TRACTOR TECHNOLOGY 374 58 LANDTECHNIK 6/2003 Control of Continuously Variable Chain Converters in Transmission Systems
T he pull type chain variator has been subject of research projects at the Department of Agricultural Machinery of Techni-sche Universität München for tractors  and passenger cars before. In 2002 the results of a mutual large research project of several TUM-departments, sponsored by the DFG ("Deutsche Forschungsgemeinschaft") have been presented with the "Autarkic Hy-brid" : A parallel hybrid concept passenger car, based on a standard Opel Astra Caravan with Diesel engine, which was
... ne, which was equipped with additional components such as an electric engine (120 V, 4 quadrants) and a specially developed chain CVT gearbox with wide spreading in i 2-design. This paper presents the control structure chosen at the Department of Agricultural Machinery to control the speed ratio of the gearbox as well as it's further development for universal use, e.g. for tractor applications. Structure of the gearbox of Autarkic Hybrid Figure 1 shows the structure of the gearbox of Autarkic Hybrid. The gearbox is characte-rised by two driving ranges. In first range (V1) the clutch L1 is closed and variator shaft C is driven by the engine(s). The syn-chronised collar coupling K1 is also closed and shaft B of the CVT is connected to the final drive (E). In overdrive of range V1 at a speed ratio of approximately i = 0,458, the rotational speed difference in all four clutches disappears and the so-called synchronous point is reached. At this point the second synchronised collar coupling (K2) is being closed and the first wet clutch (L1) can be opened. In that situation (SYN), power still can be transmitted, now without the chain variator being engaged. Since the gear-box has a fixed gear ratio at SYN, acceleration is possible only by rising the engine speed. Further acceleration leads to an operation in second range: By closing the clutch L2, while opening the clutch K1, the driving and driven pulleys are inverted, compared to first range, and second range (V2) is reached, starting in underdrive again. Thus an overall spreading of the gearbox of about 22.5 is realised. Speed ratio control for range shifts in Autarkic Hybrid In order to provide fast, comfortable and successful range shifts, it is essential to exactly control the speed ratio at the synchronous point. This is done by controlling the clamping pressures. When controlling the speed ratio at steady state, on one hand, the clamping forces need to be high enough to prevent damage by slip of the chain, on the other hand a certain ratio of clamping forces has to be achieved, the so called ζ-ratio (ratio of clamping forces between driving and driven pulley) . In standard PIV-clamping system the base level of clamping pressure in the pulleys that prevents the chain from slipping is automatically achieved by throttling a constant oil flow in the so called torque sensors. An additional valve is used to raise one of the clamping pressures by further throttling in order to reach the required ζ-ratio in steady state. The required ζ-ratio is dependent on transmitted torque and speed ratio. Moreover the actual pulley speed leads to centrifugal forces affecting the chain and clamping forces (rotating hydraulic cylinders), so it can be seen as another disturbance variable. Assuming a rather narrow speed range in normal operation when doing Continuously variable transmissions (CVT) are constantly gaining in importance in mobile machines as well as in passenger cars. Special types of CVT are PIV-based pull type chain converters. New applications are raising the requirements for controlling these gear-boxes. The most important control variable is the speed ratio and the rate of ratio change (di/dt).