The orthotic and prosthetic industry is currently seeing tremendous levels of innovation, with improvements in materials technology in particular driving the development of ever better artificial limbs, improving both the comfort and mobility of amputees and other patients. But for amputees in particular, the process of actually being fitted for a new limb has always been a slow and undignified one.

The difficulty lies in producing a socket which accurately fits the residual limb. Traditionally this has been achieved by taking a plaster cast of the residual limb - a process which is time consuming, messy and inconvenient. Once the cast had been made it would need rectifying, shaping, which can take up to two hours a very lengthy messy process for the prosthetist, a socket would be produced. An accurate first-time fit isn't always guaranteed, with the socket generally requiring modification or a complete re-cast to give a comfortable fit.

Against this background, and drawing on developments in 3D shape mapping technology, Ortho Europe set out to develop an automatic mould carving machine, which could accept a 3D software mapping of the residual limb and quickly carve a finished mould. The resulting OrthoCarve machine is a computer numerically controlled (CNC) carver combining advanced motion control technology with high quality DC servo motors to achieve the necessary accuracy and repeatability.

For an effective control system for the OrthoCarve, Ortho Europe turned to NEE Controls, the UK's leading developer and supplier of control systems for complex 3D applications. Project manager Mark Jarvis explains: "We obviously wanted to work with a controls company who had experience in 3D machining, but we also wanted a control system that would help to give us a clear competitive edge over alternative carving solutions. At the same time, we were looking for a technology partner - someone who could work with our own engineers to provide a control system precisely tailored to our needs, and who could then deliver complete control systems as virtual kits, ready to install in the OrthoCarve straight out of the box. The idea was to get the best possible control system whilst reducing our supplier base, keeping the cost down and facilitating a quick installation."

NEE Controls has over 30 years of motion control experience, and is today the UK's largest independent motion control manufacturer. Providing a design resource to many of its customers, the company's engineers regularly attend customers design meetings, working in close partnership with OEMs and helping them to minimise their design cost, risk and machine production cost.

NEE Controls AMC family of motion controllers and associated software are ideally suited to complex 3D applications where a lot of program data needs to be executed as quickly as possible. "The OrthoCarve application is characterised by the high levels of data generated by the 3D mapping," says Jarvis. "With a number of the control options we looked at, the controller simply couldn't keep up the carving speeds we needed to achieve, but this wasn't the case with the AMC products".

What Ortho Europe had recognised was that many control systems actually slow down a process by the way they try to optimise data. With any controller in continuous motion applications, the stream of linear, arc, bezier or spline commands fed to the controller will generally specify a continuous, multidimensional tool path. From this data, the 'motion planner' within the controller needs to generate a velocity profile for each axis, keeping it within its acceleration, deceleration and terminal velocity limits. In order to achieve smooth motion, the 'motion planner' must know what is to be executed ahead of what is currently being executed - a function termed 'look-ahead'. Many controllers employ a strategy of looking-ahead a fixed number of moves. A number of controllers on the market do not look more than about ten moves ahead, but if those moves are all very short then this can significantly limit how quickly the controller executes a part of the program that requires high cut speeds with accurate positioning.

The alternative strategy adopted by NEE Controls within its AMC range is to look-ahead a fixed period in time rather than a fixed number of moves. This ensures very smooth motion, accurate changes in direction, and no slowing down of the machine due to the control system having to catch up. Of course, this alternative strategy places stringent demands on the controller's microprocessor, which is why the AMC range employs high performance 32 bit RISC processors.

For the OrthoCarve machine, NEE Controls developed a customised control solution built on its AMC4 servo controller. The AMC4 servo controller provides comprehensive control functionality in a single, robust enclosure, combining a high performance RISC processor board with integrated PLC, optically isolated signal interfaces, low and medium power motor drives, safety interface and all power supplies. Supporting up to four servo or stepper axes plus two additional stepper axes, the AMC4 is fully configurable to match the specific needs of the application.

Meeting Ortho Europe's requirement for a ready-to-install kit, NEE Controls provides the control system complete with all the necessary servo motors and drives, proximity sensors, a custom made wiring harness, wiring interface panel and WinAMC interface panel software.

