Design Of Digital Air Sampler

With the economic development and improved living standards, indoor air pollution problem has been highlighted, which is considered to be the third largest category of pollution following the "soot pollution","photochemical pollution". What's more, almost 80% ~ 90% of a person's life time is spent indoors, so the government and the public have paid more and more attention on the indoor air pollution problem .In order to detect pollution in time and curb the spread of pollution, we need to monitor the indoor environment real-time and accurately. During environmental monitoring in the air, the sampling is an important part of ensuring the monitoring quality. In the environmental monitoring process, we use the air sampler to collect the measured gas timing or quantitatively, then make the gas to be dissolved in the reagent, and detected the concentration by chemical methods, so as to achieve the purpose of environmental monitoring. We can see that the air sampler play an important role in the whole process.Traditional air sampler are divided into two kinds generally, one uses the internal built-in flow meter to measure gas flow rate and displayed by the float of flow meter, and the sampling flow rate should be regulated through the knob by a man. This kind of air sampler is low-cost, but the operation is more complicated, with more reading errors; another kind of air sampler has internal flow sensor, so it has relatively high precision, but also with raise costs. The focus of this issue, therefore, is the design of a low-cost, easy to operate digital air sampler.During the design process of the air sampler, the most critical technology is the determination and control methods of the sampling flow rate. The design in this paper introduces a new method of measuring gas flow rate, that is, to calculate the gas flow rate by measuring the current and speed of the DC motor that carried in the micro vacuum pumps. At the same time the motor speed regulation circuit that is made up by operation-amplifier is used to control the sampling flow rate. The following introduction contains two aspects—hardware design and software design.In hardware design, the whole apparatus contains the micro-pump,speed detection and amplification processing circuit,A/D acquisition circuit,microcontroller STC12C5410AD,speed regulation circuit,LCD monitor and keyboard. When users press the switch to start sampling and set the flow rate and the time by the button. The speed detection and A/D acquisition circuit measure motor speed and current, then carry on the feedback control through the MCU to the speed of the DC motor that brought in micro-pumps and thereby control the value of the gas flow rate, and display in the LCD. For low-cost and portability considerations, the design chooses most of the chips for low-cost, low-power type, and to simplify as much as possible so as to reduce the size of the circuit.The design uses the micro-pump PM8001 for sampling .In the motor speed detection and amplification processing circuit part, the micro-pump DC motor speed can be measured by getting the pulse of current, voltage ripple frequency and motor speed will proportional change. The measured pulse voltage amplitude will be small and not stable, and it needs to be put into amplification processing circuit, monostable circuit, thus becoming digital signal that can be received by the MCU.The system uses a low-power 8-bit single-chip machine STC12C5410AD as microprocessor, this single-chip brings the internal A/D converter, and it can be used to complete the supply voltage and DC current collection. The hardware connection is therefore very simple, the A/D conversion can be completed as long as the middle of the two voltage-divide resistor is connected to the A/D conversion of the single-chip machine.Motor speed regulation circuit receives the flow rate control signals from the single-chip machine, and then uses the speed control circuit that is made up of DAC0832 and operational amplifier LM324 to adjust the motor speed so as to keep the flow rate of the sampler at the setting value.Human-computer interaction module includes the four-piece LCD and the keyboard circuit. In order to reduce the volume to achieve the purpose of portability, the keyboard of the system only consists of four key components to complete the set and adjust the flow rate and other functions. The users can choose to set quantities or sampling time with buttons after open the sampler, and the actual sampling flow rate can also be adjusted accordance. To reduce the costs, the design take the simple four-piece LCD which is cheap and easily controlled.The software part mainly complete the functions of speed measurement, A/D acquisition, data storage and computing, flow rate regulation, fuzzy control, keyboard and display sections.TO counter of the single-chip machine is used to get the output pulse of monostable circuit in order to get the speed of DC motor. The digital air sampler current value of flow rate is calculated by the motor speed and the A/D acquisition value for two-dimensional linear interpolation through the microprocessor, then the control volume from the fuzzy control algorithm is used to control the sampler speed through the speed control circuit to keep the flow rate of the sampler at the setting value, digital filtering method is also used to remove the effects of interference. To save data and keyboard state words and setting values, the use of single-chip built-in online flash read and write functions is also required, while software also includes the keyboard and display module man-machine dialogue.Compare with the existing products in the market, the air sampler has the advantages of low-cost, low power consumption and easy operation, which makes it get the wide market prospects.