# Motion Control and External Force Estimation of a Pneumatically Driven Multi-DOF Robotic Forceps
## 1 Introduction  
Robotic-assisted minimally invasive surgery has been significantly developed owing to its many benefits, including the reduction of the surgeon's tremble and increase in the operational accuracy [1, 2]. As the end effector of a robotic-assisted surgery system, the robotic forceps are inserted into the patient's abdominal cavity through a trocar. Hence, the size of the working part is firmly restricted. Several groups used the rigid-link mechanism to meet such requirements, wherein multiple rigid joint links independently rotate in a direction that is orthometric with the others, and the actuators are located at the proximal end outside the patient's body. [PERSON] et al. developed a forceps with four DOF [3]. Additionally, [PERSON] et al. proposed a snakelike instrument that consists of seven rigid joints for minimally invasive surgery [4]. The abovementioned devices all use electrical motors with a high reduction ratio and can realize accurate position control performance. On the other hand, the force control and sensing capabilities at the forceps part are also relevant and vital for high operational performance and safety. For example, knowing the real-time output force can help surgeons use an appropriate force to manipulate the organ [5], suture the incision [6], and do bone biopsy [7]. Moreover, the force-sensing function at the end effector is essential to realize haptic feedback [8], which is a challenging and effective improvement to the surgical robots. A standard method is embedding force sensors at the distal end of the forceps like the conventional manipulators [9; 10; 11]. However, the size of the force sensors hinders the miniaturization of the forceps, and the temperature and humidity of the working environment interfere with the accuracy of the force sensors. [PERSON] et al. developed a robotic forceps with strain gauges located at the shaft part of the forceps [12]. The working environment would not interfere with the accuracy of the force sensors, and the strain gauges did not increase the size of the distal end. However, this mechanism could not show the grasping force. [PERSON] et al. developed miniature force sensors embedded at the two fingers of the gripper [13] and proposed an algorithm to compensate for the effect of the temperature and humidity [14]. However, the miniature force sensors were expensive to the limited time of reusing the forceps. Disturbance observer is a helpful control scheme for sensorless force estimation [15]. However, all the current devices use electrical motors, wherein the high reduction ratio of their actuators hinders the estimation of the tiny external forces that arise in many surgeries. Series elastic actuator (SEA) is a classical method to realize force estimation and output force control when using electrical motors, and is commonly applied in the human interactive devices such as the exoskeleton and prosthesis. However, it is difficult to make this complicated mechanism compact for a robotic forceps. Preumatic drive, which has a higher power-weight ratio than electrical actuation, is an effective solution for this problem. Due to the high back-drivability of the pneumatic cylinders, the tiny external forces can be estimated from the air pressure exerted on the cylinder heads by using the disturbance observer scheme. Moreover, the ease of obtaining and the cleanness of the air source make pneumatic drive suitable for a surgery device than other back-drivable actuations such as hydraumatic driven. [PERSON] et al. developed a pneumatically driven robotic forceps that can estimate the external forces [16]. However, the undesirable deformation of the snakelike flexible joint in the case of large external loads was still a problem, and the magnitude of the output force was limited. The Da Vinci system adopts a rigid-link mechanism wherein the tip joint simultaneously controls the joint bending and gripping motions [17].