In minimally invasive surgery, indirect contact is established and haptic feedback is impaired. Previous research has focused on haptic feedback related to compliant objects. Stiffness has been generally related to hardness perception. Only a handful studies have taken into account the damping effect on haptic feedback. However, hardness perception in terms of viscoelasticity did not draw sufficient attention.
In this study, considering the viscoelasticity of human tissues and organs, we assessed the effect of damping on hardness perception along with spring constant. Dampers which exhibit both elastic property and viscous property were designed and manufactured with 3D printing technology. The Voight model was adopted for the design of the damper. A spring and an equivalent dashpot were positioned in parallel to simulate the viscoelasticity. The spring constants and damping coefficients of the damper were determined by compressing the damper in three testing speeds with an indentation apparatus. A series of damper with distinct spring constant and damping coefficient were produced. The psychophysical experiment was conducted by using these self-made stimuli.
In the experiment, the capacity of distinguishing hardness was evaluated by magnitude estimation method. Rating values of all stimuli in terms of hardness were collected. Perceived hardness of the combination of spring and damping were compared with spring units solely. Two different ways of holding the tool were adopted to investigate sensitivity of human perception in specific tasks. Force data were also retrieved and analyzed to determine the role of force feedback relatively with elasticity and viscosity.
The results show that the major factor in hardness perception in this context is force feedback. People tend to use the same strategy to evaluate hardness, whereas different amounts of force may be applied in a variety of ways of holding the tool. From the perspective of ergonomics, the implication of this would be the design of a laparoscopic tool. The effect of damping was confirmed to be in favor of hardness perception, but spring constant has more weight in perceived harness.