I understand that you are facing issue with the bouncing behaviour you're observing in your Simulink model of a 3-PRR delta robot while picking up a part, it is likely due to the nature of the spatial contact force and the stiffness of the robot's structure.
When the end effector applies a downward force (-20 N) to the part, the contact force pushes the part up. However, the stiffness of the robot's structure causes the end effector to recoil slightly, reducing the downward force. This creates a feedback loop where the end effector repeatedly pushes the part down, causing it to bounce.
Try reducing the magnitude of the downward force applied by the end effector. A smaller force will still be sufficient to pick up the part but may help reduce the bouncing tendency.
You can refer the below documentation to get basic idea of how to pick and place robot using forward and inverse kinematics:
