Today, reducing carbon-dioxide (CO2) emissions
is one of the hottest automotive topics. The
European Commission recently announced its
roadmap for safer and greener cars by 2012. The
strong shift in buying patterns also confirms that consumers
want the most fuel-efficient vehicles that meet their personal
and professional needs.
Using advances such as hybrid technology, car manufacturers
are working to introduce new models that reduce CO2 emissions.
Other technologies like Blue Motion, Econetic, and Efficient
Dynamics all have the same goal of reducing these emissions.
It is also a fact that diesel-engine vehicles have higher pollution
rates than gasoline-fueled cars. Specifically, diesel particulates are
harmful to human health with links to lung ailments, cancer, and
heart disease. Older diesel engines emit larger particles visible
as black smoke, while newer engines still emit particles that are
generally too small to detect with the naked eye. To further clean
up diesel particulate emissions, car manufacturers build in diesel
particulate filters (DPFs). These DPFs have been in use on offroad
vehicles since 1980 and in some automobiles since 1996.
Current DPFs trap diesel soot particles down to about 2.5
µm in diameter and, in this size range, reduce particulate emissions
by 60% to 70%. Made of porous ceramic materials, DPFs
eventually become saturated and need cleaning and regeneration.
Performing this maintenance requires heating the DPF
to an exhaust-gas temperature above 600°C. This exhaust-gas
temperature is higher than normal and, to achieve it, the eletronic control unit (ECU) temporarily introduces retarded injection
and intake-flow restriction.
This is where sensors act as critical control elements. By measuring
the pressure drop across the filter, a pressure sensor can determine
the most efficient point to start the regeneration process.
IMPLEMENTING THE INTERFACE
Using the diesel filtering model as an example, we install a differential
pressure sensor employing a classic piezoresistive element
within the DPF. This sensor detects a small pressure range of
interest, typically in the range of 0 to 15 psi. A sensor-interface
IC, such as an MLX90320 CMOS analog sensor interface, then
connects to the output of the sensor, forming a resistive type of
Wheatstone bridge circuit.
The analog sensor interface converts small changes in resistance,
usually a few millivolts, into significantly larger outputvoltage
variations. Configured in this manner, the circuit can
compare pressure signals both before and after the filter (Fig. 1).
The interface chip amplifies and corrects signals from the sensor
and converts them to a value recognizable by the ECU.
When the DPF saturates over time, the interface detects a
larger pressure differential between the signals before and after
the filter. Measured in millivolts, the interface IC amplifies and
compensates this difference and communicates it to the ECU. In
this way, the sensor interface controls the communication between
the sensing element and the ECU, guaranteeing that the filter
continues to work properly.
THE SENSOR INTERFACE
For our example, the piezoresistive sensing element connects to
the inputs of a MLX90320 sensor interface, which compensates
the signal for gain and offset to ensure a well calibrated output signal.
Besides the 3- and 10-bit digital-to-analog converter (DACs)
in the different coarse gain stages, this particular interface’s output
architecture employs an additional 10-bit DAC that makes it possible
to accurately calibrate the output span (Fig. 2).
The device’s architecture easily detects a sensor output of several
millivolts and achieves an accurate output span of 4 V. To guarantee
smooth offset tuning from the interface chip, we add a coarse offset
calibration to compensate for large offset variations of the sensing
element and use an adjustable 10-bit offset.
In the advent of thermal concerns, the sensor interface also has
the option to interface with either an internal or external temperature
sensor. However, it is advisable to use an external temperature
sensor only for applications where the temperature surrounding
the sensor differs from the temperature surrounding the interface.
By connecting an external resistor to the temperature chain
adjusted for offset and gain, the interface can perform an accurate
10-bit temperature measurement where necessary (Fig. 3). In this
way, you can connect an external temperature sensor close to the
pressure sensor.
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