Greenhouse Computer Control By Peter Ling Assistant Professor and Extension Specialist Food, Agricultural, and Biological Engineering Department The Ohio State University Computers are everywhere nowadays, from office computers to home appliances to cars. Recently I took my car to a garage because the engine light was on, and guess what, the first thing the mechanic did was to hook up their diagnostic computer to check the computer embedded in my car. As a grower, have you wondered what is in a greenhouse computer and how to take advantage of its full potential? There are three basic components in a greenhouse computer climate control system: sensors, a computer, and actuators. Sensors are used to collect information that is important for plant growth. Most commonly collect information includes temperature and relative humidity. Other environmental information such as CO2, light, vapor pressure deficit, soil tension is also used for more sophisticated plant growth regulation. Greenhouse control strategies are programmed into computer to accept set points from growers and make decisions to heat, cool, shade, or water, based on information collected by sensors. By controlling actuators such as heater/broiler, roof vent/exhaust fan, shade cloth, or irrigation/misting, one will be able to provide favorable environments for plant growth automatically. To make sure a computer controlled system works, all three components will have to work individually, and together as a system. For sensors to work properly, it is best to make sure 1) sensors are placed at a location where local environment is most important to plant growth, 2) sensors are measuring authentic environmental conditions, and 3) sensors are calibrated. Sensors are best placed right next to where the plants are, shielded from direct sun light, and in a location that is most representative to the whole growing area. An aspirated sensor can be placed anywhere in an area that has uniform climate. In the case of large climate variations from one end of the growing zone to the other, good air mixing by circulating air, is essential to achieving climate uniformity. Control strategies can be simple, single events or sophisticated operations that require well orchestrated multiple events. Turn on a heater when temperature is below set point is a simple, single event. Enrichment of carbon dioxide needs a bit more coordination to be efficient. For example, CO2 injection should be stopped when ventilation is called for cooling. There is no reason to add CO2 if it will be vent out without been used by plants. To further improve plant growth efficiency, light level, CO2 level, and nutrient concentration could all be controlled using supplemental lighting, CO2 enrichment, and fertigation, respectively. Coordination of individual control events is critical for all of them to work together effectively and efficiently. Maintenance is just as important for greenhouse computers as for cars. Sensor calibration for a greenhouse computer system is similar to tune-up for a car. Finely tuned system performs at a higher level and is more trustworthy. On the contrary, if sensor readings are not reliable, performance of the computer control system is predictably unacceptable. To be able to trust your greenhouse computer control system, one must have confidence in the accuracy and reliability of sensor readings. It is a good idea to calibrate your temperature and light sensors annually, and relative humidity sensors quarterly. It is also a good idea to install easy-to-read dial thermometers and hygrometers next to computer sensors so one can make quick checks that computer sensor readings are reasonable. Adopting a computer control system for automated greenhouse control is like learning to drive a car. The more you know about the potentials and limitations of the system, the more you can benefit from it. Greenhouse control computer system is expensive, yet, it is a proven technology that has been adopted to reduce labor expenses, to enable more precise control of plant growth and development. To justify the financial investment and be able to get the most out of a system, a maintenance schedule of checking sensors, actuators (e.g. fans, cooling pads) is warranted.