Taking the pressure-sensing technology set to transform broad range of products

Citation

(2005), "Taking the pressure-sensing technology set to transform broad range of products", Sensor Review, Vol. 25 No. 4. https://doi.org/10.1108/sr.2005.08725daf.005

Taking the pressure-sensing technology set to transform broad range of products

Taking the pressure-sensing technology set to transform broad range of products

Keywords: Pressure

Advances in fabrication technology and design are now opening up new applications for pressure sensors. VTI technologies in Finland is developing a series of sensing elements that is expected to bring new levels of performance to low power pressure sensors. The company is introducing pressure sensing elements and devices mainly intended for a wide range of battery operated and hand-held devices.

Pressure is a parameter that has an effect on our lives in numerous ways. In the atmosphere, pressure tells us about the type of weather we can expect; in our blood, it can indicate our general state of health; in machines, it can indicate how well they are functioning and, in diving, it dictates how long you can safely remain underwater.

Behind the development are advances in its own measurement techniques coupled with its ability to manufacture the devices using its own proprietary manufacturing technology called three dimensional microelectromechanical systems (3D-MEMS). VTI's technology allows the integration of tiny mechanical sensor elements and electronics in a small cost effective package (Plate 9).

Incorporation into next generation products

The company has been a major player in MEMS sensors since its foundation in the early 1990s. In 2005, it will launch a new sensor family that could be used in the next generation of consumer and industrial products from altimeters to wrist diving computers. It is based on the same technology that has allowed the development of sensors that have already been integrated into vehicle tyre pressure monitoring systems.

Plate 9 VTI's 3D MEMS technology combines accurate sensing with small size. This makes it suitable for a range of battery powered applications

The low power absolute pressure sensor concept allows sensing elements and devices that can measure pressure ranges from barometric (100 kPa) to 25 bar (2,500 kPa). The sensor is very robust and can easily withstand a pressure significantly higher than ten times the measuring range. The design of the sensor allows for optimised performance in different applications. For high-resolution applications, the sensor has resolution that is better than 2 Pa which corresponds to about 16 cm resolution in altimeter applications, for example. The rate at which the measurements are updated by reading the capacitance determines the resolution, the speed of operation and the power consumption. The sensor can also be configured to different power modes. In the low power mode it is in standby between measurements and consumes only 3-4 μA.

Separate control circuitry

The sensor has been integrated with the Application Specific Integrated Circuit (ASIC), within the device to create a package that is robust and suitable for high volume applications. The ASIC provides not only the serial interface to external equipment (SPI/I2C) but also carries out the overall control, processing, pressure calculations and temperature compensation. All this is contained in a unit that is only 6.1 mm in diameter and 1.7 mm in height.

VTI Technologies' 3D-MEMS fabrication method is a powerful approach to sensor manufacturing. It has made devices that have transformed automotive safety and stability systems, for example. Three dimensional- MEMS allows true three-dimensional structures rather than thin films to be made on a silicon substrate. This added dimension gives flexibility to the overall design such as in optimising the electrode insulation and connections to the outside world. The company's 3D-MEMS technology is used to create sensors that rely on a change in capacitance for their operation. A capacitor is a device that can store electrical charge. The classical model of a capacitor is that of two electrodes or plates on which charges builds up. At its simplest, the amount of charge that is built up on the capacitor plates can be used to determine changing pressure.

In VTI's pressure sensor, the pressure diaphragm is one plate of a capacitor that changes its value under pressure-induced displacement. In practice, two silicon wafers form the basic capacitor element with a membrane over which the force can be measured. The outer pressure exerts a force on the membrane causing it to bend towards the bottom electrode relative to a reference pressure inside the cavity between the wafers. The displacement of the membrane is detected as a change in capacitance between the membrane and bottom electrode. Using a simple mathematical calculation based on the fact that the force acting on the membrane is directly proportional to the inverse of capacitance the pressure value can be obtained.

Low noise

In this approach, the relatively wide change in capacitance, typically between 50 per cent, makes measurement relatively easy and also results in low noise systems. The results are sensing elements that are small, highly accurate and stable over a wide temperature range and have low power consumption.

One key factor with VTI's approach to its technology is that it has decided to separate the sensing elements and the ASIC, which provides the information processing and the electronic control. This gives a high degree of flexibility in the range of products that the company can offer from individual sensing elements to complete modules with on-board electronics. It also means that the company can quickly respond to market or technology changes.

For more information contact: VTI Technologies Oy, Tiina Hansson, Vice President, Corporate Communications, P.O. Box 27, (Myllykivenkuja 6), FI-01621 Vantaa, Finland, Tel: +358 50 465 4675; Fax +358 9 8791 8796; E-mail: tiina.hansson@.vti.fi, web site: www.vti.fi

Further readingBarone, P.W., Baik, S., Heller, D.A. and Strano, M.S. (2005), “Near-infrared optical sensors based on single-walled carbon nanotubes”, Nature Materials, Vol. 4.