4 wheel differential robot wrong movement

[argolomb]

Describe the bug I'm trying to reproduce this robot(https://global.agilex.ai/products/scout-2-0) in unity to replace gazebo as my simulation platform. Everything is fine with URDF-Importer and ROS-TCP Connection. I can get the joint state message and convert to movement with "ArticulationDrive drive = joint.xDrive;"

The mobile robot go forward and backward but no curves or spin around his center.

The problem occur when the wheel receive different velocities.

I've tried use cmd_vel msg and calculate the velocity in each wheel but with the same behavior.

Console logs / stack traces

void Start()
        {
            ros = ROSConnection.GetOrCreateInstance();
            ros.Subscribe<JointStateMsg>("joint_states", ReceiveROSCmd);
            rl_wheel = rear_left_wheel.GetComponent<ArticulationBody>();
            rr_wheel = rear_right_wheel.GetComponent<ArticulationBody>();
            fl_wheel = front_left_wheel.GetComponent<ArticulationBody>();
            fr_wheel = front_right_wheel.GetComponent<ArticulationBody>();

            SetParameters(rl_wheel);
            SetParameters(rr_wheel);
            SetParameters(fl_wheel);
            SetParameters(fr_wheel);
            Debug.Log("Start");
        }

        void ReceiveROSCmd(JointStateMsg joint_state_msg){
            Debug.Log("ReceiveROSCmd");
            rl_wheel_vel = joint_state_msg.velocity[2];
            rr_wheel_vel = joint_state_msg.velocity[3];
            fl_wheel_vel = joint_state_msg.velocity[0];
            fr_wheel_vel = joint_state_msg.velocity[1];

            SetSpeed(rl_wheel, (float) rl_wheel_vel);
            SetSpeed(rr_wheel, (float) rr_wheel_vel);
            SetSpeed(fl_wheel, (float) fl_wheel_vel);
            SetSpeed(fr_wheel, (float) fr_wheel_vel);

        }

        private void SetSpeed(ArticulationBody joint, float wheelSpeed = float.NaN)
        {
            ArticulationDrive drive = joint.xDrive;
            if (float.IsNaN(wheelSpeed))
            {
                drive.targetVelocity = ((2 * maxLinearSpeed) / wheelRadius) * Mathf.Rad2Deg;
            }
            else
            {
                drive.targetVelocity = wheelSpeed*Mathf.Rad2Deg;
            }
            joint.xDrive = drive;
        }

Expected behavior The same behavior in all software

Screenshots <img src="http://img.youtube.com/vi/1GrUhLVfuIg/0.jpg" alt="IMAGE ALT TEXT HERE" width="240" height="180" border="10" />

Environment (please complete the following information, where applicable):

• Unity Version:2020.3.26f1
• Unity machine OS + version:Ubuntu 20.04
• ROS machine OS + version: Ubuntu 20.04, ROS Noetic
• ROS–Unity communication: NA
• Branch or version: v0.6.0
• Agilex Ugv Gazebo Sim (https://github.com/agilexrobotics/ugv_gazebo_sim/tree/master/scout)

[vidurvij-Unity]

Can you share the repository for the project. It would help me understand the Physics settings you are using. [AIRO-1702]

[argolomb]

Can you share the repository for the project. It would help me understand the Physics settings you are using. [AIRO-1702]

Yes, sure.

scout_beta

[github-actions[bot]]

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[DLu]

I am also hoping for a solution here. @vidurvij-Unity any luck?

[argolomb]

I am also hoping for a solution here. @vidurvij-Unity any luck?

My solution was:

• Not use URDF importer
• Create the robot in unity
• Add wheel's colliders on wheels
• Create 4 wheel drive controller(using PID)

[argolomb]

The solution is set a rigidbody.inertiaTensor value.

        public Vector3 tensor;
        public Rigidbody rb;
        void Start()
        {
            ros = ROSConnection.GetOrCreateInstance();
            ros.Subscribe<TwistMsg>("cmd_vel", ReceiveROSCmd);
            rb.inertiaTensor = tensor;
            rb.centerOfMass = centerOfMass.transform.localPosition;
        }

[ptiwari0664]

@argolomb I am trying to recreate another vehicle using similar concept. Could you please illustrate how have you calculate the inertia tensor ? My vehicle is not able to climb on the hills.