IEEE Instrumentation and Measurement Technology Conference Brussels, Belgium, June 4-6, 1996.

A Low-cost Water Sensing Method for Adaptive Control of Furrow Irrigation

D.J. Turnell, G.S. Deep, RC.S. Freire COPELE - Coordenago de P6s-Graduag.i4ioy

em Engenharia El&ica, Universidade Federal da Paraiba

Campus II, Campina Grande, Paraiha, 58 100. Brazil.

Phone/Fax: +55 (083) 333 2480 E-Mail: david@dee.ufpb.br

Abstract - A computer-based optimization technique for furrow irrigation depends upon a network of water sensors to detect the water advance in the initial phase of the irrigation. This paper describes a new type of low- cast water sensing network developed specifically for this technique. Each sensing network consists of up to fourteen simple water Sensors linked using inexpensive twin-wire. The distance between the computer and the last Sensor can be up to 1OOOm. Each sensor uses the variation of capacitance to detect the presence of water and communicates its state to the base computer by means of variable-frequency current pulses.

I. INTRODUCTION

Furrow irrigation [l] is composed of three phases: advance, ponding and recession. In the advance phase the water slowly descends the furrows until it reaches their ends. For long furrows the advance phase can last up to three hours. During the ponding phase the water inflow is normally reduced to the point where the furrows remain flooded but excessive run-off of water at their ends is avoided. The ponding phase lasts for several hours. When the desired application of water is obtained the water inflow is stopped and the recession phase begins, during which the water level in the furrows slowly drops.

The main disadvantage of furrow irrigation is its inefficient utilization of water. A new computer-based control technique promises to raise the efficiency of furrow irrigation to levels comparable to those of other forms of irrigation. This technique is based upon finding a solution to the "reverse finrow advance problem" [2][3][4]. The computer registers the water progress during the advance phase of the irrigation. This is accomplished through a network of sensors installed along one of the furrows within the irrigated area. The distance versus time data obtained

!?om the sensors is used to calculate values for the water infiltration coefficients of the soil. These coefficients are then used to model the water infiltration along the furrow. The results of the modeling allow the computer to alter the total water input time (advance phase plus ponding phase) to maximize the water utilization efficiency.

For optimal results this control technique requires at least six or seven water sensors in in an irrigated area. There sensors could be connected t be hundreds of meters distant). One example is the use of radio and infka-red links between microprocessor-based field stations [5 ] . Such sophistication is acceptable within the bunds of a research project. However, for cost reasons it is totally unfeasible for real-life field applications in most of the regions of the world where irrigation is necessary.

In this paper we describe a new low-cost water sensing method developed as a part of an irrigation control system called NOS (Real-time Irrigation Optimization System). NOS is currently being developed as a joint project between the electrical and agricultural engineering departments of the Federal University of Paraih. Paraih is a poor, semi- arid Brazilian state *e economic realities are one factor that restricts the use of irrigation. Consequently, the requirement for low-cost has been impartant throughout the development of the water sensing network.

11. THE WATER SENSING NETWORK

Each sensor network consists of up to fourteen water sensors linked to the computer by up to lOOOm of twin-wire line as shown in Fig. 1. The voltage that the computer applies to the line (V) determines which one of the sensors is enabled at any given time. To scan a particular sensor the computer puts its nominal enabling voltage on the line for 40Oms and then samples the line current (I) through an A/D converter.

0-7803-3312-8/96/$5.0001996 IEEE 1360

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computer

Fig. 1. The Sensing Network

The enabled sensor generates a stream of current pulses, the period of which indicates whether the sensor is in contact with the water.

Fig. 2 shows the scanning of six sensors over time. The voltage selection is shown in figure 2a and the resultant current on the network line is shown in figure 2b. Note that for each voltage level and given instant only one sensor responds with current pulses. When a sensor detects water the period of the current pulses becomes greater by up to 90% (the amplitude of the pulses is of no relevance).

While rather unorthodox, this addressing and signaling method does have some advantages:

The current pulses generated by the sensors are more immune to noise and have a greater range than alternative voltage signals. In the prototype network the

lses pass through 600111 of thin wire with no distortion. sensor always returns a stream of current

pulses when enabled, whether wet or dry (the difference is in the frequency of the pulses). Faulty sensors can be detected and eliminated from participation before an

Just two wires are needed for the network (a network using voltage for signaling would require at least one more wire). Two wires minimizes the number of electrical connections along the line and thus reduces the greatest single potential for network failure. The circuit needed for the voltage addressing and pulse generation is simple and inexpensive. Simple sensors lead to higher network reliability.

Fig. 3 shows the sensor circuit diagram. The three operational amplifiers (A1 - A3) are part of the same integrated circuit device (LM123). The operational amplifier A3 is configured as a relaxation oscillator whose period is determined by the capacitance of the sensing element C. For the sensing elements used in the prototypes the dry capacitance was around 1nF and the period of oscillation was around lms (dry). It is the switching of the transistor T1

that causes the current pulses on th diode (LED) is used for a visual

The operational amplifiers A1 and A2 implement the voltage selection for each sensor. Amplifier A2 uses a 5 . 0 ~

the reference diode D1 (LM336). The diode D2 is used to generate a slightly lower reference voltage fix Al, thereby defining a narrow range of voltage over which the sensor is enabled. The function of Al and A2 is best understood by examination of Fig. 4, which shows their outputs as a function of the voltage on the network line.

