1. Gravity Balancing
1.1. Gravity-compensation for the SurgiScope robot
There is a large number of gravity balancing techniques that are used in order to reduce, or cancel, the actuator input efforts during a quasi-static mode of operation for mechanical system. Amongst those methods, the most usual ones are:
- The addition of counterweights: the main issue with such kind of methods is the considerable increase of the robot total mass
- The use of springs: the main issue is the increase of the spring size when the robot must carry heavy loads
Those methods may be well adapted for some types of industrial applications. However, they are undesirable for the medical operations.
Therefore, the company ISIS was looking for a new balancing system for its manipulator SurgiScope (Fig. 1) and started a partnership with the INSA of Rennes. The SurgiScope is a based on a Delta robot and is designed in order to carry out a microscope system with a weight of 70 kg.
In the case of the SurgiScope typical applications, the displacement speed of the platform is not essential because there is no need for productivity acceleration. However, as a result of the increased mass of the platform (about 70kg), the input torques became important. Thus it became evident that the platform’s mass must be balanced. In this case, the traditional approaches with counterweights and springs mounted on the moving links are not applicable. The Delta robot has a complex structure and after such balancing it becomes either very heavy or a complicated assembly with several complementary articulated dyads. That is why another means for the solution of this problem is proposed.
Fig. 1. SurgiScope robot sold by the company ISIS.
The gravity balancing of the SurgiScope was the topic of the thesis of Cédric Baradat, but I also worked in collaboration with him on the modelling of the balancing system.
It consists in the addition of a secondary mechanism between the manipulator base and moving platform. This mechanism, based on a passivepantograph linkage, can create a supplementary vertical forceon the platform to balance the gravitational forces of the robot (Fig. 2).
Fig. 2. Balancing system of the SurgiScope
Experimental results have shown that it was possible to reduce the input torques up to 80% (Fig. 3).
Moreover, the auxiliary balancing system has a good influence on the robot deformations which can be reduced by 80%.
Fig. 3. Experimental measures of input torques for three actuators of the SurgiScope robot in the balanced and unbalanced cases
More information can be found in:
C. Baradat, V. Arakelian, S. Briot and S. Guégan, "Design and Prototyping of a New Balancing Mechanism for Spatial Parallel Manipulators," ASME, Journal of Mechanical Design, 2008, Vol. 130, No. 7.
S. Briot, C. Baradat, S. Guégan, and V. Arakelian, "Contribution to the Improvement of the Medical Device SurgiScope®," Proceedings of the 2007 ASME Ine-y out a microscope system with a wei; theg;,&quenter"><1res oc;,&qropdown-tog (DETCrosco). 7.
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