Heavy tools manipulation by low powered direct-drive1.doc

article in pressavailable online at mechanism# sciencedirectandmachine theorymechanism and machine theory xxx (2007) /locate/mechmtheavy tools manipulation by low powered direct-drivefive-bar parallel robotsaeid nahavandia,*, mohammad jashim uddina, mozafar saadatb, hieu trinhaa intelligent systems research laboratory, school of engineering and information technology, deakin university, waurn ponds campus,pigdons road, geelong, victoria 3217, australia b school of engineering, the university of birmingham, edgbaston, birmingham, united kingdomreceived 18 may 2006; received in revised form 17 october 2007; accepted 19 november 2007abstractthis paper presents a simple and available system for manipulation of heavy tools by low powered manipulator for industrial applications. in the heavy manufacturing industries, sometimes, heavy tools are employed for different types of work. but the application of robots with heavy tools is not possible due to the limited torque limits of actuators. suspended tool systems (sts) have been proposed to manipulate heavy tools by low powered robot-arm for this purpose. a low powered five-bar direct-drive parallel manipulator is designed and constructed to manipulate heavy tools suspended from a spring balancer. the validity, usefulness, and effectiveness of the suspended tool system are shown by experimental results. 2007 elsevier ltd. all rights reserved.keywords: direct-drive (dd) motor; spring balancer (sb); suspended tool system (sts); torque limits; cartesian base position control; heavy object manipulation1. introductionin the last decades, robots have made tremendous development in industries for manufacturing and assembly purposes. however, work with heavy tools in heavy manufacturing industries, such as; automobiles, shipyard, and airplane hangers, application of robotics is still in its infancy. conventional robots with serial or parallel structures are impractical for these applications since a very wide range of actuator power is required for a variety of heavy work. for object handling, there is a great deal of research in the coordinated control of multiple robot arms 1,2. when two or more robot arms are used to perform a single task together, an increased load carrying, handling and manipulation capability can be achieved. however, a single manipulator cannot manipulate heavy objects because the joint torque stays within a fixed limit. for this reason, the object handling by a single manipulator is not discussed as yet. albus et al. 3 have developed a light-weight robotcorresponding author. tel.: +61 3 5227 1231; fax: +61 3 5227 1046. e-mail address: .au (s. nahavandi).0094-114x/s - see front matter 2007 elsevier ltd. all rights reserved. doi:10.1016/j.mechmachtheory.2007.11.004please cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in press2s. nahavandi et al./mechanism and machine theory xxx (2007) xxxxxxthat can operate over a large workspace. cable-suspended robots are structurally similar to the parallel-actuated robots but with the fundamental difference that cables can only pull the end-effector but not push it. the cable-suspended robot is discussed by abdullah and agrawal 4. moving a suspended load is very difficult due to both its rotation and swing effects. many researchers have worked on the issue of control of suspended load. auering and troger 5 describe the time optimal control of overhead cranes with hoisting of the load. anti-swing control of a suspended load with a robotic crane is discussed by lew and khalil 6.parallel manipulators offer significant advantages over current serial manipulators when structural stiffness and high-performance dynamic properties are required. therefore, such mechanisms have received some attention over the last two decades 7. gearing mechanisms of the conventional electromechanical robots cause many problems to the accurate motion control due to the different mechanical nonlinearities of gearing, friction, backlash, and deflection. to eliminate these problems induced by gearing, the conventional electric motors have been replaced by high torque, low speed direct-drive motors. asada et al. 8 used direct-drive motors in a serial arm mechanism. in a serial arm the relatively large weight of the dd-motor is a load for the next motor in the chain. to eliminate the inefficient payload capacity of serial arms, a closed-chain arm design has been implemented. asada and youcef-toumi 9 have designed a parallel direct-drive arm with invariant and decoupled inertia characteristics. a five-bar direct-drive manipulator designed by kazerooni 10 is statically balanced in order to eliminate the gravitational load of the arm components. huissoon and wang 11 designed a five-bar manipulator with the compensation of gravitational load and decoupling of the inertia matrix based on the use of torsion spring and counterbalance, respectively. alici 12 proposed a useful method in trajectory planning and control of five-bar planar parallel manipulators in joint space. the task based kinematic design of a two dof manipulator with a parallelogram