Implementing functional safety on new CNC machines has become an important detail for designers and builders of cutting, turning, multi-tasking and other types of machine tools. According to automation system developer NUM Corporation, during 2015 all of its OEM customers that are developing new machines using its latest Flexium+ control platform are also adopting some level of functional safety.

"Functional safety is the single most pronounced design trend we see right now," according to NUM's vice president of Customer Solutions & Quality, Peter Hutter. "However, for our markets, it's invariably being implemented at the same time as new machines are developed. Existing machine ranges with earlier-generation safety schemes tend to be left as they are.

“For NUM, a fundamental element of the trend is ease of implementation, and the simplicity of safety programming is proving to be a catalyst," he added. "Functional safety is a complex subject and many of our customers are small- to medium-sized companies, often with only one or two engineers that have PLC programming expertise. They frequently combine this work with other electrical and mechanical design roles, and they are not safety experts."

Techniques to simplify the design problem are critical, which is the basis for NUM's functional safety system, NUMSafe. Based on the use of a dedicated safety PLC, this solution is specifically designed to reduce development complexity and the time need to implement ‘safe motion’ functions according to prevailing safety standards (e.g., EN ISO 13849-1 and EN 62061.)

NUMSafe is described as a “build-to-order” functional safety program for the Flexium+ CNC platform that scales to suit the complexity of the machine control system. It includes a safety PLC, safety input and output (I/O) modules, digital servo drives with built-in safe motion monitoring facilities, and compatible brushless servo motors.

This system-wide architecture makes it possible for machine designers to install functional safety features where they are needed, with minimal additional components or wiring. It provides cost-effective protection for all types of CNC applications, from X-Y-Z-axis set-ups to complex automation for 100 or more axes.

An increasing number of NUM’s machine-tool building clients are specifying NUMSafe for new machine builds and CNC upgrades. As of late 2015, around 20% of the Flexium+ CNC systems that NUM ships now incorporate NUMSafe, and it expects this figure to rise substantially as high volume machine manufacturers (e.g., in Asia) expand their business activities via exports to markets that are subject to safety legislation.

“Many of our customers are niche players, with limited engineering resources,” Hutter explained. “Until recently, they used third-party safety solutions, which could only partially be integrated into the machines’ CNC control systems and often added many weeks of additional time to a machine's development cycle.

“By using NUMSafe, they are now able implement fully integrated, standards-compliant safety schemes in a fraction of this time and with much reduced cabling effort,” he said.

He listed several recent examples involving CNC tool grinding machines, which typically have at least five working axes (three linear and two rotary), each one requiring a variety of safe motion control functions. NUMSafe lets customers implement complete functional safety systems for their machines within a few days, in some cases even in just one day.

The most common functional safety technique for complying with basic safe-motion requirements (e.g., EN ISO 13849-1) is to implement 'Safe Torque Off' (STO) on appropriate axes. This function ensures that the drive or drives can no longer command motors to generate torque. The ‘Safely-limited Speed’ (SLS) function also is useful, especially when manual work needs to be performed on a machine with its doors open.