Pergamon Robofics & Computer-Integrated Manqfiieluring, Vol. 13, No. 4, 319-321, 1997 pp.

0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

07365845/97 517.00+0.00

PII:SO736-5845(97)00011-Z

?? Paper

UNIVERSAL COMPLIANT DEVICE BASED ON SCARA CONCEPT

DRAGAN MILUTINOVIC Mechanical Engineering Faculty, Department of Production Engineering, University of Beograd, 27 Mart 80,

Y 11000 Beograd, Yugoslavia

The paper presents the results of the research and the development of the universal adaptive compliant device for customary assembly task robot configurations. The developed system is based on a newly developed SCARA robotic mechanism and combined properties of the SCARA concept and RCC devices. It successfully operates around its true compliance center, obtained under precisely determined conditions. 0 1997 EIsevier Science Ltd. AU rights reserved.

1. INTRODUCTION 3. SYSTEM CONFIGURATION Flexible automation of the assembly process is a crucial problem in modem industiy. Assembly robots and other systems have been used to solve this problem.’ Substantial research efforts were devoted in the past to part mating mechanics, the develop- ment of mating devices, specialized assembly robots and various control strategies in order to achieve high efficiency of automatic assembly machines and robots. The combination called macro-micro robot, coarse-fine positioning devices, etc., proved very useful in various fields, as well as in assembly tasks.*

Thanks to its structural capabilities the new SCARA robotic mechanism was used for the development of universal passive adaptive compliant attachment device to customary robot configurations (Fig. 2). The mechanism with rotating discs D1 and D2 is represented by two joined links mechanism with I, and I2 segment lengths and joint rotational com- pliances cl and c2.

The paper describes the research and the develop- ment of the universal adaptive compliant device for customary robot configurations for assembly tasks. The developed prototype of the new passive adaptive compliant device3 is based on the newly developed robotic mechanism- and combined properties of the SCARA concept and RCC devices.

This passive compliant device is based on the selective compliance approach’ and the assumption that the macro robot has sufficiently smaller compliances in all directions in comparison with the compliances of the device described.

2. THE OUTLINE OF THE NEW SCARA ROBOTIC MECHANISM

The author has invented and developed a new robotic mechanism4 based on the well-known SCARA concept.’ The mechanism has two eccen- trically positioned rotating discs instead of two joined links (Fig. 1) and is suitable for small assembly robots, offering a number of advantages as described in detail in.5T6

The subject of the research presented in part here and in Ref. 3 was the application of this mechanism in the development of a universal compliant device and its use as an attachment to the macro robot wrist end. This well-known strategy, called coarse-fine positioning, offers many benefits.*

Fig. 1. SCARA robot based on newly developed robotic mechanism.

319

320 Robotics & Computer-Integrated Manufacturing ??Volume 13, Number 4, 1997

A + .

2 ,w ., lateral error

Fig. 2. Universal passive adaptive compliant device based on SCARA concept.

The device has small compliance in the ZE direction and desired large compliances in the Xn and Yu directions, providing a pure compliance center obtained under precisely determined condi- tions, 3s79 i.e. when

132 = &A rc cos(-Z,/Z2), 12 < I, (1)

and when

c,/cz = (l,/z2)2 - 1. (2)

As joint displacements are small, joint rotational compliances cl and c2 are achieved by four equal springs with compliances c situated at distances RI and Ra. The required relationship determined by Eq. (2) will then be

cl/c2 = Rf/R;. (3)

This approach facilitates the change of inserting forces by a simple change of springs, and reduces construction constraints related to link lengths Ii and 12.

As the device is always in the configuration providing a true lateral compliance center with small compliances about the Xn and Yn axes and a large one about ZE when the peg reaches the chamfered edge of the hole, it moves laterally without bending, thus preventing jamming. Since in many practical applications’ the angular error seems to be sufficiently small, the entire system, macro robot and compliant device, provides the necessary small rotational compliances, primarily from small deflections in the macro robot wrist joints.

In this way the macro robot’s lateral plane compliance center is achieved in every portion of its workspace.

