CANopen in a proton beam machine

Source: CAN Newsletter December 2005

Motion control systems, capable of positioning patients to within half a millimeter, are used for commercial operation at Europe’s first proton beam therapy facility - the Rinecker Proton Therapy Centre in Munich. An eight-axis control system was designed for the machine.
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 tons, housing a gantry with 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 tons, to provide the stiffness required for accurate and repeatable positioning of the long load. At the heart of the control system is a PC. Control is provided by a Linux-based application written in C language. This issues motion commands to the actuator using a library of C-compatible functions. The system integrator 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 the C library had to be written. As the proton beam machine has the potential to do damage if a patient is positioned incorrectly, the company 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 PCI bus card plugged into the PC, connected via CANopen to six drives and servo motors. Two further axes on the card are also used by the integrator 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.