Design and Research of LED Lighting Control System in Plant Factory

Abstract: The plant factory LED lighting control system involves multiple LED independent control, constant current drive of high power white LED, ECU unit and system control design, as well as technical issues such as monitoring circuit and unit communication. In this paper, high-power LED drive and spectral characteristics, different wavelength LED sub-control lighting and system control, and field communication technology are studied. In the design process, the high-power LED driver circuit brightness adjustment, LED light color adjustment and heat-dissipation detection debugging and verification were completed, and the basic unit of the plant factory LED lighting control system was formed. After the unit's communication interface and priority permissions are set, an ideal on-site controllable lighting system is formed. The results show that the LED light source designed by the system is not only energy-saving and environmentally friendly, but also meets the needs of the plant factory due to the brightness, color temperature and spectrum controllability of the LED.

Key words: plant illumination; LED array; illumination LED driver; CAN bus

The plant factory is an internationally recognized stage of advanced development of facility agriculture, as a symbol of nature and a symbol of plant and human industrial civilization [1]. Plant plants provide system equipment for plants to grow in a suitable environment. It is a multi-disciplinary, high-tech application, a new production method that is almost independent of natural conditions. The factory provides automatic real-time monitoring of environmental factors in the factory through artificial natural conditions to provide temperature, humidity, light, airflow, carbon dioxide concentration and other growth environments, and high-precision adjustment control. The factory provides light source for plants [2] mainly high-pressure sodium lamps and fluorescent lamps. Recently, with the technological breakthrough of high-power white LEDs and the successful application in many high-illumination occasions, LED-based light sources have become a new trend in replacing traditional lighting in agricultural production.

Each planting rack of the plant factory is equipped with one or several light panels. The light source of the light board can be composed of an LED array, wherein the high-brightness and high-power white light LED is mainly used, and the red, blue and ultraviolet LEDs are inserted at a certain distance on the light board, and are evenly distributed in different proportions. Since sunlight is full-band for plant illumination, red, blue, ultraviolet, infrared, etc. are included, so the spectrum of the panel should be close to natural light. Design a controllable light source consisting of white LED as the main light source and other light color LEDs and synthesize the spectrum according to the growth characteristics of the plant. This spectral comprehensive characteristic is more conducive to the growth of plants.

1 LED lighting system

The LED lighting system is composed of a light source composed of an LED array, an LED driving circuit, a light board management electronic control unit [3] ECU, a temperature sensor, a brightness and light color sensor, and a communication interface. The system structure is shown in FIG. The LEDs of various colors are arrayed on the lamp board at a certain interval, and the LEDs of different colors are equipped with corresponding driving circuits, and the LED driving circuit is managed by the ECU of the optical board. The ECU on the back of the light board controls the luminous intensity of different light color LEDs according to the light-emitting characteristics of the LED light board collected by the light color sensor and the brightness sensor to obtain a comprehensive spectrum of approximate sunlight. The electronic control unit ECU can independently exchange and manage the LED array of the light board, and can exchange data between the light board and the light board, and between the light board and the central control system of the plant factory through the local area network, and accept the central control system differently. Plant control instructions for spectral synthesis in different growth stages.

2 high power white LEDs and arrays

The LED light board is the key equipment of the plant factory. The main light source on the light board uses the LUXEON LAFL C4S high power white LED produced by PH ILIPS (as shown in Figure 3). The main technical parameters are as follows:

Working voltage: 12~16V; Working current: 1.0~ 1.2 A; Color temperature CCT: 5 200 ~ 6 000 K; Color rendering CR I: 70; Luminous flux: 1 000~ 1 200 lumens; Luminous angle: 60!

The spectral characteristics are shown in Figure 2.

The current and voltage parameters of high-power white LEDs have typical PN junction volt-ampere characteristics, forward voltage drop (V) and forward current (I) and their relationship with chip base temperature as shown in Figure 3.

　　

It can be seen from the VI curve in Fig. 3 that the forward voltage drop (V) decreases as the temperature of the PN junction increases, and at the same time, the forward current (I) becomes larger as the junction temperature increases. Small changes in V cause large I changes. Since the heat dissipation of high-power LEDs is the key to the application technology, when the current is controlled, the change in forward voltage drop (V) no longer affects the current change and causes a vicious cycle of temperature. Therefore, high-power LEDs must be driven by a constant current source. Controlling the LED drive current avoids the effects of temperature rise [4] on the forward voltage drop (V). As can be seen from the LUXEON specification, when the constant current of the LAFLC4S is driven to 1000 mA, the luminous flux can reach 1200 lumens. The arrangement of the LED arrays may be strip or circular depending on the occasion of the illumination. The number of white LEDs on the array can be calculated according to the ratio of the total required luminous flux to the luminous flux of a single LED according to the following formula:

Where: NLED is the number of LEDs; is the target luminous flux; is optical efficiency, 90%; is thermal efficiency, 85%; is the minimum luminous flux (lm) of LED.

