granted by the patent office on 2010-12-28 for adaptive control for a refrigeration system using pulse width modulated duty cycle scroll compressor.
This patent grant is currently assigned to Emerson Climate Technologies, Inc.. Invention is credited to Mark Bass, Jean-Luc Caillat, Hung M. Pham, Abtar Singh.
United States Patent |
RE42,006 |
Pham , et al. |
December 28, 2010 |
Adaptive control for a refrigeration system using pulse width
modulated duty cycle scroll compressor
Abstract
A diagnostic system includes a controller adapted for coupling
to a compressor or electronic stepper regulator valve. The
controller produces a variable duty cycle control signal to adjust
the capacity of the compressor or valve position of the electronic
stepper regulator valve as a function of demand for cooling. The
diagnostic system further includes a diagnostic module coupled to
the controller for monitoring and comparing the duty cycle with at
least one predetermined fault value indicative of a system fault
condition and an alert module responsive to the diagnostic module
for issuing an alert signal when the duty cycle bears a
predetermined relationship to the fault value.
Inventors: |
Pham; Hung M. (Dayton, OH),
Singh; Abtar (Kennesaw, GA), Caillat; Jean-Luc (Dayton,
OH), Bass; Mark (Bonita Springs, FL) |
Assignee: |
Emerson Climate Technologies,
Inc. (Sidney, OH)
|
Family
ID: |
25473719 |
Appl.
No.: |
10/760,173 |
Filed: |
January 16, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09886592 |
Jun 21, 2001 |
6467280 |
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09524364 |
Mar 14, 2000 |
6408635 |
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08939779 |
Sep 29, 1997 |
6047557 |
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08486118 |
Jun 7, 1995 |
5741120 |
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Reissue of: |
10147782 |
May 16, 2002 |
06499305 |
Dec 31, 2002 |
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Current U.S.
Class: |
62/126; 62/228.1;
62/217 |
Current CPC
Class: |
F04C
27/005 (20130101); F04C 28/28 (20130101); F25B
49/022 (20130101); F04C 28/06 (20130101); F04C
28/02 (20130101); F04C 28/265 (20130101); F04C
28/00 (20130101); F04C 28/22 (20130101); G05D
23/1909 (20130101); A47F 3/04 (20130101); F25B
1/04 (20130101); F04C 18/0215 (20130101); F25B
5/02 (20130101); F25B 49/005 (20130101); F04C
28/08 (20130101); F25B 41/22 (20210101); F25B
2700/21175 (20130101); F04C 2270/86 (20130101); F04C
2270/015 (20130101); Y02B 30/70 (20130101); F25B
2700/2117 (20130101); F25B 41/35 (20210101); F25B
2600/0261 (20130101); F25B 2700/21174 (20130101); F04C
23/008 (20130101); F25B 2700/193 (20130101); F25B
2700/2106 (20130101); F25B 41/31 (20210101); F25B
2400/22 (20130101); F25B 2700/1933 (20130101) |
Current International
Class: |
A47F
3/04 (20060101); F04C 18/02 (20060101); F25B
1/04 (20060101); G05D 1/08 (20060101); F25B
49/02 (20060101); F04C 27/00 (20060101); F25B
41/06 (20060101) |
Field of
Search: |
;62/125-131,204-206,210-212,222-225,217,186,179-181,183,184 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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1042406 |
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May 1990 |
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CN |
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1137614 |
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Dec 1996 |
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CN |
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1159555 |
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Sep 1997 |
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CN |
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0085246 |
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Aug 1983 |
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EP |
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0 281 317 |
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Feb 1988 |
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EP |
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0453302 |
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Oct 1991 |
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EP |
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0747597 |
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Dec 1996 |
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EP |
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0747598 |
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Dec 1996 |
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EP |
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733511 |
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Jul 1955 |
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GB |
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7190507 |
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Jul 1995 |
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JP |
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Other References
Rejection Decision regarding CN 200510064854.7 dated Feb. 6, 2009.
cited by other .
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200610128576.1, received from the Patent Office of the People's
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200610128576.1 received from the Patent Office of the People's
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European Patent Office. cited by other.
|
Primary Examiner: Jiang; Chen-Wen
Attorney, Agent or Firm: Harness, Dickey & Pierce,
P.L.C.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This is a division of U.S. Ser. No. 09/886,592, filed Jun. 21,
2001, entitled "Adaptive Control For A Refrigeration System Using
Pulse Width Modulated Duty Cycle Scroll Compressor;" which is a
division of U.S. Ser. No. 09/524,364, filed Mar. 14, 2000 U.S. Pat.
No. 6,408,635; which is a division of U.S. Ser. No. 08/939,779,
filed Sep. 29, 1997, now U.S. Pat. No. 6,047,557; which is a
continuation-in-part of U.S. Ser. No. 08/486,118, filed Jun. 7,
1995, now U.S. Pat. No. 5,741,120, each of which is incorporated
herein by reference.
Claims
We claim:
.[.1. A diagnostic system for an electronic stepper regulator
valve, comprising: a controller adapted for coupling to an
electronic stepper regulator valve, said controller producing a
variable duty cycle control signal for adjusting a valve position
of said electronic stepper regulator valve, in which said duty
cycle is a function of demand for cooling; a diagnostic module
coupled to said controller for monitoring and comparing said duty
cycle with at least one predetermined fault value indicative of a
fault condition; and an alert module responsive to said diagnostic
module for issuing an alert signal when said duty cycle bears a
predetermined relationship to said fault value..].
2. .[.The diagnostic.]. .Iadd.A control .Iaddend.system .[.of claim
1, wherein said diagnostic module monitors and compares at least
one of the following conditions.]. .Iadd.comprising.Iaddend.:
.Iadd.a controller operable to produce a variable duty cycle
control signal for controlling a cooling system device in which
said duty cycle is a function of demand for cooling; and a
diagnostic module associated with said controller and operable to
compare said duty cycle with a predetermined value indicative of a
system condition and issue a signal when said duty cycle bears a
predetermined relationship to a fault value;.Iaddend. .Iadd.wherein
said diagnostic module monitors at least one of the following
conditions:.Iaddend. .[.said.]. .Iadd.a .Iaddend.valve position of
.[.said.]. .Iadd.an .Iaddend.electronic stepper regulator; an error
value percentage indicative of the percentage of sampled error
within an accepted offset range for a defined period of time; a
moving average of said valve position for a defined period of time;
a steady state loading percentage set equal to said moving average
of said valve position for a defined period of time when said error
value percentage is less than fifty percent; a discharge cooling
fluid temperature; an evaporator coil inlet temperature; an
evaporator coil exit temperature; a moving average of a difference
between said discharge cooling fluid temperature and said
evaporator coil inlet temperature; a moving average of a difference
between said evaporator coil exit temperature and said evaporator
coil inlet temperature to approximate a superheat value; and a
length of time said evaporator coil exit temperature is less than
said evaporator coil inlet temperature during a predefined period
of time.
3. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim
.[.1.]. .Iadd.15.Iaddend., wherein said .[.diagnostic.]. module
monitors a percentage of sampled error over a defined period of
time.
4. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 3,
wherein said predetermined .[.fault.]. value is an accepted offset
range.
5. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 4,
wherein said .[.diagnostic.]. module determines an error value
percentage indicative of said percentage of sampled error within
said accepted offset range for said defined period of time.
.[.6. The diagnostic system of claim 5, wherein said diagnostic
module determines an error value percentage indicative of said
percentage of sampled error within said accepted offset range for
said defined period of time..].
7. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim
.[.6.]. .Iadd.27.Iaddend., wherein said alert module issues an
alert signal when .[.said.]. .Iadd.a .Iaddend.valve position of
said electronic stepper regulator valve is approximately zero
percent for approximately ninety percent of said defined period of
time and said error value percentage is less than one hundred
percent, said alert signal indicating said electronic stepper
regulator valve is over-sized.
8. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim
.[.6.]. .Iadd.27.Iaddend., wherein said diagnostic module further
monitors and compares a superheat value indicative of evaporator
superheat.
9. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 8,
wherein said alert module issues an alert signal when said valve
position of said electronic stepper regulator valve is
approximately one hundred percent for approximately ninety percent
of said defined period of time, said error value percentage is
approximately zero percent, and said superheat value is
approximately greater than 5.degree. F., said alert signal
indicating said electronic stepper regulator valve is
undersized.
10. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 8,
wherein said diagnostic module further monitors and compares an
evaporator coil inlet temperature value indicative of evaporator
coil inlet temperature.
11. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 10,
wherein said alert module issues an alert signal when said error
value percentage is approximately zero percent, said valve position
of said electronic stepper regulator valve is approximately zero
percent for approximately one hundred percent of said defined
period of time, said evaporator coil inlet temperature value is
less than approximately 32.degree. F., and said superheat value is
approximately greater than 5.degree. F., said alert signal
indicating said electronic stepper regulator valve is stuck
open.
12. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 10,
wherein said error value percentage is approximately zero percent,
said valve position of said electronic stepper regulator valve is
approximately one hundred percent for approximately one hundred
percent of said defined period of time, said evaporator coil inlet
temperature value is approximately greater than 32.degree. F., and
said superheat value is approximately greater than 5.degree. F.,
said alert signal indicating said electronic stepper regulator
valve is stuck closed.
13. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 10,
wherein said diagnostic module further monitors and compares an
evaporator coil exit temperature value indicative of evaporator
coil exit temperature.
14. The .[.diagnostic.]. .Iadd.control .Iaddend.system of claim 13,
wherein said alert module issues an alert signal when said valve
position of said electronic stepper regulator valve is
approximately one hundred percent for approximately one hundred
percent of said defined period of time, said error value percentage
is approximately zero, said superheat value is approximately less
than 5.degree. F., said evaporator coil inlet temperature value is
approximately less than 25.degree. F., and said evaporator coil
exit temperature value is less than said evaporator coil inlet
temperature value for greater than fifty percent of said defined
period of time, said alert signal indicating that air flow to an
evaporator is blocked or evaporator fans are not operating
properly.
.Iadd.15. The control system of claim 2, wherein said cooling
system device is selected from a group comprising: an expansion
device, a fan, a compressor, and a refrigerant control
device..Iaddend.
.Iadd.16. The control system of claim 15, wherein said expansion
device is at least one of an orifice, thermal expansion valve, and
electronic expansion valve..Iaddend.
.Iadd.17. The control system of claim 15, wherein said refrigerant
control device is an evaporator stepper regulator..Iaddend.
.Iadd.18. The control system of claim 15, wherein said fan is a
variable speed fan..Iaddend.
.Iadd.19. The control system of claim 15, wherein said fan is a
condenser fan..Iaddend.
.Iadd.20. The control system of claim 19, wherein said condenser
fan is a variable speed fan..Iaddend.
.Iadd.21. The control system of claim 2, wherein said module
includes said diagnostic module and an alert module..Iaddend.
.Iadd.22. The control system of claim 21, wherein said alert module
issues said signal..Iaddend.
.Iadd.23. The control system of claim 2, wherein said signal is an
alert signal..Iaddend.
.Iadd.24. A control system comprising: a controller operable to
produce a variable duty cycle control signal for controlling an
electronic stepper regulator valve in which said duty cycle is a
function of demand for cooling; and a module associated with said
controller and operable to compare said duty cycle with a
predetermined value indicative of a system condition and issue a
signal when said duty cycle bears a predetermined relationship to
said fault value..Iaddend.
.Iadd.25. The control system of claim 5, wherein said module
includes a diagnostic module and an alert module, said diagnostic
module comparing said duty cycle with said predetermined value, and
said alert module issuing said signal, said predetermined value
being said fault value and said signal being an alert
signal..Iaddend.
.Iadd.26. The control system of claim 24, wherein said module
includes a diagnostic module and an alert module..Iaddend.
.Iadd.27. The control system of claim 26, wherein said diagnostic
module compares said duty cycle with said predetermined
value..Iaddend.
.Iadd.28. The control system of claim 26, wherein said alert module
issues said signal..Iaddend.
.Iadd.29. The control system of claim 24, wherein said signal is an
alert signal..Iaddend.
.Iadd.30. The control system of claim 24, further comprising an
expansion device controlled by said variable duty
cycle..Iaddend.
.Iadd.31. The control system of claim 24, further comprising a fan
controlled by said variable duty cycle..Iaddend.
.Iadd.32. The control system of claim 24, wherein said controller
is operable to control a fan based on said duty cycle..Iaddend.
.Iadd.33. The control system of claim 32, further comprising an
expansion device controlled by said variable duty
cycle..Iaddend.
.Iadd.34. The control system of claim 32, wherein said fan is a
variable speed fan..Iaddend.
.Iadd.35. The control system of claim 34, wherein said fan is a
condenser fan..Iaddend.
.Iadd.36. The control system of claim 24, wherein said controller
is operable to control a compressor based on said duty
cycle..Iaddend.
.Iadd.37. The control system of claim 36, further comprising an
expansion device controlled by said variable duty
cycle..Iaddend.
.Iadd.38. The control system of claim 36, further comprising a
variable speed fan controlled by said controller..Iaddend.
.Iadd.39. The control system of claim 38, wherein said controller
controls said fan based on a current operating duty cycle of said
compressor..Iaddend.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates generally to refrigeration systems,
compressor control systems and refrigerant regulating valve control
systems. More particularly, the invention relates to a
refrigeration system employing a pulse width modulated compressor
or evaporator stepper regulator controlled by a variable duty cycle
signal derived from a load sensor. Preferably an adaptive
controller generates the variable duty cycle signal. The compressor
has two mechanical elements separated by a seal, and these
mechanical elements are cyclically movable relative to one another
to develop fluid pressure. The compressor includes a mechanism to
selectively break the seal in response to the control signal,
(hereby modulating the capacity of the system.
The refrigeration system can be deployed as a distributed system in
refrigeration cases and the like. The preferred arrangement allows
the compressor and condenser subsystems to be disposed in or
mounted on the refrigeration case, thereby greatly reducing the
length of refrigerant conduit and refrigerant required.
Conventionally, refrigeration systems for supermarket refrigeration
cases have employed air-cooled or water-cooled condensers fed by a
rack of compressors. The compressors are coupled in parallel so
that they may be switched on and off in stages to adjust the system
cooling capacity to the demands of the load. Commonly, the
condensers are located outside, on the roof, or in a machine room
adjacent the shopping area where the refrigeration cases are
located.
Within each refrigeration case is an evaporator fed by lines from
the condensers through which the expanded refrigerant circulates to
cool the case. Conventionally, a closed-loop control system
regulates refrigerant flow through the evaporator to maintain the
desired case temperature. Proportional-integral-derivative (PID)
closed loop control systems are popular for this purpose, with
temperature sensors and/or pressure sensors providing the sensed
condition inputs.
It is common practice within supermarkets to use separate systems
to supply different individual cooling temperature ranges: low
temperature (for frozen foods, ice cream, nominally -25 F.); medium
(for meat, dairy products, nominally +20 F.); high (for floral,
produce, nominally +35 to +40 F.). The separate low, medium and
high temperature systems are each optimized to their respective
temperature ranges. Normally, each will employ its own rack of
compressors and its own set of refrigerant conduits to and from the
compressors and condensers.
The conventional arrangement, described above, is very costly to
construct and maintain. Much of the cost is associated with the
long refrigerant conduit runs. Not only are long conduit runs
expensive in terms of hardware and installation costs, but the
quantity of refrigerant required to fill the conduits is also a
significant factor. The longer the conduit run, the more
refrigerant required. Adding to the cost are environmental factors.
Eventually fittings leak, allowing the refrigerant to escape to
atmosphere. Invariably, long conduit runs involve more pipefitting
joints that may potentially leak. When a leak does occur, the
longer the conduit run, the more refrigerant lost.
