Abstract:
A device for detecting an angular position of a rotating object comprises a sensor that provides a sensor output signal dependent at least on the angular position (φ) of the object, and an evaluation circuit that receives the sensor output signal and a trip threshold and provides a detection signal when the level of the sensor output signal crosses the trip threshold. The trip threshold has levels in a range of values in which the level of the sensor output signal varies rapidly as a function of the angular position of the object.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention relates to a device and a method for detecting an angular position of a rotating object, especially for detecting the time at which the rotating object passes a given angular position.  
         [0002]     Motor vehicle engines often use sensors to detect the angular position of a crankshaft or a valve position, in order to control the combustion in a cylinder of the motor vehicle engine with reference to the detected position. The pollutant content in the exhaust gases that result from the combustion process depends on the time of ignition. Accurately detecting the position allows the combustion process to occur efficiently, resulting in less pollutants.  
         [0003]     Numerous devices are known that can be used for detecting an angular position. There are generally two groups of detection errors that occur in such a device for detecting angular position. These are, on the one hand, constant angular errors that are associated with inaccuracies in the positioning of such a device in the vicinity of the object whose angular position is to be detected, or the positioning of the components of the device relative to one another. The second type of error are those that vary with time and may be caused stochastically, as a function of speed of rotation, or thermally. These errors are the result of a number of different causes. In present-day angular position detection devices, inductive magnetic sensors or Hall sensors that interact with magnets attached to the object to be detected are generally used. These sensors produce a sensor output signal that changes constantly with the position of the magnet to be detected. The amplitude of the output signal depends strongly on the distance between the object to be detected and the sensor. Changes in the speed of rotation of the object change both the frequency and the shape of the sensor output signal, so that in general no unequivocal conclusion can be drawn about a definite angular position from the level of the sensor output signal. Two techniques are known for correcting this problem. The first depends on the use of an automatically regulated amplifier to which the sensor output signal is fed and that produces from this a signal with a given fixed amplitude, in which case it is then assumed that a certain value of the amplifier-controlled signal always corresponds to the same angular position of the object to be detected. This method has the drawback that the regulation reacts to disturbances or eccentricity of the emitter wheel and introduces additional measurement errors in such situations.  
         [0004]     The second known technique depends on detecting each signal peak of the sensor output signal, and determining from them the angular position of the object to be detected. It is assumed here that the sensor output signal always reaches its maximum level—even when this value fluctuates—at the same angular position of the object to be detected. The problem arises here that the output signals of the customary inductive magnetic sensors or Hall sensors are more or less sinusoidal and consequently have very flat signal peaks, which are very sensitive to noise and interference. Therefore, there is a need for a device and a method for detecting an angular position of a rotating object that make possible precise angular detection without automatic amplification control and without detecting the peaks of the sensor output signal and thus avoid the drawbacks of the aforementioned arrangements.  
       SUMMARY OF THE INVENTION  
       [0005]     While the reference point chosen for evaluating the sensor output signal in the second arrangement described above is the value range in which the dependence of the sensor output signal on the angular position of the object to be detected reaches a relatively low value (e.g., a minimum), in the method according to an aspect of the invention a value range of the sensor output signal in which it varies rapidly with angular position is chosen for a trip threshold, exceeding which establishes that the object has the angular position to be detected. What is defined as a rapid change as a function of angular position can necessarily be determined by reference to the functional dependence of the detection signal level on the angular position of the object to be detected. Thus, for example, a value range with a rapid change of level can be defined as a value range that corresponds to angular positions at which the derivative of the sensor output signal with respect to angular position amounts to at least 10, 20, 30 or 50% of the maximum value of this derivative. For example, assuming that the sensor signal varies sinusoidally with the angular position of the object to be detected, which is true for a number of practical situations, then the ranges in which the level of the sensor output signal varies rapidly with angular position in each case lie in the vicinity of the angular positions at which the sensor output signal crosses its own mean value. Therefore, it is desirable to choose the threshold value in the vicinity of this mean value.  
         [0006]     Since the mean value of the sensor output signal for a given type of sensor can vary depending on the environment in which it is used, a device according to an aspect of the invention is provided with an averaging circuit in which an input receives the sensor output signal and an output produces a signal representative of the trip threshold. Such an averaging circuit makes the device according to the invention independent of shifts in the mean value of the sensor output signal caused by the component situation or by drifting.  
         [0007]     Since the sensor output signal varies with the angular position of the object, each level of the sensor output signal that is not an extreme value corresponds to two different angular positions of the object. To differentiate these two positions from one another and to detect only one of them selectively, the trip threshold is preferably defined by a first reference level, and the evaluation circuit produces the detection signal when the level of the detection signal crosses this first reference level from above, but not when it crosses it from below. A second reference level can of course be defined similarly, in which the evaluation circuit produces the detection signal when the level of the detection signal crosses the second reference level from below, and does not produce it when the level of the detection signal crosses the second reference level from above.  
