# Cable-Driven Mechanism Models for Sensitive and Actuated Minimally Invasive Robotic Instruments
## 1 Introduction
Some critical points preventing the use of tactile/haptic feedback in robot-assisted surgery are:  
* The small dimension of MIRIs (the diameter of the tube, shown in Figure 2, is around 8 mm);
* The MIRI sterilization that limits the life of electronic components in the sensing system (e.g., Micro-Electro-Mechanical Systems sensors [12], which could be included in MIRIs to measure a tactile interaction with the tissues [4]);
* Errors during the motion of forceps [3];
* The communication delay between the motion of surgeons' hands and the motion of MIRIs may limit perception and induce instability [3; 4; 5];
* The small forces generated by sensors, especially when using MEMS sensors [12]. Figure 1: Telerobotic Surgical System. Figure 2: Da Vinci Research Kit (dVRK) (courtesy of the University of Verona): Local Part and Remote Part. A detailed analysis on the surgical robotic instruments state of the art (published in [4]) underlined that surgeons with experience in robotic surgery are able to perform different tasks, and force feedback (FF) and haptic feedback (HF) from robotic surgical tools help novices to reduce forces on the human tissue, avoiding dangerous situations. If economically and technically feasible, FF and HF from surgical instruments could help experts and novices to increase the quality of surgical operations [20]. The dexterity of MIRIs is increased when adopting bending mechanisms, such as the one proposed in [21]. The cable-driven mechanism is used to actuate the MIRI and the bending mechanism, in the base of its design, and allows for the performance of predefined movements modifying the workspace. The bending mechanism is positioned between the end of the tube and the forceps, replacing the wrist of the dVRK MIRI. The same location between the tube and the forceps is used to include the continuum wrist proposed in [22]. One important aspect to consider is the misalignment between the command given by the local part and the motion performed by the MIRI. In [23], the misalignment is evaluated, underlining that users were able to compensate for misalignment angles up to approximately 20 degrees in both tool orientation and camera viewpoint misalignment. Cable-driven systems are used in the design of minimally invasive robotic surgery. The robotic arm of dVRK, shown in Figure 2, is an example of a robot designed with cable-driven instruments, such as MIRIs shown in the same figure. Many examples like this are available in the literature, such as in [24]. A very interesting surgical instrument is shown in [25], in which CYCLOPS is presented: a robotic tool for endoscopic surgery based on the concept of tendon-driven parallel robots. The cable-driven concept proposed in the paper is in the same line of the one we published in [5], using cables to move a mobile part with respect to the fixed part and with a direct force mapping principle involved, but applied with different methods and design. Other solutions are tested to include force-torque sensors in MIRIs, such as the one included in [26], in which fiber Bragg gratings (FBG) are used. FBG have many advantages, such as immunity to electromagnetic interference, light weight, etc. ; however, severe cross sensitivities and high non-linearities are common drawbacks, as underlined in [26]. New sensorized MIRIs must be studied and designed to reduce excessive tool-tissue interaction forces, which often result in tissue damage, as underlined in a recent review paper [27]. FBG [26; 28], capacitive sensors [29; 30], and strain-gauges [31] are only some of the sensing elements available for MEMS and MIRIs, as shown in [4; 32]. The sensitive element can be positioned in the sterilized workspace, but in this case, the electronic components of the sensor can be damaged by sterilization. Another approach is to position the sensor outside the sterilized zone, such as the work proposed in [33].