TransmissionInterfaceLoader cannot find a handle of JointStateInterface

[canalteks]

Dear developers,

I found that transmission_interface::TransmissionInterfaceLoader will throw an exception in loading new hardware_interface/Position,Velocity,EffortJointInterfaces from URDF::transmission tags.

This problem only occurs when the users used their own RobotHw with JointStateInterface.

It's because that

• transmission_interface::VelocityJointInterfaceProvider, and also Position,EffortJointInterfaceProvider, are using joint_interfaces.joint_state_interface to get a joint state handle from the resouce.
• but unfortunately, if user's RobotHw has its own JointStateInterface, the code of JointStateInterfaceProvider only populates the RobotHw's JointStateInterface but not TransmissioInterfaceLoader::loaderdata.joint_interfaces.joint_state_interface.
• it keeps TransmissioInterfaceLoader::loaderdata.joint_interfaces.joint_state_interface is empty.

Then, the inconsistency of references between two JointStateInterfaces makes this problem.

I'll suggest following approaches,

1. notify in the comment of the TransmissionInterfaceLoader --- RobotHw should not have its own JointStateInterface to use TransmissionInterfaceLoader,
2. or add this code, instead of using joint_interfaces.joint_state_interface.

// test code to make this problem
RobotHw  robot_hw;
RobotTransmissions  robot_transmissions;
JointStateInterface  joint_state_interface;
robot_hw.registerInterface( &joint_state_interface );

TransmissionInterfaceLoader  loader( &robot_hw, &robot_transmissions );

loader.load( urdf );

yours sincerely.

[littlejohnyang]

I am having similar issue here. I have my own RobotHW with JointStateInterface. It'll throw an error "Failed to load transmission 'xxxx'. It contains no valid hardware interface. I can control the motors fine but I will not be able to receive any feedback. The moment I register a JointStateInterface with my RobotHW class, it'll throw this error. But I need it in order to control the group of motors through joint trajectory controller.

But the unit test cases inside ros_control has exactly the same code. Has the developer not run into the same problem with registering JointStateInterface with RobotHW?

[mathias-luedtke]

It took me a while to understand the problem, but I can confim @canalteks's code analysis. Another problem exists for the other interface types, if you try to register them after running loader.load.

As a workaround, instead of registering the interfaces yourself, you could use the interfaces from loader.getData()->joint_interface

[jlack1987]

I too have found my way to this bug. Gotta give @canalteks a hand in actually tracking down the problem. I just started getting very strange plugin load errors that gave me no indication of what the actual problem was.

[bmagyar]

Can we afford to eliminate the local joint state interface as the (this code) section of point 2 suggests? This would be a slightly breaking change but I think it could still fly for melodic.

[ompugao]

any updates for this issue? I got a same error in melodic. How should I avoid this error?

[jlack1987]

@ompugao the way I get around it is I load all transmissions containing my custom hardware interfaces before all others. Bit hacky, but it works for me

[bmagyar]

I've been thinking about this recently working on similar stuff and I am opting for point 2. What do you guys think of such a change?