Displaying Feeling of Cutting by a Micro-Scissors Type Haptic Device

Transcription

1 28 IEEE International Conference on Robotics and Automation Pasadena, CA, USA, May 19-23, 28 Displaying Feeling of Cutting by a Micro-Scissors Type Haptic Device Shohei Fujino 1,DaisukeSato 2,KoyuAbe 1, Atsushi Konno 1, and Masaru Uchiyama 1 1 Department of Aero Space Engineering, Tohoku University, Aoba-yama 6-6-1, Sendai , Japan 2 Department of Mechanical Systems Engineering, Musashi Institute of Technology Tamazutsumi , Setagaya, Tokyo , Japan Abstract In this paper, a method for displaying feeling of cutting by a micro-scissors type haptic device is described. Micro-scissors are a type of surgical instrument, which is frequently used in brain surgery under a microscope. A prototype of a micro-scissors type haptic device consists of three components: an interface, a drive member, and force sensors. The interface is designed to equip features of a pair of microscissors and has a blade spring on its end. The drive member is composed of two DC-motors and two crank-lever mechanisms as decelerators. The force sensors are composed of strain gauges with H-slits, and implemented into each handle of the interface. For the aid of applying the force sensors to feedback control of cutting resistance forces, compensation for influence of a blade spring on the force sensors is added. Evaluation experiment for basic performance of the device is carried out, and the device is proved to be able to generate the computed cutting resistance forces. Virtual cutting experiment by six subjects is carried out, and it is proved that the device is able to display feeling of cutting. I. INTRODUCTION Surgical simulators based on virtual reality have been developing in various surgery areas for the use of preoperative planning and training of basic surgical techniques. Microsurgery performed with an operating microscope is one of such surgery areas in which installation of surgical simulators is expected [1], [2]. In surgical simulators aimed at training of surgical techniques, interface devices formed surgical instruments used in objective procedures are needed. Various types of surgical instruments are used in microsurgery: micro-forceps; micro needle holders; microscissors and so on. Among these micro-instruments, microscissors are frequently used for cutting or peeling tissues in microsurgery. Micro-scissors have some features in shape: There is no finger ring on its handles, and blade springs set at each handle s end are connected to each other. Some scissors type haptic devices for virtual reality simulator have been made before. Wakamatsu et al. developed a force display system which includes paper cutting scissors type haptic device [3]. Greenish et al. measured the forces from surgically cutting anatomical tissues of a sheep and two rats with three types of scissors, and developed scissors type haptic device formed of metzenbaum scissors which could replay the force data in the above measurement [4], [5]. These scissors type haptic devices target scissors with no blade spring, and not for microsurgery use. This research was supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Exploratory Research, Fig. 1. a drive member force sensors an interface An overview of a micro-scissors type haptic device In this paper, a method for displaying feeling of cutting by a micro-scissors type haptic device is described. A microscissors type haptic device is expected to be used in a brain surgery simulator [6], and a prototype of micro-scissors type haptic device is developed for the aid of checking the force displaying mechanism [7]. Initially, a design concept of the micro-scissors type haptic device is described. Then, a system of the micro-scissors type haptic device is shown. Furthermore, sensor compensation and control which are unique to a micro-scissors type haptic device with a blade spring are described. Next, an experiment to evaluate the basic performance of the device is carried out. Finally, a virtual cutting experiment by six subjects is conducted in order to examine the possibility of displaying feeling of cutting by the device. II. DESIGN CONCEPTS OF THE DEVICE Overview of the micro-scissors type haptic device is shown in Fig. 1. The device is attached to a mount, and the attitude of the device is adjustable. Measurements of force while cutting soft tissues with micro-scissors have not been carried out yet. Therefore, the device is designed under the assumption that the cutting resistance forces felt by an operator in cutting with microscissors are smaller than 1 N, considering that the forces applied to tissues in microsurgery are smaller than 1 N [8]. A micro-scissors type haptic device consists of three components below. The details are described individually /8/$ IEEE. 267

