Abstract:
In a method for ascertaining an offset of an output signal of an evaluation circuit integrated in a sensor, the output signal is supplied to a control unit which analyzes the output signal, and a reset of the evaluation circuit for the adjustment of the output signal to a base level is performed prior to each measuring operation of the sensor, and the control unit ascertains the offset of the output signal of the evaluation circuit upon an occurrence of a start signal output by the evaluation circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for ascertaining an offset of an output signal of an evaluation circuit integrated in a sensor, preferably of a combustion chamber pressure sensor installed in a combustion chamber of an internal combustion engine, in which the output signal is supplied to a control unit that analyzes the output signal, and a reset of the evaluation circuit for an adjustment of the output signal to a basic level is performed prior to each measuring operation of the sensor. 
         [0003]    2. Description of the Related Art 
         [0004]    Sheathed-element glow plugs including a pressure sensor are in use today, which measures the cylinder pressure during the combustion of a fuel-air mixture in an internal combustion engine. Such sheathed-element glow plugs including the pressure sensor have an evaluation circuit, which is connected downstream from the pressure sensor in the sheathed-element glow plug. The pressure sensor operates on the basis of a piezoelectric measuring method, in which the detected pressure is converted into an electrical output signal, which is output and detected by an engine control unit. Because of the piezoelectric measuring method, the evaluation circuit connected downstream from the pressure sensor must be reset to the zero level with the aid of an internal algorithm prior to each combustion process, in order to avoid drift of the output signal. 
         [0005]    As a result of the input-side electrical connection of the evaluation circuit of the pressure sensor, it can happen that the output signal exhibits an abrupt change to higher or lower voltage values following an internal reset. These abrupt changes are attributable to interference signals at the input of an integrator stage of the evaluation circuit at the instant immediately after the integrator resetting. 
         [0006]    The signal jumps following the reset result in an offset shift of the entire pressure signal, and thus also of the output signal of the pressure sensor supplied as voltage, in the following cycle. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention is therefore based on the objective of providing a method for ascertaining an offset of an output signal of the evaluation circuit integrated into a sensor, in which the offset shift of the output signal following a reset is ascertained in a reliable manner. 
         [0008]    According to the present invention, the objective is achieved in that the control unit determines the offset of the output signal of the evaluation circuit as a consequence of a start signal output by the evaluation circuit. Additional computing units in the evaluation circuit of the sensor are unnecessary because the offset is determined in the control unit, which has a cost-reducing effect. The ascertaining of the offset in the engine control unit takes place without any additional hardware outlay, because the means available in the engine control unit are employed for the software-based evaluation of the offset. In this way the offset of the output signal of the evaluation circuit can easily be determined by the engine control unit. 
         [0009]    The start signal advantageously consists of a preferably brief application of a predefined level to the output signal of the evaluation circuit. To determine the offset of the output signal, the start signal is generated in the control unit and thus in an uncomplicated manner in the evaluation circuit. 
         [0010]    In one development, the predefined level of the output signal corresponds to the level of the grounded connection of the evaluation circuit. The brief and simple pull-down of the existing level of the output signal to the level of the grounded connection signals to the control unit that a reset has taken place. In this way the control unit recognizes the reset at the correct instant and is able to carry out the offset determination. 
         [0011]    In one variant, after receiving the start signal, the control unit calculates the offset in a defined time period, the time period being started immediately after the start signal is received, in particular. As a result, the offset of the output signal is calculated in the control unit in the particular time period during which the level of the output signal is connected to ground. The offset determination in the control unit disposed externally from the sensor is actively triggered solely by the sensor. 
         [0012]    In one variant, the start signal is triggered by an internal reset of the evaluation circuit. No additional circuit element is required since this reset is already provided in the evaluation circuit. 
         [0013]    In one further development, the defined time period is selected in such a way that a diagnosis of the output signal of the evaluation circuit performed in the control unit is not detected. Existing diagnoses in the control unit are therefore not adversely affected by the offset determination of the control unit. 
         [0014]    The calculated offset is furthermore employed to correct a currently ascertained output signal of the evaluation circuit. The use of this corrected output signal therefore makes it possible to determine and further process the pressure actually arising during a combustion process in the combustion chamber of the internal combustion engine at any given time. 
