SE1100681A1 - An industrial robot containing a parallel kinomatic manipulator - Google Patents

Abstract

The present invention relates to a parallel kinematic robot to be used to manipulate a platform (102a) in at least three Degrees of Freedom having at least three rotating arms. Each arm is mounted on the outgoing shaft of a servo actuator to rotate in a horizontal plane and is connected to the manipulated platform with at least one link (109 -114). Each link has a joint with two or three degrees of freedom in each end making it possible to swing each link in all directions in relation to the arm and the platform to which it is connected. In total the platform is connected to six links allocated to the at least three arms. In the case the platform is manipulated in three Degrees of Freedom each arm is connected to two links. (Fig. 2)

Description

15 20 25 30 35 handling and assembly of components on a flat surface, such as the surface of a conveyor is typically performed by parallel kin- ematic manipulators.

A parallel kinematic manipulator (PKM) is defined as a manipulator comprising at least one stationary element, a movable element, denoted a platform, and usually three arms. Each arm comprises a link arrangement connected to the movable platform. Each arm is actuated by an actuator preferably arranged on the stationary element to reduce the moving mass. The link arrangements transfer forces to the movable platform. For a fully built-up parallel kinematic manipulator for movement of the platform with three degrees of freedom, three parallel-working arms are required. To obtain a stiff arm system with a large loading capacity and a low weight, the arms connected to the movable platform of the parallel kinematic manipulator should have a total of six links.

There are different types of manipulators for fast material handling, pick and place and assembly, such as the Scara manipulator, the Gantry manipulator, and the Delta PKM manipulator.

The Delta PKM manipulators are taking market shares from es- especially the Scara manipulators, because of its lightweight arm structure, which provides higher dynamic performance in relation to manipulator manufacturing cost. However, the Scara manipulator has some features that can not be achieved by the Del- ta manipulator, as for example small footprint, possibility to mount the manipulator beside the work space, possibilities to obtain a larger vertical stroke and the possibilities to make in - stallations with a higher manipulator density. Looking at the kin- ematics, increasing the horizontal work space size means only that the arms of the Scara manipulator need to be resized while for the Delta manipulator the whole framework and the manipu- lator must be resized, which means that the manipulator will need to be located higher above the work space. 10 15 20 25 30 35 ln order to combine the advantages of the traditional Scara kin- ematics with the performance potential of the Delta PKM manip- ulator, a Scara Tau PKM concept was developed. This concept is disclosed in WO 03/066289. The prior art Scara Tau manipu- lator has three inner arm parts mounted on three bearings, which are in turn mounted on a vertical column, which also holds the motors with its gear boxes. The output shaft from the gear box is connected to a gear wheel, which engages a gear ring mounted on the arm side of the arm bearing. Because the motor and gear box must be mounted with their shafts parallel to the rotation axes of the arms, a lot of space is needed between the arm bearings in order to get the space needed for the actuation system. This means that even if the robot obtains a small foot- print it will need a lot of space in the vertical direction. This is especially a problem if the robot is used to serve for example a conveyor, especially if the robot must be mounted over the con- veyor. Beside the need of a large space in the z-direction, the prior art robot will also be heavy because of the mounting col- umn supporting the arms and the actuation system.

The so called SCARA robots (Selective Compliance Assembly Robot Arm) are used in industry today to move and rotate ob- jects with 4 degrees of freedom. Examples of applications are mounting of electrical components, sorting of small products on a conveyor and filling boxes with products as biscuits and pra- lines. During the last 10 years the so called Delta robot con- cept has emerged taking part of the market from the SCARA robots. The competitive edge of the Delta robots is much lower arm inertia because of a parallel kinematics arm system. This robot structure is described in US patent 4,976,582 and con- sists of 3 arms arranged to manipulate 3 pairs of parallel links connected to a platform that is moved by the arms in the x-, y, and z-directions. The arm structure is mounted above the work space and a cardan transmission is used to transmit a rotation movement to the moving platform. Compared with the SCARA robots the Delta robot has the advantage of a lower arm inertia 10 15 20 25 30 35 but simultaneously the working range is smaller in relation to the footprint, the working range in the vertical direction is smaller, it is not possible to obtain the same robot density, the robot needs a large framework to hang the robot above the work space and the arm system cannot be moved out of the work space for cleaning and repair. ln order to get rid of the drawbacks with the Delta robot concept but still have low arm inertia for high speed- and acceleration capabilities, the Scara Tau robot has been proposed, see for example the Swedish patent 9700090-5. Three actuators with coinciding rotation centers drive 3 arms including a parallel link structure in such a way that the robot can be mounted beside the work space eliminating the drawbacks exemplified above with the Delta robot concept. The link structure as described in this patent is set up with 3 links to one of the actuated arms, 2 links to the second actuated arm and 1 link to the third actuated arm. An improved version of this link set up is presented in the patent US6412363. Here a symmetric link system is used with 2 links (14,15; 16,17 and 18,19) connected between each actuat- ed arm (6,7,8) and the platform (2), see Figure 1. This symmetry gives the advantages that all links can be of equal length and diameter, that all joints can be identical and that simpler joints can be used. That simpler joints can be used depends on the fact that all links form link pairs and the joints can therefore be designed as ball and socket pairs compressed by simple springs between the ends of the links. All these advantages lead to an improved modular design and the robot can be manufactured at lower cost.

