Manipulability Enhancement by an Impedance Parameter Tuning for a Remote Ultrasound Diagnostic System Norihiro Koizumi*, Takahiro Kato*, Shin’ichi Warisawa*, Hiroyuki Hashizume** and Mamoru Mitsuishi* *Department of Engineering Synthesis School of Engineering, The University of Tokyo Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan e-mail: nkoizumi, kato, warisawa, mamoru @nml.t.u-tokyo.ac.jp ** Department of Orthopaedic Surgery Okayama University Medical School Shikata-cho 2-5-1, Okayama 700-8558, Japan e-mail:
[email protected]
Abstract. A master-slave type remote ultrasound diagnostic system was developed. This paper presents its control strategy. The controller has an impedance control capability for the master and slave manipulators’ positions. It also has a continuous path(CP) control capability for the slave manipulator’s orientation. The impedance control allows the master and slave manipulators to move according to the motion control law autonomously, and allows the medical doctor to feel reaction force. Manipulability varies according to the impedance parameters. This paper makes clear the relationship among the virtual viscosity, the manipulability and the response speed. The paper proposes a method to adjust the virtual viscosity according to diagnostic tasks. The proposed impedance control strategy applied for the remote ultrasound diagnostic system was evaluated in the experiment while virtual viscosity was changed. At the first step, a slider-type linear virtual viscosity input device was developed and it became clear that the medical doctor varies the virtual viscosity value by 3 levels according to diagnosis processes. Therefore, a 3-level virtual viscosity switching system was implemented. Remote ultrasound diagnostic experiment demonstrated that the medical doctor can perform diagnosis successfully using the proposed system.
1 Introduction We have developed a remote medical system for ultrasound diagnosis in the “aged society”[1][2]. In general, reality-sensation is necessary to diagnose efficiently in remote medical systems. This paper presents the impedance controller for a masterslave type remote ultrasound diagnostic system. An impedance control is a method to control the mechanical impedance of the tip of the manipulator according to tasks, and to regulate the response characteristics regarding the motion of the manipulator properly[3]. Generally, a position controller for the slave manipulator is based on the Point To Point(PTP) control. In this system, however, the position controller for the slave manipulator is composed of a velocity-control-based impedance controller. This
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features our system and makes it possible to shorten the dead time taken at the slave manipulator side to respond for the contact force input. First, this paper presents the configuration of the system and position controller. It has an impedance controller based on velocity control. The result of the force control experiment is also discussed. Secondly, we make clear the relationship among virtual viscosity, the manipulability and the response speed. Then, we evaluate the impedance controlled remote ultrasound diagnostic system in which the virtual viscosity is variable. In this system, a slider-type linear virtual viscosity input device is presented at first. Then, we analize the variety of the input virtual viscosity during diagnosing. Finally, the problem of the device is discussed and we implement a 3-level virtual viscosity switching system. The results of the remote ultrasound diagnostic experiment using this system are also discussed.
2 Related work of conversation in the medical field. In this case, it is mainly advisory to the medical doctor with patients from a doctor in the remote site, referring diagnostic image and other data (X-ray film, computed tomography (CT), magnetic resonance imaging (MRI), electrocardiography, electromyography (EMG), endoscopy, arteriography, venography, ultrasonography and phonocardiogram). However it is necessary to realize a remote diagnostic system with a robot system that can input and realize medical doctor’s motion in diagnosis for efficient home care. Tachi’s group studies a bilateral remote control with high reality sensation including force feedback[4]. S.E.Salcudean’s group studies a remote ultrasound diagnostic system[5][6]. Furthermore, a remote ultrasound diagnostic system using pantograph mechanism is also developed[7]. The control system of our system is based on impedance control. The control law and impedance controlled master-slave system whose impedance parameter is variable feature our study. This makes it possible for the medical doctor to diagnose efficiently according to tasks in the remote ultrasound diagnosis. Furthermore, we discuss the method and device to input an impedance parameter.
3 Remote ultrasound diagnostic system The system configuration is shown in Fig.1. Specifically, a medical doctor diagnoses a patient in the remote site by manipulating the master manipulator. During diagnosing the patient, the medical doctor refers to the ultrasound diagnostic image, the ultrasound probe, the patient and the slave manipulator displayed on TV monitor. Position, orientation, force, image and audio information are transmitted between the multimedia cockpit with the medical doctor and the consulting room with the patient. The overview and degree-of-freedom configuration of the master and slave manipulators are shown in Figs.2 and 3 respectively.
