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‘Easy’ functional safety for new-generation CNC machines

28 February 2016

According to machine control vendor NUM, implementing functional safety on new CNC machinery is an overriding trend in the metalworking machinery marketplace.

"Functional safety is the single most pronounced design trend we see right now," says NUM's 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."

According to Hutter, a fundamental element of the trend is ease of implementation, and the simplicity of safety programming is proving to be a catalyst. "Functional safety is a complex subject and many of our customers are small to medium size companies, often with only one or two engineers that have PLC programming expertise," he says. "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 enablers in this large sector of the machine building market, and this is the philosophy behind NUM's functional safety system, NUMSafe. Based on the use of a dedicated safety PLC, this approach is designed to reduce development complexity and the time required to implement ‘safe motion’ functions compliant with standards such as EN ISO 13849-1 and EN 62061.
 
Introduced as a build-to-order option for NUM’s latest-generation Flexium+ CNC platform, NUMSafe is a complete functional safety system 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 enables machine designers to include functional safety features precisely where they are needed, with minimal additional components or wiring. It provides a simple, cost-effective solution for all types of CNC applications – from basic 3-axis machines through to complex automation with 100-plus axes. Peter Hutter again:

“Many of our customers are niche players, with limited engineering resources. 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 to implement fully integrated, standards-compliant safety schemes in a fraction of this time and with much reduced cabling effort.”
 
By way of an example, Hutter cites a number of recent applications involving CNC tool grinding machines – a market in which NUM has a very strong presence. Although the configuration of these machines obviously varies from one manufacturer to another, they typically have at least five working axes (three linear and two rotary), each of which requires a variety of safe motion control functions. In nearly every case, NUMSafe has enabled customers to implement complete functional safety systems for their machines within a few days – and in some cases even in just a single day.
 
The most common functional safety technique that is used to comply with the basic safe motion requirements of machinery safety standards such as EN ISO 13849-1 is the implementation of '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 is also very useful, especially when manual work needs to be performed on a machine with its doors open.
 
However, setting multi-axis machines to a safe state may also require specific axes to be driven to prescribed positions, or to satisfy other safety-related parameters, such as maintaining holding torque without moving. Also, 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, and so on. These all 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 key 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 the EtherCAT fieldbus, 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 minimises 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’, and so on, 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 behaviour. 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. They can all be implemented using either incremental or absolute encoders – apart from SLP, which requires safe homing and incremental encoders, or certified absolute encoders.


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