Design of a continuum robot for in-situ repair of aero engine.
PhD thesis, University of Nottingham.
Unlike conventional rigid-link robots, continuum robot, also known as elephant trunk and snake arm robot, has numerous numbers of degrees of freedom, which enables it to be used for accessing confined places in many fields, e.g. minimally invasive surgery, and safe robot/objective interactions, e.g. rapid handling. Up to now, most of the researches are driven to develop two kinds of continuum robots, i.e. flexible and rigid backbones, which can be structured with either small diameter but short length or long length but large diameter. Further, according to the observation of this work, the conventional flexible backbone has a twisting problem when bending in the horizontal plane with end load, rendering a poor position control. Therefore, designing a ‘slender’ continuum robot enabling to be employed in in-situ repair of gas turbine engine is still a challenge, since it requires a long length, small diameter, appropriate flexibility and variable stiffness simultaneously.
In the research of this PhD thesis, two unique concepts of continuum robot designs were proposed, i.e. double- and twin-pivot compliant joint constructions. By employing compliant joints, the continuum robot was enabled to be built with small diameter/length ratio, appropriate flexibility, stiffness, and minimised twisting angle. Further, a variable stiffness system was developed in this research, which allows the robot arm able to be articulated in a relatively low stiffness state and dramatically enhance its stiffness in a relatively high stiffness state. With these features, this system was able to be navigated into gas turbine engine (Rolls-Royce Trent XWB) and activate inspection and in-situ repair tasks.
Since the new continuum robot concepts were introduced, the fundamental modelling was developed for both design and control of the new structures. Firstly, position kinematics models were developed: one for double-pivot construction deployed a new derivation approach, which can simplify the procedure; the other for twin-pivot construction employed a two-sub bending plane model, due to unique construction of the robot, which is different to the conventional method. Secondly, the actuation force analysis was derived, enabling to calculate the action force of an arbitrary section in a multiple-section continuum robot with any bending angle. Further, buckling failure is a major obstacle for designing the compliant joints, since flexible structure can experience buckling. Hence, the analysis of compliant joint critical buckling load was introduced for guiding the hardware design. Also, a general approach for deriving Jacobian and stiffness matrix of continuum robot was presented in this work.
According to the concept and modelling of the new concepts, four demonstrators of continuum robots were built and tested. Comparing with the conventional concept, the double-pivot and twin-pivot concept can decrease the twisting angle by 67% and 98.6%, respectively. Further, in the machining trails, it has been proven that a three-section twin-pivot backbone continuum robot can provide an appropriate stiffness, control accuracy (± 1mm error for sweeping in any ± 5º area in the work volume) and repeatability (± 0.5 mm error in the whole work volume), enabling the system to blend metal materials, e.g. aluminium and titanium, which are the materials widely employed in aerospace industry. Next, a two-section variable stiffness system was tested on this demonstrator and the TCP displacement caused by end load can be decreased by up to 69%. Finally, accessing in gas turbine engines has been realised by the final full length continuum robot (1266mm). It has been proven that the system has an appropriate control accuracy to be navigated to reach the first stage of LPC (low pressure compressor) of a gas turbine engine (Rolls-Royce XWB) by following a pre-planned path.
Therefore, it can be concluded that the study of this PhD thesis provides a unique continuum robot design concept, which can be utilised for in-situ repair of gas turbine engine.
Thesis (University of Nottingham only)
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