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Because, as far as I understand the ros_control pkg require the close loop system to control it with PID.

This is actually not entirely correct.

Some controllers in ros_controllers are closed-loop, but there are quite a few which are actually open-loop (so called command forwarding controllers).

Is it possible to use ros_control pkg with open loop system.

So: yes. This would be possible.

I mean I have a stepper motor without encoder, so I don't have the position of the motor, I calculating it just by summarize every time my command in pulses to the motor.

If you implement a hardware_interface that simply copies command -> current_state (perhaps with a small delay) then none of the controllers will ever notice this.

Because, as far as I understand the ros_control pkg require the close loop system to control it with PID.

This is actually not entirely correct.

Some controllers in ros_controllers are closed-loop, but there are quite a few which are actually open-loop (so called command forwarding controllers).controllers). For instance: all controllers which take a position setpoint and output a position (ie: position_controllers/JointPositionController) are completely open-loop (see here).

Is it possible to use ros_control pkg with open loop system.

So: yes. This would be possible.

I mean I have a stepper motor without encoder, so I don't have the position of the motor, I calculating it just by summarize every time my command in pulses to the motor.

If you implement a hardware_interface that simply copies command -> current_state (perhaps with a small delay) then none of the controllers will ever notice this.