A Laparoscope Control System using a Pneumatic Robot Arm

Kei Mikami*, Kotaro Tadano*, Kenji Kawashima**

* Precision and Intelligent Laboratory, Tokyo Institute of Technology, R2-46, 4259 Nagatsuta Midori-ku, Yokohama-shi,

Kanagawa Prefecture, 226-8503 Japan **Institute of Biomaterial and Bioengineering, Tokyo Medical Dental Univ.

2-3-10 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-0062 Japan

ABSTRACT

In this research, a laparoscope control system which is controlled to follow the operator’s head movement is developed. Endoscopic image with a high realistic sensation and a high definition is important for minimally invasive laparoscopic surgery. However, the camera shake occur due to the fatigue of the scopist. The camera shake induces the surgical operator’s nausea. In the developed system, the laparoscope is held by a pneumatically-driven scope holder which provide a remote center of motion. The scope holder is driven by gyroscopes attached on the operator’s head. The scope holder is positioned at the pivot point of the laparoscope with a serial-link arm that passively moves. Misalignment between the remote center and the pivot point is decreased by the serial-link arm. Tracking performance of the scope holder was demonstrated experimentally. Validity of the developed system was confirmed by in-vivo experiments.

KEY WORDS

Laparoscopic surgery, Pneumatic drive, Scope holder

INTRODUCTION

In laparoscopic surgery, a scopist who holds the laparoscope needs to change the camera angle under the verbal instruction of an operating surgeon. The task of scopist requires dexterity and deep understanding of the surgical procedure. Also, the camera shake may occur due to the fatigue of the scopist. Therefore, robotic scope holder is needed to obtain stable and intended vision[1][2]. In this research, we develop a laparoscope control system where a laparoscope is held by the pneumatic scope holder that follows the operator’s head movements. The developed system can be highly intuitive for the operator and can be synchronized with the operation. In order to simplify installation process,

the scope holder is positioned by a serial-link arm that passively moves. The effectiveness of the system is confirmed experimentally.

LAPAROSCOPE CONTROL SYSTEM The developed laparoscope control system is shown in

Fig.1. The system mainly consists of a pneumatically-driven robotic scope holder, a serial-link arm, and a stand part. The scope holder is controlled by the head movement measured using gyroscopes attached to the operator’s head and body. As shown in Fig. 2, the view angles of the laparoscope for up and down, left and right and rotation synchronously follow the head rotations. The zoom in and out the camera’s synchronously follow by the forward and backward motions of the operator’s head.

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Proceedings of the 9th JFPS International Symposiumon Fluid Power, Matsue, 2014

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201

1C2-3

Laparoscope

Stand

Control system

Pneumatic robot arm

Serial link arm

Fig.1 Over view of laparoscope control system

Fig.2 Operation by head movement

Fig.3 System configuration

Fig. 3 shows the system configuration of the developed system. The system consists of the scope holder, two motion sensors, a foot switch, a control PC and a valve unit. The motion sensor (ZMP,IMU-Z2) is equipped with a 3-axes gyroscope, a 3-axes geomagnetic sensor and a 3-axes acceleration sensor. Measured signals from the gyroscopes in the motion sensors are taken into the control PC by CAN communication. The control signals are calculated in the control PC and are sent to the servo valves in the valve unit. The foot switch is positioned at the feet of the operator. The operation by head movement is activated only when pressing the foot switch. Therefore, the operator can move the head freely when the foot switch is released. The control system is mounted in the stand part which can be placed at an arbitrary position in the operating room.

q1

q1

q2

q4

q3

z

y

z

xPivot point

(Remote center)

Abdominal wall

Fig.4 Pneumatically-driven robot holder

Table1 Specification of the scope holder

Range of movement Generative force

q1 ±90[°] 1.57Nm q2 -50[°]~18[°] 1.55Nm q3 ±75[mm] 39.3N q4 ±170[°] 0.154Nm

PNEUMATICALLY DRIVEN SCOPE HOLDER

The robotic scope holder, which has been developed originally to hold the forceps manipulator [3], has 4-DOFs in total, consisting of 3 rotational DOFs around the inlet of a trocar cannula and 1 translational DOF along the insertion as shown in Fig. 4. In order to make the pivot point at the trocar cannula mechanically immovable without direct support, a remote center of motion is provided by a parallel link mechanism. Specification of the robotic scope holder is shown in Table1.

