Gazebo_ros_control problem

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Hello,

I am a beginner using ROS and gazebo, and my goal is to simulate a wheelchair. I have created a simple representation like this one :

I now want to make the whair moving forward by adding a velocity, using the gazebo_ros_control plugin.

I followed this tutorial : https://dvic.devinci.fr/resource/tuto...

I set the inertial values and the mass of each element : seat (+ one person) : 115kg front wheels : 250g back wheels : 1.5kg

I don't know how to set the collisions surfaces parameters (mu1 mu2 kp kd fdir1), so according to some research I put : mu1 = mu2 = 1.0 kp = 1e+8 kd = 1.0 fdir1 = '1 0 0'

I also set the launch and the config files, all the codes are below. However, when I start the simulation I got this :

All the wheels are at 0 0 0 and under the seat. I don't know why. Finally, I don't know how to configure the pid gains in the config files .yaml, I guess this is the problem, with the fact that the masse is at around 120kg.

<?xml version='1.0' ?>

<robot name="wheelchair_test" &gt;<="" p="">

<link name='base_seat_link'>

        <visual>
            <origin xyz='0 0 0.4125' rpy='0 0 0'/>
            <geometry>
                <box size='0.675 0.675 0.675'/>
            </geometry>
            <material name='orange'>
                <color rgba="1 0.5 0 1" />
            </material>
        </visual>

    <collision>
        <origin xyz='0 0 0.4125' rpy='0 0 0'/>
            <geometry>
                <box size='0.675 0.675 0.675'/>
            </geometry>
            <material name='orange'>
                <color rgba="1 0.5 0 1" />
            </material>
    </collision>

        <inertial>
        <!-- masse : complete_seat=15kg + person=100kg -->
            <mass value='115.0'/>
        <!-- full cylinder r=0.5m et h=1.3m --> 
            <inertia ixx='23.3833' iyy='23.3833' izz='14.375' ixy='0' ixz='0' iyz='0'/>
        <origin xyz='0 0 0.4125' rpy='0 0 0' />
        </inertial>    

</link>

<link name='back_seat_link'>

        <visual>
            <origin xyz='0 0 0' rpy='0 0 0'/>
            <geometry>
                <box size='0.675 0.1 0.4'/>
            </geometry>
            <material name='orange'>
                <color rgba="1 0.5 0 1" />
            </material>
        </visual>

    <collision>
        <origin xyz='0 0 0' rpy='0 0 0'/>
            <geometry>
                <box size='0.675 0.1 0.4'/>
            </geometry>
            <material name='orange'>
                <color rgba="1 0.5 0 1" />
            </material>
    </collision>

        <inertial>
            <mass value='0.1'/>
            <inertia ixx='1e-6' iyy='1e-6' izz='1e-6' ixy='0' ixz='0' iyz='0'/>
        <origin xyz='0 -0.2875 0.9500' rpy='0 0 0' />
        </inertial>    

</link>

<joint name='seat_joint' type='fixed'>
        <axis xyz='1 0 0'/>
        <parent link ='base_seat_link'/>
        <child link ='back_seat_link'/>
        <origin xyz='0 -0.2875 0.9500' rpy='0 0 0' />
</joint>

<link name='front_left_wheel'>

    <visual>
        <origin xyz='0 0 0' rpy ...