Parallel robot with variable link length mechanisms.

Moldovan, Cristian ; Dolga, Valer

1. INTRODUCTION

In this paper the authors are offering an alternative solution to the construction of a parallel robot. The presented solution consists in replacing the bar mechanism used in the parallel robots construction with a variable link length mechanism.

Also the main kinematic schema of the robot is presented in the paper, including the resulting workspace of the robot.

Theoretical works related to parallel mechanisms, and particularly hexapods, date back to centuries ago, when English and French geometricians were obsessed with polyhedra.

In 1928 James E Gwinett developed a parallel robot for the entertainment industry. This was a spherical parallel robot.

A decade later and only seventeen years after the term "robot" was coined, a new parallel robot was invented for automated spray painting by Willard L.V. Pollard.

In 1954, Dr. Eric Gough builds the first octahedral hexapodat Dunlop Rubber Co., Birmingham, England.

In 1965, Stewart's famous paper appeared in the proceedings of the (British) IMechE. In that paper, Mr. Stewart describes a 6-DOF motion platform for use as a flight simulator.

In 1971, a patent is granted to Klaus Cappel regarding a motion simulator. The patent was filed on December 7, 1964, at which time Mr. Cappel was unaware of Gough's invention (or of Stewart's paper which was not yet published).

2. THE DELTA PARALLEL ROBOT

The authors were inspired by the most successful parallel robot, the DELTA parallel robot designed by Reymond Clavel professor at EPFL--Ecole Polytechnique Federale de Lausanne. He comes up with the brilliant idea of using parallelograms to build a parallel robot with three translational and one rotational degree of freedom. In 1999, Dr. Clavel is presented with the Golden Robot Award, sponsored by ABB Flexible Automation, for his innovative work on the DELTA parallel robot.

The basic idea behind the Delta parallel robot design is the use of parallelograms. A parallelogram allows an output link to remain at a fixed orientation with respect to an input link. The use of three such parallelograms restrains completely the orientation of the mobile platform which remains only with three purely translational degrees of freedom. The input links of the three parallelograms are mounted on rotating levers via revolute joints. The revolute joints of the rotating levers are actuated in two different ways: with rotational (DC or AC Servo) motors or with linear actuators. Finally, a fourth leg is used to transmit rotary motion from the base to an end-effector mounted on the mobile platform.

[FIGURE 1 OMITTED]

The use of base-mounted actuators and low-mass links allows the mobile platform to achieve accelerations of up to 50 G in experimental environments and 12 G in industrial applications. This makes the Delta robot a perfect candidate for pick and place operations of light objects (from 10 gr to 1 kg). Ideally, its workspace is the intersection of three right circular tori. The Delta robots available on the market operate typically in a cylindrical workspace which is 1 m in diameter and 0.2 m high. In Fig.2. are the parallelograms wich are the basis elements of the DELTA robot recognizable. This figure demonstrates how orientation can be achieved using parallelogram mechanisms. (Clavel, 1991)

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. PARALLEL ROBOT WITH VARIABLE LINK LENGTH MECHANISMS

The robot has as central piece a spherical joint (1). This introduces 3 degrees of freedom to the end effector. The spherical joint is attached to the case and considered as a fixed element. To this spherical joint is attached a translation joint (2) trough a bar element. Of the translation joint (2) is the end effector (3) attached trough a bar element. Also in the case are the three motors (4) fixed which serve to positioning the end effector. The motors are linked to the end effector trough flexible and inextensible wires (5)(Perju, 2001). The roller from the motor and the flexible wire build a mechanism with variable link length (Perju, 1971). The 3 motors are situated at an angle of 120 degrees from each other. The positioning of the end effector being made by the simultaneous actuation of the motors.

The kinematic schema is presented in Fig.3.

With the three motors is the positioning achieved on a spherical surface with fixed radius of the end effector and with the translation joint is the depth positioning achieved, resulting a spherical workspace (Lovasz, 2000).

[FIGURE 4 OMITTED]

An other type of robot with mechanisms with variable link length is presented in figure 5.

[FIGURE 5 OMITTED]

The main elements remain the same:

(1) Spherical joint

(2) Translation joint

(3) End Effector

(4) Motor

(5) Flexible Wire

The difference between the first version and the second version is that only the position for the actuation of the mechanism has been changed, the main schema of the mechanism remaining the same.

4. FUTURE WORK

The authors consider that the kinematic schema stage could be in a real robot transformed and practically realised.

Another area where such robots could be used is the stepper robots. In this case of advantage is the spherical joint which takes over the weight of the body, actuation not being needed at the moment when the walking robot uses the robotic foot for support. The loads that are supported by such a structure are bigger than on the classic parallel structure.

Another ideea is to create a small compact cell, with own microcontroller, which can communicate with other similar cells, all controlled by the main computer, in order to achieve movement of the walking robot. The software in the main computer should be adaptable for walking on 2, 3, 4 or 6 legs, based on how many cells the walking robot has.

5. CONCLUSIONS

Parallel robots are usually faster than traditional articulated robots, since the motors can be mounted on the base, thus saving weight. They are also stronger than serial robots because the end effector is connected to more links.

Another benefit is that the error of the end effector is less than the errors of serial robots since the errors are averaged. However, parallel robots are usually more limited in the workspace; for instance, they generally cannot reach around obstacles (Bruyninckx, 2005).

This paper presents an alternative kinematic schema to the parallel robot, based on mechanisms with variable link length, tough the structure needs future optimization because research is in the beginning stages. The introduction of the variable link length mechanisms also replaces the use of classical linear motors.

6. REFERENCES

Bruyninckx, H. (2005). Parallel robots. Available from: http://www.roble.info/robotics/paraUel/pdf/ParallelRobots1.pdf, Accessed: 2008-03-15

Clavel, R. (1991). Conception d'un robot parallele rapide a 4 degres de liberte. PhD thesis, Ecole Politechnique Federale de Lausanne

Lovasz, E-C.; Perju, D. & Mesaros-Anghel, V (2000). On the mechanisms synthesis of centroidal type. Proceedings of IFToMM International Symposium, Liberec

Perju, D. (1971). Planar mechanisms for leading a point on a given curve (ro: Sinteza mecanismelor plane pentru conducerea unui punct pe o curba data). PhD thesis, Politechnic Institute Bucharest

Perju, D.; Modler, K-H; Lovasz, E-C & Mesaros-Anghel, V (2001). A new type of function generating mechanism with variable link length. Proceedings of IFToMM International Symposium, Bucharest.