1) The document describes a novel soot sensor design with a single layer sensing element that overcomes challenges with traditional resistive soot sensors. 2) The single layer design integrates the heating and sensing functions into a simple construction, improving regeneration efficiency and enabling robust diagnostics. 3) Testing showed the single layer sensor can regenerate effectively even at high exhaust flows, requires less voltage for regeneration than conventional designs, and responds well to varying soot concentrations.

1. P2300218
Soot Sensor to fulfill Euro6 OBD requirements
Kayvan Hedayat
Stoneridge Inc, USA
Key words- PM sensor, soot sensor, OBD, sensor regeneration, sensor diagnostics, low cost PM sensor
Abstract
Diesel Particular Filter (DPF) is the dominant solution for meeting PM level emissions as regulated by EU, EPA, and other
regulatory bodies worldwide. A soot sensor is required to assess the health of DPF and enable OEM’s to comply with On
Board Diagnostics (OBD) requirements. The platinum impedance sensor is currently the sensor technology preferred by
most OEM’s to comply with OBD requirements.
These types of resistive sensors have some specific challenges and must satisfy conflicting goals. The resistive PM sensor
must communicate with the exhaust gas flow and sense the resistance of the soot under severe conditions and then self
clean its sensing surface (i.e. regenerate) for the next sensing cycle. On one hand the sensor must allow enough flow and
soot inside its body for proper accumulation to reliably measure high resistance levels, and on the other hand the sensor
needs to restrict the flow inside its body to allow regeneration under high flow conditions (where the sensor heater
must overcome the cooler and fast exhaust flow). The sensor must also measure an open circuit under no soot
conditions and yet be able to distinguish the difference between a cracked platinum trace on the surface of the sensing
element (i.e. open circuit) to open circuit under no soot conditions. One of the most important requirements of the soot
sensor is its ability to diagnose itself routinely and to prevent “in range” failures which will have system implications.
The PM sensor must be sensitive enough to sense as low as 1 mg/m3 soot, but also its dynamic range must be high
enough to accommodate much higher levels when DPF has a significant crack or failure.
Stoneridge has developed a novel sensor that overcomes many of the challenges faced by typical resistive type soot
sensors. The sensor integrates the heater and sensing element into a simple Single Layer design which significantly
improves regeneration and enables a simple and robust diagnostics check. The sensor includes an electronic module
that controls sensor regeneration along with CAN communication. A transfer function is used to provide the proper PM
concentration to OEM’s based on flow velocity and exhaust gas temperature. Prediction techniques like finite element
analysis along with dyno testing and environmental tests have been performed to validate the Single Layer design.

2. Introduction
The diesel particulate filter in the exhaust system of a typical automobile is one of the most important components
required for protection of the environment. Therefore, it is in the focus of legislation to include PM-filter monitoring in
the regulations. The particulate filters can exhibit different types of failure modes. In some cases a very small amount of
soot leakage maybe present due to a fine crack. The soot levels can be as low as 1 mg / m³ which is close to the
measurement limit of laboratory smoke meters and soot analyzers. In other cases the soot levels can be much higher
due to a severe crack in the filter material. Another failure mode can be a “runaway” active regeneration resulting
temperatures close to 1000 C at the sensor location.
A typical platinum sensor element used in many of the resistive soot sensors is shown in figure 1 below. Under no soot
conditions, the circuit is open as there is no current flowing from one side of electrode to the other side. When soot
starts to accumulate on the surface, the gap is bridged by soot and resistance drops resulting in current flow from one
electrode the other one as shown in figure 2. It must be noted that soot’s resistance depends on many factors including
time, temperature, and exhaust constituents. Depending on the sensor construction there is a heater on the other side
of the element or an intermediate layer to regenerate the sensor. A picture of actual soot on the sensor element with 30
micron wide electrodes is shown in figure 3 for reference
Figure 1: Typical soot sensor element with open “inter-digited” construction.
Figure 2: Typical soot sensor element sense current path through soot bridges.

