Speed ratio control device and method for working vehicle

1. A speed ratio control device for a working vehicle that uses an output of an internal combustion engine as a working power and a traveling power, and comprises an accelerator pedal that increases a rotation speed of the internal combustion engine when depressed and a transmission that shifts a rotation of the internal combustion engine and applies the shifted rotation to the traveling power, comprising:

a programmable controller programmed to:

calculate a first target vehicle speed according to the rotation speed of the internal combustion engine;

calculate a second target vehicle speed according to a depression amount of the accelerator pedal;

vary the speed ratio of the transmission to achieve the first target vehicle speed when the first target vehicle speed is lower than the second target vehicle speed; and

vary the speed ratio of the transmission to achieve the second target vehicle speed when the first target vehicle speed is not lower than the second target vehicle speed.

2. The speed ratio control device for a working vehicle as defined in claim 1 , further comprising:

a sensor that detects the rotation speed of the internal combustion engine; and

a sensor that detects the depression amount of the accelerator pedal.

3. The speed ratio control device for a working vehicle as defined in claim 1 , wherein the transmission is constituted by a hydrostatic transmission combining a variable capacity hydraulic pump and a variable capacity hydraulic motor.

4. A speed ratio control device for a working vehicle that uses an output of an internal combustion engine as a working power and a traveling power, and comprises an accelerator pedal that increases a rotation speed of the internal combustion engine when depressed, a transmission constituted by a hydrostatic transmission combining a variable capacity hydraulic pump and a variable capacity hydraulic motor to shift a rotation of the internal combustion engine and applies the shifted rotation to the traveling power, and an inching pedal that applies braking to the working vehicle when depressed beyond a fixed amount, the device comprising:

a programmable controller programmed to:

calculate a first target vehicle speed according to the rotation speed of the internal combustion engine;

calculate a second target vehicle speed according to a depression amount of the accelerator pedal;

vary the speed ratio of the transmission to achieve the first target vehicle speed when the first target vehicle speed is lower than the second target vehicle speed; and

vary the speed ratio of the transmission to achieve the second target vehicle speed when the first target vehicle speed is not lower than the second target vehicle speed;

wherein the controller is further programmed to:

set the smaller of the first target vehicle speed and the second target vehicle speed as a vehicle speed command basic value;

calculate a final vehicle speed command value by reducing the vehicle speed command basic value in accordance with a depression amount of the inching pedal; and

vary the speed ratio of the transmission such that the final vehicle speed command value is realized.

5. The speed ratio control device for a working vehicle as defined in claim 4 , wherein the controller is further programmed to set the final vehicle speed command value at zero when the depression amount of the inching pedal exceeds a predetermined amount.

6. The speed ratio control device for a working vehicle as defined in claim 3 , wherein the controller is further programmed to maintain the speed ratio at infinity by setting a capacity of the hydraulic pump at zero when the final vehicle speed command value is zero.

7. The speed ratio control device for a working vehicle as defined in claim 1 , wherein the internal combustion engine comprises a mechanical throttle which is connected to the accelerator pedal via a mechanical link and varies a throttle opening in accordance with the depression amount of the accelerator pedal.

8. The speed ratio control device for a working vehicle as defined in claim 1 , wherein the working vehicle further comprises an accelerator lever that increases the rotation speed of the internal combustion engine when operated.

9. A speed ratio control method for a working vehicle that uses an output of an internal combustion engine as a working power and a traveling power, and comprises an accelerator pedal that increases a rotation speed of the internal combustion engine when operated and a transmission that shifts a rotation of the internal combustion engine and applies the shifted rotation to the traveling power, comprising:

using a calculating and modifying device to calculate a first target vehicle speed according to the rotation speed of the internal combustion engine;

using the calculating and modifying device to calculate a second target vehicle speed according to a depression amount of the accelerator pedal;

varying a speed ratio of the transmission to achieve the first target vehicle speed when the first target vehicle speed is lower than the second target vehicle speed; and

varying the speed ratio of the transmission to achieve the second target vehicle speed when the first target vehicle speed is not lower than the second target vehicle speed.