However, in order to obtain these advantages, a special drive transmission is needed between one of the motors (5) and the corresponding arm (8) that this motor (5) drives. This transmission is necessary in order to keep the tilt angles of the platform (2) constant throughout the work space. 10 15 20 25 30 35 DESCRIPTION OF THE INVENTION The transmission consists of a right angle gear train (9,10) with the second gear wheel (10) mounted on a shaft on an extension of another arm (7), actuated by the motor (4). The drawbacks with this transmission are expensive drive arrangement for the arm (8) and that the motors (4 and 5) must have larger torque capacity in order to take care of the coupled torques generated through the transmission coupling (9,10). ln this patent applica- tion the mounting of the links have been changed to get rid of this problem, see Figure 2. Thus, the horizontally mounted link pair (16, 17 in figure 1) is now replaced by a vertically mounted link pair ( 111, 112 in figure 2) and the upper arm (108) can be actuated to rotate independent of the actuation of the other up- per arms (106, 107). The mounting of the lower arm links on the upper arms according to Figure 2 makes it possible to obtain constant tilt angles for the platform (102 in Figure 2) just as for the platform 2 in Figure 1. The platform (102) is divided into 2 parts mounted together (102a and 102b). The platform part (102b) is used to connect the joints (113b and 114b) in such a way that it is possible to have angles both in the horizontal and vertical planes between a line through the joints (113b, 114b) and the platform part (102a). These angles are calculated to get appropriate rotation angles of the platform around a vertical axis when the platform is moved in radial direction relative to the central pillar (101) as well as in the vertical direction. lt should be noted that the links (113, 114) do not need to have the same length. Different lengths and deviation from parallelism will also make it possible to reduce the rotation of the platform (102) when it is moved around in the workspace. The central pillar supports the actuators (103, 104, 105), which swing the actuated arms (106, 107, 108). lt should be noted that both the pair of parallel links (109, 110) and the pair of parallel links (111, 112) must always form a vertical plane each, otherwise the platform (102) will not have a constant tilt angle in the whole 10 15 20 25 30 35 work space. This means for example that both the joint pairs (109a, 110a) and (111a, 112a) are mounted vertically and the upper arms (106, 107) are rotated around vertical axes defined by its actuators (103, 104).

By optimizing the joint positions (113a, 114a) on the upper arm (108) and the corresponding joint positions (113b, 114b) on the platform section (102b) it is possible to reduce the rotation of the platform (102) around a vertical axis when the platform is moved radially and vertically. Figure 3 shows one alternative of joint positions which reduces platform yaw Variations at both vertical and radial platform movements. ln the figure the alterna- tive mounting of the joint 103balt will further reduce the yaw var- iations of the platform. The mounting of the joint (103balt) is her made higher up in the z-direction and an optimal solution can even be achieved by having the joint 113balt above 114b. ln order to get higher stiffness of the robot at lower levels in the work space the lower arm links (113, 114) can be mounted on a beam (108b) which is more or less perpendicular to the upper arm (108), see Figure 4. This will result in the fact that when the upper arm moves towards an angle where it will be above the platform (102) corresponding to a singularity, the link (114) will have a smaller angle relative to the links (111, 112) than the link (113), reducing the forces in the links (113, 114). Therefore the stiffness of the robot will be increased in the lower parts of the work space.

