Continuously variable transmission and control method thereof

A continuously variable transmission installed in a vehicle, comprising:

• a continuously variable transmission mechanism (to be referred to hereafter as a “variator”) which modifies a speed ratio continuously;
• a subtransmission mechanism provided in series with the variator and including, as forward gear positions, a first gear position and a second gear position having a smaller speed ratio than the first gear position;
• and
• shift control means for modifying a through speed ratio, which is an overall speed ratio of the variator and the subtransmission mechanism, by modifying the speed ratio of the variator and changing the gear position of the subtransmission mechanism on a basis of an operating condition of the vehicle,
• wherein the through speed ratio when the speed ratio of the variator is a Highest speed ratio and the gear position of the subtransmission mechanism is the first gear position (to be referred to hereafter as a “low speed mode Highest speed ratio”) is smaller than the through speed ratio when the speed ratio of the variator is a Lowest speed ratio and the gear position of the subtransmission mechanism is the second gear position (to be referred to hereafter as a “high speed mode Lowest speed ratio”),
• a midway value between the low speed mode Highest speed ratio and the high speed mode Lowest speed ratio is set as a mode switch speed ratio, and
• when the through speed ratio varies across the mode switch speed ratio, the shift control means changes the gear position of the subtransmission mechanism and modifies the speed ratio of the variator in an opposite direction to a variation direction of the speed ratio of the subtransmission mechanism.

A control method for a continuously variable transmission installed in a vehicle and having a continuously variable transmission mechanism (to be referred to hereafter as a “variator”) which modifies a speed ratio continuously and a subtransmission mechanism provided in series with the variator and including, as forward gear positions, a first gear position and a second gear position having a smaller speed ratio than the first gear position;

• the control method comprising:
• a shift control step for modifying a through speed ratio, which is an overall speed ratio of the variator and the subtransmission mechanism, by modifying the speed ratio of the variator and changing the gear position of the subtransmission mechanism on a basis of an operating condition of the vehicle,
• wherein the through speed ratio when the speed ratio of the variator is a Highest speed ratio and the gear position of the subtransmission mechanism is the first gear position (to be referred to hereafter as a “low speed mode Highest speed ratio”) is smaller than the through speed ratio when the speed ratio of the variator is a Lowest speed ratio and the gear position of the subtransmission mechanism is the second gear position (to be referred to hereafter as a “high speed mode Lowest speed ratio”),
• a midway value between the low speed mode Highest speed ratio and the high speed mode Lowest speed ratio is set as a mode switch speed ratio, and
• when the through speed ratio varies across the mode switch speed ratio, the shift control step changes the gear position of the subtransmission mechanism and modifies the speed ratio of the variator in an opposite direction to a variation direction of the speed ratio of the subtransmission mechanism.