2 a lever a handle connected to the lever fulcrum pin blade spring disc spring (a) A design overview about 185 mm a coupler θc a crank connected to a motor shaft a fixed link base) (a) Usage of a crank-lever mechanism θl 14 mm 2 mm open angle: θop (b) A dimensional drawing in natural state of a blade spring Fig. 2. An interface of the micro-scissors type haptic device rotation angle of a lever θl [ ] θl R reduction ratio R The interface has the feature of micro-scissors in shape and is pinched by an operator. The drive member consists of actuators and decelerators and gives torques to the interface. Force sensors measure the forces generated by the drive member, and the obtained force data is used for feedback control. A. Design of the Interface The interface is designed to imitate spring type microscissors with straight blades and flat side face considering ease for combination with a drive member and force sensors. Both ends of a piece of blade spring are connected to handles of the interface individually, instead of connecting blade springs by a notch or a ring like actual micro-scissors. Design image of the interface is shown in Fig. 2 (a). The dimensions of the interface and adjustment of a piece of blade spring refers to a pair of actual micro-scissors from B. Braun Aesculap Co., Ltd. Fig. 2 (b) shows dimensions of the interface. Handle length is 14 mm, and distance from the fulcrum pin to the tip of the handle is 2 mm. When the blade spring of the interface is in natural state, the angle between handles θ op is 14, and overall length of the interface is about 185 mm. The minimum value of θ op is about 5.5, and at this time the restoration force of the blade spring reaches about 1.7 N at the gripping point, which is about 1 mm away from the fulcrum pin. The handles are made from aluminum alloy (JIS: A217). B. Design of the Drive Member In cutting operation with scissors, applied force to each blade from cutting object differs from state of contact, therefore a drive member must give each handle of the interface independent torques rotation angle of a crank θc [ ] (b) Result of analysis with selected parameter Fig. 3. Crank-lever mechanism as deceleration DC-motors are selected as actuators of a drive member because of its high response performance, controllability, and variety of lineups. From the aspect of friction and backdrivabillity, it is preferred that DC-motors give each handle of the interface output torque directly. However a drive member tends to be large to have enough output torque in the case of direct drive, and resolution of rotation of θ op may not be enough in the case of using a DC-motor with built in encoder. Therefore, it is decided that decelerators are used for DC-motors. For the decelerator with certain transmission performance, crank-lever mechanism which is a type of 4-bar linkage mechanism is selected. Crank-lever mechanism can realize nonslip transmission of rotation with simple structure. Usage of a crank-lever mechanism for a decelerator is shown in Fig. 3 (a). Base of a drive member is also a fixed link of crank-lever mechanism, and motor bodies and the fulcrum pin of an interface are fixed to the base. A crank is connected to a motor shaft, and a lever is connected to the handle of the interface around the fulcrum pin with appropriate shift of angle. In this configuration, rotation of the motor shaft is decelerated to rocking of the handle. In a design of decelerators using a crank-lever mechanism, range of movement and linearity of reduction ratio must be considered: Range of movement is limited between two dead points of a crank-lever mechanism, and reduction ratio of a crank-lever mechanism varies according to the state of the mechanism. Therefore, geometric analyses of a cranklever mechanism are carried out in order to decide lengths 268

3 (τdr2+τfr2) lop1 fop1 fsp(θop) strain gauges handles θop τdr1+τfr1 lop2 fop2 fsp(θop) lsp H-slits Fig. 4. Force relationship of the interface of each link. From the analysis, it is found that the range of movement and linearity of reduction ratio are in complex relation. Therefore, the combination of link parameters for a decelerator of a drive member is selected among manually given combinations, which had relatively high reduction ratio and wide range of movement: length of the fixed link is 5 mm; length of the crank is 1 mm; length of the coupler is 35 mm; length of the lever is 74 mm. Result of the analysis with above combination of link length is shown in Fig. 3 (b). There are two dead points around 12.2 and 34.1 in θ c, where the movement direction of the lever changes. Colored area in Fig. 3 (b) is selected as a range of movement of decelerator from two areas divided by dead points, because the colored area has relatively larger R than the other area. In the colored area, θ l ranges from to 173.4, this means that the range of movement is about 3 wide, and has R over about six. All links are made from aluminum alloy (JIS: A217), and hinge pins are used to connect links. Mechanical stoppers are put on the base of the drive member to prevent crank-lever mechanisms from falling into dead points. Levers are bonded with handles in order to reduce clearance. Next, relation of forces in the micro-scissors type haptic device is described. Fig. 4 depicts forces and torques applied to the interface focused on the direction of rotation. Suffix represents which handle to apply, one indicates two indicates. τ dri are output torques of a