## 1 Introduction
### Related Work
The Steady-Hand robot [19] for eye surgery is another example, where force augmentation with steady manipulation is provided. In the case of EES, hands-on systems have been proposed for specific tasks, such as commanding a drill with a modified version of the Steady-Hand robot [20] or the NeuroMate robot [21], and to control the position of the endoscope using the interaction force and virtual fixtures on a pre-planned operation [22]. The CRANEEALproject conceptualized a similar collaborative approach for controlling a robot arm in EES [23]. Additional interfaces used for EES applications include a hand tracking device (Leap Motion) used to control a continuum robotic system [24] and a voice-controlled robot for holding and maneuvering an endoscope (AESOP) [25]. However, differences in the coordinate frames between the surgeon and robot combined with limited communication channels make it difficult to control active tools precisely. Previous studies have considered specific challenges within EES, enhancing the surgeon's capabilities during the execution of certain tasks, but hindering the execution of other activities during the surgery. Teleoperation systems, for example, can provide high dexterity surgical manipulation, but require a large, time-consuming setup and the help of assistance staff for tool exchange. Thus, there is a need for a user interface that can cover the entire EES cycle, but maintaining a dexterous, intuitive, and safe operation of the robotic system. This paper aims to fill this gap by presenting a new, compact, and versatile human-robot cooperative interface. We describe the mechanical design, control strategy, and system integration with the SmartArm, a concept of a new modular robotic system for bimanual robotic-assisted endonasal surgery presented by our group [26]. The interface design considers the specific EES challenges, providing an easy setup and tool exchange, ergonomic operation, and fast access to the patient, which allow the surgeon to use additional tools within the surgical workspace. Our interface is designed to avoid the mirror effect, a common problem in surgical cooperative systems, by decoupling the interface motion during manipulation tasks. The proposed system follows a hybrid approach, where a force-controlled interface and a serial-link interface are compactly embedded in a module attached to each robot arm. We define a control strategy in a multi-state simplified surgical task scenario, with four basic instrument operations such as positioning, insertion, manipulation, and extraction. The assistance requirements are addressed through a combination of virtual remote-center-of-motion (VRCM), workspace virtual fixtures, and force based control, defined for each operation state. We demonstrate the effectiveness of the proposed user interface with reachability, object manipulation, and surgical dexterity tasks in a phantom environment designed to reproduce the constraints found in a real scenario. The results illustrate that the proposed system can improve the precision and smoothness of the forceps motion and provide safe operation by reducing the interaction forces with surrounding structures.