Abstract:
A system and method for determining an estimation of actual cam phase angle of increased accuracy are based on an observed cam phase angle derived from a cam phase sensor and a predicted cam phase angle derived from a desired or commanded cam phase angle. The estimated cam phase angle is used in the electronic control unit in computing desired settings for engine variables which depend on cam phase angle.

Description:
BACKGROUND OF INVENTION 
     The present invention relates generally to an improved method for estimating the camshaft phase angle in an engine with variable cam timing. 
     The advent of variable cam timing in internal combustion engines has complicated the engine management task. Within the engine control unit, the electronic throttle valve position (alternatively, an idle bypass valve opening if not equipped with an electronically actuated throttle valve), fuel injection pulse width, spark timing, position of the exhaust gas recirculation valve, and the cam phase angle are engine variables commanded by the engine control unit to provide the power demanded by the operator of the vehicle while also delivering high fuel efficiency, low emissions, and acceptable drivability. These engine variables are strongly coupled and have a delay time constant associated with them. Thus, the task of changing among operating conditions in a smooth manner is enabled by the engine control unit containing models of the interdependencies among the variables, dynamic models of the various actuators, accurate information from sensors about the status of the various actuators. 
     The inventors of the present invention have recognized that the accuracy of prior art methods for predicting the actual cam phase angle can be improved. As a result, the coupled parameters, i.e., spark timing, throttle position, etc. listed above, may be computed inaccurately due to being based on inaccurate input cam phase angle data. One prior method relies on the output of a sensor on the cam phaser. Because the signal from the sensor is noisy, the signal is filtered, thereby reducing the bandwidth of the signal and thus, causing a delay. Another prior method relies on a model within the engine control unit and bases the prediction on the commanded phase angle and the dynamic characteristics of the cam phaser. The cam phaser may fail or may change dynamic characteristics over its lifetime causing the prediction to be in error. 
     SUMMARY OF INVENTION 
     The drawbacks of prior art approaches are overcome by a method for determining an estimated camshaft phase angle of increased accuracy by determining a desired camshaft phase angle, determining an observed raw camshaft phase angle, and basing the estimated camshaft phase angle on the desired camshaft phase angle and the observed raw camshaft phase angle. The raw observed camshaft phase angle may be based on the output of a camshaft phase angle sensor located proximately to the camshaft. 
     A primary advantage of the invention disclosed herein is a prediction of cam angle of increased accuracy and with a lesser delay than prior art methods. 
     A further advantage of the present invention is that it provides an accurate prediction of cam phase angle even as the cam phaser performance changes due to wear, failure, ambient conditions, or other anomaly. 
     A further advantage of the present invention is that the prediction of the disclosed method provides a less noisy signal than prior art methods. 
     The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein: 
     FIG. 1 is a schematic drawing of an engine indicating salient features for practicing invention; 
     FIG. 2 is a schematic drawing of a single cylinder of an engine showing the camshaft phasing mechanism; 
     FIG. 3 is a flowchart of the steps involved according to an aspect of the present invention; 
     FIG. 4 is schematic drawing of the calculation steps in the engine control unit according to an aspect of the present invention; 
     FIG. 5 is a plot of desired camshaft phase angle, raw observed camshaft phase angle, and estimated camshaft phase angle as functions of time for a disabled camshaft phaser; 
     FIG. 6 is a plot of desired camshaft phase angle, raw observed camshaft phase angle, estimated camshaft phase angle, and filtered observed camshaft phase angle as functions of time for an operating camshaft phaser; and 
     FIG. 7 displays a portion of FIG. 6 enlarged. 
    
    
     DETAILED DESCRIPTION 
     An internal combustion engine  70  is shown in FIG.  1 . Engine  70  shown is a spark-ignition engine with spark plugs  74  installed into engine  70 . The invention may also apply to a compression-ignition engine which does not rely on spark plugs for ignition. Engine  70  is supplied fuel directly into the combustion chamber through injectors  72 , as would be the case in a direct injection gasoline or diesel engine. Fuel injectors  72  could be situated, alternatively, near the intake ports to the combustion chamber. Engine  70  is provided with a cam phaser  34 , which can alter the time at which the valves open and close relative to engine crankshaft rotation. A more detailed description is provided below with reference to FIG.  2 . Engine  70  is supplied fresh air through an inlet duct containing a throttle valve  78 . The engine discharges gases into an exhaust duct  88 . A portion of the exhaust gas stream may be routed back to the intake duct through exhaust gas recirculation (EGR) valve  90 . 
