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What is the difference between velocity and effort?

asked 2019-04-05 07:01:24 -0500

EdwardNur gravatar image

I am writing a hardware interface for my 4 wheel differential robot and I came up with this tutorial: https://github.com/ros-controls/ros_c...

I understood everything except I am confused with the effort variable. Is that the PWM to the motor from 0 to 100%? If yes, why does the interface need veloicty variable?

Does it mean that the controller handles PID? By reading the velocity and effort from the motor it finds the optimal effort to send to obtain a specific cmd_vel?

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answered 2019-04-06 04:25:45 -0500

The controllers in ROS control are described by the type of control input they receive and the type of control output they produce. So a velocity effort controller will receive desired velocity commands, monitor the joints position and produce the effort (force or torque) commands needed for the joint to achieve the desired velocity.

You are correct that the controllers almost certainly use PID to perform this control loop.

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@PeteBlackerThe3rd So the diff_drive_controller requires VelocityJointInterface and JointStateInterface. The velocityJoint only outputs cmd_vel and JointState receives pos (which I believe are the meters from encoders), vel and effort. So how does it calculate PID if it outputs only cmd_vel? It does not seem to output effort

EdwardNur gravatar image EdwardNur  ( 2019-04-06 04:32:11 -0500 )edit

I believe some things are getting conflated here @EdwardNur.

The velocityJoint only outputs cmd_vel

Assuming velocityJoint should actually read JointVelocityController, then no, it doesn't "output cmd_vel". It implements a control loop that takes in a velocity setpoint (in rad/sec) and computes either a delta-v or a delta-effort to bring the current state to the desired one (ie: the setpoint).

JointState receives pos (which I believe are the meters from encoders), vel and effort.

Are you referring to the JointStateController? That is not a controller (unfortunately named). It's a class that reads data from the hardware_interface (or really: joint handle), transforms that into a JointState message which it publishes.

So how does it calculate PID if it outputs only cmd_vel? It does not seem to output effort

none of the controllers here "output cmd_vel".

The diff_drive_controllersubscribes to cmd_vel and consumes Twists.

gvdhoorn gravatar image gvdhoorn  ( 2019-04-06 04:37:36 -0500 )edit

@gvdhoorn I am getting confused now. I thought that controllers like diff_drive_controller take input from cmd_vel and output the m/s speed for each individual wheels.

Also, from this code https://github.com/eborghi10/my_ROS_m... It seems that it only gets current velocity and position (encoders in meters) and outputs the desired velocity for each wheel back to the hardware. Am I right? So it does not perform PID anywhere

EdwardNur gravatar image EdwardNur  ( 2019-04-06 04:51:05 -0500 )edit

I thought that controllers like diff_drive_controller take input from cmd_vel and output the m/s speed for each individual wheels.

yes, that is what it does.

But you wrote:

The velocityJoint only outputs cmd_vel

and that is incorrect.

Also, from this code https://github.com/eborghi10/my_ROS_m.. .

the code you link to implements a hardware_interface. That does not do any control, but only provides an interface to the hardware. ros_control controllers use such interfaces to communicate with your hardware. So the control would happen in the controller.

gvdhoorn gravatar image gvdhoorn  ( 2019-04-06 05:03:51 -0500 )edit

@gvdhoorn Yes my bad, interface there outputs only double and not rosmsgs cmd_vel. Ok I get it now, so you are saying that I can write my controller so it can actually output the actual PWM number instead of m/s and this is my choice? So the hardware interface reads those numbers and sends it to the actual hardware?

EdwardNur gravatar image EdwardNur  ( 2019-04-06 05:19:11 -0500 )edit

No, that's not what I'm writing at all.

If using diff_drive_controller, you don't need to write your own controller.

You "just" need to write a compatible hardware_interface implementation (ie: one that exposes your joint state with a JointStateInterface (for state) and a VelocityJointInterface (for commands)).

Your hardware_interface will be responsible for converting incoming setpoints into corresponding PWM duty-cycles (per joint, so nothing to do with cmd_vel) and for converting encoder ticks into position or velocity in units that ROS expects.

gvdhoorn gravatar image gvdhoorn  ( 2019-04-06 06:12:09 -0500 )edit

@gvdhoorn that is what I mean, hardware interface will receive data in the format of double or float and I would then convert that into PWM using PID.

What I mean about the controller is that I can write my controller that essentially takes cmd_vel and based on my parameters outputs to the hardware interface the double variable with PWM values.

EdwardNur gravatar image EdwardNur  ( 2019-04-06 06:44:59 -0500 )edit

hardware interface will receive data in the format of double or float and I would then convert that into PWM using PID.

this makes little sense to me. Conversion of SI units to PWM duty cycle is (typically) a single linear conversion. There is no PID involved.

Unless you intend to run a lower-level PID controller to make sure that your velocity setpoints are being reached.

gvdhoorn gravatar image gvdhoorn  ( 2019-04-06 09:32:14 -0500 )edit

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Asked: 2019-04-05 07:01:24 -0500

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Last updated: Apr 06 '19