Electronic Design

Sensors Work To Reduce Auto Emissions

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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Another unique feature of the sensor interface is its ability to enable programmable clamping at low- and high-output levels. Enabling clamping ensures the sensor’s ability to detect minimum and maximum output levels and the ability of the ECU to check whether the sensor’s output is within an acceptable range.

Furthermore, the MLX90320 sensor interface in our example integrates fault detection, making it suitable for a range of automotive applications. Capable of detecting internal and external faults, this allows for the detection of a poor sensing-element connection.

If the chip receives an incorrect input level from the sensor, in the arenas of lower than 1.5 V or higher than 3.5 V, possibly the result of a short circuit to ground or to supply, the IC will then generate an output level outside of the clamping-level range. In turn, the ECU detects the event.

Also of interest, this particular interface can program the sensor interface through the actual connector. The module containing the sensing element and the interface are integrated into one housing with only the application pins connected to the outside of the module. It isn’t necessary to add additional communication lines since the output pin acts as both an analog output pin and as a communication pin.

Through short-circuit detection, the IC knows that the user is requesting the pin for communication. To guarantee that no changes to memory parameters occur via a short circuit, a specific timing forms the basis of this detection. Furthermore, the interface chip includes an option to lock the EEPROM to avoid accidentally changing a calibrated device. In essence, by using the output of the interface we can communicate with the chip and calibrate the parameters in EEPROM with user-defined characteristics.

Evaluation boards are available for calibrating the interface. These boards include all the hardware necessary for communicating with the interface, accompanying software, and production software. The production software can control all necessary production equipment and communicate with the device to quickly calibrate hundreds of samples.

APPLICATIONS
Sensor interfaces are ubiquitous in cars. They translate sensor signals into readable language and simultaneously filter out disturbing external elements to ensure the correct message gets sent.

We find sensors of various types in common rail, suspension, and transmission systems as well as in HVAC applications, fuel and gas injection systems, engine control (fuel, oil boost), anti-lock braking systems, and many other areas of the vehicle. They are so widely used that people tend to forget them. Understanding their capabilities will foster the use of these irreplaceable workhorses in an ever-increasing number of applications.

TAGS: Automotive
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