The use of MEMS technology has had a remarkable impact on the development of uncooled Infra-Red (IR) sensors, paving the way for the introduction of free-standing thermal-isolation structures with thin-film IR absorbers. New applications are emerging for this new class of miniature sensors, such as thermography, human detection, night vision and more.
In fact, IR thermal-sensing and imaging instruments make it possible to measure and map surface temperature and thermal distribution passively and non-intrusively. This is because IR radiation is emitted by all objects with a temperature above absolute zero. For an object that has no colour – that is, no wavelength is selectively emitted or absorbed – the radiation spectrum is completely determined by temperature alone. With rising temperature, the intensity at every wavelength of the radiation spectrum also increases. This means that it is possible to remotely determine the temperature of a body or object by measuring its radiated power.
Now, a new IR detection device, the Grid-EyE sensor array from Panasonic Automotive and Industrial Systems, uses this capability for the precise detection of the thermal signature of bodies such as human beings, as well as inanimate objects, in the array’s field of view.
IR detector based on advanced MEMS technology
The Grid-EyE sensor array uses MEMS fabrication technology to enable the implementation of an 8 pixels x 8 pixels IR sensor array in a sensor die with a footprint of just 3mm x 3mm, as shown in Figure 1. This technology makes it possible to produce a thermopile type of IR sensor consisting of hundreds of thermocouples on an area of several square millimetres. Such a sensor is extremely sensitive and has a very fast response time.
But it can also be produced at low cost because of its small size, and the ability to use standard silicon IC fabrication processes to make it in high volume.
And because the sensor is comprised of an array of sensor pixels, rather than a single sensing element as in the common pyroelectric type of IR sensor, it provides for much more sophisticated and valuable applications than were possible before.
Advantages of the Grid-EYE array sensor
Panasonic’s Grid-EyE IR array sensor is the first ever 64-pixel IR camera in a compact, integrated surface-mount package. Grid-EyE combines the MEMS sensor chip, a digital ASIC with an I2C interface, and a silicon lens, as shown in Figure 2. It has 64 thermopile elements in an 8×8 grid format which detect absolute surface temperature by sensing IR radiation, that is, without contacting the surface.
Unlike conventional sensors, Grid-EyE uses a patented 60° silicon lens etched out of a silicon wafer, which is, at less than 0.3mm high, the smallest lens on the market. The sensor package’s size is just 11.6mm x 8mm x 4.3mm, which is around 70 % smaller than competing products.
In contrast to single-element thermopile sensors and pyroelectric sensors, the Grid-EyE sensor can not only detect moving people and objects but also:
• the position and presence of motionless people and objects
• the direction of movements
• surface temperature from -20°C to 100°C
A comparison of sensing technologies is shown in Figure 3.
The Grid-EyE’s measurements are accurate: Panasonic rates the Grid-EyE’s Noise-Equivalent Temperature Difference (NETD) at ±0.5°C at room temperature. Its co-ordinated array of sensing elements can detect multiple people or inanimate objects moving in different directions. At close proximity, Grid-EyE is even capable of detecting hand movements for simple gesture control.
The applications for this more sophisticated form of IR sensing are many. They can include factory automation systems, in which the Grid-EyE sensor can monitor and count inanimate objects, and equipment maintenance, since the Grid-EyE sensor can form a thermal image of a piece of equipment: overheating is a common early-warning sign of a mechanical malfunction.
The Grid-EyE sensor can also be used in supply-chain equipment, for instance for monitoring perishable or temperature-sensitive goods. And it is also ideal for detecting the presence and motion of human beings, for instance in building-automation and security systems.
An evaluation kit for a wireless sensor node based on the Grid-EyE sensor is the Board of the Month featured on page 25.