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I. INTRODUCTION
Agricultural greenhouses are used to increase plant quality and productivity by
controlling temperature, and occasionally humidity, of the internal air. Research
greenhouses, on the other hand, are designated to control light intensity and CO2
concentration besides temperature and humidity in order to investigate their effects
on plant dynamics and growth quality [1-31.
Since the 1980s a number of researches have been devoted to integration of
microcomputer based control systems to operate greenhouses in order to improve
plant quality and growth that also lead to economic savings by efficient control of
temperature and humidity [4-9]. Saffell and Marshall controlled the temperature of a
greenhouse using a PDP-I1 computer [7], and Matthews and Saffell then extended
their work successfully to control the humidity [8]. However, their control algorithm
did not incorporate external humidity and they did not report results on simultaneous
control of the coupled variables of humidity and temperature of the resulting MIMO
process which can cause control and experimental problems.
In their work, Davis and Hooper [10] showed that robust greenhouse temperature
control can be achieved with the addition of heating pipe temperatures to proportional
and integral feedback control of the measured internal air temperatures.
Recent developments in the fields of artificial neural network theory, fuzzy control
technique, H,, synthesis and expert systems provide significant research and application
potentials in air conditioning research in general, in greenhouse control in
particular, because these are more flexible methods than the classical three term
controllers to deal with nonlinear multivariable processes [11-13].
607
608 O.S. TI]RKAY
The objective of this research is to develop the core of an expert system to control
the coupled variables of temperature and humidity of a glasshouse based on
empirically derived heuristic rules of the form
IF [inside and/or outside glasshouse[ condition
THEN [heater, fan, humidifier] actuation
To this end, the shallow knowledge of a process operator is used to formulate the
core of an expert control algorithm that is experimentally implemented in this work.
On the other hand. the presented algorithm can be improved with the deep
knowledge of specialists on plant dynamics, on the specific actuators and on the
objectives of the whole process [14. 15].
This research is a fusion of thermo-mechanical, electrical and computer control
applications which constitutes a typical application of modern mechatronics engineering.
In addition, this work enables the experimental validation of a humidity
measurement technique based on the regression analysis of the standard psychrometric
chart first proposed by Jepson [16].
2. EXPERIMENTAL SYSTEM DESIGN
2. !. Description of the system
The microcomputer based expert control system is shown in Fig. 1. The laboratory
scale glasshouse of dimensions 1.0 m x 0.5 m x 0.4 m is heated underneath by three
resistor heaters of 390 W total power.
The humidification process is realized by pumping water through a pipe with holes
onto soil of the glasshouse for a specified short period. This method is cheap but it
has the disadvantage of being a slow humidification technique compared to others. A
mains-driven ventilator of 0.2m~s -~ flow rate is used to control cooling and
dehumidification of the internal air. The effectiveness of cooling and dehumidification
mechanisms are important for glasshouse control and they can cause problems as was
the ease in this work and th~,t reported in [8].
r•R ~ T xw. ~.
Measurin8 ~ 0
Electronics~ T xd _[~
;~. ~"'~ Psychrometers
e lays ~ ' , ~ T w
z I:1 F~ _- ~. .- _TII~, I l
Itovl_ F l
I , - .... ,
Dimensions 1.0m x 0.5m x .4m
Fig. |. Microcomputer based glasshouse control system.
Expert system to control humidity and temperature 609
Since the process is a slow dynamics system an 8 MHz XT-type PC is used. A 12-bit
data acquisition and control board with 8 AD and 3 DA channels is mounted into the
microcomputer. Four of the AD channels of 5 V input each are utilized to digitize the
measured internal and external, dry-bulb and wet-bulb temperatures. The measured
temperatures are processed on-line to estimate the internal and external relative
humidities using Jepson's method as explained in the next section. The three
10 V-DA output ports have been used to actuate the relays controlling the fan, the
humidifier pump and the resistor heater according to the decision to be taken by the
expert control algorithm.
2.2. Temperature measurement
The temperature signals are measured using thermistors that are more accurate
than thermocouples but suitable only for low temperature ranges. The outputs of the
thermistors are conditioned through an in-house-made electronic circuit including four
channels. Each channel is calibrated to a gain of 0.012V°C -t in the range of
0-100 °C. Thus, the resolution of temperature measurement becomes equal to
Tr~ - - 1- °C x 5 V - 0.1 oc unit -x.
