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Module with Direct Actuation System

Abstract In order to cope with the issues arisen by analysing the first version of the system presented in Chap. 5, a new version of the module has been designed and it will be described in this chapter. It will be based on direct actuation of the joints. Furthermore, we will describe the introduction of an important feature as the possibility to regulate in real time the nominal distance between the modules. This feature allows the control sectors to slide on the strip, thus increasing the performances of the whole system. In this chapter we will also describe the kinematic analysis that have been performed, the design of the components and the developed prototype so as to to evaluate the pros and the limits of this implemented solution. Eventually, we will describe the developed control process, which allows to perform the rendering of virtual surfaces.

Issues and Solutions

The simulations realized and the prototype developed allowed us to perform a deep analysis on the issues and problems of the first solution of the device. This study allowed us to analyse the drawbacks and, on the basis of this analysis, we have designed a second solution, which will result improved if compared to the previous one in terms of performance, dimensions, resolution and implementation costs.

The main problems of the first solution of the system are:

  • • High sensibility of the transmission to the mechanic plays;
  • • Difficulty in increasing the stiffness of the whole module without increasing also the width of the element;
  • • High sensibility to the stiffness of the transmission belt;
  • • Impossibility to change the nominal distance between the control sectors.

The sensibility to the mechanic plays is due to the complexity of transmission system. Indeed, the high number of the elements needed to transfer the motion between the servomotors and the elements requires an high amount of joints and connexions, which introduce inevitable plays. We have developed a complex transmission system because our aim was to host the servomotors in charge of actuating the elements Arm1 and Arm2 at the basis of the module in order to avoid errors due to inertial © The Author(s) 2017 61

A. Mansutti et al., Tactile Display for Virtual 3D Shape Rendering,

PoliMI SpringerBriefs, DOI 10.1007/978-3-319-48986-5_6

loads. This choice requires several mechanisms to transfer the motion, in particular to the element Arm2. Also the problems related to the increase of the members stiffness and the transmission belt elasticity are caused by the configuration of the transmission system. Indeed, in order to increase the stiffness of the whole module, we need to increase the thickness of the sheets of which the elements Arm1 and Arm2 are made up. The presence of the pulleys does not allow us to increase the thickness inwards but force us to increase the width of the elements. This solution increases the minimum nominal distance between the control sectors, thus decreasing the minimum resolution of the system. As regards the transmission belt, we have installed a reinforced belt, which presents low stiffness. We have also developed a system in charge of adjusting the tension of the belt. In any case, notwithstanding all these precautions, the system is still too sensible to the stiffness and to the plays of the pulleys-belt transmission system.

Sliding control sectors

Fig. 6.1 Sliding control sectors

In order to solve the above-described issues, we have decided to redesign the whole module, so taking the opportunity to add an important feature as the possibility to regulate in real time the nominal distance among the module.

In the first solution of the system the translation along Z direction provided by the rail-runner system allows us to regulate the distance between two control sectors during the strip deformation. Indeed, this distance has to decrease when the strip is bended to follow the elastic line of the material. In any case, the control sectors are fixed on the strip. Therefore, it is possible to change in real time the nominal distance, which is the distance among the control sectors when the strip is in planar configuration.

To increase the performances of the whole system we have developed the new version of the system, which will allow the control sectors to slide on the strip. This system will allow us to place the control sector in the best position for each trajectory that has to be represented. As shown in Fig. 6.1, in the nominal configuration all the control sectors are placed at the same distance. Thereafter, thanks to the possibility to slide the control sector on the strip, it is possible to modify the nominal distance according to the trajectory, which will be represented. Thanks to this feature, it is possible to place the control sector in a given point of the trajectory, such as inflexion points. This allows us to choose in real time the best configuration of the system according to the characteristics of each trajectory that has to be rendered.

 
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