When humans and robots implement a cooperative task, information (intention) transfer between them is an important problem as the task becomes complicated. In the paper, a haptic interface using a pneumatic parallel manipulator is developed to realize “information transfer by means of contact”. The contact information given by the human, namely contact force vector and contact point on the manipulator, is detected by the interface itself and such information is transferred to the robot by being connected with some reference signal. The pneumatic parallel manipulator works as a kind of elastic body even when its position is controlled owing to the air compressibility. Focusing on this characteristic and introducing an idea of compliance center with a spherical shell, contact force vector and contact point are detected without a force sensor. The validity of the proposed method is confirmed through some experiments

Noritsugu, Toshiro

Takaiwa, Masahiro

Okayama University Digital Information Repository

Application of pneumatic parallel manipulator as haptic human interface

Okayama University Scientific Achievement Repository

EngineeringMechanical Engineering fieldsOkayama University Year 1999Application of pneumatic parallelmanipulator as haptic human interfaceMasahiro 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/10Proceedings of the 1999 IEEYASME International Conference on Advanced Intelligent Mechatronics September 19-23, 1999 Atlanta, USA Application of Pneumatic Parallel Manipulator a s  Haptic Human Interface Masahiro Takaiwa and Toshiro Noritsugu Faculty of Engineering, Okayama University S 1- 1, Tsushima-naka, Okayama, 700-8530, Japan Phone f81-86251-8062, Fax t81-86251-8024 Email t akaiwa@sys.okayama-u. ac.j p, toshiro@sys.okayama-u. ac. j p Abstract-When human and robot implement a cooper- ative task, information(intenti0n) transfer between them comes up as an important problem as the task becomes complicated. In this paper, a haptic interface using a pneumatic parallel manipulator is developed to  realize ”information transfer by means of contact”. Concretely saying, the contact information given by human, namely contact force vector and contact point on the manipula- tor, is detected by the interface itself and such informa- tion are transfered to  the robot by being connected with some reference signal. Pneumatic parallel manipulator works as a kind of elastic body even when its position is controlled owing t o  the air compressibility. Focusing on this characteristic and introducing an idea of com- pliance center with spherical shell, contact force vector and contact point are detected without force sensor. The validity of proposed method is confirmed through some experiments. K q w d -  Haptic Interface, Pneumatic Driving Sys- tem, Parallel Manipulator, Elastic Characteristic I. INTRODUCTION In an environment where human and robot coexist, such as carrying out a cooperative tasks, information transfer between human and robot is indispensable. In this study we consider an idea of ”information (in- tention) transfer by means of contact” and aim at devel- oping a haptic human interface to realize such an idea. Concretely saying, the several force information applied by human, namely the applied force vector and the con- tact point on the interface’s body, are detected by the interface itself and they are transfered to the robot by connecting with some reference signal. For the sake of this purpose, the interface itself must have a function to detect contact point and contact force vector (magnitude and direction). Distributing a tactile sensor[l] on the surface of the interface’s body may con- tribute to detect a contact point and magnitude of the contact force(if it is from normal direction), but it seems to be difficult to detect the direction of the contact force vector. In this study, Stewart[2] type parallel manipulator whose degree of freedom is driven by pneumatic cylin- /&& platform _ -  U Fig. 1. Developed pneumatic parallel manipulator ders is employed as our human interface. Parallel mech- anism is effective as this kind of haptic interface since it has multiple degree of freedom for its compact mecha- nism. Consequently, the interface can work as a kind of elastic body even when its position is controlled owing to air compressibility[3]. Utilizing this elastic character- istic, a detection scheme of the applied force vector and contact point with no use of general force/moment sen- sor is suggested. The validity of the proposed scheme is confirmed through some experiments and analysis in the bellow. Unlike a passive interface, such as distributing tac- tile sensor on a body, our interface not only act as an omni directional force sensor itself but has a function to be an multiple d.0.f. actuator, which has possibility to implement various complicated contact motion. 11. OUTLINE OF PNEUMATIC PARALLEL MANIPULATOR Fig. 1 shows the schematic diagram of the developed pneumatic parallel manipulator. 