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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