3. Figure 3: Actual sensor element with 30 micron platinum traces and accumulated soot
PM sensor’s element can be subjected to significant rapid temperature rises such as 150 degrees C / sec. The heater of
the sensor also causes a rapid temperature rise and the cumulative stresses can cause fractures in the element. There
has also been indications of small particles separating from exhaust system surfaces (e.eg pieces of platinum,...) and
mixed with exhaust gas which can potentially impact the sensor element. One very important failure mode that sensor
must be able to diagnose is fractured element and platinum traces. As shown in figures 4 and 5, the fracture can occur in
different locations resulting in different symptoms. Figure 4 shows a fracture where the sensor output is open and there
is no way to distinguish the sensor output from a healthy sensor with clean sensor element surface. One solution is to
print a metal oxide resistor across the sensor element. This solution is costly and adds the complication of a metal oxide
resistance drift over life. Figure 5 shows a fracture where the soot output is not open due to fractured platinum trace
resulting in a false signal. This type of failure is called an “in range failure” where the signal appears to be good (in valid
range), but inevitably the wrong soot concentration will be inferred.
Figure 4: Fractured platinum trace resulting in an open circuit with soot loading

4. Figure 5: Fractured platinum trace resulting in false sensor output
Sensor Construction
The Stoneridge sensor construction is shown in figure 6 below. The electronic module measures the soot impedance,
controls regeneration, measures element temperature, provides hardware / software diagnostics functions, and
provides the CAN interface to the vehicle ECU. One advantage of active sensor (sensor with electronics) compared to a
passive sensor (sensor without electronics) is the fact that high impedance soot measurement can be precisely
controlled under all conditions. An active sensor can provide a consistent and short wire length to complying with EMC
requirements as well.
Figure 6: Stoneridge active PM sensor
The sensor element is a made from a single piece alumina with thick film platinum trace only on one side. The platinum
trace loops act as both heater and sensor. The sensor measurement principle is as follow:

5. Figure 7: Single Layer electrode layout Figure 8: Single Layer electrode with soot loading
Voltage across terminals 1 and 2 (figures 7 and 8) is an indication of soot loading in the gap between the two platinum
traces. When soot s loaded, the resistance between 1 and 2 is reduced and results in a current flow as shown in figure 8.
When soot resistance reaches a certain threshold (and other criteria such as rate of change of resistance along with
certain other parameters), sensor will go into regeneration mode. In this mode the platinum traces act as heaters with
two possible configurations. Figures below show the heater current paths (figure 9) and two heater modes (figures 10
and 11) along with their equivalent circuits (figures 12 and 13).
Figure 9: Heater current paths

6. Figure 10: Parallel heater Figure 11: Series heater
Figure 12: Parallel heater Figure 13: Series heater
The series heater has higher resistance and is suited for situations when current consumption needs to be limited or
when sensor is cold and rapid heating is required. Series heater resistances also provide a better temperature
measurement as higher resistance enables higher resolution. Parallel heating is suited for situations where sensor is hot
and platinum resistance is high and there is a need to pass more current to increase heating during high flow conditions.
The advantage of this type of heater configuration is that heat is generated where soot is present and there is no need
to transfer heat between two separate surfaces. It should be noted that heater configurations are under solid state
switching and software control and it is possible to provide staged heating where heater configuration is changed real
time depending on the exhaust flow velocity and exhaust temperature.
The Single Layer sensor element design offers several performance and cost advantages:
1- It allows very effective regeneration since the heater and sensor is the same. This is described in more detail in
Sensor Regeneration section.
2- It uses about half as much platinum compared to other resistive soot sensors since heater and sensor share the
same platinum traces. With the ever rising cost of precious metals, this is an important factor.

