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15th Real Time Linux Workshop, October 28 to 31, 2013 at the Dipartimento Tecnologie Innovative, Scuola Universitaria Professionale della Svizzera Italiana in Lugano-Manno, Switzerland

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The LinuxCNC Machine Control Portable Realtime API

Michael Haberler, The LinuxCNC Project
Alexander Rössler, The Vienna Reprap Group

LinuxCNC, a package to control machine tools such as milling machines and lathes, until recently was limited to RTAI and hence x86 platforms. OS support has been abstracted to provide package-level binary compatibility over RTAI, Xenomai and RT-PREEMPT kernels, supporting RTAI and Xenomai kernel threads, and Xenomai, RT-PREEMPT, and Posix userland threads. LinuxCNC ARM7 builds are active use in the BeagleBone, Raspberry and Reprap communities.

We present an overview the "electronic circuit"-style HAL realtime component architecture, and the requirements for, and implementation experience with the portable RTAPI layer which HAL rests on: component handling, threading, unified shared memory, exception and log handling.

We discuss the impact of measured latency figures at the RTAPI layer in machine control terms: commanded path noise causing position hunting and PID loop excitation, and tool stress caused by angular noise in electronically geared motion, as well as potential software countermeasures.

A demonstration of LinuxCNC on a BeagleBone, running a MendelMax 3D printer with high-speed steppers driven by the AM3359 programmable realtime unit (PRU) will be given in the break.