6 pneumatic cylinders (31.5 mm in diameter and 35mm in stroke) are employed 0-7803-5038-3/99/$10.00 0 1999 IEEE 185 Computer pentium 11 233MHz I Pressure sensor DIA 3 converter 4 Fig. 2. Pneumatic driving circuit e displacement of piston stroke p l ,  p;! ' A,. A2 pressure of each chamber effective sectional area of chamber as its driving actuators. The position/orientation is described by a hand co- ordinate vector h = [x, y, z, @,e,+IT where x, U, z are horizontal displacement and @, 8, I) expressed by roll- pitch-yaw angle indicate the manipulator's posture. A link vector is defined as I = [e,, with a element of a displacement of each piston rod. Fig.2 shows the pneumatic driving circuit of one cylin- der. The rod side cylinder chamber is pressurized by constant pressure p2=236 kPa, while a pressure in a head side one p l  is regulated by electrepneumatic pro- portional valve. The displacement of piston rod is mea sured by potentiometer(l2.2pm in resolution) and p l  is detected by a pressure sensor(29.4 Pa in resolution). A control signal U calculated every sampling p e  riod(2.5 ms) in a computer corresponds to an input volt- age of D/A converter. 111. POSITION CONTROL SYSTEM Proposed force detection scheme is based on the ma- nipulator's elastic characteristic when position control is implemented. Fig.3 shows the position control system[4] employed in this study. It is constructed based on the pressure control system from a view that pressure con- trol is indispensable for an improvement of control per- formance in fluid power systems. A variable with upper- case letter represents Laplace transformed one and ( s )  is neglected for simplicity. Reference length of link L, is calculated through in- verse kinematics and position control system is con- structed at each link independently. State equation of air in a head side chamber and equs  tion of motion of piston rod are described as follows K p  K ,  Tp m dPl de T p x  = -pl + K ~ U  -K,- d t  static gain between U and pl  static gain between piston velocity and pl time constant of pl  against U euuivalent mass including that of Diston rod ' TABLE I SYSTEM PARAMETERS b f e  damping coefficient total amount of external force (coulomb friction, interaction force etc.) @e de ~ i A i - p 2 A ~ = m - + t - + f ~  dt2 dt ,where each parameter is represented in TABLE I. The variable with a subscript n means a nominal model of its correspondence. First, disturbance observer[5] is introduced to the me- chanical transfer part, namely from pressure PI to link velocity sL. Consequently the estimated disturbance D includes the influence of parameter change and die  turbance at the same time. Therefore if a pressure PI could be generated so as to contour pressure reference P, and D then influence of disturbance and parameter change would not affect on the control performances at all. Then pressure control system using disturbance ob- server is constructed to compensate nonlinear pressure response characteristic +d to contour P, with estimated disturbance D(actual1y D through a filter) derived from the former disturbance observer. In the case when these disturbance observer worked effectively, the transfer function between P, and sL is represented by the products of two nominal models as Supposing that either of time constant of each nomi- nal model Tp, mn/bn can be set enough small compared to the other, closed loop system can be represented by a desired second order system shown in eq.(4) by produc- ing P, through a controller C explained in eq.(5). L - wx - _  L, s2 + 2cwns + w: (4) 186 .................................................................................................................... : Fig. 3. Position control system IV. DETECTION OF FORCE VECTOR AND CONTACT POINT From Fig.3 closed loop relation between L, and L is simply described as L = QL, - I;; F, (6) where a is a closed loop transfer function and it b e  comes equivalent to eq.(4) when the position control system worked ideally. Imp corresponds to a mechan- ical impedance per one link, which is represented as a form of (7) where G is a closed loop transfer function of inner pres- sure control system. In pneumatic systems, lImpl becomes small in the low frequency range. This is one of reasons why it is not easy to improve a positioning accuracy in pneumatic system since inverse of Imp is equivalent to a sensitivity func- tion. However this characteristic gives manipulator a compliance(softness) and it must be effective for a hu- man friendly machine. From a principle of virtual work, a force/torque ap- plied at an origin of h, f m  = [ f m z ,  f m y ,  f m z  1 fm+, fms f+IT is related with a force equivalently acting on a link f e  as where J is a Jacobi matrix, which satisfy the following relation. d l  d h  d t  d t  - =  J- (9) In a parallel manipulator, contrary with a serial one, h can not be obtained analytically. In this study con- vergence calculation( Newton -Raphson method) is em- ployed to obtain h and its sufficient convergence accu- racy has been confirmed [4]. Substituting Laplace transformed eq.