Virtualization Helps CNC Machines Consolidate Real-time Processing

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Biesse’s Rover Gantry 512 installed at Simard Cuisine et Salle de Bains in Quebec City

The dual-core processor runs Windows on one core and the INtime RTOS on the other core

Computer numerical control (CNC) machines are some of the most complex systems in the manufacturing world. Even the simplest CNC machines may have four motors driving motion axes that must all move synchronously, at high speed, with very high precision in order to maximize the machine’s productivity and quality of output. The controllers that orchestrate the motion typically must make new decisions on how the motion axes must move and establish a new target position every one or two milliseconds.

The loss of a single control cycle is a mission critical failure. If the machine were to fail to update its target positions correctly for just a millisecond or two, inaccurate cuts could be made and the machine could jam or become unstable. And in order to guarantee that the machine’s outputs are produced on time, the machine’s inputs need to be sampled much faster – typically many times faster than output decisions need to be made, according to control system experts. Controls engineers call the phenomenon of missed control loop times “jitter” because if one were to look at a graph of the motion produced by imprecise control loops, it would have jagged lines or small discontinuities. Jitter is directly responsible for irregular movement of the tooling resulting in poor quality work. To avoid this, many machines are designed to enter a fault state and stop moving if the servo drives that affect the motion don’t receive at each cycle a new control message from the CNC. This causes expensive downtime while the problem is fixed or reset. But more importantly, having this type of fault at all would cause the machine to fail to meet the requirements of the market.

But even a one- or two-millisecond response time is not the fastest response that the system must make. Because of high precision timing and oversampling requirements of some fieldbus (machine network) interfaces (e.g. EtherCAT), the cycle times of control loops that manage these need to be as short as 100 microseconds. A human-directed operating system such as Microsoft Windows can not manage all of these control tasks simultaneously and guarantee that they will be processed with predictable timing (i.e., determinism), so most high-speed machines run their control loop software on real-time operating systems (RTOS).