Electrical safety with production-line robots

A fresh approach to robotic safety pioneered by Kuka Roboter and Pilz has been adopted by many of the world's largest robotic companies, including ABB, Fanuc Robots and Yaskawa Motoman.

All have partnered with Pilz to develop a direct interface between the Pilz Programmable Safety System (PSS) and the robot's Electronic Safety Circuit (ESC). Among the benefits are enhanced diagnostics, flexibility, reliability and plant availability.

The basic functions of a robot's safety system include:

• Robot status;
• Local and remote emergency stop;
• Drive status;
• Operating mode;
• Robot reset;
• Status of guarding equipment.

In addition, special functions can be assigned in a Pilz SafetyBUS Robotic Integration Specification, since large numbers of digital I/O allow for the monitoring and connection of peripheral devices, such as welding transformers and valves.

The interface is the most sophisticated of four alternative approaches to the implementation of robotic safety: centralised safety relays; a modular safety PLC with parallel hardwiring; a PSS with remote I/O using SafetyBUS; or a PSS with direct SafetyBUS interface to the robot's ESC.

Safety relays are centralised in a control cabinet close to the robot, with parallel wiring to a cabinet and within the same cabinet. The safety logic is achieved with 24 V signals and further parallel wiring is needed for limited diagnostic functions.

Using a modular safety PLC, the safety controller would be mounted in the cabinet with parallel hardwiring. This solution simplifies programming with certified software blocks for the relay logic.

Online diagnostics are available and communicated to PLCs and human machine interfaces (HMIs) over conventional fieldbus. A real advantage of this second alternative over the first is the continuous monitoring and self-testing.

Because it is electronic, this method also offers wear-free semiconductor outputs.

This has similar benefits to the centralised safety PLC approach, with the same controller and conventional fieldbus connectivity, however fieldbus wiring facilitates installation.

SafetyBUS provides three-core cabling to the control cabinet and direct interfacing to limit switches and the robot ESC via SafetyBUS remote I/O units. Additional diagnostics are also available.

Apart from the ease of programming, rapid and simplified cabling, wear-free contacts, continuous monitoring and self-testing, the added benefits spring from direct interfacing of the robotic safety circuits:

• Reduced systems complexity and comprehensive diagnostics deliver increased productivity and lower lifecycle cost of ownership.
• Increased flexibility. Safety logic can be modified without the need to change hardware and the use of fieldbus for communication minimises alterations when the system is reconfigured.
• No extra safety I/O modules are needed (limit switches are supported by the ESC gateway).
• Robot safety programming is made easy with the Pilz failsafe robot software function block.
• Reduced installation and commissioning due to easier safety logic programming, simplified wiring and testing.

The advantages of the ESC/SafetyBUS gateway have been proven by BMW Germany - the system's first installation. Kuka and Pilz began their collaboration for BMW's Body in White line in Dingolfing in response to growing payloads of more than 125 kg, wider work ranges and increasing cycle times of up to 20 cycles per minute.

The system architecture on a typical line comprises 50 assembly stations, with a personal computer for process monitoring, diagnostics, archiving, loading of programs and data exchange between the different control layers. An ethernet links the PCs to the programmable logic controller (PLC), which interfaces to the PSS safety controller via 2 MB Interbus fibre for diagnostic and status information. Other Interbus devices include operator consoles, valve terminals, drives and portable welders.

BMW has achieved a clear separation between standard control and safety, with the plant meeting the requirements of category 3 or 4 where appropriate. Up to 40 safety related devices can be connected to each SafetyBUS network to the Pilz Safety Controller, including emergency stops, light curtains, scanners and safety gates. These are interfaced with the Kuka robot ESC/SafetyBUS gateway at each workstation, robot or conveyor station.

The system has won BMW flexibility and scope for future expansion, regardless of the interfacing requirements of individual stations and location of emergency stops.

"Because it is electronic, this method also offers wear-free semiconductor outputs. This has similar benefits to the centralised safety PLC approach, with the same controller and conventional fieldbus connectivity, however fieldbus wiring facilitates installation."