Professor Dragos Axinte and Dr John Allen at the Rolls-Royce University Technology Centre (UTC) in Manufacturing Technology at The University of Nottingham and Mr. Ralph Anderson of Rolls-Royce have been awarded a Rolls-Royce Submarines Excellence through Innovation Award for a revolutionary new machining ‘robot’.
The honour recognises their work on the unique special-purpose miniature machine tool they call a ‘Free-leg Hexapod’ — or FreeHex. Their advance has created a portable version of this flexible machining system that promises to reduce costs as well as time spent fixing problems.
The engineers’ research resulted in the brand new concept of a miniature six-axis parallel kinematic platform that can apply complex computerised milling processes to a wide range of in-situ maintenance jobs.
This is one example of how Nottingham UTC researchers in collaboration with European partners, such as Tekniker in Spain, tackle ‘real-world’ engineering challenges, delivering solutions through the University’s Rolls-Royce UTCs.
Precise machining in tight spaces
The idea for the FreeHex came from a request made by Rolls-Royce’s submarine business. It required in-situ repairs to be done in a very confined workspace. Its own machine was heavy and hard to move — difficult to use in tight working environments — so Rolls-Royce asked for something smaller and more flexible with the same functions.
“Rolls-Royce presented us with a challenge that tested our abilities and gave us a chance to rethink traditional machine tools. Many of these are still ‘serial manipulators’ like robot arms; a chain of rigid links in a series. These serial kinematic machines (SKMs) stack independent stages to provide multi-axis movement; but this can lead to compound errors,” says UTC Director Professor Dragos Axinte.
“Alternative parallel kinematic mechanisms driven by actuators, often telescopic ‘jacks’ and ball-screw drives linked in pairs, are flexible but also more complex.
Our development of a Free-leg Hexapod, a unique parallel kinematic configuration without base platform so that the lower joints (feet) can be attached to the surfaces of various geometries, takes this kind of robotised in-situ processing machine a reality. This initial development will be continued as a FP7 EU-funded project, MiRoR, for which our key partner, Tekniker, will support the manufacture of the Walking FreeHex.”
Attracting interest from industry
This device design has now been patented and Rolls-Royce is looking at using the machine in other parts of its business, such as aerospace. Professor Axinte believes the portable CNC machine could create a new machine tool market segment for in-situ repair.
“Previous attempts to produce machines for in-situ maintenance resulted in bespoke machines designed for just one purpose,” says Professor Axinte. “Our machine has enough flexibility to tackle many tasks. The technology has great potential — it’s a new type of CNC machine,” he says, “and we’ve had strong interest from many businesses, including Formula One teams.”
“The Free-hex machine design is mechanically very simple. It is a versatile, lightweight machine tool and the prototype has served extremely well to demonstrate the practical potential for carrying out complex machining operations, such as profile milling and thread milling, in situ on in-service engineering plant. The technology offers us a genuine alternative to our traditional bespoke machine designs,” says Ralph Anderson of Rolls-Royce.
Stable and precise
“Part of our work analysed ‘parallel kinematics’ (describing motions of joints, bodies or objects, and systems of bodies) of a six-actuator architecture and its advantages and disadvantages,” says Professor Axinte. “These CNC (Computer Numerical Control) machines benefit from high rigidity, accuracy and dynamic response, but can experience inconsistencies of movement and lower stiffness.”
The machine they developed is a Stewart-platform-type hexapod structure. Six telescopic struts or legs with ball-screw drives work in parallel to position the moving platform. A compact high-speed spindle is mounted in the centre of this platform between the legs.
This hexapod is distinctive because the designers removed the base, or fixed platform, and allowed the positions of the joints at the lower ends of the extensible actuators to be varied — hence ‘Free-leg’ Hexapod. So the FreeHex can be positioned on different workpiece surfaces, or non-flat geometries.
Kinematics calculation model
Because the legs are gathered into pairs on three separate feet which can be temporarily attached to a workpiece in any given position, this gives the machine much greater flexibility, and consequently adds a lot more variability and complexity to the inverse kinematic model that describes it.
New calibration methods to reference the platform to the workpiece, work volume calculations, and collision assessment procedures were developed as part of the research. Axinte and Allen use a full ‘inverse kinematics model’ to describe the positions of active joints in relation to a given position and orientation of each ‘end-effector’ or tool (known as a ‘tool pose’).
To more accurately model and calibrate the free feet and legs on different surfaces, in different spaces, the researchers developed a kinematics calculation model for complex parallel structures based on ‘mean of multiple computations’ (MMC).
This somewhat complex modelling process requires input of various constraints, but can theoretically solve the kinematics problem for any structure including flexible systems and walking hexapods.
Reducing disruptive forces
The researchers looked at forces likely to act on the structure in different situations and considered how to reduce these. “To drill a large hole you need a lot of torque, or twisting force. For that you need a lot of power and a solid structure,” says Professor Axinte.
“We took a different approach, using a small tool to make lots of shallow cuts very quickly, making it a miniature low force machining system,” he says. This miniature multi-axis machine tool operates at 50,000rpm with a feed speed of 3m per minute. It can produce anything a conventional CNC machine does, such as slots, cut-offs and freeform surfaces.”
Portable, multi-functional, safe and flexible
The Nottingham machine weighs less than five kilos, has a small footprint and can be installed where the work needs to be done quickly and effectively. Its computer numerical control ensures highly accurate and repeatable machining which can operate remotely in dangerous working environments.
“It could be used in power plant repairs, to repair a diesel engine on a boat, or plant in the offshore oil and gas industry,” says Professor Axinte. “You could use it remotely in hazardous environments too.”
The FreeHex brings a new level of functionality to on-site maintenance. Rather than wait for a workshop to make and deliver a specialised part, the new portable computerised device brings workshop functionality on site and approaches repair tasks intelligently. Combined with remote monitoring of equipment and analysis of data for predictive maintenance, this adds a new element to industry efforts to boost efficiency, reduce plant downtime and minimise costs.
The work is in Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture as ‘Theoretical analysis of a special purpose miniature machine tool with parallel kinematics architecture: free leg hexapod’ (published online 21 November 2011). It can be accessed here: http://pib.sagepub.com/content/early/2011/11/16/0954405411415775
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