Robust sensor circuit controlling pumping current to oxygen sensor cell with solid electrolyte, uses combination of analog and digital circuitry, controlling errors when measuring exhaust gas nitrogen oxide and oxygen

摘要

A microcontroller (20) determines the actual value (Ip-ist) of the pumping current to the pump cell (27) under control. An actual value of the Nernst voltage (VN-ist) at the cell (34) is determined from this, and a pulse width modulated (PWM) signal is produced. An analog circuit (21) transforms the PWM signal into a pumping current for the pump cell (27). A desired value of the Nernst voltage (VN-soll) is read from a memory (40) for constants. Preferred Features: The memory for constants is programmable and integrated in the microcontroller. A measurement resistance (25) connected before the pump cell drops voltage indicative of the actual value of pumping current (Ip-ist). The microcontroller includes a third subtracter (39), forming the voltage ( DELTA VN) from actual and desired Nernst voltages. A controller (41) produces the pumping current desired value (Ip-soll). A fourth subtracter forms the pumping current difference ( DELTA Ip) in a similar manner. The PWM unit produces the signal and a second controller (43) controls pumping current difference ( DELTA Ip) from the results. Use of ADCs to determine the actual value of pumping current and the actual value of Nernst voltage is described. The analog circuit includes a filter circuit (23) converting the PWM signal to an analog signal. An impedance transformer (24) matches microcontroller output to the required value of current output. The circuit is included in the casing of the sensor plug.