&#60;P&#62;The goal of this study is to develop a mechanical system which display elastic characteristic like stiffness on the surface of human body aiming at applying to palpation simulator. Pneumatic parallel manipulator is employed as a driving mechanism, consequently, it brings capability of minute force displaying property owing to the air compressibility. Compliance control system without using force/moment sensor is constructed by introducing a disturbance observer and a compliance display scheme is proposed. The validity of the proposed scheme is verified experimentally. </p

Noritsugu, Toshiro

Takaiwa, Masahiro

Okayama University Digital Information Repository

Development of force displaying device using pneumatic parallel manipulator and application to palpation motion

Okayama University Scientific Achievement Repository

EngineeringMechanical Engineering fieldsOkayama University Year 2003Development of force displaying deviceusing pneumatic parallel manipulatorand application to palpation motionMasahiro Takaiwa Toshiro NoritsuguOkayama University Okayama UniversityThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/mechanical engineering/13Proceedings of the 2003 IEEE l o t e r n a t i ~ n a l  Conference on Robotics &Automation Taipei, Taiwao, September 14-19, 2003 Development of Force Displaying Device Using Pneumatic Parallel Manipulator and Application to Palpation Motion Masahiro Takaiwa Department of Systems Engineering, Okayama University, 3-1-1 Tsushimanaka, Okayama, 700-8530, Japan Absfracf-The goal of this study is to develop a me- chanical system which display elastic characteristic like stiffness on the surface of human body aiming at applying to palpation simulator. Pneumatic parallel manipulator is employed as a driving mechanism, consequently, it brings capability of minute force displaying property owing to the air compressibility. Compliance control system without using forcelmoment sensor is constructed by introducing a disturbance observer and a compliance display scheme is proposed. The validity of the proposed scheme is veriiied experimentally. I. INTRODUCTION A palpation motion is one of the most important medical examination to find a human disease like a cancer of the breast. There is a great need for the development of a mechanical system as a palpation training simulator[l] or a displaying device for remote diagnosis[2]. In this study we aim at developing a mechanical system which can display compliance characteristic of human skin, such as, “stiffness” for the operator’s palpation motion(pushing at a point or slide on the surface) as shown in Fig.2. A pneumatic parallel manipulator is employed as our displaying device since it can drive a multiple d.0.f. for its compactness due to the parallel link mechanism and it can execute minute force regulation owing to the air com- pressibility of Pneumatic actuator which is indispensable feature for the palpation motion. The air compressibility simultaneously implies a feature of safety and softness, which are indispensable for the mechanical system contact with human directly. In order to display a concrete compliance feeling to an operator, a control strategy is proposed where the applied force from an operator is estimated with no use of forcelmoment sensor and the contact point on which an operator is touching is detected based on that estimated force and finally realize a corresponding compliance by constructing a compliance control system[3]. By regu- lating the reference compliance value according to the movement of a contact point(motion of the finger), a palpation action is realized. The validities of the proposed control systems are confirmed through some experiments and analysis. 0-7803-7736-2/03/$17.00 02003 IEEE 4098 Toshiro Noritsugu Department of Systems Engineering, Okayama University, 3-1-1 Tsushimanaka, Okayama, 700-8530, Japan (a) Parallel Link Mechanism (h) Pneumatic Driving Circuit Fig. 1. Developed Pneumatic Parallel Manipulator Fig. 2. Concept image of compliance display 11. OUTLINE OF PNEUMATIC PARALLEL MANIPULATOR Fig. 1 (a) shows the developed pneumatic parallel manipulator. 