1.  Field of the Invention
The present invention relates generally to a variable valve timing mechanism for an internal combustion engine, which changes a rotation phase of a camshaft relative to a crankshaft so as to vary valve timing of an engine valve, and more particularly to a novel technology for detecting the rotation phase of the camshaft. 
2.  Description of the Related Art
Japanese Unexamined Patent Publication No.  2000-303865 discloses a method of detecting a rotation phase of a camshaft relative to a crankshaft in an in-line four-cylinder internal combustion engine provided with a variable valve timing mechanism. 
In the above detecting method, there are provided: a crank angle sensor which generates a crank angle signal at each angle equivalent to a stroke phase difference between cylinders; and a cam angle sensor which generates a cam angle signal at an angular interval the same as that of the crank angle sensor, so that a difference in phase of both signals, i.e., a phase difference of the cam angle signal from that of the crank angle signal is measured. 
In a region where an engine rotating speed is low, such as, at the starting of an engine operation, at an idling time or the like, it is required to achieve an improvement in the operating performance of the engine by varying valve timing. 
However, in the conventional apparatus, since the detecting cycle of the valve timing is set at each difference in the stroke phase between cylinders, a time interval in which the detecting result of the valve timing is updated becomes long in the low rotating speed region. 
Therefore, conventionally, in a feedback control of the variable valve timing mechanism, the overshooting might occur in the low rotating speed region.