Oxygen sensor signal conditioner

In an internal combustion engine having an intake system, an exhaust system, an engine block, a plurality of cylinders disposed in said engine block, a piston operatively mounted for reciprocal movement within each of said plurality of cylinders in response to the combustion of fuel and air therein, means disposed at least partially within said intake system for controlling the quantity of air supplied to said cylinders, means for supplying a controlled quantity of fuel to one or more of said plurality of cylinders, control means responsive to one or more predetermined control signals for operatively varying the ari-fuel mixture in said engine, first and second oxygen sensors, one oxygen sensor being operatively disposed in each of the two major exhaust streams of said internal combustion engine, each of said oxygen sensors being responsive to the relative air-fuel mixture existing in said corresponding exhaust stream and generating an oxygen sensor output signal having a first general voltage level if said measured air-fuel ratio is "lean" of stoichiometric and a second general voltage level if said measured air-fuel ratio is "rich" of stoichiometric, each of said oxygen sensors being characterized in that the impedance thereof decreases as its operating temperature increases and in that measurements taken below some minimum temperature such as 300.degree. Centigrade are inherently unreliable due to excessively high internal sensor impedance, an oxygen sensor output signal conditioning system for insuring that only reliable output signals indicative of said rich or lean air-fuel mixture measurements are outputted for control purposes, said oxygen sensor output signal conditioning system comprising:

• first and second means for supplying first and second test currents to said first and second oxygen sensors, respectively, and producing corresponding first and second oxygen sensor test output signals indicative of the measured sensor impedance, which is a function of sensor-operating temperatures, said first and second test current supply means being utilized to avoid cross-coupling of test currents and sensor signals between said first and second oxygen sensors;
• first and second monitoring amplifiers having relatively little current flowing into or out of their input terminals for amplifying said first and second oxygen sensor output signals and said first and second oxygen sensor test output signals from said first and second oxygen sensors, respectively, and for optimizing the test current supplied to said first and second sensors by said first and second test current supply means via the high input impedance of said first and second monitoring amplifiers;
• first and second output comparators each having first and second comparator inputs and a comparator output, the first input of each of said first and second output comparators being operatively coupled to receive said oxygen sensor output signals and said oxygen sensor test output signals from said first and second oxygen sensors, respectively;
• first and second means operatively coupled to the second comparator inputs of said first and second output comparators for establishing a reference threshold voltage level indicative of a stoichiometric air-fuel ratio, the output of each of said first and second output comparators normally having a first value when said measured air-fuel ratio is lean of stoichiometric so long as the amplified value of said corresponding oxygen sensor output signal is at said first general voltage level and therefore less than said established predetermined threshhold voltage level, the comparator output of each of said first and secon output comparators being responsive to the amplified value of said corresponding oxygen sensor output signal being at least equal to said second general voltage level and therefore being greater than said established predetermined threshhold voltage level for rapidly switching to a second value indicating that the measured air-fuel ratio is rich of stoichiometric;
• a single test comparator having first and second test comparator inputs and a test comparator output;
• first and second diode means operatively coupling the output of said first and second monitoring amplifier to said first test comparator input for receiving either of said amplified first and second oxygen sensor test output signals from said corresponding first and second monitoring amplifier means;
• means operatively coupled to said second test comparator input for establishing a test threshhold level indicative of a predetermined maximum sensor impedance value and therefore a minimum sensor temperature, the output of said test comparator normally generating a GO signal so long as the value of said amplified oxygen sensor test output signals from both of said first and second monitoring amplifiers are less than said established test threshhold voltage level, said test comparator output being responsive to the value of said amplified oxygen sensor test output signals from either of said first and second monitoring amplifiers being greater than said established test threshhold voltage level for rapidly switching to generate an INHIBIT signal, the signal at the output of said test comparator indicating whether said first and second comparator output state signals indicative of a measured lean or rich air-fuel ratio in said exhaust system are sufficiently reliable to be used for generating one or more of said predetermined control signals.

An oxygen sensor signal conditioner system wherein the oxygen sensor generates a signal voltage, said system comprising:

• means for comparing the oxygen sensor signal voltage level with a first threshold voltage level indicative of a stoichiometric air-fuel ratio for generating a first signal indicative of one of either a rich or a lean air-fuel ratio whenever the sensor signal voltage is less than said first threshold level and for generating a second signal indicative of the other one of said rich or lean air-fuel ratio whenever the sensor signal voltage is greater than said first threshold level;
• pulsed current source means for periodically testing the impedance of an oxygen sensor and operable to generate a pulsed voltage signal in response thereto which pulsed voltage signal is proportional to the impedance of the oxygen sensor;
• means for generating a second threshold voltage level which level is higher than either said first or second signal voltages;
• and
• means for comparing said pulsed voltage signal and said second threshold voltage level for generating an oxygen sensor inhibit signal whenever said pulsed voltage signal is greater than said second threshold voltage level.

In an internal combustion engine system wherein at least one oxygen sensor is operatively disposed in the exhaust system for supplying feedback information for controlling the air-fuel ratio of the system, an improved method of accurately distinguishing between oxygen sensor output signals indicative of a rich air-fuel mixture, a lean air-fuel mixture, and those due to high internal oxygen sensor impedance which often occur when said oxygen sensor is relatively cold, said method comprising the steps of:

• monitoring the relative percentage of uncombined oxygen existing in the exhaust system as a measure of the actual air-fuel ratios of said internal combustion engine system;
• generating a first oxygen sensor output signal when a "lean" air-fuel condition is detected and a second oxygen sensor output signal when a "rich" air-fuel ratio is detected;
• periodically generating a test current and transmitting said test current to said oxygen sensor for measuring the internal impedance thereof as an indication of the reliability of said oxygen sensor output signals;
• generating an oxygen sensor test output signal in response to the receipt of said test current, said oxygen sensor test output signal being indicative of the measured internal impedance of said oxygen sensor;
• amplifying said oxygen sensor output signal and said oxygen sensor test output signal;
• comparing said oxygen sensor output signals to a predetermined reference voltage level representing a stoichiometric air-fuel ratio and generating a first comparator output when said first oxygen sensor output signal proves to be less than said predetermined reference voltage to indicate a measured air-fuel ratio which is lean of stoichiometric and for generating a second comparator output signal when said second oxygen sensor output signal proves to be greater than said predetermined reference voltage indicating a measured air-fuel ratio which is rich of stoichiometric;
• comparing said oxygen sensor test output signal with a predetermined inhibit threshhold voltage level representing that oxygen sensor impedance and hence that oxygen sensor operating temperature below which said comparator outputs indicative of rich or lean measured air-fuel ratios are to be assumed unreliable and for generating an INHIBIT signal whenever said oxygen sensor test output signal is greater than said predetermined inhibit threshhold voltage level indicating that the inherent impedance of said oxygen sensor is too great to permit a reliable measurement of the air-fuel ratio existing in said exhaust system and for generating a GO signal whenever said oxygen sensor test output signal is less than said predetermined inhibit threshhold voltage level indicating that all first and second oxygen sensor output signals are reliable and may be used for generating control signals for selectively increasing and decreasing the air-fuel ratio in said engine system.