We are using ignition citadel to simulate a robot leg with 3 motors. This leg ends in a screw connected at 4 points (see image below).

We have one revolute joint per motor - motor frame (joints MX in the picture), one revolute joint per motor frame - screw frame (RX in the picture) and we want one fixed joint between screw frame -screw (SX in the picture), so the joint hierarchy is the following:

• M1 -> R1 -> S1
• M1 -> R4 -> S4
• M2 -> R2 -> S2
• M3 -> R3 -> S3

All SX joints are in the same body (screw).

Is there any possible way to do this?

How could we define this structure in the SDF for ignition?

Thank in advance :)