# Incorrect normal reaction force in Sphere on Plane contact model

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Himanshu Prabhat on 21 May 2020
Answered: Steve Miller on 8 Aug 2020
I have a body made of cylinder with sphere at one base. It's constrained to move in z-direction though prismatic joint. Spherical base of the body is in contact with a plane as shown in the figure.
A time varying external force F(t) pushes the body towards the plane. F(t) is a perodic in nature and is described as:
where Tp is time period of F(t)
Ideally it's expected that normal reaction should eventually converge to applied external force. This holds true when Tp is greater than 0.12 sec. Figure below shows the case of 0.12 sec:
Pressure force is essentially applied external force. When Tp gets smaller, normal reaction is not able to rise & fall in given amount of time. Figure below shows the case of Tp = 0.01 sec:
Values of contact parameters are as follow:
• Mass: 0.031 lbf (0.014 kg)
• Contact stiffness: 5.7e06 lbf/in (1e09 N/m)
• Contact damping: 5.7lbf/(in/s) (1e03 N/m/s)
• Penetration exponent: 2.2 (standard for steel to steel contact)
To check whether contact parameters are suitable or not, I solved the same problem in 1D using simulink. There,the normal reaction was converging to applied external force in steady state even for low value of Tp. I'm not sure if this is a solver issue or something else. Currently I'm using daessc solver.

Steve Miller on 21 May 2020
Can you share the model? There are a number of reasons you could see this behavior. --Steve
Himanshu Prabhat on 21 May 2020
Thank you very much for your response, Steve. Somehow I figured out the issue in the model. I had used a S-PS converter which was applying second order filtering with time constant = 0.001 sec to external force input. That's why the external force to model iself was having sluggish rise and fall. Also, since I was tapping the external force signal before S-Ps converter, I couldn't see what actual force is going into the model. Now it's working fine. I have removed the filtering and providing the input as it is. All this happenhed because I copied this S-PS converter block from other place in the model and I forgot that it was not in the default state. Sorry for troubling you. Thanks again for quick response.

Steve Miller on 8 Aug 2020
Looks like you managed to answer this question yourself - that's great!

R2019a

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