温室水肥一体化营养液调控装备设计与试验

针对当前温室生产中用水效率普遍较低、肥料使用量较高的问题,有必要对水肥一体化营养液调控及灌溉技术进行深入研究,探索适合中国具体情况的,低成本的水肥一体化营养液灌溉模式和相关农业装备。该文设计了一种水肥一体化营养液调控装备,根据营养液制备装置特点和制备过程,建立了营养液制备系统二阶数学模型,并利用增量式PID(proportion integration differentiation,比例-积分-微分)算法和改进Smith预估器对营养液的制备过程进行精确控制;利用EC传感器和p H传感器实时检测营养液的制备情况,并且通过驱动相应的继电器实现营养液的调控。试验结果表明,该水肥一体化营养液调控装备30 s左右可以进入营养液调控的稳定阶段(EC值误差在±0.1 m S/cm),系统达到规定的EC值的响应时间不超过60 s,使用该装备能够快速、精确的实现营养液的调控。

Design and test of regulation and control equipment for nutrient solution of water and fertilizer integration in greenhouse

Abstract: Excessive amounts of fertilizers and irrigation water are used in agriculture, which will lead to some negative effects. Not only the resource utilization rate is reduced, but also the liquid nitrate content exceeds because of drainage, and the underground water source is polluted finally. Integration technology of water and fertilizers can improve the resource utilization, while the pollution of the surrounding natural environment can be also reduced. Automatic control and regulation of irrigation water and fertilizers integration have been implemented in the greenhouse production in Europe and North America, the resulting result is reasonable fertilizers and water application, and the control accuracy is higher. However, excessive irrigation and use of fertilizers still exist in China's greenhouse production. The result of this production mode is that the proportion of nutrient inputs is unreasonable and the nutrient is in serious loss due to excessive irrigation. It is necessary to conduct the thorough research on the integration technology of water and fertilizers to develop the regulation technology and equipment of water and fertilizers integration suitable for Chinese actual situation and with low cost. Preparation of nutrient solution is a process that fully mixes fertilizers and water and makes the nutrient solution reach the preseted value. There are 2 different modes in the mixed tank, including advection and ideal mixing, where all kinds of liquids are mixed, and the actual system can be considered as the combination of these 2 modes. Moreover, considering the time delay caused by the flow of liquid in the pipers, the process of the preparation of the nutrient solution can be simulated by using a first-order lag transfer function model. However, in order to achieve higher control accuracy the mixing tank should have sufficient space. Such nutrient solution's electric conductivity (EC) value will not change dramatically when injected into the liquid fertilizers, and therefore, the mixing tank should have a larger space. But it will bring a lot of other problems, for example, EC values' adjustment speed and flexibility will become worse. To resolve this conflict, a Venturi device is added to the mixing tank in this paper, so a virtual volume is added to the mixing tank, and the whole nutrient solution preparation system is converted to a second-order lag system. According to the theoretical analysis of the nutrient solution preparation system, a regulation and control equipment of water and fertilizers integration nutrient solution is designed in this paper. Besides the nutrient solution preparation function, the equipment also has irrigation control and environmental control functions, and the system structure is composed of computer, master control module, wireless transmission module, EC and pH measurement module and driver control module. The wireless transmission module includes 433 MHz wireless transmission, third generation (3G) wireless transmission and short message service (SMS) alarm; the drive control module is divided into nutrient solution ratio drive, environmental control drive and irrigation control drive; and EC and pH value of the nutrient solution are detected by the EC and pH measurement module. The control of nutrient solution is implemented in the following ways: first, the EC and pH values of the nutrient solution in mixing tank are detected online in realtime, and then the detection results are sent to the master control module through the RS-485 bus. The different value between computer's set value and current detection value is calculated by the master control module, and the nutrient solution ratio drive module is controlled by master control module according to the calculated results, so the injection ratios of fertilizer mother liquid are controlled by solenoid opening and closing. The nutrient solution adjustment process is executed finally. The regulation algorithm of nutrient solution is realized by the incremental PID (proportion, integration, differentiation) algorithm and a gain adaptive Smith predictor. The experimental results show that the system enters a stable stage in about 30 s and is infinitely close to the set value of the 2.0 mS/cm EC. This shows that the nutrient solution regulation equipment is not only faster, but also more accurate for the system response.