System and method for controlling a continuously variable transmission

1. A method of controlling a continuously variable transmission, comprising:

sensing an output speed of a motor;

determining an upper speed limit based on the output speed of the motor and a positive torque limit associated with the motor;

determining a limited motor speed command based on the lesser of a motor speed command and the upper speed limit; and

controlling the motor based on the limited motor speed command.

2. The method of claim 1 , further comprising:

receiving an input signal from an operator representative of a requested speed; and

determining the motor speed command based on the input signal received from the operator.

3. The method of claim 1 , wherein the determining of the upper speed limit includes accessing a look-up table to determine the positive torque limit as a function of the output speed of the motor.

4. The method of claim 1 , wherein for an actual motor speed less than X and greater than −Y, the upper speed limit is a constant value.

5. The method of claim 4 , wherein X is chosen to be slightly less than an actual motor speed that marks a transition between the constant power region of the motor and a torque limited region of the motor.

6. The method of claim 1 , further comprising:

sensing an output speed of an engine and producing an engine speed signal indicative of the engine speed; and

reducing the limited motor speed command when the engine speed signal drops below a threshold value.

7. A method of controlling a continuously variable transmission, comprising:

sensing an output speed of a motor;

determining an upper speed limit based on the output speed of the motor and a positive torque limit associated with the motor;

determining a limited motor sneed command based on the lesser of a motor speed command and the upper speed limit;

controlling the motor based on the limited motor speed command;

determining a lower speed limit based on the output speed of the motor and a negative torque limit of the motor, wherein the determining of the limited motor speed command includes:

setting the limited motor speed command equal to the upper speed limit, if the motor speed command is equal to or greater than the upper speed limit;

setting the limited motor speed command equal to the motor speed command, if the motor speed command is less than the upper speed limit and greater than the lower speed limit; and

setting the limited motor speed command equal to the lower speed limit, if the motor speed command is equal to or less than the lower speed limit.

8. The method of claim 7 , wherein the determining of the upper and lower speed limits includes selecting an alternate lower speed limit if service brakes are used for retarding and the motor speed is greater than zero, and selecting an alternate upper speed limit if service brakes are used for retarding and the motor speed is less than zero.

9. A method of controlling a continuously variable transmission, comprising:

sensing an output speed of a motor;

determining an upper speed limit based on the output speed of the motor and a positive torque limit associated with the motor;

determining a limited motor speed command based on the lesser of a motor speed command and the upper speed limit; and

controlling the motor based on the limited motor speed command;

wherein the controlling of the motor based on the limited motor speed command includes calculating a motor torque command based on a difference between the limited motor speed command and the output speed of the motor, and controlling the motor based on the motor torque command.

10. The method of claim 9 , wherein the calculation of the motor torque command is performed by a proportional and integral (PI) control algorithm.

11. The method of claim 9 , wherein the upper speed limit is calculated as a value of the motor speed command that will cause the motor torque command to equal the positive torque limit of the motor.

12. The method of claim 11 , wherein the upper speed limit is calculated according to:

Upper Spd Limit=[(Motor Trq Limit/Lgain)−Iterm]/Pgain+Actual Motor Spd,

wherein Iterm=∫(Igain·(Motor Speed Cmd−Actual Motor Speed)·dt,

wherein Motor Trq Limit is the positive motor torque limit, Motor Speed Cmd is the motor speed command, and wherein Lgain, Pgain, and Igain are constants.

13. A method of controlling a continuously variable transmission, comprising:

sensing an output speed of a motor;

determining an upper speed limit based on the output speed of the motor and a positive torque limit associated with the motor;

determining a limited motor speed command based on the lesser of a motor speed command and the upper speed limit; and

controlling the motor based on the limited motor speed command;

wherein for an actual motor speed less than X and greater than −Y, the upper speed limit is a constant value,

wherein the constant value is equal to

(T_Lim_Pos_Shelf/(Lgain·Pgain)+X),

wherein T_Lim_Pos_Shelf is the maximum positive torque limit of the motor in a torque limited region of the motor, and wherein Lgain and Pgain are constants.

14. A method of controlling a continuously variable transmission including a motor, comprising:

determining an upper speed limit based on i) a sensed output speed of the motor, and ii) a positive torque limit associated with the motor; and

controlling the motor based on the lesser of a motor speed command and the upper speed limit.

15. A system for controlling a continuously variable transmission, comprising:

a power source;

a continuously variable transmission powered by the power source, the continuously variable transmission including a motor;

an electronic controller operative to (i) determine an upper speed limit based on an actual motor speed and a positive torque limit associated with the motor, (ii) determine a limited motor speed command based on the lesser of a motor speed command and the upper speed limit, and (iii) control the motor based on the limited motor speed command.

16. The system of claim 15 , further comprising: a motor speed sensor operatively coupled to sense an output speed of the motor and to provide an actual motor speed signal to the electronic controller.

17. The system of claim 15 , wherein the power source is an engine, and further including:

an engine speed sensor operatively coupled to sense an output speed of the engine and to provide an engine speed signal indicative of the engine speed to the electronic controller.

18. The system of claim 15 , further comprising:

an electric generator electrically coupled to provide electric power to the continuously variable transmission, wherein the motor is an electric motor.

19. The system of claim 15 , wherein the continuously variable transmission is a hydrostatic transmission and the motor is a hydraulic motor.

20. A system for controlling a continuously variable transmission, comprising:

an engine;

an electric generator operatively coupled to the engine;

a continuously variable transmission coupled to receive electric power from the electric generator, the continuously variable transmission including an inverter and an electric motor; and

an electronic controller operative to (i) determine an upper speed limit based on an actual motor speed and a positive torque limit associated with the electric motor, (ii) determine a limited motor speed command based on the lesser of a motor speed command and the upper speed limit, and (iii) control the electric motor based on the limited motor speed command.