Cable Stiffened Flexible Link Manipulator: Theory and Experiments

Dixit, Rahul and Kumar, R Prasanth (2019) Cable Stiffened Flexible Link Manipulator: Theory and Experiments. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Robotic manipulators play an important role in industry for tracking as well as pick and place applications. Manipulators are required to carry out the commanded task with good repeatability and precision. With the help of robotic manipulators, very high speeds of operation are possible thus increasing the production rate. In order to achieve good position accuracy and reduce the vibrations of the end effector, these manipulators are made to have high stiffness of links. High stiffness designs make the manipulators heavy and bulky. Therefore, the existing heavy Rigid Link Manipulators (RLM) are shown to be inefficient in terms of power consumption or speed of operation with respect to the operating payload. Also, the operation of high precision robots is limited by the tip positioning accuracy requirement. Space manipulators due to their long arms have difficulties in controlling the tip positions and therefore have to work at sufficiently low speeds in order to reduce tip positioning error. Flexible link manipulators (FLM), which are much lighter and highly flexible compared to RLM, have been proposed in the past as means to reduce energy consumption and increase the speed of operation. Unlike RLM, the FLM has infinite degrees of freedom. Due to the distributed flexibility affecting the precision of operation, special control algorithms are required to make them usable. But FLMs due to their distributed flexibility have challenges in overcoming tip deflections for larger payloads and higher speeds of operation. Structural failures and larger tip errors for sufficiently large payload and high speed of operation limit the usage of FLMs. Though several methods to control tip error were studied by researchers but FLM could not make a place in industry today. In this thesis, FLM was modeled using finite segment approach due to its easy implementation and simulations were carried out to study tip oscillations for various payloads and speeds of operations. Position as well as torque trajectories were given as inputs to the hub and tip position was evaluated. Hub torque requirement for given trajectory was studied for FLM and RLM and the comparison is made. Study was also made to show that if FLM thickness is increase to tolerate higher stresses, an equivalent RLM can be constructed for the same mass having better tip position accuracy. Concept of dynamic workspace was introduced and it was shown that a single link FLM can be commanded at the hub to avoid obstacle which otherwise is not possible with single link RLM. A method to stiffen FLMs using thin cables, without adding significant inertia or adversely affecting the advantages of FLMs, has been proposed as a possible solution. FLM stiffened using thin cables can use existing control algorithms designed for RLMs. Cable stiffened flexible link manipulator (CSFLM) has been studied in detail in terms of the extent of stiffening and acceptable tip position error. Optimal string attachment locations was studied and results validated with the help of experiments. Spatial CSFLM was modeled using finite segment approach and simulation studies were presented. Limitations of CSFLM were discussed along with the methods to overcome. The CSFLM was shown as the potential candidate to replace the RLM in the industry and space applications saving manipulator cost, energy consumption, etc. Such cable stiffened flexible link manipulators have the advantage of allowing the use of existing control techniques designed for RLMs. Effectiveness of this new approach is shown through simulations and experiments.

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