A MEMS based air quality sensor
individual CMUT cells on our sensor chip
the area of the primary sensor
the diameter of each CMUT cell
Air pollution is estimated to cause up to 500,000 early deaths in the EU alone. Air pollution has many different components including oxides of nitrogen (NOx) and sulphur (SO2), ozone (O3), heavy metals such as lead and cadmium, and microparticles.
AQSense is a multi-gas sensor, meaning it can detect a wide range of compounds within a single sensor. The micro scale of the sensor results in a small unit with low power requirements that can be easily retrofitted to the modern urban environment, for example to lamp posts and wiring cabinets. The ability to use the existing cellular network as well as other local and metropolitan wireless networks negates the requirement for data cabling. Current pollution sensors fall into two categories: Firstly, large units with high accuracy which are used by local governments and industry to fulfil their legal obligations. Firstly, large units with high accuracy which are used by local governments and industry to fulfil their legal obligations. These units are highly accurate, but expensive and require significant infrastructure to install (secure cabinets with power and data). Due to cost most cities operate just a few of these units, resulting in a very low-resolution picture of pollution.
Secondly, miniaturised pollution sensors are available, but these do not typically meet the high accuracy levels required so are of more interest to consumers and citizen scientists.
Augvald seeks to break down this barrier, providing both governments and consumers with a highly accurate, miniaturised pollution sensor with a lower lifetime cost than traditional systems.
The low cost will allow local government to deploy a much larger network of sensors, resulting in a higher resolution image of air pollution across our cities. When combined with other data such as traffic patterns and weather, actionable and predictive maps can be generated.
Meanwhile, citizen scientists can generate highly accurate date for their location, and share this with others and even with local government as part of a sensing network.
The Augvald sensor
Imagine a small, low cost and highly accurate pollution sensor which can be deployed in a vast network across the urban landscape. The AQSense gives an accurate picture of air quality at a street-by-street level, helping governments plans and intervene to reduce pollution levels.
Estimated deaths in EU alone due to Air Pollution
An overview of the technology
CMUTs were first presented in 1994 by Haller and Khuri-Yakub as air transducers, and then later as immersion transducers. A CMUT consists of a plate suspended over a cavity, usually filled with vacuum. Ultrasound can be generated or detected electrostatically using the capacitance between the plate and the substrate. A single CMUT element may consist of a single cell, or of many cells connected in parallel. Unlike conventional piezoelectric ultrasound transducers, CMUTs are microelectromechanical systems (MEMS) devices, fabricated using well-developed technologies from the integrated circuit industry.
This allows transducers to be made with very small size and pitch and to be closely integrated with the supporting electronics. Since their invention, CMUTs have been used in a wide variety of applications including intracardiac imaging, high intensity focused ultrasound therapy, thermoacoustic detection of embedded objects, and real-time volumetric ultrasound imaging.
The key technology behind the Augvald chemical sensors is a capacitive micromachined ultrasonic transducer (CMUT)
A schematic of a single CMUT sensor
Schematic of the CMUT chemical sensor operating principal
CMUTs can be used as gravimetric chemical sensors by coating the plate with a functionalization layer. The functionalization layer absorbs chemicals from the air, which increases the plate mass and decreases the resonance frequency. Readout is entirely electrical, unlike some gravimetric sensors, which use optical readout.
Sensitivity of the CMUT is dependent on several factors
- The functionalization layer and its ability to selectively adhere to molecules of analyte
- The CMUT design and fabrication
- The associated circuitry
- The machine learning work which is required to “teach” the system to identify the analyte