There is considerable interest today in environmentally friendly
refrigeration systems. Shortening the conduit run is seen as one
way to achieve a more environmentally friendly system. To achieve
this, new condenser/compressor configurations and new control
systems will need to be engineered.
Re-engineering condenser/compressor configurations for more
environmentally friendly systems is not a simple task, because
system efficiency should not be sacrificed. Generally, the
conventional roof-mounted condenser system, supplied by condensers,
benefits from economies of scale and is quite efficient. These
systems serve as the benchmark against which more environmentally
friendly systems of the future will need to be measured.
To appreciate why re-engineering an environmentally yet efficient
system has proven so difficult, consider these thermodynamic
issues. The typical refrigeration case operates in a very
unpredictable environment. From a design standpoint, the thermal
mass being cooled is rarely constant. Within the supermarket
environment, the temperature and humidity may vary widely at
different times of day and over different seasons throughout the
year. The product load (items in the refrigeration case) can also
change unpredictably. Customers removing product and store clerks
replenishing product rarely synchronize. Outside the supermarket
environment, the outdoor air temperature and humidity may also vary
quite widely between day and night and/or between summer and
winter. The capacity of the system must be designed for the
harshest conditions (when the condenser environment is the
hottest). Thus systems may experience excess capacity in less harsh
conditions, such as in the cool evenings or during the winter.
Periodic defrosting also introduces thermal fluctuations into the
system. Unlike thermal fluctuations due to environmental
conditions, the thermal fluctuations induced by the defrost cycle
are caused by the control system itself and not by the surrounding
environment.
In a similar fashion, the control system for handling multiple
refrigeration cases can induce thermal fluctuations that are quite
difficult to predict. If all cases within a multi-case system are
suddenly turned on at once--to meet their respective cooling
demands--the cooling capacity must rapidly be ramped up to maximum.
Likewise, if all cases are suddenly switched off, the cooling
capacity should be ramped down accordingly. However, given that
individual refrigeration cases may operate independently of one
another, the instantaneous demand for cooling capacity will tend to
vary widely and unpredictably.
These are all problems that have made the engineering of
environmentally friendly systems more difficult. Adding to these
difficulties are user engineering/ergonomic problems. The present
day PID controller can be difficult to adapt to distributed
refrigeration systems. Experienced controls engineers know that a
well-tuned PID controller can involve a degree of artistry in
selecting the proper control constants used in the PID algorithm.
In a large refrigeration system of the conventional architecture
(non-distributed) the size of the system justifies having a
controls engineer visit the site (perhaps repeatedly) to fine tune
the control constant parameters.
This may not be practical for distributed systems in which the
components are individually of a much smaller scale and far more
numerous. By way of comparison, a conventional system might employ
one controller for an entire multi-case, store-wide system. A
distributed system for the same store might involve a controller
for each case or adjacent group of cases within the store.
Distributed systems need to be designed to minimize end user
involvement. It would therefore be desirable if the controller were
able to auto configure. Currently control systems lack this
capability.
The present invention provides a distributed refrigeration system
in which the condenser is disposed on the refrigeration case and
serviced by a special pulse width modulated compressor that may be
also disposed within the case. If desired, the condenser and
compressor can be coupled to service a group of adjacent
refrigeration cases, each case having its own evaporator. The pulse
width modulated compressor employs two mechanical elements, such as
scroll members, that move rotationally relative to one another to
develop fluid pressure for pumping the refrigerant. The compressor
includes a mechanism that will selectively break the seal between
the two mechanical elements, thereby altering the fluid pressure
developed by the compressor while allowing the mechanical elements
to maintain substantially constant relative movement with one
another. The compressor can be pulse width modulated by making and
breaking the fluid seal without the need to start and stop the
electric motor driving the mechanical elements.
The pulse width modulated compressor is driven by a control system
that supplies a variable duty cycle control signal based on
measured system load. The controller may also regulate the
frequency (or cycle time) of the control signal to minimize
pressure fluctuations in the refrigerant system. The on time is
thus equal to the duty cycle multiplied by the cycle time, where
the cycle time is the inverse of the frequency.
The refrigeration system of the invention has a number of
advantages. Because the instantaneous capacity of the system is
easily regulated by variable duty cycle control, an oversized
compressor can be used to achieve faster temperature pull down at
startup and after defrost, without causing short cycling as
conventional compressor systems would. Another benefit of variable
duty cycle control is that the system can respond quickly to sudden
changes in condenser temperature or case temperature set point. The
controller adjusts capacity in response to disturbances without
producing unstable oscillations and without significant overshoot.
Also, the ability to match instantaneous capacity to the demand
allows the system to operate at higher evaporator temperatures.
(Deep drops in temperature experienced by conventional systems at
overcapacity are avoided.)
Operating at higher evaporator temperatures reduces the defrost
energy required because the system develops frost more slowly at
higher temperatures. Also, the time between defrosts can be
lengthened by a percentage proportional to the accumulated runtime
as dictated by the actual variable duty cycle control signal. For
example, a sixty percent duty cycle would increase a standard
three-hour time between defrosts to five hours (3/0.60=5).
The pulse width modulated operation of the system yields improved
oil return. The refrigerant flow pulsates between high capacity and
low capacity (e.g. 100% and 0%), creating more turbulence which
breaks down the oil boundary layer in the heat exchangers.
Another benefit of the variable duty cycle control system is its
ability to operate with a variety of expansion devices, including
the simple orifice, the thermal expansion valve (TXV) and the
electronic expansion valve. A signal derived from the expansion
device controller can be fed to the compressor controller of the
invention. This signal allows the variable duty cycle control
signal and/or its frequency to be adjusted to match the
instantaneous operating conditions of the expansion device. A
similar approach may be used to operate variable speed fans in air
cooled condenser systems. In such case the controller of the
invention may provide a signal to control fan speed based on the
current operating duty cycle of the compressor.
Yet another benefit of the invention is its ability to detect when
the system is low on refrigerant charge, an important environmental
concern. Low refrigerant charge can indicate the presence of leaks
in the system. Low charge may be detected by observing the change
in error between actual temperature and set point temperature as
the system duty cycle is modulated. The control system may be
configured to detect when the modulation in duty cycle does not
have the desired effect on temperature maintenance. This can be due
to a loss of refrigerant charge, a stuck thermal expansion valve or
other malfunctions.
For a more complete understanding of the invention, its objects and
advantages, refer to the following specification and to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system block diagram of a prior art refrigeration
system configuration;
FIG. 2 is a block diagram of a refrigeration system in accordance
with the present invention;
FIG. 3 is a cross-sectional view of an embodiment of the pulse
width modulated compressor, shown in the loaded state;
FIG. 4 is a cross-sectional view of the compressor of FIG. 3, shown
in the unloaded state;
FIG. 5 is another embodiment of a refrigeration or cooling system
in accordance with the present invention;
FIG. 6 is a block diagram of the controller;
FIG. 7 is a block diagram showing how the controller may be used to
modulate an evaporator stepper regulator;
FIG. 8 is a block diagram of the signal conditioning module of the
controller of FIG. 6;
FIG. 9 is a block diagram of the control module of the controller
of FIG. 6;
FIG. 10 is a state diagram depicting the operating states of the
controller;
FIG. 11 is a flowchart diagram illustrating the presently preferred
PI control algorithm;
FIG. 12 is a waveform diagram illustrating the variable duty cycle
signal produced by (be controller and illustrating the operation at
a constant frequency;
FIG. 13 is a waveform diagram of the variable duty cycle signal,
illustrating variable frequency operation;
FIG. 14 is a series of graphs comparing temperature and pressure
dynamics of system employing the invention with a system of
conventional design;
FIG. 15 is a block diagram illustrating the adaptive tuning module
of the invention;
FIG. 16a is a flowchart diagram illustrating the presently
preferred operation of the adaptive tuning module, specifically
with respect to the decision whether to start tuning;
FIG. 16b is a flowchart diagram illustrating the presently
preferred process performed by the adaptive tuning module in the
integration mode;
FIG. 16c is a flowchart diagram illustrating the operation of the
adaptive tuning module in the calculation mode;
FIG. 17 is a state diagram illustrating the operative states of the
adaptive tuning module;
FIG. 18 is a block diagram illustrating the fuzzy logic block of
the adaptive tuning loop;
FIG. 19 is a membership function diagram for the fuzzy logic block
of FIG. 18;
FIG. 20 is a truth table relating to the membership function of
FIG. 19 as used by the fuzzy logic block of FIG. 18;
FIG. 21 is an output membership function diagram for the fuzzy
logic block of FIG. 18; and
FIG. 22 is a schematic illustrating exemplary sensor locations for
control-related and diagnostic-related functions of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates how a conventional supermarket refrigeration
system is configured. As previously discussed, it is conventional
practice to place the compressors 30 and the condenser 32 in a
location remote from the refrigeration cases 34. In this
illustration, the compressors 30 are configured in a parallel bank
located on the roof 36 of the building. The bank of compressors
supply a large condenser 32, which may be air cooled or water
cooled. The condenser supplies liquid refrigerant to a receiver 38.
