As the first step, we can obtain vessel motions by pushing and pulling the vessel and analyze the characteristics of the vessel。 From the simulations and experiments repeated thus, we can define the parameter values described in each dynamic equation。

The second step is the motion analysis process of the winch with cable。 The cable tension is changed by winding and un- winding the winch。 The tension variations can induce various cable motions which may be unpredictable and  uncertain in the sea state, as shown in Fig。 4。 This means that the cable tension affects the vessel motions as an unpredictable input force。 Then, the input force made from cable tension variation can be considered as a disturbance。 Therefore, if we have the frequency and amplitude information about the uncertain ca- ble motions, we can provide a useful control strategy to cope with the uncertainty。 Based on this fact, the authors found the information required for designing the control system by ex- periment。

3。2 Experiment apparatus and modeling

3。2。1 Apparatus setup for experiment

To evaluate the proposed control strategy, a pilot model is made to simulate a mooring winch system as shown in Fig。 5。 In  Fig。  5,  ③~⑤  and  ⑦  represent  the  vessel  part  which

Fig。 4。 Various uncertain cable motions produced by winch operation。

Fig。 5。 Single type pilot model for mooring winch system。

nominal winch model first because the winch dynamics is mixed with the cable dynamics in the experimental data。 If the winch model is achieved first, it is possible to extract the cable motions from the experimental data。 Hence, the identification process to extract the cable dynamics is illustrated in Fig。 6。

slides  on  the  frame。  The  vessel  part  is  connected  to    ①

Following this idea, a winch model Gw (s)

is obtained from

(damper) and ② (spring) which are equipped to cope with the passive cable motions as shown in Figs。 2 and 3。 As ex- plained earlier, the cable motions are identified from the ex- periments of winding and unwinding the winch。 These actions

the experiment in the load free condition。 It means that the

cable is not connected to the winch。 As a result, the winch model is obtained as follows:

induce various cable motions which give input force changes to the vessel, and thus are considered as uncertainties。

3。2。2 Modeling

First, let us define the parameter values of the vessel part given in Eq。 (1)。 These are Ks and Ds except Ms which can be measured easily。 From the initial response experiments, the two parameters including the vessel mass are obtained as fol- lows:

To obtain the cable motions, we fixed the vessel part to the frame。 After winding (pulling) and unwinding (releasing) the winch in the various operating conditions where the cable length are changed within 2~5 m, we can obtain persuadable information about the cable motions。 In the result, we obtained the experiment results considering the winch operating condi- tions as shown in Fig。  7。 Fig。  7 describes the obtained    force

variations measured by load cell which is installed on the winch。 It is natural and clear that there are complicated    cable

Then we try to obtain the characteristics of the active  cable文献综述

motions。

To  obtain  the  cable  motions,  it is necessary to define  the

motions with wide frequency and amplitude range。 Analyzing the frequency responses, we can obtain a representative model ( Gd (s) ) which appears in Fig。 8 as solid line and finally given