EQUIPMENT DEVELOPMENT FOR AUTOMATIC COLLECTION OF GREENHOUSE

ENVIRONMENTAL DATA USING MOVING

SENSORSJ.W. von Beckmann

Agricultural Engineering DepartmentUniversity of British ColumbiaVancouver, British Columbia

N. Ross Bulley

Member CSAE

Agricultural Engineering DepartmentUniversity of British ColumbiaVancouver, British Columbia

INTRODUCTION

There are many variables in the designof a greenhouse that affect plant growthand development, such as building orientation, design, shape, and constructionmaterials (1, 8). The effects of thesevariables on the environment in a

greenhouse are readily measured at asingle location and can be used for thecontrol of the environment within presetlimits. However, the conditions at onelocation are not truly representative ofthe conditions encountered by the plantsthroughout the house.

In evaluating any new concepts ingreenhouse design, or structural material,it is necessary to theoretically analyze thepossible effects of the change as well as tosupport the calculations with data collected from prototypes and finishedstructures (3, 7, 9, 11, 12, 14). Presentmethods for measuring and recordingenvironmental conditions are either verytime-consuming or require many costlysensors. For these reasons it is believed

that many publications report only aminimum amount of environmental data,such as inlet and outlet air temperaturesand humidities, with little informationabout light distribution (9, 12). Somepapers report several light intensities andlittle about temperature and humidityvariations (4) or an author assumes thatmeasurements at a single location canadequately represent the overall conditions. Usually, the control of thegreenhouse environment is based on thisassumption (1, 2, 5, 6, 1-3).

A system has been designed to allowseveral locations in a greenhouse to besampled for temperature, humidity, light,or other environmental variables thatshould prove useful for those researchers

RECEIVED FOR PUBLICATION FEBRUARY14,1973

who were previously limited because ofthe high cost of "better" data loggingsystems. The purpose of the system beingdescribed is:

(1) To provide a relatively inexpensive method of measuring a fixednumber of environmental vari

ables at several locations.

(2) To collect the information fromstation to station automatically;and

(3) To employ commonly foundsensors and instruments, withoutthe need of a complete datalogging system.

The design specifications selected forthis prototype were the abilities tomeasure from 1 to 15 environmentalparameters (some combination of light,temperature, and humidity) at each of 10locations in about 10 min. The actualnumber of locations encountered in the10 min will vary with the number ofmeasurements taken per location, thedistance between locations, and theequilibration time required by the sensors.

THE SYSTEM

Overview

The data-collecting system consists ofa module, or mobile transmitting station,and a stationary data recording station, orreceiver station, remote from the module.The module travels along a fixedtwo-railed track, independent of cables orother connections to the receiver station.Besides carrying sensors, the modulecontains its own propulsion system andelectronic circuits that control its operation. The receiver station consists of onesensor amplifier per type of sensor carriedby the module, a recorder, as well aselectronic circuitry similar to that on themodule. The track, approximately 7 cmwide, is constructed of two rails (the type

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

used for large sliding doors) boltedtogether and is suspended from the roofstructure of the greenhouse to form aclosed loop. A fixed electrically shieldedcable (transmission cable) runs midwaybetween the rails. At the locations alongthe track where sampling is to take place(sampling sites), small mercury troughsmounted between the rails enable an

electrical connection to be made betweenthe module and receiver station via the

cable. The troughs are machined into apiece of Plexiglas and each is partly filledwith mercury. Each trough is connectedto its own wire in the transmission cable.

The sensor that requires the most wiresfor data transmission (N wires) determines the number of wires that must bein the cable (N 4- 1). The additional wireis required to carry a signal for synchronizing the operations of the module andreceiving station.

The module (Figure 1) travels alongthe track until it reaches a sampling site.Upon being stopped by a wedge on thetrack, the module's fork connector(Figure 2) drops into the mercury troughsand completes the module-to-receiverconnection via the transmission cable.

The sensors equilibrate to their newenvironment for the length of time (avariable) preset in the module circuit.After equilibration, the sensors aresequentially connected to their respectiveamplifiers at the recording station and thedata are recorded. On completion of therecording, the fork is lifted from thetroughs, and the module advances to thenext sampling site.

THE MODULE

The basic components of the moduleare shown in Figure 1. At a sampling site,microswitch MSI (A) is tripped by awedge on the track, and stops the motor(B), which propels the module. Anautomobile windshield-wiper motor is

99

Figure 1. Transmitting module. Legend: A,microswitch MSI; B, motor; C,motor-reversing switch (test modelonly); D, sensor connector strip; E,electronic circuit box; F, microswitchMS2; G, timer relay; H, relay 16; I,fork lifter solenoid; J, spring suspension assembly.

used to drive one of the four wheels. The

motor reversing switch (C) was used onlyon the prototype, as a short straight trackwas used in preliminary testing.

