NUM (Naperville, Ill.) has released a sophisticated system-wide functional safety architecture for its CNC system, providing an exceptionally scalable solution that can be applied very easily to almost any type of machine tool — regardless of complexity or number of axes. Known as NUMSafe, it includes a dedicated safety PLC, safe input and output modules, and digital servo drives with built-in safe motion monitoring. It is compatible with NUM's new-generation Flexium+ CNC platform.

A key advantage of the NUMSafe architecture is that safe devices such as the safety PLC and I/O modules can be contained within the same standard terminal line up as other elements of the control system. All communication between the machine’s control system, operator panel and servo drives is handled via EtherCAT field bus, using a Fail Safe over EtherCAT (FSoE) protocol to ensure integrity of safety-related data. NUMSafe complies with the EN ISO 13849-1 machinery safety standard up to PL e, and with the EN 61800-5-2 functional safety standard for variable speed drives, up to SIL 3.

To help machine builders minimize development time the application programs for the safety PLC are created using the same suite of powerful software tools that is used to commission the entire system, including the CNC, PLC, drives and I/O modules. The logic of the safety application is programmed using function blocks such as ‘E-Stop’, ‘Operation Mode’, ‘AND’, ‘OR, etc, linked to safe inputs and outputs.

Complex safety functions can be set up easily by chaining function blocks. For example, requesting the machine operator to perform an action requiring use of both hands – such as operating two control switches simultaneously – to prevent exposure to moving parts, and verifying this before allowing execution of any motion command, only requires a couple of instructions. The safety application is a self-contained program; as soon as the developer is satisfied with its logic it can be downloaded to the safety PLC.

NUM’s latest NUMDrive X digital servo drives, which are available in mono- and bi-axes versions, can incorporate a safe motion monitoring module that operates in conjunction with the safety PLC to oversee and control all safety-related aspects of drive and motor behavior. By allowing safety functions to be implemented on individual machine axes, and only where required, this modular approach helps reduce system cost. Furthermore, NUM offers two versions of the module, enabling designers to match their application needs very precisely. Both versions feature redundant channel architecture and use cross-monitoring techniques to ensure data integrity.

The basic safe motion monitoring module provides a Safe Torque Off (STO) function; this is the most commonly used safety function and ensures that the drive can longer command the motor to generate torque, with continuous monitoring to prevent unexpected start-up. In addition to STO, the extended version of the module implements the EN 61800-5-2 compliant functions of Safe Operating Stop (SOS), Safe Stop 1 (SS1), Safe Stop 2 (SS2), Safely­Limited Speed (SLS) and Safely-Limited Position (SLP) either by means of safe homing and incremental encoders, or by employing certified absolute encoders such as those used with NUM’s new line of innovative single-cable servo motors.

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