Method for predicting a phase position of a camshaft

Abstract

A method for predicting a future camshaft position includes approximating a regulating circuit or a part of a regulating circuit that includes at least an adjusting device by a transfer function, and ascertaining a future camshaft position on the basis of the transfer function. An engine control unit is also provided.

Claims

1. A method for predicting a future camshaft position, the method comprising: approximating one of a regulating circuit and a part of a regulating circuit that includes at least an adjusting device by a transfer function, wherein the transfer function is a PT1 transfer element; and ascertaining a future camshaft position based on the transfer function, wherein the step of ascertaining the future camshaft position includes ascertaining the future camshaft position as a predicted camshaft position in accordance with the following equation: $y_{\mathrm{pred}}=A^{n}y+\underset{i=0}{\overset{n-1}{.Math.}}{A}^{i}B{u}_{\mathrm{set}}$ where y.sub.pred denotes the predicted camshaft position, u.sub.set denotes a set value for a camshaft position, y denotes a camshaft position measured on a last flank, A and B are constants of a time-discrete state-space model of the PT1 transfer element, and n specifies a number of prediction steps.

2. The method according to claim 1, wherein the number n of prediction steps preferentially amounts to n=5.

3. The method according to claim 1, wherein the step of ascertaining the future camshaft position includes ascertaining a predicted camshaft position at a time of one of opening and closing a charge-cycle valve.

4. The method according to claim 1, wherein the step of ascertaining the future camshaft position includes ascertaining a predicted camshaft position at a time of one of opening and closing a charge-cycle valve, and wherein the predicted camshaft position at the time of one of opening and closing the charge-cycle valve is used in a charge-capture function in order to optimize a proportion of fresh air in a combustion chamber of a motor vehicle.

5. The method according to claim 1, which comprises determining a filter-time of the transfer function by using a characteristic map.

6. An engine configuration, comprising: one of a regulating circuit and a part of a regulating circuit that includes at least an adjusting device; an engine control unit including a camshaft prediction unit, said camshaft prediction unit being configured to approximate said one of said regulating circuit and said part of said regulating circuit by a transfer function, wherein the transfer function is a PT1 transfer element; and said camshaft prediction unit ascertaining a future camshaft position based on said transfer function, wherein ascertaining the future camshaft position includes ascertaining the future camshaft position as a predicted camshaft position in accordance with the following equation: $y_{\mathrm{pred}}=A^{n}y+\underset{i=0}{\overset{n-1}{.Math.}}{A}^{i}B{u}_{\mathrm{set}}$ where y.sub.pred denotes the predicted camshaft position, u denotes a set value for a camshaft position, y denotes a camshaft position measured on a last flank, A and B are constants of a time-discrete state-space model of the PT1 transfer element, and n specifies a number of prediction steps.

7. An engine control unit, comprising: a camshaft prediction unit configured to approximate one of a regulating circuit and a part of a regulating circuit that includes at least an adjusting device by a transfer function, wherein the transfer function is a PT1 transfer element; and said camshaft prediction unit ascertaining a future camshaft position based on said transfer function, wherein ascertaining the future camshaft position includes ascertaining the future camshaft position as a predicted camshaft position in accordance with the following equation: $y_{\mathrm{pred}}=A^{n}y+\underset{i=0}{\overset{n-1}{.Math.}}{A}^{i}B{u}_{\mathrm{set}}$ where y.sub.pred denotes the predicted camshaft position, u denotes a set value for a camshaft position, y denotes a camshaft position measured on a last flank, A and B are constants of a time-discrete state-space model of the PT1 transfer element, and n specifies a number of prediction steps.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) FIG. 1 is a schematic illustration of an exemplary embodiment of a transfer function in accordance with the invention;

(2) FIG. 2 is a schematic illustration of an exemplary embodiment of a method for predicting a future camshaft position in accordance with the invention; and

(3) FIG. 3 is a schematic view of an exemplary embodiment of an engine control unit for predicting a future camshaft position in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) In the case of a method according to the invention for predicting a future camshaft position, a regulating circuit including a camshaft-position regulator and an adjusting device is approximated by a transfer function, and a future camshaft position is ascertained on the basis of the transfer function.

(5) An exemplary embodiment of a transfer function is shown in FIG. 1. The transfer function G approximates the behavior of a system. The transfer function G has an input variable u and an output variable y. The transfer function G models the behavior of the systemthat is, how the output variable y of the system reacts to alterations of the input variable u of the system.

(6) In the exemplary embodiments described in the following, the transfer function describes, in particular, the regulating circuit formed of the camshaft-position regulator and the adjusting device of a motor vehicle.

(7) As described in the introduction; the proportion of fresh air in the combustion chamber is crucially influenced by the times (points in time) of opening and closing of the gas-exchange valve activated by the camshaft. These times may be interpreted in equivalent manner as angular positions of the crankshaft. These angles are changed by the camshaft adjustment. A camshaft regulation system optimizes the proportion of fresh air in the combustion chamber of a motor vehicle by ascertainment of an optimized charge and by appropriate regulation of the times of opening and closing of the gas-exchange valve activated by the camshaft.