4. PROTOTYPE DEVELOPMENT AND TESTING

In order to compensate for lateral errors located in the circle with radius r,, = 1 mm, enabling the use of the device on robots with low positioning accuracy, link lengths Ii and l2 have to be carefully selected to provide sufficiently small dimensions and mass of the device, and sufficiently small joint rotations.

By adoption of a well-accepted link lengths ratio” 11/l;? = 1.6, computer simulations and analysis of the error circle rmax= 1 mm, including the values of several link lengths, were made. According to Eqs (l)--(3) the following link lengths satisfy previously quoted requirements through a compromise Ii = 8 mm, Z2= 5 mm, so that d2= 128.6”, cl/ c2=1.56, RI=25 mm and R2=20 mm.

On the basis of the adopted concept and estimated design parameters, the first prototype of the device has been developed (Fig. 3).

The device has a compact structure with small dimensions and mass, and a significant degree of modularity and universality, and may, therefore, be attached to different macro robots and toolplates as the upper interface plates and toolplates are standar- dized.

An important feature is the easy exchange of springs, and, thus, extensive changes in insertion force, as well as inclusion of the locking system during the movement to the assembly position enabled by the specific structure of the device.

The prototype was tested on a vertical milling machine and on an articulated robot (Fig. 3). Hole diameters were D = 20 mm, with chamfers of 1 x45” and clearances of l/10 mm, 5/100 mm and 2/100 mm between peg and hole. The insertions were made on lateral errors up to 1 mm for eight significant angle positions of contact points between the peg and the chamfered edge of the hole.

The experiments on the vertical milling machine, as a rigid and precise machine at which angle insertion errors may be considered negligible, have shown that the device easily and very quickly compensates all lateral errors smaller than chamfer dimensions.

Numerous insertion experiments with the device

Universal compliant device based on SCARA concept ??D. MILUTINOVIC 321

universal passive adaptive compliant device for customary robot configurations for assembly tasks. It successfully operates around its true compliance center, obtained under precisely deter- mined conditions. This enables the macro robot’s lateral plane compliance in every portion of its workspace.

The theoretical and practical approach to this problem as well as its solution represent a step toward the development of a more sophisticated sensorized and motorized device to be used as an attachment to the macro robot’s wrist end, which is already the subject of current research.

Fig. 3. The prototype of the device installed on an articulated robot.

mounted on articulated robots have also given the same results when angle insertion error was kept at a minimum, because small rotational compliances were primarily due to small deflections in macro robot wrist joints.

5. CONCLUSION Combined effects of the SCARA concept and RCC devices were used in the development of a new

1.

2.

3.

4. 5.

6.

7.

8.

9.

10.

REFERENCES Makino, H. and Arai, T., New developments in assembly systems. Annals of the CIRP, 1994, 43(2), 501-512. Kochekali, H., Nowrouzi, A., Kavina, Y. B. and Whitaker, R. A., Factors affecting robot performance. Industrial Robot, 1991, 1, 9-13. Milutinovic, D., Micro SCARA robot as universal adaptive compliant wrist. Annals of the CIRP, 1996, 4J( l), 3 l-34. Milutinovic, D., YU Patent 47846. 1996. Milutinovic, D., Potkonjak, V. A new concept of the SCARA robot. Robotics and Computer-Integrated Manufacturing, 1990, 314, 337-343. Milutinovic, D. and Makino, H., A new robotic mechanism based on the SCARA concept. In Proceedings of the 24th International Symposium on Industrial Robots, Tokyo. 1993, pp. 801-806. Japan Industrial Robot Association. Makino, H. and Furuya, N., Selective compliance assembly robot arm. In Proceedings of the 1st International Conference on Assembly Automation, Brighton. 1980, pp. 77-86. IFS Publications. Asada, H. and Slotine, J. J., Statics. In Robot Analysis and Control. John Wiley, New York, 1986, pp. 73-92. Bourriers, J. P., Jeannier, P. and Lhote, F., Intrinsic compliance of position-controlled robots-applications in assembly. In Proceedings of the 5th International Conference on Assembly Automation, Paris. 1984, pp. 133-142. IFS Publications. Makino, H., Standard performance test methods for planar positioning assembly robots. Annals of the CIRP, 1985, 34(l), 33-36.