6 pneumatic cylinders are employed to form so called Stewart type platform[41. The positionlorientation of the upper platform is ex- pressed by a hand vector h = [z ,y ,z ,$ ,O,$~]~  us- ing roll-pitch-yaw angle notation. The origin of hand coordinate frame h is set at a center point of upper platform when manipulator stands in a standard posture. TABLE 1 SYSTEMS PARAMETERS Time constant of pressure response Nominal time constant of pressure respanre Steady gain of pressure response Nominal steady gain of pressure response Steady gain between piston velocity and pressure Equivalent m m  for one cylinder VISCOUS cnfficient External force applied on a link cross sectional area of cylinder chamber air pressure in chamber displacement of piston rod Jacobian matnx ! Time constant of filter Similarly a link vector is defined as = [e,, ...., e,]T with an element of a displacement of each piston rod. Force/moment vector works at an origin of h is defined as fm = [fz, f,, fZ,r+,r~,r+lT. he equivalent force vector acts on piston rod is denoted with f e which satisfy the following relation. fm .= J T f e  (1) , where J is Jacobian matnx and it forms the next relation in a parallel manipulator mechanism. (2) de d h  - = J -  d t  dt In the mean while, Fig.l(b) shows the pneumatic driv- ing circuit of one cylinder. Low friction type pneumatic cylinder is employed (Airpel Co. Ltd.. 9.3mm in internal diameter, 50mm in rod stroke). Pressure in each cylinder's chamber, p l ,  p Z  are detected by pressure sensors and the displacement of piston rod e is measured by wire type h e a r  encoder. The A D  converter is of 12 bit resolution. A control signal U calculated every sampling pericd(l0 ms) in a computer corresponds to an input voltage of a servo valve (FESTO, 50 elmin) through DIA con- verteflresolution of 12 bit), which regulates the difference pressure of each cylinder. Supply pressure p ,  is set to be 400 kPa. Table I shows the control parameters. The linearized state equations of pressure in cylinder's chamber are described by the following equation[5]. Equation of motion of piston rod is expressed by Eq.(4). d2e de dt2 dt p l A l  ~ p2A2 = f s  = m- + b- + f e  (4) 111. RECOGNITION OF ELASTIC CHARACTERlSTlC A. Conceptional image Fig2 shows the concept image of compliance display. Human touches at an any point on the surface of an ma- nipulator with their fingertip and implements a palpation motion by applying force for a various direction. The manipulator displays a corresponding force for the pushing motion of a fingertip by regulating compliance of the manipulator itself based on the displacement of fingertip and applied force. In order to realize such an action, a manipulator should have a function to detect which point a fingertip is pushing at and how much force is being applied. In the next section the strategy of compliance display including these detecting function is described. B. Compliance contml system Fig.3 shows the proposed position based compliance control system[6]. The inner position control system is designed in order that the closed loop transfer function may follow the 3rd order system shown in Eq.(5). { s3 + A s Z  +Bs +C - G, = ding H -- HT The inner block with a doublet represents a control system of generating force Fg as shown in Fig.4, which works to lower the influence of piston rod velocity that acts as disturbance on pressure response as shown in Eq.(3) as well as to make Fg to follow to the reference value with time constant T,,[6]. First of all, the applied external force which works on a link equivalently is estimated by introducing a disturbance observer[7] for the transfer, part PI;(s),  in- stead of measuring by installing a forcelmoment sen- sor which may loose a feature of compactness. The estimated disturbance D(s ) (=  -F,(s)) is transfered to the hand coordinate forcelmoment vector f ,, through a transpose of Jacobian matrix J T  and then fed hack by being multiplied with a compliance matrix K-' = diag{K,- ' ,K,- ' ,K,- ' ,K~',KK',K~'}T. C. Detecting contact force and contact point Fig.5 shows a geometrical model where contact force vector f is applying at a contact point represented by position vector R = [XO, yo, 201~. So the first purpose of this study is to detect these vector f and R based on the estimated forcelmoment vector f-. Here we consider f- = [ftT, T ~ ] ~  with transient force vector ft  and moment one r. As you see that, force vector f is simply derived from the balance of translational force around the origin as f = f t  (6) In the mean while, if the equation of manipulator's surface is known as Eq.(7), then the contact point can be derived based on the balance of moment shown by Eq.(8) in the following manner[8]. ! d X O ,  YO, 20)  = 0 (7) Rx f = r  (8) 4099 Fig. 3. Proposed compliance control system Fig. 4. Generation force control system From FigS, position vector R can be described using a parameter X as ft x ?- ft R = -  + X- If# l f t l  (9) And R is obtained by substituting Eq.(9) into Eq.