camera robot and a redundant robotic grasper. ... Placing the camera in a miniature robot that is inserted into .... divided in a set of subtasks or atomic action.
Robotic System for Single Incision Laparoscopic Surgery I. Rivas-Blanco, P. del Saz-Orozco, I. García-Morales and V. Muñoz, Member, IEEE Abstract— This paper proposes a robotic system to assist and collaborate with surgeons in Single Incision Laparoscopic Surgery (SILS) operations. The system, aim at solving the main drawbacks of this kind of surgery, is composed of a miniature camera robot and a redundant robotic grasper. Positioning of both robots inside the patient’s abdomen is done by means of magnetic control. External magnetic sources are placed at the end effector of two robotic arms, and permanent magnets are integrated in the robots. Camera robot is provided with three permanent magnets, so both position and orientation can be controlled. Sliding control, which is robust against perturbations and parameter uncertainties, is chosen. Robotic grasper’s redundancy makes possible autonomously obstacle avoidance and increases its workspace. The haptic device is designed so as surgeons can handle the grasper as if it were a conventional tool. In order to this aim, augmented reality is used to simulate a traditional tool in the visual feedback system, in substitution of the robotic grasper. Besides the telemanipulation, requirements for autonomously functions to assist surgeons in the specific tasks of suturing are discussed.
I.
INTRODUCTION
Nowadays Minimally Invasive Surgery (MIS) is a widely accepted technique all over the world, not only by surgeons but also by patients, who find small incisions more aesthetical. The next natural step for laparoscopic surgery seems to be minimizing the number of incisions in the patient. In this sense, SILS is a new technique in which all of the laparoscopic working ports enter the abdominal wall through the same incision. This can be done thanks to the development of new access devices, such as the TriportTM, the SILSTM Port, the Uni-X TM Single Port Sytem, and the Airseal TM [1]. These new devices consist of a single plastic 2 to 3 cm disk holding the working ports, connected to a plastic ring by a clear plastic sheath [2]. Benefits of this new generation method include less incisional pain, better patient’s recovery thanks to lower postoperative narcotic requirements and faster return to normal life, improved cosmetics, and higher patient satisfaction [3]. Despite these advantages, SILS have some challenges that cannot be overlooked. Close proximity of the laparoscope and instruments, as they are sharing the same port, entails a loss of triangulation between the camera and the working ports, and limits the range of motion [4]. Many researches are addressing the solutions to these problems via the development of miniature robots and special semiflexible laparoscopic instruments. Placing the camera in a miniature robot that is inserted into the patient reduces the number of instruments sharing the port,
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and moves apart the camera from the surgical instruments, avoiding the loss of triangulation. Additionally, the loss of degrees of freedom (DOF) due to the fulcrum effect is also avoided. The first commercial miniature robots were aimed at inspecting the GatroIntestinal (GI) tract by means of swallowable capsules capable of recording and transmitting images [5]. The main drawback of this kind of systems is their passive locomotion, which does not enable active control of the pill movement. With the aim of improving capsule dexterity, different methods for active locomotion have been studied, such as legged capsules [6], inchworm-like locomotion systems [7][8], adhesives systems [9][10], and electrical stimulation [11]. Different micro actuators have been considered in literature, from piezoelectric actuators [12] to Shape Memory Alloys [13] and electrolytic actuators [14]. The presence of actuators, transmission mechanism and power modules makes it difficult to maintain a minimal size of the miniature robots. Several authors have demonstrated the feasibility of using external magnetic fields to move and rotate endoscopic capsules for GI tract diagnosis and manipulation [15]-[17]. Lehman et al. [18] proposed a miniature robot with two arms with unfold and refold capabilities, which is anchored to the abdominal wall via magnetic interaction. In Simi et al. [19], a wired miniature surgical camera robot controlled via a Magnetic Levitation System (MLS) is proposed. MLSs provide an easy external control of the robot, which position depends on the intensity of an external magnetic source. Besides loss of triangulation, SILS procedures suffer from a reduction in the range of motion of instruments outside the abdomen, due to their close proximity. This fact reveals the need of using special semiflexible or curved laparoscopic instrument, so that a larger space can be achieved without crashing tools. The use of these new instruments makes necessary specific skills training periods for surgeons to be comfortable with them, even for experienced surgeons in traditional laparoscopy [20]. Flexible endoscopes are composed of a flexible shaft and a bending tip, which angle of deflection is controlled by means of navigation wheels. Handling of flexible endoscopes requires combining various actions to perform the desired movements, which results really complex for surgeons [21]. New generation of teleoperated flexible endoscopes are presented in [22]. This paper is organized as follows. Next section presents a robotic system, conceived to overcome SILS’s drawbacks, composed of a miniature camera robot and a redundant robotic
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grasper. Requirements to collaborate with surgeons in the specific task of suturing are analyzed. Camera robot design and control strategy are discussed in section III. Section IV describes the kinematic design, actuators characteristics and master console of the robotic grasper. Conclusions are reported in section V. II.
