The present invention relates generally to a circuit arrangement used as an interface for a sensor, in particular, for a pumped reference oxygen sensor used in combination with a combustion engine.
Oxygen sensors are particularly used in combination with combustion engines using controlled catalytic converters. The use of catalytic converters in cars started much earlier in the USA in comparison with Europe. However, innovation cycles are often slower in the US automotive technology. Therefore, often older technology is used for longer periods in the USA as compared to European countries. For example, the binary lambda oxygen sensor which is used to regulate the gasoline mixture in a combustion engine, comprises the negative terminal being electrically coupled with the sensor housing in embodiments of the first generation. The reasons for this connection relates to an easier and cheaper construction. However, this construction also results in an electrical coupling between the sensor housing and the engine through the fixture of the sensor within the muffler arrangement. This connection is disadvantageous because shifted ground potentials within a motor vehicle. For example, the engine ground is usually more negative than the ground of the motor control unit. This potential shift is due to switching of high load currents, e. g., 10 . . . 15A, and the inner resistance of the ground wires, e. g. 20 . . . 40 mΩ. Typical voltage shifts are in the range of −300 . . . +600 mV. In addition, any on/off switching of high loads causes voltage overlay peaks of up to multiple 100 m Vss. These ground distortions and overlay voltages can cause serious problems with respect to evaluation of the respective sensor signals and may render the actual sensor signals completely useless.
Oxygen sensors according to newer technology are, therefore, fully isolated and, thus, avoid any electrical coupling with the engine. However, oxygen sensors of the first generation are still widely used, in particular, in the United States because of their lower manufacturing costs. Therefore, modem motor control units must be able to interface with these kind of sensors which are not fully isolated. To this end, specific interface circuits used to be available which allowed for the evaluation of sensors whose housing is electrically coupled with the engine and thus with the motor vehicle ground.
FIG. 4 shows an example of such a known circuit. A sensor 400 which is connected with the engine ground is coupled through resistors 430 and 415 with the input terminals of the interface circuit proper. Capacitors 425 and 435 are coupled between the input terminals of the interface circuit and the interface ground. One of the sensor connections is furthermore coupled through resistor 410 with a supply voltage Vcc and through capacitor 420 with the interface ground. The interface circuit comprises a first switch 440 which is coupled with the input terminals. The switch output is coupled through a capacitor 445 with the input of a second switch 460 and through capacitor 450 with the input of a third switch 455. The first output of switch 455 is connected with the supply voltage Vcc. The second output of switch 455 and the first output of switch 460 are coupled with the interface ground. The second output of switch 460 is connected to the inverting input of an integrator consisting of operational amplifier 475 and capacitor 470 in its feedback loop. A fourth switch couples the inverting input of the integrator with either the supply voltage or the output of the integrator. The output of the integrator is coupled with the first input of a fifth switch 495 which is controlled by the output signal of a comparator 405. The second input of switch 495 is coupled through resistor 490 with the supply voltage. The first input of comparator 405 is coupled with the second interface input terminal and the second input of comparator 405 receives a voltage signal being equal to half the supply voltage. Furthermore, a timing circuit is provided which generates control signals for switches 440, 455, 460, and 480.
All switches are implemented as CMOS switches. Capacitor 445 is used as a transfer capacitor for eliminating the common mode of the input signal. To this end, the capacitor is switched in a first position between interface ground and the second input terminal and in a second position between the inverting input of operational amplifier 475 and the first interface input terminal with a high frequency. Thus, the CMOS switch operates like a resistor. Capacitor 465 operates as a feedback capacitor in a similar way. These two capacitors operating as resistors form together with the operational amplifier/integrator an inverting amplifier. The bias capacitor 450 together with CMOS switch 455 are used to generate a small bias current which is fed to the sensor 400 and which will not influence the measurement when the sensor is in operating mode, i.e. the sensor has low resistance.
One of the disadvantages of this circuit arrangement is that the CMOS switches at the input of the circuit must comply with a high standard. This renders this circuit expensive and interference-prone. In addition, this circuit must withstand the required negative input voltages. thus, additional protective measurements, such as, isolation and charge pumps (not shown) must be provided. Furthermore, the CMOS switches must be able to tolerate a relatively high input voltage of up to 12V in case of a short circuit of the sensor. This is particularly difficult because the supply voltage is usually only 5V. Integrated circuits using this technology need furthermore additional isolation/separation measurements if more than one interface circuit is provided to prevent any cross over influence of the channels and to prevent a latch-up.
The bias current generated by switch 455 and capacitor 450 is used to detect a connection failure between the first input terminal and the sensor. In such a case, the bias current will overdrive the operational amplifier. A similar scenario takes place in case of a short circuit between the first input terminal and the positive terminal of the battery. The output voltage in both cases will be approximately 0V. To detect any interruption between the second input terminal and the sensor additional circuitry is necessary. This additional circuitry is shown in FIG. 4 with resistor 410, capacitor 420, comparator 405 and CMOS switch 495. During normal operation the current generated by resistor 410 will flow to the engine ground through the electrical coupling of the housing of sensor 400 and will not influence the measurement. However, in case of an interruption of this connection the potential at the second input terminal will raise to the supply voltage, for example, 5V. In case of a normal operational temperature of the sensor, i.e. low resistance of the sensor, the operational amplifier will be driven to its positive limit, e.g., 5V. However, in case of a cold sensor (during the start up phase of the engine) the sensor will have a high resistance and the internally generated bias current will put the circuit into an undefined state. To prevent such a state, comparator 405 will compare the potential at the second input terminal with Vcc/2. If the potential is above this threshold, comparator 405 will control switch 490 to select a constant output voltage to signalize this error.
As described above, the prior art interface circuit is highly cumbersome and requires additional evaluation of the generated output signal. Furthermore, this type of interface circuit is not in production anymore and, thus, not available for new construction which specifies the use of a non-isolated sensor.