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Fig. 1. System configuration
Fig. 2. Overview of the master manipu- Fig. 3. Overview of the slave manipulalator tor
4 Impedance controller 4.1 Controllers for the master and slave manipulators Position controllers for the master and slave manipulators are implemented using an impedance controller based on velocity control. The controllers for the master and slave manipulators are illustrated in Figs.4 and 5 respectively.
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First, we illustrate the controller for the master manipulator. A driving force of the master manipulator is calculated by Eq.(1). Here, is the driving force of the master manipulator. is the measured force of the master manipulator, is the measured force of the slave manipulator. is the force-scale ratio.
if if and if and
(1)
Generally, only the first equation of Eq.(1) is implemented. Therefore, in the first implementation of our system, the driving force of the master manipulator was calculated by the first equation. However, a medical doctor commented that he couldn’t control the master manipulator very well when the ultrasound probe contacted with a patient’s body. Then, the control law is designed as shown in Eq.(1). According to the second equation of Eq.(1), the medical doctor cannot push the probe against the patient’s body anymore. According to the third equation of Eq.(1), a medical doctor can pull back the probe. The medical doctor advised us to add the second control law. In short, if the slave-force is larger than the master-force, the driving force of the master manipulator is set 0[N]. This makes it possible to enhance stability of the motion of the master manipulator when the medical doctor pushes against the patient’s body. As a result, unexpected motion against the intention of the medical doctor should be avoided.
!#"$& %
Fig. 4. Impedance controller for the master manipulator
Fig. 5. Impedance controller for the slave manipulator
(2)
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! !
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(3) (4)
(5)
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is the desired position of the master manipulator, is the virtual mass of the master manipulator, " is the virtual viscosity of the master manipulator, The desired velocity can be calculated by solving Eq.(2) using Eqs.(3)and (10) by Runge-Kutta method. And represents sampling time of force information. In Eq.(4), and are calculated from Eqs.(5)and (8). To compensate the dead time ! caused by sampling time, !+* , and%$&) !- are calculated using the !" master force and the previous one in Eqs.(9) and (10). Those forces are used in Eqs.(6) and (8). Based on the desired velocity, this system can control proper angular velocity which is sent to the motor driver. Then, we illustrate the controller for the slave manipulator. Same as the master manipulator, the slave manipulator is also controlled by velocity control based impedance controller. And a position error between the master and the slave manipulators is compensated by position feedback.
5 Determination strategy of an impedance parameter 5.1 Impedance parameter determination strategy
is set as 10[kg], considering the mass of the tip of the slave manipulator. A transmission function of the slave manipulator .0/ 21#3 is expressed as the following equation: .4/ 21#3 9 (11) 53 !#"$ "$ 6387 "$ ! "$ & 3 !
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Fig. 6. Scene of an experiment (uppers:patient and slave, left bottom:diagnostic image, right bottom:doctor and master)
9 Here, is a time constant. The time constant is one of indicators that represent ( the settling time. It represents the time that takes for of the final change. In this system, Time constants for each virtual viscosity are calculated as follows: 9 3 for "$
[N s/m]. of final change takes 9 . Impulse response and unit step response of impedance controller are expressed in Eqs.(12) and (13). $& $&
.4/ 21#3 3 $& $&
.4/ 21#3 3
"$
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"$
9
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(13)
As is shown, the impedance controller is the first-order system. Considering step and impulse responses, the settling time is shorter when the value of 57 "$ is smaller. As a result, the manipulability becomes enhanced. However, a large force is needed to move the master manipulator when " is large. 5.2 Impedance parameter determination experiments The purpose of this experiment is to make clear the relationship between the impedance parameter and the manipulability. A medical doctor diagnosed using this system, and then evaluated the manipulability(Fig.6). In the experiments, " is set as
[N s/m]. In Figs.7 and (9, driving forces of the master manipulator( ) and velocity( % ) calculated by Eq.(2) are shown. Time in each figure represents the elapsed time from the start of diagnosis.
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When the driving force is input in case of " =100[N s/m], the settling time of velocity is long(Fig.7(a2) and (b2)). In other words, the response speed is too small. As a result, the medical doctor felt stress against the response speed. As shown in Fig.9(a2) and (b2), the response speed is not small when " is [N s/m]. As a result, the medical doctor felt no stress against response speed. However, the output velocity is small against the magnitude of the driving force input. Because of it, the medical doctor felt stress against the small magnitude of the velocity. Actually, the medical doctor didn’t move the master manipulator so dynamically as compared with the case that " is
[N s/m]. However, the medical doctor moved the master manipulator dynamically when " is [N s/m]. According to the medical doctor, "
[N s/m] is the best to diagnose if " is constant through the diagnosis. Specifically, as shown in Fig.8(a2) and (b2), both response speed and driving power to move master manipulator are agreeable.