The high-power white light distributed on the circuit board and the LEDs of other colors and the lamp housing maintain good heat dissipation. The lamp housing is usually made of aluminum with good heat dissipation performance, and a cooling fan supplemented with ventilation is installed in the lamp housing. A temperature sensor and a light sensor are installed on the lamp board to monitor the operating temperature and illumination parameters of the LED.

3 PWM controllable LED constant current drive

Linear's LT3755 family of ICs is designed to drive constant current drive for high power, high current LEDs. They drive an external N-channel power MOSFET from a 7. V supply inside the IC to achieve a high current of 1 500mA to drive the LAFLC4S high power white LED. The LT3755 current control mode provides constant current stabilization over a wide input voltage range of 45 to 40 V and an output voltage from 29 to 70 V. The FB pin acts as an input to the LED current sense and protection functions and also enables the converter to function as a constant voltage source. Adjusting the resistance of the RT pin results in a frequency variation range of 100 kH z to 1 MH z. Changing the frequency setting optimizes efficiency and further improves performance. The current sense circuit connected in series with the upper end of the LED is the most flexible solution for driving LEDs, suitable for boost, buck mode or buck-boost mode configuration. The PWM input port provides an LED dimming ratio of up to 3000∃1. Figure 4 shows the drive circuit of the white LED LAFLC4S.

Compared with the large current of LUXEON LAFLC4S white LED, the LED drive current of other wavelengths on the lamp board is only between 100~300 mA. AM IS's automotive-grade LED driver series is adopted, and the AM IS 39100 integrated circuit is very high. It is resistant to high temperatures and high reliability and can provide 8 LED output drives separately. Each output of the driver can reach 350mA, and a higher drive current can be obtained if the external MOSFET of the port is used. The AM IS39100's external circuitry is simple enough to require only one external capacitor. The chip features overtemperature protection, overcurrent protection, short circuit protection and low voltage protection. When the chip's internal circuit detects a potential hazard, the driver is automatically turned off and the diagnostic status of the fault is read through the serial port.

The AM IS 39100 can drive up to 8 LEDs simultaneously (as shown in Figure 5). The driving current of each branch is programmed and controlled by the ECU control unit. The protection function and sub-control features of this IC are suitable for the controlled illumination of other wavelength LEDs on the panel. The AM IS39100 simultaneously controls the combined driving of eight different wavelength LEDs and the LT3755 high power white LED. It can obtain a comprehensive spectrum of approximate sunlight on the lamp board to meet the light color characteristics of plant growth.

4 LED light board control unit

The light board control unit is responsible for independent lighting of different wavelength LEDs, adjustable control of luminous flux and spectral color quality, temperature detection and system protection. According to the dimming instructions of the central management system of the factory, the integrated spectrum of the different characteristics of the light board can be managed. The controllable LED light source of the plant factory not only provides the illumination required by the plant but also utilizes the specific wavelength of the LED monochromatic light to the specific The attraction of pests is targeted to attract insects and insects. The circuit diagram of the LED light board control unit is shown in Figure 6.

Since the LEDs are densely packed with LEDs of various wavelengths, photometric sensors and their driving chips for each LED, each chip inputs different PWM signals and brightness and temperature management, so a high-performance multi-port ECU control is required. chip. The ECU control unit not only manages the brightness control and light color synthesis of the LEDs of various wavelengths of the light board, but also protects the working state and external interference of the light board according to the working characteristics of the LED driving circuit, and also synthesizes the spectrum and brightness of the light board. The temperature is detected. Therefore, the processing power, reliability and stability of the chip are very important. The system ECU control unit was designed with Infineon's XC164 CS series of 16-bit single-chip microprocessors. This chip not only has good high temperature resistance and stability, but also has a built-in CAN bus port for field control communication.

The input port of the XC164 monitors the temperature of the high-power LED base substrate. This observation point is close to the junction temperature of the LED. It is the key to protect the LED from thermal damage. At the same time, it detects the temperature cycle of each LED heat sink and manages the thermal balance of the lamp. Protection control. The input port of the XC164 also processes the monitoring data of the illuminance sensor and the spectrum sensor probe to calculate the light color synthesis effect and compare it with the factory central controller light distribution command for real-time control. The output ports of the XC164 respectively output different PWM signals to control the drive current of the AM IS 39100 to achieve brightness control of LEDs of different wavelengths and drive of the LT3755 white high power LUXEON LAFLC4S. The lamp control unit also implements over-voltage protection, over-current protection, reverse protection, short-circuit protection, surge protection, system diagnosis, etc., while controlling the brightness of the light source.

