The Design Of Surface Acoustic Wave Sensor Control Circuit Based On PIC MCU

The research of surface acoustic wave(SAW) gas sensor began in seventies of thelast century. The SAW sensor has advantages of small size, low power consumption,high sensitivity, high stability. It does not need an analog-to-digital conversion. It alsohas a short response time and low cost. The SAW gas sensor has a huge potential inambient air detection and military toxic vapor detection. In the United States, chemicalagents detector based on SAW gas sensor array is planing to equip USA army. While inour own country, the study of this sensor is still in its infancy.Based on the SAW gas sensor array, the control circuit for a handhold SAW gasdetector is designed in this thesis. The detector included a variety of components suchas a sensor array, a preconcentrator, two pumps, two valves, a Peltier device and so on,controlled by a peripheral interface controller (PIC) microcontroller unit (MCU). Thefrequency signal processing circuit, the data transfer circuit, the pneumatic systemcontrol circuit, the constant temperature control circuit, the LCD display circuit and thedetector alarm circuit are designed in this thesis. In the control procedures, thefrequency signal processing program is the main procedure. Other procedures such asthe data transfer procedure, the pneumatic system control procedure as well as theLCD display procedure are designed as a subroutines.The performance of the control circuit is tested under the pass-through mode andthe enriching mode. At the pass-through mode, the responses of the SAW devicescoated on different polymer coatings are tested under different vapors, demonstratingthat the frequency signal processing circuit can record the sensor singal correctly. Atthe enriching mode, the influence of the preconcentrator on the SAW array wasinvestigated. Meanwhile, the effect of the air pump on or off on the SAW sensorfrequency change was also studied. A sharp and narrow peak is recorded when thepump is turned on or turned off. When the reference SAW sensor is uncovered, thepeak heights can decrease about50%.As a result, a good balance on the sensitivity and the detection time of the SAW detector can be realized by adjusting the collection time of the preconcentrator.