The receiver 38, in turn, supplies the individual refrigeration
cases, which are connected in parallel, as illustrated. In most
implementations a liquid line solenoid valve 40 is used to regulate
flow to the associated evaporator 42. The refrigerant is supplied
to the evaporator through a suitable expansion device such as
expansion valve 44. The expansion valve 44 provides a restricted
orifice that causes the liquid refrigerant to atomize into liquid
droplets that are introduced into the inlet side of the evaporator
42. The evaporator 42, located within the refrigeration case 34,
extracts heat from the case and its contents by vaporization of the
liquid refrigerant droplets into a gas. The compressors 30 extract
this gas by suction and compress it back into the liquid state. The
liquid refrigerant is then cooled in the condenser 32 and returned
to the receiver 38, whereupon the cycle continues.
To match cooling capacity to the load, the compressors 30 may be
switched on and off individually or in groups, as required. In a
typical supermarket arrangement there may be several independent
systems, each configured as shown in FIG. 1, to handle different
operating temperature ranges. Note that the liquid line 46 and the
suction line 48 may each need to be quite lengthy (e.g., up to 150
feet) to span the distance from refrigeration case to roof.
FIG. 2 shows a refrigeration case 34 configured according to the
principles of the present invention. The condenser 32 and
compressor 30 are both disposed within case 34 or attached thereto.
Evaporator 42 and the associated expansion valve 44 are likewise
disposed within case 34. The condenser 32 is provided with a heat
removal mechanism 50 by which heat is transferred to ambient. The
heat removal mechanism can be a water jacket connected to suitable
plumbing for carrying waste heat to a water cooling tower located
on the building roof. Alternatively, the heat removal mechanism can
be a forced-air cooling system or a passive convection-air cooling
system.
The refrigeration system of the invention employs a compressor
controller 52 that supplies a pulse width modulated control signal
on line 54 to a solenoid valve 56 on compressor 30. The compressor
controller adjusts the pulse width of the control signal using an
algorithm described 1 below. A suitable load sensor such as
temperature sensor 58 supplies the input signal used by the
controller to determine pulse width.
FIGS. 3 and 4 show the details of compressor 30. FIG. 3 shows the
compressor in its loaded state and FIG. 4 shows the compressor in
its unloaded state. The solenoid valve 56 switches the compressor
between these two states while the compressor motor remains
energized. One important advantage of this configuration is that
the compressor can be pulse width modulated very rapidly between
the loaded and unloaded states without interrupting power to the
compressor motor. This pulse width modulated cycling exerts less
wear on the compressor, because the motor is not subjected to
sudden changes in angular momentum.
Referring to FIGS. 3 and 4, there is shown an exemplary compressor
30. Compressor 30 may be used within a hermetic scroll compressor
such as generally of the type described in assignee's U.S. Pat. No.
5,102,316.
The exemplary compressor 30 includes an outer shell 61 and an
orbiting scroll member 64 supported on upper bearing housing 63 and
drivingly connected to crankshaft 62 via crank pin 65 and drive
bushing 60. A second non-orbiting scroll member 67 is positioned in
meshing engagement with scroll member 64 and axially movably
secured to upper bearing housing 63. A partition plate 69 is
provided adjacent the upper end of shell 61 and serves to define a
discharge chamber 70 at the upper end thereof.
In operation, as orbiting scroll member 64 orbits with respect to
scroll member 67, suction gas is drawn into shell 61 via suction
inlet 71 and thence into compressor 30 through inlet 72 provided in
non-orbiting scroll member 67. The intermeshing wraps provided on
scroll members 64 and 67 define moving fluid pockets which
progressively decrease in size and move radially inwardly as a
result of the orbiting motion of scroll member 64 thus compressing
the suction gas entering via inlet 72. The compressed gas is then
discharged into discharge chamber 70 via discharge port 73 provided
in scroll member 67 and passage 74.
In order to unload compressor 30, solenoid valve 56 will be
actuated in response to a signal from control module 87 to
interrupt fluid communication to increase the pressure within
chamber 77 to that of the discharge gas. The biasing force
resulting from this discharge pressure will overcome the sealing
biasing force thereby causing scroll member 67 to move axially
upwardly away from orbiting scroll member 64. This axial movement
will result in the creation of a leakage path between the
respective wrap tips and end plates of scroll members 64 and 67
thereby substantially eliminating continued compression of the
suction gas.
A flexible fluid line 91 extends from the outer end of passage 90
to a fitting 92 extending through shell 61 with a second line 93
connecting fitting 92 to solenoid valve 56. Solenoid valve 56 has
fluid lines 82 and 84 connected to suction line 83 and discharge
line 85 and is controlled by control module 87 in response to
conditions sensed by sensor 88 to effect movement of non-orbiting
scroll member 67 between the positions shown in FIGS. 3 and 4.
When compression of the suction gas is to be resumed, solenoid
valve 56 will be actuated so as to move scroll member 67 into
sealing engagement with scroll member 64.
The refrigeration case embodiment of FIG. 2 may be packaged as a
self-contained unit. While that may be a desirable configuration
for many applications, the invention is not restricted to stand
alone, self-contained refrigeration case configurations. Rather,
the invention lends itself to a variety of different distributed
refrigeration systems. FIG. 5 shows an example of such a
distributed system.
Referring to FIG. 5, a single compressor 30 and condenser 32 can
service several distributed refrigeration cases or several
distributed cooling units in a heating and cooling (HVAC) system.
In FIG. 5 the refrigeration cases or cooling system housings are
shown as dashed boxes, designated 34a, 34b, and 34c. Conveniently,
the compressor 30 and condenser 32 may be disposed within or
attached to one of the refrigeration cases or housings, such as
refrigeration case or housing 34a.
Each refrigeration case or housing has its own evaporator and
associated expansion valve as illustrated at 42(a, b, c) and 44(a,
b, c). In addition, each refrigeration case or housing may have its
own temperature sensor 58(a, b, c) supplying input information to
the compressor controller 52. Finally, a pressure sensor 60
monitors the pressure of the suction line 48 and supplies this
information to compressor controller 52. The compressor controller
supplies a variable duty cycle signal to the solenoid valve 56 as
previously described.
The multiple case or multiple cooling unit embodiment of FIG. 5
shows how a single compressor can be pulse width modulated by
compressor controller 52 to supply the instantaneous demand for
cooling. The temperature sensors 58(a, b, c) collectively provide
an indication of the load on the system, as does pressure sensor
60. The controller adjusts the pulse width of the control signal to
modulate the compressor between its high capacity and low capacity
states (100%, 0%) to meet the instantaneous demand for
refrigerant.
