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Functional safety architecture cuts costs for CNC machine-builders

04 February, 2013

NUM has released a scalable functional safety architecture for CNC applications that can be applied to almost any machine tool – regardless of its complexity or number of axes.

The NUMSafe system includes a dedicated safety PLC, safe I/O modules, and digital servodrives with built-in safe motion monitoring. It is compatible with NUM`s new Flexium+ CNC platform.

Until recently, designers wanting to implement even basic functional safety on CNC machines had to use electromechanical technologies such as special contactors and relays. This involved extra component and wiring costs, as well as longer machine development and build times. By integrating safety functions across a CNC platform, NUMSafe is said to minimise the need for additional hardware and to simplify software development.

End-users will benefit from increased machine productivity because they no longer need to power off a whole machine whenever there is a human interaction. Instead, protection can be ensured by dedicated functions which limit the movement, speed and position of the axes.

“Functional safety is becoming a mandatory requirement for most types of machine tool,” explains NUM`s chief technical officer, Massimiliano Menegotto. “As well as improving operator safety, it also increases machine productivity and availability. NUMSafe`s inherent scalability and versatility of control gives OEMs a simple solution, regardless of whether the end-product is a basic three-axis machine, or a highly complex machine with 100-plus axes, for applications such as rotary transfer machining.”

The new architecture (above) allows safe devices, such as the safety PLC and I/O modules, to be contained in the same standard terminal line-up as other elements of the control system. All communications between the machine`s control system, operator panels and servodrives are done via EtherCat, using the Fail Safe over EtherCat (FSoE) protocol to ensure integrity of the safety-related data. The system is suitable for applications up to PLe / SIL 3.

NUM’s NUMDrive X digital servodrives can incorporate a safe motion monitoring module that works with the safety PLC to oversee all safety-related aspects of drive and motor behaviour. By allowing safety functions to be implemented on individual axes, this modular approach helps to cut costs.

The basic safe motion monitoring module provides a Safe Torque Off (STO) function. An extended version adds other functions including SOS, SS1, SS2, SLS and SLP, using either safe homing and incremental encoders, or certified absolute encoders such as those used with NUM`s new single-cable servomotors.

Used individually or in combination, the safety functions allow designers to implement various machine safety concepts. For example, SS2 can monitor that the braking of an axis is as quick as possible in an emergency. This can be followed by the SOS function which instructs the drive/motor to maintain a fixed position by generating a degree of holding torque, but not to move – the axis is monitored continuously to ensure that it remains stationary.

There is no need for an extra encoder on axes that implement safety functions. Motor feedback can be derived from safe encoders which return position and redundant position data to the drive using a secure certified protocol or, in the case of synchronous motors, any sin/cos encoder.

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