## 1 Introduction  
Robotic surgery is the latest trend in minimally invasive surgery. The Da Vinci System (Intuitive Surgical Inc., Sunnyvale, CA, USA) is well known as the world's most successful surgical robot. As the Da Vinci System is a remote-control console in which the operator controls the robot, the operator has no choice but to separate it from the operating table. However, many surgeons want to be able to freely intervene and flexibly cope with unexpected situations during robotic surgery. Since surgical robots are super expensive, only a small number of tertiary hospitals have them, and they are used for serious surgery considering the overall procedures such as installation and recovery of robots during surgery. Therefore, considering the entire medical field including the primary hospital, a robot that can be used for laparoscopic surgery, which requires two hours of operation, is needed. In addition, medical personnel are complaining of fatigue from surgery due to a shortage of medical assistants, and demands for surgical assistance robots as shown in Figure (a)a are increasing. In this study, we developed an entry-level surgical assistant robot with an end-effector attached to a six-joint cooperative robot [1]. The robot works with the surgeon on the operating table by holding a laparoscopic camera and pulling the internal tissue with forceps. To evaluate the validity and usability of the design of the prototype, animal experiments were conducted using pigs as shown in Figure 1b. The most basic function of laparoscopic surgery assistance robots is remote center motion (RCM), and direct teaching is provided on how the operator can intuitively manipulate them without additional training. The stability of RCM and the sensitivity to direct teaching are important indicators for performance evaluation from the point of view of surgeons. The movement of an imaginary point outside the organization is called remote center motion. The RCM is an essential function for maintaining the position of the insertion port while the surgical robot inserts the surgical tool into the human body with a trocar and operates it. Medical robots have a variety of mechanisms for RCM [2]. The method of mechanically limiting the remote center and always maintaining the remote center in any movement of the robot is widely used for high safety. Many surgical robots, including Da Vinci, maintain a remote center by decoupled motion in a z-bar-shaped parallelogram link structure [3; 4]. Some robots maintain the remote center by using circular guides [5; 6]. Microsurgical robots such as eye surgery have parallel mechanisms with closed linkage [7]. However, the mechanically constrained system is not suitable for a surgical assistant robot that works in the same space as the operator because it consists of parallel mechanisms and support mechanisms and has a large possibility of colliding with the surgeon when placed on the operating table. To minimize the size of the robot, there is also a non-mechanical method of maintaining a virtual remote center through the control of articulated robots [8]. With the recent development of cooperative robots, various direct teaching methods are being studied. The most widely used manual traction method for low-cost cooperative robots is to control current without sensors [9; 10]. However, since it operates only when the applied torque is large enough considering the reduction ratio and friction of the reduction gear, it does not meet the requirements of the surgical site, and because it is not position-based control, remote center maintenance performance is unreliable. There is a system that allows sensitive and precise operation by attaching a torque sensor to each rotating shaft of the robot, but it is mainly used for expensive cooperative robots [11; 12]. Another method is to attach a force/torque sensor to the tip of a cooperative robot and teach it directly based on the sensor [13; 14].