(7) to In our manipulator, as shown in Fig.l(a), a hemispher- determine A. ical shell, which is made of plaster, is introduced as the outer shape, whose center matches to the origin of h. Therefore giving an equation of hemisiherical shell as Eq.(lO) with radius T, contact point can he represented'hy Eq.(ll), where [XI, YI, aIT is a first term of right hand side of Eq.(9). x Fig. 6. S p k g  model In the next, displaying corresponding compliance to the contact point is considered. Fig.6 shows a geomet- rical compliance model, where Kd is a desired stiffness along with the direction of the contact point. Hence the remaining problem is how much each element K , ,  Ky, K ,  should he set to realize the desired stiffness Kd for any contact point. From a balance of translational force at the origin of h, the desired stiffness K d  for the direction of the contact point (20. yo, 20) and the stiffness for each axis K,, Kv,  K ,  satisfy the relation represented by Eq.(12), where [Az, Ay, ArIT is a displacement vector generated by an applied contact force. The left hand side corresponds to the desired force converted to the direction of contact point vector and the right one do the resultant force of each axis. 41 00 In order that K,, Ky and K ,  may satisfy Eq.(12), we introduce a constraint condition which makes the compliance control characteristic be normalized for each direction in the following manner. The closed loop relation of the control system shown in Fig.3 is described as 70 .= 50 F, = I,,JTJCIG,~'(I+I,,JTJCIK-l)~l(HdG,-H) - ?1.5 0 0 0 0 0 0 1.5 0 0 0 0 0 0 2.9 0 0 0 0 0 0 3 . 0 ~  lo3 0 0 o o o o 1.4 x 103 o 0 0 0  0 o 1.4 103 - - s io so 0.1 0.5 I w [rad/sl Fig. 1. Frequency charactenstic Seeing from Eq.(15), diagonal element comsponding to the z axis in J T J  is almost 2 times as much as that for x and y axis. Therefore by setting each element of K according to the ratio in Eq.(15), the frequency characteristic between F, and HdG, - H for x,y,r direction becomes equivalent except for the influence of a static gain. K,: KY : K,  = 1 : 1 : 2 (15) Substituting Eq.(15) into Eq.(12), desired stiffness for each axis is given as In this study, the stiffness for the rotational direction X,, K O ,  Kg are all set to be 0, which means positioning control is implemented for the rotational direction. Fig.7 shows the frequency characteristic of the coef- ficient of HdG, - H in Eq.(13). By normalizing the frequency characteristic of the compliance control perfor- mance for each axis, the frequency characteristic of the desired compliance (admittance) can he also prescribed by the same frequency characteristic, which is useful in evaluating the realization of the desired compliance (admittance) in a frequency domain. :y/ ~, iLr! ...-. ...... f [SI (a) Horizontal direction 0.3, , , . , , , 7 l I e 0.2 :o.y, , , ; M r ,  .....~ 1 ...... f IS1 (b) Rotational direction Fig. 8. Position control performance IV. EXPERIMENTS AND DISCUSSION A. Position control performances Fig.8 shows the positioning step response, where (a) and (h) corresponds to the horizontal and rotational direction, respectively. A small circle o indicates the response of a model shown in Eq.(5), where parameters are chosen as A = 38, B = 410, C = 1400 in order that the step response may he almost the same with that of 2nd order system with un = 8.0 rad/s and 6 = 1.0. In the both figures, a little ovenhoot are confirmed but the obtained response for each direction is almost the same with that of the desired model, which proves an effectiveness of a proposed position control system. B. Detection of contact force Vector and contact point Fig.9 shows the estimation performances of the contact force and contact point. Contact force is applied through a force sensor continuously 3 times for the same point (xo,yo,zo)=(-21.4, 37.0, 74.0) [mm] as shown in the 41 01 (a) Experimental situation I O  8 5 4  2 '0 20 40 60 80 IW f I S 1  (b) contact force . .  .............. . ..:i . , .  . I  : * ', : I : : * I f IS1 (c) contact point Fig. 9. Estimation performance of contact force and contact point . .  Fig. 10. Stiffness model figure (a). The estimation performance of contact force and contact point are shown in (b) and (c). respectively. In spite that force is applied from various direction, both of the contact force and contact point can be confirmed to be estimated well, which proves the effectiveness of the proposed detection scheme. C. Compliance display performance Compliance displaying performances are verified. We introduce a geometrical model of a stiffness on a human skin as Eq.