     Continuing with FIG. 1, engine control unit (ECU)  18  has a microprocessor  50 , called a central processing unit (CPU), in communication with memory management unit (MMU)  60 . MMU  60  controls the movement of data among the various computer readable storage media and communicates data to and from CPU  50 . The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM)  58 , random-access memory (RAM)  56 , and keep-alive memory (KAM)  54 , for example. KAM  54  may be used to store various operating variables while CPU  50  is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory capable of storing data, some of which represent executable instructions, used by CPU  50  in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU  50  communicates with various sensors and actuators via an input/output (I/O) interface  52 . Examples of items that are actuated under control of CPU  50  through I/O interface  52 , are fuel injection timing, fuel injection rate, fuel injection duration, EGR valve  90  position, throttle valve  78  position, and cam phaser  34  position. Sensors communicating input through I/O interface  52  may be indicating engine speed, vehicle speed, coolant temperature, manifold pressure, pedal position, camshaft phase sensor  36 , throttle valve  78  position, EGR valve  90  position, air temperature, exhaust temperature, mass air flow  82 , and others; some of which are shown explicitly in FIG.  1  and others are shown as other sensors  38 . Some ECU  18  architectures do not contain MMU  60 . If no MMU  60  is employed, CPU  50  manages data and connects directly to ROM  58 , RAM  56 , and KAM  54 . Of course, the present invention could utilize more than one CPU  50  to provide engine/vehicle control and ECU  18  may contain multiple ROM  58 , RAM  56 , and KAM  54  coupled to MMU  60  or CPU  50  depending upon the particular application. 
     An electronically-controlled throttle, such as throttle valve  78  shown in FIG. 1, provides an example of a system delay. When ECU  18  receives a signal from a pedal position sensor indicating a driver demand for additional power, ECU  18  commands throttle valve  78  to open. The additional power to the driving wheels is delayed by: ECU  18  in interpreting the signal (due to filtering) from the pedal position as a demand for power, computational delays in ECU  18  due to computational traffic, the limitations imposed by the time step at which computations are performed within ECU  18 , mechanical delay in throttle valve  78  attaining the commanded position, and inertial delay in filling the intake manifold to the new, higher manifold pressure. It is known to those skilled in the art to model the air delivered to the engine accounting for system delays. The model relies on accurate information of many system variables, including valve timing, which is related to camshaft phasing. The ability of the model to provide the desired functionality depends on the accuracy of the models in capturing the phenomena and their interactions. The subject of the present invention is increasing the accuracy of cam phase angle data within the ECU  18 . 
     FIG. 2 shows a single piston  68  disposed in engine  70 . Camshaft  84  of engine  70  is shown in FIG. 2 communicating with rocker arm  86  which is fixed at end  88  for actuating intake valve  64 . Exhaust valve  66  may be similarly equipped as intake valve  64  (cam phasing hardware not shown). Alternatively, camshaft  84  may be used to actuate both intake valve  64  and exhaust valve  66 , in which case a phase change in camshaft  84  affects both intake valve  64  and exhaust valve  66  timings. Camshaft  84  is directly coupled to cam phaser  34 . Cam phaser  34  forms a toothed wheel having a plurality of teeth  92 . Camshaft  84  is hydraulically coupled to an inner camshaft (not shown), which is in turn directly linked to camshaft  84  via a timing chain (not shown). Therefore, cam phaser  34  and camshaft  84  rotate at a speed substantially equivalent to the inner camshaft. The inner camshaft rotates at a constant speed ratio to crankshaft  100 . However, by manipulation of a hydraulic coupling (not shown), the relative phase of camshaft  84  to crankshaft  100  can be varied by applying a hydraulic pressure in advance chamber  96  or retard chamber  98 . By allowing high pressure hydraulic fluid to enter advance chamber  96 , intake valve  64  opens and closes at a time earlier relative to crankshaft  100 . Similarly, by allowing high pressure hydraulic fluid to enter retard chamber  98 , intake valve  64  opens and closes at a time later relative to crankshaft  100 . 
     Teeth  92 , being coupled to cam phaser  34  and camshaft  84 , allow for measurement of cam phase angle via cam timing sensor  92  providing a signal to ECU  18 . Four equally spaced teeth on cam phaser  34  are preferably used for measurement of cam timing for a bank of four cylinders, eg., an inline four cylinder engine or one bank of a V-8 engine. ECU  18  sends control signals to conventional solenoid valves (not shown) to control the flow of hydraulic fluid either into advance chamber  96 , retard chamber  98 , or neither. 
     Camshaft phase angle may be measured using the method described in U.S. Pat. No. 5,548,995, which is incorporated herein by reference. In general terms, the rotation angle between the rising edge of a signal from sensor  102  which senses a tooth (not shown) coupled to crankshaft  100  and a signal detected by camshaft phase sensor  36  from one of the plurality of teeth  92  on cam phaser  34  provides a measure of the relative cam timing. For the particular example of an inline four cylinder engine, with a four-toothed wheel on cam phaser  36 , a measure of cam timing for each bank is received four times per revolution. 
