Center-pivot automatization for agrochemical use

Computers and Electronics in Agriculture 49 (2005) 419–430

Center-pivot automatization for agrochemical use

J. Palacına,∗, J.A. Salsea, X. Cluaa, J. Arnob, R. Blancob, C. Zanuyc

a Department of Informatics, Universitat de Lleida, Jaume II, 69, 25001 Lleida, Spainb Department of Agro-forestry Engineering, Universitat de Lleida, Rovira Roure, 191, 25198 Lleida, Spain

c Companıa de Agroquımicos, S.A. N-240 Km 110, Almacelles, Lleida, Spain

Received 16 January 2005; received in revised form 1 August 2005; accepted 2 August 2005

Abstract

This paper presents an agrochemical application system that could be incorporated into existingirrigating center-pivots. The system consisted of a hydraulic circuit with emitters, where the dosewas individually controlled with variable valve timing. This paper describes the electronic controlsystem, designed to be modular and distributed along the pivot. The main problem solved was thedetermination of the emitter’s absolute displacement to generate their adequate timing control. Thepaper ends with the test of a low-cost implementation of the complete system under real center-pivotoperation.© 2005 Elsevier B.V. All rights reserved.

Keywords: Agriculture; Control applications; Center-pivot; Variable valve timing control

1. Introduction

Center-pivot irrigation machines are among the most popular systems for irrigatinggeneral field crops and are widely used on sprinkler-irrigated land. They facilitate efficientirrigation (King et al., 1999) on many areas where surface or conventional sprinkler irrigationmethods are not adaptable.

The center-pivot irrigation system (Fig. 1) rotates around a fixed pivot point. The lateralsare supported above the crop by a series of towers, usually supported on two rubber-tired

∗ Corresponding author.E-mail address: [email protected] (J. Palacın).

URL: http://robotica.udl.es.

0168-1699/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.compag.2005.08.006

420 J. Palacın et al. / Computers and Electronics in Agriculture 49 (2005) 419–430

Fig. 1. Image of a center-pivot with mounted agrochemical application system prototype.

wheels powered by low-speed electric or hydraulic motors. Typical span length is 30–50 m,with a total lateral length of 60–800 m.

Most center-pivot irrigation systems currently being used are designed to apply a con-stant rate of water over the entire irrigated area. The turn displacement of center-pivotsrequires that the precipitation rate must be adjusted throughout the lateral, increasing asthe distance from the fixed pivot point. This can be accomplished using several sprinkler-spacing configurations (Allen et al., 2000), or varying sprinkler spacing, sprinkler flow orboth. Center-pivots have a good uniformity for irrigation, with a Christiansen uniformitycoefficient of 70–88%.

Center-pivots are also currently being used for chemigation (Sumner et al., 1997), i.e.,the application of agrochemicals mixed with the water during irrigation. Although chemi-gation practice has become quite widespread, it has some important disadvantages. Themore important disadvantages are the high application volumes because you irrigate whenyou chemigate (Solanelles et al., 2002). Large droplet size gives poor agrochemical cropcoverage because sprinklers are designed to reduce drift and increase irrigated area, but notfor agrochemical spraying (Sumner et al., 2000).

In this paper, we introduce a precision agrochemical application system that could beadded to almost any kind of running center-pivot irrigation system (Justribo et al., 2002).The application system is based in a variable-rate technology, depending on the emitter’sradial absolute position and speed to maintain a uniform application dose along the coveredarea. The application system is truly independent and can apply the agrochemical with noirrigation, just using the center-pivot to transport the mounted agrochemical applicationsystem. This paper is centered in the control methodology and the electronic implementa-tion, while the hydraulic design will be described elsewhere. However, the paper concludeswith some functional tests of a low-cost implementation of the complete design, where theresults are influenced by the hydraulic system used.

J. Palacın et al. / Computers and Electronics in Agriculture 49 (2005) 419–430 421

Fig. 2. Usual application rate depending on the absolute speed for different typical emitters.

2. Problem definition

The circular shape of the center-pivot-irrigated area with different linear speeds alongthe lateral displacement increases the difficulty of the hydraulic design to obtain a constantrate of water over the entire irrigated area. There are many hydraulic possibilities to obtainreasonable solutions to this irrigation problem. However, from the agrochemical point ofview, the problem becomes more complex because the sprinkler dose at a nominal pressurecombined with the center-pivot linear speed produces an application rate which is suitablefor irrigation but normally too high for agrochemical application (Fig. 2). This problempersists even if specific agrochemical emitters are used, which means that a variable-ratecontrol technology must be used to obtain the desired agrochemical dose with each emitter’stotal activation time proportional to its absolute linear speed.

