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Underwater Robots - Gianluca Antonelli.pdf

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[-]<br />

1<br />

0.8<br />

0.6<br />

0.4<br />

0.2<br />

0<br />

close<br />

0 20 40<br />

[deg]<br />

60 80<br />

6.6 Fuzzy Inverse Kinematics 131<br />

Fig. 6.18. Membership function of the linguistic variable: joint limits used in the<br />

fuzzy singularity-robust task priority technique<br />

variables α i ’s. Roughly speaking, the motion has to be normally assigned to<br />

the manipulator, i.e., if all the α i ’s are null. Hence, the value of β is related<br />

to the value of the α i ’s and the first set of rules to be designed concerns<br />

the 3linguistic variables α 1 , α 2 and α 3 .Acomplete and consistent set of<br />

fuzzy rules for 3linguistic variables each ofthem defined intwo fuzzy sets<br />

requires 2 3 rules for every output leading to32total rules. The rules have<br />

been arranged in ahierarchical structure that gives higher priority tothe<br />

kinematic singularity ofthe manipulator and lower tothe vehicle attitude.<br />

Hence, the following 8rules have been used:<br />

1. if (manipulator is singular) then ( α 1 is high);<br />

2. if (manipulator is not singular) then ( α 1 is low);<br />

3. if (manipulator is singular) then ( α 2 is low);<br />

4. if (joint limits is not close) then ( α 2 is low);<br />

5. if (manipulator is not singular) and (joint limits is close)<br />

then ( α 2 is high);<br />

6. if (manipulator is singular) or (joint limits is close)<br />

then ( α 3 is low);<br />

7. if (vehicle attitude is small) then ( α 3 is low);<br />

8. if (manipulator is not singular)<br />

and (joint limits is not close)<br />

and (vehicle attitude is not small) then ( α 3 is high).<br />

In detail, the rules are developed as follows:<br />

• The first two rules concern the primary task. Inthis case, the manipulator<br />

singularity isofconcern and the variable α 1 is activated when the<br />

manipulator isclose to asingularity.

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