Solution for exhaust control system applied to laboratory cabinet

introduction

The fume hood controller is a microcomputer controller on the fume hood. The microcomputer is used to perform complex logical calculations and controls to manage the operation of various electromechanical equipment on the fume hood. The sensor is used to obtain various physical parameters of the fume hood for the operator to use. The use of microcomputers can greatly improve the operating reliability of various equipment and simplify the complexity of system equipment. This design uses PID control to stabilize the exhaust wind speed at the face wind speed set by the user. Due to the use of a liquid crystal display, the operation is simple and intuitive. Multiple working modes (standby, forced exhaust, manned/unmanned exhaust) make this device more energy-saving and practical. The advanced wind speed calibration method is used to more accurately detect the actual wind speed, thereby achieving real-time and accurate exhaust control.

1 Working Principle

The toxic and harmful gases generated in the laboratory must be removed in time. The standard for measuring fume hoods is to meet the following requirements: harmful gases cannot overflow and turbulence cannot be generated in the hood. Therefore, the surface wind speed, that is, the average wind speed at the fume hood window, is the main technical parameter for measuring the performance of the fume hood.

The national standard for the wind speed of the fume hood is: 0.4~0.6 m/s. The calculation formula for exhaust volume (unit: m3/h) is:

L=3 600×SVβ (1)

Where: S is the opening area of ​​the operating port (unit: m2); V is the surface wind speed (unit: m/s); β is the safety factor (1.05~1.1).

It can be seen from formula (1) that for a fume hood without a regulating system, when the operating door is fully open, if the face velocity meets the requirement of 0.3-0.5 m/s, when the operating door is half-open or fully closed, the face velocity will exceed the design value, greatly affecting the exhaust and experimental results.

Therefore, how to effectively control the surface wind speed is an effective guarantee for the exhaust of the fume hood. This design adopts the door travel and wind speed control method, which can keep the fume hood surface wind speed at the set value when the door is at different heights.

The actual ventilation volume of the fume hood can be expressed by the following formula:

Q=SV=SWH (2)

Where: S is the opening area of ​​the operation port (unit: m2); V is the face wind speed (unit: m/s); W is the opening width of the fume hood (unit: m); H is the window height of the fume hood (unit: m). The sensor on the cabinet door is used to transmit the door travel signal to the controller. The controller calculates the set air volume signal according to the set face wind speed and formula (2), and then it can be obtained that the valve opening required to obtain the set wind speed under the current cabinet door height. Then the real-time air volume measured by the wind speed sensor is converted into an electrical signal of the corresponding valve opening. An output voltage is obtained through PID control in the ARM to control the valve opening, thereby realizing wind speed control.

2 System Structure

The main function of the fume hood controller circuit is to complete the response to the input of the external environment, such as the input of the manned or unmanned status, the window opening, the feedback input, the manually set input, etc. Through judgment and calculation, a certain voltage is output to control the Venturi valve to form a closed-loop control. The controller transmits the current exhaust volume of the fume hood to the air supply controller through the RS 485 bus, and communicates with the host computer through the ModBus protocol, also using the RS 485 bus form, and selects the RSM 485 low-power chip for level conversion. When implementing monitoring and parameter setting, it can be done in batches, which greatly simplifies the amount of operation. The host computer software can be developed using LabVIEW or VB.

The hardware circuit includes human-computer interaction circuit, such as keyboard, LCD, buzzer; A/D sampling circuit, such as man-or-no man A/D circuit, window opening A/D circuit, feedback input A/D circuit; D/A circuit; wind speed detection circuit and serial communication part. The system structure is shown in Figure 1.

2.1 A/D sampling and D/A output

The A/D and D/A circuits mainly realize functions such as voltage conversion matching and analog signal filtering. The analog signals are window height input Hx: the height of the control cabinet window is collected in real time through a pull-wire rheostat or ultrasonic ranging, and the input signal word changes linearly in the range of 0~10 V to obtain the desired air volume; valve feedback input f: valve opening feedback, reflecting the current real-time air volume. 0~10 V linear change; occupant/unoccupied input Z: detect whether there is anyone in the fan-shaped area in front of the fume hood through the human body sensor, and convert the occupant/unoccupied state into a +12 V/0 V voltage value. The output analog signal is the valve drive signal, and the linearly changing voltage controls the valve opening.

2.2 Human-machine interface

It includes a keyboard, LED indicator and LCD display to realize information input, data display and alarm. The circuit is completed by the PCA9555 unit alone. The PCA9555 is a 24-pin CM OS device that provides an expansion of the 16-bit general parallel input/output port GPIO in I2C-Bus/SMBus applications. And the communication frequency can always reach 400 kHz.

The button uses low-level interrupt triggering mode.

The power supply circuit realizes the input of AC/DC 24V at the same port, and outputs DC ±12V, +5V and +3.3V. The input end adopts a half-bridge rectifier circuit to realize the input of AC or DC power at the same port at the same time, which can adapt to the needs of different environments. After rectification, the voltage is stabilized to +12V through the TL2575HV-12IKV unit circuit. The subsequent voltage conversion circuit is used to generate +5V and +3.3V power supplies.

2.3 ModBus

ModBus serial link protocol is a master-slave protocol. RTU mode has a higher data throughput than ASCII mode at the same baud rate, so RTU mode is used. RTU mode distinguishes characters and message frames by judging the time interval. The idle interval of at least 3.5 character times distinguishes the message frames. At the same time, the entire message must be sent in a continuous character stream, and the interval between 2 characters in the frame is less than 1.5 character times. The RTU message frame is as follows:

The ModBus protocol specifies more than 20 function codes, and this design only uses a few of them. The host computer sends three commands: read holding register (function code 03), write single register (function code 06) and write multiple registers (function code 16) to complete the read and write functions of ARM. The slave computer program is responsible for responding to the host computer command and sending data back to the host.

3 System operation and implementation

The system mainly realizes information collection, communication, parameter setting and information display. According to the national standard of 0.4~0.6 m/s wind speed across the fume hood, the controller calculates the valve control voltage Vout (fx) based on the fume hood window opening Hx, valve feedback Fb and set wind speed Vf to control the wind speed to remain within this range. And the actual air volume L and wind speed V are displayed on the LCD panel. The calculation formula is as follows:

In the formula: k=0.06~0.42; n=0.6~1.4, is the air leakage adjustment; Hmax, Hmin are the highest and lowest positions of the window opening, Zmax, Zmin are their corresponding voltage signals; Hx is the actual value of the window opening; a, b are the parameters of the relationship between valve opening and air volume, which are calibrated on site; Fb is the valve feedback voltage.

4 Conclusion

This device includes 5 parts: micro control unit, actuator unit, sensor input unit, display unit and operation unit, which make the fume hood intelligent and variable air volume control function. The ventilation volume is completely controlled by computer, which is easy to operate and ensures that the effect of exhausting harmful gases is always in the best state. It can not only accurately control the exhaust volume, but also reduce energy consumption, reduce noise, and even work without noise (there are multiple working states).

The emission of laboratory polluted gases affects the health of laboratory personnel and the safety of the working environment. According to the national regulations on wind speed and taking into account the impact of the environment on wind speed in actual use, this system can also be extended to other exhaust fields.