Smoke Particulate Sensors for OBD and High Precision Measuring

Carbon particles have the capability to be electrically charged. A very high static electrical field attracts particles through Coulomb forces in that they touch statistically the high voltage electrode of a capacitor incorporated into the main exhaust gas flow. By touching the electrode they are charged. Having the same polarity they are repelled and move to the grounded electrode where they are discharged. This procedure produces an electrical current. The RMS value of this amplified signal represents the particle mass. The dependency on the gas flow speed is corrected by a mathematical procedure incorporating the intake parameters of the engine. For high accurate measurements a low constant exhaust gas speed is necessary achieved by a pump sucking the gas through a bypass. To avoid interference from capacitor blade vibrations and gas flow oscillations inside the exhaust silencers a low-pass filter with a cut-off frequency of 2 Hz e.g. is necessary.

Another method of measuring smoke particles consists of using the property of carbon conducting electrical current. Between bifilar winded electrodes carbon particles produce electrical resistance bridges and gradually reduce the isolation resistance. This principle is used for measuring the mass of particles in total. By deriving the gradient of the resistance changes the dynamic particle concentration is measured. The bifilar coil consists of resistance material. By simply shortening the ends of the coils the measuring device is also used for burning the particles. The smallest device has the size of a spark plug. The surface of a ceramic heater is sputtered with electrodes having a very small gap in order to optimise the reaction time. This particle summarizing measurement technology is used for monitoring particle traps.