The goal of this study is to develop a mechanical system that can display elastic characteristic of an object aiming at the application in the field of virtual reality. A pneumatic parallel manipulator is introduced as a driving mechanism, which consequently brings the capability of minute force displaying property owing to the air compressibility. A 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

Fujie, Kenichi

Noritsugu, Toshiro

Takaiwa, Masahiro

Okayama University Digital Information Repository

Recognition of elastic characteristic of object using pneumatic parallel manipulator

Okayama University Scientific Achievement Repository

EngineeringMechanical Engineering fieldsOkayama University Year 2002Recognition of elastic characteristic ofobject using pneumatic parallelmanipulatorMasahiro Takaiwa Toshiro NoritsuguOkayama University Okayama UniversityKenichi FujieOkayama UniveristyThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/mechanical engineering/12TEA1 5-4 sic$OZ-0058 Recognition of Elastic Characteristic of Object Using Pneumatic Parallel Manipulator Masahiro Takaiwa, Toshiro Noritsugu, Kenichi h j i e  Okayama University, 3-1-1 Tsushimanaka Okayama, JAPAN takaiwaQsys.okayama-u.ac. j p  toshiro@sys.okayama-u.ac.jp Abstract: The goal of this study is to develop a mechanical system that can display elastic characteristic of an object aiming at the application in the field of virtual reality. Pneumatic parallel manipulator is introduced as a driving mechanism, consequently, which 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. Keywords: Pneumatic driving system, Parallel Manipulator, VR, Compliance control, Distur- bance observer 1. Introdution Virtual reality technologies have become one of the re- cent attracts in the industrial field, where, such as, the application to the virtual prototyping in mechanical CAD or the surgery simulation') are expected. Among the virtual reality technologies, the development of the instruments, which display force or tactile feeling, is important because such a feeling, besides of the vision, plays an important role for human to recognize an euvi- ronment. Pneumatic actuators are effective for this kind of mechanical s ~ s t e m ~ ) ~ )  since its inherent features of softness and safety are indispensable for the mechanical device which contact with human directly. In this paper, we aim at developing a mechanical equipment that displays elastic characteristic of an ob- ject. Concretely, Stewart type parallel manipulator is introduced as a mechanical structure from a view that it can drive multiple d.0.f. for its compactness and pneu- matic cylinders are employed as the driving actuators, which bring capability of minute force control property owing to the air compressibility. The compliance display scheme is proposed, where a compliance control is constructed based on the es- timated external force and its validity is confirmed through some experiments and analysis. 2. Outline of Pneumatic Parallel Manipulator Fig. 1 (a) shows the developed pneumatic parallel ma- nipulator. 6 pneumatic cylinders (10 mm in diameter and 15 mm in stroke) are employed and they drive an upper platform in 6 d.0.f.. The position/orientation of the upper platform is ex- pressed by a hand vector h = [z, y, z, ++, 8 ,  $IT using roll-pitch-yaw angle notation. The origin of hand coor- dinate frame h is set at a center point of upper plat- (a) Parallel Link (b) Pneumatic Driving Mechanism Circuit Figure 1: Developed Pneumatic Parallel Manipulator form when manipulator stands in a standard posture. Similarly a link vector is defined as = [e, . . . . , ! f ]T with an element of a displacement of each piston rod. Force/moment vector works at an origin of h is defined as fm = [ ~ * , ~ ~ , ~ = , T ~ , T ~ , T ~ ] ~ .  The equivalent force vector acts on piston rod is denoted with f. which sat- isfy the following relation. fm = J T f e  (1) , where J is a Jacobi matrix which forms the next rela- tion dl dh - =  J- dt dt In the mean while, Fig.l(b) shows the pneumatic driving circuit of one cylinder. Low friction type pneu- matic cylinder is employed (Airpel Co. Ltd., 9mm in diameter, 50" in rod stroke). Pressure in each cylin- der's chamber, p l ,  p2 are detected by pressure sensors and the displacement of piston rod ! is measured by pc- tentiometer. The A/D converter is of 12 bit resolution. A control signal U calculated every sampling pe- riod(l0 ms) in a computer corresponds to an input voltage of a servo valve (FESTO, 50 !/mine) through D/A converter(reso1ution of 12 bit), which regulates the difference pressure of each cylinder. Supply pressure p,=400 kPa. Table 1 shows the control parameters. The linearized state equations of pressure in cylinder's chamber are m b f. AI,  A2 p l , p z  e Tp Figure 4: Generation force control system Equivalent mass for one cylinder Viscous coefficient External force applied on a link cross sectional area of cylinder chamber pressure in chamber displacement of piston rod Time constant of pressure response Figure 2 Concept image of compliance display described by the following equation. de - dt = -pz - kpu+ k,- dt ~ P Z  Equation of motion of piston rod is expressed by W 4 ) .  d2e de dt2 dt  plA1 - pzAz = f, = m- + b- + f e  (4) 3. Recognition of Elastic charac- teristic Fig.2 shows the concept image of compliance display. Human contacts with their fingertip at a center point of upper platform of manipulator and apply force for a various direction. The manipulator displays a prospec- tive compliance in arbitrary direction and make human feel(recognize) an object through a force feeling. Fig.3 shows the proposed position based compliance control system4). The inner position control system is designed in order that the closed loop transfer function may follow the 3rd order system shown in F.445). 