drive member, and τ fri are torques resulting from friction among components of the device. l opi are distances from the fulcrum pin to griping points, and f opi are applied forces of the operator to griping points. l sp is distance from fulcrum pin to mounting point of blade spring, and f sp (θ op ) are restoration forces of blade spring at mounting points of blade spring. For the equilibrium of force in Fig. 4, f opi is given as follows: f opi = τ dr i l opi + l sp l opi f sp (θ op )+ τ fr i l opi (1) An operator feels forces equivalent to f opi in the opposite direction when using the device. In (1), only the forces which result from τ dri are defined as cutting resistance force in this research. Hence, cutting resistance forces do not contain the influence of blade springs and frictions. C. Design of Force Sensors Strain gauges are applied to the handles of the interface to measure forces in the direction of cutting [4]. But sensitivity to strain of the handles is not enough to measure small Fig. 5. interface An overview of force sensors implemented to handles of the 1 mm load strain [μst] load [N] (a) Calibration condition (b) Calibration curves Fig. 6. Calibration of the force sensors forces that are expected to arise from cutting soft tissues with the micro-scissors, hence H-slits are processed on handles in order to gain more sensitivity and directivity to strain in cutting direction [9]. H-slit structure is an extension of parallel beam structure, and is used for a sensor with highly unidirectional force/torque sensor in order to measure the forces and moment in only one direction. Fig. 5 shows an overview of force sensors implemented to handles of the interface. Strain gauges are fixed on strain generating areas for both sides of each handle, and their resistance changes are detected by half-bridge configuration. Calibrations of the sensors implemented to each handle are executed individually: The interface is disassembled while the calibrations are executed. Calibration condition is shown in Fig. 6 (a). The handle is fixed to a table at the state of cantilever,and load is put on the griping point of the handle 1 mm far from the fulcrum pin. Load is increased from 1 g to 1 g gradually. Fig. 6 (b) shows the calibration curves obtained from five calibrations. From these calibrations, linearity of sensor sensitivity is confirmed. III. SYSTEM OF THE DEVICE System of the device is shown in Fig. 7. The system consists of the main body of the device, a relay box which contains power supply and circuits, and a control computer. In the device, two Maxon A max 26 mm diameter/1 W motor with HEDL 554 encoders are used. With the combination of decelerators of crank-lever mechanism, the device has an angular resolution of under about.3 (not constant) and maximum force output 2.7 N at griping points without considering the range of the force sensors. Maxon line amplifier LSC 3/2 is used for each motor with current control mode. Bridged circuit with half-bridge 269

4 micro-scissors type haptic device motor force sensor encoder Fig. 7. relay box servo amp. bridge circuit line receiver amp. control computer multifunction borad (A/D, D/A) counter board System of the micro-scissors type haptic device LAN strain [μst] "714strain.dat" u 2:7 "714strain.dat" u 2:8 closing opening opening closing open angle θop [ ] 15 configuration is used for each force sensor implemented in handles. Line receiver circuit is used for each encoder in order to translate line driver signals to TTL signals. The control computer is equipped with a K MHz running VxWorks 6.1, and control frequency is set to 248 Hz. The counter board PCI 621E from Interface Corporation is used in order to count the pulse of rotary encoders, and multifunction board PCI from Interface Corporation is used for analog digital and digital analog conversion. IV. SENSOR COMPENSATION AND CONTROL OF THE DEVICE Measured strains by the force sensors implemented to handles contain not only strains resulting from cutting resistance forces, but also strains resulting from restoration force of blade spring and friction among components. Therefore, in order to realize feedback control of cutting resistance forces, the strains resulting from restoration force of blade spring and friction among components must be compensated for measured strain in real time. In this section, acquisition of cutting resistance forces with sensor compensation and control of the micro-scissors type haptic device is described. A. Study of Strains in Blank Run Measurement of strains of handles in blank run is carried out in order to study the influence of restoration forces from the blade spring and friction among components. Fig. 8 shows the measurement result of consecutive five times blank run by an operator. Relation between strains of handles to θ op changes by phase of motional state of the interface. This feature is also observed in blank run of scissors that have no blade spring [4], therefore the main cause of this phenomenon is thought to be the friction among the components which changes with motional state of the interface. When the interface is closing, relation