0.012 V 12-"
2.3. Humidity measurement
The internal and external humidities could have been measured using humidity
transmitters. However, this research permitted the experimental validation of a
humidity measurement technique first proposed by Jepson [16l and described in the
next paragraph.
The dry-bulb temperature, T, is simply that indicated by an ordinary bare
transducer such as a thermometer or a thermistor. The wet-bulb temperature, Tw, is
that indicated by the thermistor bulb covered with a wet absorbent wick and exposed
to an unsaturated air-water vapor mixture moving between 2.5 and 5 m s -t. [17]. To
this end, a permanent magnet D.C. motor driven fan has air continuously blown onto
the thermistor bulb Covered with an absorbent wick embedded in distilled water
(Fig. 1).
For air-water vapor mixtures the wet-bulb temperature and the adiabatic saturation
temperature, T*, differ by only a few tenths of a degree at atmospheric pressure.
Thus, T* can be substituted by T,, in psychrometric calculations. Corresponding to a
given combination of a dry-bulb temperature T and a wet-bulb temperature Tw, the
nonlinear psychrometric relations specify the value of relative humidity • as shown on
the schematic psychrometric chart of Fig. 2.
The conv.entional methods of humidity measurement rely upon the use of a large
matrix table of T, T,, and ~0 calculated from the nonlinear psychrometric relations, or
upon an interative procedure of these relations [18]. Jepson used the least squares
regression analysis to obtain a set of linear equations approximating the standard
psychrometrie chart curves under an atmospheric pressure assumption. The resulting
linear equation
T,, = b(#,) T + a(,t,), (1)
610 O.S. TURKAY
Tw ~ T* I ~=0.9~
I*Ci
I "~ = 0.3 : 30 T [ °C] 80
Fig. 2. Relations of T. T* and q) of schematic psychrometric chart.
where (^) denotes "'estimate", together with the regression coefficients bop ) and a(<p)
of Table 1, establishes the linearized relationship between the variables T, T,+ and ~p.
Thus, Eqn (1) and the entries of Table 1 can be used to estimate one of the variables
with sufficient accuracy for most engineering applications knowing the other two [16].
In this work. the measured temperatures T and T~, are used to estimate the humidity.
Since the regression coefficients in Table 1 are given for q~ = 30, 40, 50, etc., a simple
interpolation is used to compute the intermediate regression coefficients corresponding
to each integer relative humidity. The interpolation is iterated within a subroutine
until the condition
If T,+ (measured) - 7"+ [from Eqn (i)] < 0.2 °C
Then Output the calculated relative humidity as tile measured one.
3. EXPERT CONTROL ALGORITHM
The expert system to control the temperature and humidity of the glasshouse is
shown in Fig. 3. The desired ranges of internal relative humidity and temperature of
the glasshouse are specified as inputs to the computer code with their minimum and
maximum values. The measured internal states together with the measured external
relative humidity are fed back to the rule-based expert control algorithm consisting of
three levels of hierarchy.
In commercial greenhouse applications the internal temperature is usually greater
than the outside one. Hence. the external temperature is not included within the
decision tree. However. its inclusion is straightforward and this could be done by
incorporating one more level of hierarchy.
Table 1. Regression coefficients of linearizcd psychrometric relation
¢~ h(~) [°F/~FI a(~) I°FI Corr. coeff.
0.3 0.750256 - I. 15645{] 0.999897
0.4 O. 18112{)2 -2.491510 0.999945
0.5 0.858568 -2.911680 0.999973
O. 6 O. 896106 - 2.69237{) O. 999993
O. 7 0.928161 - 2. 6967311 I.(~)<M)IM]
0.8 0.955fM6 - 1.561950 I .{)00000
0.9 0.978813 -0.805760 1 .[k')O000
1.0 1 0 1
Expert system to control humidity and temperature 611
Rel'. Inputs [ Measurement1s
r,-r T P!
H0=Tmin Hl=Hmax H : Internal Rel. Hum]
Fix :External ReI.Hum]
IF
ACTUATE
I^ew,t., [Q:~ter V: F~n H: Humidir~ I ~, 1
I°'.-"-- I
Fig. 3. Decision tree of expert control algorithm.