Variable-rate operation was based on calculation of the adequate spray activation timeand the distance between consecutive activation. The activation time can be evaluatedusing:

tON = D × EA

EF, (1)

whereD is the selected agrochemical dose (l/m2), EA the area covered in static operationby the emitter selected (m2) at the working pressure,EF the emitter flow (l/s) at the workingpressure andtON is the activation time (s).

Sprinkler activation must be produced when the distance covered by the emitter reachesthe distance between consecutive activations,dAC, a fixed value (in meters) that depends onthe diffusion shape of the emitters used. The distance covered by an emitter was obtained


Fig. 3. Emitter distance estimation obtained from the distance traveled by its adjacent towers.

analytically from the distance covered by its adjacent towers (Fig. 3):

Edi(t) = Tdn(t) + (Tdn+1(t) − Tdn(t))ESi

TSPAN, (2)

whereTSPAN is the distance between two adjacent towers,ESi the relative position of theemitteri in the span,Tdn(t) the evolution of the distance covered by the towern andEdi(t)is the evolution of the distance covered by the emitteri.

The distance covered by each tower was obtained by numerical integration of its speed.However, the pivot movement is quite complex. Each tower was motorized with a three-phase asynchronous motor that runs at a constant displacement speed, while mechanicalangular sensors were used to modulate the motor activation (like a pulse width modulationspeed control) to obtain the adequate rotating mean speed to maintain alignment of thepivot’s spans.

3. Proposed control system

The proposed solution for agrochemical dispensation using the existing pivot structureis summarized inFig. 4. The system was based on a control module in each tower span, acentral dose module and a dedicated network for internal communications.

3.1. Control module

Available center-pivot structures can have from 1 to more than 10 towers. Therefore,the main control module was designed in a modular manner to be distributed along thepivot and placed in each span tower (Fig. 4). Every control module had to operate the inneragrochemical emitters of its span, toward the center of the pivot. Moreover, each emitterhad an electrovalve associated for variable-rate individual control.

Variable-rate control depended on the absolute emitter dose and position. The mainresponsibility of the control module was to activate the emitters according todAC duringtON.An estimate of the distance covered by the emitter can be obtained (Eq.(2)) if the distancecovered by the tower and some structural parameters of the pivot are known. The controlmodule was provided with these structural parameters during an initial configuration stage,whereas the distance covered by the tower can be obtained in two different calculations. Thefirst method used an additional wheel with an electronic encoder for maximum precision


Fig. 4. Modular agrochemical control system.

in the distance measurement,

Tdn(t) = K

∫ t

0DEn(t) × dt, (3)

whereDEn(t) are the pulses (p) generated by the encoder of the towern andK is the distanceratio of the wheel of the encoder (m/p).

The second method detected the motorized tower activation and assumed that the towerwould run at a fixed speed without acceleration or deceleration delays,

Tdn(t) = TS

∫ t

0An(t) × dt (4)

whereTS is speed of the motorized wheels of the towers (m/s) andAn(t) is the motor’sactivation function of the towern, a digital time function with value 1 when the motor ison and 0 when the motor is off. This second method can be considered because the tower’sspeed is very low (usually between 1 and 3 m/min) and has a small variance if slope isrelatively constant in the radial direction of pivot movement. This option is lower cost thana dedicated structure to hold a wheel with an encoder for every tower because the originalmechanical interrupters can be used for sensing tower-motor activation. Nevertheless, thisoption was theoretically less accurate and also required that the expected tower speed beentered into the system during initial configuration.

Moreover, each control module also needed the distance covered by the adjacent innertower to analytically compute the individual emitter’s distance. Therefore, another importanttask of the control module was to communicate with adjacent modules by sending itsdistance covered.


Fig. 5. Control module schema.

The control module consisted of a low-cost microcontroller with two communicationsinterfaces, a sensor interface and a power output driver for variable-rate control of up to 24electrovalves (Fig. 5). The main task of this module was the real-time computation of thedistance covered by each emitter according to the pivot parameters established in the initialconfiguration stage. Simulated floating-point arithmetic was used to improve the accuracyof all computations. Additionally, each control module stored several functional parametersthat could be obtained remotely for statistical purposes.

The number of electrovalves,N, used in each span must be carefully analyzed, becauseof the cost of the cabling between the control unit and each electrovalve. The total cablelength, CLSPAN, can be estimated depending on the span length,TSPAN, between towersand the number of towers,N:

CLSPAN = TSPAN × 1

2× N. (5)

Considering the usual span lengths, a trial number of 11 electrovalves is recommended,although each control module can handle up to 24 electrovalves.

3.2. Central dose module

The central dose module, located in the pivot center, gets the user’s selected applicationdose and sends it to all the pivot control modules. This module consists of a microcontrollerand two communications interfaces (Fig. 6). The module can be used as a bridge between


Fig. 6. Central dose module schema.

an external RS232 serial port and the internal communication network. The module alsohas several inputs to stop agrochemical application in case of wind or other unfavorablespraying conditions detected by means of convenient sensors.