between strains of handles to θ op is nearly linear. Moreover, reproducibility of relation between strains of handles to θ op is confirmed in five times trial in Fig. 8. B. Modeling of Strains in Blank Run The strains resulting from the restoration force of blade spring and friction among components of the device are simply modeled based on the measuring result of blank run. strain Fig. 8. phase (d) Fig. 9. Measured strain in blank run phase (a) phase (c) open angle phase (b) Model of strain in blank run An image of the model of strain in blank run shown in Fig. 9 consists of two principles. Classification of motional state of the interface: Motional state of the interface is classified into four phases; (a) opening phase; (b) resting before closing phase; (c) closing phase; (d) resting before opening phase. This type of classification is often used in analysis of cutting with scissors [4]. Transition condition of motional state of the interface is empirically described by θ op and angular velocity of θ op. Linear approximation of relations of strain to θ op :Relation of strain to θ op is approximated to be linear in each of the above four phases of motional state. This is because relation of strain to θ op in phase (c) with high possibility of cutting is nearly linear, and for simplicity. With the above modeling of the strain in blank run, strains of handles in blank run can be calculated while using the device. To verify the propriety of the model of strains in blank run, blank run by an operator is carried out, and a comparison between measured strains and calculated strains by the model is done. Results of the comparison are shown in Fig. 1. The calculated strains agree with the measured strains well. Calculated strains are particularly close to measured strains in phase (c), therefore it would be possible to use this model to compensate the influence of blade spring and friction. However, it is observed that there are delays of transition in phase (b) and visible errors in phase (d) and phase (a). Therefore, improvement of transition condition and approximation of the relation of strain to θ op is required for the proposed model. 27

5 strain [μst] (model) -6 (model) time [s] -4-6 (model) open angle θop [ ] (a) Time strain relationship (b) θ op strain relationship Fig. 1. Comparison between measured strain and calculated strain strain [μst] (model) 15 cutting resistance force [N] time [s] fref θop open angle θop [ ] Fig. 11. Result of an evaluation experiment when f max =.5 N Accordingly, cutting resistance forces generated by a drive member are obtained from compensated strains using calibration curves in Fig.6 (b). C. Control of a Micro-Scissors Type Haptic Device f refi are desirable cutting resistance forces to display an operator, and f curi are cutting resistance forces obtained from the force sensors including the above compensation. When the device is displaying cutting resistance forces to an operator, torques in motors τ mi are given as follows: τ mi = l op i R i (θ op ) f ref i + K Pi (f refi f curi ) + K Ii (f refi f curi )dt + K Di ( f refi f curi ) (2) Where R i (θ op ) are reduction ratio of crank-lever mechanism, K Pi are proportional gains, K Ii are integral gains and K Di are derivative gains. The first term on the right side member in (2) is calculated from static equilibrium of forces. If there is no requirement for f refi,thenτ mi are zero. V. EVALUATION EXPERIMENT FOR BASIC PERFORMANCE OF THE DEVICE An evaluation experiment for basic performance of the device is conducted. The device s capability to display cutting resistance forces to an operator based on simple model, and the capability to track cutting resistance forces is confirmed. A. Experimental Conditions of the Evaluation Experiment In the experiment, f ref of each handle is equal, and the model for calculating f ref is given as: 13. θ op f max (9 <θ op 13 ) 4. f ref = sin π(θ op 7.) f max (7 <θ op 9 (3) ) 4. (θ op 7 ) f ref increases as the interface is closed, and reaches a peak of cutting resistance force f max at 9.Thenf ref decreases as the interface is closed, and becomes zero at 7. f ref is set to zero after once θ op < 7 in order not to produce cutting resistance force in opening the interface. There are three models with different f max :.2 N;.5 N; 1. N. Haptic update rate is 248 Hz in the experiment. B. Experimental Results of the Evaluation Experiment Fig. 11 shows a result of the experiment in the case of f max is.5 N. f ref exists only in colored area in the graphs, hence the changes of f curi in non-colored area are caused by errors of compensation in force sensors. It is observed that f curi follows f ref well, and the other variety of f max show the same tendency. Relatively visible oscillations are observed in case f max is.2 N, although an operator couldn t sense it. For this experiment, it is understood that the developed micro-scissors type haptic device can generate cutting resistance forces accurately. VI. VIRTUAL CUTTING EXPERIMENT Virtual cutting experiments to display the feeling of cutting are conducted to examine whether human could feel the sense of cutting. A. Experimental