In the first level of hierarchical control, the measured internal temperature T is
compared with the reference range of T,,i,-Tm~x. Assume, for example, that T > Tm~
at a given sampling time. Then the algorithm goes to the second level of hierarchy to
compare the measured relative humidity H with the specified H,,~,-Hm~, range.
Assuming that H > H,,~x, the algorithm based on the shallow knowledge of an expert
operator takes the decisions to put the heater off, the fan on and the humidifier off.
Thus, the temperature and the relative humidity of the glasshouse decrease until they
fall within the specified ranges.
4. RESULTS AND DISCUSSION
The experiments were conducted in March during a week period when the
temperature and humidity of the laboratory were around 14 °C and 70% relative
humidity, respectively. The experimental temperature measurement was checked
using a thermometer of 0.1 °C accuracy. The relative humidity measurement was
calibrated using a commercial psychrometer. Since the process dynamics is slow the
sampling period was chosen as 3 s.
First, the heating dynamics of the glasshouse was investigated. To this end, at
initial internal conditions of 24.1 °C and 75% RH, the desired range of temperature,
[Tmi, = 25.9°C, Tm~ = 26.00C], and the uncontrolled range of relative humidity,
[RHm,, = 3, RHm~ = 99], were specified as inputs. This way the temperature was
controlled alone without humidity control. In Fig. 4, it is seen that the internal
temperature reaches its reference range in 4 min after a time delay of approximately
1 min. Since bang-bang control is used, the recorded signal shows a limit cycle
612 O.S. TORKAY
27
26.5
26
T 25.5
['C] 25
24.5
24
Tmax=26.0 RHmax=99]
Tmin=25.9 R Hmin=30 i 78
' 76
0 1 2 3 4 5 6 7 8 9
Time [ Min. ]
Fig. 4. Heating dynamics of the glasshouse without control of humidity.
oscillation of approximately 45 s period. The temperature remained within the actual
range of [T = 25.8 °C. T = 26.2 °C] with an error of 0.1 °C which is due to the overall
thermal capacitance of the glasshouse. The achieved control is excellent for a
glasshouse application. It is noted that while the temperature increases, the uncontrolled
relative humidity decreases and finally reaches a steady-state value of 69%.
A similar experiment was conducted to observe the dehumidification dynamics. At
internal initial conditions of 79% RH and 19.2 °C, the reference range of relative
humidity [RHm,, = 67, RH,,a~ = 69] and the uncontrolled range of temperature
[Tmi, = 10°C, Tm,,x =30°C] were specified as the inputs. Figure 5 shows that the
dehumidification process is very slow. The reference range is reached in about
10 rain. In fact, the humidification and dehumidification processes of the experimental
set-up had very slow response times due to the filirness of the physical devices used
for these purposes.
The relative humidity measurement method proposed by Jcpson is experimentally
verified also in Fig. 5. It is seen that the rchttive humidity is measured with a
T
[ "C]
24
23
22
21
20
19
18
17
rmax=ao RHmax=69
~ Tmin=10 RHmin=67
PSV ~
RH
" RH Range
i
0 1 2 3 4 5 6 7 8 9
Time [ MIn. ]
85
80
75 %
70 RH
65
Fig. 5. Dehumidification dynamics of the glasshouse without control of temperature.
Expert system to control humidity and temperature 613
resolution of 1%. The accuracy of this method is significantly dependent upon the
correctness of the measurement of the wet-bulb temperature. However, this drawback
is also true when a psychrometric matrix table or iterative calculations of psychrometric
relations are used for humidity measurement. Hence, Jepson's method is a
good alternative technique compared to the other two methods due to its memory
storage advantage and possible computational speed advantage.
The simultaneous control of temperature and humidity are shown in Figs 6-8. In
these cases, the process becomes a MIMO control system.
At initial conditions of [13.5 °C, 79% RH], a step change increase of [Tmm =
15.6 °C, T,,~., = 15.8 °C] and a step change decrease of [RHmi. = 72, RHr, a, = 74] for
the desired ranges of temperature and relative humidity were specified as the inputs,
respectively. The resulting curves of temperature and relative humidity together with
16
15.5
15
T 14.5
[ °C] 14
13.5
13
on
off
Tmax= 15.8 RHmax=74 ]
Tm n = 15.6 RHmin = 72
[
I ,.,,~ T Range ~ [80
~ 76%
on ~._ te~-F-an-~ RH Flange }
%ff ÷ " --~_~__~-÷L_,__÷._r~.__ ; ;t 70
0 2 4 6 8 10 12 14 16 18
Tlme [ Min. ]
Fig. 6. Simultaneous control of increasing the temperature and decreasing the relative humidity.
