Force vs Position Spring Graph Discrepancy Question

[Robert Hilderman]

I oscillated a spring attached to a force sensor above a position sensor. The position and force sensors were zeroed at the spring's equilibrium. The graph below was my result. I'm wondering as to why the graph isn't a perfect line. It makes sense that the force applied on the force sensor would be lower when the spring is rebounding up but I'm not sure why from a physics standpoint. Thanks so much
https://lh5.googleusercontent.com/pSJvRaEHX9vfzt94ZjtmS-5aAOg45ejXglriQTtsYUT41xvIa9L3Bi2X4oOJeyybPKxDadH0pVmdPIjFUcGHuOV7yZYkHGTTekWdoxLd3qzJrCBnrxPUXoD-c93zRtKOwPTHa38a

 

[Dale]

That is from all of the dissipation mechanisms. Friction, internal heating, sound, etc. I believe that the area inside the loop is equal to the energy lost each cycle.

 

[robphy]

https://en.wikipedia.org/wiki/Hysteresis#Elastic_hysteresis

 

[nasu]

If your force sensor is piezoelectric, it has its own hysteresis as well.