five-bar link mechanism presented by kim 13. a direct-drive master arm was described by kotoku et al. 14. balancing of an inverted type pendulum with a redundant direct-drive robot was explained by jung et al. 15.this paper first describes the design of a low powered direct-drive five-bar parallel manipulator for precise motion control of the manipulator. the robot-arm has 2dof and can move in horizontal plane freely. in traditional robotic operations, the cutting tool is directly mounted on the manipulator, and hence the tool weight directly affects the manipulator dynamics. to manipulate heavy tools by a low powered manipulator, a new tool strategy, suspended tool system (sts) has been addressed and verified experimentally.2. system descriptionthe robot-arm can be driven directly or indirectly. with direct-drive, the link joint is coupled to the rotor of the driving motor directly. with indirect-drive, the link is connected to the driving motor through a transmission mechanism. direct-drive method provides better positioning accuracy since the intermediate gearing system is eliminated and consequently the mechanism is free of backlash and hysteresis. another advantage is the improved reliability because of the smaller number of mechanical parts. direct-drive arms, in general, tend to have excessively fast operating ranges, whereas the output forces are extremely small (asada and ro 16). in this section, a five-bar direct-drive manipulator is described. the hardware and singularity problems are also addressed. the suspension mechanism is explained for robotic manipulation.2.1. five-bar direct-drive robot with suspension systemthe experimental system consists of a robot with two degrees of freedom (dof) having a five-bar link configuration and a suspension mechanism. the robot-arm can move in the horizontal plane (xy). the suspension system consists of a spring balancer and an overhead rail. a roller slider moves on the overhead rail freely with the spring balancer. the overhead rail acts as a positioning device. the spring balancer suspends the tools by its lifting force. the lifting force of the spring can adjust easily by an adjusting screw manually. here it is important to note that the lifting force of the spring balancer must be equal to the gravity forces of the tools, otherwise it will affect the motion of the manipulator severely. fig. 1 shows the cad design of direct-drive five-bar parallel manipulator with a suspension mechanism. table 1 shows some important properties of five-bar link mechanism.please cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in presss. nahavandi et al. / mechanism and machine theory xxx (2007) xxxxxx overhead rail-pring balancer wire ropeforce sensorrinding toolpneumatic hand 0)3fig. 1. five-bar direct-drive parallel robot with suspension mechanism.table 1properties of five-bar manipulatorpropertieslink1link2link3link4length (m) mass (kg) c.m (m) m.i, cm (kg m2)0.26 0.43 0.154 0.0050.12 0.60 0.038 0.0020.26 0.72 0.130 0.010(0.12 + 0.187) 1.50 0.1564 0.01362.2. hardware descriptiona hardware schematic diagram of the control system is shown in fig. 2. it illustrates the various connections between the controller and the components of the system. a pentium based microcomputer (nec, pc-98), 133 mhz, is used in the control system. the a/d and d/a converter has 8 channels and 12-bit resolution. the feature of servo driver is that it can be preset to operate in three different modes of control, i.e., position control, velocity control, and torque control. for our experimental system, the servo driver is set to the torquepentium pc i/o portcontrol boxdirect drive robotd/a convertercounter boardservo drivermotorresolvera/d converterampliferforce sensorfig. 2. hardware of robot system.please cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in press4s. nahavandi et al./mechanism and machine theory xxx (2007) xxxxxxcontrol mode. the counter board has 3 ports and 24-bit pulse resolution. a low capacity three-axis force sensor is mounted between the end of robot-arm and the tool holder, which is calibrated to work up to 19.62 n. an operational amplifier with low pass filter is designed to eliminate unexpected noise from the output signal of the force sensor. table 2 shows some important properties of direct-drive motors.2.3. work space and singularityasada and ro 16 and ting 17 have pointed out the singularity problem for the five-bar closed link manipulator. the workspace of a robot-arm is the total volume swept out by the end-effector as the robot-arm executes all possible motions. the workspace is constrained by the geometry of the robot-arm. for a given end-effector position, there are in general two possible solutions to the inverse kinematics. the singular configuration separates these two solutions. at singular configuration, the manipulator cannot move in certain directions. there are two types of singularities, stationary singularity and uncertainty singularity. a closed-loop manipulator may have both stationary and uncertainty singularities. at a stationary singularity, the jacobian matrix has zero determinant, whereas