Abstract:
A detector circuit for detecting the presence of a remote capacitive sensor having at least two terminals connected via a protection circuit that includes one or more capacitors, the detector circuit comprising: a current supply for changing the charge on the sensor and the protection circuit, a detector for measuring the voltage on one or more of the terminals; wherein the presence of the sensor is determined by changing the charge on the capacitive sensor and the one or more capacitors of the protection circuit in a predetermined manner such that the voltage measurement on the one or more terminals when the sensor is present is significantly different than when the sensor is absent.

Description:
FIELD OF THE INVENTION 
     This invention relates to improvements in or relating to diagnostics of a capacitive sensor, for example a knock sensor in an engine control system. 
     BACKGROUND OF THE INVENTION 
     Abnormal combustion, more commonly known as knock, is a limiting factor on power generation in an internal combustion engine. The name knock refers to the noise that is transmitted from the collision of multiple flame fronts and the increased cylinder pressure that causes the pistons, connecting rods and bearings to resonate. A knock sensor detects engine knock and sends the voltage signal to an engine control unit (ECU). The ECU uses the knock sensor signal to control timing of the engine. Engine knock generally occurs within a specific frequency range and the knock sensor is located on the engine block, cylinder head or intake manifold and is able to detect that frequency range. 
     Detecting the presence of a capacitive sensor, such as the knock sensor, at the end of a capacitive cable, through an engine control unit (ECU) that is protected by an electromagnetic compatibility (EMC) capacitor is a long existing problem. This is because the knock sensor has high impedance and does not generate a very strong output signal, which means that the sensitivity of the ECU is critical. In addition, due to legislation relating to emission controls in engines there is a legal requirement to know if the knock sensor is working or not. Knock is a relatively quiet noise which can be easily confused with other engine noises and the determination of whether the knock sensor is working or not is made particularly difficult by this. Another problem that exists is that the capacitance of the knock sensor is relatively low and is often hard to measure due to the values of the EMC capacitor and the capacitance in the cabling. 
     A number of different methods have been proposed to overcome the problem of detecting the presence of the capacitive sensor. One relates to diagnostics using threshold detection as is described in for example EP 0720007 (Bosch). In this method the normal output of the sensor is characterised with respect to engine speed, and output higher or lower than expected is taken to indicate a fault with the knock sensor. The issue with this is that the point of the knock detection system is to minimise knock, which therefore means the normal condition is nascent or no knock and hence low output from the sensor. Since the ECU input is high impedance it tends to be subject to significant pickup of electrical noise. Thus low noise from a connector sensor and electrical pickup by the wires when the sensor is disconnected can be indistinguishable. 
     A second method of overcoming the problem that has been disclosed is a diagnostic using self resonance as is described in U.S. Pat. No. 5,421,191 a (Chrysler). In this method the sensor is excited with pulses matching the sensor&#39;s predetermined resonance and the amplitude of these is measured after excitation is removed. This method requires knowledge of the resonant frequency of the sensor, wiring and load combination as installed, which can vary considerably. If the driving frequency is wrong then no resonance will be induced. 
     A third method of overcoming the problem that has been disclosed is a diagnostic using capacitor oscillation, as is described in EP 0704706 (Marelli). In this scheme the sensor is used as part of the oscillator. If the sensor is connected then a particular frequency should be presumed. This scheme suffers from tolerance issues. The sensor is around 800 pF and the connector connecting it might be 1 m to 5 m of coaxial cable at 300 pF/m. The EMC protection capacitors are typically between 470 pF and 1000 pF. As these capacitor are usually low tolerance it becomes difficult to identify whether the sensor itself is connected due to the greater amount of low tolerance capacitance in the connection to the sensor. 
     One object of the present invention is to overcome at least some of the problems associated with the prior art. In addition, a further object is to provide a diagnostic of capacitive sensing by charge sharing. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and an apparatus as described in the accompanying claims. 