As an alternate control technique, one or more of the suction lines
exiting the evaporator can be equipped with an electrically
controlled valve, such as an evaporator pressure regulator valve
45c. Valve 45c is coupled to controller 52, as illustrated. It may
be supplied with a suitable control signal, depending on the type
of the valve. A stepper motor valve may be used for this purpose,
in which case controller 30 would supply a suitable signal to
increment or decrement the setting of the stepper motor to thereby
adjust the orifice size of the valve. Alternatively, a pulse width
modulated valve could be used, in which case it may be controlled
with the same variable duty cycle signal as supplied to the
compressor 30.
Controller 52 is not limited to solely compressor control
applications. The variable duty cycle control signal can also be
used to control other types of refrigerant flow and pressure
control devices, such as refrigerant regulating valves. FIG. 7
shows such an application, where the output of controller 52
supplies control signals to evaporator stepper regulator 43. This
device is a fluid pressure regulator that is adjusted by stepper
motor 45. The evaporator stepper regulator (ESR) valve 43 adjusts
the suction pressure to thereby adjust the capacity of the
system.
A block diagram of the presently preferred compressor controller is
illustrated in FIG. 6. A description of the various signals and
data values shown in this and successive figures is summarized in
Table I below.
TABLE-US-00001 TABLE I Default No. Variable Name Value Description
1 Signal Conditioning: Sensor Alarm False Indicates Sensor Reading
is not within expected range Sensor Mode Min User configuration to
indicate if Min/Max/Avg is performed for all temperature Sensors
Sampling Time (Ts) 0.5 sec Rate at which Signal condition- ing
block is executed Control Type T/F Type if controlled by only Temp.
or both Temp. & Pressure 2 Control Block: Sensor Alarm False
Same as before System Alarm False Generated by Adaptive Block
indicative some system problem SSL 0 Steady state loading % Defrost
Status False Whether system is in defrost Pull_Down_Time 0 Time
taken to pull down after defrost Gain (K) 7 Gain used in PI
algorithm Integral Time (TI) 100 used in PI Control Time (Tc) 10
Sec used in PI Control Set Pt. (St) 0 F. used in PI Operating State
1 What state the machine is operating at 3 Defrost Control Defrost
Status False If defrost status of the case Defrost Type External If
the defrost is from external timer or Internal clock of controller
Defrost Interval 8 hrs Time between defrost Defrost Duration 1 hr
Defrost Duration Defrost Termination 50 F. Termination temperature
for Temp. defrost
At the heart of the controller is control block module 102. This
module is responsible for supplying the variable duty cycle control
signal on lead 104. Module 102 also supplies the compressor ON/OFF
signal on lead 106 and an operating state command signal on lead
108. The compressor ON/OFF signal drives the contactors that supply
operating current to the compressor motor. The operating state
signal indicates what state the state machine (FIG. 10) is in
currently.
The control block module receives inputs from several sources,
including temperature and pressure readings from the temperature
and pressure sensors previously described. These temperature
readings are passed through signal conditioning module 110, the
details of which are shown in the pseudocode Appendix. The control
block module also receives a defrost status signal from defrost
control module 112. Defrost control module 112 contains logic to
determine when defrost is performed. The present embodiment allows
defrost to be controlled either by an external logic signal
(supplied through lead 114) or by an internal logic signal
generated by the defrost control module itself. The choice of
whether to use external or internal defrost control logic is user
selectable through user input 116. The internal defrost control
uses user-supplied parameters supplied through user input 118.
The preferred compressor controller in one form is
auto-configurable. The controller includes an optional adaptive
tuning module 120 that automatically adjusts the control algorithm
parameters (the proportional constant K) based on operating
conditions of the system. The adaptive tuning module senses the
percent loading (on lead 104) and the operating state (on lead 108)
as well as the measured temperature after signal conditioning (on
lead 122). Module 120 supplies the adaptive tuning parameters to
control block 102, as illustrated. The current embodiment supplies
proportional constant K on lead 124 and SSL parameter on lead 126,
indicative of steady-state loading percent. A system alarm signal
on lead 126 alerts the control block module when the system is not
responding as expected to changes in the adaptively tuned
parameters. The alarm thus signals when there may be a system
malfunction or loss of refrigerant charge. The alarm can trigger
more sophisticated diagnostic routines, if desired. The compressor
controller provides a number of user interface points through which
user-supplied settings are input. The defrost type
(internal/external) input 116 and the internal defrost parameters
on input 118 have already been discussed. A user input 128 allows
the user to specify the temperature set point to the adaptive
tuning module 120. The same information is supplied on user input
130 to the control block module 102. The user can also interact
directly with the control block module in a number of ways. User
input 132 allows the user to switch the compressor on or off during
defrost mode. User input 134 allows the user to specify the initial
controller parameters, including the initial proportional constant
K. The proportional constant K may thereafter be modified by the
adaptive tuning module 120. User input 136 allows the user to
specify the pressure differential (dP) that the system uses as a
set point.
In addition to these user inputs, several user inputs are provided
for interacting with the signal conditioning module 110. User input
138 selects the sensor mode of operation for the signal
conditioning module. This will be described in more detail below.
User input 140 allows the user to specify the sampling time used by
the signal conditioning module. User input 142 allows the user to
specify whether the controller shall be operated using temperature
sensors only (T) or temperature and pressure sensors (T/P).
Referring now to FIG. 8, the signal conditioning module is shown in
detail. The inputs (temperature and/or pressure sensors) are shown
diagrammatically at 144. These inputs are processed through analog
to digital convertor 146 and then supplied to the control type
selector 148. Temperature readings from the temperature and/or
pressure sensors are taken sequentially and supplied serially
through the analog to digital convertor. The control type selector
codes or stores the data so that pressure and temperature values
are properly interpreted.
Digital filtering is then applied to the signal at 150 to remove
spurious fluctuations and noise. Next, the data are checked in
module 152 to ensure that all readings are within expected sensor
range limits. This may be done by converting the digital count data
to the corresponding temperature or pressure values and checking
these values against the pre-stored sensor range limits. If the
readings are not within sensor range an alarm signal is generated
for output on output 154.
Next a data manipulation operation is performed at 156 to supply
the temperature and/or pressure data in the form selected by the
sensor mode user input 138. The current embodiment will selectively
average the data or determine the minimum or maximum of the data
(Min/Max/Avg). The Min/Max/Avg mode can be used to calculate the
swing in pressure differential, or a conditioned temperature value.
The average mode can be used to supply a conditioned temperature
value. These are shown as outputs 158 and 160, respectively.
FIG. 9 shows the control block module in greater detail. The
conditioned temperature or pressure signal is fed to calculation
module 162 that calculates the error between the actual temperature
or pressure and the set point temperature or pressure. Module 162
also calculates the rate of change in those values.
The control block module is designed to update the operating state
of the system on a periodic basis (every Tc seconds, nominally once
every second). The Find Operating State module 164 performs this
update function. The state diagram of FIG. 10 provides the details
on how this is performed. Essentially, the operating state
advances, from state to state, based on whether there is a sensor
alarm (SA) present, whether there is a defrost status signal (DS)
present and what the calculated error value is. The Find Operating
State module 164 supplies the operating state parameter and the
Pull Down Time parameter to the decision logic module 166.
Referring to FIG. 10, the Find Operating State module 164 advances
from state to state as follows. Beginning in the initial state 168
the module advances to the normal operating state 170 after
initialization. It remains in that state until certain conditions
are met. FIG. 10 shows by label arrows what conditions are required
to cycle from the normal operating state 170 to the defrost state
172; to the pull down state 174; to the sensor alarm pull down
state 176; to the sensor alarm operating state 178 and to the
sensor alarm defrost state 180.