at an uncertainty singularity, the determinant of jacobian matrix is infinity. for the five-bar link configuration, the determinant of jacobian matrix j, is defined asj = 14sin(92 o1)(1)where 1 and 4 are the links length, 91 and 62 are the actuator input angles and stays within the limits: 0 b1 180 and 0 92 180.the stationary singularity occurs whensin($2 01) = 0(2)from eq. (2), the stationary singularity occurs on the boundary of the workspace. thus, by selecting link dimensions, a wide singularity free workspace can be obtained. fig. 3 shows the workspace of five-bar link mechanism for the link configuration as shown in table 1.table 2properties of direct-drive motorspropertiesdirect-drive motorsmotor #1motor #2maximum torque (nm) rated rps (rps) encoder resolution (p/rev) weight (kg) rotor inertia (kg m2)2.04.51024002.42.0 x 103.94.51024004.52.0 x 100.60.4 0.20-0.2 l-0.6 -0.4 -0.2 0 0.2 0.4 0.6 x axis mfig. 3. workspace of five-bar manipulator.please cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in presss. nahavandi et al./ mechanism and machine theory xxx (2007) xxxxxx5ryz spring balancerwire rope, v spring balancer force, fbc_._. u grinding tool - f force, f(x, y)fig. 4. model of suspended tool system.table 3parameters of spring balancerparametersvaluemass of roller slider, ms mass of spring balancer, ms mass of tool, mt length of wire rope, acceleration of gravity, g1.44 (kg) 1.41 (kg) 2.00 (kg) 1.55 (m) 9.81 (m/s2)2.4. suspension mechanisma model of a suspended tool system is shown in fig. 4. ari et al. 18 pointed out that the passive angles, sewing angle and orientation angle, are not controllable since a single wire suspends the system. the properties of spring balancer are shown in table 3. in fig. 4, / = swing angle, and w = orientation angle. in order to simplify the suspension system, the following assumptions are considered. the elastic deformation of overhead rail, mass of rope, rolling resistance, wind forces, and noise are neglected. the cartesian coordinates of the end-effector isx = x0 + sin (f cos i/ y = y0 +1 sin sin i/r(3) (4)the suspension force, fb, in wire rope depends on the suspended mass but independent of the variation of rope length. the active forces on suspended tool areffa = fj sin (f cos i/r fy = fj sin (/ sin i/r(5) (6)now, the suspension force vector in planar motion isffe = ffa fhy (7)please cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in press6s. nahavandi et al. i mechanism and machine theory xxx (2007) xxx-xxx3. system dynamicsin order to accommodate the control derivation, dynamics of the robot and some properties of the dynamic equation, both in joint space and cartesian space, as well as the relations between them are described in this section.3.1.problem formulationthe equation of motion of a planar robot manipulator is defined in joint coordinate asm9)9 + h9,9) = x(8)where 9 e r x denotes the vector generalized joint coordinates of the manipulator, m(ff) e r x denotes the symmetric and positive definite inertia matrix, h9, 9) e r2x1 denotes the vector of nonlinearities which includes the coriolis centripetal and gravitational forces, x x , x(t) xd eq. (16) yieldsrt) = xd + a2x11 + a1xd(17)from eqs. (16) and (17) we get,xd - x + a2(xd -x)+a1 xd - x) = ri) - r(18)e(t) + a2e(t) + a1e(t) = 0(19)where the position tracking error, e(t) = x xbased on control law (19), linearizing and decouple cartesian base feedback control law yieldsplease cite this article in press as: s. nahavandi et al., heavy tools manipulation by low powered direct-drive ., mech. mach. theory (2007), doi:10.1016/j.mechmachtheory.2007.11.004article in presss. nahavandi et al./ mechanism and machine theory xxx (2007) xxxxxx7m(6)j1 (xrf - a2e - a1 e) + m(9)j1xd + h(9, &) = x(20)from eq. (20) the position tracking error e(t) approaches zero exponentially. fig. 5 shows a block diagram of cartesian base position control model.due to its structure simplicity and performance robustness, the proportional, integral, and derivative (pid) controller has been widely used in various fields of control applications 19. in practice, the primary concern with the use of pid controllers is the tuning, that is, the determination of pid controller parameters to produce satisfactory and robust control performance. the longstanding use of pid controllers has motivated the development of various tuning methods or rules for setting suitable pid controller parameters 20,21. the methods differ in complexity, flexibility, and in the amount of process knowledge used. in recent years, the optimization-based pid controller tuning methods have been receiving increasing attention 2226. in the most proposed optimal tuning methods, the three pid controller parameters are searched to minimize a certain integral performance criterion while satisfying stability and/or sensitivity constraints. in 27,28, shubladze proposed to determine the pi and pid controller parameters for maximizing the degree of stability. tuning the pid controller parameters maximizes the degree of stability of the parallel manipulator.trajectory generatorccartesian positionxt),y(t)(,x,y)inverse dynamict i