An automatic crew system has been implemented that is composed of three sub-systems, organized as shown in Table 2. The automatic crew has the task of sailing the yacht on a given racecourse, according to a set of basic strategic rules. Details on the sub-systems are provided in the following paragraphs. Table 2 - Automatic crew system
Sub-system Helmsman Input
yacht state variables; navigator decisions (e.g. target heading when sailing in a straight line).
Sail Trimmer
yacht state variables; navigator decisions (e.g. target sheeting angle).
Output rudder angle
; rudder rate d
dt . sheeting
angle ; sheeting rate d
dt .
Navigator yacht state variables; decisions. 5.1 HELMSMAN
An attempt of simulating human actions on a yacht rudder can be found in [5], where a proportional- derivative (PD) controller is adopted that controls the error between the actual and target yacht heading. Weather helm effect is accounted for by applying an open-loop rudder offset expressed as a predetermined function of true wind angle (twa). The PD controller is switched off while manoeuvring, when rudder position is being supplied as a function of time.
Two steering modes have been adopted here, in order to allow the yacht to sail a complete racecourse: a ‘fixed- awa’ mode for upwind and ‘dead downwind’ legs, when beating is necessary to get to the next mark, and a ‘fixed- heading’ mode for reaching legs, when it is possible to sail to the next mark in a straight line. Both steering fashions are based on simple PID controllers, whose gains have been adjusted in order to mimic actual time- histories of rudder angle. Upwind steering based on the ‘fixed-awa’ PID yields a straightforward, yet effective, model for tacking: the sign of target awa is changed and the PID lets the yacht tack around without exhibiting unrealistic overshoots.
Figure. 4 - Yacht reaching (t=1000s) towards Mark #3 at a fixed heading; WT = true wind vector 5.2 SAIL TRIMMER
A mainsail-genoa combination is considered here, whose details can be found in [8]. Two sail trimming modes have been implemented in the simulator: the first one provides ‘directly’ the sail plan CL and CD as a function of awa. Since no human judgement is involved, this can be regarded as the optimal trimming mode. A second trimming mode provides CL and CD ‘indirectly’: the sail trimmer module takes awa as an input and returns the sheeting angle with respect to the yacht centreline. Once is known, the sail angle of attack is calculated out of awa, the yacht leeway and through the formula below, according to Fig. 5
awa () (5)
CL and CD can then be calculated through lookup tables. Linear (awa) trimming ‘rules’ are adopted here: this is to say that sails are eased off as the helmsman bears away, which is a basic sail trim technique.
A PID controller has been implemented alongside with the latter sail trim sub-system. The PID is required to take over in strong breezes, when the sail plan is overpowered and sails have to be eased off in order to keep the yacht heel angle within acceptable limits. The choice of such a threshold for the heel angle represents a further aspect open to human judgement.
©2008: Royal Institution of Naval Architects
B-15
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