Even though certain technical limitations of the haptic device itself make it quite challenging to reproduce the exact tactile sensation obtained when performing the surgical procedure on real tissues, a cohort of ten volunteer, experienced urologists from University of Illinois at Chicago (UIC) was given the possibility of using the simulator for 30 minutes and tuning the parameters relative to tactile feedback based on their experience performing this surgical procedure on real patients, as done in [22]. They tune values (numbers between 0 and 1) for stiffness, damping, static and dynamic friction, and “pop-through”, i.e., the force required to penetrate the abdominal structures with the VN, through the GUI, until the sensation was considered convincing. In CHAI3D, even though more limited in terms of haptic properties that can be assigned to meshes (damping and pop-through are not available), the values were tuned so that the perception remained as similar as possible to the OH simulator. We used a 3-degree-of-freedom (DoF) haptic device, the TouchTM 3D Stylus [30], to provide the force feedback from the organs. The sensation experienced when penetrating the tissues with a sharp instrument is emulated with the so-called haptic fulcrum effect (Figure 2). The surgeon can freely move the VN in the direction of insertion, back and forth, but not laterally. Given the insertion point r, the proxy position p, the proxy orientation Q=(qw,qx,qy,qz), the perceived force is F=k(p-p), where p is the projection of p onto l, i.e., line passing for r and r=r+(qx,qy,qz).

Fulcrum point effect

When the skin is traversed, the user perceives a constant damping effect. As soon as the VN soaks into the underlying tissues’ surface, constant friction and increased damping are added until the user feels a give in-between linea alba and peritoneum and when reaching the peritoneal cavity to simulate the two “pops”. The collisions between the VN and the organs are detected using the proxy-based algorithm [31]. In CHAI3D, this is significantly sped up through the so-called bounding volumes. For the deformable objects, only impacts between one or more filling spheres and the proxy are considered [32].

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