Cantilever Gas Sensor Based On Webcam Imaging

With the development of science and technology,the monitoring of gases and humidity is closely related to people’s life,health,and industrial production.For example,volatile organic compounds(VOCs),as indoor and outdoor air pollutants,are serious health threats.Therefore,the demand for such sensors is increasing.Among these sensors,MEMS cantilever-based gas sensors are developing rapidly because of their high sensitivity,real-time monitoring,small size,and compatibility with semiconductor fabrication techniques.However,the fabrication of this micro-cantilever requires bulky and expensive semiconductor fabrication equipment such as lithography and etching machines,so its high processing threshold is a serious impediment to its scope of application.Therefore,we hereby designed a simple,low-cost,cantilever,whose fabrication does not require large equipment,and used webcam imaging to detect cantilever beam displacements.Those improvements result in significant cost savings and make this type of sensor inexpensive and portable,facilitating the extension of its application to people’s life.The cantilevers are composed of three-layers: a substrate layer of 6 μm copper foil,an adhesive layer(1 μm double-sided adhesive or Kafuter glue),and a functional layer(12 μm polyethylene film or 6 μm polyimide film).When exposed to target gas,the cantilever would bend and inducing the laser spot or marker particle on the free end of the cantilever to move,which would be online monitored by webcam and analyzed by our image processing apps.There are two image processing algorithms to recognize cantilever displacement in this thesis.One is a low-precision centroid algorithm,and the other is a high-precision four-quadrant brightness difference algorithm.This thesis is divided into three main parts as follows:1.Cantilevers composed of copper foil,double-sided adhesive,and polyethylene(PE)film,were prepared.A laser spot as target,and a general webcam was used to detect cantilever curvature.The cantilever-based gas sensor was thus developed for xylene detection.When the target gas molecules diffused into the inner molecular gap of the polyethylene film,polymer swelling happened and induced bending of the cantilever,which would induce the movement of the laser spot.The results show a good linear response between the sensor signal and xylene concentration in the range of 0-30 ppm with a correlation coefficient of 0.992 and a low detection limit of 1.5 ppm.However,the sensor is susceptible to humidity fluctuance.The response increases with increasing humidity,and shows good linearity with a correlation coefficient of 0.997 in the range of 0-3% humidity change.Therefore,experiments were carried out to correct the data for the effect of humidity,and the correction curves also showed a good linear relationship.2.Cantilevers based on copper foil,double-sided adhesive film,and polyimide film were prepared.Reflective particles were used as labels and a general electron microscope composed of a 2X lens and webcam was used for imaging.A humidity sensor was thus developed.The cantilever would bend when water molecules absorb on the polyimide film through hydrogen bond.Results show that the sensor exhibits good linearity between the sensor response and the 0-6% humidity difference,with a correlation coefficient of 0.998 and a low detection limit of 0.1%.The sensor also has good stability and selectivity,maintaining good detection characteristics in different environments.However,the gas flow rate affects the sensor,which was also studied in this thesis.3.Cantilever sensors based on copper foil,silicone sealant,and polyethylene film were prepared with the same detection method used in 1.Results also show that the sensor response shows good linearity with xylene in the range of 0-82 ppm with a correlation coefficient of 0.998 and a low detection limit of 4 ppm.Most importantly,the sensor is also extremely resistant to humidity,with a response of 4 ppm xylene almost twice the response of 20% humidity difference(ΔRH).This study would help explore the functions of each layer and diverse the type of the cantilever sensor.And moreover,it would facilitate performance adjustment in demand by changing the materials of each layer.