VCT System Having Closed Loop Control Employing Spool Valve Actuated by a Stepper Motor

This invention relates to an internal combustion engine in which the timing of the camshaft of a single camshaft engine, or the timing of one or both of the camshafts of a dual camshaft engine, relative to the crankshaft, is varied to improve one or more of the operating characteristics of the engine.

It is known that the performance of an internal combustion engine can be improved by the use of dual camshafts, one to operate the intake valves of the various cylinders of the engine and the other to operate the exhaust valves. Typically, one of such camshafts is driven by the crankshaft of the engine, through a sprocket and chain drive or a belt drive, and the other of such camshafts is driven by the first, through a second sprocket and chain drive or a second belt drive. Alternatively, both of the camshafts can be driven by a single crankshaft powered chain drive or belt drive. It is also known that engine performance in an engine with dual camshafts can be further improved, in terms of idle quality, fuel economy, reduced emissions or increased torque, by changing the positional relationship of one of the camshafts, usually the camshaft which operates the intake valves of the engine, relative to the other camshaft and relative to the crankshaft, to thereby vary the timing of the engine in terms of the operation of intake valves relative to its exhaust valves or in terms of the operation of its valves relative to the position of the crankshaft. Heretofore, such changes in engine valve timing have been accomplished by a separate hydraulic motor operated by engine lubricating oil. However, this actuating arrangement consumes significant additional energy and it increases the required size of the engine lubricating pump because of the required rapid response time for proper operation of the camshaft phasing actuator. Further, these arrangements are typically limited to a total of 20.degree. of phase adjustment between crankshaft position and camshaft position, and typically such arrangements are two-position arrangements, that is, on, or fully phase adjusted as one position, and off, or no phase adjustment, as a second position.

The present invention is designed to overcome the aforesaid problems associated with prior art variable camshaft timing arrangements by providing a self-actuating, variable camshaft timing arrangement which does not require external energy for the operation thereof, which does not add to the required size of the engine lubricating pump to meet transient hydraulic operation requirements of such variable camshaft timing arrangement, which provides for continuously variable camshaft to crankshaft phase relationship within its operating limits, and which provides substantially more than 20.degree. of phase adjustment between the crankshaft position and the camshaft position. Prior U.S. Pat. Nos. which describe various systems of the foregoing type are 5,046,460 and 5,002,023, the disclosures of each of which are incorporated by reference. Those disclosures provide a pulse-width modulated (PWM) solenoid for varying the position of a control valve which actuates the VCT phase adjustment mechanism. Closed-loop control of those VCT systems was disclosed in U.S. patent application Ser. No. 07/847,577, the disclosure of which is incorporated herein by reference. This technology is acceptable for chain driven engines, but it is less than desirable in belt-driven engines where the PWM equipment may leak oil on the belt thus causing slippage and a decrease in engine efficiency.

The present invention provides a method for phase adjustment of an internal combustion engine in which the position of the camshaft, or the positions of one or both of the camshafts in a dual camshaft system, is phase adjusted relative to the crankshaft by an actuating arrangement which is controlled by a closed loop system which commands a stepper motor. A predetermined set point dictates the desired camshaft phase angle for certain engine performance criteria. This variable camshaft timing (VCT) system can be used to improve important engine operating characteristics such as idle quality, fuel economy, emissions or torque. A preferred embodiment of a camshaft mounted hydraulic VCT mechanism uses one or more radially extending vanes which are circumferentially fixed relative to the camshaft and which are receivable in cavities of a sprocket housing that is oscillatable on the camshaft. Hydraulic fluid is selectively pumped through a proportional (spool) valve to one side or another of each vane to advance or retard the position of the camshaft relative to the sprocket. A pumping action occurs in reaction to a signal generated by a closed loop feedback system. Closed loop feedback control is imperative for any but the "two-position" case, i.e., fully advanced or fully retarded. This is because camshaft phase is controlled by the integral of the spool valve position. That is, spool position corresponds not to camshaft phase, but to its rate of change. Thus, any steady state spool position other than null (centered) will cause the VCT to eventually go to one of its physical limits in phase. Closed loop control allows the spool to be returned to null as the camshaft phase reaches its commanded position or set point. An additional result of using feedback control is that the system performance is desensitized to mechanical and environmental variations. This results in a reduction of the effects of short term changes, such as changes in oil pressure or temperature, or long term variations due to tolerances or wear. In addition, set point tracking error in the presence of unanticipated disturbances (e.g. torque shifts) is reduced. The use of VCT state variable feedback opens up the possibility of using "optimal" control to best achieve a wide variety of performance objectives. In addition, integral control can be incorporated into this technique to ensure zero steady-state tracking error.

