Patent Publication Number: US-2002005060-A1

Title: Circuit arrangement for interference-free signal evaluation

Description:
[0001] The invention relates to an evaluation circuit for the interference-free evaluation of signals, in particular output signals of knocking sensors, as generically defined by the preamble to the main claim.  
       PRIOR ART  
       [0002] It is known for signals which are carried for instance by a sensor on to an evaluation unit, to be transmitted between the sensor and the evaluation unit with the aid of a shielded line. The shielding protects the signal transmission against interference and adulteration of the signal. Shielding a line is relatively expensive, however, and still is no guarantee that no signal will not be adulterated after all.  
       [0003] German Published, Non-Examined Patent Application DE-OS 42 22 475 therefore proposes an arrangement for blanking out interference signals on signal lines, which assures that an interference-free original signal can be evaluated. In this known arrangement, the transmission of an original signal is done in a first signal line, and the transmission of an inverted or differentiated original signal is done in a second signal line. The previously inverted or differentiated signal is reinverted or integrated again at the end of the signal line. After that, this signal is linked in an AND gate with the unaltered transmitted signal, and as a result an interference-free original signal is obtained at the output of the AND gate. If a ground cable is additionally carried along in the cable between the sensor and the evaluation arrangement, then overvoltage protection can be achieved by incorporating Zener diodes between the signal lines and ground.  
       ADVANTAGES OF THE INVENTION  
       [0004] The circuit arrangement of the invention for the interference-free evaluation of signals, having the characteristics of claim  1 , has the advantage over the prior art that economical signal evaluation which is nevertheless secure against interference can be accomplished, and common-mode interference is largely avoided. This advantage is attained by constructing the signal evaluation circuit using differential circuitry in what is known as the “switched-capacitor technique”. It is especially advantageous that the operating mode of the evaluation circuit can be selected to be either symmetrical or asymmetrical, or in other words that the sensors whose output signals are to be evaluated can be connected to the evaluation circuit in such a way that in the asymmetrical operating mode, a connection with ground reference is possible, while in the symmetrical operating mode a potential-free connection is possible. Because of the advantageous selectability of the operating mode, the circuit arrangement of the invention can be operated both with conventional sensors, such as conventional knocking sensors with shielded connection lines, and with sensors with unshielded (twisted-pair) lines. This combination of the circuit arrangement with sensors, such as knocking sensors with unshielded lines, is an advantageous way of enabling economical and yet interference-proof realization of a knocking evaluation system.  
       [0005] A particular advantage of the circuit arrangement of the invention is attained by the capability of defining the operating mode from outside. This definition can be done for instance via a programmable control pin. It is thus possible to adapt the evaluation circuit from outside to the desired requirements, that is, for the symmetrical or asymmetrical operating mode, and as a result if there are four signal inputs, for instance, either four sensors are connectable asymmetrically or in other words with ground reference, or two sensors are connectable symmetrically or in other words in potential-free fashion.  
       [0006] The further advantages of the invention are attained by the provisions recited in the dependent claims.  
       DRAWING  
       [0007] One exemplary embodiment of the invention is shown in the sole figure of the drawing and is explained in further detail in the ensuing description. 
     
    
    
     DESCRIPTION  
     [0008] In the drawing, an exemplary embodiment of the invention is shown in which the output signals of a knocking sensor K 1  are to be evaluated. The knocking sensor K 1  has two connection terminals A 1 , A 2 , by way of which the knocking sensor K 1  is connected to the signal processing circuit S. Between the knocking sensor K 1  and signal processing circuit S is a passive coupling network. This passive coupling network includes a capacitor CL, which is located between the connection terminals A 1 , A 2  of the knocking sensor K 1 , and two parallel-connected capacitors C 1   a  and C 2   a.  The resistors R 1   b,  R 2   b  are located between these two capacitors. The capacitor C 1   a  is also connected to the input E 1  via a resistor R 1   a,  ad the capacitor C 2   a  is connected to the input E 2  of the signal processing circuit S via the resistor R 2   a.  The connection between the two resistors R 1   b  and R 2   b  leads to a reference voltage input, to which the voltage Ur is applied.  
     [0009] The signal processing circuit S includes a differential input amplifier V, which is connected to the two input E 1  and E 2 . Downstream of the amplifier V are signal processing stages, not shown, which for instance include at least one bandpass filter and/or one rectifier and/or one integrator and are for instance also designed using differential circuitry.  
     [0010] These signal processing stages are followed by an output stage A, which is capable of converting the differential signal into an asymmetrical signal with ground reference and makes this signal available for further evaluation via an analog/digital converter of a microcomputer μc.  
     [0011] In the exemplary embodiment shown in the drawing, the unshielded terminals of a knocking sensor K 1  are connected potential-free, for instance via twisted lines, to the two inputs E 1 , E 2  of the signal processing circuit S. The input stage of this evaluation circuit is embodied, for the sake of suppressing common-mode interference, as a differential amplifier stage, and in particular as a switched-capacitor amplifier with multistage-adjustable amplification. The downstream signal processing stages can also be designed using differential circuitry. This is true particularly for band pass filters, rectifiers or integrators. The embodiment employing the switched-capacitor technique allows an economical space-saving integration of the requisite analog functions in semiconductor processors, for instance using HCMOS technology.  
     [0012] Since the output stage of the signal processing circuit can convert the differential signal into an asymmetrical signal with ground reference that is then finally supplied to the microcomputer, various signal evaluations can be achieved. In a knocking detection system that includes a plurality of knocking sensors, such as two knocking sensors, which are each connected via two inputs to the signal processing circuit, these two knocking sensors can be connected symmetrically or in other words in potential-free fashion. In this mode, the knocking sensor signals are then processed fully differentially.  
     [0013] If conversely a signal processing circuit with four inputs is used, so as to evaluate up to four knocking sensors, an asymmetrical operating mode is selected, in which the knocking sensors are connected with ground reference to the signal processing circuit. In this mode, a fixed reference voltage Ur generated in the signal processing circuit serves as a reference voltage for the differential signal processing.  
     [0014] If the signal processing circuit has an additional externally accessible control pin, then the operating mode can be programmed from outside in each case via this control pin. Accordingly the operating mode can be chosen from outside, so that with the same number of inputs and the same signal processing circuit, either two knocking sensors can be connected potential-free, or up to four knocking sensors with ground reference can be connected, and suitable signal processing can be done in one or the other operating mode.  
     [0015] By the choice of operating mode, sensors with unshielded (twisted-pair) lines between the sensor and signal processing, or also sensors with shielded connection lines, can be connected. The combination of the circuit arrangement, which can be constructed as an evaluation component, with knocking sensors with unshielded lines makes a simple, economical and yet interference-proof realization of a knocking evaluation system possible. In the exemplary embodiment, an evaluation of knocking sensor signals is described, but the use of the circuit arrangement of the invention is not limited to knocking sensors; instead, it can also be employed in conjunction with the evaluation of other sensors.