SYSTEM APPROACH
A. System description The system proposed in this work is composed of a miniature robot to position and orientates the camera and a redundant robotic grasper to assist a SILS procedure, as it can be seen in Fig. 1. The miniature robot is provided with permanent magnets, so it can be controlled via an external magnetic field. The robotic grasper is designed with 7 DOF so it has the possibility to avoid obstacles, as other surgical tools or patient’s organs, as well as to increase its workspace. Grasper is also provided with a permanent magnet located at the base to position it. The wires to power robotic grasper’s actuators are passed through one of the entries of the singleport and connected to the master console. Both camera robot and grasper robot are controlled by external magnetic sources, located at the end effector of two robotic arms.
Figure 1. Complete system scheme
The approach of placing the camera in a miniature robot that is inserted into the patient reduces de number of instruments sharing the port and moves apart the camera from the surgical tools, avoiding the loss of triangulation. The robotic grasper is provided with a permanent magnet to position it away from the entry point. This way, the grasper’s handle (and so the master console) can be attached to the robotic arm structure, solving the problem of the limitation of the range of motion of tools due to their close proximity outside the abdomen. Thus, the system described above is aimed at solving the main drawbacks of SILS procedures, i.e., loss of triangulation and reduction in range of motion. Magnetic driven in the miniature robot has numerous advantages versus the use of actuators, which need a power module that increases the overall robot volume. The main advantages of magnetic levitation techniques are that friction can be removed - as there is no direct contact -, actuators
breakdown possibility is very weak due to its simple structure, and manipulator can operate as a rigid body that does not use jointed parts, so position errors do not compound and dynamic behavior is simple to model. On the other hand, its principal disadvantage on small scales is that this kind of systems is inherently unstable, thus control can be computationally intensive [23]. Redundant structure of grasper robot increases its workspace up to the whole abdomen volume (section IV), so further locations than with traditional surgical tools can be achieved. Moreover, redundancy makes the grasper capable of adopting an appropriate configuration to avoid obstacles inside the abdomen, which increases the flexibility of the system to perform different tasks. In this sense, a semiautonomous control of the robot is proposed [24], i.e. some DOF are controlled by the surgeon, while the remaining DOF are optimally controlled by the system according to a specific task of obstacle avoidance. Robotic grasper is handled by a haptic device attached to the structure of the robotic arm hosting the magnetic source. This way, surgeon can develop the operation as if it were a conventional multi-port laparoscopic surgery, which is much more natural for them. Hence, the specific skills training period to adapt surgeon’s movements to SILS procedures can be avoided. Besides the telemanipulation control, autonomous functions to collaborate with surgeons in the suture task will be implemented. This is further discussed in next section. B. Scenario definition Suturing is one of the most challenging tasks occurring in minimally invasive surgery and takes a significant percentage of the operating time [25]. So automating suture procedure would reduce the overall surgery duration and decrease the demand on surgeons during this task. Rosser suture method uses a primary tool and a support tool, as shown in Fig. 2, which is very useful to automate the procedure, as the support tool can be handled autonomously by the redundant grasper while the surgeon handles the primary tool. Overall suture task is modeled as a finite sequence of states that should be accomplished to guarantee the success of the surgical action [26]. In order to simplify the process, suture is divided in five primary tasks: stitching, first knotting, second knotting, third knotting and thread cutting. These primary tasks are also divided in a set of subtasks or atomic actions so as the control system is capable of following the current state of the procedure by means of transitions conditions. The analysis of transitions between tasks and subtasks reveals the need of four sensory systems. A voice recognition system and a maneuver recognition system will be integrated in the control module. The former is needed in order to enable the interaction of the surgeon with the system by means of voice commands, while the latter makes the system capable to recognize the specific maneuver surgeon is performing. Thus, controller is able to identify the procedure state according to the primary tool’s movement. A force sensor system will be integrated at the tip of the robotic grasper to detect pressure over the tissue. Finally, images transmitted by the camera robot will be
This work was supported in part by the Spanish national project DPI201021126. 2763
analyzed in the vision system via appropriate vision algorithms capable of identifying the different elements which are part of the scenario.
precise movement with no chattering. Nevertheless, it requires enough bandwidth to communicate not only with the camera but also with motors. In addition, in case of a motor failure the whole robot has to be removed from patient in order to repair or change it, making necessary to vacate the single-port, and so consuming an extra time that prolong surgery duration.
Figure 2. Rosser suture method: use of a primary and a support tool
III.
