Design of plant growth dynamic supplementary lighting control system based on LED light source

In each growth period of tomatoes, weak light environment will have a great impact on the growth of tomatoes, so it is necessary to supplement light in each growth period of tomatoes. As for how to supplement more scientifically and how much is the most appropriate, scholars have been studying the problem.

The dynamic supplement light system based on fluorescence detection uses MINI-PAM to monitor the actual photosynthetic efficiency of tomatoes and LED combination light source supplement light to guide greenhouse tomato planting, quantitatively analyze fluorescence parameters and solve the problem of scientific and appropriate supplement light. The system consists of the host computer general control center, the greenhouse information monitoring system of the STM32 microcontroller independently developed and designed, a programmable constant current source that provides stable current, a high-power red and blue LED combination light source and a MINI-PAM portable modulated fluorescence detector. The STM32 microcontroller greenhouse environmental factor monitoring system and the portable modulated fluorescence detector measure and record environmental information and fluorescence parameter information respectively, and analyze and calculate the brightness of the LED combination light source and the optimal red and blue light ratio from this information. Finally, the host computer controls the programmable constant current source to drive the LED combination light source, and finally achieves the purpose of dynamic supplement light.

0 Introduction

Photosynthesis is the material basis of all material metabolism and energy in the biological world. In the original reaction of photosynthesis, plants release the absorbed light energy in a large fluorescence form. Therefore, chlorophyll fluorescence is closely related to photosynthesis. In the past 30 years, the research on LED artificial light sources in facility horticulture, plant facility cultivation, and space agriculture has attracted widespread attention worldwide. The results of studies on plants such as strawberries and orchids with different light qualities show that red light has an important influence on plant morphology and regulating plant height, and has a promoting effect on the growth process of leaves. The leaves, stems and petioles of plants will elongate, but the chlorophyll content is low, and the growth index and dry matter accumulation will also decrease. The regulatory effect of blue light on photosynthesis is mainly concentrated on the opening of stomata, the differentiation of chloroplasts, and the regulation of the activity of photosynthetic enzymes. POSPA and other studies have found that under the action of monochromatic blue light LEDs, the chlorophyll content is the highest and the number of leaf stomata is the largest. At present, in the study of the composite spectrum on the photosynthetic physiological characteristics of plants, the combination of red and blue LEDs can have a positive impact on the growth and development of plants. Due to the monochromatic light processing, the system uses the combination of red and blue light as an auxiliary light source for tomato growth. Through the software, the host computer, the LED array controlled by the programmable power supply and the MINIPAM modulated fluorescence instrument are closed-loop controlled to analyze under what lighting conditions are most conducive to the growth of tomatoes.

1. The overall structure of the system

The dynamic fill light control system uses MINI-PAM to monitor the actual photosynthetic efficiency of tomatoes and LED combination light source fill light to guide the greenhouse tomato fill light production. The system consists of five modules. The host computer decision module is an industrial-grade server, a greenhouse environmental factor monitoring system of the STM32 microcontroller independently developed and designed, a JBP-7510 programmable control constant current source that provides a stable current for the fill light source, a high-power red and blue LED combination light source and a MINI-PAM modulated fluorescence detector. Taking the fluorescence parameters as the target, the output current of the programmable constant current source is continuously adjusted in the greenhouse environment to change the illumination intensity of the LED combination light source, thereby changing the fluorescence parameter ΦPSⅡ of the tomato to the expected value, so as to achieve the effect of artificial regulation of tomato photosynthesis. The basic schematic diagram of the system is shown in Figure 1.


Figure 1 Overall principle structure diagram of the system

2. Hardware design of the monitoring system

2.1 Power supply module of the LED combination light source The JBP-7510 programmable control constant current power supply is a current power supply. Under the rated working state, it can output a stable expected current. In order to accurately control the driving current of the LED combination light source and achieve the purpose of controlling the illumination conditions of the light source, it is very important to choose a current source with high stability. The programmable constant current source has a constant output current, so the current is not affected by the load. The size of the driving current value directly controls the brightness of the LED lamp beads, so the current source can accurately control the light intensity of the LED combination light source. The maximum voltage output of the programmable constant current source is 75V, the voltage accuracy is 0.05% + 37.5mV, the maximum current output is 10A, and the current accuracy is 0.1% + 10mA. According to the fluorescence parameters, the host computer calculates the appropriate fill light current value, sends instructions to the power supply through serial communication, and finally the programmable constant current source provides a stable output current to drive the LED combination light source.