The dose can be selected by an embedded dial, by a small remote infrared digital displaywith a security code, or by serial order. Therefore, a PC can be used to remotely controlthe agrochemical application system and for real-time monitoring. The pivot parametersestablished at initial configuration were used to computetON, depending on the selecteddose, and then this value was sent to all control modules.

Eq.(1) is correct, as long as pressure is constant over the entire application area. However,this condition is not true in long hydraulic circuits, such as were used in this system and apressure loss must be expected. Therefore, the entire hydraulic circuit must be designed tominimize this error source, although this aspect was not covered in this work.

The central dose module can also sense its tower displacement. This option is not usedin a center-pivot irrigation system, because center displacement is always zero. However,this capability is very useful to adapt the proposed system to other irrigating structures thatrun in parallel, i.e., the longitudinal-pivot.

3.3. Communication network

The system modules used a serial network for communication. The network was basedon a RS485 bidirectional two-wire standard working at very low speed (9600 bps) to uselow-cost cable in the implementation.

The central dose module is the network end-node. When the dose is selected, this modulesends the computedtON to the closest control module. Every control module acts as a repeater


to avoid communications problems because of the pivot length. Therefore, when receivingtON, the module stores its value and sends it to the next control module. The last pivotcontrol module receives the dose-time and acknowledges the data responding with an ‘OK’that propagates through the other modules up to the central module. When the central dosemodule receives this acknowledgement, the system is ready to run according to center-pivotrotational movements.

In normal operation, the communications interface oriented to the center-pivot is alwayslistening and the communications interface oriented to the outer part is in write mode. Thisorientation is because every control module has to measure the distance run by its tower andsend it to the outer neighboring tower. That is, the normal message flux is always from theinner modules to the outer modules. This configuration also allows immediate propagationof emergency stop messages originated in the central dose module.

4. Test

The proposed system was implemented and tested on a medium-sized center-pivot irriga-tion system located in Almacelles (Spain) with four spans of 50 and 20 m overhang (Fig. 1).The pivot was equipped with an agrochemical application pipe supplied by means of a pumpthat injects the agrochemical products at a predefined pressure. Mechanical supports wereadded to the chassis of the pivot in order to support the agrochemical polyethylene pipe atan appropriate height of approximately 50 cm above the crop canopy (Fig. 1). A total of44 mist sprinklers (EINTAL Vibrospray Green) were used as emitters and were connectedto the electrovalves for the pulsed spray of agrochemical products. A central dose unit wasinstalled in the pivot center of rotation and a control module was installed in each tower forthe variable-rate control of each span’s electrovalve. The control board was isolated inside acase to protect it from the outdoor conditions and each case has been subjected in the outertower of the span. The design and implementation of the hydraulic circuit are not discussedin this work, although in this first implementation, the hydraulic system was constructedunder the low-cost (non-optimal and worst case) paradigm.

Two parameters are needed for a correct agrochemical application:tON anddAC. Thevalue ofdAC was computed during the initial configuration of the control system dependingon the emitters used and is always the same to guarantee adequate spacing between thespraying spots for optimal overlapping. Therefore, the user only needs to select the doseand then the central dose module computes and sends the value oftON to all control modulesto start the agrochemical application.

The first test was conducted to evaluate the distance estimation for the individual emitters.The distance traveled by the emitters is computed from the speed of the span towers, whichcan be obtained using an encoder (high cost) or by sensing the tower-motor activation (lowcost). In this test, the distance was obtained from the low-cost speed estimation alternative.This distance was evaluated by comparing the distance between consecutive activations ofthe emitters with the theoreticaldAC value.

The development of the test was as follows: (i) 12 of the 44 available pivot emitterswere selected for the measurement; (ii) a targetdAC distance of 74.2 cm was obtainedaccording the emitters used; (iii) the center-pivot was activated for normal operation; (iv)


Fig. 7. Non-cumulative mean error when measuring the distance between consecutive activation of the emitters.

the activations of the selected emitters were marked in the ground; (v) the test concludedafter one complete turn of the center-pivot; (vi) the distances between two consecutiveactivations of the selected electrovalves were measured.Fig. 7 shows the relative meanerror for all distances measured with respect to the target distance in the selected pivotemitters.

The measurement of the realtON in the electrovalves is meaningless because a modernmicroprocessor can generate an activation pulse with a precision of several microseconds.However, the quantity of agrochemical applied during the activation of the electrovalvedepends on the activation time,tON, but also on the pressure. Therefore, a second test wasdeveloped to estimate the influence of the pressure loss as a consequence of the low-costhydraulic circuit used.