(b) Elevation view
CAMERA ROBOT
A. Robot design A miniature robot to be introduced into the patient through the single-port, having the possibility of orientating it in a reliable and easy way, must satisfy the following specific requirements: 1) A very small structure is needed, so robot can fit the up to 3 cm single-port disk by which it is going to be introduced. Spy cams are suitable in our system due to their small dimensions and the high quality of the images transmitted, essential for surgeons to perform a successfully surgery. 2) Flexibility to orientate the camera in the desired direction, thanks to its two degrees of freedom. Camera pan and tilt are obtained by varying the external magnetic fields. 3) Low power supply so no extra space for batteries is needed. As actuators do not require power, system can be fed with a set of three 3V small size batteries. 4) Lighting system to illuminate the operation area. Structured LED illumination offers good properties as light source such as high brightness, low cost, small dimensions, low coherence, uniform illumination, high efficiency and long lifetime. Moreover, this lighting system can be used not only to illuminate but also to estimate distances. 5) Magnetic isolation is required in order to separate the internal magnets, so that magnetic fields are as independent as possible and therefore easier to control. 6) Communication with the camera is done via an Xbee module, which provides a bi-directional wireless connectivity that can be used both to transmit camera images to surgeon and to specify zoom options to camera through a microphone. 7) An inclinometer is used to determine the inclination of the micro robot inside the patient. 8) Frame material must be intrinsically safe and biocompatible. A block diagram of the overall system is shown in Fig. 4. Controller and robot communicate via wireless communication techniques, while feedback is done with the measures provided by an inclinometer and the information from the camera. Equipping the robot with actuators for locating and moving it inside the patient provides a more
(a) General view
(c) Plan view
Figure 3. Miniature camera robot
Figure 4. System’s block diagram.
On the other hand, with the magnetic system approach, if one of the magnets fails, a fail-safe algorithm is implemented, making possible to work with just two magnets. Moreover, communication is simpler and requires less bandwidth since only camera zoom and illumination options have to be transmitted. Spatial movement is very simple and intuitive since it can be achieved translating the magnetic source. The main disadvantage of magnetic control is that chattering worsens the image, as it affects to the camera stability. So among the magnetic levitation challenges is the chattering-free control, assuring camera stability as image high quality is crucial in a laparoscopic surgery. Control strategy is further discussed in next section. B. Control Strategy Camera robot is controlled from outside by means of three electromagnets, which intensity determines the distance from the robot to the magnetic source. In Fig. 5, an orientation of an angle α in the robot is represented, where l1 = 1 cm and l2 = 4 cm. Robot is fixed to the abdominal wall by the central magnet, so distance from this magnet to the magnetic source (xd) is constant. Robot orientation is obtained by varying the distances from magnets A and C to the source (xL and xR, respectively). A goal expressed in angle of orientation can be translated into a goal in distances by the following equations:
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xL xd l1 sin 90 l2 sin
(1)
xR xd l1 sin 90 l2 sin
(2)
Figure 5. Camera robot orientation
The main challenge in controlling the system is that it is non-linear and unstable [27]. Other difficulties are related to the specific surgical application the system is aimed at, where there are different sources of unknown parameters. The first one is the composition and width of the patient’s body, which may affect the value of the magnetic constant C. Another aspect that should be taken into account is the mass each magnet has to support. As the miniature robot is designed for surgical purposes, a fault tolerant control system is a need. In this sense, if one of the magnets fails the other two should be able to compensate its fault. In normal conditions, each magnet is responsible of controlling a third part of the total mass of the robot m, which is considered to be around 150 grams, taking into account the approximate weight of the electronics components composing the robot and the frame material estimate. In case a magnet fails, the other two will have to control the half of the total mass. So, the mass each magnet have to control may vary from m/3 – all magnets working properly – to m/2 – in case of a magnet failure -. Thus, a robust control strategy is required. Sliding control is a simple robust approach for non-linear systems with constrained perturbations and uncertainties. This control strategy consists in defining a sliding surface and a condition of not leaving it once reached. The main advantages of this approach are: flexible design, as any control is suitable so long as the resulting trajectories are directed towards the switching surface; robustness, as dynamic behavior of the closed-loop system is independent of changes in the plant parameters as long as the system stays on the sliding surface; and invariance, as motion on the sliding surface is invariant with respect to bounded disturbances that are in the range space of the input vector [28]. On the other hand, in order to take into account the presence of uncertainties, the control law has to be discontinuous across the sliding surface, which leads to chattering, a phenomenon defined as fast and finite-amplitude oscillations, which result in loud noise. This fact makes necessary to modify the control law so as to avoid chattering, maintaining the system close to the sliding surface. In this sense, a thin boundary layer is used to adjust for the trade-off between robustness and chatter elimination, leading to a smooth control law of the system.
IV.
