why do we need the (pid) controllers in ros_control?

asked 2021-12-25 21:41:58 -0500

zhixin
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For example, I have a robot with a motor controlled by a embeded board and I want to use ros applications to control the robot. Usually, there is a pid , let's say, speed controller implemented in the embeded board(or in the driver of motor). The SBC send(write) speed control signals to the embeded board. So I believe all we need is a hardware interface to translate ros msgs to embeded's control signal. what is the point of controllers in ros_control? Can I only implement a hardware interface and how?

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answered 2021-12-28 10:12:10 -0500

osilva

1650 ●3 ●172 ●19 https://github.com/rob...

Hi @zhixin, let's start with ros_control package:

The ros_control packages are a rewrite of the pr2_mechanism packages to make controllers generic to all robots beyond just the PR2.

The ros_control packages takes as input the joint state data from your robot's actuator's encoders and an input set point. It uses a generic control loop feedback mechanism, typically a PID controller, to control the output, typically effort, sent to your actuators. ros_control gets more complicated for physical mechanisms that do not have one-to-one mappings of joint positions, efforts, etc but these scenarios are accounted for using transmission

Source: http://wiki.ros.org/ros_control One of the goals of ROS is not to re-invent the wheel so ros_controlpackage allows for wide range of applications. However it's not the only way to interact with Hardware, you could do that through ROS1 messages and services. Refer to this tutorial: http://wiki.ros.org/ROS/Tutorials/Cre...

And take a look at example of robot implementation, you can communicate with drivers or other interfaces. https://docs.niryo.com/dev/ros/v3.2.0...

Finally I suggest you look at what is also available with ROS2 control: http://control.ros.org/

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Thank you, Very informative anwser. So there are many ways to interact with the hardware. For the message and services as you mentioned, say I want controll a motor by sending the signal through a series port, I think the "actual send signal code" should be written in a node "control_signal_publisher", the messge is given to the topic not the actual SBC port. And I should assure the sended signal and the messge is the same. Is that right?

zhixin ( 2022-01-02 02:25:59 -0500 )edit

Take a look at this example using Arduino and rosserial: https://www.google.com/amp/s/atadiat....

I think you can do something similar. Not sure what do you mean by assuring the sent signal and message are the same, can you explain further pls

osilva ( 2022-01-02 05:10:43 -0500 )edit

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osilva ( 2022-01-02 05:12:13 -0500 )edit

First thanks for the rosserial example, it solve my confusion. Then forget about "assuring the sent signal and message are the same". I just misundertand the communication between SBC and Arduino board: not realized that there is ros interface not only on SBC but also on Arduino. So rosseriel and ros_lib do the stuff I meant. User needn't to directly handle the seriel port(connect SBC and Arduino) stuff.

zhixin ( 2022-01-02 06:59:38 -0500 )edit

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why do we need the (pid) controllers in ros_control?

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