Modeling remote driven link

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Hi Everyone,

I'm modeling a robot simulation using gazebo, and have a question about the parent child relationship of the links.

My robot consists of a base and 6 legs. Each leg consist of three links/joints. So lets say I have a configuration which consist of : (base – hip – upper leg – lower leg) all connected by a revolute joint. So in first place you would say that:

Base is parent of the Hip. Hip is parent of the upper leg. Upper leg is parent of the lower leg.

But in real robot it is somewhat different. Here the lower leg is remotely driven. So when the upper leg rotates about the (hip-upper leg) joint, the lower leg will only translate along with the upper leg, but does not change its orientation! w.r.t. the hip.

Here also a figure of the upper and lower leg is given, which should make the question more clear. So when theta_1 changes, vartheta_2 will remain the same. So theta_2 will change!

Anyone an idea to implement this in Gazebo? Can I do something with the parent-child relations, add a massless assistant link to accomplish this, or do I have to do something totally different?

Thanks in advance!

Erik