(S) into eq.(6) by considering eq. (9), yields eq. (10) Therefore if mechanical impedance of link Imp is known, then force/torque Fm at an origin of h can be estimated without using a force/moment sensor. Utilizing the Fm calculated in eq.(lO), the detection scheme of the applied force vector and its contact point are described bellow. Fig.4 shows a geometrical model, in which contact point of the force vector f is expressed by poler coordinates (T,  a, 0). As you see that, concerning to the force vector f, it is simply derived from the balance of translational force as f =z [fino, f m y ,  f m z I T  (11) On the other hand, contact point (T ,  a, 0) is d e  rived by considering the balance of moment around the origin of h. The moment around an origin made by f is described by R x f, where R = [T cos 0 cos a, T cos 0 sin a,  T sin PIT is a position vector of 1 87 x -1.6 0 0 0 0 o 7  0 1.6 0 0 0 0 0 0 2.9 0 0 0 0 0 0 3 .7x  lo-* 0 0 0 0 0  0 1 . 7 ~  10W2 0 - 0  0 0 0 0 1.7 x drn+ Fig. 4. Geometrical model contact point. Therefore equations of a balance of m e  ment around the origin of h is described as the follow- ing. - fmzr sin Q cos + fmyr cos Q cos P = fm+ (12.a) fm,rsinp - f m r r c o s a c o S ~  = fme (12.b) - fmyr sin /3 + fmzr cos Psin Q = fm+ (12.c) In this study a convergence calculation (Newton- Raphson method) is also employed to solve eq.(12) for the variables T ,  Q, p since it is too complicated to solve analytically. This method needs an inverse of Jacobi matrix of the equation, however it proves to be irregu- lar. Therefore we assume T is constant by considering that contact force f is applied on a newly introduced spherical shell which cover the manipulator and whose center matches the origin of h. Consequently Q! and ,B can be obtained by the following iteration. n = 0,1,2, ... where JZ is a pseudo inverse of the Jacobi matrix shown in the following equation. Here, as you see that the accuracy of the pro- posed estimation method depends on that of calculated force/moment fm works at origin of h. Then a new hand coordinate system is introduced so that Jr J in eq.( 10) becomes diagonal matrix at its origin. In generally such a point is called "compliance center". It is expected to Fig. 5. Manipulator covered by shell improve the estimation accuracy of f m  by considering at point of compliance center. In our manipulator, a compliance center was found to be at 15mm under the center of bottom plane and FJ at that point is represented as follows, where non- diagonal element is neglected to be 0 since their con- tribution for the force generation are significantly low compared to the diagonal one. Hence the spherical shell is constructed so as to make its center to match the compliance center, which is shown in Fig.5(a part in the figure). Here if the force vector is previously known to be applied through the normal direction for the spherical shell, contact point(a,p) is simply and directly given by on line as shown in eq.(16) since no moment is gen- erated around compliance center owing to its physical characteristic. v. EXPERIMENTS AND DISCUSSION Fig.6 shows the positioning step response at horizon- tal direction, in which 0 indicates the response of a model shown in eq.(4). For all links, U,  and < are set to 8.0 rad/s and 1.0, respectively. From identification, the 188 -20 E E Lu - x d i r e c t i o n  I 3 1 0  - m o d e l  0 0 . 5  1 1 . 5  Fig. 6. Position control performance 1 1 0  I . . . . ,  . . . , . . . . I  I .  . . .  1 0 - i  1 o n  Hz l o '  Fig. 7. Identification of impedance characteristic time constant of Pk in Fig.3 is much smaller than that of PPI so the controller given in eq.(5.b) is employed. The obtained responses for each direction are almost the same with that of the desired model, which proves an effectiveness of a proposed position control system. A .  Detection of Force Vector and Contact Point For the force detection, mechanical impedance char- acteristic of a link Imp must be previously known. It may be essential to use eq.(7) as a mathematical model of impedance characteristic, however it seems to be so complicated for implementation that an approximated model obtained from the following experimental identi- fication is employed for a simplicity. In the static state of positioning step response, random frequency force is applied on a piston rod through a temporarily attached force sensor. In Fig.7, 0 indicates a bode diagram of a mechanical impedance characteristic analyzed by FFT algorithm, while solid line corresponds to that of the approximated model described by eq.(17) where K=6.0 x lo4 N/m and TI =O. 13 s. K(1 + T I S )  I m p  = S Fig.8 indicates the estimation performance when con- tact force is constrained to be applied from normal di- rection for the shell at the point of (U = 120 deg. p = 42 deg. as shown in (a). Magnitude of the applied force is shown in (b), in which dotted and solid line indicate the force measured by force sensor and that calculated according to eq.