Then, let us verify the proposed control approach through our experiment where we evaluate the control performance of our approach by comparing it with PID control, and the final distance target of moving plate (vessel part) is set as 0。08 m。

Fig。 16。 Response to the sinusoidal disturbance : simulation。

Fig。 17。 Control input made by controller : simulation。

Fig。 18。 Tracking and disturbance rejection performance with the pro- posed controller : experiment (distance : above, tension variation: bellow)。

First, Fig。 18 shows the step response (above) and cable ten- sion (below) of the proposed control strategy where the step type disturbance works continuously from 20 s。 This result shows that the proposed controller works well, such that good control performance is achieved without tracking error。

On the other hand, Fig。 19 shows the PID control results (the tracking performance above, cable tension below) where the step type disturbance are input to the controlled system irregularly。 This result explains that PID control is weak in coping with disturbance。

Especially, let us look at the cable tension in the rising state region (transient state), and compare it with the proposed con- trol case。 In this state, the cable tension force variation of PID (in Fig。 19 (below)) is rough and violent。

On the other hand, there is smooth and natural tension force variation in the proposed control system as shown in Fig。 18 (above)。 In fact, because the harsh tension variation  is the main cause of breakdowns of the mooring cable, it should be avoided in real applications。

Fig。 19。 Tracking and disturbance rejection performance with a PID controller : experiment (distance: above, tension variation: bellow)。

Fig。 20。 Frequency responses (open loop system : above, closed loop system : below)。

As shown in Fig。 20, the frequency responses of the open- loop and the closed loop indicate that the frequencies of the closed- loop have improved。

5。 Conclusion

This paper deals with the problems in designing a mooring winch control system。 When a mooring winch is in operation, the cable tension changes, and the variations in the cable ten- sion function as a force capable of restraining the motion or position of the ship。 However, the winding and unwinding of the cable creates a variety of unpredictable motions because of the physical characteristics (stiffness and viscosity) of the cable。

How to normalize the dynamic features of the cable and how to reflect them in designing the controller is an important issue。 The conventional FEM techniques that are used to de- duce detailed models represent the static characteristics of the cable, but those conventional models containing too many variables are too complex to be applied to designing a control- ler。 In fact, they are hardly used for that purpose。

Therefore, in this study, we constructed a practical cable model  to be directly applied  to  designing  a cable controller。

Since it is difficult to predict the dynamic characteristics of the cable, we proposed a method of normalizing the dynamic characteristics of the cables as a transfer function, regarding it as a factor of uncertainty。

The characteristics of cable motions (variations in tension) was measured and analyzed through several experiments of winding and unwinding of the cable using the winch under various conditions of cable lengths。 From the experimental results, a representative model was obtained, and this model was regarded an uncertainty in designing the controller using the strength control technique。

To verify the effectiveness of the proposed method of de- signing the controller, it was compared with experimental results obtained by applying the PID controller to prove the effective performance of the proposed designing technique。