(8) In the exemplary embodiment of the invention now being described, the closed regulating circuit made up of the camshaft-position regulator and the adjusting device is approximated as a PT1 transfer element (first-order low-pass) which is based on a time-discrete state-space model of the form:
k(k+1)=Ax(k)+Bu(k)
y(k)=Cx(k)+Du(k)
where x(k) is a system state, y(k) is the regulated variable, here the camshaft position at time-step k, and u(k) is the correcting variable, here accordingly the set value for the camshaft position at time-step k.

(9) For a PT1 transfer element, a state-space representation can be chosen in which C=1 and D=0, so that the state-space model simplifies into:
x(k+1)=y(k+1)=Ax(k)+Bu(k)

(10) Hence, for the PT1 example, the system state x corresponds to the regulated variable ythat is, here the actual position of the camshaft.

(11) On the basis of this time-discrete state-space model, the future camshaft position y.sub.pred of the PT1 transfer element can be ascertained in n prediction steps on the basis of the equation:

(12) $y\left(k+n\right)=A^{n}y\left(k\right)+\underset{i=0}{\overset{n-1}{.Math.}}{A}^{i}Bu\left(k+n-1-i\right)$

(13) Assuming a constant set value u(k)=u.sub.set for the camshaft position over the prediction period, this simplifies into:

(14) $y\left(k+n\right)=A^{n}y\left(k\right)+\underset{i=0}{\overset{n-1}{.Math.}}{A}^{i}B{u}_{\mathrm{set}}$
where y(k+n) denotes the predicted camshaft position after n prediction steps, and y(k) is the camshaft position measured on the last flank.

(15) The constants A and B of the time-discrete state-space model of the PT1 transfer element are determined in this embodiment as follows:

(16) $A=1-\frac{h}{T_1}$$B=\frac{h}{T_1}$$h=\frac{60}{n_{\mathrm{eng}}}.Math.\frac{w_{\mathrm{pred}}}{360}.Math.\frac{1}{n}$

(17) Here, w.sub.pred denotes the angular distance between the last measured flank of the camshaft transducer wheel and the points valve opens or, respectively, valve closes. T.sub.1 denotes the time constant of the PT1 transfer element, and n.sub.eng is the engine speedthat is, the rotational speed of the crankshaft as made available by the engine management system (engine control system)n is a predefined integer that specifies the number of prediction steps and h may be interpreted as a prediction-step duration.

(18) Since the angles for the opening and closing of the gas-exchange valve have been defined with the aid of the cam contour of the camshaft, an equivalent prediction-time can be ascertained on the basis of the crank angle distance between the flank and the opening-angle with the aid of the engine speed n.sub.eng, and the PT1 behavior can be extrapolated for this period.

(19) The filter-time, for instance the above time constant T.sub.1 of the PT1 transfer element, may depend on the signal for activation of the camshaft-adjusting unitthat is to say, on the pulse duty ratio with which the output stage of the regulating valve of the corresponding camshaft-adjusting unit is activated, and also on variables influencing the behavior of the process, such as oil temperature or oil pressure. The filter-time can, for instance, be ascertained through the use of a characteristic map on the basis of the process variables made available by the engine management system. Alternatively, the time constant T.sub.1 can also be ascertained through the use of equations from the process variables made available.

(20) The following table shows an example of how, for exemplary 3000 rpm and retard adjustment, a time constant T.sub.1 can be derived from a given oil temperature:

(21) TABLE-US-00001 Oil Temperature Time Constant T.sub.1 15 C. 0.02677 30 C. 0.02616 60 C. 0.02497 105 C. 0.01702 130 C. 0.01628

(22) As can be gathered from the table, the time constant T.sub.1 here is chosen in such a way that it decreases with the oil temperature. This is due to the fact that at low temperatures the oil is still relatively viscous and therefore the speed of adjustment is low. Corresponding values for varied influencing factors can also be saved in a characteristic map.

(23) Hence the future camshaft position can be ascertained in, for instance, five prediction steps on the basis of the equation:

(24) $y_{\mathrm{pred}}=A^{5}y+\underset{i=0}{\overset{4}{.Math.}}{A}^{i}B{u}_{\mathrm{set}}$
where y.sub.pred denotes the predicted camshaft position after five prediction steps, and y is the camshaft position measured on the last flank.

(25) Depending upon whether the angular distance between the last measured flank of the camshaft transducer wheel and the points valve opens or valve closes is employed for w.sub.pred, the predicted camshaft positions for inlet valve opens or, respectively, inlet valve closes can be ascertained through the use of the above function. Equally, a prediction for the future position of the camshaft transducer wheel for outlet valve closes can also be determined.

(26) The predicted camshaft positions ascertained in such a way can be made available to a charge-capture function which can thereby better ascertain the time of opening or closing of the gas-exchange valve activated by the camshaft.