Accordingly, it is an object of the present invention to provide an improved VCT method and apparatus which utilizes an electronic driver and stepper motor for spool position control and an advanced control algorithm that yields a prescribed set point tracking behavior. Specifically, it is an object of the present invention to provide a VCT system which utilizes a predetermined set point to dictate the desired camshaft phase angle to effectuate certain engine performance criteria. A further object of the present invention is to provide an improved VCT method and apparatus which does not rely upon oil pressure for spool valve control thus allowing the system to be accurately positioned at engine start-up when system oil pressure is usually low. Another object of the current invention is to provide a VCT system for belt-driven engines which utilizes a stepper motor for spool valve control thus avoiding leakage of hydraulic fluid onto the belt from a hydraulically controlled spool valve, such as a pulse-width-modulated solenoid.

For a further understanding of the present invention and the objects thereof, attention is directed to the drawings and to the following brief descriptions thereof, to the detailed description of the preferred embodiment, and to the appended claims.

FIG. 1a is a block diagram of a basic closed loop feedback system for a VCT system utilizing a stepper motor for phase shift actuation;

FIG. 1b is a block diagram of a closed loop feedback system for a VCT system similar to FIG. 1a with additional detail of control elements;

FIG. 1c is a block diagram of the VCT control law used in a closed loop feedback system of a preferred embodiment of the present invention;

FIG. 1d is a block diagram of a control system for the stepper motor and spool valve of a VCT system of the present invention;

FIG. 1e is a block diagram showing closed loop control of a VCT system using a stepper motor;

FIG. 1f is a block diagram of the control law with integral control and state feedback for a stepper motor in a VCT system of the present invention;

FIG. 1g is a block diagram of an alternate closed-loop feedback system similar to FIG. la but also having a synchronous filter in the controller;

FIG. 1h is a block diagram illustrating the component stages of a synchronous feedback filter;

FIG. 2 is an end elevational view of a camshaft with an embodiment of a variable camshaft timing system applied thereto;

FIG. 3 is a view similar to FIG. 2 with a portion of the structure thereof removed to more clearly illustrate other portions thereof;

FIG. 4 is a sectional view taken on line 4--4 of FIG. 3;

FIG. 5 is a sectional view taken on line 5--5 of FIG. 3;

FIG. 6 is a sectional view taken on line 6--6 of FIG. 3;

FIG. 7 is a end elevational view of an element of the variable camshaft timing system of FIGS. 2--6;

FIG. 8 is an elevational view of the element of FIG. 7 from the opposite end thereof;

FIG. 9 is a side elevational view of the element of FIGS. 7 and 8;

FIG. 10 is an elevational view of the element of FIG. 9 from the opposite side thereof; and

FIG. 11 is a simplified schematic view of the VCT arrangement of FIGS. 2-10 .

A simplified diagram of a closed-loop control system for a VCT mechanism using a stepper motor is shown in FIG. 1a. The phase feedback is compared to a desired phase set point and the difference is input to a phase loop controller. The controller calculates a stepper motor position which is compared to present stepper motor position. A stepper motor control unit issues a signal to the stepper motor to move it an appropriate distance. The VCT mechanism is then actuated by the stepper motor movement which changes its position relative to a crankshaft.

As is schematically shown in FIGS. 1b and 11, the control objective of the present invention is to have the VCT mechanism at the phase angle given by the set point 35 with the spool 100 stationary in its null position. That is, the VCT mechanism is at the correct phase and the phase rate of change is zero. A sophisticated control algorithm which utilizes the dynamic state of the VCT mechanism is used to accomplish this result. Closed-loop control of the VCT mechanism is achieved by measuring the camshaft phase shift .THETA..sub.o 20, comparing it to the desired set point r 35, and adjusting the VCT mechanism so the phase follows the set point r 35. The control law 108 compares the set point r 35 to the phase shift .THETA..sub.o 20 and issues commands to stepper motor 134 in order to position the spool 100 when the phase error (set point r 35 minus phase shift 20) is not zero. The spool 100 is stepped to the right if the phase error is positive (retard) and to the left if the phase error is negative (advance). When the phase error is zero, the VCT phase equals the set point r 35 so the spool 100 is held in the null position (as shown in FIG. 11).

Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 27 and 28, respectively, as the crankshaft (not shown) and camshaft 26 are rotated, and these can be used to actuate the operation of one or more hydraulic elements of a hydraulically operated VCT system. According to the present invention, in a VCT system, for example, a system as described in the aforesaid U.S. Pat. Nos. 5,046,460 and 5,002,023, and U.S. patent application Ser. No. 07/847,577, or according to the embodiment of FIGS. 2-10 as hereinafter described, the measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 27a and 28a, respectively, and issued to a phase measurement device 107. The time from successive crank-to-cam pulses, divided by the time for an entire revolution and multiplied by 360.degree., gives the measured phase difference, .THETA..sub.o 20. This phase difference is then supplied to the control law 108 for processing.

A closed-loop block diagram of the control law 108 is described in detail in FIG. 1c. The stepper motor 134 acceleration command is uniquely determined from a control algorithm which is a function of the following three VCT system state variables: VCT phase difference .THETA..sub.o 20, spool 100 position (rate of VCT phase change), and spool 100 velocity. The set point r 35 and the phase difference 20 are each multiplied by proportional gain K.sub.73. The difference between those computed values, less the computed velocity and position feedback values, yields a raw acceleration.

The acceleration is then limited and integrated to yield a raw velocity. The control algorithm maintains spool 100 velocity as an internal variable. The spool 100 velocity is limited by three factors. First, it cannot exceed its maximum velocity. Second, its derivative must be less than its maximum acceleration. Finally, as the spool 100 approaches its physical limits, its velocity must ramp to zero so that it does not introduce an error in the step count. The equations which correspond to these limits are as follows:

The feedback gains, K.sub.v, K.sub.x, and K.sub.o, are selected to give a one (1) Hertz sinusoidal response of -3dB (decibels) at -45 degree phase lag, but other gains can be selected to obtain a different response. In addition, spool 100 position is tracked by counting the step commands as described above. In essence, although the control algorithm is not based on commanding the spool to a specific position, the ability to keep track of the spool 100 position and velocity allows better closed-loop control. A model for stepper motor 134 position based on the control algorithm discussed above is shown in FIG 1d. As long as the stepper motor 134 step rate does not exceed the prescribed velocity, the stepper motor 134 position corresponds exactly to spool 100 position. FIG 1e is a block diagram of an alternative stepper motor step computation and tracking method.

Integral control of the VCT system can be introduced in order to ensure zero steady-state tracking error. That is, a constant set point r 35 will be reached exactly. The integral of the phase error, set point minus phase feedback, becomes a fourth state variable. It is multiplied by an additional feedback gain, K.sub.I, and added into the acceleration command as shown in FIG. 1f. Optimal control laws can also be developed for this system.

An alternate embodiment of the present invention consists of an expanded closed loop feedback system including variation compensation and disturbance feed forward. The gain of this hydromechanical system depends on a number of variables such as hydraulic supply pressure, engine speed, oil temperature and natural crankshaft/camshaft orientation. In order to counteract the phenomena in the controller, the net effect of all the variables is estimated and a proportional gain is increased as response decreases.

FIG. 1g illustrates an embodiment of the present invention with a synchronous filter 25 in the controller 208 for filtering the measured phase shift .THETA..sub.o 20. As the camshaft rotates, the torque pulses 10 superimpose a high frequency disturbance on the measured phase shift, .THETA..sub.o 20. Thus, there is an exact synchronization between the torque pulses 10 and the high frequency disturbance. Likewise, the camshaft measurement pulses 27a are also synchronized with the disturbance. It is possible to take advantage of this synchronization to efficiently filter the phase measurement, .THETA..sub.o 20, so that the high frequency disturbance is isolated from the control action. As the camshaft speed varies, the filter frequency automatically tracks the disturbance frequency. The filter itself is a discrete-time notch filter with a sampling frequency equal to that of the camshaft measurement pulse frequency 27a. The filtered phase measurement, .THETA..sub.f 30, is supplied to control law 108 and processed as discussed above. Since the high frequency disturbance is isolated, the control law 108 does not attempt to compensate for it. This further makes it possible to save actuation power, reduce wear and enhance signal linearity by such a filtering step.