The JBP-7510 programmable control constant current power supply tests the control current and actual current when driving the LED fill light source at room temperature of 25 degrees Celsius. The 12 data in the test results are shown in Table 1. From the analysis of the data results, it is found that the current accuracy is better than ± 0.2%, so the current accuracy of the JBP-7510 programmable control constant current power supply can meet the output accuracy of the system fill light current.



2.2 LED array design

Using the characteristics of LED light concentration and fixed direction, reasonable planning and optimization must be carried out to achieve uniformity of red and blue LEDs in the irradiation area. The supplementary light of tomatoes mainly uses red and blue LEDs. The designed combined light source must ensure the uniformity of red and blue light in the light received by tomatoes. The two design schemes have a red and blue LED number ratio of 1:1 and 4:1. Array 1: The red and blue LED number ratio is 1:1, and the LEDs are evenly arranged. The distance between adjacent LEDs is 10mm. The array is shown in Figure 2 (a). Array 2: The red and blue LED number ratio is 4:1. The red LED and the blue LED form an area, and the blue LED is in the center. The array is shown in Figure 2 (b).


Figure 2 Schematic diagram of LED array

2.3 MINIPAM modulated fluorescence detector

MINIPAM pulse modulated chlorophyll fluorescence instrument is manufactured by WALZ. This system uses it to measure the steady-state fluorescence yield Fs under light adaptation of plants, the maximum fluorescence yield Fm under light adaptation and other related fluorescence parameter values. Two-way communication with the host computer is carried out through the RS-232 interface to realize the closed-loop control of the host computer MINIPAM and LED light array.

3. Software design of dynamic supplementary lighting control system for tomato growth

Dynamic supplementary lighting is realized based on information collection and storage, data analysis, and command issuance by the host computer. The dynamic cycle process is completed by the environmental factor monitoring system, portable modulated fluorescence instrument, and programmable constant current source. The specific process is shown in Figure 3.


Figure 3 System workflow diagram

The environmental factor monitoring module based on the STM32F103VCT6 microcontroller is used to monitor the tomato growth environment in real time, and the environmental factors obtained by the sensor are transmitted to the host computer. When the light in the greenhouse is insufficient, the host computer starts MINIPAM and obtains the return value of the fluorescence parameter (ΦPSⅡ value), calculates the light intensity required for supplementary lighting and the appropriate red and blue LED combination ratio, and issues the LED combination light source working current value command to the JBP-7510 programmable constant current source communicating with COM1, and issues the red and blue LED ratio command to the STM32F103VCT6 microcontroller. The red and blue LED combination light sources adjust their brightness by driving current, and the STM32F103VCT6 microcontroller controls the current of each LED to achieve the control ratio. The MCU continues to monitor the lighting conditions and cyclically compares the dynamic fill light until the fluorescence parameters are maintained at the preset value. MINI-PAM regularly transmits the collected tomato fluorescence parameters through the COM2 port of the host computer and stores them on the host computer. The host computer determines whether the fluorescence parameters are higher or lower than the preset value, and then makes decisions to control the MCU, actuators and programmable control current source to adjust the greenhouse environment in real time to ensure the optimal LED combination light source red and blue light combination ratio and brightness, making the greenhouse environmental factor control model the best tomato growth model.

4. Conclusion

At present, a dynamic supplementary lighting system has been preliminarily constructed, and the system has been used to complete the 24-hour monitoring of fluorescence parameters. According to actual needs, based on LED light sources, according to the effects of monochromatic LED and combined LED light sources on plants, LED combination ratio parameters suitable for different growth stages of plants are designed, and through the design of the best LED light source, the relationship between LED output light intensity and fluorescence parameters is used to input the best matching light source information and light cycle into the computer system in real time. The computer guides the dynamic output based on all the collected fluorescence parameters and environmental factor information to realize the dynamic supplementary lighting system. This system mainly targets the characteristics of low temperature and weak light in northern greenhouses, takes tomatoes as the research object, and supplements the lighting methods under low temperature and weak light, and establishes a tomato growth control model under the best light source, different growth periods, and different environmental parameters (water, temperature).