The decision logic module 166 (FIG. 9) determines the duty cycle of
the variable duty cycle signal. This is output on lead 182,
designated % Loading. The decision logic module also generates the
compressor ON/OFF signal on lead 184. The actual decision logic
will be described below in connection with FIG. 11. The decision
logic module is a form of proportional integral (PI) control that
is based on an adaptively calculated cycle time T.sub.cyc. This
cycle time is calculated by the calculation module 186 based on a
calculated error value generated by module 188. Referring back to
FIG. 6, the conditioned pressure differential signal on lead 122
(Cond dP) is supplied to the Calculate Error module 188 (FIG. 9)
along with the pressure differential set point value as supplied
through user input 136 (FIG. 6). The difference between actual and
set point pressure differentials is calculated by module 188 and
fed to the calculation module 186. The adaptive cycle time
T.sub.cyc is a function of the pressure differential error and the
operating state as determined by the find operating state module
164 according to the following calculation:
T.sub.cyc(new)=T.sub.cyc(old)+K.sub.c*Error (1) where: K.sub.c:
proportional constant; and Error: (actual-set point) suction
pressure swing. The presently preferred PI control algorithm
implemented by the decision logic module 166 is illustrated in FIG.
11. The routine begins at step 200 by reading the user supplied
parameters K, T.sub.i, T.sub.c and S.sub.t. See FIG. 6 for a
description of these user supplied values. The constant K.sub.p is
calculated as being equal to the initially supplied value K; and
the constant K.sub.i is calculated as the product of the initially
supplied constant K and the ratio T.sub.c/T.sub.i.
Next, at step 202 a decision is made whether the absolute value of
the error between set point temperature and conditioned temperature
(on lead 190, FIG. 9) is greater than 5.degree. F. If so, the
constant K.sub.p is set equal to zero in step 204. If not, the
routine simply proceeds to step 206 where a new loading percent
value is calculated as described by the equation in step 206 of
FIG. 11. If the load percent is greater than 100 (step 208), then
the load percent is set equal to 100% at step 210. If the load
percent is not greater than 100% but is less than 0% (step 212) the
load percent is set equal to 0% at step 214. If the load percent is
between the 0% and 100% limits, the load percent is set equal to
the new load percent at step 216.
The variable duty cycle control signal generated by the controller
can take several forms. FIGS. 12 and 13 give two examples. FIG. 12
shows the variable duty cycle signal in which the duty cycle
varies, but the frequency remains constant. In FIG. 12, note that
the cycle time, indicated by hash marks 220, are equally spaced. By
comparison, FIG. 13 illustrates the variable duty cycle signal
wherein the frequency is also varied. In FIG. 13, note that the
hash marks 220 are not equally spaced. Rather, the waveform
exhibits regions of constant frequency, regions of increasing
frequency and regions of decreasing frequency. The variable
frequency illustrated in FIG. 13 is the result of the adaptive
modulation of the cycle time T.sub.cyc.
FIG. 14 graphically shows the benefits that the control system of
the invention has in maintaining tighter temperature control and
higher suction pressure with improved system efficiency. Note how
the temperature curve 222 of the invention exhibits considerably
less fluctuation than the corresponding temperature curve 224 of a
conventional controller. Similarly, note that the pressure curve
226 of the invention has a baseline well above that of pressure
curve 228 of the conventional controller. Also, the peak-to-peak
fluctuation in pressure exhibited by the invention (curve 226) is
much smaller than that of the conventional controller (curve
228).
The controller of the invention operates at a rate that is at least
four times faster (typically on (be order of at least eight times
faster) than the thermal time constant of the load. In the
presently preferred embodiment the cycle time of the variable duty
cycle signal is about eight times shorter than the time constant of
the load. By way of non-limiting example, the cycle time of the
variable duty cycle signal might be on the order of 10 to 15
seconds, whereas the time constant of the system being cooled might
be on the order of 1 to 3 minutes. The thermal time constant of a
system being cooled is generally dictated by physical or
thermodynamic properties of the system. Although various models can
be used to describe the physical or thermodynamic response of a
heating or cooling system, the following analysis will demonstrate
the principle.
Modeling the Thermal Time Constant of the System Being Cooled
One can model the temperature change across the evaporator coil of
a refrigeration system or heat pump as a first order system,
wherein the temperature change may be modeled according to the
following equation:
.DELTA.T=.DELTA.T.sub.ss[1-exp(-t/.gamma.)]+.DELTA.T.sub.0exp(=.tau./.gam-
ma.). where: .DELTA.T=air temperature change across coil
.DELTA.T.sub.ss=steady state air temperature change across coil
.DELTA.T.sub.0=air temperature change across the coil at time zero
t=time .gamma.=time constant of coil. The transient capacity of the
unit can be obtained by multiplying the above equation by the air
mass flow rate (m) and specific heat at constant pressure (C.sub.p)
and integrating with respect to time.
Generally, it is the removal of the refrigerant from the evaporator
that controls the time required to reach steady state operating
condition, and thus the steady state temperature change across the
condenser coil. If desired, the system can be modeled using two
time constants, one based on the mass of the coil and another based
on the time required to get the excess refrigerant from the
evaporator into the rest of the system. In addition, it may also be
desirable to take into account, as a further time delay, the time
lag due to the large physical distance between evaporator and
condenser coils in some systems.
The thermal response of the evaporator coil may be modeled by the
following equation:
.E-backward.=1/2[(1-e.sup.1/.gamma.1)+(1-e.sup.1/.gamma.2)] where:
.E-backward.=temperature change across coil/steady state
temperature change across coil t=time .gamma.1=time constant based
on mass of coil .gamma.2=time constant based on time required to
remove excess refrigerant from evaporator
In practice, the controller of the invention cycles at a rate
significantly faster than conventional controllers. This is because
the conventional controller cycles on and off in direct response to
the comparison of actual and set-point temperatures (or pressures).
In other words, the conventional controller cycles on when there is
demand for cooling, and cycles off when the error between actual
and set-point temperature is below a predetermined limit. Thus the
on-off cycle of the conventional controller is very highly
dependent on the time constant of the system being cooled.
In contrast, the controller of the invention cycles on and off at a
rate dictated by calculated values that are not directly tied to
the instantaneous relation between actual and set-point
temperatures or pressures. Rather, the cycle time is dictated by
both the cycle rate and the duty cycle of the variable duty cycle
signal supplied by the controller. Notably, the point at which the
controller cycles from on to off in each cycle is not necessarily
the point at which the demand for cooling has been met, but rather
the point dictated by the duty cycle needed to meet the demand.
Adaptive Tuning
The controller Geneva described above can be configured to perform
a classic control algorithm, such as a conventional
proportional-integral-derivative (PID) control algorithm. In the
conventional configuration the user would typically need to set the
control parameters through suitable programming. The controller may
also be of an adaptive type, described here, to eliminate the need
for the user to determine and program the proper control
parameters.
Thus, one important advantage of the adaptive controller is its
ability to perform adaptive tuning. In general, tuning involves
selecting the appropriate control parameters so that the closed
loop system is stable over a wide range of operating conditions,
responds quickly to reduce the effect of disturbance on the control
loop and docs not cause excessive wear of mechanical components
through continuous cycling. These are often mutually exclusive
criteria, and a compromise must generally be made. In FIG. 18 (and
also FIG. 6) there are two basic control loops: the refrigeration
control loop and the adaptive tuning loop. The refrigeration
control loop is administered by control block module 102; the
adaptive loop is administered by adaptive tuning module 120.
Details of the adaptive tuning module 120 are shown in FIGS. 15,
16a-16c and 17. The presently preferred adaptive tuning module uses
a fuzzy logic control algorithm that will be described in
connection with FIGS. 18-20.
Referring to FIG. 15, the adaptive tuning module performs basically
three functions. First, it decides whether to perform adaptive
tuning. This is handled by module 240. Second, it gathers the
needed parameters for performing adaptive tuning. This is handled
by module 242. Third, it calculates the adaptive gain used by the
control loop. This is handled by module 244.