The cables from the sensors, which aresupported by a rigid plastic tube atdifferent elevations under the module, areconnected to the sensor connector strip(D). The electronic circuit box (E)electronically connects each sensor se-

100

Figure 2. Fork connector. Top, fork connectorA about to drop into mercury troughB; bottom, connection completed.

quentially to the fork connector used totransfer the sensor signals via thetransmission cable to the sensor amplifiers.

Microswitch MS2 (F) is used to startthe motor and advance the module to the

next sampling site. The timer relay (G)closes after the preset sensor equilibrationtime has elapsed. The connection of thesensors to their amplifiers then takesplace, and the connecting sequence isterminated by the closing of relay 16 (H).This relay activates the lifter solenoid (I),which removes the fork connector from

the mercury troughs. The lifting of thefork results in the closing of MS2 and theadvancement of the module.

Three of the four wheels that supportthe module are idlers, one of which ismounted in a spring suspension assembly(J), which assures that all four wheels arealways in contact with the track.

The module obtains its electrical

power from the main direct-currentpower supply located at the recordingstation. The track acts as the negative(and ground) connection and a power-conducting strip adjacent to one of therails is the positive connection. Electricmotor brushes are used to transfer the

current from these conductors to themodule. A small voltage regulatormounted on the module supplies thecurrent for the module's circuitry.

The block diagram of the module'scircuit is shown in Figure 3. The module

EQUILIBRATION

TIMER

LuJtimer

F9601SQUARE WAVE

GENERATOR

F9016HEX. INVERTERS

I c

REED RELAYS &SENSORS

^ ^TO FORK CONNECTOR

>.CTS

VOLTAGE

REGULATOR

MS 2

ff~r—'

c-Lll_it

TTT -ABC

FORK CONNECTOR

/'̂ ROTARY^"" JswiTCH

Figure 3. Module circuit block diagram: circuitry for module propulsion andsequential connection of sensors tocommon cable.

would be moving when the switches arein the positions shown. A more detaileddescription of the circuitry has beenreported previously (10).

THE RECORDING STATION

RECEIVER

The switching of a sensor to its properamplifier is controlled by the integratedcircuits of the module. However, becausethe 15 sensor signals have been convertedinto one signal path, the wires of thetransmission cable, they must be divertedagain to their respective amplifiers at therecording station. For this reason, asimilar circuit to the module circuit is

required at the recording station, and thisis the recording station receiver. The 15sensor signals are received sequentially viathe cable and are routed to their proper

amplifiers. To- simplify the receivercircuit, it was arbitrarily decided to divertthe first three sensor signals to oneamplifier, the next three to another, andso on up to the 15th.

In order for the sensors and amplifiersto be connected at the right time, it isessential that the module's switchingcircuits are synchronized with those ofthe receiver's. A synchronizing signal thatis transmitted via its own wire in the

transmission cable from the module to

the receiver assures that this will occur.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

\Aensor\/SIGNALINPUTS

\ / SYNC.YINPUT

AUTO.

RESET

F7492DIVIDE

BY 3

IF 9390

BY 5

REED RELAYS

nRECORDER

Figure 4. Record station block diagram: circuitry for the sequential connectionof the sensor signals from the cableto their respective amplifiers andrecorder.

As well as connecting the sensors andamplifiers, the receiver connects theindividual amplifiers to the recording unitone at a time. This eliminates the

interference between amplifier outputs. Ifa pen recorder is used as the recordingdevice, a straight horizontal line will beobtained for the length of time that asensor is connected to its amplifier. Thistime may be varied by an adjustment inthe module circuit. A digital print-out ormagnetic recording device that may betriggered to take a reading at a giveninstant would be much more convenient,because transferring of data from thechart paper would be unnecessary.

The block diagram of the recordingstation receiver is shown in Figure 4 andhas been reported in more detail previously (10).

Preliminary Tests

The only errors that might beencountered in the output data of thissystem that cannot be corrected bycalibration of the sensors are those that

might occur as a result of a badconnection between the sensor and itsamphfier. Because the mercury connectoris the only place where a variation ofcontact resistance should be found, a testwas made on the reproducibility of theresistance of a sensor-to-amplifier connection. A digital ohm meter (Weston 1240)was connected at the receiver station

where normally the sensor's amplifier

would be. The module was allowed to gothrough its normal procedure of approaching the mercury connector anddropping the prongs into the troughs,switching in the sensors, and advancing.The resistance of the dummy sensor wasmeasured through the switching systemevery time the module stopped at thesampling station.