         [0008]     It is preferred for the two reference levels to be defined simultaneously in the device according to the invention, since this permits a hysteresis to be generated in a simple manner that suppresses faulty multiple detections of the given angular position of the object derived from signal noise during a rotation of the object. The evaluation circuit that produces the detection signal preferably comprises a comparator with two inputs that receives the sensor output signal at a first input and produces the detection signal at its output when the levels of the signals at its two inputs cross one another, and a selection circuit for the optional input of the first or second reference level at the second input of the comparator.  
         [0009]     In a first version, the two reference levels are chosen in each case on the two sides, respectively, of a mean value of the sensor output signal. Knowing the functional relationship between the angular position of the object and the sensor output signal, the spacing of each of the two reference levels from the mean value can be chosen so that they correspond to two angular positions of the object with the same spacing from the angular position at which the sensor output signal crosses the mean value. In the case of a sinusoidal relationship between the angular position and the sensor output signal, for this purpose each of the two reference levels must have the same spacing above and below the mean value, respectively.  
         [0010]     In a refined version of the invention, the two reference levels are variable between a central level and each of two levels separated from the central level upward and downward, respectively, and the first reference level is at the central level when the sensor output signal crosses the first reference level from above and the second reference level is at the central level when the sensor output signal crosses the second reference level from below. In this way, in the one case exactly as in the other, the passage of the sensor output signal through the central level is detected and a hysteresis is nevertheless realized.  
         [0011]     There are different possibilities for providing that each of the two reference levels is at the central level at the proper time. One is a phase-shifting circuit or a differentiation circuit (with there being no difference between the two in the case of a sinusoidal relationship between the angular position and the sensor output signal) for differentiating the change in the reference levels from the sensor output signal. The differentiated sensor output signal obtained with such a circuit may be utilized to make either the first or the second reference level the same as the central level, depending on its algebraic sign.  
         [0012]     Alternatively, the evaluation circuit may comprise at least a second comparator and a switch controlled by the comparator, with the sensor output signal being fed to one input of the comparator and with the one of the two reference levels that is not fed to the first comparator at the same time being fed to the other input of the comparator, and the second comparator trips the switch over when the levels of the two input signals of the second comparator cross one another.  
         [0013]     The device is preferably also provided with an amplitude detection circuit for detecting the amplitude of the sensor output signal. Such an amplitude detection circuit can be used in particular to derive from it the two reference levels, or if the reference levels are variable., to derive their level separated from the central level.  
         [0014]     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0015]      FIG. 1  is a block diagram illustration of a first embodiment of a angular position sensing device;  
         [0016]      FIG. 2  is a plot of various signals as a function of angular position at various locations in the sensing device illustrated in  FIG. 1 ;  
         [0017]      FIG. 3  is a block diagram illustration of a second embodiment of a angular position sensing device;  
         [0018]      FIG. 4  is a block diagram illustration of a third embodiment of a angular position sensing device;  
         [0019]      FIG. 5  is a block diagram illustration of a fourth embodiment of a angular position sensing device;  
         [0020]      FIG. 6  is a plot of various signals as a function of angular position at various locations in the sensing device illustrated in  FIG. 6 ;  
         [0021]      FIG. 7  is a block diagram illustration of a fifth embodiment of a angular position sensing device; and  
         [0022]      FIG. 8  is a plot of various signals as a function of angular position at various locations in the sensing device illustrated in  FIG. 7 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]     In  FIG. 1 , the reference symbol  1  designates a rotating object, such as an emitter wheel of a crankshaft of an internal combustion engine. The emitter wheel  1  carries a magnet  2 . The detection device comprises a magnetic field sensor  3 , for example an inductive sensor or a Hall sensor, which produces a sensor output signal A. The sensor output signal A is shown in  FIG. 2  as a function of the angle of rotation p of the emitter wheel and has a curve similar to a sine function, with each period of the sine function corresponding to one rotation of the object.  
         [0024]     An amplifier  4  is connected to the output of the magnetic field sensor  3  and produces an amplified sensor output signal B. As shown in  FIG. 2 , the amplifier  2  can go to saturation at large amplitudes of the sensor output signal A. This is useful since a signal with steep, readily detected zero passages for further processing is obtained in this way, but it is not necessary for the applicability of the invention.  
