There is known a valve apparatus that drives a spool valve through a linear solenoid (see, for example, JPH10-198431A).
JPH10-198431A discloses a technique of generating dither, which has a sufficiently small amplitude, in a spool of an electromagnetic spool valve device in the valve apparatus to place the spool into a state of dynamic friction, so that hysteresis is reduced.
The electromagnetic spool valve device has a zone, which is located around each of a movement start location of a plunger of the linear solenoid and a movement end location of the plunger. In this zone, even when the spool is moved in an axial direction, an output oil pressure of the electromagnetic spool valve device does not change.
In the following discussion, the movable range of the spool relative to a sleeve is assumed to have a control zone and two dead zones. The control zone is defined as a zone, in which the output oil pressure changes in response to movement of the spool in the axial direction. The dead zone is defined as a zone, in which the output oil pressure does not change even though the spool is moved.
(First Disadvantage)
Even in the dead zone, clamping of a foreign object between the sleeve and the spool can be limited through application of appropriate dither to the spool.
Therefore, it is desirable to apply the dither to the spool even in the dead zone.
Hereinafter, a signal, which is supplied to the linear solenoid to generate the dither in the spool, is defined as a dither signal.
A pulse height and a frequency of the dither signal are set such that the dither signal causes generation of the dither, which places the spool into the state of the dynamic friction, and a change in the output oil pressure, which is caused by the dither, is sufficiently reduced. Specifically, the pulse height and the frequency of the dither signal are set for the purpose of alleviating the hysteresis, and thereby the pulse height and the frequency of the dither signal are set to be appropriate in the control zone.
In the control zone, at the time of driving the spool, a spring force of a spring, a drive force of the linear solenoid, and a feedback (F/B) force exerted in a F/B chamber by a feedback oil pressure are changed (see FIG. 3A).
In contrast, in the dead zone, the F/B force does not change (see FIG. 3B).
The inventors of the present application have found that a spring constant of a spring system (including the spring and the F/B chamber), which applies a spring force against oscillation of the spool, differs between the control zone and the dead zone. Specifically, the spring constant of the spring system in the dead zone is smaller than the spring constant of the spring system in the control zone.
Therefore, at the time of changing the axial position of the spool from the control zone to the dead zone, since the spring constant is reduced in the dead zone, the amplitude of the dither of the spool is disadvantageously increased in the dead zone. Thereby, in a case where the dither is applied to the spool in the dead zone, at the time of changing a position of the spool from the control zone to the dead zone, the amplitude of the dither is increased to cause a large change in the output oil pressure.
(Second Disadvantage)
The inventors of the present application have found that due to the difference in the spring constant of the spring system, which applies the spring force to the spool, between the control zone and the dead zone, an eigenfrequency of the spool, the plunger and a push rod also differs between the control zone and the dead zone. Specifically, the eigenfrequency in the dead zone is reduced in comparison to the eigenfrequency in the control zone (see FIG. 6).
In order to ease the control of the amplitude of the dither, the frequency, which generates the dither, is normally set to a frequency that is slightly smaller than the eigenfrequency. Specifically, the frequency, which generates the dither, is set to be a frequency that is slightly smaller than the eigenfrequency in the control zone (see a dotted line C and reference sign F2 in FIG. 6).
However, since the eigenfrequency is reduced in the dead zone, the amplitude of the dither in the dead zone is reduced due to the influence of the eigenfrequency. Therefore, the amplitude of the dither, which can limit the clamping of the foreign object in the dead zone, cannot be applied to the spool in the dead zone.