The actuators (103, 104, 105) shown in Figures 2 - 4 are expen- sive. Either they are implemented with very costly direct drive ring motors or with large bearing pairs connected to ring formed gear trains driven by electrical motors. ln order to obtain lower cost of the actuators, an actuation arrangement as in Figure 5 can be used. Here the actuated arms (106, 107, 108) are mounted directly on the output shafts (122, 124, 126) of low cost servo actuators (121, 123, 125). The servo actuators consist of 10 15 20 25 30 35 mass produced motors, gears, brakes and resolvers or encod- ers. The servo actuators are mounted on a holder (127), which in turn is mounted on a low cost tube arrangement consisting of one horizontal tube (128), one vertical tube (129) and a foot plate (130). ln order to obtain maximum work space the arm (106) is mounted lower than the arm (107), making it possible for at least the inner part of arm (106) to pass under the inner part of arm (107) to obtain as large work space as possible.

Moreover, the tube (128) is made as long as needed for the arms (106, 107) to pass under the tube (128). lt should be ob- served that the shafts (122, 124) must be parallel and vertical to preserve constant tilt angles of the platform. ln most applications the platform (102) shown in figures 2 - 5 is equipped with a wrist to control the orientation of the tool or ob- ject handled by the robot. ln most pick and place operations it is enough to have a motor with a gear box to rotate the objects to be picked and placed. lf the orientation changes needed are less than 90 degrees it is possible to perform this with parallel kinematics as shown in Figures 8 and 9. ln Figure 9 a fourth servo actuator (300) mounted on the robot foot (130) or on the robot pillar (180) rotates a fourth arm (106b) connected to the platform by the link (114). Compared with Figure 7 the only dif- ference is that the link (114) has been moved from the upper arm (106a), which it shared with the links (109, 110) to the sep- arate arm (106b) which is used to change the yaw angle of the platform. ln the same way link (114) in Figure 6 can be moved to an added upper arm (108b) as illustrated in Figure 9. The new upper arm (108b) in Figure 9 is rotated by the servo actuator (131) mounted on the arrangement (127) via the beam (132).

Combining the solutions with extra servo actuators in Figures 8 and 9 and adding an extra servo actuator also under the arm 107 it is possible to actuate 6 degrees of freedom of the platform as shown in Figure 10. Here each lower arm link is actuat - ed by its own upper arm (106a, 106b, 107a, 107b, 108a, 108b) using the servo actuators (121, 300, 123, 301, 125, 131) respec- 10 15 20 tively. For best kinematics all servo actuator shafts are parallel and for each pair of servo actuators the shafts should coincide.

Figure 11 shows a roof mounted version of the robot in Figure 7.

The beam 200 belongs to for example a robot cell and the actuator platform 127 is mounted on the beam by means of a rod 128. The work space will be limited by collisions between the arm 108 and the beam 128 and the direction of the platform 128 in relation to the beam 200 determines the where the non sym- metric work space is obtained in relation to for example a con- veyor. Moreover, the placement of the beam on the platform 127 should be made in such a way that the collisions between the upper arm 108 and the beam 128 should occur when the upper arm 107 collides with the shaft 122. To avoid collisions between the upper arm 108 and the beam 128, a actuator concept, as il- lustrated in Figure 12, can be used for the actuator 125 in Fig- ure 11 Here the arm 108 is mounted on the bearing 220 driven by the motor 250 having the pinion 240 engaging the gear teeth of 230. The beam 210 is used to mount the platform 127 on the roof beam 200.

Claims (1)

10 15 20 CLAIM 1. An industrial robot including a manipulator for movement of an object in space, where the manipulator comprises: - a movable platform arranged for carrying the object, - a first arm arranged for influencing the platform in a first direc- tion and including a first inner arm part rotatable about a first axis, an outer arm part pivotally connected to the inner arm part and to the platform, and a first actuator for actuating the arm parts, - a second arm arranged for influencing the platform in a second direction and including a second inner arm part rotatable about a second axis, an outer arm part pivotally connected to the inner arm part and to the platform, and a second actuator for actuat- ing the arm parts, - a third arm arranged for influencing the platform in a third di- rection and including a third inner arm part rotatable about a third axis, an outer arm part pivotally connected to the inner arm part and to the platform, and a third actuator for actuating the arm parts.