Response control system for hydrostatic-mechanical transmissions

A response control system for a hydrostatic-mechanical transmission equipped with a mechanical transmission unit actuated through an input shaft connectable to a power source;

• a hydrostatic transmission unit which is connectable to the input shaft and comprises a pump and motor having their respective discharge controlling swash plates, the angle of at least either of the swash plates being variable;
• and a differential unit, having an output shaft, for actuating both the mechanical transmission unit and the hydrostatic transmission unit, the control system comprising:
• (a) speed ratio detecting means for detecting, in digging/moving operation, an actual speed ratio that is the ratio of the revolution speed of the output shaft to the revolution speed of the power source in order to determine if the actual speed ratio is equal to a specified value or less;
• (b) revolution speed difference computing means for computing the difference between the actual revolution speed of the power source and a target revolution speed for the power source;
• (c) target ratio computing means, operably coupled to the speed ratio detecting means and to the revolution speed difference computing means, for computing a target speed ratio that is a target value for the ratio of the revolution speed of the output shaft to the revolution speed of the power source and for setting a constant to substantially zero when the actual speed ratio detected by the speed ratio detecting means is equal to the specified value or less and that the difference between the actual revolution speed and the target revolution speed of the power source, which has been computed by the revolution speed difference computing means, is small, wherein the constant set by the target speed ratio computing means indicates the response of the transmission to a change in the revolution speed of the power source;
• and
• (d) swash plate angle controlling means, operably coupled to the target speed ratio computing means, for controlling the angle of at least either of the discharge controlling swash plates according to the target speed ratio computed by the target speed ratio computing means.