FIG. 1h is a block diagram of the synchronous filter with variables as follows:

FIGS. 2-10 illustrate an embodiment of a hydraulic vane system in which a housing in the form of a sprocket 32 is oscillatingly journalled on a camshaft 26. The camshaft 26 may be considered to be the only camshaft of a single camshaft engine, either of the overhead camshaft type or the inblock camshaft type. Alternatively, the camshaft 26 may be considered to be either the intake valve operating camshaft or the exhaust valve operating camshaft of the dual camshaft engine. In any case, the sprocket 32 and the camshaft 26 are rotatable together, and are caused to rotate by the application of torque to the sprocket 32 by an endless roller chain 38, shown fragmentarily, which is trained around the sprocket 32 and also around a crankshaft not shown. As will be here after described in greater detail, the sprocket 32 is oscillatingly journalled on the camshaft 26 so that it is oscillatable at least through a limited arc with respect to the camshaft 26 during the rotation of the camshaft, an action which will adjust the phase of the camshaft 26 relative to the crankshaft.

An annular pumping vane 60 is fixedly positioned on the camshaft 26, the vane 60 having a diametrically opposed pair of radially outwardly projecting lobes 60a, 60b and being attached to an enlarged end portion 26a of the camshaft by bolts 62 which pass through the vane 60 into the end portion 26a. In that regard, the camshaft 26 is also provided with a thrust shoulder 26b to permit the camshaft to be accurately positioned relative to an associated engine block, not shown. The pumping vane 60 is also precisely positioned relative to the end portion 26a by a dowel pin 64 which extends therebetween. The lobes 60a, 60b are received in radially outwardly projecting recesses 32a, 32b, respectively, of the sprocket 32, the circumferential extent of each of the recesses 32a, 32b being somewhat greater than the circumferential extent of the vane lobes 60a, 60b which are received in such recesses to permit limited oscillating movement of the sprocket 32 relative to the vane 60. The recesses 32a, 32b are closed around the lobes 60a, 60b, respectively, by spaced apart, transversely extending annular plates 66, 68 which are fixed relative to the vane 60, and, thus, relative to the camshaft 60, by bolts 70 which extend from one to the other through the same lobe, 60a or 60b. Further, the inside diameter 32 c of the sprocket 32 is sealed with respect to the outside diameter of the portion 60d of the vane 60 which is between the lobe 60a, 60b, and the tips of the lobes 60a, 60b of the vane 60 are provided with sealed receiving slots 60e, 60f, respectively. Thus, each of the recesses 32a, 32b of the sprocket 32 is capable of sustaining hydraulic pressure, and within each recess 32a, 32b, the portion on each side of the lobe 60a, 60b, respectively, is capable of sustaining hydraulic pressure.

The functioning of the structure of the embodiment of FIGS. 2-10, as thus far described, may be understood by reference to FIG. 11. Hydraulic fluid, illustratively in the form of engine lubricating oil, flows into the recesses 32a, 32b by way of a common inlet line 82. The inlet line 82 terminates at a juncture between opposed check valves 84 and 86 which are connected to the recesses 32a, 32b, respectively, by branch lines 88, 90, respectively. The check valves 84, 86 have annular seats 84a, 86a, respectively, to permit the flow of hydraulic fluid through the check valves 84, 86 into the recesses 32a, 32b, respectively. The flow of hydraulic fluids through the check valves 84, 86, is blocked by floating balls 84b, 86b, respectively, which are resiliently urged against the seats 84a, 86a, respectively, by springs 84c, 86c, respectively. The check valves 84, 86, thus, permit the initial filling of the recesses 32a, 32b and provide for a continuous supply of makeup hydraulic fluid to compensate for leakage therefrom. Hydraulic fluid enters the line 82 by way of a spool valve 92, which is incorporated within the camshaft 26, and hydraulic fluid is returned to the spool valve 92 from the recesses 32a, 32b by return lines 94, 96, respectively. Because of the location of the check valves 84 and 86 which block the backflow of hydraulic fluid, the need for the spool 100 to return to the null (centered) position to prevent such backflow is eliminated.

The spool valve 92 is made up of a cylindrical member 98 and a spool 100 which is slidable to and fro within the member 98. The spool 100 has cylindrical lands 100a and 100b on opposed ends thereof, and the lands 100a and 100b, which fit snugly within the member 98, are positioned so that the land 100b will block the exit of hydraulic fluid from the return line 96, or the land 100a will block the exit of hydraulic fluid from the return line 94, or the lands 100a and 100b will block the exit of hydraulic fluid from both return lines 94 and 96, as is shown in FIG. 11, where the camshaft 26 is being maintained in a selective intermediate position relative to the crankshaft of the associated engine.