The key to safe and reliable operation of the panel is the heat dissipation of the LED, which is related to important indicators such as LED lifetime, light decay, and color shift. The temperature monitoring sensor monitors the multi-point temperature on the high-power LED base substrate and the heat sink respectively. When the temperature of the base substrate rises to the XC164 preset warning value, the system reduces the driving current when the LED-driven PWM signal is adjusted, so that the high-power LED is lowered. The power consumption reaches the cooling effect. When the LED light board is thermally managed, when the temperature of the heat sink reaches the critical temperature, the system starts the cooling fan to force the convection of the heat source of the light board. The ECU can determine the fan position and wind speed of the forced air cooling area according to the location and temperature value of the detection point. Adjust the PWM drive control signal as necessary to reduce the LED power consumption. When the temperature is too high, the monitoring point is abnormal or the LED light board is faulty, the light board control unit issues the fault information to the surrounding light board through the on-site local area network and provides real-time data to the factory central control system and issues an alarm signal.

Figure 6 circuit board control unit circuit diagram

5 LED lighting board management

Plant factories often change cultivated plants, and different spectral ratios and illumination control procedures are used depending on the plants in the cultivation area. The cultivation area also often stacks the cultivation racks and increases or decreases the light boards depending on the plant species. Since the illumination in the same plant cultivation area is completed by several or dozens of light panels, it is required to achieve uniform illumination, mutual unity, and regional management of the light panels. And realize on-site network management between the light board and other monitoring equipment, between the light board and the light board, and between the light board and the central control system. Therefore, it is necessary to construct a flexible, real-time, reliable communication method and a network structure suitable for plug-and-play flexible and changeable.

The CAN (Contro ller Area N etwo rk ) controller area network belongs to the field bus field. It is a serial communication network that effectively supports distributed control or real-time control. Originally developed as a serial data communication protocol for the data exchange between numerous control and test instruments in modern automobiles, the CAN bus data communication has outstanding reliability and real-time performance compared with the general communication bus. And flexibility. Therefore, it is gradually being applied in other fields. After the plant factory lighting adopts the LED light source system, the plant lighting control technology is gradually becoming intelligent. The drive and protection of the light source, the monitoring of the spectrum and brightness are handled by the light panel control unit and the light panel control unit is set to a lower network level. The monitoring equipment of the environmental management factors such as the area management equipment and the light board management equipment of the cultivation area and the temperature, humidity, CO2 concentration, nutrients, water quality, and light irradiation amount of the cultivation area are set to a higher level, and the communication priority is enjoyed. Any node device on the network, including each light board, can actively send information to other nodes on the network at any time, regardless of master-slave, flexible communication mode, and no need for site address and other information. When multiple nodes send information to the bus at the same time, the lower priority node will actively exit the transmission, and the highest priority node can continue to transmit data without affection. The communication distance between devices can be up to 10 km (rate 5 kb% s- 1 ); the communication rate is up to 1 Mb% s- 1 (the communication distance is up to 40 m at this time). The communication medium can be a low cost twisted pair, coaxial cable or optical fiber. Since the network transmission adopts a short frame structure, the transmission time is short, the probability of interference is low, and the interference resistance is good.

The system CAN interface uses Infineon's TLE6250 dedicated CAN transceiver chip. They are independent CAN bus network control chips that can be combined with various MCUs to form a product that supports the CAN bus. In Figure 7, two CAN transceiver interface chips can be connected to the high-speed network and low-speed network of the plant factory LED lighting system.

6 Conclusion

Plant factory LED lighting control system design involves multi-port separately controlled LED driving, spectral synthesis, light source thermal management, system control and communication and many other technical issues. After designing the spectral characteristics of different wavelength LEDs, the multi-circuit design of the PWM circuit, the design of the ECU unit and the monitoring circuit, the light source panel is formed. A separate lighting unit for the plant factory LED lighting system is provided on the light panel with corresponding sensors and a control lighting program. The lighting unit realizes the functions of adjustable brightness, adjustable light color, heat dissipation protection, and communication between management units of the high-power LED driving circuit. The research results show that the plant factory uses LED as the artificial light source, which is not only energy-saving and environmentally friendly, but also makes the LED lighting control system more suitable for the plant factory due to the brightness, color temperature, spectral controllability and independent lighting, system control, flexibility and convenience of the LED. Demand. When these independent lighting unit communication interfaces are given different priority rights, flexibly built and grouped into the network architecture of the factory, under the unified management of the central control system, the ideal controllable lighting system of the plant factory is formed. Synthetic light is more conducive to plant growth.

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