how to make the surface like a spring?

edit

Hi, I am simulating hybrid position force control in ignition gazebo, and try to let the tool on robot flange to approach a wall.

I have set kp to a low value for both the tool and the wall surface. But when they contact, there is still an abrupt contact force according to the force_torque sensor attached on the flange. I want to make the wall has a much less stiffness. How should I set the simulation parameters?

    <contact>
      <ode>
        <min_depth>0.00</min_depth>
        <kp>1e+03</kp>
        <kd>100</kd>
      </ode>
    </contact>

Thanks! Jiying Jia