( ll), respectively. Almost satisfactory (i)Top view (ii) Side view (a) Experimental situation ._" calculated force - - z 'I I - 0 2 4 6 8 1 [SI Magnitude of applied force - ob 5 150 100 50 0 -50 d 0 2 4 6 t [SI Contact point Fig. 8. Estimation performance (normal direction) estimation performance can be confirmed, but it is get- ting worse while a force is not applied. It causes that positioning error are still remaining even a force is not applied, which yield a 'lie' force according to eq.(ll). Contact point calculated from eq.(16) is shown in (c), where solid and dotted line correspond to a and p, r e  spectively. From these results, almost the satisfactory estimation performance in both of the magnitude and the contact point can be confirmed. Subsequently, Fig.9 shows an estimation performance of a force vector(arbitrary direction) and contact point. A human holds a force sensor and gives an impulsive force 3 times continuously at the point of a=120 deg. and p= 42 deg. as shown in Fig.S(a) in the sequence 0, @ and 0. All of the 3 force vectors 0 @ @ lie in the 0'-0-A' plane but angle from x-y plane is about 20 deg. 189 (i)Top view (ii) Side view (a) Experimental situation (b) Magnitude of applied force (c) EXtimated force vector and contact point Fig. 9. Estimation performance (arbitrary direction) for 0, about 40 deg. for @I and about 60 deg. for 0. Fig.9 (b) shows the magnitude of the applied force, in which dotted and solid line are the force measured by force sensor and that calculated according to  eq.(ll),  respectively. Almost the satisfactory estimation perfor- mance can be confirmed, except while a force is not a p  plied from the same reason mensioned in Fig.8(b). Fig.9 (c) shows the estimated contact point and force vector which are calculated from eq.(12) by ”off-line” convergence calculation method using fm at the time when each applied force has a maximum value. Here, contact point(a, p) was converged to be (125 deg., 32 deg.) for 0, (120 deg., 42 deg.) for @ and (121 deg., 51 deg.) for 0. Through the all cases, almost sufficinet estimation r e  sults were obtained in the directions of force vector and an element of contact point of a. However, except for the case 0, the element of contact point p is differerent from its original. In generally it has been confirmed that the estimation performance of contact point (especially p) is getting worse as the direction of force vector d e  viated from the normal direction for the spherical shell. There is much left to improve the estimation perfor- mances concerning to the contact point. VI. CONCLUSION In this paper a human-robot haptic interface which detect a contact information given by human, namely applied force vector and contact point, is proposed to realize an idea of ”information(intenti0n) transfer by means of contact”. In order to fulfil the intended function, a parallel ma- nipulator driven by pneumatic actuator is employed by focusing that it works as a kind of elastic body even when its position is controlled owing to air compress ibility. Utilizing this elastic characteristic and newly intro- duced combination of spherical shell and compliance center, one detection scheme of applied force vector and contact point is proposed with no use of general force/moment sensor. In the case force vector is constrained to be normal for the spherical shell, the magnitude of a force and the contact point are almost accurately estimated owing to the characteristic of compliance center. On the other hand, in the case when the applied direcion of the force is arbitrary, estimation accuracy of contact point get worse compared to that of force vector. It can be said that the obtained experimental perfor- mance proved the effectiveness of the proposed scheme, but there is still much room for further improvement of the estimation accuracy. This is the matter to be settled at 111 PI I31 [41 I51 present. REFERENCES D.Johnston, P. Zhang, J. Hollerbach, S.Jacobsen, A f i l l  Tac- tile Sensing Suite for Dextrwu.9 Robot Hands and Use in Con- tact Force Control, Proc. IEEE ICRA Minneapolis,Minnesota, vol. 4, 322213227, 1996 D.Stewart, A Platfom with Six degrees of Reedom, Proc. Inst. Mechanical Engineers, 180-15, 371/386, 1965-1966 M. Takaiwa and T. Noritsugu, Sensor-less Force Detection by  Pneumatic Parallel Link Manipulator and Its Applimtion for Contact Task, Proc. of FLUCOME’97, vol.1, 469/474, 1997 T. Noritsugu and M. Takaiwa, Motion Control of Pneumatic Parallel Manipulator Using Disturbance obsmrer, Japan- U.S.A. Flexible Automations, 1986. T. Murakami and K. Ohnishi, Advanced Control Technique in Motion Control, The Nikkan Kogyo Shinbun Ltd., Japan, 1990 1 90 

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Application of pneumatic parallel manipulator as haptic human interface

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