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Vol. 188, Issue 5, May 2015, pp. 61-68




A Method for Precision Closed-loop Irrigation Using a Modified PID Control Algorithm

M. S. Goodchild, K. D. Kühn, M. D. Jenkins, K. J. Burek & A. J. Dutton

Delta-T Devices Ltd., 130 Low Road, Burwell, Cambridge, CB25 0EJ, UK
Tel.: +44 1638 742922, fax: +44 1638 743155

E-mail: martin.goodchild@delta-t.co.uk


Received: 16 March 2015 /Accepted: 30 April 2015 /Published: 29 May 2015

Digital Sensors and Sensor Sysstems


Abstract: The benefits of closed-loop irrigation control have been demonstrated in grower trials which show the potential for improved crop yields and resource usage. Managing water use by controlling irrigation in response to soil moisture changes to meet crop water demands is a popular approach but requires knowledge of closed-loop control practice. In theory, to obtain precise closed- loop control of a system it is necessary to characterise every component in the control loop to derive the appropriate controller parameters, i.e. proportional, integral & derivative (PID) parameters in a classic PID controller. In practice this is often difficult to achieve. Empirical methods are employed to estimate the PID parameters by observing how the system performs under open-loop conditions. In this paper we present a modified PID controller, with a constrained integral function, that delivers excellent regulation of soil moisture by supplying the appropriate amount of water to meet the needs of the plant during the diurnal cycle. Furthermore, the modified PID controller responds quickly to changes in environmental conditions, including rainfall events which can result in: controller windup, under- watering and plant stress conditions. The experimental work successfully demonstrates the functionality of a constrained integral PID controller that delivers robust and precise irrigation control. Coir substrate strawberry growing trial data is also presented illustrating soil moisture control and the ability to match water deliver to solar radiation.


Keywords: Control theory, Precision irrigation, Soil moisture, Closed-loop PID control, Data logging.


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