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.
Specific Safety Logic, Functions
However, setting multi-axis machines to a safe state also may require specific axes to be driven to prescribed positions, or to satisfy other safety-related parameters, such as maintaining holding torque without moving. And, each type of machine will have different protection devices – such as protective doors, interlock systems and light barriers – as well as a different structure in terms of its loading area, geometry of axes, etc. All these require specific safety logic and specific safety functions for correct machine management.
However, if the CNC system’s PLC is involved it can lead to unnecessarily complex control schemes with redundant logic, causing significant additional set-up and programming overheads.
NUMSafe overcomes these issues by using a dedicated safety PLC and clearly differentiating between standard and safety-related logic. Machine designers only need to implement safety functions that are pertinent to the task in hand, without worrying about extraneous functions for motion axes that are not involved in orderly shutdown routines.
Another advantage of NUMSafe is that safe devices such as the safety PLC and safety I/O modules can be housed in the same standard terminal as other elements of the control system, further simplifying integration. All communication between the machine’s control system and servo drives is handled via EtherCAT field bus, using a Fail Safe over EtherCAT (FSoE) protocol to ensure the integrity of safety-related data.
The application program for the safety PLC is created using the same suite of software development tools that is used to commission the overall system, including CNC, PLC, drives and I/O modules. This unified development environment minimizes learning time and reduces code generation overheads. The logic of the safety application is programmed using function blocks such as ‘E-Stop’, ‘Operation Mode’, ‘AND’, ‘OR’, etc., linked to safety inputs and outputs. Complex safety functions can be set up easily by chaining function blocks.
NUM’s latest NUMDrive X digital servo drives, which are available in single- and dual-axis versions, offer an optional safe motion monitoring module that operates with the safety PLC to oversee and control all safety-related aspects of drive and motor behavior. This modular approach helps to reduce system costs, by allowing safety functions to be implemented on individual machine axes, and only where required.
There are two versions of safe motion monitoring module, enabling designers to match their application needs very precisely. The basic module provides the STO function. The extended version also offers STO, as well as the additional EN 61800-5-2 compliant functions of Safe Operating Stop (SOS), Safe Stop 1 (SS1), Safe Stop 2 (SS2), SafelyLimited Speed (SLS), Safely-Limited Position (SLP) and Safe Direction Monitoring (SDM). Used singly or in combination, these functions enable designers to embrace a wide range of machine safety concepts. All of them can be implemented using either incremental or absolute encoders – apart from SLP, which requires safe homing and incremental encoders, or certified absolute encoders.
NUM offers a range of compatible brushless servo motors, too. These include single-cable models developed specifically for use with NUMDrive X servo drives, which use a dedicated two-wire link embedded with the motor’s power cable to carry power, position and redundant position feedback data to/from a safe digital encoder.