(17) apploximately, which means reference (a) Experimental situation 10 Contact point L E E 4  Contact poim y I-2 Contact point of r 45 50 55 60 , [ $ ] 6 5  -" (b) Locus of contact point 1. ~ Stiffness model 1 Based on estimated tom I (c) Realized stiffness Fig. 11. Compliance display performance stiffness Kd displaying for a fingertip has a maximum value of K,,, at a point [xp,yp,tp] and according to going away from that point Kd closes to the minimum one K,,,",,. + Kmin (17) Fig.10 shows the geometrical image of Eq.(17), where restriction of zo = 0 is introduced to make possible it to be shown in the actual 3-d space, where (xp, yp, zp)=(O,O.O), K,,, = l.O[N/mm], K,i,=0.5[N/mm], and spreading parameter s p  = 200. Fig.11 shows the experimental result of compliance display by palpation motion. As shown in'figure (a), human holds a force sensor and execute a round trip motion in order that contact point may go over the most rigid point with applying a force for normal direction con- tinuously. The most rigid point is set to be (zp, yp, +,)=(- I 4102 Force sensor (a) Experimental situation Contact point x 50 60 70 (b) Locus of contact pa%’ Stiffness model Eased on estimated lorce 0. 0.  70 f [SI 80 50 60 0.d ’ ’ ’ ’ , ’ -1 (c) Realized stiffness Fig. 12. Compliance display performance 40.0, 0.0, 75.5)[mm] and the stiffness is determined as K,,, = 1.0 [N/mm], Kmi, = 0.5 [N/mm] based on the prior measurement stiffness data of a forearm. Fig. 11 (h) shows the locus of the contact point. Seeing from the figure, round trip motion is confirmed to be done for the direction along with y axis. Fig.11 (c) shows the realized stiffness, where solid line corresponds to the value obtained from a models shown in Eq.(17), while dotted one indicates the actual realized stiffness obtained by a calculation of Iftl/J(A.)2 +(AY)’ + (Az)’. A response lag can be confirmed, which is considered tn be resulted from dynamics lag of inner position control system, but almost satisfactory displaying property can be obtained, which proves that the proposed displaying scheme works properly. Further improvement of the dis- playing performance is the matter to he settled at present. Fig.12 shows the same experimental results with Fig.11 except that the difference stiffness K,,, - K,i, is quite small of 0.1 [N/mm] for the contact point of (zpr yp, zp)=(O.O, -40.0, 75.5)[mm]. It is confirmed that even small variation of stiffness of 0.1 [N/mm] can be displayed, which is owing to the air compressibility and it is the advantage of employing a pneumatic driving system. V. CONCLUSION In this study, we developed a mechanical system using a pneumatic parallel manipulator, aiming at displaying a compliance characteristic on a human skin. In order to realize such a motion, we proposed a com- pliance displaying scheme, where a contact point(which point on the surface of the device an operator is touching at) and cnntact force(how much force an operator is applying) are detected using an estimated force/moment with no use of general force/moment sensor and then display the target compliance determined according to the contact point by consuucting a compliance control system. Through some experiments, almost satisfactory control performances can be confirmed both in estimating cnntact force and contact point and in displaying the reference stiffness. The small variation of stiffness of 0.1 [N/mm] can he displayed, which is owing to the air compressibility of pneumatic actuator. In addition to the further improvement of compliance displaying performance, the concrete recognition using not only sense of force feeling reported here but that of vision by consuucting the deseased part in a computer using graphics image is under the current investigation. VI. REFERENCES [ I ]  J.Kim,S.De,Slinivasan, “Computationally efficient techniques for real time surgical simulation with force feedback, Proc. 10th Symp. on Haptic In- terface for Virtual Environment and Teleoperator Systems, HAPTICS 2002, 2002, pp.51-57 [2] S.Majima and K.Matsushima, “Fuzzy Evaluation of Stiffness of Tissue by means of Micro-manipulator”, Proc. SPIE- Int. Soc. Opt. Eng. (USA), V01.2101, No.1, 1993, pp. 521-526, [3] M. Takaiwa and T. Noritsugu, “Development of Pneumatic Human Interface and Its Application to Compliance Display”, J. of Robotics and Mechatmn- ics, Vo1.13, No.5,2001,pp.472-478 141 D.Stewart. “A ulatfom with Six Deerees of . 1  Freedom”: Pmc. ‘Inst. 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Development of force displaying device using pneumatic parallel manipulator and application to palpation motion

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