Control of the spool 100 within the member 98 is in response to a force from lead screw 142 of stepper motor 134 which bears against the land 100b of the spool 100. The stepper motor 134 is an electromechanical device in which lead screw 142 can achieve a number of discrete positions. These positions are available sequentially as a result of a series of step commands. That is, a sequence of "n" step pulses moves the motor, and thus lead screw 142, "n" steps. These steps can be in either a forward or reverse direction. In the present VCT system, spool 100 position corresponds directly with the stepper motor 134 step position through lead screw 142 and is controlled by controller 208 as described above. As is shown in FIGS. 4 and 5, the stepper motor 134 assembly may be mounted at an exposed end of the camshaft 26 so that the lead screw 142 can bear against an end of land 100b.

The position of the spool 100 within the member 98 is influenced by spring 102 which acts on the end of land 100a to resiliently urge the spool 100 to the left, in the orientation illustrated in FIG. 11. The position of the spool 100 within the member 98 is further influenced by pressurized hydraulic fluid within a portion 98a of the member 98, on the outside of the land 100a, which urges the spool 100 to the left. The portion 98a of the member 98 receives its pressurized fluid (engine oil) directly from the main oil gallery ("MOG") 130 of the engine, and this oil is also used to lubricate a bearing 132 in which the camshaft 26 of the engine rotates.

The vane 60 is alternating urged in clockwise and counter clockwise directions by the torque pulsation in the camshaft 26 and these torque pulsations tend to oscillate the vane 60, and, thus, the camshaft 26, relative to the sprocket 32. However, in the FIG. 11 position of the spool 100 within the cylindrical member 98, such oscillation is prevented by the hydraulic fluid within the recesses 32a, 32b of the sprocket 32 on opposite sides of the lobes 60a, 60b, respectively, of the vane 60, because no hydraulic fluid can leave either of the recesses 32a, 32b, since both return lines 94, 98 are blocked by the position of the spool 100. If, for example, it is desired to permit the camshaft 26 and vane 60 to move in a counter clockwise with respect to the sprocket 32, it is only necessary to step the motor 134 such that lead screw 142 moves to the right. This will urge the spool 100 to right and thereby unblock the return line 94. In this condition of the apparatus, counter clockwise torque pulsations in the camshaft 26 will put fluid out of the portion of the recess 32a and allow the lobe 60a of vane 60 to move into the portion of the recess which has been emptied of hydraulic fluid. However, reverse movement of the vane will not occur as the pulsations in the camshaft become oppositely directed unless and until the spool 100 moves to the left, because of the blockage of the fluid flow through the return line 96 by the land 100b of the spool 100. Thus, large pressure variations induced by camshaft torque pulses will not affect the condition of the system, eliminating the need to synchronize the opening and closing of the spool valve 92 with individual torque pulses. While illustrated as a separate closed passage in FIG. 11, the periphery of the vane 60 actually has an open oil passage slot, element 60c in FIGS. 2-10, which permits the transfer of oil between the portion of the recess 32a on the right side of the lobe 60a and the portion of the recess 32b on the right side of the lobe 60b, which are the nonactive sides of the lobes 60a and 60b; thus, counter clockwise movement of the vane 60 relative to the sprocket 32 will occur when flow is permitted through return line 94 and clockwise movement will occur when flow is permitted through return line 96.

Further, the passage 82 is provided with an extension 82a to the nonactive side of one of the lobes 60a or 60b, shown as the lobe 60b, to permit a continuous supply of makeup oil to the nonactive sides of the lobes 62a and 62b for better rotational balance, improved damping of vane motion, and improved lubrication of the bearing surfaces of the vane 60.

Makeup oil for the recesses 32a, 32b of the sprocket 32 to compensate for leakage therefrom is provided by way of a small, internal passage 120 within the spool 100, from the passage 98a to annular space 98b of the cylindrical member 98, from which it can flow into the inlet line 82. A check valve 122 is positioned within the passage 120 to block the flow of oil from the annular space 98b to the portion 98a of the cylindrical member 98.

The elements of the structure of FIGS. 2-10 which correspond to the elements of FIG. 11, as described above, are identified in FIGS. 2-10 by the referenced numerals which were used in FIG. 11, it being noted that the check valves 84 and 86 are disc type check valves in FIGS. 2-10 as opposed to the ball type check valves of FIG. 11. While this type check valves are preferred for the embodiment of FIGS. 2-10, it is to be understood that other types of check valves can also be used.

Although the best mode contemplated by the inventors for carrying out the present invention as of the filing date hereof has been shown and described herein, it will be apparent to those skilled in the art that suitable modifications, variations, and equivalents may be made without departing from the scope of the invention, such scope being limited solely by the terms of the following claims.