1. Field of the Invention
The present invention relates to a method of controlling a clutch pressure of a transmission structured so as to engage a hydraulic clutch so as to set a stage for shifting a speed, and a control apparatus therefor.
2. Description of the Related Art
The transmission generally has a main transmission which is provided with a clutch for a plurality of stages for shifting the speed including a backward movement, and a sub transmission which is provided with a clutch for a high speed and a low speed, and there has been known a structure which performs a speed change in forward and backward directions by a combination between the main transmission and the sub transmission and a structure which is provided with a clutch for a forward movement stage for shifting the speed and a clutch for a backward movement stage for shifting a speed.
In recent years, a large-size transmission has been developed together with an enlargement of a vehicle.
The applicant of the present application has filed Japanese Patent Publication No. 5-65731 and PCT International Publication No. WO98/01687 with respect to the following technique, as a clutch hydraulic pressure control apparatus in a transmission corresponding to an enlargement of the vehicle.
There has been disclosed a method which is provided with an electromagnetic proportional pressure control valve and a flow amount detecting valve for each of speed change clutches of a clutch hydraulic pressure control apparatus, the method detecting a filling state of a clutch operating fluid within a piston volume of the clutch by the flow amount detecting valve, operating the electromagnetic proportional pressure control valve in real time at the same time of being filled with fluid so as to perform a gradual increase of a fluid pressure, performing a clutch engagement control, and measuring a time for being filled with the fluid at the preceding clutch engagement so that a timing for the clutch engagement does not become different even when the time for filling a clutch chamber is changed due to an abrasion of a clutch disc, a dispersion of a part processing accuracy and a variation of an engine speed so as to automatically set to a suitable target filling time after comparing with the suitable target filling time.
However, because in the technique described in Japanese Patent Publication No. 5-65731 and PCT International Publication No. WO98/01687, the fluid is caused to flow into the clutch chamber through throttling passages of the electromagnetic proportional pressure control valve and the flow amount detecting valve, the filling time of the fluid into the clutch chamber is elongated and the clutch engagement is delayed when it is applied to a clutch of a large capacity piston volume in a large-size transmission, so that an engagement time lag is generated. In particular, in the case of a construction vehicle, it is necessary to frequently repeat a speed change between a forward movement first speed and a forward movement second speed for excavating earth and sand. Accordingly, it is necessary that an amount of fluid to be supplied to the clutch chamber is set to a large amount when starting the speed change and to a small amount immediately before the clutch is engaged, thereby performing a rapid speed change and a reduction of the speed change shock.
Further, because the operation is performed while frequently repeating an engagement and a disengagement of the clutch, the oil is again started to be supplied to the clutch before the discharge of the oil is competed in the case that a discharging time of the oil within the clutch chamber at the disengaging time is long, so that a dispersion is generated in the time for the clutch engagement in the same clutch, an there is a problem that a speed change shock is easily generated because a lot of oil is supplied to the clutch chamber in which the oil is still left.
It is possible to supply a lot of oil at a time of starting the speed change and make the discharging time of the soil short at a time of disengaging the clutch by enlarging a capacity of the electromagnetic proportional pressure control valve and the flow amount detecting valve. However, a large space is required and costs are high.