Number 31 October 92
The Huntington Technical Brief
By David Brubaker Ph.D.
FUZZY ENVIRONMENTAL CONTROL
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INTRODUCTION
In January 1992, Liebert Corporation of Columbus, Ohio,
specialists in environment control units for computer
installations, introduced the LogiCool, a precision temperature
and humidity controller with a fuzzy control unit at its heart.
This Technical Brief briefly discusses both the design of
LogiCool's fuzzy controller, and the process through which
Liebert elected to use a fuzzy approach.
DESIGN GOALS
Control of the environment for large computing systems is often
a far greater challenge than for rooms inhabited by people. Not
only do the systems themselves generate heat, but they are often
specified by their manufacturers to be maintained in as little as
a plus-or-minus 1 degree (Fahrenheit) range. Humidity is also a
challenge, causing, for example, corrosion and jamming of
associated mechanical systems at high humidity levels and the
enhanced possibility of static discharge with low levels.
Humidity control is often specified to be 50% relative humidity,
with a maximum swing of plus-or-minus 3% per hour.
In addition, the design of a precision environmental control
system also faces nonlinearities, caused by such system behavior
as air flow delay and dead times, uneven airflow distribution
patterns, and duct work layouts. Uncertainties in system
parameters are often present, for example, room size and shape,
location of heat-producing equipment, thermal mass of equipment
and walls, and amount and timing of external air introduction.
Recognizing these challenges, Liebert undertook the design of a
control system requiring (in general terms):
* Precision temperature and humidity control;
* Minimization of cycling times (i.e., the opening and closing of
the damper and turning on and off of the compressor), thereby
increasing reliability and component life, and also resulting in
increased energy efficiency;
* Straightforward and therefore inexpensive control electronics.
In short, Liebert wanted to precisely control with simple
hardware a nonlinear system with significant uncertainties.
Several traditional linear approaches were considered but proved
inadequate. A fuzzy logic approach was investigated and
ultimately implemented. Design specifics - The LogiCool control
system has six fuzzy inputs, three fuzzy outputs, and 144
principles (rules). It runs on a Motorola 6803 microprocessor,
and is programmed in C.
LogiCool's fuzzy input variables are: e_temperature, the
temperature relative to a setpoint; delta_T/delta_t, the rate of
temperature change; e_humidity, the humidity relative to a
setpoint; delta_H/delta_t, the rate of humidity change; and two
proprietary variables associated with the action of the
controllers.
Fuzzy outputs control: 1) amount of cooling, 2) amount of
dehumidification, and 3) heat. Outputs can also be treated as
fedback input variables, and time delays are treated as fuzzy
outputs as well. Each fuzzy variable is assigned seven
membership functions as values, with the traditional
Large_Negative, Medium_Negative, Small_Negative, Near_Zero,
Small_ Positive, Medium_Positive, and Large_Positive as labels.
Ranges for the values of each variable are proprietary.
An example of a temperature control principle, using the as ...then
... (rather than the if ... then ...) syntax, is:
as temperature relative to set point is small_positive
and temperature rate of change is medium_positive
then amount of cooling is small_positive;
The Liebert design also incorporates time delays into their
principles. The following demonstrates both this as well as the
use of a fuzzy output as a feedback variable.
as temperature relative to setpoint is small_negative
and amount of cooling is small_positive
then wait delay to cooling change is medium_positive;
A fuzzy OR operator (maximizer) is used as the defuzzification
technique, avoiding the complicated calculations associated with
a centroid approach. Liebert has found that with the large
number of principles, a more elaborate approach is unnecessary.
Inputs are sampled, the principle-base accessed, and outputs are
updated once a second. The "long" inter-sample delay allows the
6803, a simple eight-bit microprocessor, to implement this
rather large fuzzy system.
ECONOMIZER
A key feature of the system is LogiCool's Economizer, which also
runs under fuzzy control. When appropriate (for example in the
cool of the morning), the outdoor ambient temperature is used to
assist in internal temperature control. While on/off control of
the Economizer could have been used, fuzzy logic greatly reduced
the number of system cycles, thereby significantly reducing wear
on the damper.
DESIGN PROCESS
The sequence that resulted in the decision to use fuzzy logic
started roughly eighteen months ago. From the engineering side,
the decision was driven by being able to meet difficult
requirements. Two Liebert engineers, Terry Bush and Dennis Weber,
had already read, become interested in, and familiarized
themselves with fuzzy technology, and were therefore able to
recognize that a fuzzy approach could satisfy the stringent
requirements. Simultaneously, Liebert marketing was both aware
of fuzzy logic controlled air conditioning systems available
outside the United States, and was also getting feedback from
customers indicating that an improved method of handling complex
internal environmental control was needed. Putting the two
together, engineering was asked to investigate a fuzzy
controller.
Liebert engineers designed and implemented the system in-house,
with the entire fuzzy logic controller portion completed in two
to three months. A commercially available fuzzy development tool
was not used, primarily because in the beginning there was
insufficient confidence in the approach to justify the expense
of such a tool. Liebert is still satisfied with the decision to
"roll its own" system, as it resulted in a design that
incorporates a number of features not easily provided in the
tools available at the time, for example the delaying of output
actions by fuzzily defined wait delays.
Liebert engineers did ultimately write a PC-based simulator to
test the fuzzy design under simulation prior to committing it to
hardware. Conclusion - Although quantitative metrics are not
available, Liebert reports that LogiCool has fully met its
design goals. Damper and compressor cycling times have been
greatly reduced, especially during Economizer operation. This
reduction in cycling times results in increased reliability and
increased expected component life. LogiCool also meets
Liebert's operational requirements associated with precisely
controlling temperature and humidity in rooms with uncertain and
nonlinear characteristics. Moreover, installation includes no
tuning procedure - the same set of principles satisfies all
installations.
Liebert is completely satisfied with the response to LogiCool.
Sales since its introduction last January are better than
expected, and production run sizes are being increased to
respond to the demand. In addition, in recognition of the
innovation in overall design, the LogiCool has been recognized
by HVAC News as a 1992 Design Winner. For more information on
the LogiCool, call Liebert Corporation, at 1-800-877-9222.
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The Huntington Technical Brief is published, monthly and free
of charge, as part of the marketing effort of Dr. David Brubaker
of The Huntington Group. A full collection of past issues
(starting with number 5 -- issues 1 through 4 are unrelated to
fuzzy logic and are unavailable) may be obtained for $10.00. The
42-page report "Introduction to Fuzzy Logic Systems" is
available for $35.00.
For the past sixteen years Dr. Brubaker has provided
technical consulting services in the design of complex systems,
real-time, embedded processor systems, and for the past five
years, fuzzy logic systems. If you need out-of-house expertise
in any of these, please call 415-325-7554.
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Copyright 1992 by The Huntington Group
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