As the line voltage rises and r ches the lower limit at which led (VI) the output of amplifier A1 As the line voltage rises even more

and passes the upper limit (v2) the output of A2 also goes from low to high. From the way that the transistor T1 is configured the sensor only generates current pulses when the output of A1 is high and output of A2 is low, This only happens when the voltage on the line is within the sensor enabling range vl to v2. The range vl to v2 is different and non-overlapping for each sensor.

A.. The Sensing Element

Most water sensors detect the presence of water. oach is not possible with the NOS sensor due to interaction between sensors in a &row. Early field trials with resistance-based sensor prototypes showed that the low-imp path fontled by the water in the f i a r r ~ ~ interferes with the operation of oscillator circuit.

To avoid this problem the proposed sensor uses capacitance to detect water. This allows the sensors to be electrically isolated from the water and, ~ s e ~ ~ ~ t ~ y , each other. The

v

I.-.--..____*

Fig.2. Sensor Se1

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Fig. 3 . The Sensor Circuit Diagram

sensing element used in the final prototype is an 8Ocm

on rubber). The result is a crude form of

y Ehe capacitance of the ribbon cable element ut 90% when totally immersed in water. In

increase ranges fiom 40% to 70% unt of soil clinging to the wire. The causes a comparable change in thus in the period of the current

ulses ate^ on the line. Even a 40% change in pulse

time

e Voltage Selection Mechanism

period is easily detected by the scanfihg sofhvare on the computer.

B. Current Consumption

One of the critical aspects of the sensor design is the current consumption. If the sensors were to consume too much current then the excessive voltage drop along the network line would interfere with the voltage selection mechanism. The prototype sensor has a standby consumption of 2.4 mA at 1Ov. This is sufficiently low to allow the use of inexpensive 18 AWG wire on the prototype network (which uses 8 sensors spaced over 680m). Longer networks, however, will require slightly thicker wire -- at least between the computer and the first few sensors where the current is greater.

C. The Interjke Circuit

Each sensor network requires an interface circuit at the computer. This circuit has two functions: 1. To generate the variable line voltage (0 - 24v) used to

enable the sensors. 2. To provide a voltage that is proportional to the line

cment and from which the computer can detect the pulses generated by a sensor.

Fig. 5 shows the interface circuit used in the prototype system. The computer controls the output voltage V,, by changing the voltage V, generated by an %bit D/A converter. The 47kW resistor and the 1QnF capacitor form a low-pass filter to eliminate noise fiom the Vb signal. The operational amplifier A1 maintains the V,,, voltage proportional to V,. The transistor TI increases the current output capacity.

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24v AI” LM123

VI

’IN

to sensors

OV 1 . Fig. 5 . The Interface Circuit

The function of the operational amplifier A2 is to ~roduce a voltage VI proportional to the current being supplied by the circuit. The output ofA2 is a voltage proportional to the line current. This voltage comes fkom the amplification of the voltage drop across the J.9M resistor. In the prot output A2 is connected to an $-bit A/D converter. the current pulses the computer takes a burst of 200 samples and then analyses the data to identi@ the pulse period. Scanning is repeated several times to prove the accuracy with which the period is d e t e ~ i n e ~ .

could monitor the water advance of a furrow irrigation. The potentially long distances involved and the number of sensors needed tend to suggest sophisticated solutions based upon microprocessor-based field stations, digital networks, radio links, etc. However, as this investigation has shown, the problem can also be solved in a simple and low-cost manner consistent with the economic realities of many developing countries. The simple nature of the network is a positive factor in terms of reliability and most problems that do arise can be resolved by a walk-along inspection armed with pliers and electrical tape.

D. Sensor Construction and Installation

The original idea for the sensor construction was to use small plastic boxes. However, this was ruled out due to the cost of the water-proof boxes that are capable of [l]A.Benami and A.Ofen, 1984, Irrigation Engineering,

REFERENCES

withstanding the harsh conditions out in the fie1 boxes, the final sensor prototypes are encapsulated in clear plastic resin. The ribbon cable used as a sensing element emerges fiom one side of the plastic block while on the other side are two brass screw terminals that are used to connect to the Bine wire. The sensor’s light emitting diode (Fig.3) can through the clear plastic, allowing sensor activation to \se confinned in the field. The sensors are installed along the ridge ofthe b r o w and are protected by small plastic bags. The network line wire also runs along the ridge of the b o w . The sensing element ribbon cable drops down fkom each sensor into the trough ofthe firrow.

111. CONCLUSIONS

[2

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Irrigation Engineering Scientific Publications (IESP), Maifa, Israel, 1984. ISBN 965-222-030-2. Azevedo,C., 1992, “Real-time solution of the inverse furrow advance problem”, PHd thesis, Agricultural and Irrigation Engineering Department, Utah State University, 1992. Walker.W. and J.D.Busman, 1990, “Real-time estimation of h o w infiltration”, Journal of Irrigation and Drainage Engineering, Vol. 116, May 1990. Izadi,B. and D.Hmann, 1987, “Real-time estimation of infiltration parameters for controlling an irrigation”, Summer Meeting of the American Society of Agricultural Engineers, Baltimore, 1987. Latimer, E.A. and D.L.Reddell, 1990, “Components for an advance rate feedback irrigation system (MWIS)”, Transactions of the ASAE, July 1990.

The water sensing method presente here is one of many possible ways in which a computer-based control system

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