Notes

Greenhouse controller and Electronic Leaf Mist Control
The greenhouse environment must be accurately controlled to produce optimal conditions for crop production. In addition to controlling the humidity and temperature, environmental control systems can manage shade systems and supplemental lighting, control carbon dioxide concentration, schedule and sequence irrigation valves, and control the pH, electrical conductivity (EC), and temperature of the irrigation water to name a few parameters. There are essentially four types of environmental control systems available: (1) thermostats and timers, (2) analog step controllers, (3) computer zone controllers, and (4) integrated computer controls.

Thermostats and timers are simple and low-cost devices that provide limited control. Thermostats allow manual adjustment of on/off settings (Figure 7.4). Timers are used as on/off switches. A typical greenhouse zone may require 3 or more individual thermostats to control heating and cooling functions, plus timers for irrigation and lighting control. Additional relays are often necessary to interconnect fans and louvers and other devices that must work together. Apart from being inexpensive and simple to install, there is a little if any benefit. They aren't very accurate—plus or minus 2- or 3-degrees F (1–1.5°C) is the typical accuracy out of the box, and they get worse with age. They usually cannot be calibrated to get the accurate temperature. They only allow one temperature, period, so you are not able to have different day and night temperature.


The name step controller has an origin from the ability to step the control in multiple stages (Figure 7.5). Stage controllers use a solid-state integrated circuit to monitor environmental data in the greenhouse and create output signals that activate equipment based on a set of internal programmed instructions. The microprocessor is a simple, low-cost device that is reliable, accurate, and works well in the greenhouse environment. A stage controller combines the functions of several thermostats into one unit, eliminating many of their drawbacks. Because of the inherent flexibility of staged controllers, these controllers tend to be very flexible and provide many features at a relatively low cost compared to electromechanical controllers. Compared to electromechanical controllers, they provide better control of the greenhouse environment, save money on energy, and provide some conveniences to the grower. In addition to being able to give a stepped response to a single sensor such as a temperature sensor, analog controllers can receive signals from two or more sensors, such as temperature and humidity.

Computer zone controllers bring the benefits of computerization to the stage controller concept, providing greater control flexibility and programmability, and the ability to provide improved accuracy and better equipment coordination. They are generally designed to provide control and coordination of temperature and humidity in a single zone, although some companies offer units that are expandable to 2 or 3 zones. Instead of stages, they are described based on outputs. An output can operate a piece of equipment or a group of that equipment. For Electronic Leaf Mist Control , an output might be controlling one exhaust fan or multiple exhaust fans. Equipment that modulates (can operate at various speeds or levels) requires two outputs, one to send the open signal the other to close. Procom Greenhouse Controller allows equipment to open proportionally.




Integrated computer controls (ICC) combine the capability of several stage or computer zone controllers and various other individual control devices such as timers, into a single, integrated computer system. These systems often have no user interface in the greenhouse; instead, they are run completely from a PC. They might have easy remote accessibility through smart phones, iPads, laptops, or other PCs via the Internet, or a local area network (LAN. Integrated computer controls can provide coordinated control of virtually all greenhouse environment functions. Not only can temperature, ventilation, and humidity be controlled, but many other factors, such as propagating bed temperatures, application of liquid fertilizers through the irrigation system, daylength lighting, light-intensity regulation with mechanically operated shade cloth (and thermal sheets or curtains), operation of a mist or fog system, and carbon dioxide enrichment—all can be varied for different times of the day and night.





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