         [0025]     The amplified sensor output signal B is fed to a first input of a comparator  5 . A reference level is applied to the second input of the comparator  5 , which is selected by a switch  6  from two reference levels E and F applied to its inputs. Each of the reference levels E, F is chosen so that it intersects the curve of the amplified sensor output signal B at angular positions φ 1  or φ 3 , respectively, which are at the same angular spacing before or after a zero passage φ 1 . At the time origin, which is chosen arbitrarily so that the signal B has the value of zero there, the reference level C has the high value F. The comparator  5  produces the output signal D=0. When the value of the signal B crosses the level F from below at an angle of rotation φ 0 , the output signal D of the comparator  5  trips from zero to one, and thus indicates the fact that the emitter wheel is in the position of rotation φ 0 . At the same time, the switch  6  is switched over by the change of the output signal D, so that the low reference level E is then applied to the second input of the comparator  5 . At first this changes nothing at the output signal D of the comparator. The output signal D of the comparator  5  reverts to zero only at the rotational position φ 3 , when the signal B drops below the low reference level E; the switch  6  is again switched over, and the reference level F is again applied to the second input of the comparator. When the signal B again grows beyond the level F, the device completes one operating cycle. Since the device according to the invention responds to rapid changes of the sensor output signal A, exact determination of the time at which the emitter wheel is in the position φ 0  (or φ 3 ) is possible.  
         [0026]      FIG. 3  shows a first refinement of the device from  FIG. 1 . In this refinement, an averaging circuit is connected to the output of the amplifier  4 , in this case in the form of an RC network including a resistor  7  and a capacitor  8  that connect the output of the amplifier  4  in series to ground. The potential that is reached during the operation of the circuit at a center point  9  between the resistor and the capacitor, with suitable choice of the time constant of the RC network, corresponds to the mean voltage of the amplified sensor output signal B. This center point  9  is connected to a first input of an adder  10  whose second input is connected to the output of the switch  6  and whose output is connected to the second input of the comparator  5 . The adder  10  thus superimposes the mean voltage of the amplified sensor output signal B on each reference level switched through by the switch  6 , and thus makes the switching insensitive to drifting of the sensor  3 , of the amplifier  4 , or any imbalance of the emitter wheel  1  that would lead to a mean value of the sensor output signal A differing from zero. This ensures that despite any such drifting, the output signal D of the comparator  5  changes at the same angular position of the emitter wheel  1 .  
         [0027]     A further refinement is shown in  FIG. 4 . This configuration differs from the two shown previously in that the reference levels E, F in this case are derived from the sensor output signal A. For this purpose, the device is equipped with an amplitude detection circuit for the (saturated or unsaturated) amplified sensor output signal B. The amplitude detection circuit here is made in the form of two series circuits, each including a diode  11 ,  12 , and a capacitor  13 ,  14 , which are connected on the one side to the output of the amplifier  4  and on the other side to the center point  9  of the RC network  7 ,  8 . The two diodes  11 ,  12  are connected antiparallel, so that the capacitor  13  is charged through the diode  11  as long as the signal B has a potential below the averaged potential O at the center point  9 , and the capacitor  14  is charged through the diode  12  when the level of the signal B is above the potential O of the center point  9 . Two voltage dividers that include resistors  15 ,  16 , and  17 ,  18  are each connected in parallel to the capacitors  13  and  14 , respectively. The two voltage dividers divide the voltages applied through the capacitors  13 ,  14  in equal parts and feed them to the switch  6  as reference levels E and F. The circuit provides that each of the reference levels E, F is in a fixed ratio, set by the division ratio of the voltage dividers  15  to  18 , to the amplitude of the amplified sensor output signal B, so that even with fluctuating amplitude of this signal it is ensured that the device responds at the same angular positions φ 0  (or φ 3 ) of the emitter wheel  1 .  
         [0028]     In the configurations considered up to now, the necessity of implementing a hysteresis compels the choice of different reference levels for the switching of the output signal D to zero or to one. It would be desirable to choose these reference levels as close as possible to the average value O of B, since the closer they are to the average value O, the smaller are detection errors that can arise with fluctuating speeds of rotation of the emitter wheel  1  from the dependence of the amplitude of A on this speed of rotation. However, the smaller the difference between the two reference levels, the greater is the risk that a noise component in the sensor output signal A will lead to detection errors to which the device responds too soon or too late.  
         [0029]      FIG. 5  shows a refinement of the detection device according to the invention that permits it to reconcile these requirements that are contradictory at first glance, and to detect exactly the angular position at which the sensor output signal A crosses its mean value, and nevertheless to realize a hysteresis. With this configuration, the reference potentials fed to the switch  6 , here labeled H, G, are not constant, or constant except for the drifting already mentioned, but they are switched over during each rotation of the emitter wheel  1  between the mean potential O of the amplified sensor output signal B obtained through the RC network  7 ,  8 , and the potential E, F delivered by the amplitude detection circuit  11  to  18 , each differing from the mean potential O by the same amount upward or downward, respectively. The switching occurs with the assistance of two switches  19   920 , which in turn are controlled by the output signal K of an RS-flip flop  22 . Setting and resetting inputs of the RS-flip flop  22  are each connected through a capacitor  23  to the outputs of comparators  24 ,  25 . The inputs of the comparator  24  are wired to the amplified sensor output signal B and the reference level E; those of the comparator  25  are wired to the amplified sensor output signal B and the reference level F.  