         [0015]    One further refinement of the present invention relates to a sensor, preferably a combustion chamber pressure sensor installed in a combustion chamber of an internal combustion engine; the sensor includes an evaluation circuit, which transmits an output signal to a control unit for analysis, and a reset of the evaluation circuit for an adjustment of the output signal to a base level is performed prior to each measuring operation of the sensor. A sensor which makes it possible to determine the offset of the output signal of the evaluation circuit in a reliable manner includes means that output to the control unit a start signal output by the evaluation circuit for ascertaining the offset of the output signal of the evaluation circuit. These means have simple circuits that require no higher intelligence comparable to a bus system. 
         [0016]    The means advantageously include a switching circuit logic, which applies the start signal to an output of the evaluation circuit. This simple switching circuit logic can easily be inserted into an evaluation circuit developed as ASIC in the course of a sample revision. 
         [0017]    In one variant, the switching circuit logic is developed as transistor. The use of a transistor allows a cost-effective development of the circuit, which can able be added to the evaluation circuit quite easily in terms of construction. 
         [0018]    In one development, a flip-flop circuit for setting a defined time period for the offset determination is connected upstream from the switching circuit logic. The use of the flip-flop circuit for actuating the switching circuit logic in the evaluation circuit of the sensor results in active signaling to the control unit, which ascertains the offset after receiving these signals. 
         [0019]    Another further refinement of the present invention relates to a control unit for ascertaining an offset of an output signal of an evaluation circuit integrated into a sensor, preferably of a combustion chamber pressure sensor installed in a combustion chamber of an internal combustion engine, which analyzes an output signal of the evaluation circuit. In a control unit that is able to ascertain an offset of the output signal of the evaluation circuit in an uncomplicated manner, means are provided which ascertain an offset of the output signal of the evaluation circuit after detecting a start signal output by the evaluation circuit of the sensor. This has the advantage that the offset determination function can be integrated into the software of the control unit without any need to provide additional hardware elements. 
         [0020]    In one further specific embodiment, the means determine the offset in a time period that is specified by a duration of the occurrence of the start signal. The method of operation of the control unit is controlled by the active output of the reset by the evaluation circuit. The control unit requires no additional intelligence in this procedure. 
         [0021]    The means, preferably a microprocessor, in particular correct a currently measured output signal of the evaluation circuit with the aid of the offset and supply the corrected output signal to the further evaluation. Because of this correction, the actual value of the pressure in the combustion chamber of the internal combustion engine measured by the sensor is always available for the further evaluation. 
         [0022]    The present invention permits numerous specific embodiments. One of these embodiments will be explained in greater detail with reference to the figures shown in the drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  shows a basic representation of the system of a sheathed-element glow plug in a combustion engine. 
           [0024]      FIG. 2  shows an output signal of the evaluation circuit over a crankshaft angle range following an internal sensor reset. 
           [0025]      FIG. 3  shows a schematic representation of the evaluation circuit of the sensor. 
           [0026]      FIG. 4  shows a schematic sequence of the signal conditioning in a control unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    Cold combustion engines, especially diesel engines, require starting assistance in igniting the fuel-air mixture introduced into the diesel engine at ambient temperatures of &lt;40° C. Glow systems made up of sheathed-element glow plugs, a glow-time control unit and glow software stored in an engine control unit are used as starting aids. 
         [0028]      FIG. 1  schematically illustrates such a glow system  1 . Sheathed element glow plug  2  projects into combustion chamber  3  of diesel engine  4 . On one side, sheathed-element glow plug  2  is connected to glow-time control unit  5 , and on the other side it leads to a vehicle system voltage  6 , which actuates sheathed-element glow plug  2  using a nominal voltage of 4.4 or 7 Volt, for example. Glow-time control unit  5  is connected to engine control unit  7 , which in turn leads to a diesel engine  4 . In addition, a combustion chamber pressure sensor  8  is integrated into sheathed-element glow plug  2 , which therefore likewise projects into combustion chamber  3  of diesel engine  4  and is connected to engine control unit  7 . Combustion chamber pressure sensor  8  operates according to a piezoelectric measuring method, in which the pressure detected by combustion chamber pressure sensor  8  is converted into an electrical output signal that is detected by engine control unit  7 . 