(27) FIG. 2 shows schematically an exemplary embodiment of a method for predicting a future camshaft position. In a step S1, a time constant T.sub.1 of the transfer function is determined, for instance by evaluation of a characteristic map and of corresponding process variables that are made available by the engine control unit. In step S2, a set value u.sub.set for the camshaft position is retrieved from the camshaft regulation system. In step S3, the angular distance w.sub.pred between the last measured flank of the camshaft transducer wheel and the point valve opens is ascertained. In step S4, the current engine speed n.sub.eng is retrieved from the engine management system. In step S5, the constant h is calculated on the basis of the engine speed n.sub.eng and the angular distance w.sub.pred. In steps S6 and S7, two parameters A and B of the transfer function are calculated on the basis of the prediction-step duration h and the time constant T.sub.1. In step S8, a predicted future camshaft position y.sub.pred is calculated from the current camshaft position y on the basis of the calculated parameters A and B and on the basis of the set value u.sub.set for the camshaft position. In step S9, this predicted future camshaft position y.sub.pred is made available to a charge-capture function.

(28) FIG. 3 shows schematically an exemplary embodiment of an engine control unit for predicting a future camshaft position. An engine control unit 1 includes a camshaft regulation unit 2, a charge-capture functional unit 3 and a camshaft prediction unit 4. The camshaft prediction unit 4 ascertains a predicted future camshaft position y.sub.pred in accordance with the method described above, and makes the position available to the charge-capture functional unit 3. The camshaft regulation unit 2 regulates a camshaft position on the basis of the output of the charge-capture functional unit 3, and outputs corresponding control signals to a camshaft-adjuster 9. A camshaft transducer wheel 5 with an appropriate sensor makes measuring signals available to the engine control unit that permit inferences as to the current position of the camshaft. A crankshaft transducer wheel 6 with an appropriate sensor makes measuring signals available to the engine control unit 1 that permit inferences as to the current position of the crankshaft. The engine control unit 1 can, for instance, ascertain the current engine speed n.sub.eng on the basis of these signals. Further components, such as an oil-temperature sensor 7 and an oil-pressure sensor 8, make variables available to the engine control unit that influence the behavior of the process, such as, for instance, oil temperature or oil pressure. On the basis of these variables, the camshaft prediction unit 4 can, for instance, ascertain a time constant of the transfer function.

(29) In the above exemplary embodiment, a closed regulating circuit made up of a camshaft-position regulator and an adjusting device was approximated by a PT1 transfer function. On the basis of the teaching described above, however, alternative embodiments of the teaching of the invention are possible.

(30) For instance, instead of a PT1 transfer function, other transfer functionssuch as, for example, PT2 or otherscould also come into operation. In such cases, a person skilled in the art will adapt the state-space model to the transfer function being used. For instance, it is known to a person skilled in the art that in the case of more complex transfer functions C is generally represented as a matrix. In the case of PT1, this output matrix C, which maps the states of the transfer element onto the output, as described above, is reduced to a scalar. In the case of SISO systems (single input, single output), the output matrix C is reduced to a vector.

(31) Furthermore, instead of a complete regulating circuit merely a part of a regulating circuit may also be approximated by the transfer function in alternative embodiments. For instance, in particular the positioning device (phase positioning, phase adjustment) can be approximated through the use of an I-element that exhibits an integrative transfer behavior. The prediction proceeds similarly in the case of the approximation of the positioning device (phase positioning, phase adjustment) through the use of an I-element, as in the case of the PT1 embodiment described above. The speed of adjustment is stored in a characteristic map as a function of various engine parameters such as rotational speed or oil pressure, oil temperature and pulse duty ratio for the phase-positioner. Instead of the 5-step prediction described above, in one embodiment only one prediction step is carried outthat is to say, n=1. The input u in this case corresponds not to the set camshaft value (camshaft setpoint value) but rather to the PWM (Pulse-Width Modulation) pulse duty ratio for the phase-positioner, which is output from the camshaft-position regulator.

LIST OF REFERENCE CHARACTERS

(32) 1 engine control unit 2 camshaft regulation unit 3 charge-capture functional unit 4 camshaft prediction unit 5 camshaft transducer wheel 6 crankshaft transducer wheel 7 oil-temperature sensor 8 oil-pressure sensor 9 camshaft-adjuster S1 determination of the time constant T.sub.1 S2 determination of the set value u.sub.set for the camshaft position S3 determination of the angular distance w.sub.pred S4 retrieval of the engine speed n.sub.end S5 calculation of the constant h of the PT1 transfer element S6 calculation of parameter A of the PT1 transfer element S7 calculation of parameter B of the PT1 transfer element S8 calculation of the predicted future camshaft position y.sub.pred S9 output of the predicted future camshaft position y.sub.pred G transfer function n.sub.eng engine speed w.sub.pred angular distance x state of the transfer function y output value of the transfer function (current camshaft position) y.sub.pred predicted camshaft position A, B parameters of the transfer function C, D further parameters of the transfer function h prediction-step duration T.sub.1 time constant u.sub.set set value for the camshaft position