I know that a redundant link is a link that doesn't affect the motion of the mechanism even if it is removed . But how could we recognize these links ?

I was solving a problem abuout finding the mobility of a mechanism. I have noticed that the mechanism is negative , although the configuration of the mechanism appears as if the mechanism is able to move , so this is a contradiction. Is this method used to investigate if there are redundant links ? (By finding the mobility first and checking whether the mechanism is able to move or not )

After this step , I think we must recalculate the true mobility , by first removing the redundant links . How could we determine these links ? Is there a certain method ? or I must check all the links , by imagining what will happen to the motion of the mechanism if each link was removed ?

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    $\begingroup$ Could you give a more specific example of a scenario that causes you difficulty? Your question is really difficult to understand $\endgroup$ – BarbalatsDilemma Jul 31 '17 at 13:34

The mobility criterion is not perfect. If there is a redundant link then either the mechanism is locked or the link is replicating a different links movement.

Thespecial cases are somewhat self evident because it follows one of the predefined patterns.

  1. essentially same joint is calculated several times:
    1. A Slider constraint that is connected from multiple points, you can identify it by two parallel sliders.
    2. A revolute constraint that connects in multiple places of the same axis. For example a realworld door hinge is connected in 2 places but has only one joint!
    3. A planar constraint constrained to same rigid section on same plane
    4. ...
  2. Its a parallellogram of somekind.

Also quite often, for beginners, its not actually redundant but you have miscalculated or misunderstood the notation.

Anyway you can find the redunant joint by searching for sub graphs that are locked apparently locked and replacing them with rigid links if they dont fall under special cases. Usually you find triangles. Another way is to split and calculate mobility on both sides and infer from that.

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