Custom robot hardware_interface implementation

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Clearpath's Husky is also using ros_control. The main reason for most people to implement a ros_control layer is to gain access to the standard set of controllers & a fairly seamless gazebo integration.

I agree that relying on ROS services in a RobotHW is not a great idea but if you can do it with topics, you suddenly have a non-blocking setup which, provided that the rest of the system manages to keep up, should work out fairly well. Of course you won't have realtime-safety guarantees but if you are running something low-rate of fairly safe, you are probably going to be fine.

The most extreme non-realtime ros_control implementation I made had a texas instruments microcontroller board talked to ros_control over rosserial and some topics. It was fine, ros_control is not only for industrial communication back-ends.

Here's a great example from Clearpath on how to make a very simple RobotHW implementation with topics, no realtime guarantees: https://github.com/jackal/jackal_robot

The short answer is yes you could have service calls or a continually updating topic subscription populate your read() method and in your write() method have a topic publish or action service request. If you did want something simple like this I wouldn't bother using ros_control however, it would be more suitable to just write your own py/cpp node that doesn't implement the robot_hw interface.

The main reason why this isn't a good idea is that your control loop is going to be slow as you are using non-realtime operations within it. Whilst its possible to avoid this with realtime publishers and subscribers or just having a slow control loop, as I said previously, it wouldn't be worth the effort to use ros_control.

You are right that there is no specific need for a i2c or CAN or etherCAT communication system in your robot_hardware but you will find that most research and commercial implementations will have this. I would recommend looking through some real world examples on github like Clearpath's Husky.