MEMS DESIGN
This involves assuming that the CO2 level is caused primarily
by the persons present. As a result, it provides an estimate
based on numerous assumptions. Consequently, with this
eCO2 value, the regulation of the indoor air quality can only be
performed on the basis of this potentially inaccurate information.
This leads to air-conditioning systems consuming an unnecessary
amount of energy or not ventilating properly at all,
precisely when it is needed. As a result, air quality is not effectively
improved and users lose confidence in products that
work with such eCO2 sensors.
MEMS-based photoacoustic spectroscopy
Thanks to its experience with MEMS microphones and
through experimental processes, Infineon has succeeded in
developing a new CO2 sensor based on photoacoustic spectroscopy
Fig. 3: The compact XENSIV PAS CO2
sensor from Infineon.
(PAS) – a physical method that is suitable for detecting
gas components in a mixture and, for example, determining the
CO2 concentration in indoor air.
Photoacoustic spectroscopy utilises the fact that gas molecules
only absorb light with a specific wavelength; in the case
of carbon dioxide, this wavelength is 4.2μm. An infrared source
with an optical filter supplies the gas with energy in a rapid succession
of light pulses at precisely this wavelength. This leads
to the rapid heating and cooling of a gas sample, which in turn
leads to thermal expansion and contraction. The sound generated
by this can be recorded with a microphone, evaluated
and used to draw conclusions about the amount of CO2 in the
gas. The higher the CO2 concentration, the stronger the signal.
The use of a highly sensitive MEMS microphone as a detector
allows for significant miniaturisation compared to NDIR-CO2
sensors.
Challenges in sensor development
The Infineon CO2 sensor integrates a photoacoustic transducer
with detector, infrared source and optical filter on a printed circuit
board. The sensor has a small microcontroller for on-board
signal processing, sophisticated algorithms and a MOSFET
for operating the infrared source. A modulated IR light source
radiates onto the gas mixture in the sampling chamber. The CO2
present absorbs the IR light, heats up and increases the pressure
in the sampling chamber and these pressure changes can
be measured by a MEMS microphone.
A major challenge in developing a PAS-CO2 sensor was to
push the performance
of the microphone to its
limits and minimise system
noise, i.e. to isolate the
MEMS detector from external
noise so that only the
pressure change originating
from the CO2 molecules
in the chamber is detected.
Infineon modelled the
MEMS microphone response
before prototyping
some units to validate the
modelling results.
Features and advantages
Infineon’s new XENSIV PAS CO2 sensor features the IM69D130
XENSIV MEMS microphone with a signal-to-noise ratio of 69
dB. It is designed for applications where low self-noise, wide
dynamic range, low distortion, and high acoustic overload point
are required. Thanks to the IM69D130, the slightest pressure
fluctuations can be measured in the gas sensor, so that even
a small amount of gas is sufficient for the exact determination
of the gas concentration. As a result, the sampling chamber
could also be designed small. Offering true CO2 measurements,
the new sensor is more than 75 % smaller than conventional
CO2 sensors with comparable performance parameters. The
integrated microcontroller converts the signal at the MEMS microphone
output into a ppm value that is available via three interfaces:
the serial I²C, UART or PWM interface. The direct ppm
readings, surface mount capability and simple design allow
easy and fast integration with flexible production numbers. All
components are designed and produced in-house to Infineon’s
high quality standards.
The extremely robust sensor covers a measuring range
from 0 ppm to 10,000 ppm with a measurement accuracy in
the range up to 5000 ppm (± 3 %) at ± 30 ppm. It operates in
a temperature range from 0°C to 50°C at a relative humidity of
0 % to 85 % (non-condensing). Drift is less than 1 % per year
(with active self-calibration). In pulsed mode, the CO2 sensor
is designed to last ten years. This makes the XENSIV PAS
CO2 ideal for demand-oriented ventilation control in building
automation and for controlling indoor air quality in smart home
applications.
Infineon is planning
several sensor variants
to meet very specific
requirements in various
applications, such as
low power consumption
in battery-powered
applications, smaller
size at lower cost for
portable devices, and
even more reliable
variants for extremely
harsh industrial applications.
Sensor variants
for other gases
are on the roadmap
too.
Fig. 4: Typical applications of the XENSIV PAS CO2 sensor.
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