Irrigating Greenhouse Crops
Irrigation System Controllers
Basic Control Strategies
Two general types of controllers are used to control irrigation systems: Open control loop systems, and closed control loop systems. The difference between these is that closed control loops have feedback from sensors, make decisions and apply decisions to the irrigation system.
Open Control Loop Systems
In greenhouses with no feedback system control (i.e., open loop system), irrigation scheduling is determined according to the grower’s perspective. Open loop control systems use irrigation duration or applied volume for control purposes.
Closed Control Loop Systems
In a closed control loop the operator sets up a general strategy for control. Once the general strategy is defined, the control system takes over and makes detailed decisions of when to apply water and how much water to apply. This type of system requires that feedback be given back to the controller by one or more sensors. Depending on the feedback of the sensors, the irrigation decisions are made and actions are carried out if necessary. It is important to note that in this type of systems the feedback and control of the system is done continuously.
Timers
Regardless of the type of irrigation control system in use, the system will be paired with an irrigation timer, which is necessary to make sure that the irrigation system turns on and off at the correct times. Irrigation timers are simple controllers consisting of clock units capable of activating one or more subunits of the irrigation system at specified times. The two most common types of controller designs are electromechanical and electronic.
Electromechanical Controllers
Electromechanical controllers use an electrically driven clock and mechanical switching (gear arrays) to activate the irrigation stations. These types of controllers are generally very reliable and not too sensitive to the quality of the power available.
Electronic Controllers
Electronic controllers rely on solid state and Integrated circuits to provide the clock/timer, memory, and control functions. These types of systems are more sensitive to power line quality than electromechanical controllers and may be affected by spikes, surges and brownouts.
Computer-Based Control Systems
A computer-based control system consists of a combination of hardware and software that acts as a supervisor with the purpose of managing irrigation and other related practices such as fertigation and maintenance. This is done by the use of a closed control loop.
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