     In order to overcome the problems of the prior art the sensor diagnostics components make use of the same components used for bias. This has the added advantage of minimising the overall number of external components required. This is achieved by sourcing current from the Analogue to Digital Converter (ADC) pin through the network of external components, including the knock sensor. The ratio of connected capacitances results in a ratio of voltages that can be read using the ADC. Connection of the knock sensor is indicated by presence of its capacitance. The manner in which this can be measured is accurate and consistent. The problems associated with the prior art relating to the ability to “see” the knock sensor are overcome by not looking at absolute values but instead looking at relative values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example to the accompanying drawings, in which: 
         FIG. 1  is a circuit diagram in bias state of the diagnostics circuit for a knock sensor in accordance with one embodiment of the invention, given by way of example, 
         FIG. 2  is a circuit diagram in discharge state of the diagnostics circuit knock sensor in accordance with one embodiment of the invention, given by way of example, 
         FIG. 3  is a circuit diagram in charge share state of the diagnostics circuit for a knock sensor in accordance with one embodiment of the invention, given by way of example, and 
         FIG. 4  is a graph of the simulation results comparing the existence of the sensor and no sensor in accordance with one embodiment of the invention, given by way of example, 
         FIG. 5  is example of a pin pull resistor configuration in accordance with one embodiment of the invention, given by way of example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1  a knock sensor  100  is shown. In another embodiment, the knock sensor  100  can be a remote capacitive sensor, a pressure sensor, or the like. The knock sensor  100  is generally a piezoelectric element, which electrically tends to have the appearance of a capacitor. The knock sensor is connected to an engine control unit (ECU)  102  through a capacitive cable  104 . Typically the knock sensor will have a capacitance of 800 pF and the cable will have a capacitance of the order of 100 pF. The first elements within the engine control unit  102  are two EMC protection capacitors C 2  and C 3 . These capacitors C 2  and C 3  are each of the order of about 1000 pF. C 2  and C 3  are protection capacitors for the engine control unit. Two resistors R 1  and R 2  of about 20 KOhm are connected in series with the capacitors C 2  and C 3 , these resistors limit current and protect an analogue to digital converter ADC  106 . Further capacitor C 4  is connected in parallel with the knock sensor. This capacitor C 4  is of the order of 100 pF and has the effect of the low pass anti-aliasing filter which removes high frequency noise from the signal received from the knock sensor. The circuit includes sensor bias circuitry shown generally as  108 , the function of this will be described in greater detail below. The sensor bias circuitry is then connected to the analogue to digital converter (ADC)  106 . 
     In the circuit shown the sensor bias circuitry is designed to bias the ADC  106  at the voltage between 5V and 0V depending on the values of the resistances. In this case because the resistances are all equal the bias voltage on the ADC is 2.5V. This has an advantage in that the ADC does not need to operate at negative voltages and can thus be less complex than would otherwise be the case. The ADC in this case is a differential ADC which enables output measurements from either output pin or a differential measurement from both output pins. The ADC could be replaced by a differential amplifier and an analogue detection circuit if required. This circuit also includes a number of N and P MOSFETs (not shown) which are used in the knock sensor diagnostics and will be described in greater detail below. 
     In order to determine whether the knock sensor is connected or operating the circuit is first placed in a discharged state. This will be described now with reference to  FIG. 2 . This is achieved by forcing both pins (ANP and ANM in the figures) to ground (0V). The knock sensor, cable and EMC capacitors are all discharged by ANP and ANM being forced to ground. This grounding is achieved by connecting two MOSFETs  110  and  112  between the pins and ground. In this discharged state the output on the ADC will be zero, this can be measured either singly on each pin or differentially. The manner in which the pins are forced to ground is not limited to the example shown in the diagrams, but may be any other appropriate means. 
     It will be appreciated that the sensor biasing circuitry is included in  FIG. 2 , although the detail is not shown. In the same way the MOSFETs  110  and  112  are included in  FIGS. 1 and 3  even though they are not shown therein. 
