Research Project

Tactile exploratory strategies with natural soft fruit

Perception of naturalistic and ecological softness

August 27, 2020

Chang Xu, Hankun He, Steven C. Hauser, and Gregory J. Gerling

When interacting with deformable objects, tactile cues at the finger pad help inform our perception of material compliance. Nearly all prior studies have relied on highly homogeneous, engineered materials such as silicone-elastomers and foams. In contrast, we employ soft fruit varying in ripeness; ecological substances associated with tasks of everyday life. In this work, we investigate volitional exploratory strategies and contact interactions with natural soft objects, for comparison to engineered materials.

The newly defined metric of virtual stiffness can differentiate between the fruits’ ripeness, amidst their local variations in geometry, stiffness, and viscoelasticity.

New techniques and designs for experimenting with ecologically soft objects

Experimental setup developed for finger palpation and pinch grasp, including an ink-based method to capture contact interactions, and instrumented force and optical sensors to measure contact force and displacement. Soft and hard plums are used in force-choice human subjects experiments.

Virtual stiffness cues may encode the naturalistic softness by integrating force, displacement, and exploratory strategies

Compliance cannot be discriminated solely by applying touch force or fingertip displacement. In contrast, virtual stiffness could optimally afford discrimination between soft fruit, independent of the touch force, amidst overlapped fingertip displacement.

For thumb-index pinch, biomechanical measurements of physical cues and multidimensional clustering analysis. Normalized results of A) peak grasp force , B) fingertip displacement, and C) overall estimate on virtual stiffness for the soft and hard plums. D) All trial data are partitioned into four exclusive clusters based on peak grasp force and displacement.


  • Xu, C., He, H., Hauser, S.C., and Gerling, G.J., Tactile exploration strategies with natural compliant objects elicit virtual stiffness cues [DOI][PDF]. IEEE Transactions on Haptics, 2020.
  • Xu, C., He, H., Hauser, S.C., and Gerling, G.J., Measurement of touch interaction cues in discriminating soft fruit [PDF]. IEEE World Haptics Conference, 2019.
  • Xu, C., Hauser, S.C., Wang, Y., and Gerling, G.J., Roles of force cues and proprioceptive joint angles in active exploration of compliant objects [DOI][PDF]. IEEE World Haptics Conference, 2019.
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