Controlling Ultra-Precise Movement

A high-integrity motion control system, capable of positioning patients to within half a millimeter, is currently being commissioned for commercial operation at Europes first proton beam therapy facility - the Rinecker Proton Therapy Centre in Munich.

A novel eight-axis control system has been designed for the machine by IPG of Switzerland. It is based on motion control components from Baldor, and the companys Mint-language motion commands - specially ported to run under Linux for this project to provide an open and maintainable system.

Proton beam irradiation therapy is used for the treatment of tumors, and offers considerable advantages over X-rays as it does less collateral damage. This is because the energy that accelerated protons deposit increases as they slow down, reaching a peak at the target point - allowing unhealthy tissue to be destroyed with great precision.

The proton beam machine consists of a large steel cylinder weighing 100 tonnes, housing a gantry with very large magnets that guide the accelerated proton beam. The patient is driven into the cylinder and positioned in the path of the beam to within a half millimeter, by means of a table with control of X, Y, Z, rotation, pitch and roll axes based on closed-loop servo motors. The table itself weighs 4.5 tonnes, to provide the stiffness required for accurate and repeatable positioning of the long load.

At the heart of the control system is a PC fitted with Baldors PCIbus-based multi-axis motion controller, NextMove-PCI. Control is provided by a Linux-based application written in C. This issues motion commands - compatible with sophisticated Mint motion language - to NextMove-PCI using Baldors library of C-compatible Mint functions.

IPG wanted to use Linux for the project in order to obtain open access to the source code - for maintainability and a long lifetime - but in order to do this, a new Linux driver for Baldors Mint-compatible C library had to be written. As IPG had a considerable experience with Linux, Baldor commissioned them to do this, and provided training and technical support to aid the project.

The result gave IPG a very flexible motion software environment, with a library of high-level commands providing sophisticated movement functions. Among the advanced Mint features that IPG employs in the control system are Mints BLEND, CONTOURING and S-RAMP commands. These allow moves to be overlapped with no jerky transitions - in this instance to ensure ultra-smooth movement and positioning of the patient.

Behind this smooth motion, however, is a highly complex control program that converts polar coordinates into the Cartesian coordinates required for the three-dimensional patient positioning, and which also corrects for the non-linearities of the table and the slight flexing that occurs depending on the extension from its mountings. IPG simplified this problem by dividing movements down into sequences of smaller steps. IPG also uses Mints Feedrate Override commands, a facility that allows users to set high and low velocity rates, and then switch the entire motion system between them with a single command.

As the proton beam machine has the potential to do damage if a patient is positioned incorrectly, IPG designed safeguards into the motion system. Each axis has at least two absolute position sensors - one of which is connected to a security monitoring application program running on a separate PC. The movement commands issued by the control system are monitored by the security PC. At the end of a movement, for fail-safe operation, the two must agree that the system is in the right place before the proton beam can be switched on.

The motion subsystem consists of a NextMove-PCI bus card plugged into the PC, connected to six Baldor FlexDrives and BSM servo motors. Two further axes on the card are also used by IPG to control the linear extension of the proton beam nozzle as it is set up for a treatment session, and a heavy-duty industrial motor that adjusts the magnet gantry.

The Mint languages sophisticated motion functions helped to simplify this complex project, notes Alexander Ferro, IPGs Head of Automation Engineering. It allowed us to concentrate our efforts on developing the application-specific algorithms that underpin the positioning movements, and on the overall control system.

Weve worked with IPG on projects before, and our willingness to provide strong technical support to help resolve their problems proved a decisive factor in winning the contract for this innovative machine, adds Daniel Berg of Baldor Switzerland, who managed the project for Baldor. In this case we helped them to port Mint onto the Linux platform - a technology that is now available to our wider customer base.

Three of the proton beam machines have been built, and are now installed at the Rinecker Proton Therapy Centre in Munich. They are currently being commissioned in preparation for the start of patient treatment in March 2005.

IPG is a subcontractor of Schaer Engineering AG (Flaach, Switzerland), who are employed by ACCEL, who in turn work for the main project contractor m+w zander of Stuttgart, Germany.