6 Variable virtual viscosity system 6.1 slider-type linear virtual viscosity input device implemented at first as a device to change the virtual viscosity of the system(Fig.10(a)). A medical doctor can change the virtual viscosity while diagnosing by manipulating the slider on the device(Fig.10(b)). In Fig.10(c), a variation of the virtual impedance input in a diagnosis is shown. In Fig 10(d), allotted time for each virtual impedance value through 8 diagnoses. In this figure, 3 peaks of impedance
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value(about 50,200,350[ 7 ]) can be observed. The medical doctor pointed out that it deteriorates efficiency of diagnosis to manipulate the master manipulator and the device concurrently.
6.2 Evaluation of a 3-level virtual viscosity switching device manipulation and the characteristic of the allotted time for each virtual impedance value, a 3-level virtual viscosity switching device is newly implemented. Specifically, the virtual viscosity value for each level of the switches can be set by each slider installed on the switching device in Fig.11(a). The medical doctor can change the virtual viscosity of the system by switching the device(Fig.11(b)). At first, we set 3 virtual viscosity levels such as 50, 200 and 350 [ 7 ] in a diagnostic experiment. The medical doctor switches the virtual viscosity as shown in Fig.11(c). Considering the stability of the system, 3 virtual viscosity levels are adjusted to 120, 200, 350 [ 7 ] respectively by the medical doctor after the experiment. The variation of the virtual viscosity after the adjustment is shown in Fig.11(d). In the diagnosis, the medical doctor switches the virtual viscosity as follows: (i)When the medical doctor moves the ultrasound probe roughly, "$ [N s/m] is mainly selected. (ii)When the precise motion is needed, " [N s/m] is mainly used. (iii)When the doctor wants to move probe more carefully ( and push it strongly against the patient, " [N s/m] is selected.
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7 Conclusions The position controllers of the master and slave manipulators have been developed based on an impedance controller. It is suggested that the motion control law and impedance parameters should be designed well corresponding to the diagnosis processes. The relationship between an impedance parameter and manipulability has been made clear. Furthermore, we implemented a variable impedance adjustable remote ultrasound diagnostic system. At first, the slider-type linear virtual viscosity input device is presented and evaluated. Then, the problem of the system is pointed out and we implemented a 3-level virtual viscosity switching system. Finally, it is confirmed that the medical doctor can diagnose successfully with the variable impedance device.
References 1. M.Mitsuishi, S.Warisawa, T.Tsuda, T.Higuchi, N.Koizumi, H.Hashizume and K.Fujiwara, “Remote Ultrasound Diagnostic System,” Proc. of 2001 Int. Conf. Robotics and Automation, Vol.2, pp.1567-1573, 2001. 2. M.Mitsuishi, T.Tsuda, Takuya Higuchi, N.Koizumi, H.Hashizume and K.Fujiwara, “Development of a Remote Ultrasound Diagnostic System,” Proc. of the 18th Annual Conf. of the Robotics Society of Japan, Vol.1, pp.435-436, 2000(in Japanese). 3. T.Tuji, M.Hatagi, H.Akamatsu and M.Kaneko, “Non-Contact Impedance Control for Manipulators,” Advanced Robotics, Vol.15, No.4, pp.136-143, 1997.
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(a) A slider-type linear virtual viscosity input
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4. S.Tachi and T.Sasaki, “Impedance Controlled Master-slave Manipulation System. Part I. Basic Concept and Application to the System with a Time Delay, Advanced Robotics,” Advanced Robotics, Vol.6, No.4, pp.483-503, 1992. 5. W.-H. Zhu, S.E.Salcudean, S.Bachman, and P.Abolmaesumi, “Motion/Force/Image Control of A Diagnostic Ultrasound Robot,” Proc. of 2000 Int. Conf. Robotics and Automation, pp.1580-1585, 2000. 6. P.Abolmaesumi, S.E.Salcudean, W.-H.Zhu, S.P.DiMaio and M.R.Sirouspour, “A User Interface for Robot-Assisted Diagnostic Ultrasound,” Proc. of 2001 Int. Conf. Robotics and Automation, Vol.2, pp.1549-1554, 2001. 7. K.Masuda, K.Ishihara, “Development of a robot for ultrasound diagnosis and its clinical applications by considering safety contact on body surface,” Proc. of the 18th Annual Conf. of the Robotics Society of Japan, Vol.1, pp.439-440, 2000(in Japanese).
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