SERIAL LINK ARM The scope holder requires to be positioned so that the

remote center of the parallel link mechanism is coincident with the trocar cannula. However, it is difficult to conduct rapid and accurate positioning during the installation process. When there is misalignment between both points, the abdominal wall is subjected to an external force Fext as shown in Fig.5. This external force also causes friction forces that negatively affect the

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movement of the robotic holder. In this paper, in order to decrease the misalignment, the

scope holder is positioned by a serial-link arm that passively moves. The serial-link arm consists of two links to produce motion in the horizontal plane, a link that swings in the vertical plane and a vertical slider mechanism as shown in Fig.6. The slider mechanism has two tension springs and a constant force spring. Self-weight of the scope holder is supported by the constant force spring. The external force Fext caused by the misalignment and errors of the self-weight compensation in the vertical direction are absorbed by the tension springs. Also, the passive movements by the horizontal links reduce the misalignment in the horizontal direction. Then, the position of the remote center is aligned automatically according to Fext as shown in Fig.7.

q2

Pivot point

Movement by the serial-link arm

Fext

Fig.7 Self-alignment of pivot position of the scope holder

by serial-link arm

CONTROL SYSTEM Fig. 6 shows the block diagram of the control system of the robotic scope holder. The reference velocities q& ref

obtained with the gyroscopes are integrated during the foot switch is on to have the reference position vector qref . When the foot switch is off, the reference velocities are given as 0. As shown in Fig. 8, the reference torque τref to be generated at each joint of the scope holder is given by a PD controller with the feed-forward compensation of inverse dynamics calculated from the reference position. Kpp and Kpd in Fig. 8 denote the proportional and the differential gain of position, respectively. The reference torque is transformed to the reference force of each pneumatic actuator Fref by the Jacobian Ja which is the mapping from the force to the torque. The reference force Fref is controlled by a PID controller with the feedback force F calculated from the pressures in the actuator. The control signal is sent to the pneumatic servo valves and the force is generated by charging the compressed air to the actuator. This process is conducted in the block of Driving Force Controller shown in Fig. 8. All of the calculations are performed with a period of 1 ms in the computer shown in Fig. 3.

Pivot point

Remote center

q2

Fext

Fig.5 Misalignment between remote center

and pivot point

zy

x

tension springs

constant force spring

Serial-link arm

robot arm

UP

DownSlider

YawYaw

Pitch

Yaw

Slider

Serial-link arm

Fig.6 Schematic of serial-link arm

Fig. 8 Block diagram of control system

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Pivot point

Remote center

(a) Initial condition

Pivot point

Remote center

(b) After adjustment

Fig.9 Experimental set up

EXPERIMENTAL RESULTS

An experiment was conducted to evaluate the effects of the serial link arm. In the experiment, the remote center of the scope holder was initially displaced 25 mm from the pivot point of the trocar cannula as shown in Fig.9 (a). Scope operation by head movements was started with this situation. Fig.10 shows the experimental results of velocity

responses of each joint of the scope holder during the operation. The upper, middle and lower graphs show the velocities in q1, q2 and q3, respectively, where red line indicates references from the user and blue actual responses. While tracking errors due to the misalignment can be seen before around 10 seconds, the error disappeared after 10 seconds. This is because the external forces were removed by the self-alignment of the serial-link arm. The serial-link arm was passively moved by several perturbations of the external force from the trocar cannula as shown in Fig.7. As a result, the remote center of the scope holder corresponded to the pivot point as shown in Fig.9 (b). To demonstrate the effectiveness of the developed

system, in-vivo experiments using pigs are conducted with several surgeons (Fig.11). In the experiments, we found that the serial-link arm also contributed to easy and quick positioning during the system setup. The scope angle was changed smoothly as the operator intended throughout the experiments.

responsereference

-80

-60

-40

-20

0

20

40

60

80

0 5 10 15 20

-80

-60

-40

-20

0

20

40

60

80

0 5 10 15 20

-20

-15

-10

-5

0

5

10

15

20

0 5 10 15 20

・ q 3[m

m/s

]・ q 2

[deg

/s]

Time[s]・ q 1

[deg

/s]

Fig.10 Velocity responses during operation

Fig.11 In vivo experiments

CONCLUSIONS

In this paper, laparoscope control system was developed. To simplify installation process, we propose the laparoscope control system with the serial-link arm. The scope holder require to match the remote center of the parallel link mechanism and the trocar cannula. The serial-link arm absorb the misalignment automatically. Its validity was verified experimentally. In the future, we will evaluate the system in clinical trials.

REFERENCES 1. Sackier JM, Wang Y: Robotically assisted

laparoscopic surgery; From concept to development, Surgical Endoscopy, Vol. 8, pp.63-66, 1994 2. Finlay PA: A robotic camera holder for laparoscopy,

Proceedings of 10th International Conference on Advanced Robotics. Workshop 2 on Medical Robotics, pp129-132, 2001. 3. Tadano K, Kawashima K, Kojima K, Tanaka N: Development of a pneumatic surgical manipulator IBIS IV, Journal of Robotics and Mechatronics, Vol. 22, No.2, pp. 179-187, 2010.

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