Search Results

Historically slip in a CVT was regarded as destructive. The reason for this was that slip was not controllable and since it is unstable always resulted in damage to the variator. Recent publications suggest that limited amounts of slip in a pushbelt type variator can be allowed [6]. This opens the door to other strategies for lowering the powerconsumption of CVT's. Not only can slip be used for optimizing variator efficiency [1], actuation efficiency can also be greatly improved. If the safety margin is eliminated, the clamping force can be reduced by more than 25%. This can be directly translated into a 25% decrease in actuation power. Shifting behaviour is also influenced by slip [3]. This effect can be used to greatly reduce the power needed for fast shifting during emergency stopping, tip-shifting and kickdown actions. Using this strategy the force needed for shifting is reduced, and with it the power needed from the actuation system is reduced.

Apart from enabling continuous ratio change under load, the Continuously Variable Transmission (CVT) offers, between limits, the ability to choose engine rotational speed independently of vehicle speed. Here lies a potential efficiency benefit, because the engine can operate more fuel efficiently. There are unfortunately considerable power losses within the CVT itself, causing many CVT equipped cars to be less fuel-efficient than cars with a manual transmission. The internal losses are caused for a substantial part by the CVT’s hydraulic actuation. An electromechanical CVT pulley actuation system was designed to overcome the hydraulic power loss and hence improve CVT efficiency. Spindles are driven from the fixed world through epicyclic gearings by electric motors that are placed outside the transmission housing in a cool environment. A mechanical link between the adjustment mechanisms on the two shafts provides energy exchange, thus lowering shifting power demand and actuator size.

For analysis, control design and testing of an electromechanically actuated metal V-belt type CVT, a simulation model is built. Due to its complexity and nonlinear behavior, this model is not suitable for control design. To use control design techniques like H1 or μ-synthesis, linear transfer functions from all inputs to all outputs must be known. Using approximate realization techniques, step responses simulated with the model are analyzed. In this way, a linearized state-space representation of the system is obtained. The transfer functions show resonances around 8−10 [Hz], depending on the CVT ratio. Using MIMO-control, closed loop bandwidths that could be obtained are up to 10 [Hz] for ratio control and 15 [Hz] for slip control.