Ratio and torque control
Torque control is achieved by controlling the force at the roller; ratio control is achieved by controlling the operating position of the roller.
Torotrak variators normally operate in ‘torque control’ mode. In this mode, hydraulic pressure is applied to the roller pistons. The force generated by this hydraulic pressure is reacted by traction fluid shear forces at the input and output contact patches, which in turn give rise to input and output disc torques. If the disc speed ratio changes as a result of the applied disc torque (or indeed any externally applied torques), the rollers automatically ‘steer’ to a new position, thereby accommodating the new speed ratio. This self-steering occurs as a result of the special geometry employed within the variator.
Torque control is an enabler for ‘geared neutral’ when the variator is used in an IVT configuration. One benefit of torque control is that it eliminates any risk of ‘nervous neutral’ which otherwise requires extremely fine ratio control. Furthermore, hydraulic actuation ensures that each roller contributes equally to the overall power transfer, thereby ensuring that no one roller is overloaded. This maximises power transfer capability and efficiency. It also eliminates the need for uneconomic levels of variator manufacturing accuracy.
Torque control can provide a similar feel to that offered by a torque convertor (but without the inefficiency), and is therefore suited to a range of applications, including automotive, truck and bus main drive transmissions. It also offers the ideal control system for the Kinetic Energy Recovery System (KERS), which requires a CVT for the control of power flow to and from the flywheel.
Ratio control involves controlling the speed ratio of the variator. There are multiple options for achieving ratio control, and the choice of system is governed by the transmission application and its requirements. A common approach is the addition of a control loop to the standard torque-controlled variator. This closed loop may be implemented either by using a hydro-mechanical arrangement (thereby eliminating the need for electronics), or by using speed sensors and control software. In both cases the desired speed ratio is ultimately achieved through the application of hydraulic pressure to the rollers.
An agricultural tractor carrying out a soil engaging activity, such as ploughing, is an ideal candidate for a ratio control variator. As soil conditions vary dynamically, ratio control ensures a constant operating speed irrespective of the changing loads on the plough; a ratio control system is also fully compatible with existing tractor draft-control systems. It is also possible to revert to torque control mode at any point. Taking the agricultural tractor example, this ensures optimum performance during other tasks such as front loading (where tractive effort, rather than wheel speed control is desirable). In this way, the transmission offers two distinct functional modes in a single cost-effective solution.