Conditions of the Virtual Cutting Experiment The experimental setup is shown in Fig. 12. A host computer calculates f ref according to (3) and displays a CG model of the micro-scissors without blade spring. The host computer is equipped with a Pentium4 (3.73 GHz), and communicates with the control computer via TCP/IP at the frequency of 124 Hz. When the operator opens/closes the micro-scissors type device, the open angle θ op is reproduced in the CG model of the micro-scissors. The micro-scissors type device is fixed on the base, however since the device is equipped with a force sensor (see Fig. 12), the operator can move the CG model horizontally by applying horizontal force to the device. The movable area of the CG model is divided into the five areas. The five different cutting resistance forces,.2,.4,.6,.8, and 1. N, are randomly assigned to the divided area. The assignment is kept in secrecy for subjects. There are six subjects for the experiments, and all of them are in their twenties and have no experience in using microscissors. First, the usage of micro-scissors and how to grip it is explained to the subjects, and blank runs with a pair of actual micro-scissors are carried out. Then, blank runs with the micro-scissors type haptic device are carried out by the subjects, and operational feeling of the device is assessed. Next, cutting operations by the virtual micro-scissors within 271

6 haptic device TABLE I RESULT OF VIRTUAL CUTTING EXPERIMENT f max [N] A B subject C D E F : feel no resistance CG model of a pair of micro-scissors Fig. 12. force sensor Experimental Setup of virtual cutting experiments any of the five areas are performed. The subjects are only informed that the five areas have different peaks of cutting resistance force from. to 1. N, and told to permute the numbers of these five areas based on the peak of cutting resistance force. Subjects are allowed to do the operation until satisfied. B. Experimental Results of the Virtual Cutting Experiment Subjects are seemed to be unaccustomed to the device without any DOF other than rotational for cutting at first. After some practice, they reply that there is no uncomfortable feeling in blank runs with the device, therefore friction and loosening of the device are thought to be small enough. A result of virtual cutting with the device is shown in Table I. Two subjects permute the numbers of the five areas completely, and three subjects permute four models correctly. The subjects except for F reply that they get the feeling of cutting rubber sheets, thin plates made of plastic, coated conductors and nails. VII. CONCLUSIONS Feeling of cutting is displayed by a micro-scissors type haptic device in virtual cutting experiments. The device is designed in order to generate cutting resistance forces and consisted of three components, the interface, the drive member and force sensors. The blade spring of the interface affects the force sensors implemented in each handles, therefore compensation based on calibration is done in order to obtain cutting resistance force. Using the obtained cutting resistance forces to feedback control, the device can generate cutting resistance force accurately. For sense of reality, it is desirable that a pair of actual micro-scissors could be used as an interface of a microscissors type haptic device. The presented method for displaying feeling of cutting will be able to be applied in the case, however, calibration condition of force sensors and compensation on force sensors should be changed accordingly. A joint screw of a pair of micro-scissors has much influence on maneuvering feeling of the instrument, hence calibration with disassembling micro-scissors may change its maneuvering feeling. Connection between two blade springs of a pair of actual micro-scissors, which is a notch or a ring, is expected to be a complicating factor to cause strain in handles. Miniaturization of a drive member is necessary to install a micro-scissors type device in a force display with multi DOFs to display feeling of operating a pair of microscissors with multi DOFs [1]. Naturally, measurement and analysis of forces in cutting soft tissues with micro-scissors are needed to decide specific performance requirements of a micro-scissors type haptic device and to make accurate models of cutting soft tissues with a pair of micro-scissors in order to display more realistic feeling of cutting. REFERENCES [1] J. Brown, K. Montogomery, J C. Latombe and M. Stephanides, A Microsurgery Simulation System, Proceedings of the 4th International Conference on Medical Image Computing and Computer- Assisted Intervention, pp , 21. [2] N. Mukai, M. Harada, K. Muroi, Y. Miyamoto, A. Uratani and T. Yano, Development of a PC Based Real Time Surgical Simulator, Systems and Computers in Japan, vol. 33, no. 7, 22. [3] H. Wakamatsu, X. Zhang and S. Honma, Teleoperational Force Display System in Manipulation of Virtual Object Using Scissors- Type Cutting Device, Proceeding of the 3rd Asia Pacific Conference on Control and Measurement, pp , [4] S. Greenish, V. Hayward, V. Chial, A. 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