         [0030]     The graphs of  FIG. 6  illustrate more precisely the method of operation of this configuration. The signal B is assumed to be identical with that of  FIG. 2 ; the direct output signal A of the magnetic field sensor  3  is not shown in  FIG. 6 . The two comparators  24 ,  25 , each derive a logic signal L or M, respectively, from B. The signal L has the value one when B is below the level F; otherwise the value is zero. The signal M has the value one when B is above the level E; otherwise the value is zero. The capacitors  23  each allow passage of a pulse on a rising flank of the signals L, M to the RS-flip flop, to set it or reset it. These pulses are shown as broken lines in the graphs of the signals L, M in  FIG. 6 . Negative pulses, which correspond to the falling flanks of the signals L, M, have no effect on the RS-flip flop  22  and are not shown in the FIG.; they can also be diverted to ground through diodes (not shown).  
         [0031]     As seen from the curve of the output signal K of the flip flop  22 , it is set in each case by a rising flank of the signal L at the angular position φ 1  and is reset by a falling flank of the signal M at the angular position φ 4 . The flip flop  22  thus flips the switches  19 ,  22  in each case just prior to a zero passage φ 2  or φ 5 ) of the signal B. More precisely, the flip flop  22  switches over the output signal G of the switch  20  from the level E to zero and the output signal H of the switch  19  from the level zero to F just prior to a falling zero passage (at φ 2 ) of the signal B.  
         [0032]     At the origin of the graphs, where φ=0, the switch  6  passes the signal G to the comparator  5  as signal J. Consequently, up to the angle φ 1  the signal J has the level E and is thus lower than signal B, and the output signal D of the comparator  4  is constantly logical one. When the signal G is at zero at the angle φ 1 , the signal J does the same but the magnitude ratios of the input signals of the comparator S are not thereby changed, so that the output signal D of the comparator  5  retains the level  1 . Only when the signal B passes through zero at φ 2  do the magnitude ratios change, and the output signal D then goes to zero. This leads to the flipping of the switch  6 , which then switches the signal H through to the comparator  5 . The rise of the level of the signal J to F associated with this does not change the output signal D, which remains zero, but a hysteresis is reached to the effect that a disturbance of the signal B must have at least the amplitude F to lead to another flipping of the output signal D.  
         [0033]     When the signal B again rises above the level E at the angle φ 4 , the flip flop  22  is reset, the signal G goes from zero to E, and the signals H. J go from F to zero. Thus the trip threshold of the comparator at the proper time prior to the zero passage of the signal B at φ 3  again lies at zero, and the position φ 5  is correctly detected and is manifested as a rising flank of the output signal D.  
         [0034]     Another circuit variant with which an angular position of the emitter wheel  1  corresponding to a zero passage of the signal B has been detected exactly is shown in  FIG. 7 . In this case the comparators  24 ,  25  and the flip flop  22  are replaced by a differentiation element  26 , which computes the derivative of the signal B with respect to time dB/dt, and a trigger circuit  27 .  
         [0035]     Curves of signals in the circuit of  FIG. 7  are illustrated in  FIG. 8 . With the temporarily saturated curve of the signal B assumed in the present description as an example, the curve for dB/dt shown in the figure is obtained, with alternating positive and negative intervals separated by intervals in which the derivative disappears. The trigger circuit  27  forms therefrom the signal K controlling the switches  19 ,  20 , which assumes the value of zero or one in each angular interval in which the amplifier  4  is not saturated, depending on the algebraic sign of the derivative B, and in principle can assume arbitrary values in the intermediate intervals, since these intervals are of no significance for the operating method of the circuit.  
         [0036]     The shape of the switch signal K results in the levels E or zero, or zero or F, respectively, shown in  FIG. 8  as solid lines, for the output signals H. G of the switches  19 ,  20  when the amplifier  4  is not saturated. As with the configuration of  FIG. 5 , the signal G with zero level is applied at first as signal J to the input of the comparator  5  during a falling flank of signal B in the angular interval [φ 1 , φ 3 ]; upon zero passage at φ 2  there is a switch to signal H with level F. During the negative half-wave of signal B, the signal H and with it the signal J goes to zero, so that when the rising flank of the signal B goes through zero at the angular position φ 5 , the zero level is again applied to the comparator  5  as the comparison level.  
         [0037]     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.