         [0029]      FIG. 2  illustrates the characteristic of combustion chamber pressure p measured by combustion chamber pressure sensor  8  over a crankshaft angle range. Combustion chamber pressure p assumes a maximum in the combustion of the fuel-air mixture taking place in combustion chamber  3  of diesel engine  4  and then flattens again. Prior to each new measurement, an evaluation circuit  9  performs an internal reset R in combustion chamber pressure sensor  8 , which causes an abrupt change in output signal A of evaluation circuit  9  integrated into combustion chamber pressure sensor  8 , which is illustrated in  FIG. 2  by Δp, that falsifies output signal A by this offset. 
         [0030]    Evaluation circuit  9  disposed in combustion chamber pressure sensor  8  is illustrated in  FIG. 3 . It follows combustion chamber pressure sensor  8  with a charge amplifier  10 , which in turn is connected to a monoflop  11 . A reference voltage  12  of 0.7 Volt, which is switched to ground, is present between charge amplifier  10  and monoflop  11 , in an input of monoflop  11 . Monoflop  11  leads to output  14  of evaluation circuit  9 , where output signal A is tapped off by engine control unit  7 . A transistor  13 , which is triggered by charge amplifier  10 , is situated between the output of monoflop  11  and output  14 . Charge amplifier  10  includes a reset logic and sets the output signal of evaluation circuit  9  to ground prior to each new combustion that is to be detected by sheathed-element glow plug  2  and combustion chamber pressure sensor  8  in combustion chamber  3  of diesel engine  4 . In this way the level of output signal A of evaluation circuit  9  drops for the period during which monoflop  11  is active. This brief pulldown of the level of output signal A is detected at output  14  by engine control unit  7  and evaluated as start signal for ascertaining the offset of output signal A of the evaluation circuit. During time period Δt, in which output signal A is switched to ground by transistor  13 , engine control unit  7  starts the offset detection and, simultaneously, the offset compensation. The switching to ground causes no EMC interference whatsoever, since the level of output signal A at the switching instant is already very low (700 mV or less). 
         [0031]    Because of the introduction of switching transistor  13  in evaluation circuit  9 , which is developed as ASIC, output signal A of evaluation circuit  9  is related to ground in an uncomplicated manner. This switching instant takes place directly upon reset R of the integrator stage included in charge amplifier  10 . Reset R is followed by defined time interval Δt, such as in the range of a few 10 μs. This time interval Δt is defined by monoflop  11 , which is likewise triggered by internal reset R. 
         [0032]    The abrupt change of output signal A to ground generates a flank of output signal A at output  14  of evaluation circuit  9 , which starts a counter  16  in engine control unit  7  ( FIG. 4 ). Counter  16  counts a predefined number of signal values of output voltage A of evaluation circuit  9 , which are written into an array  17 . If the required number of signal values has been reached in array  17 , then an average value is formed from the signal values stored in array  17  in an offset calculation  18  and compared with a nominal offset of 0.7 Volt, for example. The difference between the calculated average value and the nominal offset forms offset  19  of output signal A of evaluation circuit  9  that is caused by the abrupt change in output signal A. In point  20 , this offset  19  is subtracted from output signal A currently applied at output  14  of evaluation circuit  9 . Because of this subtraction, a corrected output signal A k  is calculated in engine control unit  7 , which corresponds to the pressure in combustion chamber  3  of diesel engine  4  actually measured by combustion chamber pressure sensor  8 . Corrected output signal A k  is then forwarded to the further evaluation. 
         [0033]    By selecting the suitable time interval At of the monoflop, which simultaneously corresponds to the offset evaluation period of engine control unit  7 , it is possible that there will be no need to carry out a signal diagnosis “signal below minimum” in engine control unit  7 . The proposed solution is therefore application-neutral. Diagnoses available in engine control unit  7  anyway remain unaffected by this method. 
         [0034]    Because of reset R executed by evaluation circuit  9 , engine control unit  7  detects the correct instant for determining the offset. A new offset is ascertained during time interval Δt, and this new offset is subtracted from the actual output signal A for the following remaining cycle.