The development of the test was as follows: (i) 12 of the 44 available pivot emitters wereselected for the measurement; (ii) a specific dose was selected to inject 40 ml during elec-trovalve activation (tON = 3.233 s); (iii) the center-pivot was activated for normal operation;(iv) the agrochemical injected during the activation of the selected emitters was collected inindividual test tubes; (v) the experiment ended after one complete turn of the center-pivot;(vi) the volumetric quantity in all test tubes was measured.Fig. 8shows the mean relativevolumetric error obtained in the agrochemical application, andFig. 9 shows the pressuremeasured during operation at some radial distances of the hydraulic circuit used.

5. Results

The results of the first experiment show a very small relative error in the estimation of thedistance traveled by the emitters with a total mean error of +1.4%. This total positive meanerror also indicates that the real tower speeds were slower than those initially assigned during


Fig. 8. Mean error for the agrochemical emitted volumetric application.

configuration and suggests that this estimation can be improved by reducing the declaredtower speed by 1.4%. However, this conclusion needs further verification by intensive testson different pivot systems.

The results of the second experiment illustrate the effects of a pressure loss along thepivot. There was a reduction in the volume measured when increasing the relative emitters’position from the center of the pivot.Fig. 9 shows the shape of the pressure loss curvein the hydraulic circuit at some distances from the center of the pivot, e.g., at 160 m, the

Fig. 9. Hydraulic pressure measured during operation along radial pivot distance.


pressure loss was 7% and the volumetric error was 5%, although at some meters before andafter this position, the error was 12 and 10%, respectively. The maximum volumetric errorobtained was worse than expected and was caused by the low-cost hydraulic circuit usedand the density of the agrochemical products used. Despite this maximum error, the meanvolumetric application for all the emitters was 40.17 ml, i.e., a mean error of only 0.4%.The mean volumetric error was so low that in certain applications, a maximum error of 10%in the dose may be tolerable, considering the lower cost of the hydraulic circuit used.

However, uniformity of the volumetric application can be improved with threeapproaches: (i) improvement of the hydraulic system design, especially in the most criticalparts to reduce pressure loss; (ii) estimating hydraulic pressure loss in the pipe and usingthe control modules for compensation by modifying appropriately the activation time,tON;(iii) measuring the real pressure in the hydraulic circuit and using the control modulesto compute the optimum activation time,tON, for maximum uniformity in the volumetricapplication.

6. Conclusions and future work

In this work, a new agrochemical application system was presented. The system consistedof two basic electronic modules and a hydraulic circuit with electrovalves and emitters. Emit-ter activation was controlled individually with a variable valve timing control that dependedon their absolute rotational displacement. The system was designed to be incorporated intoexisting center-pivot irrigation systems, but can also be incorporated into longitudinal-pivots. Initial test results confirmed that the activation status of the motorized tower can beused to estimate the distance covered by an emitter with an acceptable error. Moreover, alow-cost hydraulic implementation was tested, resulting in a maximum error of 10% in thedose applied. This error may be considered acceptable for cost critical applications.

In the near future, several improvements are planned for the system. An internal sub-network to access the emitters of each span will be used to reduce the total cable length andinstallation costs. Onboard accelerometers are planned to improve tower-speed measure-ment while avoiding the use of external sensors. Finally, the parameters of the hydraulic sys-tem will be incorporated into the initial system configuration to estimate pressure loss alongthe pivot and then adapt the variable-rate timing control for a more uniform application.

Acknowledgement

The authors wish to thanks to Antonio Justribo for his enthusiasm and leadership in thepractical implementation and test of the system.

References

Allen, R., Keller, G.J., Martin, D., 2000. Center Pivot Design. The Irrigation Association, Falls Church,Virginia.


Justribo, A., Palacın, J., Zanuy, C., Arno, J., 2002. Device for applying agrochemicals that can be installed on amobile structure. European Patent Application EP1410711A1.

King, B.A., McCann, I.R., Eberlein, C.V., Stark, J.C., 1999. Computer control system for spatially varied water andchemical application studies with continuous-move irrigation systems. Comput. Electron. Agric. 24, 177–194.

Solanelles, F., Zanuy, C., Arno, J., Martınez, E., Gracia, F.J., 2002. Assessment of intermittent control of liquidflow for spray applications using fan nozzles and mist sprinklers, Basilea, Book of Abstracts of the 10th IUPACInternational Congress on the Chemistry of Crop Protection, p. 411.

Sumner, H.R., Garvey, P.M., Heermann, D.F., Chandler, L.D., 1997. Center pivot irrigation attached sprayer. Appl.Eng. Agric. 13 (3), 323–327.

Sumner, H.R., Dowler, C.C., Garvey, P.M., 2000. Application of agrichemicals by chemigation, pivot-attachedsprayer systems, and conventional sprayers. Appl. Eng. Agric. 16 (2), 103–107.

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Center-pivot automatization for agrochemical use