Model Based Control for by Ax2YAZF



                                           CVT 2004

                                 Category: CVT-systems
       Title : Model Based Control for a Pressure Control Type CVT

Wansik Ryu: Ph.D Student, School of Mechanical Engineering, Sungkyunkwan University,
              300 Chunchun-dong, Suwon, 440-746, Korea, e-mail :
Juhyun Nam, Engineer, Jatco, Korea
Yongjoon Lee, General Manager, Hyundai Motor Company
Hyunsoo Kim*: Professor, School of Mechanical Engineering,           Sungkyunkwan University,
           300 Chunchun-dong, Suwon, 440-746, Korea, e-mail :

        Hydraulic control for a metal belt CVT ratio control can be classified into two types : (1)
flow control and (2) pressure control. In the flow control system, the CVT ratio control is
carried out by controlling the oil flow to and from the primary actuator. On the other hand, in
the pressure control system, the desired speed ratio is obtained by controlling the primary
actuator pressure. It is known that the pressure control is easier than the flow control in
maintaining the desired speed ratio.         Furthermore, the pressure control system has an
advantage of preventing the belt form slippage in active manner since it is possible to control
the primary belt clamping force directly through the pressure control. In the flow control
system, the belt-pulley slippage can be controlled in passive manner, for instance, by adopting
the outlet orifice to restrict the exhaust oil flow, which provides the residual pressure.
        In this paper, a model based control algorithm for a pressure control type CVT has been
developed. Firstly, the P-line is proposed to predict the shift performance using the steady
state pressure characteristics. The P-line shows the pressure difference which can be used to
evaluate the shift speed. Two types of ratio control valves : type(I) without line pressure
area on the valve spool and type(II) with line pressure area are designed using the P-line.
For the two types of the ratio control valves, shift speed is evaluated by the P-line.       It is
found from the experiments that the larger the pressure difference from the P-line, the faster

    To whom all the correspondence concerning this paper should be addressed.

the shift speed. To design the model based control, steady state characteristics for the
pressure control type CVT ratio valve are obtained by the simulation and experiments and the
model based controller is designed.   It is found from the experimental results that the linear
controller such as PID control can be applied in the pressure control type CVT with
satisfactory tracking performance and robustness meanwhile nonlinear control algorithm or
adaptive control gains are required for the flow control type CVT. Moreover, the model based
controller shows superior robustness to the PID controller especially for the overdrive range
while having similar tracking performance with the PID control. Taking the advantage of
the pressure control type CVT, a model based primary pressure control algorithm is proposed
to prevent the belt-pulley slip. It is also found from the experiments that the model based
control provides higher primary pressure to prevent the slip.        However, the increased
primary pressure causes slower downshift response, which requires compromise between the
safety and the performance.

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