Module 240 bases the decision on whether to start tuning upon two
factors: the current operating state of the system and the control
set point. The flowchart of FIG. 16a shows the steps involved in
this decision. Module 242 integrates key parameters needed for the
calculations performed by module 244. Essentially, module 242
inputs the percent loading, the temperature and pressure values and
the set point temperature. It outputs the following data: S_ER (the
total number of conditioned temperature and pressure data points
that are within 0.5 degrees or 1 Psig of the set point value),
S_Close (the total number of percent loading data points that goes
to zero percent during a given sampling interval, e.g. 30 min.),
S_Open (the total number of percent loading data points that goes
to 100% in the sampling interval) and SSLP (a moving average or
rolling average of the percent loading during the sampling
interval). Module 242 is responsive to a tuning flag that is set by
module 240. Module 242 performs the integration of these key
parameters when signaled to do so by the tuning flag. FIG. 16b
shows the steps involved in performing integration of these key
parameters.
Finally, the calculation block takes the data supplied by module
242 and calculates the adaptive gain using the process illustrated
in FIG. 16c.
The adaptive tuning module 120 will cycle through various operating
states, depending on the state of a timer. FIG. 17 is a state
diagram showing how the presently preferred embodiment will
function. Note that the sequence transitions from the
initialization mode to either the integration mode or the no tuning
mode, depending on whether the tuning flag has been set. Once in
the integration mode, the system performs integration until the
timer lapses (nominally 30 minutes), whereupon the calculation mode
is entered. Once the calculations are completed the timer is reset
and the system returns to the initialization mode.
The block diagram of the adaptive scheme is shown in FIG. 18. There
are two basic loops--The first one is the PID control loop 260 that
runs every "dt" second and the second is the adaptive loop 262 that
runs every "ta" second. When the control system starts, the PID
control loop 260 uses a default value of gain (K) to calculate the
control output. The adaptive loop 262, checks the error e(t) 264
every "ta" seconds 266 (preferably less than 0.2 * dt seconds). At
module 268 if the absolute value of error, e(t), is less than
desired offset (OS), a counter Er_new is incremented. The Offset
(OS) is the acceptable steady-state error (e.g. for temperature
control it may be +/1.degree. F.). This checking process continues
for "tsum" seconds 270 (preferably 200 to 500 times dt seconds).
After "tsum" seconds 270, the value Er_new is converted into
percentage (Er_new% 272). The parameter Er_new% 272 indicates the
percentage of sampled e(t) that was within accepted offset (OS) for
"tsum" time. In other words, it is a measure of how well the
control variable was controlled for past "tsum" seconds. A value of
100% means "light" control and 0% means "poor" control. Whenever
Er_new% is 100%, the gain remains substantially unchanged as it
indicates lighter control. However, if Er_new happens to be between
0 and 100%, adaptive fuzzy-logic algorithm module 274 calculates a
new gain (K_new 276) that is used for next "tsum" seconds by the
control algorithm module 278.
In the preferred embodiment, there is one output and two inputs to
the fuzzy-logic algorithm module 274. The output is the new gain
(K_new) calculated using the input, Er_new%, and a variable, Dir,
defined as follows: Dir=Sign[(Er_new%-ER_old%)*(K_new-K_old)] (2)
where: Sign stands for the sign (+ve, -ve or zero) of the term
inside the bracket; Er_new% is the percentage of e(t) that is
within the offset for past "tsum" seconds; Er_old% is the value of
Er_new% in "(tsum-1)" iteration; K_new is the gain used in "tsum"
time; and K_old is the gain in (tsum-1) time.
For example, suppose the controller starts at 0 seconds with a
default value of K=10 and, ta=1 seconds, tsum=1000 seconds and
OS=1. Suppose 600 e(t) data out of a possible 1000 data was within
the offset. Therefore, after 1000 sec. Er_new%=60 (i.e.,
600/1000*100), K_new=10. Er_old% and K_old is set to zero when the
adaptive fuzzy-logic algorithm module 274 is used the first time.
Plugging these numbers in Eq.(2) gives the sign of the variable
"Dir" as positive. Accordingly, the inputs to the adaptive
fuzzy-logic module 274 for the first iteration are respectively,
Er_new%=60 and Dir=+ve.
The next step is to perform fuzzification of these inputs into
fuzzy inputs by using membership functions.
Fuzzification
A membership function is a mapping between the universe of
discourse (x-axis) and the grade space (y-axis). The universe of
discourse is the range of possible values for the inputs or
outputs. For ER_new% it is preferably from 0 to 100. The value in
the grade space typically ranges from 0 to 1 and is called a fuzzy
input, truth value, or a degree of membership. FIG. 19 shows graph
300 which contains the membership functions for the input, Er_new%.
Er_new% is divided into three linguistic variables--LARGE (304),
MEDIUM (306) AND SMALL (308). For Er_new%=60, the fuzzy inputs (or
degree of membership function) are -0.25 of LARGE and 0.75 of
MEDIUM. The input variable "Dir" is well defined (+ve, -ve or zero)
and thus does not require a membership function in this
application. The next step is to create the "Truth Table" or Rule
Evaluation.
Rule Evaluation
Rule evaluation takes the fuzzy inputs from the fuzzification step
and the rules from the knowledge base and calculates fuzzy outputs.
FIG. 20 shows the rules as truth table. For the first column and
first row, the rule is: "IF ER_new% is LARGE AND Dir is NEGATIVE
THEN New Gain is NO CHANGE (NC)" (i.e. if the percentage of e(t)
data that is within the offset (OS) for last "tsum" seconds is
LARGE and the direction (DIR) is NEGATIVE/ZERO then do not change
the existing K value (NO CHANGE)).
In the example, because ER_new% has fuzzy inputs LARGE (0.25) AND
MEDIUM (0.75) with POSITIVE Dir, the rules that will be used
are:
IF ER_new% is LARGE (0.25) AND Dir is POSITIVE THEN New Gain is NO
CHANGE (NC=1)
IF ER_new% is MEDIUM (0.75) AND Dir is POSITIVE THEN New Gain is
POSITIVE SMALL CHANGE (PSC=1.2)
Defuzzification
Finally, the defuzzification process converts the fuzzy outputs
from the rule evaluation step into the final output by using Graph
310 of FIG. 21. Graph 310, uses the following labels ="NBC" for
negative big change; "NSC" for negative small change; "NC" for no
change; "PSC" for positive small change; and PBC for positive big
change. The Center of Gravity or centroid method is used in the
preferred embodiment for defuzzification. The output membership
function for change in gain is shown in FIG. 21.
The centroid (the Fuzzy-Logic Output) is calculated as: .times.
.times..mu..function..SIGMA..times. .times..mu..function.
##EQU00001## where: (x) is the fuzzy output value for universe of
discourse value x. In our example, the output (K_new) becomes
.times..times..apprxeq. ##EQU00002##
Once the three steps of fuzzification, rule evaluation, and
defuzzification are finished and the output has been calculated,
the process is repeated again for new set of Er_new%.
In the above example, after the first 1000 sec, the adaptive
algorithm calculates a new gain of K_new=11.50. This new gain is
used for the next 1000 sec (i.e. from t=1000 to 2000 sec in real
time) by the PID control loop. At t=1001 sec, counter Er_new is set
to zero to perform counting for the next 1000 seconds. At the end
of another 1000 seconds (ie. at t=2000 seconds), Er_new% is
calculated again.
Suppose this time, Er_new% happens to be 25. This means, by
changing K from 10 to 11.5, the control became worse. Therefore, it
would be better to change gain in the other direction, i.e.,
decrease the gain rather than increase. Thus, at t=2000 sec,
Er_new%=25, Er_old%=60 (previous value of Er_new%), K_new=11.5 and
K_old=10 (previous value of K). Applying Eq.(2), a negative "Dir"
is obtained. With Er_new% of 25 and Dir=Negative, the fuzzy-logic
calculation is performed again to calculate a new gain for the next
1000 seconds. The new value of gain is K_new=7.76 and is used from
t=2000 to 3000 seconds by the PID Loop.