WinAMC is a fully flexible operator interface that has been specifically developed to generate virtual control panels for use with NEE's AMC controllers. WinAMC provides both AMC control and status monitoring, and runs on any PC using Windows 95/98/NT/Me/XP, including industrial PCs, touch screen systems, laptops and desktop computers.

The package provides a set of virtual switches, buttons and display components which can be assigned to specific functions and grouped together to create a control panel that provides the exact functionality required for the particular application. The control panel is displayed in its own window, with multiple control panels being supported. Control panel combinations can be saved as 'user profiles' with a user name and password to create, for example, separate sets of operator panels and maintenance engineer panels.

WinAMC uses a proprietary secure communications protocol which automatically detects the presence of an AMC controller on the serial link, and communicates with it at baud rates up to 115,200 baud.

"WinAMC provides us with very powerful software for creating operator panels", says Jarvis. "The package was very quick and easy to learn, and has allowed us to develop an operator interface which offers clear screens and simple menus". Making the OrthoCarve simple to operate is one of keys to raising overall productivity. In addition, because WinAMC is multi-lingual - supporting any language that Microsoft Windows supports - we know we can develop machines for use anywhere in the world.

"NEE Controls have worked very closely with us throughout the development process", Jarvis continues. "They scored over competitors not just on the quality of the products, but also because these systems are completely customisable, with the company designing special features in hardware and software to match the needs of our application. The result is that OrthoCarve is delivering new benchmark standards for mould carving, producing a high quality, accurate, finished mould is just three and a half minutes".

Whilst the vast majority of profiles that the OrthoCarve produces are moulds for above or below knee amputees, it has the flexibility to handle everything from small upper extremity shapes to large spinal shapes, carving at a very fine resolution for a high degree of accuracy and best patient fit.

OrthoCarve has been optimised for use with TracerCAD, the market leading prosthetic and orthotic CAD system for producing 3D models of residual limbs. Indeed, this optimisation can extend to full integration with the TracerCAD package, allowing Ortho Europe to offer a complete turnkey CAD/CAM solution for mould production.

The process of creating the socket for the artificial limb begins in the TracerCAD software, using either a 'Tracing Pen' or 'T-Ring' to capture a 3D representation of the patient's residual limb. With the Tracing Pen, the prosthetic practitioner literally traces the shape of the residual limb, whilst the alternative T-Ring (using synchronized digital imagers) provides automated 3D shape captured in less than one hundredth of a second with a simple push of the button. TracerCAD turns the captured data into a 3D representation, this can then be transferred to the OrthoCarve machine as an ISO G&M code file.

WinAMC panel

Here, WinAMC provides an operator interface between TracerCAD and the AMC4 controller, allowing the operator to download the TracerCAD generated file to the AMC4, with the controller subsequently generating the required machining profiles.

The moulds are carved from polyurethane foam blanks, with the finished mould representing an exact likeness of the residual limb. The mould is then transferred to the clinics workshop and draped or laminated with plastic to form a snug and comfortable socket for the artificial limb.

"The reduction in time between taking the initial 3D data and fitting the finished limb is dramatic", says Jarvis. "A number of orthotic and prosthetic centres are buying OrthoCarvers for in-house use. Once the socket has been produced, the rest of the limb is made up from off-the-shelf items products. Therefore it is likely that an amputee could arrive at the centre for an initial consultation in the morning, and in theory walk out the same day with a fitted limb".

Since its launch, OrthoCarve has quickly established itself as one of the most advanced and reliable carvers in the marketplace. Currently the company is producing around two machines a month, but is expecting to quickly ramp up production to eight machines a month as demand increases. In addition to the standard OrthoCarve machine, Ortho Europe is now working with NEE Controls to develop faster and smaller models.

"I don't believe we've yet reached the limit in carving speed for the machine as it stands", says Jarvis. "And NEE Controls tells us that it also has a new 64 bit engine board for the AMC range on the way, which is backwardly compatible, and will provide a 20-fold performance improvement within the controller. Now that could really offer some exciting possibilities…"