How to move iiwa robot in Drake?

I'm trying to add a second iiwa robot to my simulation after figuring out how to add the first one. I'm currently attempting to move the first one using

iiwa_robot = plant.GetBodyByName("iiwa_link_0")
        X_B = plant.EvalBodyPoseInWorld(plant_context, iiwa_robot)

        
        tf = RigidTransform(p=[15, 10, 5]) 
plant.SetFreeBodyPose(plant_context,iiwa_robot,X_B.multiply(tf))

However I keep getting the error

Body 'iiwa_link_0' is not a free floating body

I've also tried the other iiwa_links_ which produced the same error. Am I supposed to change something in the sdf file? Is there a better way to do this that I'm just not thinking of?

Solution

When you add your first IIWA arm, you can call WeldFrames to put the robot at the desired location.

Here is the pseudo code

iiwa1 = Parser(plant).AddModelFromFile(iiwa_file, "iiwa1")
# Weld the link0 of iiwa1 with the world at a given location.
plant.WeldFrames(plant.world_frame(), plant.GetBodyByName("iiwa_link_0", iiwa1).body_frame(), RigidTransform(p=[15, 10, 5]))

You can similarly add the second IIWA arm and weld its link_0 to the world to a different location.

iiwa2 = Parser(plant).AddModelFromFile(iiwa_file, "iiwa2")
# Weld the link0 of iiwa2 with the world at a given location.
plant.WeldFrames(plant.world_frame(), plant.GetBodyByName("iiwa_link_0", iiwa2).body_frame(), RigidTransform(p=[10, 10, 5]))