1 (5) H C - = G, = diag{ H,  s3 + AsZ + BS t C The inner block with a doublet represents a control s y s  tem of generating force F. 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 F. to follow to the reference value with time constant Tpn4). Figure 5:  Compliance model First of all, the applied extemal force which works on a link equivalently is not measured by a force sen- sor but estimated by introducing a disturbance observer for the transfer part A(s). The estimated disturbance D(s)(= -F.(s)) is transfered to the hand coordinate forcejmoment vector f,,, through a transpose of Jacobi matrix JT and then fed back by being multiplied with a compliance matrix K-' = diag{K;', KF1, ._., K;'P. 4. Experimental results A recognizing of compliance change in local area is car- ried out experimentally. We set a compliance model as shown in Fig.5, where the stiffness for t  direction K, be- come larger when the contact point closes to the origin (z = y = 0) as shown in Eq.(6). K, = 1.0 x exp { -(21~')> + 0.5 (6) Fig.7 shows the obtained control performances for this case. Figure (a) shows the experimental situation. Figure 6 Graphic scene 1725 Figure 3 Position control system Human contact with a manipulator at a center point of upper platform and moves it with a reciprocation mo- tion according with y axis giving a constant force for z axis continuously. The stiffness for horizontal direction is set as a small value of K, = Ky = 0.1 N/mm not to prevent the motion for the horizontal direction. Figure (b) shows the applied force on manipulator for z axis, where solid and dotted line indicate the esti- mated force and measured one hy using a force sensor as shown in figure (a). An estimation error(ahout 1 N) is confirmed, which is mainly resulted from identifica- tion error between Pk(s) and P,,(s). The improvement of estimation accuracy is the matter to  be settled at present. Figure (c) shows the displacement of manipulator, where solid line indicates that for y direction while dot- ted one corresponds to  that for z direction. When the contact point across over the most rigid point(x = y = 0) at t=40 s and 60 s ,  since applying force is almost constant, an increase of displacement for z axis can be confirmed, which makes human feel getting over an pro. jection. Figure (d) shows the realized s t i e s ,  where dotted line is a value from a model shown in Eq.(6) and dashed line solid one shows the calculated stiffness based on the estimated force and based on the measured one hy a force sensor, respectively. These obtained stiffness are almost the same and match with the model quantita- tively. Fig.8 is the results of the same experiment with Fig.7 except that the stiffness model shown in Fig5 is set for the x direction K, in y - I plane as 10.0 K. = 1.5 x exp In this case, at the point (y=O and z=-10 mm) the stiffness hecome most large value of 2.0 Nfmm. Fig.6 shows the graphics model corresponding to  the experiment of Fig.'/ constructed with OpenGL. The dis placement of manipulator is reflected on graphics model in real time. The effect of visual aid in recognizing an object is also a matter of fntnre problem. 5. Conclusion In this study, we developed a pneumatic parallel manip ulator to  display a compliance of an object for human. A compliance displaying scheme is proposed, where the applied force is estimated using a disturbance observer and the desired compliance is realized by constructing a basic compliance control system. The compliance displaying properties has been con- firmed through the basic direction in hand coordinate frame, which proves the effectiveness of the proposed control system. In the next step, development of a con- trol scheme to display an arbitrary compliance for the arbitrary direction and verifying the effectiveness of o k  displaying equipment by applying to  the practical recog- nition motion are the matter to be settled at present. References [l] R.Baumann and R. Clauel, Haptic Interface for Vir- tual Reality Based Minimally Invasive Surgery Sim- ulation, Proc. IEEE E R A ,  381/386, 1998 121 Y. Tanaka, T. Kikuchi and A.Kaneko, Force Display in Virtual World by Fluid Power Glove, hoc .  of the Fourth JHPS Int. Symp. on Fluid Power, 187/192, 1999 [3] Y.Tanaka, T. Kanamori, Dynamic Force Display Device by Pneumatic Pressure Control, Pmc. of Fifth Triennial Int. Symp. on Fluid Control, Mea- surement and Visualization, ~01.2, 719/'723, 1997 [4] T. Noritsugu and M. Takaiwa, Motion Control of Pneumatic Parallel Manipulator Using Disturbance observer, Japan-U.S.A. Flezible Automations, 1986 [5] T. MuraEami and K. Ohnishi, Advanced Control Technique in Motion Control, The Nikkan Kogyo Shinbun Ltd., Japan, 1990 1726 1 - 1  paraltel manipulatoi (a) Experimental situation 10 . , . , . I . , .---krsrr- . - Ettimfld bru 8 -  2 6 :  I -. 4- 2-  OLJ io ' 4a ' Bo io I I S 1  (b) Contact force for z axis 20, . , ..... , . .. i -10 -200 U 20 40 60 80 r 14 (e) Displacement in hand coordinate frame .---- B s d o n s l l m z C d r y l  0 20 40 W 80 X I 4  (d) Compliance display performance Figure 7: Recognition of object in y-z plane I parallel manipulator (a) Experimental situation 0 20 40 61 I Id (b) Contact force for z axis ...... - I E O   'om_.. % i 7-10 ... -20 ., ... . ,  . ;  ., / 0 20 40 60 I IS1 (c) Displacement in hand coordinate frame 01; " 20 ' ' 40 ' ' 60 ' I  I I S 1  (d) Compliance display performance Figure 8: Recognition of object in x-z plane 1121 

Recognition of elastic characteristic of object using pneumatic parallel manipulator

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