Tmax=18.2 RHmax=77 1
Tin!n=18.0 RHmin=75 ] 78
20 I ~ RH Ra~nge
19.5 t ""~'~.~
19 t . "~~, ~..~~ %,%IU- ~_~ [~ - 7 4 %
18.5 .~-=~C(~=j T Range . . . . . . . ~"-~'-~;,-~t RH
["C] 18 ~ - ~ C . . . . " ~'v~.70
17 ~f ~ ÷: ÷ 66
0 1.5 3.0 4.5 6.0 7.5 9.0 10.5
Time [ Min. ]
Fig. 7. Simultaneous control of decreasing the tcmlrmrature and increasing the relative humidity.
614 O.S. TORKAY
27
26
25
T 24
[°C] 23
22
21
2O
!Tmax=24.0 RHmax=72 t
!Train=23.8 RHmin=70 I
! 76 r - - - - "
RH . . . . - -~/~- ~-~ 72
T Range =- ~-; ' P ~ ;
. . . . , - . -_~-.-~-4~' 70 %
,~ _ _.-/..n,,'i,.' ~ Heater • Fan i I 66
0 1 2 3 4 5 6 7 8 9 10
Time [ Min. ]
Fig. 8. Simultaneous control of increasing both the temperature and the relative humidity.
the control actions of the heater and the fan are depicted in Fig. 6. The actual
temperature and relative humidity reach their reference ranges in 6 and 10 min,
respectively. After 14 min the internal temperature remains within the desired range,
thus the heater becomes off. On the other hand, the fan switches on and off to
maintain the relative humidity within the reference range.
The results of a similar experiment conducted by decreasing the temperature and
increasing the relative humidity from the initial values of [19.8°C, 72% RH] are
displayed in Fig. 7. The desired ranges of temperature and relative humidity,
[T,,i, = 18.0°C, T,,~ = 18.2 °C] and [RI-I,,,,~ = 75. RH ...... = 77] are reached successfully.
Note that the cooling mechanism of the glasshouse is inefficient and this renders
the implementation of the expert control algorithm a more difficult task.
Figure 8 displays the results obtained by increasing both the temperature and the
relative humidity of the glasshouse from the initial values of [21.0 °C, 64% RH] to the
reference ranges of [7",,,,, = 23.8 °C, T ...... = 24.0 °C] and [RH,,,, = 70, RHm,x = 72]
which are also achieved successfully in approximately 10 min.
With the experimental facilities available it was not possible, however, to decrease
the temperature and the relative humidity simultaneously. This was due to the
ineffectiveness of the dehumidification process and to the lack of an effective cooling
device which are both realized by replacing the wet air inside the glasshouse with the
dry air from outside. A similar ambiguity is reported by Matthews and Saffell [8].
5. CONCLUSIONS
The temperature and relative humidity of a laboratory scale greenhouse have been
controlled using a rule-based expert system. The expert control decision tree has been
devised upon the shallow knowledge of a process operator. The experimental results
demonstrated that the expert algorithm works successfully for SISO or MIMO control
of the coupled variables of temperature and relative humidity of the glasshouse.
However, it was not possible to decrease temperature and humidity simultaneously.
This was mainly due to the ineffectiveness of the ventilator to cool the internal air
Expert system to control humidity and temperature 615
and to decrease the humidity of the glasshouse. However, this does not shade the
validity of the expert control algorithm which can be improved further with the deep
knowledge of experts for specific applications.
This experimental work has further validated the humidity measurement technique
proposed by Jepson. The described method is applicable with sufficient accuracy for
most engineering implementations as long as the wet-bulb temperature is measured
accurately. This, however, is a necessity for other conventional methods such as
iterative method of psychrometric relations or using psychrometric matrix tables.
Thus, Jepson's method is assessed to be a good alternative to these methods.
Acknowledgements--The support of the Research Fund of Bogazifi..University is acknowledged. The author
would like also to thank Bogaziqi University Alumni Society (BUMED) for their financial contribution.
Special thanks are due to S. Giilener and G. Aysun who initiated this research as their graduation project
and to Assoc. Prof. V. Kalenderoglu for his advice.
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