     After having been discharged the diagnosis circuit, the circuit is then pulled up to maximum voltage (5V) on one side. In the diagram this is shown on the ANP side of the circuit but could equally well be on the ANM side of the circuit. Similarly it would be possible to pull up one side of the circuit and then the other in a sequential manner. The ANP side is pulled up by connecting a MOSFET  116  to the 5V supply. This places the circuit in a charge share state and due to the symmetry of the circuit the output pin of the ADC can be read out to determine whether the knock sensor is connected and/or operating. 
     In this state R 1 , R 2 , and C 2  have no effect. R 1  and R 2  cancel out due to the fact that once the circuit is charged up there is no current flowing. As C 2  has one side connected to ground and one side connected to 5V the capacitance is irrelevant to the voltage on ANM. 
     The voltage on C 3  is charge shared through the knock sensor and the cable. In terms of capacitance, the capacitance of the knock sensor, cable and capacitor C 4  are in parallel with each other. These three capacitances are on the high side of capacitor C 3 . The values of capacitance for the knock sensor, cable and C 4  are substantially similar to the capacitance of C 3 . This means that once this circuit is stable the voltage on pin ANM of the ADC is going to be approximately 2.5V. The value of capacitance of the knock sensor is significantly greater than that of the cable and C 4 . Thus, if the knock sensor is not there or faulty there will be a difference between the capacitance of the knock sensor, cable and C 4  and that of C 3 . The effect of this is a larger voltage on C 4  than on C 3 , which means a difference output on pin ANM at the ADC. To make the readings of the circuit in this charge shared state the input to the ADC is preferably measured in single ended mode, although differential mode may also be used. 
       FIG. 4  shows a graph of voltage against time of the output from the ADC at the different stages illustrated in  FIGS. 1 ,  2  and  3 . The graph shows the three stages with the knock sensor working in one example and the knock sensor not working in another. The first stage is the sensor bias with knock signal and noise, this stage lasts between zero seconds and 200 μs in the graph, but on a vehicle would be the normal case for a sensor in use. The voltage is at about 2.5 V throughout this stage as indicated above. The second stage is the discharged phase and last between 200 μs and 250 μs in this embodiment. At this point the voltage goes down towards zero. The third and final stage is the charge sharing phase this lasts from about 250 μs to 500 μs. In the graph shown there are two distinct lines, one at the voltage of about 2.5 V and the other at a lower voltage of approximately 1.8 V. The difference in the voltage measurements are sufficiently significant to be measured on the ADC output pin. The output at 2.5 V is that achieved when the knock sensor is working and the overall capacitance of the wiring and sensor is of the order of 1000 pF. The lower voltage output is achieved when the knock sensor is not working or not connected in this case the overall capacitance is of the order of 200 pF. 
     This invention has a significant advantage over the prior art, in that it provides a definitive means of determining whether the knock sensor is connected or not. The difference in voltage on the output pin of the ADC is easily measurable as can be seen in  FIG. 4 . As the difference in voltage is at a reasonably substantial level it is not necessary to use expensive, sensitive equipment to measure the different voltages on the output pin. 
     As has been previously mentioned the knock sensor diagnostics circuit includes a number of MOSFETs which are required at various stages in the process. To provide a high degree of adjustability a pin pull resistor configuration as is shown in  FIG. 5  may be used. The configuration includes an input pin through which the knock sensor signal is received  500 ; a number of resistors  502  a, b, c and d; number of level shifters  504 ; and a plurality of MOSFETs  508 . The MOSFETs include certain n-type MOSFETs and certain p-type MOSFETs. This circuit shows a number of resistors and switches although not all are required to run the diagnostic. They are included to make the knock sensor diagnostics circuit adjustable in terms of different knock sensors in different engines. The manner in which the resistors and MOSFETs are connected is controlled through the level shifters from the digital logic of the engine control unit and is adjusted according to the engine and knock sensor requirements. 
     It will be appreciated that the example shown is merely an example and many variations of the circuit may exist which fall within the scope of this invention. For example, different values of capacitance, resistance or circuit design can be used. It is a common feature of them all that the resulting voltage on the ECU pins is substantially and measurably different when the knock sensor is there than when it is not.