REDUNDANT ROBOTIC GRASPER
A. Kinematic Design Robotic grasper is designed with a redundant structure of 7 DOF, as depicted in Fig. 6. Grasper tool is attached to the robot’s end effector in order to perform a surgical task. Denavit-Hartenberg (D-H) parameters’ lengths are determined accordingly to the human abdomen volume, so as grasper can achieve any location inside of it. For design purposes, abdomen is represented as a parallelepiped of dimensions 460 x 300 x 200 mm. Therefore, lengths of D-H parameters are chosen as follows: d1 = 80 mm, a2 = 110 mm, d4 = 110 mm, d4 = 110 mm, a7 = 110 mm. Thus, a total length of 520 mm with all actuators at initial values is obtained.
Figure 6. Kinematic structure of the robotic grasper
As mentioned in section II, the redundant structure of the robot increases its workspace up to the whole patient’s abdomen. This is shown in Fig. 7, where it can be seen that external robot’s workspace cover’s the whole parallelepiped that simulates the abdomen.
Figure 7. External Workspace
With respect to the internal workspace, challenging locations to reach are those that are close to the robot’s base, which are not reachable with a non-redundant structure. Fig. 8 shows the internal workspace of the robot. As it can be seen, redundancy makes the grasper capable of achieving any
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location around the first link. Thus, with the exception of the space occupied by first link, any location inside the abdomen is reachable for the robotic grasper.
Figure 8. Internal Workspace
B. Actuators One of the main challenges in developing a robot to be introduced into the patient’s abdomen is the small size of the incision through which it is inserted. In this sense, small dimensions actuators that do not increase the size of the grasper are needed. Another challenge in the grasper design is that actuators must withstand high torques to develop the required tasks during the surgery, so a high reduction ratio is needed. Therefore, a tradeoff between actuators volume and output torque must be achieved [29]. Micromotors have been selected from the international company Faulhaber & Co. For joints 2, 3, 5 and 7 a flat micromotor is needed to obtained a low robot diameter, so the gearmotor with integrated encoder Series 1512 012SR IE2-8 is chosen. For joints 1, 4 and 6 a micromotor Series 1224 012S is selected, with a gearhead Series 12/4 and an encoder Series HEM3-256-W. Micromotors technical specifications are shown in Table 1, where type 1 is that of joints 2, 3, 5 and 7, and type 2 that of the rest joints. TABLE I.
would be greatly increased with appropriates master manipulation and visual feedback. Laparoscopic surgery requires skill training periods for surgeons to be comfortable with this new way of operating. Nowadays, most surgeons are very skilled with laparoscopic tools. However, the robotic grasper proposed in this work is not a conventional tool, and so it does not move in the same way. Additional training periods can be avoided if master console is designed so that surgeons move the handle in the same way as if it were a conventional tool. Master structure transforms these natural movements for surgeons into an appropriate configuration of the robotic grasper. Tool tip’s position can be easily calculated with geometric relations. Images from the camera are an essential part of any laparoscopic surgery, as it is the only visual feedback surgeons have to be able to operate. Thus, a high quality and intuitive image is a need. In this sense, the robotic grasper may disturb surgeons, who are not used to its presence in the scene. As surgeons handle the grasper as if it were a conventional tool, a visual feedback according to it would be very convenient. In this sense, augmented reality (AR) is aimed to fuse virtual objects into a real scene under a realistic manner [30]. Thus, with an appropriate AR algorithm, the robotic grasper can be substitute in the scene by a virtual conventional grasper, which moves accordingly to surgeon’s movements in the handle (Fig. 9).
MICROMOTORS TECHNICAL SPECIFICATION
Micromotor
Diameter (mm)
Length (mm)
Output Torque (mNm)
Reduction ratio
Weight (g)
Type 1
15
12
30
324:1
7.7
Type 2
12
53.1
300
256:1
34.5
Figure 9. Virtual scene using Augmented Reality
V.
With 300 mNm of output torque, maximum force of 2.72 N can be executed when joints 5 and 7 are not aligned. This force is enough to develop any suturing tasks, which maximum force requirements are those for tasks of tissue holding and tissue pulling. C. Master Console In order to develop a SILS operation as comfortable as possible for surgeons, procedure must be as similar as possible to a traditional multi-port surgery. Having the grasper’s handle separated from the other surgical instruments is a convenient advantage for surgeons. Nevertheless, surgeons’ comfort
CONCLUSIONS
In this work, a robotic system composed of a miniature camera robot and a redundant robotic grasper to assist a SILS procedure has been presented. Both robots are inserted into the abdomen through the single incision, and fixed to the abdominal wall by means of magnetic interaction. External magnetic sources are located at the end effector of two robotic arms. Miniature camera robot avoids the loss of triangulation typical of SILS, as image sensor is separated from the surgical tools. The haptic to control the robotic grasper is also separated from the other surgical tools, augmenting the range of motion of the instruments. Sliding mode control has been proposed for the camera robot to deal with perturbations and uncertainties. The master console of the robotic grasper is
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