A Variable Stiffness Robotic Probe for Soft Tissue Palpation

During abdominal palpation diagnosis, a medical practitioner would change the stiffness of their fingers in order to improve the detection of hard nodules or abnormalities in soft tissue to maximize the haptic information gain via tendons. Our recent experiments using a controllable stiffness robotic probe representing a human finger also confirmed that such stiffness control in the finger can enhance the accuracy of detecting hard nodules in soft tissue. However, the limited range of stiffness achieved by the antagonistic springs variable stiffness joint subject to size constraints made it unsuitable for a wide range of physical examination scenarios spanning from breast to abdominal examination. In this letter, we present a new robotic probe based on a variable lever mechanism able to achieve stiffness ranging from <inline-formula><tex-math notation="LaTeX">$\text{0.64}$</tex-math></inline-formula> to <inline-formula><tex-math notation="LaTeX">$\text{1.06}$</tex-math></inline-formula> N<inline-formula> <tex-math notation="LaTeX">$\cdot$</tex-math></inline-formula>m/rad that extends the maximum stiffness by around <inline-formula><tex-math notation="LaTeX">$\text{16}$</tex-math></inline-formula> times and the stiffness range by <inline-formula><tex-math notation="LaTeX">$\text{33}$</tex-math></inline-formula> times. This letter presents the mechanical model of the novel probe, the finite element simulation as well as experimental characterization of the stiffness response for lever actuation.