Suppose for the third iteration, i.e., from (t=2000 to 3000
seconds, Er_new% comes out to be 95% (which represents tighter
control). Performing the same fuzzy-logic operation gives the same
value of K_new, and the gain remains unchanged until Er_new% again
degrades.
Exemplary Applications
Both pulse width modulated (PWM) Compressors and electronic stepper
regulator (ESR) Valves can be used to control evaporator
temperature/pressure or evaporator cooling fluid (air or water)
temperature. The former controls by modulating the refrigerant flow
and the latter restricts the suction side to control the flow.
Referring back to FIG. 18, the block diagram of the control system
for such an actuator working in a refrigeration system 279 is
shown. In FIG. 18 one and preferably up to four temperatures of
evaporator cooling fluid or one evaporator suction pressure
(generally shown at 282) is sampled every dt seconds. A sampling
time of dt=10 seconds was found to be optimum for both the
applications. After processing by the analog to digital module 284,
the sampled signal is then reduced to one number by taking the
average or the minimum or the maximum of the four temperatures
depending on the system configuration or the user preference at
module 286. Typically, in a single actuator (PWM/ESR) systems where
the complete evaporator coil goes into defrost at one time,
averaging of control signal is preferred. In a multiple
evaporator-single actuator system where defrost of evaporator coils
does not occur at the same time, minimum is the preferred mode. The
value obtained after avg/min/max is called conditioned signal. At
comparison module 288 this is compared with the desired set point
to calculate the error, e(t).
The control algorithm used in the loop is a Proportion-Integral
(PI) control technique (PID). The PI algorithm calculates the valve
position (0-100%) in case of ESR or calculates the percentage
loading (0 to 100%) in case of PWM compressor. A typical integral
reset time, Ti, for both the actuators is 60 seconds. The gain is
tuned adaptively by the adaptive loop. The adaptive algorithm is
turned off in the preferred embodiment whenever the system is in
defrost; is going through pull-down; there a big set point change;
sensor failure has been detected; or any other system failure is
detected.
Consequently, the adaptive algorithm is typically used when the
system is working under normal mode. The time "ta" preferably used
is about 1 seconds and "tsum" is about 1800 seconds (30
minutes).
Diagnostics Related to PWM Compressor/ESR Valves:
Referring to FIG. 22, a discharge cooling fluid temperature sensor
312 (Ta), an evaporator coil inlet temperature sensor 314 (Ti) and
an evaporator coil exit temperature sensor 316 (To) can provide
diagnostic features for the evaporator control using PWM/ESR. The
Inlet temperature sensor 314 can be anywhere in evaporator coil
318. However, the preferred location is about one third of the
total evaporator length from the evaporator coil distributor
320.
Using these three temperature sensors, system learning can be
performed that can be used for diagnostics. For example,
diagnostics can be performed for ESR/PWM when it is used in a
single evaporator along with an expansion valve. In this example,
the following variables are tracked every "tsum" second in the
adaptive loop. The variables can be integrated just after ER_new
integration is done in the adaptive loop. N-Close: Number of times
Valve position /PWM loading was 0%. N-Open: Number of times Valve
position/PWM loading was 100%. MAVP: The moving average of the
Valve position /PWM loading for "tsum" seconds. SSLP: The
steady-state Valve position /PWM loading is set equal to MAVP if
for the "tsum" duration ER_new% is greater than 50%. dT: Moving
average of the difference between Ta and Ti (Ta-Ti). SH: Moving
average of the difference between To and Ti (To-Ti) in the said
duration. This is approximately the evaporator superheat. N_FL:
Number of times To was less than Ti during the said duration, i.e.,
"tsum" seconds. This number will indicate bow much the expansion
valve is flooding the evaporator.
In addition, Pull-down time after defrost, tpd, is also learnt.
Based on these variables, the following diagnostics are performed:
temperature sensor failure; degraded expansion valve; degraded ESR
valve/PWM Compressor; oversized ESR/PWM; undersized ESR/PWM; and no
air flow.
Temperature Sensor Failure
Failures of temperature sensors are detected by checking whether
the temperature reading falls within the expected range. If PWM/ESR
is controlled using Ta as the control variable, then when it fails,
the control is done as follows. The above said actuator is
controlled based on Ti, or the Ta values are estimated using the
learned dT (i.e., add dT to Ti value to estimate Ta). During pull
down, the valve/PWM can be set to full-open/load for the learned
pull-down time (tpd). If Ti also fails at the same time or is not
available, the actuator is opened 100% during pull down time and
then set to steady-state loading percent (SSLP) after
pull-down-time. An alarm is sent to the supervisor upon such a
condition.
Degraded Expansion Valve
If an expansion valve sticks or is off-tuned or is
undersized/oversized, the following combinations of the tracked
variable can be used to diagnose such problems. N_FL>50% and
ER_new%>10% indicate the expansion valve is stuck open or is
off-tuned or may be even oversized and thus is flooding the
evaporator coil. An alarm is sent upon such a condition. Moreover,
SH>20 and N_FL=0% indicate an off-tuned expansion valve or an
undersized valve or the valve is stuck closed.
Degraded ESR Valve/PWM Compressor
A degraded ESR is one that misses steps or is stuck. A degraded PWM
Compressor is one whose solenoid is stuck closed or stuck open.
These problems are detected in a configuration where defrost is
performed by setting the ESR/PWM to 0%. The problem is detected as
follows.
If ER_new%>50% before defrost and during defrost
Ti<32.quadrature. F. and SH>5.quadrature. F., then the valve
is determined to be missing steps. Accordingly, the valve is closed
by another 100% and if Ti and SH remain the same then this is
highly indicative that the valve is stuck.
If ER_new%=0 and N_Close is 100% and Ti<32 F. and SH>5 F.
then PWM/ESR is determined to be stuck open. If ER_new%=0 and
N_Open is 100% and Ti>32 F. and SH>5 F. then PWM/ESR is
determined to be stuck closed.
Over-sized ESR/PWM
If N_Close>90% and 30%<ER_new%<100%, then an alarm is sent
for oversized valve/PWM Compressor.
Under-sized ESR/PWM
If N_Open>90% and ER_new%=0 and SH>5, then an alarm is sent
for undersized valve/PWM Compressor.
No Air Flow
If N_Open=100%, ER_new%=0, SH<5 F. and Ti<25 F and
N_FL>50%, then either the air is blocked or the fans are not
working properly.
Additionally, these diagnostic strategies can also be applied to an
electronic expansion valve controller.
The embodiments which have been set forth above were for the
purpose of illustration and were not intended to limit the
invention. It will be appreciated by those skilled in the art that
various changes and modifications may be made to the embodiments
discussed in this specification without departing from the spirit
and scope of the invention as defined by the appended claims.
APPENDIX
Pseudocode for Performing Signal Conditioning
Repeat the following every Ts Seconds: Read User Inputs: Sampling
Time (Ts) Control Type (P or T) Sensor Mode (Avg/Min/Max) Perform
Analog to Digital Conversion (ADC) on all (four) Temp. Sensor
Channels output data as Counts Digitally Filter Counts Ynew=0.75 *
Yold+0.25 * Counts output data as Filtered Counts Convert Filtered
Counts to Deg F. Test if at least one Sensor is within normal
operating limits e.g. within -40 and +90 F. If none are within
limit--Set Sensor Alarm to TRUE Else Perform Avg/Min/Max operation
based on Sensor Mode If Control Type is NOT a T/P Control Type Then
End Signal Conditioning Routine (until next Ts cycle) Else (Control
Type is T/P) Do the Following: Perform ADC on Pressure Sensor
Channel output data as Counts Digitally Filter Counts Ynew=0.75 *
Yold+0.25 * Counts output data as Filtered Counts Convert Filtered
Counts to Psig Test if pressure Sensor is within normal operating
limits e.g. within 0 and +200 If not within limit: Set dP=dP Set
Pt. Else: Calculate dP=Pmax-Pmin Set Sensor Alarm to Conditioned
T/dP End Signal Conditioning Routine (until next Ts cycle)
* * * * *