One proposal of a control mechanism for a hydrostatic-mechanical power transmission system of the above type is disclosed in Japanese Patent Publication No. 31660/1987. The control operation of the control mechanism for a hydrostatic-mechanical power transmission system in the above patent publication will be hereinbelow described.
A target revolution speed for the engine working as a power source is firstly obtained from throttle position, and then the deviation of the actual revolution speed of the engine from the target engine revolution speed is obtained. In accordance with the above deviation, the angle of the displacement setting swash plate of the pump disposed in the hydrostatic transmission unit is controlled, thereby making the actual engine revolution speed approach to the target engine revolution speed.
The above-described control mechanism, however, has poor response performance because of its feed-back control in which the amount of controlling the angle of the displacement setting swash plate of the pump in the hydrostatic transmission unit is based on the deviation of engine revolution speed.
In order to overcome such a drawback, the invention seeks to provide an improved control mechanism for a hydrostatic-mechanical power transmission system having good response performance, wherein the revolution speed of the power source changes in accordance with variations in throttle position to readily meet a target revolution speed.