     Referring now to FIG. 3, ECU  18  schedules cam phaser  34 , in block  10 , according to models within ECU  18 , one example of which is described in U.S. Pat. No. 6,006,725, which is incorporated herein by reference. This provides the desired phase of the camshaft, which is denoted as cam_ph_d herein. Within ECU  18  is a dynamic model  16  of cam phaser  34 . The dynamic model  16  may incorporate system inertias, compliances, compressibilities, actuator delays, material characteristics, and other factors to describe the behavior of camshaft  84  in response to a command to cam phaser  34  to make an angle change. Based on dynamic model  16 , a predicted cam phase can be computed, denoted as cam_ph_pred. In block  42 , cam_ph_pred and cam_ph_obs_corr are summed to yield cam_ph_est, which is the estimated cam phase angle with increased accuracy compared to prior art methods. The observer leg of the computation begins with a measurement of the cam phase angle, cam_ph_obs_raw, which is computed in block  29  based on signals from the camshaft phase sensor  34  and the crankshaft phase sensor  102 . In block  30 , the raw signal (cam_ph_obs_raw) is compared with cam_ph_est. An error signal, cam_ph_obs_err is the output of block  30 . In block  32 , cam_ph_obs_err is integrated, which filters the signal and provides a corrected signal, called cam_ph_obs_corr herein. As discussed above, cam_ph_obs_corr is used in block  42  as one of the inputs to provide the output, cam_ph_est. 
     FIG. 3 is a simplified version of the invention to clearly indicate that two inputs are used to arrive at cam_ph_est. FIG. 4 shows the method in more detail and in context within ECU  18 . ECU  18  receives input from sensors  38  and camshaft sensor  36  and crankshaft sensor  102 ; from the latter two sensors, ECU  18  computes cam_ph_obs_raw in block  29 . ECU  18  computes cam_ph_d, the desired cam phase, based on a model such as taught in U.S. Pat. No. 6,006,725. Cam_ph_d and cam_ph_obs_raw are compared in operation  22 , which provides the value of cam_ph_err, that is the difference between the commanded signal and the measured signal. Cam_ph_err is used as feedback control to camshaft phaser  34 , as in prior art. Cam_ph_d, block  12 , is used in dynamic model  16  to determine cam_ph_pred. Cam_ph_pred is summed in block  42  with the output of blocks  30  and  32 , previously described in conjunction with FIG.  3 . The output of summing operation  42  yields cam_ph_est, the subject of the present invention. Cam ph_est is used within ECU  18  in relevant actuator models. These may be models which compute desired throttle valve  78  position, desired EGR valve  90  position, spark timing, fuel injection timing, and fuel injection pulse width, as examples. Output of the actuator models  60  is fed to actuators  62 . 
     The present invention is demonstrated in FIGS. 5-7, in which experimental data are used to illustrate the present invention and compare it with prior art solutions. In FIG. 5, an inoperable camshaft phaser  34  is commanded a camshaft position, i.e., the desired camshaft phase angle, cam_ph_d, shown as curve  110 . Because the camshaft phaser  34  is inoperable, the camshaft does not respond. Curve  112  is the cam_ph_obs_raw, i.e., the measured cam phase angle. Curve  112  does not deviate from the initial value since the camshaft phase does not change. Curve  112 , however, does indicate a typical noise level on the signal. If cam_ph_obs_raw were used as the basis to compute other engine parameters, such as throttle position, these parameters would constantly vary. Eg., throttle plate  78  would flutter in response to the noise appearing on curve  112 . The estimate of cam phase, as provided by the present invention cam_ph_est, shown in curve  114 , is based on both cam_ph_obs_raw and cam_ph_d. As such, it does deviate from a steady value in response to the command to camshaft phaser  34 . However, it readily returns to the steady value. Also, curve  114  is not a noisy signal. 
     In FIG. 6, a working camshaft phaser  34  is commanded to assume a new desired phase angle, cam_ph_d which is shown as curve  120 . Curve  122  shows the output of the measurement, cam_ph_obs_raw. Again, there is noise on the measured signal, curve  122 . Curve  124  shows the estimated camshaft phase angle, according to the present invention. Curve  126  shows a filtered version of curve  122 . As mentioned above, a problem with cam_ph_obs_raw is that due to its noise, control of other engine parameters is degraded. A common technique to remove noise from a signal is to filter the signal with the undesired consequence that the signal is time delayed. Curve  126  is a filtered version of curve  122 . It can be seen in FIG. 6 that curve  124 , the subject of the present invention lags behind the unfiltered measured signal, curve  122 , but precedes the filtered measured signal, curve  126 . FIG. 7 is an enlarged version of a portion of FIG.  6 . The noise of curve  122  is even more evident in FIG.  7 . The stepwise nature of curve  124 , cam_ph_est, is due to the computation time step, which is 100 msec. Similarly, the filtered version of the measured signal, curve  126 , changes on a 100 ms time scale; thus similar to curve  122 , curve  126  displays a stepwise character. Curve  126  lags curve  122  by about one computation step, or 100 msec. Thus, the present invention provides a clear advantage over filtering a measured signal. 
     While a preferred mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. The above-described embodiment is intended to be illustrative of the invention, which may be modified within the scope of the following claims.