7. 3- provides a simple one piece robust construction
4- One unused surface (back side of sensor element) provides future opportunity for another sensor such as a
precision temperature sensor or other emissions related sensors.
5- Simple diagnostics check as described below.
Sensor Diagnostics
PM sensor must perform self diagnostics for conditions such as sensor element integrity, clogged tip, wire harness
shorted, wire harness cut open, over voltage, under voltage, and a variety of other hardware and software diagnostics.
PM sensor shall not report an output signal in the valid range that is not correct (i.e. in range failure mode). It is
expected that the PM sensor routinely goes through a self diagnostics check and report its status to the vehicle ECU.
Stoneridge sensor element consists of two loops and diagnostics is achieved by passing a current through each loop
occasionally to assess platinum trace integrity. Figure below shows the concept.
Figure 14: Any fracture prevents current flow
Sensor Regeneration
PM sensor must regenerate to clean its sensing element surface when the threshold criteria are met. Regeneration is
also required at some pre-determined frequency for diagnostics purposes. The sensing element surface temperature
generated by the heater must be high enough to burn soot, but cannot be too high to damage the ceramic substrate,
fixing glass, and platinum traces. A very small or no thermal gradient is highly desired on the active sensing element
surface to reduce thermally induced stresses and to minimize hot spots.
The oxidation temperature of soot is around 650 degrees C. Literature suggests that oxidation temperature depends on
several factors such as amount of NO2 present .Tests at Stoneridge show that it takes temperatures as high as 750
degrees C to fully oxidize soot in an aftertreatment system with DOC, DPF, and SCR. Therefore the temperature window
that sensor must maintain is very important in light of the fact that measuring the temperature on the surface of the

8. sensing element using the platinum traces has limited resolution due to low resistance values and thick film technology
limitations.
The single layer design heater is where soot is present which make the heat transfer very efficient. In the Single Layer
design the heat does not have to transfer from one surface to another. Analysis and testing is shown in section below.
Analysis and testing
Effective sensor regeneration during high flow conditions and low voltages has been a challenge for resistive sensors.
Analysis and testing were performed to validate regeneration capability and heater stability of the Single Layer design.
Since the heater is exposed, particular care was taken in thick film material system and testing to insure its stability.
Basic sensor functionality and output sensitivity were also assessed in Stoneridge chassis dyno.
Computational Fluid Dynamic simulation was first performed to assess the regeneration capability of the Single Layer
design. Sensor assembly including the tip was simulated with 9 Volts heater input in the presence of 50 m/s exhaust
flow velocity and 200 degrees C for exhaust gas temperature. Temperature profile of the sensing element is shown in
two configurations in figure 15. The configuration on left is the traditional PM sensor element with heater on the back
side of the active sensing element. The configuration on the right is the Single Layer design showing a higher
temperature profile on the active surface with the same inputs to the model.
Figure 15: CFD simulation of conventional PM sensor element (on left) and Single Layer design (on right)
Testing was performed in the high flow high temperature stand at Stoneridge with settings of 200 degrees C gas
temperature while flow velocity was increased (shown on X axis of figure 16). The heater voltage required to take sensor
element temperature to 750 degrees C was measured and is shown on the Y axis of the graph in figure 16. Voltage levels
required are much less than the conventional type sensor element where at 10 Volts regeneration would be very
difficult at high flows.

9. Figure 16: High temperature high flow stand and graph showing regeneration voltage versus flow velocity
The Single Layer design was also tested in the Stoneridge chassis dyno with several soot concentrations. Sensor output is
shown at 1 mg/ m³, 7 mg/m³, and 12 mg/m³. As it can be seen in figure 17, the sensor responds with good signal to
noise ratio. Exhaust temperature was at 270 degrees C and flow velocity was at 15 m/s.
Figure 17: Sensor output at Stoneridge chassis dyno

10. Conclusion
For the foreseeable future, diesel particulate filter continues to be the paramount choice among OEM’s to meet the
emissions requirements as mandated by regulatory bodies. Resistive soot sensors are capable distinguish a healthy DPF
from a damaged DPF; and due to their lowest cost and performance attributes, they continue to be the preferred choice
for OEM’s. Alternate resistive soot sensor with a Single Layer element is shown to provide robust diagnostics capability
and effective regeneration. Validation testing has been performed to qualify the basic principle of the single element
design.