The test was repeated 15 times. Theresistance varied ±0.5 ohms over the 15readings. This does not imply that thevariation obtained from station to stationwill be as small as the variation at a singlestation. But, any variation in readings dueto change in transmission cable lengthfrom station to station can be readilycorrected mathematically. Sensors connected to the module, and suspendedunder it, showed that results from a givensampling station were obtainable in theirproper sequence and were reproducible.Calibration of the sensors was omitted in

the trial and consequently the results arenot reported.

Due to the toxic effect that mercuryvapors have on some plants, it wasproposed that a thin layer of lubricationoil covering each mercury trough shouldsubstantially reduce evaporation of mercury. It was necessary, then, to study theeffect of the oil layer addition to themercury trough resistance reproducibility. A dummy sensor having a 105.5-ohmresistance out of circuit was placed onthe module* as in the previous experiment.Ten trials were performed using no oiland repeated with the oil covering thetroughs. The lack of reproducibility ofthe resistances due to the added oil is

evident by comparing the results listed inTable I.

The results of this experiment indicatethat the addition of oil to the mercuryproduced random increases in the conductor resistance, and for that reasonshould not be used.

DISCUSSION

Probably the most controversial designaspect of the system is the mercurytrough connector. The use of thesolid-liquid-solid connection was chosenover sliding contacts due to the variabilityof electrical contact resistance of thelatter, resulting from dust collection andcorrosion. Even with a heavy dust layeron the mercury, electrical connectionusing this technique was unaffected.Great care in handling mercury in thegreenhouse is required, but it is felt thatwith adequate ventilation, the accumulation of mercury vapors in the atmospherewill be negligible, as the surface area ofthe troughs is small.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

TABLE I EFFECT OF OIL LAYER ONMERCURY TROUGH RESISTANCE

Resistance (ohms) ofdummy sensor in circuit

Trial No oil Lub. oil

1 105.8 106.3

2 105.6 117.4

3 105.7 107.5

4 105.6 111.1

5 105.6 127.1

6 105.6 105.9

7 105.6 106.8

8 105.8 106.0

9 105.6 108.9

10 105.6 110.7

Mean = 105.6 Mean = 110.8

SD = 0.0850 SD = 6.726

SEof SEof

mean = =0.0269 mean = 2.127

The total cost of the complete systemcannot be accurately estimated, as it willvary with sensors, amplifiers, recordingdevices, and length of track used.However, the material cost of thetransmitter, receiver, and carrier is approximately $500. The cost of thetwo-railed track is about $1.00 per foot,not including the transmission cable.

SUMMARY

One of the major problems encountered in collecting data for use inanalyzing the internal environment of agreenhouse is the time required to readand record the many environmentalparameters. This problem severely restricts the analysis of factors affecting theenvironment by limiting the number oflocations that can be conveniently sampled and the frequency with which eachlocation can be sampled. The time andcost factors also limit the number ofparameters that can be measured at eachlocation.

A sensor-carrying module has beendesigned and developed that will automatically measure environmental parameters at any number of predeterminedpoints along a fixed track. The module iscapable of sampling 15 variables at asingle location and transmitting via onecable all 15 signals for interpretation by amain decoding and recording stationremote from the module.

The sampling rate for 15 parametersper location is approximately one location per minute, which is considerablyfaster and less expensive than othermethods.

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Originally the design was intended foruse in the greenhouse, but there is noreason why it may not be incorporatedinto other agricultural buildings such aspoultry barns, where environmental gradients exist that could be monitored.

ACKNOWLEDGMENTS

The authors acknowledge the technicalassistance in the construction of the

equipment by J. Pehlke and the late W.Gleave of the Agricultural EngineeringDepartment, University of BritishColumbia. This research was financed bythe National Research Council of Canada.

REFERENCES

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Cotton, R.F. 1969. Some measurementsof micro-climate in a glasshouse with a

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11. Walker, J.N. 1965. Predicting temperatures in ventilated greenhouses. Trans.Amer. Soc. Agr. Eng. 8(3): 445-448.

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14. Walker, J.N. and L.R. Walton. 1971.Effect of condensation of greenhouse heatrequirement. Trans. Amer. Soc. Agr. Eng.14(2): 282-284.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973