Independent Identification of Friction Characteristics for Parallel Manipulators

Abstract

The compensation for friction or joint losses in robotic manipulators contributes to an important improvement of the control quality. Besides appropriate friction modeling, experimental identification of the model parameters is fundamental toward better control performance. Conventionally steady-state friction characteristics are investigated for mechanical systems in the first step. However, and due to the high kinematic coupling, such procedure is already complicated for complex multiple closed-loop mechanisms, like parallel manipulators. Actuation friction of such mechanisms becomes configuration dependent. This paper presents a methodology that deals with such challenge. The kinematic coupling is regarded in the friction model and therefore in the design of the experimental identification. With the proposed strategy, it is possible to identify the steady-state friction parameters independently from any knowledge about inertial or rigid-body dynamics. Friction models for sensorless passive joints can also be provided. Besides, the method is kept very practical, since there is no need for any additional hardware devices or interfaces than a standard industrial control. The suitability for the industrial field is proven by experimental application to PaLiDA that is a six degrees of freedom parallel manipulator equipped with linear directly driven actuators.

Practical Model-based and Robust Control of Parallel Manipulators Using Passivity and Sliding Mode Theory

“…The overall friction that occurs in each actuator is obtained from (33) by means of kinematic transformation (Abdelllatif et al, 2007):…”

“…This is especially the case for parallel robots, since the friction forces discussed here depend on the system's configuration. As demonstrated in (Abdelllatif et al, 2007), the resulting friction that is to be counteracted by an actuator j is expressed as:…”

“…to open-chain robots, where the friction forces depend only on the actuator's velocity. The interested reader is referred to the article (Abdelllatif et al, 2007) for deeper insight into the consideration of friction for parallel manipulators.…”

“…Friction in the passive joints is modeled only as dry friction with a common parameter for all α j (the first revolute joint of each strut) and another one for all β j -joints (the second revolute joint of each strut). The friction model contains therefore 14 different parameters (see Table 1 for an overview and (Abdelllatif et al, 2007) for more details). For the experimental validation of the proposed control, two exemplarily test motions (see Fig.…”

Dynamic Parameter Identification for Parallel Manipulators

“…For parallel manipulators, the direct approach has been applied (Renaud et al, 1993;Guegan et al, 2003;Farhat et al, 2008). Meanwhile, the indirect approach has been proposed (Grotjahn et al, 2004;Abdellatif et al, 2007). However, apart from error accumulation in each step, the separation of the parameters of a different nature is not straightforward as for open chain manipulators.…”