Accuracy improvement is an important research topic in the field of cable-driven parallel robots (*CDPRS). One reason for inaccuracies of *CDPRS are deviations in the cable lengths. Such deviations can be caused by the elongation of the cable due to its elasticity or creep behavior. For most common *CDPRS, the cable lengths are controlled using motor encoders of the winches, without feedback about the actual elongation of the cables. To address this problem, this paper proposes a direct cable length measurement sensor based on a laser distance sensor. We present the mechanical design, the first prototype and an experimental evaluation. As a result, the measurement principle works well and the accuracy of the measured cable lengths is within −2.32 mm to +1.86 mm compared to a range from −5.19 mm to +6.02 mm of the cable length set with the motor encoders. The standard deviation of the cable length error of the direct cable length measurement sensor is 58% lower compared to the one set with the motor encoders. Equipping all cables of the cable robot with direct cable length measurement sensors results in the possibility to correct cable length deviations and thus increase the accuracy of *CDPRS. Furthermore, it enables new possibilities like the automatic recalibration of the home pose.
The installation of curtain wall modules (CWMs) is a risky activity carried out in the heights and often under unfavorable weather conditions. CWMs are heavy prefabricated walls that are lifted normally with bindings and cranes. High stability is needed while positioning in order not to damage the fragile CWMs. Moreover, this activity requires high precision while positioning brackets, the modules, and for that reason, intensive survey and marking are necessary. In order to avoid such inconveniences, there were experiences to install façade modules in automatic mode using robotic devices. In the research project HEPHAESTUS, a novel system has been developed in order to install CWMs automatically. The system consists of two sub-systems: a cable driven parallel robot (CDPR) and a set of robotic tools named as Modular End Effector (MEE). The platform of the CDPR hosts the MEE. This MEE performs the necessary tasks of installing the curtain wall modules. There are two main tasks that the CDPR and MEE need to achieve: first is the fixation of the brackets onto the concrete slab, and second is the picking and placing of the CWMs onto the brackets. The first integration of the aforementioned system was carried out in a controlled environment that resembled a building structure. The results of this first test show that there are minor deviations when positioning the CDPR platform. In future steps, the deviations will be compensated by the tools of the MEE and the installation of the CWM will be carried out with the required accuracy automatically.
Using fully-constrained cable robots as manipulators for 3D-printing, there is the risk of collisions between the cables and the printing part.
This paper presents a method to calculate the shape of the workspace volume within which a part can be printed without such collisions. The presented method is based on the fact that the printing part is produced in a sequence of horizontal layers. The areas occupied by the cables in the layers are scaled similar mappings of the cross-sections of the printing part. There is no collision if the 2D-shapes occupied by the cables in the printing layer do not overlap with the cross-sections of the printing part in the same layer. A procedure to find the largest printable 2D-shapes within the class of parallelograms for each layer is developed. The maximum printable 3D-volume is then given by stacking the 2D-shapes of each layer. Figures show the results of the method applied on the cable robot IPAnema 3. Finally, a guideline for the design of fully-constrained cable robots to maximize their printable volume is given.
Cable-driven parallel robots can have a much larger workspace than other parallel robots with rigid links or conventional serial robots. This property comes at the cost of more complex workspace calculations and control schemes that are necessary to account for the elasticity and unilateral force transmission of their parallel cable links. In practice, most cable-driven parallel robots cannot achieve the full workspace that is predicted by theoretical models. This is due to calibration errors and simplified modelling assumptions in the control schemes. While most previous works on this subject have focused on creating more accurate and complex models, the goal of this work is to increase the workspace volume that cable-driven parallel robots can realize in practice by using a simple model coupled with a new force correction method that is robust to modelling errors and uncertainties. The new method applies force corrections within the nullspace of the structure matrix in order to keep the cable forces within their limits. Experiments show that the new method can significantly increase the workspace when used in addition to basic kinematic codes. In simulations this combination achieves the same workspace as complex controllers that require the precise knowledge of many additional model parameters.
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